Danfoss: Facts Worth Knowing About Frequency Converters (wordt in een nieuw venster geopend)

Danfoss: Facts Worth Knowing About Frequency Converters (wordt in een nieuw venster geopend)
Handbook | VLT® Frequency Converters
Facts Worth Knowing
about Frequency Converters
Preface
In 1968, Danfoss was the first company in the world to commence mass production of
Frequency Converters, for variable speed control of three-phase induction motors.
Today FC’s are an increasingly important component for optimising motor operation,
and the system attached to the motor. FC’s are now used in an expanding range of
applications, with the following main objectives in mind:
• Energy efficiency optimisation: Converting from fixed to variable speed in
applications with varying load, delivers a step change in energy savings. In fact these
days, modern motor technology always requires advanced control in order to run
optimally at all speeds.
• Factory automation: Continuously escalating demand for factory throughput
leading to a higher degree of automation implies a growing need for variable speed
solutions.
• Process control and optimisation: Improved process control often requires variable
speed motor control and leads to more precise control, higher throughput, or
comfort, depending on the application.
The fundamentals of FC technology persist, but many elements are also rapidly
changing. Increasingly, software is embedded in today’s products, offering new
functionalities and enabling the FC to play a larger role in the system. New motor types
are appearing, placing additional demands on motor control. This in turn means the FC
must be able to control an expanding variety of motor types, without burdening the
end user with more complexity. In addition, new energy efficiency requirements lead
to more variable speed applications, eventually making all motors variable speed and
controlled by a FC.
With this latest update of “Facts worth knowing about frequency converters”, we at
Danfoss would like to continue the heritage from previous versions of this book. We are
proud of what we do and are enthusiastic about FC’s. With this book we hope to convey
some of this enthusiasm to you!
If you would like to learn more, please feel free to contact Danfoss.
Jakob Fredsted
Vice President, Research & Development
Danfoss Power Electronics A/S
© Copyright Danfoss
Content
3
Content
Page
0 Introduction ......................................................................................................................................... 7
0.1 Speed Control of Electrical Motors ....................................................................................... 7
0.2 Why use Speed Control? .......................................................................................................... 8
0.3 How to Adjust the Motor Speed ............................................................................................ 8
0.4 Frequency Convereters ............................................................................................................. 9
1 Electric Motors...................................................................................................................................10
1.1 Overview ......................................................................................................................................10
1.2 Fundamentals ............................................................................................................................11
1.2.1 Stator and Rotor .............................................................................................................11
1.2.2 Power and Torque ..........................................................................................................12
1.2.3 AC and DC Motors .........................................................................................................13
1.2.4 Electromagnetic Induction.........................................................................................13
1.2.5 Poles, Synchronous Speed and Asynchronous Speed .....................................14
1.2.6 Efficiency and losses .....................................................................................................15
1.3 Asynchronous Motors .............................................................................................................17
1.3.1 Rotating Field ..................................................................................................................17
1.3.2 Squirrel Cage Motor ......................................................................................................19
1.3.3 Slip, Torque and Speed.................................................................................................21
1.3.4 Typical Operating Conditions ....................................................................................23
1.3.5 Changing Speed .............................................................................................................24
1.3.6 Motor Nameplate and Star or Delta Configuration ...........................................27
1.4 Synchronous Motors................................................................................................................29
1.4.1 Permanent Magnet (PM) Motors ..............................................................................29
1.4.1.1 Back EMF ...........................................................................................................31
1.4.1.2 Torque and Speed Range ...........................................................................32
1.4.2 Brushless DC (BLDC) or Electronically Commutated (EC) Motors ................33
1.4.3 Line Start PM Motor (LSPM Motor) ..........................................................................34
1.4.4 Reluctance Motors.........................................................................................................35
1.4.5 Synchronous Reluctance Motor with Squirrel Cage .........................................36
1.4.6 Synchronous Reluctance Motor (SynRM) ..............................................................37
1.4.7 Switched Reluctance Motor (SRM) ..........................................................................39
2 Frequency Converters....................................................................................................................41
2.1 Direct Converters ......................................................................................................................41
2.2 Converters with Intermediate Circuit ................................................................................42
2.3 Rectifier.........................................................................................................................................44
2.3.1 Uncontrolled Rectifiers ................................................................................................45
Content
4
2.4
2.5
2.6
2.7
2.8
2.9
Page
2.3.2 Semi-controlled Rectifiers ..........................................................................................46
2.3.3 Fully-controlled Rectifiers ...........................................................................................47
2.3.4 Active Front-End/Active Infeed.................................................................................48
Intermediate Circuit .................................................................................................................48
2.4.1 Variable Intermediate Circuit .....................................................................................49
2.4.2 Constant Intermediate Circuit ...................................................................................51
Inverter .........................................................................................................................................52
Modulation Principles .............................................................................................................55
2.6.1 Pulse Amplitude Modulation (PAM) ........................................................................55
2.6.2 Pulse with Modulation (PWM) ...................................................................................56
2.6.3 Asynchronous PMV .......................................................................................................59
2.6.3.1 SFAVM ................................................................................................................59
2.6.3.2 60° AVM .............................................................................................................62
Control Circuit and Methods ................................................................................................63
2.7.1 Simple Control Method ...............................................................................................65
2.7.2 Scalar Control with Compensation..........................................................................66
2.7.3 Space Vector with and without Feedback ............................................................67
2.7.3.1 Space Vector (Open Loop) ..........................................................................67
2.7.3.2 Space Vector (Closed Loop) .......................................................................68
2.7.4 Open Loop and Closed Flux Vector Control .........................................................69
2.7.4.1 Flux Vector (Open Loop) .............................................................................69
2.7.4.2 Flux Vector (Closed Loop) ...........................................................................70
2.7.5 Servo Drive Control .......................................................................................................70
2.7.6 Control Conclusions ......................................................................................................71
Danfoss Control Principles.....................................................................................................71
2.8.1 Danfoss VVCplus Control Principle ..........................................................................72
2.8.2 Danfoss Flux Vector Control Principle ....................................................................76
Standards and Legislations ...................................................................................................77
3 Frequency Converters and Motors ..........................................................................................78
3.1 Basic Principles...........................................................................................................................78
3.1.1 U/f Operation and Field Weakening........................................................................78
3.1.2 87 Hz Characteristics.....................................................................................................80
3.1.3 Running in Current Limit .............................................................................................82
3.2 Compensations..........................................................................................................................82
3.2.1 Load-independent Start Compensations .............................................................83
3.2.2 Load-dependent Start Compensations .................................................................83
3.2.3 Load Compensations ....................................................................................................84
3.2.4 Slip Compensations ......................................................................................................84
3.2.5 PM Motor and SynRM compensations ...................................................................85
Content
5
Page
3.3 Automatic Motor Adaption (AMA)......................................................................................86
3.4 Operation.....................................................................................................................................87
3.4.1 Motor Speed Control ....................................................................................................87
3.4.2 Reversing ..........................................................................................................................88
3.4.3 Acceleration and Deceleration Ramps (Ramp Up and Down).......................89
3.4.4 Motor Torque Control ...................................................................................................91
3.4.5 Watchdog .........................................................................................................................92
3.5 Dynamic Brake Operation......................................................................................................93
3.5.1 Extending Deceleration Ramp ..................................................................................94
3.5.2 Motor as a Breaking Resistor......................................................................................94
3.5.3 Brake Chopper Circuit (Brake Module) and Resistor .........................................95
3.5.4 Use of a Regenerative Braking Unit .........................................................................95
3.6 Static Brake Operation ............................................................................................................96
3.6.1 Coasting to Stop .............................................................................................................97
3.6.2 DC Braking ........................................................................................................................97
3.6.3 DC Hold .............................................................................................................................97
3.6.4 Electromechanical Brake .............................................................................................97
3.7 Motor Heating and Thermal Monitoring ..........................................................................98
3.8 Functional Safety ................................................................................................................... 100
4 Saving Energy with Frequency Converters ...................................................................... 104
4.1 Potential .................................................................................................................................... 104
4.2 Motor + Frequency Converter Efficiency ...................................................................... 105
4.3 Classification of Energy Efficiency ................................................................................... 106
4.4 Energy Efficient Motor Start ............................................................................................... 109
4.5 Energy Efficient Motor Control ......................................................................................... 111
4.6 Load over Time........................................................................................................................ 113
4.6.1 Applications with Variable Torque ........................................................................ 113
4.6.2 Applications with Constant Torque ...................................................................... 115
4.7 Life Cycle Costs ....................................................................................................................... 116
4.8 System Savings ....................................................................................................................... 117
4.9 Using Renegerated Power .................................................................................................. 119
5 Electromagnetic Compatibility .............................................................................................. 122
5.1 EMI and EMC ............................................................................................................................ 122
5.2 EMC and Frequency Converters ....................................................................................... 123
5.3 Grounding and Shielding ................................................................................................... 125
5.4 Installations with Frequency Converters ....................................................................... 130
5.5 Legislation and Standards .................................................................................................. 131
6
Content
Page
6 Protection against Electric Shock and Energy Hazards.............................................. 133
6.1 General ...................................................................................................................................... 133
6.2 Mains Supply System............................................................................................................ 134
6.3 Additional Protection ........................................................................................................... 137
6.4 Fuses and Circuit Breakers .................................................................................................. 139
7 Mains Interference ....................................................................................................................... 142
7.1 What are Harmonics?............................................................................................................ 142
7.1.1 Linear Loads .................................................................................................................. 142
7.1.2 Non-linear Loads ......................................................................................................... 143
7.1.3 The Effect of Harmonics in a Power Distribution System ............................. 145
7.2 Harmonic Limitation Standards and Requirements.................................................. 147
7.3 Harmonic Reduction Methods in Frequency Converters ........................................ 147
7.3.1 Passive Harmonic Mitigation .................................................................................. 149
7.3.2 Active Harmonic Mitigation .................................................................................... 150
7.4 Harmonic Analysis Tools ...................................................................................................... 152
7.4.1 VLT® Motion Control Tool MCT 31 ......................................................................... 153
7.4.2 Harmonic Calculation Software (HCS) ................................................................. 153
8 Interfaces .......................................................................................................................................... 154
8.1 Human Machine Interface (HMI) ...................................................................................... 154
8.2 Operating Principles of Serial Interfaces ....................................................................... 156
8.3 Standard Serial Interfaces in Frequency Converters ................................................. 158
8.4 Fieldbus Interfaces in Frequecy Converters ................................................................. 159
8.5 Fieldbuses Standardisation ................................................................................................ 161
9 Sizing and Selection of Frequency Converters ............................................................... 164
9.1 Get the Drive Rating Right.................................................................................................. 164
9.2 Rating of the Frequency Converters from Motor Specification ............................ 165
9.3 Overload Capacity ................................................................................................................. 166
9.3.1 Energy Efficiency Concerns ..................................................................................... 168
9.4 Control Range ......................................................................................................................... 169
9.5 Derating of FC ......................................................................................................................... 170
9.6 Regenerative Energy............................................................................................................. 171
9.7 Motor Cables ........................................................................................................................... 172
9.8 Environment ........................................................................................................................... 173
9.9 Centralised versus Decentralised Installation.............................................................. 175
9.10 Examples ................................................................................................................................... 177
Introduction
7
0 Introduction
By definition, a Frequency Converter (FC) (or frequency changer) is an electronic
device that converts alternating current (AC) of one frequency to another frequency.
Traditionally, these devices were electro-mechanical machines (motor-generator set).
They are sometimes referred to as “dynamic” FC’s. With the invention of solid state
electronics, it has become possible to build completely electronic FC’s, which are often
referred to as “static” FC’s (no moving parts).
Whilst the principle of converting fixed mains voltage and frequency into variable
quantities has always remained virtually the same, there have been many
improvements from the first FC’s, which featured thyristors and analogue technology,
to today’s microprocessor-controlled, digital units.
Because of the ever-increasing degree of automation in industry, there is a constant need
for more automated control and a steady increase in production speeds, so better methods
to further improve the efficiency of production plants are being developed all the time.
Today, the FC-controlled, three-phase motor is a standard element in all automated
process plants, commercial and public buildings. High-efficiency Induction motors,
but especially motor designs such as Permanent Magnet motors, EC motors and
Synchronous Reluctance Motors, need regulation with FC’s, many motors cannot even
be operated directly from the 3 phase standard power supply.
0.1 Speed Control of Electrical Motors
Different terminologies are used for systems that can control or alter the speed of
electrical motors. The most commonly used ones are:
• Frequency Converter (FC)
• Variable Speed Drive(VSD)
• Adjustable Speed Drive (ASD)
• Adjustable Frequency Drive (AFD)
• Variable Frequency Drive (VFD)
While VSD and ASD refer to speed control in general, AFD and VFD are directly
connected to adjusting the feeding frequency of a motor. In this context, the
abbreviation “drive” is used as well. Throughout this book, the terminology Frequency
Converter will be widely used. This wording covers the power electronic part of the
devices and the supporting components like current sensors, I/Os and Human Machine
Interface (HMI).
8
Introduction
0.2 Why use Speed Control?
There are numerous reasons for adjusting the speed of an application:
• Save energy and improve efficiency of systems
• Match the speed of the drive to the process requirements
• Match the torque or power of a drive to the process requirements
• Improve the working environment
• Reduce mechanical stress on machines
• Lower noise levels, for example on fans and pumps
Depending on the application one or the other benefit is predominant. However, speed
control is proven to bring significant advantages in many different applications.
0.3 How to Adjust the Motor Speed?
There are three main technologies to realise speed control used in industry. Each has its
unique features:
Hydraulic
• Hydro-dynamic type
• Static types
They are often favoured in conveyor applications especially for earth-moving and
mining equipment. This is basically due to inherent “soft start” capability of the
hydraulic unit.
Mechanical
• Belt and chain drives (with adjustable diameters).
• Friction drives (metallic)
• Variable speed gear
Mechanical solutions are still favoured by many engineers – especially mechanical
engineers – for some applications, mainly because of their simplicity and low cost
Electrical
• FC with electrical motor
• Servo systems (for example servo amplifier and servo PM motor)
• DC motor with control electronics
• Slip-ring motor (slip control with wound-rotor induction motor)
Historically, electrical devices for speed control were complex to handle and expensive.
They were used for the most challenging tasks where no alternatives were available.
The provided list of technical solutions for speed control of motors is not exhaustive
Introduction
9
and shall give an insight of the possibilities only. This book will focus on speed control
of electrical motors by FC’s.
0.4 Frequency Converters
Modern Frequency Converters can be applied to adjust and maintain the speed or
torque of a driven machine with an accuracy within ±0.5%. This is independent of the
load when compared to fixed speed operation of the induction motor, where the speed
can vary by as much as 3 – 5% (slip) from no-load to full-load operation.
Motor manufacturers employ a variety of concepts to achieve high efficiency in
electrical motors. For users it can be difficult to see the main benefit from one
technology to another, but the user will surely observe that energy efficient motors
need high technology controls.
In principle, nearly all motors can be operated with control algorithms specially
adapted to each motor type. Some manufacturers of FC’s relate their design to a narrow
group of motor technologies, but many manufacturers have the different algorithms
built-in and selectable during commissioning.
For the commissioner it is important that the FC is easy to commission based on data,
normally available for the motor type which is used. After commissioning the user must
be confident that the system is really as easy as expected, thus online measurements of
actual energy consumption and easy access to important data about the operation is
essential.
To ease the selection and ensure the various Government aims of reduction of energy
consumption, there is a big motivation for a complete set of regulations.
It must be borne in mind that all system components are important for potential
energy savings. According to the German Association of Electrical and Electronics
Manufacturers (ZVEI), approximately 10% of the savings can be achieved by using
high-efficiency motors, 30% of the savings are achieved by variable speed, but as
much as 60% of the potential savings are achieved by looking at the overall system and
optimising accordingly.
With that in mind, please read all chapters in this book and remember you cannot
judge a system by looking at only one or few of the components involved.
We wish you an interesting read.
Electric Motors
10
1 Electric Motors
1.1 Overview
An electric motor is an electromechanical device that converts electrical energy into
mechanical energy. The reverse process of producing electrical energy from mechanical
energy is performed by a generator.
The operating demands of the electric motor, especially in industry, have been
enormous. Robustness, reliability, size, energy efficiency, and price are only some
of these criteria. The differing needs have resulted in the development of different
types of electric motors. The following diagram gives a general overview of the most
commonly used electric motor technologies.
With permanent
magnet
DC
Without
permanent
magnet
Electric motor
Asynchronous
Aluminium
rotor
Induction
motor
Copper
rotor
Synchronous
reluctance
AC
Reluctance
Switched
reluctance
Synchronous
BLDC (EC)
Permanent
magnet
Fig. 1.1 Overview of the most common electric motor technologies
Interior
mounted
magnets
Surface
mounted
magnets
Electric Motors
11
1.2 Fundamentals
1.2.1 Stator and Rotor
The construction of all rotating electric motors consists in principle of two main
components.
2
1
Fig. 1.2 Construction of the asynchronous motor
Stator
The stator (1) is the stationary part of the motor which holds packages of laminations
where the electrical windings are placed.
Rotor
The rotor (2) is the rotating part of the motor which is mounted on the motor shaft. Like
the stator, the rotor is made of thin iron laminations which hold the rotor windings.
One variation is the outer rotor motor. Unlike the inner rotor design, the stator is placed
in the middle of the motor and the rotor rotates around the stator. This construction is
used in some fan applications where the fan blades are directly mounted on the rotor.
Unless otherwise mentioned, all the following explanations are related to inner rotor
design.
The connection dimensions of typical industrial motors are defined in IEC standards.
However not all motors fulfill these requirements. For example, NEMA frame motor
dimensions differ from IEC standards, due to the conversion from the metric to the
imperial system.
Electric Motors
12
1.2.2 Power and Torque
The rated output of electric motors is defined within a standard range. This
standardisation allows users to choose between different motor manufacturers
for specific applications. The “standard” output range and its increments differ
from country to country and region to region. It is recommended to find out what
manufacturers define as standard in their catalogues. On average, motors with
frame size up to 315 (ca. 200 kW) can be regarded as standard motors with standard
dimensions.
Horsepower [hp] is the imperial unit used for motor power. If this unit is specified, it can
be converted as follows: 1 hp = 0.736 kW or 1 kW = 1.341 hp.
Table 1.1 shows the typical industrial standard rated output power in [kW] and [hp].
kW
0.18
0.25
0.37
0.55
hp
0.75
1.10
1.00
kW
15.0
hp
20.0
18.5
1.50
2.20
2.00
3.00
22.0
30.0
37.0
45.0
55.0
75.0
30.0
40.0
50.0
60.0
75.0
100
3.00
4.00
5.50
7.50
11.0
5.00
7.00 10.00 15.0
90.0 110.0 132.0 160.0 200.0
Table 1.1 Rated motor output power
Besides power, torque is an important characteristic of the motor. Torque indicates the
strength of rotation of the motor shaft. Power has a direct relationship to torque and
can be calculated when torque and speed are known.
P=
Txn
9.550
P = Power [kW]
T = Torque [Nm]
n = Speed [RPM]
The factor 9.550 used in the formula results from the conversion of units:
• Power from the base units W (watt) to nameplate units kW (kilowatt)
• Speed from the base unit s-1 (revolutions per second) to nameplate min-1 (revolutions
per minute)
Electric Motors
13
1.2.3 AC and DC Motors
The first electric motor, a DC motor, was built around 1833. Speed control of this type
of motor is simple, and met the requirements of many different types of applications
at the time. The DC motor is controlled by supplying a DC voltage whose magnitude
influences the speed of the rotor. Voltage applied to stator and rotor windings results
in magnetic fields which attract or repel each other, leading to rotor movement.
Energy supplied to the rotor is transmitted via brushes, typically made of graphite, to
a commutator. The commutator ensures that the next winding is energised to achieve
a continuous rotation. The brushes are subject to mechanical abrasion and require
maintenance or periodic replacement. The importance of DC motors has decreased
over time and they are rarely used in power ranges above a few hundred watts today.
Compared to DC motors, AC motors are much simpler and more robust. However, AC
motors typically have a fixed speed and torque characteristic. Because of these fixed
characteristics, for many years AC motors could not be used for many diverse or special
applications. They are nonetheless used in most applications to transform electrical
energy into mechanical energy.
The functional principle of AC motors is based on the effects of a rotating magnetic
field. The rotating field is generated either from a multi-phase fed AC source (typically
three-phase) or from a single phase source assisted by capacitors or inductances to
achieve phase shift.
This book focuses on AC motors, particularly on asynchronous motors, as the
requirements for operation with FC’s in adjustable speed drive applications for various
motor types can be derived from this motor technology. DC motors will not be
addressed further.
1.2.4 Electromagnetic Induction
Most electric motors operate through the interaction of magnetic fields and currentcarrying conductors to generate force. This is the reverse process of producing
electrical energy from mechanical energy, performed by generators such as an
alternator or a dynamo on a bicycle.
a) Generator principle, induction by motion
When a force (F) acts on a conductor and moves it across a magnetic field (B), a voltage
is induced. If the conductor is part of a closed circuit, a current (I) flows, see Fig. 1.3
Principle for electromagnetic induction.
Electric Motors
14
b) Motor principle
In motors, the induction principle is utilised in the reverse order: a current-carrying
conductor located in a magnetic field is influenced by a force (F) which results in a
movement.
a) Generator principle
b) Motor principle
Fig. 1.3 Principle for electromagnetic induction
In both cases a magnetic field is required. In Fig. 1.3 Principle for electromagnetic
induction the magnetic field originates from a permanent magnet, but in a motor the
magnetic field is generated in the stator. Typically, this is achieved by applying voltage
to the stator windings. The conductors affected by the electromagnetic force are
located in the rotor.
1.2.5 Poles, Synchronous Speed and Asynchronous Speed
The synchronous speed of a motor can be calculated when the supply frequency and
number of pole pairs are known.
n0 =
f  60
p
f = frequency [Hz]
n0 = synchronous speed [min-1]
p = pole pair number
While the frequency is determined by the grid or the FC, the number of poles is
determined by the way the stator coils are connected.
Electric Motors
15
a)
b)
Fig. 1.4 Two coils in one phase connected in series to a) two poles b) four poles
Table 1.2 Pole pairs (p) or pole number and synchronous motor speed – lists the
number of poles corresponding to synchronous speed (n0) at 50 and 60 Hz supply.
Higher pole numbers are possible but rarely used nowadays.
Pole pairs (p)
1
2
3
4
6
Pole number (2p)
2
4
6
8
12
[min-1] (50 Hz supply)
3000
1500
1000
750
500
n0 [min-1] (60 Hz supply)
3600
1800
1200
900
600
n0
Table 1.2 Pole pairs (p) or pole number and synchronous motor speed
Synchronous means “simultaneous” or “the same”. This means in synchronous motors
the speed of the rotor is the same as the speed of the rotating field. If the rotor speed is
affected by slip (see also section 1.3.3 Slip, Torque and Speed) and therefore lower than
the speed of the rotating field, the motor is classified as asynchronous, meaning “not
simultaneous” or “not the same”.
1.2.6 Efficiency and Losses
The motor draws electrical power from the mains. At a constant load, this power is
greater than the mechanical power the motor can output to the shaft, due to various
losses in the motor. The ratio between the output power P2 and input power P1 is the
motor efficiency:
η=
P2
P1
= output power
input power
The efficiency depends on the motor principle, components (for example lamination
quality), amount of active material (for example, due to lamination or use of magnets),
size of the motor (rated power) and number of poles.
Electric Motors
16
P1
Copper loss
Iron loss
Fan loss
Friction loss
P2
Shaft output
Fig. 1.5 Typical losses in the motor
The losses in the motor illustrated in Fig. 1.5 Typical losses in the motor comprise:
• Copper losses as a result of the resistances of the stator and rotor windings
• Iron losses consisting of hysteresis losses and eddy-current losses
Hysteresis losses occur when iron is magnetised by an alternating current (AC).
The iron is magnetised and demagnetised repeatedly (that is, 100 times per second
with a 50 Hz supply). Magnetising and demagnetising both require energy. The
motor supplies power to cover the hysteresis losses, which increase with frequency
and the strength of magnetic induction.
Eddy-current losses occur because the magnetic fields induce electric voltages in
the iron core as in any other conductor (see Fig. 1.6 Eddy-currents are reduced by
the laminated form of the motor core). These voltages produce currents that cause
heat losses. The currents flow in circuits at right angles to the magnetic fields.
The eddy-current losses are dramatically reduced by dividing the iron core into thin
laminations.
Fig. 1.6 Eddy-currents are reduced by the laminated form of the motor core
Electric Motors
17
• Fan losses occur due to the air resistance of the motor fan
• Friction losses occur in the ball bearings holding the rotor
When determining the efficiency and motor output power, the losses in the motor
are normally subtracted from the supplied power. The supplied power is measured,
whereas the losses are often calculated or determined experimentally.
1.3 Asynchronous Motors
To understand clearly how an adjustable speed drive system works, it is necessary to
understand the principles of operation of this type of motor. Although the basic design
has not changed much in the last decades, modern insulation materials, computerbased design optimisation techniques as well as automated manufacturing methods
have resulted in lower cost per kilowatt power and higher efficiency for a given motor
size.
The information in this book will apply mainly to the so-called “squirrel-cage“
three-phase asynchronous motor, which is the type commonly used with FC’s.
1.3.1 Rotating Field
When applying a multi-phase AC source (typically three-phase) to a suitable winding
system, a rotary magnetic field is generated which rotates in the air gap between the
stator and the rotor. If one of the phase windings is connected to a supply phase, a
magnetic field is induced.
Fig. 1.7 One phase produces an alternating field
18
Electric Motors
The magnetic field in the stator core has a fixed location, but its direction varies, as
shown in Fig. 1.7 One phase produces an alternating field. The speed of rotation is
determined by the supply frequency. At a frequency of 50 Hz, the field changes
direction 50 times per second.
If two phase windings are connected to the respective supply phases, two magnetic
fields are induced in the stator core. In a two-pole motor, one field is displaced by 120
degrees relative to the other. The maximum field values are also displaced in time, as
shown in Fig. 1.8 Two phases produce an asymmetrical rotating field.
Fig. 1.8 Two phases produce an asymmetrical rotating field
This produces a rotating magnetic field in the stator which is highly asymmetrical
until the third phase is connected. When the third phase is connected, there are three
magnetic fields in the stator core. There is a 120° displacement between the three
phases, as shown in Fig. 1.9 Three phases produce a symmetrical rotating field.
Fig. 1.9 Three phases produce a symmetrical rotating field
The stator is now connected to the three-phase supply. The magnetic fields of the
individual phase windings form a symmetrical rotating magnetic field. This magnetic
field is called the rotating field of the motor.
Electric Motors
19
The amplitude of the rotating field (φ) is constant and 1.5 times the maximum value
(max ) of the alternating fields. It rotates at the synchronous speed resulting from the
pole pair number and supply frequency (see also section 1.3.3 Slip, Torque and Speed).
a)
b)
Fig. 1.10 Magnetic field components
The representation of the rotating field as a vector with a corresponding angular
velocity describes a circle, shown in Fig. 1.10 Magnetic field components. The
magnitude of the magnetic field φ as result of the components (φ1, φ2, φ3) is constant
also at different moments (a and b). As a function of time in a coordinate system, the
rotating field describes a sinusoidal curve. The rotating field becomes elliptical if the
amplitude changes during a rotation.
With single phase motors the phase shift which determines the rotation direction of
the motor is created by a capacitor or an inductance which also results in an elliptical
field.
1.3.2 Squirrel-cage Motor
The squirrel-cage rotor is the most frequently-used rotor type, and is used in the
squirrel-cage motor. Unlike the stator, where the coils have many windings, in the
squirrel-cage motor, only one winding is placed in the slots of the rotor lamination. This
is typically done with aluminium or copper rods. The rods are short- circuited at each
end of the rotor by a ring made out of the same material. Copper has the advantage
that it has a better conductivity than aluminium which results in lower losses and a
higher efficiency. Drawbacks compared to aluminium are higher prices, lower starting
torques and higher melting temperature which complicate the casting and leads to a
higher tooling efforts.
Electric Motors
20
A variant of the squirrel-cage rotor is the slip-ring rotor which has wound coils for each
phase. The coils are connected to slip-rings. Brushes sliding on the slip-ring allow the
connection of external resistors which modifies the motor behaviour (see also section
1.3.5 Changing Speed). If the slip-rings are short-circuited, the rotor acts as a squirrelcage rotor.
Iw (A)
Fig. 1.11 Operational field and squirrel-cage rotor
The rotor movement of the squirrel-cage motor is created as follows:
A rotor rod placed in the rotating field is passed by a series of magnetic poles, as shown
in Fig. 1.11. The magnetic field of each pole induces a current (IW) in the rotor rod, which
is influenced by a force (F). This force is determined by the flux density (B), the induced
current (IW), the length (L) of the rotor within the stator, and the angle (θ) between the
force and the flux density. Assuming that θ = 90°, the force is:
F = B x IW  L
The next pole passing the rod has an opposite polarity. It induces a current in the
opposite direction to the previous one. Since the direction of the magnetic field has
also changed, the force acts in the same direction as before as shown in Fig. 1.12b
Induction in the rotor rods.
Fig. 1.12 Induction in the rotor rods
Electric Motors
21
When the entire rotor is located in the rotating field, see Fig. 1.12c Induction in the
rotor rods, the rotor rods are affected by forces that cause the rotor to rotate. The
rotor speed (2) does not reach the speed of the rotating field (1) since no currents are
induced in the cage bars when it is rotating at the same speed as the field.
1.3.3 Slip, Torque and Speed
As described in sections 1.2.5 Poles, Synchronous Speed and Asynchronous Speed
and 1.3.2 Squirrel Cage Motor, under normal circumstances the rotor speed (nn) of
asynchronous motors is slightly lower than the speed (n0) of the rotating field. The
difference between the speed of the rotating field and the rotor is called slip (s) where:
s = n0 – nn
The slip is often expressed as a percentage of the synchronous speed and is typically
between 1 and 10 percent.
s = (n0 – nn)  100
n0
The individual forces in the rotor rods combine to form the torque (T) on the motor
shaft (see section 1.3.2 Squirrel Cage Motor). With a given value of force (F) and radius
(r) the motor torque is: T = F  r.
Fig. 1.13 Torque on the motor shaft is the force (F) x radius (r)
The relationship between motor torque, speed and current of asynchronous motors
has a characteristic curve, shown in Fig. 1.14 Principal motor current and torque
characteristics. This curve depends on the rotor slot design and the rod material.
22
Electric Motors
Fig. 1.14 Principal motor current and torque characteristics
The motor operating range (0 < n/n0 < 1) can be split up into two ranges:
• Starting range (0 < n/n0 < nB /n0 )
• Operating range (nB /n0 < n/n0 < 1)
These ranges have the following characteristics:
Starting torque Ta. This is the torque the motor produces with the rated voltage and
rated frequency applied at standstill.
Stall torque TB at stall speed nB. This is the highest torque the motor can produce when
the rated voltage and rated frequency are applied.
Rated motor torque Tn at nominal speed nn.
The rated values of the motor are the mechanical and electrical values for which the
motor was designed in accordance with the IEC 60034 standard. The rated values, also
called motor specifications or motor ratings, are stated on the motor nameplate.
The rated values indicate the optimal operating point for the motor, when connected
directly to the mains.
Electric Motors
23
Apart from the normal motor operating range, there are two braking ranges.
• n/n0 > 1: the motor is driven by the load above its synchronous speed (n0)
operating as a generator. In this region, the motor produces a counter torque and
simultaneously returns power to the supply grid.
• n/n0 < 0: braking is called regenerative braking or plugging.
If two phases of a motor are suddenly interchanged, the rotating field changes
direction. Immediately afterwards, the speed ratio n/n0 is 1. The motor, previously
loaded with torque T, now brakes with its braking torque. If the motor is not
disconnected at n = 0, it will continue to run in the new rotational direction of the
magnetic field.
1.3.4 Typical Operating Conditions
In principle, asynchronous motors have six coils: three coils in the stator and three coils
in the squirrel-cage rotor (which behaves magnetically as if consisted of three coils). A
subset of these coils can be used as the basis for generating an equivalent circuit that
makes the operating principle of the motor easier to understand, especially when the
frequency of the supply voltage changes or varies.
RFe
Fig. 1.15 Equivalent circuit diagram (one phase) for a motor operating under load
Applying a supply voltage (U1) results in a current in the stator (I1) and the rotor (I2)
which is limited by the resistance in stator (R1) and rotor (R2) and the reactance in stator
(X1 σ) and rotor (X2 σ). While the resistance is independent of the supply frequency the
reactance has an influence.
XL = 2 x π x f x L
XL = reactance [Ω]
f = frequency [Hz]
L = inductance [H]
24
Electric Motors
The coils mutually influence each other by means of magnetic induction. The rotor coil
induces a current in the stator coil and vice versa. This mutual effect means that the
two electric circuits can be interconnected via a common element consisting of RFe
and Xh, which are called the transverse resistance and reactance. The current the motor
draws for magnetising the stator and the rotor flows through this common element.
The voltage drop across the ”transverse link” is the induction voltage (Uq). As RFe is very
small and is neglected in the following explanations.
Standard operation
When the motor operates in its normal operating range, the rotor frequency is, due to
the slip, lower than the rotating field frequency. In the equivalent circuit diagram, the
effect is described by a change in the rotor resistance R2 by the factor 1/s. R2/s can be
expressed as R2 + R2  (1 – s)/s where R2  (1 – s)/s represent the mechanical motor load.
No-load situation
The slip s is small at no-load (idle) operation. This means that R2 x (1 – s)/s is high.
Consequently, almost no current can flow through the rotor. Ideally, this is comparable
to removing the resistor that represents the mechanical load from the equivalent
circuit.
The induced voltage (Uq) is often confused with the motor terminal voltage. This is due
to the simplification of the equivalent circuit diagram to make it easier to understand
various motor conditions. However, the induced voltage only approximately
corresponds to the terminal voltage in no-load operation.
Locked rotor situation
The slip increases when the motor is operating under load. Therefore R2 x (1 – s)/s will
decrease. When the rotor is locked the slip is 1 and hence the current which increases
with the load reaches its maximum.
The equivalent circuit diagram thus corresponds to the conditions applicable to
the asynchronous motor in normal practice. It can be used in numerous cases for
describing conditions in the motor.
1.3.5 Changing Speed
The motor speed n is dependent upon the rotational speed of the magnetic field and
can be expressed as:
(1 – s) x f
n = n0 – ns =
p
Electric Motors
25
The motor speed can therefore be changed by changing:
• The pole pair number p of the motor (for example, pole-changing motors)
• The motor slip s (for example, slip-ring motors)
• The motor supply frequency f (for the motor)
n = (1 - s) x f
p
No. of pole pairs
Slip
Rotor
Resistance
Frequency
Starter voltage
Cascade
coupling
Fig. 1.16 Different options for changing the motor speed
Pole number control
The rotational speed of the magnetic field is determined by the number of pole pairs
in the stator. In the case of a two-pole motor, the rotational speed of the magnetic field
is 3000 RPM at a motor supply frequency of 50 Hz. For a four-pole motor the speed is
1500 RPM.
Fig. 1.17 Torque characteristics when changing pole number
Motors can be designed to have two or more different pole-pair numbers. This is done
by using a special arrangement of the stator windings (Dahlander winding) in the slots
and/or by using more separate and isolated windings in the slot.
26
Electric Motors
The speed is changed by switching the stator windings to change the number of pole
pairs in the stator. By switching from a small pole-pair number (high speed) to a high
pole-pair number (low speed), the actual motor speed can be dramatically reduced,
for example, from 1500 to 750 RPM. With rapid switching from higher to lower speed,
the motor runs through the regenerative range. This places a considerable load on the
motor and the mechanism of the driven machine which can cause damage to motor
and machinery.
Slip control
Controlling the motor speed using slip can take place in two different ways: either by
changing the stator supply voltage or by modifying the rotor. It should be mentioned
that these methods involve considerable thermal losses. Please refer to other sources of
information if more is needed.
Rotor control
Controlling the motor speed using the rotor can be made in two different ways:
• Resistors are inserted in the rotor circuit. These types of motors are called “slip-ring”
motors. The trade-off using this method is higher power losses in the rotor.
• Rotor circuits are cascaded with other electrical machines or rectifier circuits. The
rotor circuit is then connected via slip rings to DC machines or to controlled rectifier
circuits instead of resistors. The DC machine supplies the rotor circuit with additional
variable voltage making it possible to change the rotor speed and magnetisation.
Frequency regulation
With a variable frequency supply, it is possible to control the motor speed with minor
additional losses. The rotational speed of the magnetic field and hence the rotor speed
changes with the frequency. To maintain the motor torque, the motor voltage must
change together with the frequency as shown in Fig. 1.18 Torque characteristics with
voltage/frequency control.
With a constant ratio of motor supply voltage to frequency, the magnetisation in the
rated motor operating range is also constant.
Fig. 1.18 Torque characteristics with voltage/frequency control
Electric Motors
27
At low speed the ratio must be adjusted to compensate for the ohmic losses. Further
forced cooling may be required in this speed range.
1.3.6 Motor Nameplate and Star or Delta Configuration
Normally the motor has a nameplate on it which has all essential motor data.
Additional data are available in the motor catalogue or can be obtained from the
manufacturer.
IEC 60034-6
RPM
Fig. 1.19 Motor nameplate shows essential data
The nameplate shown has the following information:
1. It is a three-phase AC motor with a rated frequency of 50 Hz
2. Rated output (shaft) power is 15 kW
3. The stator windings can be connected in series (star) with a rated voltage of 400 V
and rated (apparent) current of 27.5 A
4. Alternatively, the stator windings can be connected in parallel (delta) with a rated
voltage of 230 V and rated (apparent) current of 48.7 A
5. It has an IP 54 protection
6. Insulation class F (155 °C) and a power factor (cos. φ) of 0.90.
7. Rated speed 2910 RPM (a two-pole motor) is the motor speed at the rated voltage,
rated frequency and rated load
8. Fulfils the IEC 60034-6 standards
Some motor data (torque, efficiency, etc.) can be calculated using the nameplate
data. For example the power factor can be used to calculate the active and reactive
components of the motor current.
Electric Motors
28
Pay special attention to the rated motor voltages in star and delta. If the supply
voltage is higher than the rated voltage of the applied configuration, the motor will be
damaged. The connection itself can be often changed by rearranging the jumpers at
the motor terminal.
U1
V1
W1
U1
V1
W1
W2
U2
V2
W2
U2
V2
a)
b)
Fig. 1.20 Star (a) and delta (b) configuration of motors via jumpers on the terminal block
In delta connection the full supply voltage is applied to each motor phase but the
current is reduced by the factor 3. In star connection the current is maintained, and
the voltage is reduced. Therefore the power is the same regardless of the connection
due to the fact that the feeding voltages are different.
L1
Upp
lpp = lp
L1
Up = Upp
33
lpp = 33 = lp
Upp
L2
lp
L2
L3
L3
a)
b)
Fig. 1.21 Current and voltage distribution in star (a) and delta (b) configuration
So-called star/delta starters utilise this behaviour for reducing the starting current of
a motor. In delta connection, the motor must suit the supplying mains. This means
on 400 V mains the motor must have a 690 V star and a 400 V delta rating. At start the
motor will be connected in star, reducing current, power and torque to one-third. After
the motor has been accelerated the connection will be changed to delta.
Motor voltages in catalogues are often expressed by mentioning the star and delta
voltages together (example: 400/230 V Y/Δ or 690/400 V Y/Δ). The lower voltage is
always related to delta and the higher to the star connection.
The relation of the current is vice versa: the lower current relates to star configuration,
and the higher current relates to delta configuration.
Electric Motors
29
1.4 Synchronous Motors
The synchronous motor is defined by the fact that the rotor rotates at the same speed
as the magnetic field created by the stator windings. The design of the stator is in
many cases similar to that of asynchronous motors, with distributed windings. Some
manufacturers use concentric windings (in slot) which enable a more compact motor
design and require less copper. The energy savings achieved by the reduced use of
copper are however often eaten up by additional losses, which result in harmonics in
the air gap flux caused by the construction.
a)b)
Fig. 1.22 Distributed windings.
1.4.1 Permanent Magnet (PM) Motors
The simplest way to build a permanent magnet motor (PM motor) is to replace the
squirrel-cage rotor of an asynchronous motor with a rotor which is equipped with
permanent magnets. When applying a suitable voltage to the stator, a rotating
magnetic field will be created in the air gap. The rotor will follow the field at
synchronous speed because the magnets are attracted by the rotating field. If the
difference between rotor speed and the speed of the magnetic field is too big the
motor falls out of synchronicity and the motor will stop. Therefore a suitable controller
is required which ensures that speed changes are done by adjusting the feeding
frequency continuously and not by switching from one speed to another.
In the past PM motors were often used in servo applications with focus on fast and
precise operation. These servo motors are typically slim and long in order to have a low
inertia for high dynamic applications. To utilise the high-efficiency characteristic of PM
motors in other applications the principle has been transferred to motors in IEC frame
sizes. Standard frequency converters can be used in the majority of PM motor systems
for operation if suitable control algorithms are implemented in the device.
Electric Motors
30
In order to magnetise the motor in the best way the controller needs to know the rotor
angle at any point in time. In many applications sensorless strategies for determining
the rotor angle are sufficient. If the controller is not capable of sensorless control or in
high dynamic servo applications, external position feedback devices are used.
In the equivalent diagram the magnets are represented by a voltage source Up
because turning the rotor will result in a voltage induced in the stator. This voltage
is called back EMF, see section 1.4.1.1 Back EMF. The absence of motor slip, rotor
resistance and inductance indicates that no losses are created in the rotor which results
in the very good efficiency.
supply voltage
stator current
Voltage generated by permanent
magnets
R1, X1 describe the coils in the stator
using X1 = X1h + X1σ
R1
U1
I1
UP
U1
I1
X1
Up
~
Fig. 1.23 Simplified PM motor equivalent circuit diagram
ln general PM motors can be divided into motors with rotors where the magnets are
placed on the surface (SPM motor) or internally (IPM motor). The placement of the
magnet results in different shapes of the resulting magnetic field and is described by
the inductances Ld and Lq.
Fig. 1.24 Magnet placement a) SPM and b) IPM
Electric Motors
31
As the magnets behave like air in relation to the resulting magnetic field, salient
and non-salient fields are created. With SPM motors Ld and Lq have the same value
resulting in a non-salient field while the different Ld and Lq of an IPM creates a salient
field which produces an additional torque in field-weakening.
1.4.1.1 Back EMF
When the shaft of a PM motor is turned, the motor produces a voltage at its terminals.
This voltage is called back EMF (EMF = electromotive force), and describes an
important characteristic of the motor. The higher the voltage, the better the motor
efficiency. Depending on the connection and placement of the windings, the shape of
the back EMF can be trapezoidal or sinusoidal. For trapezoidal voltage so-called block
commutation is required which is easy to realise in the electronics but has drawbacks
like noise and torque ripples. Typically PM motors have sinusoidal back EMF and will be
operated via sinusoidal commutation.
Given that the motor actively generates a voltage must be considered, not only
during operation but also when the feeding FC is disconnected from mains (power
loss, breakdown, switched off ), because the motor can potentially generate sufficient
energy to power up the device while the shaft is rotating (for example, when coasting).
The voltage needed for powering the FC depends on the mains voltage the FC is
designed for.
Example: Required speed of a PM motor with 200 V back EMF to power on a 400 V
mains FC (required DC link voltage approx. 320 V).
npower on =
UDC on
32
[email protected]
320V
=1000 RPM = 32 =1000 RPM = 1134 RPM
200V
If the voltage generated by the motor is too high the converter can be destroyed.
Practically this can happen when the controlling FC is switched off while the motor is
operating at very high speed. During operation the FC limits the voltage coming back
from the motor. When the control is suddenly switched off the full back EMF voltage
can be seen at the terminals immediately. This critical speed depends on the back EMF
of the motor and the voltage the FC is designed for.
Electric Motors
32
Example: 400 V mains, UBack EMF @ 1000 RPM = 100 V, UDC critical = 1000 V
ncritical =
UDC critical
[email protected] RPM u—2
u1000 RPM =
1000 V u1000 RPM = 5656 RPM
100V u —2
A brake resistor can be used to overcome such critical situations.
Unfortunately there is no standard used by motor manufacturers to provide
information about the back EMF. Some manufacturers state back EMF related to 1000
RPM while others use nominal speed of the motor. Sometimes the value of factor ke is
given in radians and must be converted to RPM.
UEMF = ke =
1000
=2/
60
Where peak values are provided the voltage must be divided by square root of two in
order to get the RMS value.
Also advanced motor data like motor resistance and inductances are stated in differing
ways. Sometimes they are given as phase/phase values, and sometimes as phase/star
values.
URMS =
UPeak
32
1.4.1.2 Torque and Speed Range
The torque of a PM motor is proportional to the motor current, and its speed is
proportional to the feeding frequency. At nominal torque and speed, a certain voltage
is required. If the FC can deliver a higher voltage, the speed can be increased further.
This results in a higher power at constant torque. When the voltage has reached an
upper limit, the motor enters the field weakening area. Operation in field weakening is
only possible with suitable frequency converters. Motor mechanics and insulation must
support the higher speed and withstand the higher voltage.
Electric Motors
33
T [Nm]
Nominal power
Nominal
torque
Nominal speed
range
Above nominal
speed
Field
weakening
n [min-1]
Nominal speed
Related
to back EMF
Critical speed
Fig. 1.25 Operation in field weakening area
The greatest risk in field weakening operation is switching off the motor control at too
high speed, as the high back EMF can destroy the FC (see section 1.4.1.1 Back EMF).
Another possibility for extending the speed range is to change the star configuration of
a motor to delta, if the motor provides this feature. Similar to asynchronous motors, a
delta connection results in a higher voltage on the windings, because it is not reduced
by the factor 1.73 or 3 as for a star configuration.
1.4.2 Brushless DC (BLDC) or Electronically Commutated (EC) Motors
EC (Electronically Commutated Motor) and BLDC (Brushless DC) are basically different
names for the same technology. In the original BLDC concept only two phases were
energised with a trapezoidal voltage. Compared to a distribution over three phases this
result in 1.22 time higher current. For determining the rotor position Hall sensors have
been used. Drawbacks of the concept were worse torque ripples and iron losses.
In practice there are many different types of EC motors, such as small servo motors
with power ratings of a few watts or motors in building automation systems up
to approximately 10 kW. In general BLDC/EC has a reputation for extremely high
efficiency. This is fully deserved, in particular for very small devices – the original
application area for these motors – where they are distinctly better than universal
or split-pole motors (efficiency approximately 30%). Above a few hundred watts the
efficiency is comparable to standard PM motors.
34
Electric Motors
Modern EC/ECM utilise the same control principles as the PM motors. In building
automation EC motors are often used as hubs in EC fans. This results in a very compact
fan unit with a very efficient motor. Unfortunately the placement of the motor in the
middle of a centrifugal fan creates air turbulences which reduce the total fan efficiency.
In comparison to a direct-driven fan the difference at same motor efficiency can be in
the range of 3-6%.
1.4.3 Line Start PM Motor (LSPM motor)
A line start PM motor is a hybrid of a squirrel-cage asynchronous motor and a PM
motor where the magnets are placed internally to the rotor.
Fig. 1.26 The position of magnets in the rotor influences the motor characteristics
When connected to a three phase grid the motor develops a torque and accelerates
like a standard asynchronous motor to near synchronous speed, if the motor torque
is greater than the load torque throughout acceleration. When the rotor has roughly
reached the speed of the rotating field, a synchronising torque (reaction torque) is
produced due to magnetic coupling between the rotating stator field and the rotor
poles, which pulls the rotor into synchronism.
After synchronisation, the motor continues to run at synchronous speed. As there is no
speed difference between the magnetic field and the rotor, no currents are induced in
the cage. This results in a high efficiency with a good power factor. When load changes
take place the squirrel cage is still working as a damper. This is also the case when the
motor is operated by a FC where the additional damper can reduce the efficiency by
approximately 5-10 %.
If the motor is loaded with a torque that is greater than its synchronous stalling torque,
it is pulled out of synchronism and continues to operate like an asynchronous motor
at a load-dependent speed. Depending on the design, the motor is more or less
Electric Motors
35
sensitive to under-voltage situations which can also result in falling out of synchronism.
Renewed synchronisation takes place automatically when the load torque is lower than
the synchronising torque. However, the rotor will stop if the motor is loaded with a
torque that is greater than its induction stalling torque.
Torque
Drawbacks of the concept are the influence of the magnets while starting the motor.
Torque oscillations and torque peaks, paired with noise, arise during the start up.
Furthermore the starting torque is lower compared to an asynchronous motor as the
magnets create a negative torque component (1).
Tcage
Tmagnet
TLSPM
S=0
Speed
Fig. 1.27 Starting torque of LSMP is reduced compared to the pure squirrel cage torque
LSPM motors are typically used in fans and pumps, available in the power range up to
approximately some 10 kW, but can also be used in low inertia applications.
1.4.4 Reluctance Motors
For creating a motor movement these types of motors utilise magnetic reluctance,
which is also called magnetic resistance. Similar to electric circuits the magnetic flux
follows the path of the lowest resistance. As in asynchronous motors, the magnetic
field is created by applying a suitable voltage to the stator windings. The rotor rotates
towards the position with minimum magnetic reluctance. If the rotor is now forced out
of this position a torque is created in order to move it back to the position where the
reluctance is minimised. The torque resulting from the magnetomotive force depends
on the relationship between the inductances in the d-axis and q-axis, known as the
saliency ratio.
The saliency ratio results directly from the rotor lamination design. Cut-offs in the
lamination are utilised to shape the equivalent air gap of the machine by controlling
(1) Source – 2014 .J Sorgdrage, A.J Grobler and R-J Wang, Design procedure of a line-start permanent magnet
synchronous machine.
Electric Motors
36
the flux paths. They also influence how the d-axis and q-axis inductances vary with
the magnetisation current. As these cut-offs increase the equivalent air gap, a higher
magnetising current is required which leads to a worse cos φ. As illustrated in Fig. 1.28
Maximum power factor vs. saliency ratio, the maximum power factor depends on Ld/Lq
ratio. The higher the ratio the better the cos φ becomes. Modern rotor designs have a
ratio in the range from 4 to 10.
1
0.9
Power factor
0.8
0.7
0.6
cos 0.5
0.4
Ld
-1
Lq
Ld
+1
Lq
0.3
0,2
0.1
0
1
2
3
4
5
6
7
8
9
10
Saliency ratio, (Ld/Lq)
Fig. 1.28 Maximum power factor vs. saliency ratio
Even if reluctance motors require a higher cos φ, the energy efficiency is reasonably
high. Losses arise in the rotor mainly by harmonics in the air gap between stator and
rotor.
The reluctance principle was first used around the year 1840. Over time various
optimisations resulted in different motor principles and designs. In the next chapters
the three most common types of reluctance machines are described.
1.4.5 Synchronous Reluctance Motor with Squirrel Cage
The stator of this three-phase reluctance motor is identical to that of a standard threephase squirrel-cage motor. The rotor design is modified by removing the windings and
cutting pole gaps on the circumference of the laminated rotor core. The gaps are filled
again with aluminium and the end windings are shorted.
Fig. 1.29 Rotor with pole gaps on the circumference placed in the stator
Electric Motors
37
Similar to a LSPM motor design, (see section 1.4.3 Line Start PM Motor (LSPM Motor))
the motor accelerates to near synchronous speed when connected to a three
phase grid, if the produced torque is sufficient for the load. When approaching the
synchronous speed the rotor is pulled into synchronism and runs at synchronous speed
despite the absence of rotor excitation.
Fig. 1.30 Torque characteristic of a reluctance motor
Under load, the salient rotor poles lag behind the stator rotating field by the load angle.
Again the behaviour is similar to LSPM when the load torque becomes too high. The
motor is pulled out of synchronism, continues to operate like an asynchronous motor
and regains synchronisation automatically when the load torque is lower than the
synchronising torque.
The possibility to start direct on line (DOL) and run at synchronous speed make the
motor interesting for several applications. Power range ends often at approximately
10 kW. The drawback is a reduced efficiency, especially when operated by FC’s, as the
rotor windings act as an additional damper.
1.4.6 Synchronous Reluctance Motor (SynRM)
The design of a new generation of reluctance motors focuses on energy efficiency. This
highly efficient motor type is often meant when synchronous reluctance motors are
addressed and should not be confused with reluctance motors which focus on high
torque density or the possibility to start on mains. The key to the efficiency is the new
rotor design.
Electric Motors
38
Fig. 1.31 Special rotor lamination design results in high efficiency at low torque ripples
The stator construction and the windings are similar to an asynchronous motor. By
applying a suitable voltage to the distributed windings, a harmonic field is created
which creates low harmonic losses. Also the design of the rotor is optimised to reduce
harmonic losses and operate with low torque ripples.
As the motor cannot start directly on mains, a frequency converter is required to
control the motor. For magnetising the cut-offs in the rotor lamination, higher apparent
power is required than for an Asynchronous motor (see section 1.4.4 Reluctance
Motors). If the converter and the capacitors in the intermediate circuit are suitably sized
they will deliver the additional apparent current. In this case the grid is not loaded with
the higher apparent power and the low cos φ.
For operating the motor, the FC needs to know the rotor angle. Depending on the
angle, the converter will energise the different windings. The determination of the rotor
angle is often done sensorless without an additional device. In order to achieve an
energy efficient control, the converter must also take care of the Ld and Lq behaviour in
operation.
Inductance
Ld
D-axis inductance (Ld)
1/2
1/2
LdSat
Lq
Q-axis inductance (Lq)
LdSat
Id/Iq in % of nom.
motor current
saturation factor
Fig. 1.32 Example of Ld/Lq relationship to Id/Iq
100%
Electric Motors
39
The inductance components of the SynRM rotor change depending on the load
because of saturation effects. Therefore the individual inductances Ld and Lq depend
on Id and Iq current (Ld(Id,Iq) and Lq(Id,Iq)). If this is taken into account, very high energy
efficiency operation of the motor is possible. Over a certain power range the part-load
efficiency has advantages against other concepts.
For decades, asynchronous motors were state of the art, while other technologies
were only used in niches. The trend towards more energy efficient motors and the
opportunities provided by FC’s has resulted in innovative technologies like the
improved SynRM. More improvements and optimisations are in development.
1.4.7 Switched Reluctance Motor (SRM)
Construction of the stator is very similar to that of DC motors as concentric windings
are used. This can result in a compact housing. The rotor lamination design has a very
clear shape with low inertia where the number of poles can easily be counted. While
on two pole motors the rotor poles are aligned with the stator poles, the pole ratio
is typically different. This principle is also applied on other motor types but it is very
obvious on switched reluctance motors.
6/4 pole
8/6 pole
Fig. 1.33 Switched reluctance motor configuration examples
To run the motor a suitable controller is required, which energises the stator coils in a
sophisticated way. The phases are energised one after the other. When the coils of a
phase are supplied with a voltage, a flux is established through the stator poles and the
rotor, which results in rotor movement. After the rotor has started moving the voltage
will be switched to the next phase and so on.
40
Electric Motors
Starting the motor directly on mains is not possible. The design allows 100% torque at
stall indefinitely and achieves high efficiency even in part-load operation. The double
salient construction in rotor and stator is very robust, but results typically in high
torque ripples and low dynamics at higher noise.
Frequency Converters
41
2 Frequency Converters
Since the late 1960s, the FC has developed at a tremendous rate. Major advances
have been made thanks to developments within the fields of microprocessor and
semiconductor technology, in particular, and the associated price reduction. However,
the basic principles of the FC remain the same.
As stated in the introduction, the main function of a FC is to generate a variable
supply (for example, 0 to 400 V / 0 to 50 Hz) from a supply with “fixed” parameters (for
example, 400 V and 50 Hz). There are two approaches to performing the conversion,
defining two types of FC’s: Direct converters and converters with intermediate circuit.
Frequency Converters
Direct converters
Converters with
intermediate circuit
Constant
Variable
CSI
PAM
Current-source
frequency converters
I-converters
PWM
Voltage-source
frequency converters
U-converters
CSI: Current Source Intverter
PAM: Pulse Amplitude Modulation
PWM: Pulse Width Modulation
Fig. 2.1 Overview of frequency converter types
2.1 Direct Converters
The direct converter performs the conversion with no intermediate storage.
Direct converters are generally only used in high-power applications (megawatt range).
This book does not deal with this type of converter in detail, but several features are
worth mentioning.
42
Frequency Converters
Direct converters are characterised by:
• Reduced frequency control range (approximately 25 to 30 Hz) with 50 Hz mains
frequency
• Common use with synchronous motors.
• Suitability for applications with stringent dynamic performance requirements.
2.2 Converters with Intermediate Circuit
In the vast majority of cases, the FC is equipped with an intermediate circuit. Another
term for intermediate circuit is “DC bus”. Within the category of converters with an
intermediate circuit, there are two subtypes:
• constant intermediate circuit
• variable intermediate circuit.
FCs with an intermediate circuit can be broken down into four main components as
shown in Fig. 2.2 Block diagram of a frequency converter with an intermediate circuit.
Fig. 2.2. Block diagram of a frequency converter with an intermediate circuit
Rectifier
The rectifier is connected to a single-phase or three-phase AC mains supply and
generates a pulsating DC voltage. There are four basic types of rectifier, as shown in
Fig. 2.3 Main component topologies:
• controlled
• semi-controlled
• uncontrolled
• active front-end
Frequency Converters
43
Intermediate circuit
The intermediate circuit can function in three different ways, as shown in Fig. 2.3 Main
component topologies Intermediate circuit:
• Conversion of the rectifier voltage into a DC voltage
• Stabilisation or smoothing of the pulsating DC voltage to make it available to the
inverter
Inverter
Conversion of the constant DC voltage of the rectifier into a variable AC voltage. The
inverter generates the frequency of the motor voltage. Alternatively, some inverters
may additionally convert the constant DC voltage into a variable AC voltage. See Fig.
2.3 Main component topologies Inverter.
Control circuit
The control circuit transmits signals to – and receives signals from – the rectifier, the
intermediate circuit and the inverter. The design of the individual FC determines
specifically which parts are controlled.
Single fase
2
3
4
Rectifier
1
Triple fase
Intermed.
circuit
Un-controlled
Semi-controlled
Fully controlled
Constant
Variable
5
6
Inverter
9
Fig. 2.3 Main component topologies
“Active Front End”
7
8
10
Frequency Converters
44
Configuration of the FC involves selection between different main components. See
table 2.1 Frequency converter configuration examples.
Abbreviation
Configuration:
Reference to
components in
Fig. 2.3
Pulse amplitude modulated converter
PAM
1 or 2 or 3
and 6
and 9 or 10
Pulse width modulated converter
PWM
1 or 2 or 3 or 4
and 7 or 8
and 9 or 19
CSI
3, 5, and 9
Configuration example
Current-source converter
Table 2.1 Frequency converter configuration examples
What all FC’s have in common is that the control circuit uses signals to switch the
inverter semiconductors on and off. This switching pattern is based on a variety of
principles. FC’s can further be broken down into types according to the switching
pattern that controls the supply voltage to the motor.
2.3 Rectifier
Depending on the power involved, the power supply takes the form of a three- phase
AC voltage or a single-phase AC voltage with a fixed frequency.
For example:
Three-phase AC voltage: 3 x 400 V/50 Hz
Single-phase AC voltage: 1 x 240 V/50 Hz
The rectifier of a FC consists of diodes or thyristors, a combination of both, or bipolar
transistors (IGBTs).
Fig. 2.3 Main component topologies shows the four different rectification approaches
that are available today. In low-power applications (up to 30 kW, depending on the
manufacturer), uncontrolled B6 bridge rectifiers are generally used. Half-controlled
rectifiers are used in the power range 37 kW and above.
The rectifier circuits described above allow energy to flow in one direction, from the
supply to the intermediate circuit.
Frequency Converters
45
2.3.1 Uncontrolled rectifiers
Uncontrolled rectifiers consist of diodes as shown in Fig. 2.4 How diodes work.
Fig. 2.4 How diodes work
A diode allows current to flow in one direction only: from the anode (A) to the cathode
(K). The current is blocked if it attempts to flow from the cathode to the anode. It is not
possible to control the current strength, as is the case with some other semiconductors.
An AC voltage across a diode is converted into a pulsating DC voltage. If a three-phase
AC voltage is supplied to an uncontrolled three-phase rectifier, the DC voltage will
pulsate continuously.
Fig. 2.5 Uncontrolled rectifier (B6-diode bridge)
Fig. 2.5 Uncontrolled rectifier (B6-diode bridge)shows an uncontrolled three-phase
rectifier consisting of two groups of diodes. One group consists of diodes D1, D3 and
D5. The other group consists of diodes D4, D6 and D2. Each diode conducts for onethird of the period T (120°).
In both groups, the diodes conduct in sequence. Periods during which both groups
conduct are offset in relation to each other by one sixth of the period T (60°).
Diode group D1,3,5 conducts the positive voltage. When the voltage of phase L1 reaches
the positive peak value, terminal (A) takes on the value of phase L1. Reverse voltages of
the magnitude UL1-2 and UL1-3 are present across the other two diodes.
Frequency Converters
46
The same principle applies to diode group D4,6,2. Here terminal (B) takes on the
negative phase voltage. If, at a given time, L3 reaches the negative threshold value,
diode D6 conducts.
The other two diodes are subject to reverse voltages of the magnitude UL3-1 and
UL3-2.
The DC output voltage of the uncontrolled rectifier is constant and represents the
difference between the voltages of the two diode groups. The average value of the
pulsating DC voltage is approximately 1.31 to 1.41 times the mains voltage with a
three-phase supply or approximately 0.9 to 1.2 times the AC voltage in the case of a
single-phase supply.
The current consumption of the diodes is not sinusoidal. Consequently, uncontrolled
rectifiers generate mains interference. To counteract this, FCs with B12 and B18
rectifiers are increasingly used. B12 and B18 rectifiers comprise 12 or 18 diodes
respectively, organised in groups of 6.
2.3.2 Semi-controlled Rectifiers
In the case of semi-controlled rectifiers, a thyristor group takes the place of one of the
diode groups (for example, D4,6,2 as shown in Fig. 2.5 – Uncontrolled rectifier (B6-diode
bridge). The thyristors are also referred to as silicon controlled rectifiers (SCR). SCRs are
found in many applications in electronics, and in particular for power control.
By controlling the firing times of the thyristors, it is possible to limit the inrush current
of the units and achieve soft-charging of the capacitors in the intermediate circuit.
The output voltage of these rectifiers is identical to that produced by uncontrolled
rectifiers. Typically, semi-controlled rectifiers are found in FCs of power size 37 kW and
greater.
+
Fig. 2.6 How thyristors work
Frequency Converters
47
Referring to Fig. 2.6 How thyristors work, when α is between 0° and 90°, the thyristor
circuit is used as a rectifier. When the α value is between 90° and 300° the thyristor
circuit is used as an inverter.
2.3.3 Fully-controlled Rectifiers
Fully-controlled rectifiers involve the use of thyristors. As with a diode, a thyristor
permits the current to flow from the anode (A) to the cathode (K) only. However, the
difference is that the thyristor has a third terminal known as the gate (G). When the
gate is triggered by a signal, the thyristor will conduct. Once current starts flowing
through the thyristor, it will continue conducting until the current drops to zero. The
current cannot be interrupted by sending a signal to the gate.
Thyristors are used in rectifiers. The signal sent to the gate is known as the α control
signal of the thyristor. α is a time delay, which is specified in degrees. The degree value
indicates the delay between the voltage zero crossing and the time when the thyristor
is triggered.
Fig. 2.7 Fully-controlled three-phase rectifier
Fully-controlled three-phase rectifiers can be broken down into two groups of
thyristors: T1, T3 and T5, on the one hand, and T4, T6 and T2 on the other. With fully
controlled rectifiers, α is calculated from the moment when the corresponding diode in
an uncontrolled rectifier would normally begin to conduct, that is, 30° after the voltage
zero crossing. In all other respects, controlled rectifiers behave like uncontrolled
rectifiers.
The amplitude of the rectified voltage can be varied by controlling α. Fully-controlled
rectifiers supply a DC voltage with an average value U, where
U = 1.35 x mains voltage x cos α.
48
Frequency Converters
Compared to uncontrolled rectifiers, fully-controlled rectifiers result in major losses
and disturbances in the supply network, because they draw a high reactive current
when the thyristors conduct for short periods. This is one of the reasons why thyristors
are mainly used in the inverter section of the FC. However, the advantage of fullycontrolled rectifiers is that they enable regenerative braking power in the intermediate
circuit to be fed back to the supply network.
2.3.4 Active Front-End / Active Infeed
For certain FC applications the motor sometimes works as a generator. In these cases
the energy balance can be improved by returning energy to the supply grid.
Such FC’s require a controlled (active) rectifier, which allows energy to flow backwards.
Therefore these devices are called Active Front End (AFE) or Active Infeed Converters
(AIC). Precondition for feeding back energy to the supply grid is that the voltage level
in the intermediate circuit is higher than the grid voltage. This higher voltage must be
maintained in all operating conditions. Various strategies are available to reduce the
losses during standby and motor operation but none can completely eliminate losses.
Further additional filtering is required in generative mode as the generated voltage
does not fit the sine wave shape of the supply grid without.
2.4 Intermediate Circuit
Depending on the design, the functions performed by the intermediate circuit include:
• Acting as an energy buffer so that the motor can draw energy from/return energy to
the grid via the inverter and as a means of accommodating intermittent load surges
• Decoupling the rectifier from the inverter
• Reducing mains interference
The intermediate circuit is based on one of four different basic circuits, shown in Fig. 2.3
Main component topologies. The type of intermediate circuit used is determined by
the nature of the rectifier and inverter with which it is to be combined.
The basic differences between the various types of intermediate circuit are explained in
the following sections.
Frequency Converters
49
2.4.1 Variable Intermediate Circuit
Fig. 2.8 Variable DC intermediate circuit
This type of intermediate circuit consists of a very large inductor, also known as a
“choke”, and is combined with a fully controlled rectifier as shown in Fig. 2.3 Main
component topologies part 5, and Fig. 2.8 Variable DC intermediate circuit.
The inductor converts the variable voltage from the fully controlled rectifier into
a variable direct current. The load determines the size of the motor voltage. The
advantage of this kind of intermediate circuit is that braking energy from the motor can
be fed back into the supply network without the need for additional components. The
inductor is used in current-source FCs (I-converters).
Fig. 2.9 Variable DC voltage intermediate circuit
Finally, a chopper can be inserted in front of a filter, as shown in Fig. 2.9 Variable DC
voltage intermediate circuit. The chopper contains a transistor which acts as a switch
for turning the rectified voltage on and off. The control circuit regulates the chopper by
comparing the variable voltage after the filter (UV ) with the input signal.
If there is a difference between these values, then the ratio of the time ton (when the
transistor is conducting) to the time toff (when the transistor is blocking) is adjusted.
Frequency Converters
50
This makes it possible to vary the effective value of the DC voltage depending on how
long the transistor conducts. This can be expressed as:
UV = U x
toff
ton + toff
When the chopper transistor interrupts the current, the filter inductor (or “choke”)
attempts to produce an infinitely high voltage across the transistor. To prevent this
from happening, the chopper is protected by a freewheeling diode, as shown in Fig. 2.9
Variable DC voltage intermediate circuit.
Fig. 2.10 Chopper transistor regulates the intermediate circuit voltage with corresponding effective value
The filter in the intermediate circuit smooths the square-wave voltage after the
chopper , while keeping the voltage constant at a given frequency. The frequency
associated with the voltage is generated in the inverter.
Frequency Converters
51
2.4.2 Constant Intermediate Circuit
Fig. 2.11 Constant DC intermediate circuit
The intermediate circuit can consist of a filter comprising a capacitor and/or an
inductor (choke). Typically, electrolytic capacitors are used due to their high energy
density. Although capacitors have a limited service life, they offer the following
benefits:
• Smoothing of pulsating DC voltage (UZ1)
• Availability as an energy reserve for supply voltage drops
• Availability for energy storage for load surges and generative operation of the motor
DC inductors offer the following advantages
• The FC is protected against mains transients
• Smoothing of current ripple, which in turn increases the service life of the
intermediate circuit components, especially the capacitors
• Reduction of mains interference and the option of smaller supply conductor cross
sections. This function can also be implemented by means of line chokes upstream of
the FC
When planning an installation it is important to note that the inductors are heavy and
can get hot. Hot spots can arise.
This form of intermediate circuit can be combined with various types of rectifier. In
the case of fully controlled rectifiers, the voltage is kept constant at a given frequency.
Thus, the voltage that is supplied to the inverter is a pure DC voltage (UZ2) with variable
amplitude.
With semi-controlled or uncontrolled rectifiers, the voltage at the inverter input is a
DC voltage with constant amplitude (approximately 2 times the mains voltage). The
anticipated voltage and frequency are both generated in the inverter.
52
Frequency Converters
In the last few years manufacturers have devised intermediate circuits without
capacitors and inductors (chokes). This has been generally termed “capacitor less” or
“slim” intermediate circuit. The control circuit controls the rectification of the supply
voltage in a way that lower inrush currents can be achieved and so that mains
interference can be limited to values of less than 40% (fifth harmonic). This results in
the following characteristics:
• Lower building cost
• No charging circuit required
• More compact and lower weight construction
• Susceptibility to supply system voltage dips. That is, the FC is more likely to trip in the
event of voltage dips, due to transients in the supply system
• Mains interference can occur in the high frequency spectrum
• The high ripple associated with the intermediate circuit reduces the output voltage
by approximately 10% and results in higher motor power consumption
• The restart time for operation may be longer, due to three processes occurring:
– Re-initialisation of the FC
– Magnetisation of the motor
– Ramping up to the required reference for the application
2.5 Inverter
The inverter is the last of the main elements making up the FC. The inverter processes
represent the final stage in terms of generating the output voltage and frequency.
When the motor is connected directly to the mains, the ideal operating conditions
apply at the rated operating point.
The FC guarantees good operating conditions throughout the whole speed range by
adapting the output voltage to the load conditions. It is thus possible to maintain the
magnetisation of the motor at the optimal value.
From the intermediate circuit, the inverter obtains one of the following:
• Variable direct current
• Variable DC voltage
• Constant DC voltage
In each case, the inverter must ensure that the supply to the motor is an AC voltage.
In other words, the frequency of the motor voltage must be generated in the inverter.
The inverter control method depends on whether it receives a variable or a constant
value. With a variable current or voltage, the inverter only needs to generate the
Frequency Converters
53
corresponding frequency. With a constant voltage, the inverter generates both the
frequency and amplitude of the voltage.
Even though inverters work in different ways, the basic design is always the same. The
main components are controlled semiconductors, arranged in pairs in three branches,
as shown in Fig. 2.3 Main component topologies.
Transistors are increasingly taking the place of thyristors in the inverter stage of FCs
for several good reasons. Firstly, transistors are now available for large currents, high
voltages and high switching frequencies. Furthermore, unlike thyristors and diodes,
transistors are not affected by the current zero crossing. Transistors can enter the
conducting or blocking state at any time simply by changing the polarity of the voltage
applied to the control terminals. The advances made in the field of semiconductor
technology over recent years have increased the switching frequency of transistors
significantly. The upper switching limit is now several hundred kHz.
Thus, magnetic interference caused by pulse magnetisation within the motor can be
avoided. Another advantage of the high switching frequency is the fact that it allows
variable modulation of the FC output voltage. This means that a sinusoidal motor
current can be achieved, as shown in Fig. 2.12 Effect of switching frequency on motor
current. The control circuit of the FC merely has to switch the inverter transistors on
and off in accordance with a suitable pattern.
Fig. 2.12 Effect of switching frequency on motor current
Frequency Converters
54
The choice of the inverter switching frequency is a trade-off between losses in the
motor (sine shape of motor current) and losses in the inverter. As the switching
frequency increases, so do the losses in the inverter, in line with the number of
semiconductor circuits.
High-frequency transistors can be divided into three main types:
• Bipolar (LTR)
• Unipolar (MOSFET )
• Insulated Gate Bipolar (IGBT )
Table 2.2 Comparison of power transistor characteristics shows the key differences
between MOSFET, IGBT and LTR transistors.
Properties
Semi-conductor
MOSFET
IGBT
LTR
Low
High
High
Insignificant
High
Insignificant
Low
High
Medium
Short
Short
Insignificant
Medium
Medium
Medium
Medium
Low
High
Medium
Voltage
Medium
Voltage
High
Current
Symbol
Design
Conductivity
Current conductivity
Losses
Blocking conditions
Upper limit
Switching conditions
Turn-on time
Turn-off time
Losses
Control conditions
Power
Driver
Table 2.2 Comparison of power transistor characteristics.
IGBT transistors are a good choice for FCs in terms of the power range, the high level
of conductivity, the high switching frequency and ease of control. They combine the
features of MOSFET transistors with the output properties of bipolar transistors. The
actual switching components and inverter control are normally combined to create a
single module called an “intelligent power module” (IPM).
Frequency Converters
55
A freewheeling diode is connected in parallel with each transistor, because high
induced voltages can occur across the inductive output load. The diodes force the
motor currents to continue flowing in their direction and protect the switching
components against imposed voltages. The reactive power required by the motor is
also handled by the freewheeling diodes.
2.6 Modulation Principles
The semiconductors in the inverter either conduct or block according to the signals
generated by the control circuit. The variable voltages and frequencies are generated
using two basic principles (types of modulation):
• Pulse Amplitude Modulation (PAM) and
• Pulse Width Modulation (PWM)
Fig. 2.13 Modulation of amplitude and pulse width
2.6.1. Pulse Amplitude Modulation (PAM)
PAM is used in FC’s with variable intermediate circuit voltage or current. In FC’s with
uncontrolled or half-controlled rectifiers, the amplitude of the output voltage is
generated by the intermediate circuit chopper, shown in Fig. 2.9 Variable DC voltage
intermediate circuit. In a case where the rectifier is fully controlled, the amplitude is
generated directly. This means that the output voltage for the motor is made available
in the intermediate circuit.
The intervals during which the individual semiconductors should be on or off are
stored in a pattern, and this pattern is read out at a rate dependent on the desired
output frequency.
This semiconductor switching pattern is controlled by the magnitude of the
intermediate circuit variable voltage or current. If a voltage-controlled oscillator is used,
the frequency always follows the amplitude of the voltage.
56
Frequency Converters
Using PAM can result in lower motor noise and very minor efficiency advantages in
special applications like high speed motors (10.000 – 100.000 RPM). However, this often
does not overrule the drawbacks like higher costs for the more sophisticated hardware
and control issues like higher torque ripples at low speed.
2.6.2 Pulse width Modulation (PWM)
PWM is used in FC’s with constant intermediate circuit voltage. This is the most
widely-established and best developed method. Compared with PAM, the hardware
requirements for this modulation method are lower, control performance at low speed
is better and brake resistor operation is always possible. Some manufacturers dispense
with electrolytic capacitors and inductors (chokes) (slim intermediate circuit).
The motor voltage can be varied by applying the intermediate circuit (DC) voltage
to the motor windings for a certain length of time. The frequency can be varied by
shifting the positive and negative voltage pulses for the two half periods along the
time axis.
Because the technology varies the width of the voltage pulses, it is called ‘”Pulse
Width Modulation” or PWM. With conventional PWM techniques, the control circuit
determines the on and off times of the semiconductors in a way which makes the
motor voltage waveform as sinusoidal as possible. Thus the losses in the motor winding
can be reduced and a smooth motor operation, even at low speed is achieved.
The output frequency is varied by connecting the motor to half the intermediate
circuit voltage for a specific period of time. The output voltage is varied by dividing the
voltage pulses of the FC output terminals into a series of narrower individual pulses
with pauses in between. The pulse-to-pause ratio can be modified depending on the
required voltage level. This means that the amplitude of the negative and positive
voltage pulses always corresponds to half the intermediate circuit voltage.
Frequency Converters
57
1.00
0.866
U-V
U-W
W-U
0.50
0
0
60
120
180
240
300
360
-0.50
-0.866
-1.00
Switching pattern of phase U
Phase voltage (0-point & half the intermediate circuit voltage)
Combined voltage to motor
Fig. 2.14 Output voltage PWM
Low stator frequencies result in longer periods. The period can increase to such
an extent that it is no longer possible to maintain the frequency of the triangular
waveform.
This makes the voltage-free period too long, causing the motor to run irregularly.
To prevent this, the frequency of the triangular waveform can be doubled at low
frequencies.
The low switching frequency leads to an increase in acoustic motor noise. To limit the
amount of noise produced, the switching frequency can be increased. This has been
made possible thanks to advances in the field of semiconductor technology, which
mean that modulation of an approximately sinusoidal output voltage and generation
of an approximately sinusoidal current are now achievable. A PWM FC that relies
exclusively on sinusoidal reference modulation can provide up to 86.6% of the rated
voltage (see Fig. 2.14 Output voltage PWM).
The phase voltage at the FC output terminals corresponds to half the intermediate
circuit voltage divided by 2, and is thus equal to half the mains
supply voltage. The mains voltage of the output terminals is equal to 3 times the
phase voltage and is thus equal to 0.866 times the mains supply voltage.
58
Frequency Converters
The output voltage of the FC cannot equal the motor voltage if full sinusoidal wave
form is needed, as the output voltage would be roughly 13 % too low. However, the
extra voltage needed can be obtained by reducing the number of pulses when the
frequency exceeds approximately 45 Hz. The disadvantage of using this method is that
it makes the voltage alternate step-wise and the motor current becomes unstable. If
the number of pulses is reduced, the harmonic content at the FC output increases. This
results in higher losses in the motor.
Another way of dealing with the problem involves using other reference voltages
instead of the three sine references. These voltages could have any shape of waveform,
for example, trapezoidal or step-shaped.
For example, one common reference voltage uses the third harmonic of the sine
reference. By increasing the amplitude of the sine reference by 15.5 % and adding the
third harmonic, a switching pattern for the inverter semiconductors can be obtained
which increases the output voltage of the FC. All control values of the inverter are
transmitted from the control card, and the various reference signals for determining the
switching times are stored in a table in memory and are then read out and processed
according to the reference value.
There are other ways of determining and optimising the on and off switching times
of the semiconductors. The Danfoss VVC and VVCplus control principles are based
on microprocessor calculations which identify the optimum switching times for the
inverter semiconductors.
The specifications for the software involved in calculating the switching times are
manufacturer-specific and will not be covered here.
If more stringent requirements are imposed on the FC speed setting range and smooth
running characteristics, then the PWM switching times need to be determined by an
additional digital IC rather than the microprocessor. For example, an ASIC (Application
Specific Integrated Circuit) can determine the PWM switching times. This component
incorporates the manufacturer’s proven knowledge. Meanwhile, the microprocessors
are responsible for handling other control tasks.
Frequency Converters
59
2.6.3 Asynchronous PWM
Two asynchronous PWM methods are described below:
• SFAVM (Stator Flux-oriented Asynchronous Vector Modulation)
• 60° AVM (Asynchronous Vector Modulation)
These enable the amplitude and angle of the inverter voltage to be changed in steps.
2.6.3.1 SFAVM
Stator Flux-oriented Asynchronous Vector Modulation (SFAVM) is a space-vector
modulation method that makes it possible to change the inverter voltage arbitrarily,
but step-wise within the switching time (in other words, asynchronously).The main
purpose of this type of modulation is to maintain the stator flux at the optimum level
throughout the stator voltage range, ensuring no torque ripple. Compared with the
mains supply, a “standard” PWM supply will result in deviations in the stator flux vector
amplitude and the flux angle. These deviations will affect the rotating field (torque) in
the motor air gap and will cause torque ripple. The effect produced by the deviation
in amplitude is negligible and can be reduced by increasing the switching frequency.
The deviation in the angle depends on the switching sequence and can result in higher
levels of torque ripple. Consequently, the switching sequence must be calculated in
such a way as to minimise the deviation in the vector angle.
Each inverter branch of a 3-phase PWM inverter can assume two switch states, ON
or OFF, as shown in Fig. 2.15 Inverter switch states. The three switches result in eight
possible switch combinations, leading in turn to eight discrete voltage vectors at the
inverter output or at the stator winding of the connected motor. As shown below, these
vectors (100, 110, 010, 011, 001, 101) mark the corners of a hexagon, where 000 and
111 are zero vectors.
Fig. 2.15 Inverter switch states
Frequency Converters
60
With switch combinations 000 and 111, the same potential occurs at all three output
terminals of the inverter. This will be either the positive or negative potential from the
intermediate circuit, as shown in Fig. 2.15 Inverter switch states. As far as the motor is
concerned, this is the equivalent to a terminal short circuit and so a voltage of 0 V is
applied to the motor windings.
Generation of motor voltage
Steady-state operation involves controlling the machine voltage vector Uωt on a
circular path. The length of the voltage vector is a measure of the value of the motor
voltage and the speed of rotation, and corresponds to the operating frequency at a
specific time. The motor voltage is generated by briefly pulsing adjacent vectors to
produce an average value.
Some of the features of the Danfoss SFAVM method are as follows:
• The amplitude and angle of the voltage vector can be controlled in relation to the
preset reference without deviations occurring
• The starting point for a switching sequence is always 000 or 111. This enables each
voltage vector generated to have three switch states
• The voltage vector is averaged by means of short pulses on adjacent vectors as well
as the zero vectors 000 and 111
1,00
U-V
V-W
W-U
0,50
0,00
0
60
120
180
240
300
360
-0,50
-1,00
Switching pattern of phase U
Phase voltage (0-point & half the intermediate circuit voltage)
Combined voltage to motor
Fig. 2.16 With the synchronous 60° PWM principle the full output voltage is obtained directly
Frequency Converters
61
SFAVM provides a link between the control system and the power circuit of the
inverter. The modulation is synchronous to the control frequency of the controller and
asynchronous to the fundamental frequency of the motor voltage. Synchronisation
between control and modulation is an advantage for high- power control (for example,
voltage vector, or flux vector control), since the control system can control the voltage
vector directly and without limitations. Amplitude, angle and angular speed are
controllable.
In order to dramatically reduce the on-line calculation time, the voltage values for
different angles are listed in a table. Fig. 2.17 Output voltage (motor) – (phase-phase)
shows the motor voltage at full speed.
Fig. 2.17 Output voltage (motor) – (phase-phase).
62
Frequency Converters
2.6.3.2 60° AVM
If 60° AVM (Asynchronous Vector Modulation) is used – as opposed to the SFAVM
principle – the voltage vectors are determined as follows:
• Within one switching period, only one zero vector (000 or 111) is used
• A zero vector (000 or 111) is not always used as the starting point for a switching
sequence
• One phase of the inverter is held constant for 1/6 of the period (60°). The switch state
(0 or 1) remains the same during this interval. In the two other phases, switching is
performed in the normal way
Fig. 2.18 Switching sequence of the 60° AVM and SFAVM methods for a number of 60°
intervals and Fig. 2.19 Switching sequence of the 60° AVM and SFAVM methods for
several periods compare the switching sequence of the 60° AVM method with that of
the SFAVM method – for a short interval (Fig. 2.18) and for several periods (Fig. 2.19).
Fig. 2.18 Switching sequence of the 60° AVM and SFAVM methods for a number of 60° intervals
Fig. 2.19 Switching sequence of the 60° AVM and SFAVM methods for several periods
Frequency Converters
63
2.7 Control Circuit and Methods
The control circuit, or control card, is the fourth main component of the FC. The three
hardware components dealt with so far (rectifier, intermediate circuit and inverter)
are nearly always based on the same principles and components regardless of the
manufacturer. In the majority of cases, these components are standard, nearly always
purchased from the same external manufacturers.
The control circuit design stands in contrast to these, as the area where the FC
manufacturer concentrates all its acquired knowledge.
In principle, the control circuit has four main tasks:
• Controlling the FC semiconductors. The semiconductors determine the anticipated
dynamic characteristics or accuracy
• Exchanging data between the FC and peripherals (PLCs, encoders)
• Measuring, detecting and displaying faults, conditions and warnings
• Performing protective functions for the FC and motor
Using microprocessor technology, with single or dual processors, it is possible to
increase control circuit speeds using ready-made pulse patterns that are stored in
memory. As a result, there is a significant reduction in the number of calculations
required.
With this type of control, the processor is integrated into the FC and is always able to
determine the optimum pulse pattern for each operating stage. There are a variety of
control methods available for determining the dynamic characteristics and response
time in the event of a change in reference or torque as well as the positioning accuracy
of the motor shaft.
In general, the basic functions of a FC can be summed up as follows:
• Rotating and positioning the rotor
• Open or closed-loop speed control of the AC motor
• Open or closed-loop torque control of the AC motor
• Monitoring and signalling operating states
Categorising the various voltage-source FC’s available on the market (according to the
form of control), at least six different types can be identified:
• Simple (scalar) without compensation control
• Scalar with compensation control
• Space vector control
• Open loop flux (field-oriented) control
• Closed loop flux (field-oriented) control
• Servo-controlled systems
Frequency Converters
64
This classification is illustrated in Fig. 2.20 Speed control performance control
classification and Fig. 2.21 Torque performance control classification. Here, the response
time refers to how long the FC needs to calculate a corresponding signal change to its
output when there is a signal change at the input. The motor characteristics determine
how long it takes to register a response on the motor shaft when an input signal is
applied to the input of the FC.
[ms]
Servo
10
Flux vector with
Flux
vector wo. feedback
feedback
100
Simple
Reaction time
1
Space
vector
Scalar with
compensation
10
1
0.1
0.01
Precision: Speed (% of rated speed)
[%]
Fig. 2.20 Speed control performance control classification
The rated motor speed is used as the basis for establishing the speed accuracy. The
rated motor speed is 50 Hz in most countries, and 60 Hz in the US.
FCs can be classified according to price/performance ratio. That is, a FC that uses a
simple control method is better value for money for performing very simple tasks, than
one featuring field-oriented control.
[ms]
0.1
Flux-vector
without feedback
Servo
Reaction time
1
Flux-vector
with
feed-back
10
100
Space vector
without
feedback
Scalar
allaarr with
wit
ith
compensation
Space vector with
feed-back
100
10
1
Precision: Torque (% of rated torque)
Fig. 2.21 Torque performance control classification
0.1 [%]
Frequency Converters
65
The speed setting ranges associated with the individual FC types are roughly as follows:
• Simple (scalar) without compensation
1:15
• Scalar with compensation
1:25
• Space vector
1:100(0)
• Flux (field-oriented) open loop
1:1000
• Flux (field-oriented) close loop
1:10.000
• Servo
1:10.000
The torque control performance can be classified as follows:
• The reaction time may be defined in the same way as for speed control
• The accuracy is determined in relation to the motor’s rated torque
Please note that FC’s that rely on a simple control method cannot be used for either
open-loop or closed-loop control of the motor torque
2.7.1 Simple control method
This type of control is rarely used today. The control is in principle a fixed relationship
between desired motor speed and a motor voltage. The model can be more or less
refined, but the major disadvantages are:
• Unstable motor speed
• Difficult start of the motor
• No protection of the motor
The only advantage of simple control could be the low price, but since basic
components for sensing motor are relatively low-cost, very few manufacturers now
pursue this method.
Frequency Converters
66
2.7.2 Scalar Control with Compensation
+
-
f stator
f REF
Δf
u0
Voltage
generator ΔU
Ramp
Inverter control
Load
compensator
I wirk
Slip
compensation
u stator
Current
compens.
calculation
Fig. 2.22 Structure Scalar type Frequency Converter with compensation
When compared with simple control, FC with compensations adds three new control
function blocks as illustrated in Fig. 2.22 Structure Scalar type Frequency Converter
with compensation.
The load compensator uses the current measurement to calculate the additional
voltage (ΔU) required to compensate for the load on the motor shaft.
The current is typically measured by means of a resistor (shunt) in the intermediate
circuit. It is assumed that the power in the intermediate circuit is equal to the power
consumed by the motor. If several active switch positions are combined, these can be
used to reconstruct all the phase current information.
Basic features:
• Voltage/frequency [U/f ] control with load and slip compensation
• Control of voltage amplitude and frequency
Typical shaft output:
• Speed setting range
• Speed accuracy
• Acceleration torque
• Speed change response time
• Torque control response time
1:25
±1% of rated frequency
40-90% of rated torque
200-500 ms
Not available
Frequency Converters
67
Typical features:
• Improved control properties compared with simple scalar control
• Able to withstand sudden changes in load
• No external feedback signal required
• Unable to solve resonance problems
• No torque control properties
• Problems occur when attempting to control high-power motors
• Problems in the event of load changes in the low speed range
2.7.3 Space Vector with and without Feedback
The space vector control method is available with (“closed loop” ) or without (“open
loop”) an external motor speed feedback. As illustrated below, a feature allowing motor
current polar transformation is added to the control (in the components responsible for
magnetisation and torque-generating current).
The voltage angle (θ) is regulated in addition to the voltage (U) and frequency (f ).
Basic features:
• Voltage vector control in relation to steady-state characteristic values (static)
Typical features:
• Improved dynamic performance compared with scalar control
• Very good at withstanding sudden changes in load (compared with scalar plus
compensation)
• Operation at the current limit
• Possibility of active resonance dampening
• Possibility of open-loop/closed-loop torque control
• High starting and holding torque
• Problems during rapid reversing compared with flux vector
• No “rapid” current control
2.7.3.1 Space Vector (Open Loop)
If the space vector without external speed feedback speed and position will be
calculated by the control software and is based on information about motor current
and motor frequency which is measured (see example on page 74, Fig. 2.26 Basic
principles of Danfoss VVCplus control).
68
Frequency Converters
Basic features:
• Voltage vector control in relation to steady-state characteristic values (static)
Typical shaft output:
• Speed setting range
1:100
• Speed accuracy (steady state) ± 0.5% of rated frequency
• Acceleration torque
80-130% of rated torque
• Speed change response time 50-300 ms
• Torque change response time 20-50 ms
2.7.3.2 Space Vector (Closed Loop)
For the closed loop space vector method, an encoder or other device to detect the
motor speed or position is required. It is the control software, the resolution on the
feedback input and encoder’s resolution that determines the accuracy of motor
control.
Typical shaft output:
• Speed setting range
1 : 1000 – 10,000
• Speed accuracy (steady state) Depends on resolution of feedback
component used
• Acceleration torque
80 – 130% of rated torque
• Speed change response time 50 – 300 ms
• Torque change response time 20 – 50 ms
Frequency Converters
69
2.7.4 Open Loop and Closed Loop Flux Vector Control
Flux vector control is also referred to as field-oriented control. The control methods
referred to above control the motor magnetic flux via the stator. With field-oriented
control, the rotor flux is controlled directly. The following motor variables are controlled
within this context:
• Speed
• Torque
Once the rated data for the motor has been entered, a magnetic flux model can be
used to determine the necessary voltage and angle for ensuring optimum motor
magnetisation. The measured motor current is converted into a torque- generating
current and a magnetising current. An internal PID controller is responsible for
controlling the speed, with the feedback value being estimated on the basis of the
measured motor current.
2.7.4.1 Flux Vector (Open loop)
Flux control requires accurate information about the condition, temperature, rotor
position of the motor. It is a challenge to run open loop while the motor condition is
being simulated. Obtaining optimum performance can be difficult, especially at low
motor speed.
Typical shaft output:
• Speed setting range
1:50
• Speed accuracy (steady state) ± 0.5% of rated frequency
• Acceleration torque
100-150% of rated torque
• Speed change response time 50-200 ms
• Torque change response time 0.5-5 ms
Frequency Converters
70
2.7.4.2 Flux Vector (Closed Loop )
For the closed loop flux vector control method, an encoder or similar sensor is required
on the motor shaft. The control software and the feedback resolution determine the
accuracy of motor control.
Control is performed in exactly the same way as with open loop methods. However, in
this case the speed is calculated from the encoder signals rather than being estimated.
Flux vector control is illustrated in Fig. 2.23 Structure closed loop flux vector control.
Typical shaft output:
• Speed setting range
• Speed accuracy (steady state)
• Acceleration torque
• Speed control response time
• Torque control response time
1:1000 to 10,000
Dependent on the feedback signal (encoder) used
100 – 150% of rated torque
5.00 – 50 ms
0.50 – 5 ms
=
Mains
3¾
Ÿr.ref
Speed
controller
Ÿr
Is q.ref
Is d.ref
Field
weakening
Imr.ref
Us q.ref
Current
controller Us d.ref
d, q
Inverter control
Encoder
Motor
a, b
Flux
controller
Istator-3
kty
Imr
Is d.ref
Is q.ref
Flux
model
d, q
a, b
Speed
calc.
2
3
Position
Fig. 2.23 Structure closed loop flux vector control
2.7.5 Servo Drive Control
The servo converter control method will not be explored in depth here.
One common method is very similar to closed loop flux control, but to ensure high
Frequency Converters
71
dynamic response, the power components and hardware may be upgraded as much
as two, three or four times the power components in a FC to ensure available current
and torque.
2.7.6. Control Conclusions
In conclusion, all control methods are primarily handled by the software. The more
dynamic the motor control needs to be, the more complex the control algorithm
required.
A similar principle applies for initial use of a FC. Initial use of a simple FC does not
involve a great deal of programming. In most cases, all you have to do is enter the
motor data. However, for applications that require a flux vector control or critical
applications like hoists, more complex programming is required, right from initial use.
Due to the fact that the control is mainly a software issue, many manufacturers have
implemented several control methods in their units, for example U/f, space vector, or
field-oriented control. Parameters are needed to switch from one control method to
another, for example from space vector control to the flux vector method. Pop-up
menus help the operator to set the parameters needed for each control method, in
order to meet the application demands.
2.8 Danfoss Control Principles
A general overview of the standard current control principles for Danfoss FC’s is
illustrated in Fig. 2.24 Basic principles of current standard frequency converters from
Danfoss.
Fig 2.24 Basic principles of current standard frequency converters from Danfoss
72
Frequency Converters
The PWM switching patterns are calculated for the inverter using the selected control
algorithm. U/f control is suitable for applications involving
• Special motors (for example, sliding rotor motor)
• Motors connected in parallel
In the case of the applications referred to above, no compensation of the motor is
required. With the VVCplus control principle, the amplitude and angle of the voltage
vector are controlled directly, as is the frequency. At the heart of this method lies a
straightforward, yet robust, motor model. The type of control method involved is called
Voltage Vector Control (VVC).
Some of the features offered include:
• Improved dynamic properties in the low speed range (0 - 10 Hz)
• Improved motor magnetisation
• Speed control range: 1:100 opened loop
• Speed accuracy: ±0.5% of the rated speed without feedback
• Active resonance dampening
• Torque control
• Operation at the motor current limit
2.8.1 Danfoss VVCplus Control Principle
The Danfoss VVCplus control principle uses a vector modulation method for constant
voltage-source PWM inverters. Depending on the application control demands, the
motor equivalent diagram can be simplified (that is, the iron, copper and air flow losses
are ignored) or used in its full complexity.
Example:
A simple fan or pump application control uses a simplified motor diagram. However
a dynamic hoist application requiring flux vector control requires the complex motor
equivalent diagram, accounting for all losses in the control algorithm.
The inverter switching pattern is calculated using either the SFAVM or 60° AVM
principle, to keep the pulsating torque in the air gap very small. The user can select
the preferred operating principle, or allow the control to select one automatically
on the basis of the heatsink temperature. When the temperature is below 75° C, the
SFAVM principle is used for control. At temperatures above 75°, the 60° AVM principle is
applied.
Frequency Converters
73
The control algorithm takes two operating conditions into consideration:
• No-load state (idle state), see Fig. 2.25a Motor equivalent circuit diagram under “noload”. In the no-load state, there is no load on the motor shaft. For conveyors the noload state literally means no products are being transported. It is simply assumed the
current drawn by the motor is only needed for magnetisation and compensation for
losses. The active current is considered to be nearly zero. The no-load voltage (UL) is
determined on the basis of the motor data (rated voltage, current, frequency, speed).
Fig. 2.25a Motor equivalent circuit diagram under “no-load”
• Loaded state
The motor shaft is loaded, implying that products are being transported, as shown in
Fig. 2.25b Motor equivalent circuit diagram under “load”.
The motor draws more current when it is loaded. In order to produce the required
torque the active current (IW) is needed. Losses in the motor (especially in lower
speed range) need to be compensated for. A load-dependent additional voltage
(UComp) is made available to the motor:
U = ULOAD = UL + UComp
Fig. 2.25b Motor equivalent circuit diagram under “load”
74
Frequency Converters
The additional voltage UComp is determined using the currents measured under the
two conditions mentioned above (loaded and no-load) as well as the speed range: low
or high speed. The voltage value and the speed range are then determined on the basis
of the rated motor data.
The control principle is illustrated in the block diagram below:
Fig. 2.26 Basic principles of Danfoss VVCplus control
As shown in Fig. 2.26 Basic principles of Danfoss VVCplus control, the motor model
calculates the no-load references (currents and angles) for the load compensator (ISX,
ISY ) and the voltage vector generator (I0, θ0).
The voltage vector generator calculates the no-load voltage (UL) and the angle
(θL) of the voltage vector on the basis of the no-load current, stator resistance
and stator inductance.
The measured motor currents (Iu, Iv and Iw) are used to calculate the reactive current
(ISX) and active current (ISY) components.
Based on the calculated currents (ISX0, ISY0, ISX, ISY) and the voltage vector actual values,
the load compensator estimates the air-gap torque and calculates how much extra
voltage (UComp) is required to maintain the magnetic field strength at the reference
value. It then corrects the angle deviation (Δθ) that is to be expected due to the load on
the motor shaft. The output voltage vector is represented in polar form (p). This enables
direct overmodulation and facilitates connection to the PWM ASIC.
Frequency Converters
75
Voltage vector control is particularly useful for low speeds, where the dynamic
performance of the drive can be significantly improved (compared with U/f control)
by means of appropriate control of the voltage vector angle. In addition, steady-state
behaviour improves, since the control system can make better estimates for the load
torque on the basis of the vector values for both voltage and current than it would be
able to on the basis of the scalar signals (amplitude values).
f
Internal frequency
fs
Reference frequency set
Δf
Calculated slip frequency
ISX
Reactive current (calculated)
ISY
Active current (calculated)
ISXO, ISYO
No-load current of x/y axis (calculated)
Iu, Iy, Iw
Measured phase current (U, V, W )
Rs
Stator resistance
Rr
Rotor resistance
θ
Voltage vectors angle
θ0
“No-load” theta value
Δθ
Load-dependent angular compensation
Tc
Heat-sink temperature (measured)
UDC
Intermediate circuit voltage
UL
No-load voltage vector
Us
Stator voltage vector
UComp
Load-dependent voltage compensation
U
Motor supply voltage
Xh
Reactance
X1
Stator leakage reactance
X2
Rotor leakage reactance
ωs
Stator frequency
Ls
Stator inductance
LSs
Stator leakage inductance
LRs
Rotor leakage inductance
Table 2.3 explanations of symbols used in:
Fig. 2.23 Structure closed loop flux vector control
Fig. 2.24 Basic principles of current standard frequency converters from Danfoss
Fig 2.25a Motor equivalent circuit diagram under “no-load
Fig. 2.25b Motor equivalent circuit diagram under “load”
Fig. 2.26 Basic principles of Danfoss VVCplus control
Frequency Converters
76
2.8.2 Danfoss Flux Vector Control Principle
The principle of flux vector control assumes that a complete equivalent circuit diagram
data is available. With this approach, all the relevant motor parameters are taken into
account by the control algorithms. Considerably more motor data needs to be specified
than is the case with the basic VVCplus control.
Changing a single parameter during commissioning switches the control algorithm
from VVCplus control to flux vector control. Here more information needs to be fed in
to the drive for smooth control of the motor. All parameters will not be explained here
as they are fully explained in the operation manuals.
A brief description of the control strategy is shown in Fig. 2.27 Basic principles of
Danfoss Flux Vector control. A flux database is stored in the frequency converter. The
currents measured in all 3 phases are transformed in to polar coordinates (xy).
Flux
set point
Speed
control
nset
nest
ψset
Flux
control
Torque
control
Usx
USY
Flux
model
Speed
estimation
Fig. 2.27 Basic principles of Danfoss Flux Vector control
Frequency Converters
77
2.9 Standards and Legislations
As for all other products legislations and technical standards are worldwide available to
ensure safe operation of FC’s.
Legislation is issued by the legislative branch of national or local government and
can of course be different in the different countries around the globe. However it is
mandatory to comply with – it is law. It is a political document, typically free of specific
technical details – these details can be found in standards.
Standards are written by experts in relevant standardisation bodies (such as the
International Electrotechnical Commission IEC or the European Committee for
Electrotechnical Standardisation CENELEC) and reflect the technical state of the art.
Their role is to establish a technical common ground for cooperation between market
players. Typically IEC standards are accepted in the majority of countries and local
standards (EN, NEMA) will be harmonised to fit them.
Manufacturers have to demonstrate and document compliance with the local
legislations by following the standards otherwise they are not allowed to sell their
product in the local market. On the product itself the compliance is indicated by
symbols.
a)
b)
Fig. 2.28 CE- Marking (a) and ul listing (b)
Which standards have been applied and which legislative conformance has been
stated is noted for example in Europe in the Declaration of Conformity. For a better
understanding this book address several standards connected to FC’s and some
relevant legislative measures.
Frequency Converters and Motors
78
3 Frequency Converters and Motors
In the previous chapters, the motor and the FC were each presented in isolation. This
chapter explains the interaction between the two components.
3.1 Basic Principles
3.1.1 U/f Operation and Field Weakening
The major technical characteristics of a motor are found on its nameplate. The
information shown is very relevant for the electrical installer, because values for
voltage, frequency and full load current are given, but important information for the
mechanical design is missing and can be found in the datasheet, catalogue, or by direct
contact to the motor manufacturer.
This mechanical design information includes data related to motor start and
intermittent operation, and also the available torque at the motor shaft. The shaft
torque is easy to calculate from the nameplate data.
For a given load, the following expression applies:
T=
P u 9550
n u —3 u V u I u cos M u 9550 k u V u I
=
=
n
f u 60/pu (1-s)
f
This results in the principle relation:
T~
V
uI
f
This relation is utilised in voltage source FC’s which maintain a constant ratio between
the voltage (U) and the frequency (f ) . This constant ratio (U/f ) determines the
magnetic flux density (Φ) of the motor and is determined by the motor nameplate
data (for example, 400 V/50 Hz = 8 [V/Hz]). The constant flux density ensures optimum
torque from the motor. Ideally the ratio 8 [V/ Hz] means that each 1 Hz change in
the output frequency will result in an 8 V change in the output voltage. This way of
controlling the output values of the FC is called “U“ to “f ” characteristic control.
Frequency Converters and Motors
79
U [V]
T [Nm]
800
T Uxl
f
1
f max. torque decrease with factor
TLoad
400
10%
MN
230
Tmotor
0
50
f [Hz]
a) U/f characteristic control (ideal)
0
50
f [Hz]
b) T/n characteristic (ideal)
Fig. 3.1 Principle U/f characteristic and torque
The ideal curve of the U/f characteristic for a star connected 50 Hz motor is shown in
Fig. 3.1 Principle U/f characteristic a) applied motor voltage b) resulting torque. Up to
50 Hz the FC applies a constant U/f ratio to the motor which result in the possibility to
get a constant torque out of the motor.
For operating the motor at 100 Hz ideally the output voltage should be increased to
800 V to maintain a constant U/f ratio (dotted line in Fig. 3.1a Principle U/f characteristic
and torque). As this high voltage is critical for the motor insulation this is not an applied
strategy. Typically the FC limit its output voltage to the value of the input (for example
400 ±10%)
This means that the FC can maintain a constant U/f ratio to a certain frequency only.
After this frequency it can continue to the frequency but not the voltage anymore.
As this is affecting the U/f ration the magnetic flux density is reduced. Therefore this
speed range is also called field weakening area (Fig. 3.1b Principle U/f characteristic and
torque). The reduced magnetic field results in a reduced maximum motor torque. While
the nominal torque is reduced by 1/f the stall torque decreases by 1/f2.
Please note that the shown curves are ideal and require some compensation which are
described in the following sections.
80
Frequency Converters and Motors
3.1.2 87 Hz Characteristics
Typically asynchronous motors operated with FC’s are configured to the nominal
voltage of the mains. This means that 400 V/230 V motor will be configured in star
when operated by a 400 V FC. As described in the previous section a 50 Hz motor will
enter field weakening when the voltage can’t be increased any more. For extending the
speed range the motor can be configured in delta.
Example
Motor: 15 kW, 400 V/230 V Y/Δ, 27.5A/48, 7A, 50 Hz
At 50 Hz the power in star and delta configuration is 15 kW because of the different
mains voltage which result in different motor currents.
PY (50 Hz) = 3  400 V  27.50 A  cos φ  η = 14.92 [kW]
PΔ (50 Hz) = 3  230 V  48.70 A  cos φ x η = 15.19 [kW]
With delta connection it can be seen in Fig. 3.2 87 Hz characteristic that in contradiction
to the start configuration the motor runs with constant U/f ratio up to 230 Volt, but if
the FC is powered from a 400 Volt supply, we are actually able to keep the constant U/f
ratio up to 400 Volt and the high current,
PΔ (87 Hz) = 3  400 V  48.70 A  cos φ  η = 26.42 [kW]
Frequency Converters and Motors
81
U [V]
T [Nm]
800
T Uxl
f
TLoad
Star configuration
400
1
f max. torque decrease with factor
10%
MN
230
Delta configuration
0
50
87
Tmotor 87 Hz
Tmotor
f [Hz]
a) 87 Hz U/f characteristic control (ideal)
0
50
87
f [Hz]
b) T/n characteristic (ideal)
Fig. 3.2 87 Hz characteristic
This means we have the rated flux density (Φ) up to 400 Volt even the motor is
configured for 230 Volt. With this higher voltage we can increase the maximum
frequency with rated flux to 87 Hz.
The use of this knowledge presupposes the following:
• The selected FC must easily be able to handle the higher delta current (48.70 A)
• The motor must be wound such that it can withstand the required operating voltage
(typically higher in the star configuration) supplied by the FC (i.e. with a 690 V supply
voltage and a 690 V FC, this application is only possible with a motor wound for
690 V / 400 V Y/Δ)
• The torque on the motor shaft remains the same for both configurations up to
50 Hz. Hz. Above 50 Hz, a star-connected motor enters the field weakening range.
When it is delta-connected, this does not happen until approximately 90 Hz. If the
±10% tolerance of the FC is used, the field weakening range begins at 55 Hz or 95 Hz
respectively. The torque decreases because the motor voltage is not increased
The benefits of this increased motor capacity utilisation are:
• An existing FC can be operated with a greater speed control range.
• A lower-power rating motor can be used. Such a motor can have lower moment of
inertia which allows higher dynamics. This improves the dynamic characteristics of
the system.
Please note that operation of a 400V/230V Y/Δ motor in delta at 400 V is only possible
on a FC because of the higher feeding frequency of 87 Hz. Operation direct on 400 V/
50 Hz mains will destroy the motor!
82
Frequency Converters and Motors
3.1.3 Running in Current Limit
As seen, the relationship between motor shaft torque and motor current indicates
that if motor current can be controlled, then the torque is also under control. If an
application temporarily needs torque up to maximum it is essential that the FC is
designed for continuous operation current up to the current limit, and not exceed it or
trip.
There are different strategies for designing the FC to run in current limit situations. The
most typical strategy is the fact that torque will be reduced when the speed is reduced.
But as we shall see later there can be applications where this strategy cannot be utilised
and can even cause bigger problems.
3.2 Compensations
It used to be difficult to tune a FC to a motor because some of the compensation
functions, such as “start voltage”, “start compensation” and “slip compensation”, are
difficult to relate to practice.
These compensations are required because motor characteristics are not linear. For
example an asynchronous motor requires a greater current at low speed to accomplish
both magnetising current and torque-producing current for the motor. The built-in
compensation parameters ensure optimum magnetisation and hence maximum torque:
• During start
• At low speeds
• In the range up to the rated speed of the motor
In the latest generation of FC’s, the device automatically sets the necessary
compensation parameters once the motor rating details of the motor have been
programmed into the FC. These details include voltage, frequency, current and speed.
This applies to approximately 80% of standard applications such as conveyors and
centrifugal pumps. Normally, these compensation settings can also be changed
manually for fine tuning applications such as hoisting or positive displacement pumps
if required.
3.2.1 Load-independent Start Compensations
Increase the output voltage in the lower speed range by manually setting an additional
voltage, often called start voltage.
Frequency Converters and Motors
83
Example
A motor which is much smaller than the recommended motor size of an FC may
require an additional, manually adjustable voltage boost in order to overcome static
friction or ensure optimum magnetisation in the low speed range.
If several motors are controlled by one FC (parallel operation), it is recommended to
de-activate the load-independent compensation.
The load-independent supplement (start voltage) ensures an optimum torque during
start.
3.2.2 Load-dependent Start Compensations
The load-dependent voltage supplement (start and slip compensation) is determined
via the current measurement (active current).
This compensation is normally called the I  R compensation, boost, torque increase, or,
– at Danfoss, – start compensation.
This type of control reaches its limits when the disturbances are difficult to measure
and the load is highly variable (for example in motors with operational change in the
winding resistance of up to 25 % between the hot and cold states).
The voltage increase may have different results. Under no-load operation, it may lead
to saturation of the motor flux. In the event of saturation, a high reactive current will
flow that leads to heating of the motor. If the motor is operating with a load, it will
develop little torque because of the weak main flux and may stop running.
The real U/f and T/n characteristics are generally as shown in Fig. 3.3 Real U/f and T/n
characteristic.
Frequency Converters and Motors
84
UMotor [V]
800
Without load
400
Load
compensation
230
Compensating
voltage
f [Hz]
50
100
T [Nm]
100%
50%
n [min-1]
Fig. 3.3 Real U/f and T/n characteristic
In Fig. 3.3, additional voltage is supplied to the motor at low speeds for the purpose of
compensation.
3.2.3 Load Compensations
The motor voltage is increased under load ascertained from the measured motor
current.
The output voltage receives a voltage boost which effectively overcomes the influence
of the DC resistance of the motor windings at low frequencies and during start.
An increase in output voltage leads to over-magnetisation of the motor. This increases
the thermal load on the motor such that a reduction in torque is to be expected. The
motor voltage is reduced in no-load operation.
3.2.4. Slip compensation
The slip of an asynchronous motor is load-dependent and typically amounts to some
5% of the rated speed. For a two-pole motor, this means that the slip will be around
150 RPM.
Frequency Converters and Motors
85
However, the slip will be approximately 50% of the required speed if the FC is
controlling a motor at 300 RPM (10 % of the rated synchronous speed of 3000 RPM).
If the FC has to control the motor at 5 % of the rated speed, the motor will stall if it is
loaded. This load dependence is undesirable, and the FC is able to fully compensate for
this slip by effectively measuring the active current to the motor.
The FC then compensates for the slip by increasing the frequency according to the
actual measured current. This is called active slip compensation.
The FC calculates the slip frequency (fslip) and the magnetisation or no-load current
(I) from the motor data. The slip frequency is scaled linearly to the active current
(difference between no-load and measured current).
Example
A 4-pole motor with a rated speed of 1455 RPM has a slip frequency of 1.5 Hz and a
magnetisation current of approximately 12 A.
With a load current of 27.5 A and 50 Hz, the FC will output a frequency of about
51.5 Hz. At a load current between IΦ (12 A) and IN (27.5 A), the frequency will be
adjusted accordingly between zero and 1.5 Hz.
As demonstrated in the example, factory setting of slip compensation is often scaled
such that the motor runs at the theoretical synchronous speed. In this case,
51.5 Hz - 1.5 Hz = 50 Hz.
3.2.5 PM Motor and SynRM Compensations
For Permanent Magnet motors the start and slip compensations are irrelevant, but
other parameters are essential.
The magnetising profile differs of course from the asynchronous motor, but other
important data and compensations are:
• Nominal motor speed and frequency
• Back EMF
• Max speed before back EMF damages the FC
• Field weakening
• Dynamic details relevant for the control
For SynRM motors other parameters are essential, for instance:
• Stator resistance
• d and q axis inductances
• Saturation inductances and
• Saturation point
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Frequency Converters and Motors
3.3 Automatic Motor Adaptation (AMA)
Motor data on the motor nameplate or from the motor manufacturer’s datasheet are
given for a specific range of motors, or a specific design, but rarely do those values
refer to the individual motor. Due to variations in the production of motors and the
installation, those motor data are not always accurate enough to ensure optimal
operation.
Also as seen above there are several compensations which require setting.
For modern FC’s, fine-tuning to the actual motor and installation can be a complicated
and troublesome task.
In order to make installation and initial commissioning easier, automatic configuration
functions like the Automatic Motor Adaption (AMA) from Danfoss are becoming
increasingly common. These functions measure for example the stator resistance and
inductance. The effect of the cable length between FC and motor is also taken into
account.
The parameters required for different motor types differ in important details. For
instance, the back EMF value is essential for PM motors and saturation point level is
important for SynRM motors. Therefore different types of AMA are required. Note that
not all FC’s support the AMA function for all motor types.
In principle two types of AMA are used:
Dynamic
The function accelerates the motor to a certain speed to perform the measurements.
Typically the motor must be disconnected from the load /machine for “identification
run”.
Static
The motor is measured at standstill. This means there is no requirement to disconnect
the motor shaft from the machine. It is important, however, that the motor shaft is not
rotated by external influences during measurement.
Frequency Converters and Motors
87
3.4 Operation
3.4.1 Motor Speed Control
The output frequency of the FC, and thus the motor speed, is controlled by one or
more signals (0-10 V; 4-20 mA, or voltage pulses) as a speed reference. If the speed
reference increases, the motor speed goes up and the vertical part of the motor
torque characteristics is shifted to the right (Fig. 3.4 Reference signal and motor torque
relation).
Fig. 3.4 Reference signal and motor torque relation
If the load torque is less than the motor torque, the speed will reach the required value.
As shown in Fig. 3.5 Relation current limit and over current limit, the load torque curve
intersects the motor torque curve in the vertical part (at point A). If the intersection
is in the horizontal part (point B), the motor speed cannot continuously exceed the
corresponding value. The FC allows brief current limit overshoots without tripping
(point C), but it is necessary to limit the duration of the overshoot.
t
Fig. 3.5 Relation current limit and over current limit
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Frequency Converters and Motors
3.4.2 Reversing
The direction of rotation of asynchronous and many synchronous motors is determined
by the phase sequence of the supply voltage. If two phases are swapped, the direction
of rotation of the motor changes (the motor reverses).
Fig. 3.6 The rotation direction of the motor reverses when the phase sequence is changed
A FC can reverse the motor by electronically changing the phase sequence. Reversing
is accomplished by either using a negative speed reference or a digital input signal.
If the motor must have a specific direction of rotation when first put into service, it is
important to know the factory default setting of the FC.
Since a FC limits the motor current to the rated value, a motor controlled by a FC can be
reversed more frequently than a motor connected directly to the mains.
N
Fig. 3.7 Braking torque of the frequency converter during reversing
Frequency Converters and Motors
89
3.4.3. Acceleration and Deceleration Ramps (Ramp Up and Down)
For many applications there are various reasons why the speed must not change too
quickly but instead must be changed slowly or with smooth transitions. All modern FC’s
have ramp functions to facilitate this. These ramps are adjustable and ensure that the
speed reference is able to increase or decrease only at a preset rate.
The acceleration ramp (ramp up) indicates how quickly the speed is increased. It is
stated in the form of an acceleration time tacc and indicates how quickly the motor
should reach the new speed. These ramps are mostly based on the rated motor
frequency, e.g. an acceleration ramp of 5 seconds means that the FC will take 5 seconds
to go from standstill to the rated motor frequency (fn = 50 Hz).
However there are some manufacturers who relate the acceleration and deceleration to
the values between the minimum and maximum frequency.
Fig. 3.8 Acceleration and deceleration times
The deceleration ramp (ramp down) indicates how quickly the speed is decreased. It
is stated in the form of a deceleration time tdec and indicates how quickly the motor
should reach the new reduced speed.
It is possible to go directly from acceleration to deceleration, since the motor always
follows the output frequency of the inverter.
Ramp times can be set to such low values that in some situations the motor cannot
follow the preset speed.
This leads to an increase in the motor current until the current limit is reached. In the
case of short ramp-down times, the voltage in the intermediate circuit may increase to
such a level that the protective circuit will stop the FC.
If the inertia of the motor shaft and the referred inertia of the load are known, the
optimum acceleration and deceleration times can be calculated.
Frequency Converters and Motors
90
tacc = J =
n2–n1
(Tacc – Tfric) = 9.55
tdec = J =
n2–n1
(Tacc + Tfric) = 9.55
J
Tfric
Tacc
Tdec
n1 and n2
is the moment of inertia of the motor shaft and load [kgm2].
is the friction torque of the system [Nm].
is the overshoot torque used for acceleration [s].
is the braking torque that occurs when speed reference is reduced [s].
are the speeds at frequencies f1 and f2 [min-1].
If the FC allows an overload torque for a brief time, the acceleration and deceleration
torques are set to the rated motor torque T. In practice, the acceleration and
deceleration times are normally identical.
Example
A machine has the following specifications:
J
= 0.042 kgm2
n1 = 500 min-1
n2 = 1000 min-1
Tfric = 0.05  TN
TN = 27 Nm
Theoretical acceleration time can be calculated as follows:
n2–n1
0.042 =(1000–500)
tacc = J =
(Tacc – Tfric) = 9.55 (27.0 – (0.05 = 27.0)) = 9.55
The ramp functions ensure that there is no abrupt change of speed, provided the FC is
set to the calculated acceleration. This is essential for many applications like:
• Ensuring bottles do not topple over on bottle transporting conveyors
• Preventing water hammer in pump systems
• Comfort in escalators or lifts
Most often linear ramps are used. However different characteristics are possible for
different applications, for example, an “S” or “S2” ramp. With the “S” ramp, the transitions
to and from standstill are particularly gentle.
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91
Frequency
Fmax
Accel.
T1
Run
Decel.
T2
a) Without smoothing
Accel.
T1
Run
Decel.
T2
b) With smoothing
Fig. 3.9 Linear ramp a) and S-ramp (b)
3.4.4 Motor Torque Control
Motor torque is another parameter which is important for the application, as shown in
Fig. 3.5 The motor current can overshoot the current limit briefly.
Torque is the basis for the rotation or movement of a load. Reasons for controlling the
torque include:
• Limiting the torque to prevent damage on machine etc.
• Control the torque to make more motors share the load.
If an application is suddenly overloaded, and the FC is sized for overload, the machine
can work for a given time in the overload mode. However this excessive torque can be
fatal for the machine or reduce the lifetime. Therefore, many FC’s can be programmed
to send a warning in case of overload, but also limit the torque under specific
conditions.
As described in section 3.1 Basic Principles there is a relationship between current and
torque. This relationship is not direct, but depends on slip, cos phi and temperature in
the motor. The limitation based on measuring the current is not accurate. If the FC is
the Space Vector type or the flux type (see chapter 2 Frequency Converters) the current
is measured vectorially in all three motor phases, and the distribution of the current
components is easy. With this information the FC can calculate the torque precisely
enough to make sure the machine is protected.
In situations where more motors are on a common mechanical system, it is essential
that the motors share the load equally. If the slip compensation factor is reduced, the
motors will automatically balance their torque, but not necessarily maintain the desired
speed.
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Frequency Converters and Motors
Another function in some FC’s is called the Droop function. Droop function means
that one motor is controlling the speed and additional FC’s follow the same speed and
automatically share the load.
Example
A 100 meter long conveyor belt has numerous drive stations distributed along the belt.
If one of the motors tends to run a bit faster than the other, this motor will have to give
more torque. The result can be:
• The motor can be overloaded and overheated
• The belt can be damaged because of the partial higher torque
• Pulleys and drive drums may slip with excessive wear as result
In such situations, torque and torque sharing is important.
3.4.5 Watchdog
FC’s can monitor the process being controlled and intervene in case of operational
disturbance. This monitoring can be divided into three areas: machinery, motor, and FC.
The machinery is monitored by
• Output frequency
• Output current
• Motor torque
Based on these values, a number of limits can be set which intervene in the control
function if they are exceeded. These limits could be the lowest permissible motor
speed (minimum frequency), the highest permissible motor current (current limit) or
the highest permissible motor torque (torque limit). If the limits are exceeded, the FC
can, for example, be programmed to:
• give a warning signal,
• decrease the motor speed or
• stop the motor as fast as possible
Example
In an installation using a V-belt as a connection between the motor and the rest of the
installation, the FC can be programmed to monitor the V-belt.
As expected, the output frequency increases more quickly than the preset ramp. If the
V-belt breaks, the frequency can be used to either give a warning or stop the motor.
Frequency Converters and Motors
93
3.5 Dynamic Brake Operation
Machines can create potential or kinetic energy which we want to remove from the
machine.
Potential energy is caused by gravity, for example when a load is hoisted to a position
and being held in position.
Kinetic energy is caused by movement, for example a centrifuge running at a given
speed which we want to reduce or a trolley to be stopped.
The dynamic characteristics of some loads require 4-quadrant operation. A reduction in
the stator frequency (and voltage) by the FC allows the motor to act as a generator and
convert mechanical energy into electrical energy.
N
CW rotation
CW rotation
N
N
T
-T
T
Braking
Driving
Driving
Braking
T
N
N
T
T
CCW rotation
T - Torque
N - Speed
-N
Fig. 3.10 Four Quadrant operation: Clockwise (CW) and Counter Clockwise (CCW)
Motors connected directly to the mains deliver the braking energy straight back to the
mains.
If a motor is controlled by a FC, the braking energy is stored in the intermediate circuit
of the FC. If the braking energy exceeds the power loss of the FC, the voltage in the
intermediate circuit increases dramatically (in some cases exceeding 1000 V DC).
If the voltage exceeds the internal voltage limit, the FC is then switched off for selfprotection and usually issues an alarm message or error code “over voltage”. Measures
must be taken to prevent the FC being tripped if the motor feeds back excessive
braking energy.
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Frequency Converters and Motors
The following measures are typically used:
• Extend the deceleration ramp time
• Dissipate the energy in the motor. That is, the motor is used as a braking resistor
• The FC is fitted with a “brake chopper” electronic circuit and appropriate braking
resistors
• Use of a regenerative braking unit to feed energy back to mains
• Use of FC’s with an active rectifier to feed energy back to mains
The first two measures require no additional hardware components. All the other
measures do require additional components and must be taken into account at the
design stage of the machinery.
3.5.1 Extending Deceleration Ramp
The deceleration ramp time can be extended by the operator by changing the relevant
parameter setting. However the operator must judge the load ratios himself.
Example
An attempt to brake a 22 kW motor operated by a FC from 50 Hz to 10 Hz within one
second will end up with the FC tripping because the motor, acting as a generator, will
feed back too much energy. The user can prevent the FC from tripping by changing the
ramp-down time (for example, to 10 seconds).
Alternatively, the modern FC has control functions such as overvoltage control (OVC)
that must be enabled to prevent the FC tripping or to automatically extend the ramps.
The FC itself determines then the appropriate ramp time. This type of ramp extension
automatically takes account of varying load inertias. Care must be taken when this kind
of function is used on machines with vertical or horizontal movement (such as hoists,
lifts, and portal cranes) as extending the ramp time does also mean that the traveling
distance will be prolonged.
3.5.2 Motor as a Braking Resistor
Manufacturers use various methods for using the motor as a braking resistor. The basic
principle is based on re-magnetising the motor. Every manufacturer gives its method a
different name, such as AC brake, flux brake, or compound braking. This type of braking
is not recommended for highly dynamic applications (such as hoists or lifts) because
the more frequent the braking, the hotter the motor becomes and consequently it can
fail to perform as expected.
Frequency Converters and Motors
95
3.5.3 Brake Chopper Circuit (Brake Module) and Resistor
The circuit essentially consists of a transistor (for example, IGBT ) that eliminates the
excess voltage by “chopping” it and sending it to the connected resistor. The control
circuit must be fed with the appropriate information during commissioning that a
braking resistor is connected. The control circuit can also check whether the resistor
is still in working order. Typically it must be specified if an FC is equipped with a brake
chopper or not, at the point of ordering.
Braking
resistor
Control logic
Above a certain power level, the use of a braking module and resistor will cause heat,
space and weight problems.
Fig. 3.11 Brake “chopper” and resistor
3.5.4 Use of a Regenerative Braking Unit
If the load often generates a great deal of regenerative energy, it may be useful to use a
fully-regenerative braking unit.
If the voltage in the intermediate circuit rises to a given level, the DC voltage in the
circuit is fed back to the mains, synchronously in both amplitude and phase, by an
inverter.
This feedback of energy can be accomplished by:
• FC’s with an active rectifier. In this FC type, the rectifier can transmit energy from the
intermediate DC circuit to the power supply
• External regenerative braking units integrally connected to the intermediate circuit
of one or more FC’s, monitor the voltage in the intermediate circuit.
Frequency Converters and Motors
96
Fig. 3.12 Regenerative braking unit shows a simplified version of the operating
principle.
AC line supply
Motoring
Generating
DC link
a)
Fig. 3.12
AC line supply
Motoring
Generating
DC link
b)
Regenerative braking unit: motoring phase control on (a), motoring phase off (b)
For evaluation when it makes economic sense using these kind of devices please refer
to chapter 4 Saving Energy with Frequency Converters.
3.6 Static Brake Operation
The FC has several functionalities for locking or coasting the motor shaft, such as:
• Coasting to stop
• DC brake
• DC hold
• Electromechanical brake
The last three of these functions can typically only be performed after a stop command
has been issued. This is often misunderstood in practice. It is important to note that a
reference value of 0 Hz does not function as a stop command.
In general, do not use these functions when the direction of rotation is reversed.
3.6.1 Coasting to Stop
With the motor coasting, the voltage and frequency are immediately interrupted
(0 V/0 Hz) and the motor is “released”. As the motor is no longer energised, it will
typically spin down to zero speed. Depending on the speed and inertia of the load this
can take from seconds to hours (for example, for huge separators).
Frequency Converters and Motors
97
3.6.2 DC Braking
A DC voltage across any two of the three motor phases is used to generate a stationary
magnetic field in the stator. This field cannot generate high braking torque at the rated
frequency. The braking power remains in the motor and may cause overheating.
Three parameters are required for DC braking:
• The frequency at which the brake should be activated. A frequency value below 10
Hz is recommended. Use the motor slip frequency as a guide. A frequency of 0 Hz
means that the function is disabled
• The braking current used for holding the motor shaft. The recommendation is not to
exceed the rated current of the motor in order to prevent possible thermal overload
• The duration of DC braking. This setting depends on the application
3.6.3 DC Hold
Unlike the DC brake, the DC hold has no time limit. Otherwise the above
recommendations for the DC brake apply. This function can also be used when
“auxiliary heating” is implemented for a motor placed in a very cold environment. As
constant current flows through the motor, do not exceed the rated motor current. This
minimises the thermal load on the motor.
3.6.4 Electromechanical Brake
The electromechanical brake is an aid for bringing the motor shaft to a standstill.
This can be controlled from the FC by means of a relay and there are various possible
control options.
It is important to establish when the brake can be released, as well as hold the motor
shaft.
Some of the points to consider are:
• Motor pre-magnetisation, meaning a minimum amount of current is needed
• The frequency at which activation or deactivation occur
• Reaction times (delay times) of the relay coils
For critical applications such as hoists or lifts, once the start command has been given
the brake may only be released after ensuring optimum pre-magnetisation of the
motor; otherwise the load could fall. A minimum current, usually the magnetising
current, should flow first to ensure that the motor cannot drop the load.
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Frequency Converters and Motors
3.7 Motor Heating and Thermal Monitoring
Energy lost in motors during operation will warm up the motor. If the motor is heavily
loaded, some cooling is needed. Depending on the system, motors can be cooled in
different ways:
• Self-ventilation
• Forced air cooling
• Liquid cooling
To maintain the motor lifetime, keep the motor within the specified temperature range.
The most common cooling method is self-ventilation, where the motor is cooled by a
fan mounted on the shaft.
The temperature conditions of the motor are subject to two influences:
• When the speed decreases, the cooling air volume also decreases.
• When a non-sinusoidal motor current is present, more heat is generated in the motor.
At low speeds the motor fan is not able to supply enough air for cooling. This problem
arises when the load torque is constant throughout the control range. This lower
ventilation determines the permissible level of torque during continuous loading.
When the motor runs continuously at 100% rated torque, at a speed which is less than
half the rated speed, the motor requires extra air for cooling. This extra air cooling is
indicated by the shaded areas in Fig. 3.13 T/N characteristics with and w/o external
cooling.
Frequency Converters and Motors
99
T
Fig. 3.13
T/N characteristics with and w/o external cooling
Alternatively, instead of providing extra cooling, it is possible to reduce the motor
load ratio. To reduce the motor load ratio, select a larger motor. However, the FC
specification imposes limits on the size of motor that can be connected.
When the motor current is non-sinusoidal, the motor receives harmonic currents that
increase the motor temperature, as shown in Fig. 3.14 Maximum continues torque
and current shape relation. The magnitude of the harmonic currents determines the
amount of heat increase. Therefore, do not operate a motor continuously at 100% load
when the current is non-sinusoidal.
Fig. 3.14
Maximum continues torque and current shape relation
When the application predominantly requires low speeds, an additional fan to cool
the motor is recommended to ensure full torque. However, the fan should be powered
from a separate supply and should not be connected to the output of the FC.
As an alternative to air, liquid can be used for cooling the motor. Liquid cooling is
typically implemented in special motor designs.
Frequency Converters and Motors
100
Two temperature monitoring methods are implemented in the FC, in order to protect
the motor:
Calculation:
The motor temperature is calculated based on a mathematical motor model
Measurement:
Thermistors or PTCs placed inside the motor can be connected to the device, to
monitor temperature
If motor overheating occurs, the remedial action required is programmed to fit the
application needs.
3.8 Functional Safety
Functional safety defines protection against hazards caused by incorrect functioning of
components or systems. In Europe, functional safety falls under the Machinery Directive 2006/42/EC.
The Machinery Directive describes the purpose of functional safety as follows:
“Machinery must be designed and constructed so that it is fitted for its function, and can be
operated, adjusted and maintained without putting persons at risk when these operations
are carried out under the conditions foreseen but also taking into account any reasonably
foreseeable misuse thereof.”
Depending on which application standard has to be fulfilled, the system must reach a
defined safety level. The required safety level is defined through the risk assessment.
The Machinery Directive refers to different standards, according to the safety level
required.
Safety Level
Category
Performance Level
Safety Integrity Level
Abbreviation
Cat
PL
SIL
Standard
EN 954-1
EN ISO 13849-1
IEC 61508 / IEC 62061
The European functional safety regulations are comparable to many others around
the globe. For example, in North America the OSHA (Occupational Safety and Health
Act) applies, and in Canada the CCOHS (Canadian Centre for Occupational Health and
Safety) provide the framework for applying safety measures. Although the relevant
standards differ between the various regions, the safety principles are closely related.
Frequency Converters and Motors
101
In general it is common to use abbreviations in the different legislative frameworks and
the standards to describe the safety function and the safety level.
Function
Description
Safe Torque Off
STO
The motor does not get
energy to produce torque/
rotation.
This function complies to
stop category 0 according to
IEC 60204-1.
Illustration
Frequency
Activation
of STO
Actual frequency
Time
Safe Stop 1
SS1
Activation
of STO
Frequency
A controlled stop, in which
the drive elements of the
machine are kept energised
in order to stop it. The power
is only disconnected when
standstill has been reached.
This function complies to
stop category 1 according to
IEC 60204-1.
Actual frequency
SS1 time supervision
Time
SS1 time
Safe Limited Speed
SLS
Frequency
SLS activated
A safe state of speed is called
Safe Limited Speed.
This ensures that a machine
runs at a constant safe speed.
If it runs faster a Stop function
will be activated
Actual frequency
SLS max. speed limit
Time
Safe Speed Monitor
SSM
SSM monitors for zero speed
and sets an output signal
high if zero speed is reached.
This function can be used
to unlock doors or simply to
display that the machine is in
standstill.
Frequency
SMS always active
Actual frequency
SMS max. speed limit
Time
Standstill output
active
Frequency
Safe Maximum Speed
SMS
Ensures that the machine
does not run at a higher level
than a defined maximum
speed.
It prevents machine damage
and reduces hazards.
Function-wise it is the same
principle as SLS
Actual frequency
Speed motor limit
Time
Table 3.15 General FC safety functions and their functionality
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Frequency Converters and Motors
The FC has several additional functional safety functions:
• SOS Safe Operating Stop
• SS2 Safe Stop 2
• SDI Safe Direction
• SBC Safe Brake Control
• SAM Safe Acceleration Monitor
• SLP Safe Limited Position
• SCA Safe Cam
• SLI
Safe Level increment
• SSR Safe Speed Range
• SBT Safe Break Test
SISTEMA
Independent Software tools such as SISTEMA (Safety Integrity Software Tool for the
Evaluation of Machine Applications) help the machine builder to make all the calculations of the safety application.
Fig. 3.16 Screenshot of the SISTEMA start page
Frequency Converters and Motors
103
The SISTEMA software utility provides support to developers and testers of safety-related machine controls, in evaluation of safety in the context of ISO 13849-1. The tool
enables modelling of the structure of the safety-related control components based
upon the designated architectures. This modelling enables automated calculation of
the reliability values with various levels of detail, including that of the attained Performance Level (PL).
Relevant parameters are entered step-by-step in input dialogs, for example:
• risk parameters for determining the required performance level (PLr)
• the category of the SRP/CS
• measures against common-cause failures (CCF) on multi-channel systems
• average component quality (MTTFd)
• average test quality (DCavg) of components and blocks
The impact of each parameter change upon the entire system is reflected immediately
in the user interface. The final results are printable in a summary document.
Saving Energy with Frequency Converters
104
4 Saving Energy with Frequency Converters
4.1 Potential
Electric motors account about 48% of global electrical energy consumption (1). In
Industrial applications the ratio is even higher. Depending on the region and the
industrial area, 65-75% of electrical energy is used for electric motors. Therefore
electrical drive technology holds a great deal of potential for reducing the worldwide
energy consumption.
Frequency Converters enable the development and improvement of more energyefficient motor technologies. Even more beneficial is applying the main reason why
FC’s were developed: adjustable speed control. Speed control helps to optimise
processes and operate motors at optimal speed and torque.
When total potential savings that could be made in a system are defined as 100%,
roughly 10% of that potential could be obtained through the use of more efficient
components, such as motors. Operation with adjustable speed control offers potential
energy savings of approx. 30%. However, the greatest savings (approx. 60%) are to be
made by optimising the entire system.
Increased efficiency
10%
System
optimization
60%
30%
Adjustable
speed control
Fig. 4.1 Potential energy savings
If a few key points are taken into consideration, FC’s can lead to high energy savings
being quickly and easily realised as the majority of applications (approx. 60-70%) are
suitable for speed control. In particular, fans and pumps - which cover almost 50% of
the applications - are obvious targets because of their huge saving potential.
(1) Source: 2008 – International Energy Agency
Saving Energy with Frequency Converters
105
4.2 Motor + Frequency Converter Efficiency
The efficiency of a system consisting of a motor operated by a FC can be calculated by
multiplying the single efficiencies.
ηSystem = ηMotor * ηFrequency converter
Typically FC efficiency curves at two different loads are shown in Fig. 4.2 Efficiency
example of frequency converters (A = 100% load / B = 25% load). The efficiency of the
FC is high throughout the control range, both at high and at low load levels.
Fig. 4.2 Efficiency example of frequency converters
Beside the economical aspect that high efficiencies of FC’s result in lower energy
consumption, the dissipated power that has to be removed from the installation is
reduced. This is important if the FC is integrated into a cabinet. If the losses are too high
separate cooling devices are required which consume energy as well.
Normal and part load efficiencies of motors are compared to the FC as illustrated in Fig.
4. 3 Efficiency example of a 2-pole motor ( A = 100% load / B = 25% load).
Fig. 4.3 Efficiency example of a 2-pole motor
106
Saving Energy with Frequency Converters
Consequently the motor has a major influence on the system efficiency (Fig. 4.4
Efficiency example of a frequency converter and motor combination ( A = 100% load /
B = 25% load).
Fig. 4.4 Efficiency example of a frequency converter and motor combination
Although it’s common practice to qualify the efficiency of the different components,
the accuracy of the value depends strongly on the amount of decimal places used . So
often the losses of the different components are provided as well. For example 143W
losses is much simpler to handle than 90.467% efficiency (related to 1.5 kW).
4.3 Classification of Energy Efficient Components
Matching individual components to a particular drive system has several advantages
over pre-configured systems because this allows the engineer to optimise the
system to his requirements Pre-configured systems are always optimised for general
applications and can never fit all. If available, an indicator for the efficiency of
components is efficiency classes.
Frequency Converter
The standard EN 50598-2 defines efficiency classes for FC’s. As power electronics can
have several configurations the classes IE0-IE2 are defined for Complete Drive Modules
(CDM) consisting of rectifier, intermediate circuit and inverter (see Fig. 4.5 Definition
of CDM and PDS). CDM with ability to feed back e.g. braking energy to the mains are
addressed but not covered because they have typically twice the losses.
Saving Energy with Frequency Converters
107
Power Drive System (PDS)
Complete Drive Module (CDM)
Mains &
cabling
Feeding
section
Auxiliaries
Driven Equipment
Basic
Drive
Module
(BDM)
Motor
Transmission
Loadmachine
Often named as Frequency Converter
Fig. 4.5 Definition of CDM and PDS
The IE classes are defined in relation to a reference CDM (RCDM). By having the same
scale for all power sizes the classes are defined by relative losses. CDM with relative
losses in the range of ± 25% of the RCDM is classified as IE1. CDM with higher losses are
grouped in IE0 while CDM with lower losses are in class IE2 (see Fig. 4.7 Definition of
CDM and PDS efficiency classes).
The rating does not reflect the CDM efficiency at lower speed / torque as it’s
determined at 100% relative speed and 90% relative torque-producing current.
For verification the CDM is tested with all included components at a defined test load.
Fine tuning or a special test mode is not allowed.
Transmission
Even though the kind of transmission can have a huge impact on the system efficiency,
no efficiency classes are defined. The following table gives an indication of typical
efficiencies:
Direct driven
Spur gear
Bevel gear
Worm gear
100%
98%
98%
95%
Flat belt
V-belt
Tooth belt
Chain
96...98%
92...94%
96...98%
96...98%
Table 4.1 Typical transmission
efficiencies.
Fig. 4.6 One Gear Drive is a direct driven motor which comes without gearbox but provides high torque at high
efficiency
Saving Energy with Frequency Converters
108
Motors
For the power range 0,12-1000 kW, efficiency classes IE1-IE4 for electric motors are
defined in the standard IEC/EN 60034-30-1. Although the standard is valid for all motor
types some motor constructions (e.g. brake motors) are excluded from the standard.
Several countries and regions use the IE class limits to define Minimum Efficiency
Performance Standards (MEPS) to restrict the use of low efficiency motors. The
efficiency class is related to the nominal operating point of the motor. Efficiencies at full
speed but reduced torque must be stated on the nameplate or in the documentation.
Limits are different for supply frequencies (50/60Hz) and the number of motor poles (2,
4 or 6 poles).
Classes for motors operated with FC’s are under discussion and will be defined in IEC/
EN 60034-30-2.
Frequency Converter + Motor Combination
Efficiency classes for Frequency Converter and Motor combination are defined in the
standard EN 50598-2 via an IES rating. Similar to the CDM, the classes of the so called
Power Drive System (PDS), which is the motor + FC combination (see Fig 4.5 Definition
of CDM and PDS), are related to a reference system (see Fig. 4.7 Definition of CDM and
PDS efficiency classes). PDS with 20% higher losses than the reference are in class IE0
while systems with 20% lower losses are in class IES2.
Losses related
to reference
CDM
IE0
Losses related
to reference
PDS
ISE0
125%
120%
Reference
100%
IE1
IES1
100%
80%
75%
IE2
0%
IES2
0%
Fig. 4.7 Definition of CDM and PDS efficiency classes
The classification is made for 100% relative speed and 100% relative torque. If the FC
is designed for a shorter cable or it’s directly mounted on the motor where shorter
Saving Energy with Frequency Converters
109
cable can be used this must be stated in the documentation. In general all kind of
optimisations are possible as long they are noted in the documentation. Consequently
comparing two PDS ratings is difficult because they will most likely have different
bases.
The IES class for FC and drive combinations illustrates the difficulty in optimising a
system and that all components must be carefully selected, in order to optimise the
application. The difference between pre-configured and non-optimised free combined
systems will most often be minor, but matching different components generally allows
finer adjustment to the machine, giving the machine builder a competitive advantage.
4.4 Energy Efficient Motor Start
The energy for starting a motor can be split into 3 major parts:
• Energy required for operating the load
• Energy required for accelerating the load and the motor
• Losses in motors and control
The simplest way for starting a motor is to connect the motor direct on line (DOL) but
this is also an inefficient solution. The motor will have high losses when starting due
to the huge slip when applying the voltage. While accelerating the motor, the slip and
hence the losses are reduced.
800
% Full load current
700
600
4
500
400
3
300
200
2
100
0
1
0
12.5
25
37.5
Full load
50 Hz & speed
Fig. 4.8 Typical motors currents curves started by (1): Frequency converter at VT load (2): star-delta starter (3):
Softstarter (4): direct on line (DOL)
Saving Energy with Frequency Converters
110
Softstarters can be used which adjust the motor voltage like Star/Delta starters, but
linearly. The device increases the voltage until a programmed current limit is hit. The
limit is application-dependent, typically in the range of 300-500% FLC. While the motor
is accelerating the current drops and the device increases the voltage further. This
sequence continues until the mains voltage level is applied to the motor.
300%
800%
Start at mains
Start at mains
700%
250%
600%
Current [%]
Torque [%]
200%
150%
Softstarter
100%
500%
400%
300%
Softstarter
200%
50%
0%
0%
100%
20%
40%
60%
Speed [%]
80%
0%
0%
100%
20%
40%
60%
Speed [%]
80%
100%
Fig. 4.9 Comparison motor start direct on mains with motor started by a softstarter (400% current limit)
For minimising the losses Softstarters are typically operated in by-pass after the motor
has been started. During the starting phase the losses are approx. 4.5 W per A.
The most efficient way for starting a motor is the use of FC’s. As voltage and frequency
are controlled the slip and hence the losses are reduced. Using a by-pass like on
Softstarters is possible but seldom used.
300%
800%
Start at mains
Start at mains
700%
250%
600%
Current [%]
Torque [%]
200%
150%
Frequency Converter
100%
500%
400%
300%
Frequency Converter
200%
50%
0%
0%
100%
20%
40%
60%
Speed [%]
80%
100%
0%
0%
20%
40%
60%
Speed [%]
Fig. 4.10 Comparison motor start direct on mains with a motor started by a frequency converter
at 160% overload
80%
100%
Saving Energy with Frequency Converters
111
Principle torque and current curves for starting a motor with constant load direct on
mains, by a softstarter and by a frequency converter are shown in Fig. 4.9 Comparison
motor start direct on mains with motor started by a softstarter and Fig. 4.10
Comparison motor start direct on mains with a motor started by a frequency converter
at 160% overload. The curves will look different with different loads.
4.5 Energy Efficient Motor Control
All motors operate by applying the correct voltage at a given frequency. A rotating
shaft does not mean, however, that the motor is operating efficiently. For controlling
a motor, a control algorithm (U/f, voltage vector, flux vector,…) and a control strategy
are required. That both components must suit a motor type can easily be seen with
motors using permanent magnets. For energy-optimum operation the controller must
match the supply voltage waveform as closely as possible to the waveform of the back
EMF. Block commutation is used for trapezoidal back EMF and sine commutation for
sinusoidal.
0°
60°
120°
180°
240°
300°
360°
Block commutation
Sine commutation
Fig. 4.11 Block vs. Sine-wave commutation
Block commutation is known to have some disadvantages like torque ripple and
excessive noise. However both technologies are comparable when it comes to
efficiency.
Control strategies which are often used in different control algorithms are:
Constant Torque angle
Maximum torque is created when the torque angle is kept constant at 90°. The
constant Torque angle strategy keeps the angle constant by controlling the rotor d-axis
current to zero while leaving the current vector on the y-axis.
Saving Energy with Frequency Converters
112
Maximum Torque Per Ampere
This strategy minimises stator current magnitude for a required torque while
considering reluctance torques. Variations in inductances during operation must be
considered to obtain best results.
Constant Unity Power Factor control
The angle between current and voltage vector is kept constant under this strategy so
the apparent power rating of the inverter can be reduced.
In addition FC’s provide extra functionalities for reducing the magnetic field strength
at reduced load. This can be done by special U/f characteristics or by Automatic Energy
Optimization (AEO) functions.
Automatic Energy Optimisation
Start with high acceleration
AEO-adjustment
Speed
Automatic load
adjustment
Motor voltage
Motor current
Fig. 4.12 Automatic Energy Optimisation
Automatic adjustments take place after the application reaches a steady state.
The applied control strategy reduces the magnetisation level and thus the energy
consumption. An optimised balance between energy saving and having enough
magnetisation for sudden load peaks must be given to ensure reliable operation. See
fig 4.12 Automatic Energy Optimisation.
The average energy saving potential for small to medium-sized drives is 3 to 5 % of
the rated motor power during operation at low loads. As a very important side-effect,
the motor runs almost noiselessly at low loads – even at low to medium switching
frequencies.
Saving Energy with Frequency Converters
113
4.6 Load over Time
Every component in a system has some losses, so adding components to a system
should be avoided if possible. This applies also to FC’s. Adding a unit to a motor which
has to run all day at full load and full speed will only result in additional losses. But as
soon as reducing speed and torque make sense to the application, the use of a FC will
reduce the energy consumption. The achievable savings depend on the load profile
over time, the torque characteristics and the efficiency of the motor and drive system
at the given part load points.
40
35
Time fraction
30
25
20
15
10
5
0
20%
40%
60%
Load
80%
100%
Fig. 4.13 Load over Time fraction diagram indicates how long a load is operated at part load
Part load is used in two different contexts. When a motor is operated from the mains,
the feeding motor frequency is fixed and the speed only varies with load. When the
motor is operated by a FC, part load describes torque at a certain speed where the
torque characteristic is given by the application. Actually the majority of applications
are operated at part load. This is also true for mains driven motors as they are typically
oversized.
4.6.1 Applications with Variable Torque
Variable torque applications often involve pumps and fans. However, a distinction
has to be made in the case of pumps. Although the most popular types of centrifugal
pump have a quadratic torque characteristic, eccentric, vacuum or positive
displacement pumps have a constant torque characteristic.
The energy saving potential of pumps and fans is very high as these machines follow
the affinity laws.
Saving Energy with Frequency Converters
114
Q1
¾
Q2
H1
¾
H2
P1
¾
P2
n1
n2
( )
( )
( )
n1
n2
n1
n2
Flow is proportional to speed
2
Pressure or head is proportional to square of speed
3
Power is proportional to cube of speed
The flow Q increases linearly with increasing speed (RPM), while the pressure/head H
increases quadratically and the power consumption P increases cubically. In theory a
reduction in speed of 20% results in an energy reduction of 50%.
Pressure [bar]
Operating
point
System
characteristic
Pump
characteristic
Energy required
Flow [Q]
Fig. 4.14 Energy required in a variable torque pump application for throttle control
In many fan and pumps systems, swirl flaps, dampers or throttles are used for
controlling the flow of the system. If a centrifugal pump is controlled using a throttle
valve, throttling moves the machine’s working point along the pump characteristic.
The reduction in energy requirement achieved is minimal compared with the pump’s
nominal operating point.
Pump characteristic
System
characteristic
Pressure [bar]
Operating
point
Energy required
Flow [Q]
Fig. 4.15 Energy required in a variable torque pump application for speed control
Saving Energy with Frequency Converters
115
If a fan/pump is speed-controlled, the operating point moves along the system characteristic. This moves the unit out of its best efficiency point and efficiency will typically
decrease slightly but the energy saving due to the reduced speed is still much higher
compared to throttle or other mechanical controls. In real applications, the achieved
energy savings will differ from the theoretical because losses in piping and duct -work
result in a basic load and thus additional losses.
In pump applications often a minimum speed (application and pump type/makerelated) is required for avoiding sedimentation of solids and ensuring sufficient
lubrication of the pump. If the range between minimum speed and speed for the
maximum required power is too big the system can be cascaded. When pumps
are cascaded, one speed-controlled pump covers the base load. If consumption
increases, the frequency converter will switch in more pumps sequentially. The pumps
accordingly operate at maximum efficiency whenever possible. Pump control ensures
that the system is always as energy-efficient as possible. In some applications more
than one pump is speed controlled. Cascades can be used in a similar way for other
applications like fans or compressors.
4.6.2 Applications with Constant Torque
Applications with constant torque are applications for which the load is typically not
significantly altered by the speed. This includes conveyor belts, hoists and mixers.
If, for example, an engine block is positioned on a horizontal conveyor belt, the weight
of that engine block will not change, regardless of the conveyor belt speed. The torque
required to move this engine block is always the same. Of course, the friction and
acceleration torque would change according to the operating conditions, but the
torque needed to move the load still remains constant.
50% speed, 100% torque
Operating point
Energy required
Speed [min-1]
Fig. 4.16 Energy required at different speed and loads
Torque [Nm]
Torque [Nm]
100% speed, 100% torque
Operating point
Energy required
Speed [min-1]
Saving Energy with Frequency Converters
116
The energy required by such a system is proportional to the required torque and the
speed of the motor.
P~Txn
If the speed can be reduced with a constant load as is the case in refrigeration cycles,
one of the direct results will be energy savings. In other constant load applications,
reduced speed will not have a huge impact. If, for example, the speed of a conveyor
belt is reduced, the energy required to transport the goods from A to B stays
approximately the same as the distance stays the same. Small savings are achieved
through such factors as reduced frictional loss or optimised acceleration. Nevertheless
the use of speed control in constant torque applications is continuously increasing
because of the benefits to the process itself.
4.7 Life Cycle Costs
Costs
Potential ways to save energy can be found in almost all sectors, like building services,
conveyor belt systems or chemical processes.
Disposal costs
Energy costs
Operation and maintenance
Investment
Time
Fig. 4.17 Initial costs usually account for only approximately 10% of overall life cycle costs. The higher initial
costs of an energy-saving device often pay for themselves in next to no time
Saving Energy with Frequency Converters
117
4.8 System Savings
Regardless of whether the energy efficiency of a new or existing process/machine shall
be improved, the whole system must always be considered. Existing installations have
the advantage that measurements can be made to determine the losses, creating a
benchmark as to whether improvements to the system are working as expected
te
r
ad
ap / p
p l ro c
ica es
ti o s /
n
Lo
tr a M e
ns c h
m an
iss i c
i o al
n
ot
or
M
V
M olta
ot g
or e
c a d ro
bl p
e
Co
nv
er
(S Ou
in t p
us u
or t fi
dU lte
/d r
t)
Au
to
m
at
io
n
E
fil MC
te
r
s
V
M olta
ai g
ns e
c a d ro
bl p
es
Fu
se
rm
ns
fo
ai
M
Tr
a
ns
er
(a
ct M
i v ai
e / ns
pa fi
ss lte
iv r
e)
Fig. 4.18 Overview of motor drive system with various accessories – illustrates a drive
system operating a conveyor showing the majority of components which can be found
in a drive system.
Fig. 4.18 Overview of motor drive system with various accessories
The setup and the whole dimensioning of the system depend on the application
(transmission, motor, output filter and motor cable) and its environment (EMC filter,
output filters, cables, mains, climate, etc.). Therefore the engineering and the energy
saving assessment should always start with the application assessment. It makes no
sense to select one or two highly- efficient components if they have a negative impact
on the system efficiency. This is illustrated in the following example.
Before deciding to make an investment, it is necessary to examine not only the
technical, but also the commercial and logistical aspects, so that measures which are
not cost-effective, or which are counter-productive, can be avoided or minimised. TCO
(Total Cost of Ownership = total costs within a certain timeframe) and LCC (Life Cycle
Costs = costs incurred within a lifecycle) are methods used for such an evaluation.
A life cycle cost analysis includes not only the procurement and installation costs, but
also the costs of energy, operation, maintenance, downtime, the environment and
disposal. Two factors – energy cost and maintenance cost, have a decisive effect on the
life cycle cost.
Saving Energy with Frequency Converters
118
LCC = Cic + Cin + Ce + Co + Cm + Cs + Cenv + Cd
Cic = initial capital cost (procurement cost)
Cin = installation and commissioning costs
Ce = energy cost Cs = downtime and lost production costs
Co = operating cost Cenv = environmental cost
Cm = maintenance cost Cd = decommissioning and disposal costs
One of the biggest factors in the life cycle cost formula is the energy cost. Higher
Investments which bring the energy consumption down will, in many applications,
have only a minor impact
System efficiency [%]
• Fan1
Fan1 (IE4
(IE4)motor)
• Fan1
Fan1 (IE2
(IE2)motor)
• Fan1
Fan1 (IE3
(IE3)motor)
• Fan2
Fan2 (IE4
(IE4)motor)
Part load ratio
Fig. 4.19 Measurement of different ventilator fan systems with 3 kW acc. DIN EN ISO 5801 in same ventilation unit
Fan 1 is a direct-driven type and the system efficiency increases when more efficient
motors (better IE class) are used. Fan 2 is EC fan with a high-efficiency motor. The lower
system efficiency results from the fan design. As the motor is placed as hub in the EC
fan the air flow is disturbed and the system efficiency decreases.
The majority of applications are suitable for speed control but it must be validated
case by case. For example, not all compressors are designed for speed control and their
minimum and maximum speed limits must be respected and too short or too fast ramp
times can be critical.
Saving Energy with Frequency Converters
119
4. 9 Using Regenerated Power
Electric motors can be operated in generative mode when e.g. an asynchronous motor
runs faster than its synchronous speed. This can happen when decelerating the motor
from one speed to another. In most cases, the user will then channel the generated
energy into braking resistors which will convert it into heat. In practice, there are two
common technical solutions to feed this energy back into the grid or to supply it to
other machines:
Intermediate circuit coupling
Some FC’s are able to interlink their DC intermediate circuit with the intermediate
circuits of other devices. This enables other devices to be directly supplied with
regenerated energy. However, there are some constraints that should be noted.
For example, users should always ensure that, if one device short circuits, it will not
damage the others. Users must, of course, also take note of what happens if all coupled
devices emit regenerated energy at the same time.
Power feedback (regenerating)
Active Front End or Active Infeed Converters can feed regenerated power back into the
grid. If the use of regenerative devices is economic depends on 3 factors:
Available energy
Most applications generate energy during deceleration processes. This energy
decreases continuously during the speed change. Theoretically, the regenerated
energy is equal to 50% of the difference between the energy which is in the system
when starting and stopping the deceleration, but in reality this figure lies somewhere
between 10 and 20%. Exceptions to this are seen in lifts, cranes and hoists. Furthermore
the nominal motor performance is not equal to the regenerated energy as oversizing of
motors is common practice. Only very rarely does the nominal motor power hit exactly
the required application power.
Losses
Motor, cables, gears and even the AIC itself create losses which reduce the energy
which can be fed back into the grid.
Saving Energy with Frequency Converters
120
13.16 kW
10.23 kW
11 kW
550 W
451 W
1155 W
770 W
Fig. 4.20 System losses during motor operation
7.73 kW
10.23 kW
9.51 kW
476 W
312 W
999 W
716 W
Fig. 4.21 System losses at regenerative operation
The losses caused by the AFE or AIC itself are much higher than for a standard FC
due to the active rectifier whose losses can be twice as high in operation but also in
standby. Depending on the construction, regenerative FC’s without the necessary
filters create more harmonic currents, which can also lead to higher losses in the grid.
Occurrence
The more often the motor is operated in regenerative mode the more energy is fed
back to the grid. Therefore situations during a load cycle where energy is generated
must be considered. As well as the load cycle itself, the number of load cycles defines
the resulting amount of energy for a given time.
The majority of applications will never justify an investment in AIC which are typically
more costly than standard inverters. The example of an elevator illustrates that AIC
can also have a negative impact even though elevators are usually seen as the optimal
application for AIC.
Saving Energy with Frequency Converters
121
Application:
Elevator in residential building
Load:
1100 kg
Operation:
1h per day
ηgear=90% ηHoist way=80% ηMotor=88% (IE2) ηAIC=95%
Standby losses: AIC = 40 W, VSD = 40W
ηVSD=97%
Result:
AFE/AIC
Standard VSD
Losses motor per year
47 kWh
34 kWh
Losses standby per year
336 kWh
168 kWh
Generated Energy per year
170 kWh
–
Balance
213 kWh
202 kWh
Table 4.2 Energy consumption for elevator example
The values used in the example are very conservative. It may be surprising but one
hour operation is very high for an elevator in a typical residential building. Nevertheless
the energy balance is negative. This illustrates that applications where potentially AIC
can be used require special consideration.
Electromagnetic Compatibility
122
5 Electromagnetic Compatibility
5.1 EMI and EMC
Electromagnetic interference (EMI) is the degradation of the performance of
equipment caused by electromagnetic disturbance.
An example of EMI is when random dots and lines (commonly called “snow”) appear on
the screen of a television when a vacuum cleaner is operated in the same room. In this
example the vacuum cleaner is the source of interference and the TV set is the victim
equipment.
Radiated interference
Conducted interference
Fig. 5.1 Difference between radiated and conducted interference
Electromagnetic noise can be propagated through conductors (conducted
interference) or through electromagnetic waves (radiated interference). There are four
interference coupling mechanisms:
• Galvanic coupling occurs when two circuits (noise source and victim) share a
common electrically conductive connection
• Capacitive coupling (also known as electric coupling) occurs when two electric
circuits have a common reference and the noise couples between two conductors
through parasitic capacitances
• Inductive coupling (also known as magnetic coupling) occurs when the magnetic
field around a current carrying conductor is induced in another conductor
• Electromagnetic coupling occurs when the noise source radiates electromagnetic
energy through a conductor that acts as a transmitting antenna. The victim circuit
receives the disturbance through a conductor that acts like a receiving antenna
There can be various sources of electromagnetic interference, such as:
• Natural sources such as lightning
Electrical equipment which is not intended to produce electromagnetic radiation: for
example a frequency converter or power supply
Electromagnetic Compatibility
123
• Electrical equipment intended to produce electromagnetic radiation: for example a
portable radio transmitter
The art of EMI troubleshooting consists of identifying the noise source, coupling
mechanism and reducing the interference coupling to an acceptable level.
When a piece of equipment or system is able to function satisfactorily in its
electromagnetic environment without introducing intolerable disturbances in that
environment, it is called electromagnetic compatibility (EMC). It is important to note
that the definition of EMC contains two aspects:
• Immunity: the ability of equipment to function in the presence of some level of
electromagnetic interference
• Emission: the unintended emissions from equipment need to be limited to a
tolerable level
The difference between the emission margin and the immunity margin is called
compatibility gap.
Level
Immunity margin
Compatibility
gap
Emission margin
Fig. 5.2 Explanation of compatibility gap
RFI or EMI?
The term radio frequency interference (RFI) is often used interchangeably with EMI. RFI
is an older term and refers to the interference of the reception of radio signals (radio,
TV, wireless communication). EMI is a newer term which refers broadly to interference
of any electrical equipment, including Frequency Converters.
Electromagnetic Compatibility
124
Common-mode and differential mode
When referring to conducted interference the terms common-mode (CM) and
differential-mode (DM) are often used.
CM
DM
Source
Load
CM
DM
Reference ground
Fig. 5.3 Common-mode and differential-mode
The differential mode (DM) noise is conducted on both lines of the current loop in
opposite directions, in series with the desired signal. The common-mode (CM) noise is
conducted on both lines in the same direction and its return path is through a common
reference ground.
5.2 EMC and Frequency Converters
Emission
FC’s involve fast switching of voltages (high du/dt rates) in the thousands of V/μs range
with amplitudes in the 500 V – 1000 V range FC’s (depending on supply voltage) and
high current levels. This makes FC a potential source of EMI and their EMC-correct
installation needs to be carefully followed.
Rectifier
L
(+)
Inverter
High
du/dt
C
Mains line RFI filter
Vdc/2
W
Icm
(o)
V
U
C
Ccm1
Ccm2
L
Shielded
motor cable
Vdc/2
( -)
Heatsink/
chassis
Fig. 5.4 Propagation of interference in a Frequency Converter
M
Load
Electromagnetic Compatibility
125
The noise source is the voltage source inverter that produces a pulse-shaped output
voltage with very short rise- and fall times (also expressed as high du/dt). This voltage is
applied across parasitic capacitances to ground in the motor cable and motor results in
a common-mode current:
Icm = Ccm × du
dt
where Ccm is the parasitic capacitance to ground.
The common-mode current needs to close the loop and return to its source, the DClink. Controlling the return path of the common-mode current is a key element of
keeping electromagnetic interference under control. Inside the FC there are commonmode capacitors – that means capacitors between the FC circuit and ground/earth.
The common-mode capacitors can be found in the RFI circuit (Ccm1) or as decoupling
capacitors in the DC-link (Ccm2). If a shielded motor cable is used and the motor end
of the cable is connected to the motor chassis and the FC end is connected to the
FC chassis then, ideally, the common mode current will return to the DC link via the
common-mode capacitors. The common-mode current returning through the mains
supply is unwanted because it can cause interference in other equipment connected
to the mains. Therefore this current must be minimised, for example by using RFI
filters. When unshielded motor cables are used, then only a part of the common-mode
current returns through the FC’s chassis and common-mode capacitors thus causing
more interference on the mains grid.
Immunity
Immunity, as well as noise emission need be considered in a FC application. The control
signals connected to a FC can be quite susceptible to noise. In general, analogue
signals are more susceptible than digital signals. Therefore it is better to use digital bus
communication instead of analogue reference signals. If analogue signals cannot be
avoided, a 4 – 20 mA current reference signal is preferred to a 0 – 10 V voltage reference
signal because it is less susceptible to noise.
5.3 Grounding and Shielding
Grounding
Grounding means to connect electrical equipment to a common reference ground or
earth. There are two reasons for doing this:
Electromagnetic Compatibility
126
• Electrical safety: Safety grounding ensures that in the case of the degradation of
electrical isolation no live voltage is present on conductive parts that can be touched
by humans – thus avoiding the risk of electric shock.
• Reduce interference: Signal grounding reduces voltage differences that might cause
noise emission or susceptibility problems.
It is very important to note that electrical safety always has the highest priority – higher
than EMC.
Various types of grounding are common.
Series
Equipment
1
Equipment
2
Equipment
3
Parallel
Equipment
1
Equipment
2
Equipment
3
Multi-point grounding
Equipment
1
Equipment
2
Equipment
3
Fig. 5.5 Single point grounding in series or parallel and multi-point ground is possible
The different types of grounding have advantages and disadvantages, but what
matters at the end of the day is that the impedance of the grounding connection is as
low as possible in order to provide potential equalisation of the connected equipment.
Shielding
Shielding is used both for immunity (protecting against external interference)
and emission (preventing interference to be radiated). In FC applications, shielded
cables are used both for power (motor cable and brake resistor cable) and for signals
(analogue reference signals, bus communication).
Electromagnetic Compatibility
127
The shielding performance of a cable is indicated by its transfer impedance ZT. The
transfer impedance relates a current on the surface of the shield to the voltage drop
generated by this current on the opposite surface of the shield:
U2
I1
Fig. 5.6 Illustration of transfer impedance
ZT =
U2
, where L is the cable length
I1 • L
The lower the transfer impedance value the better the shielding performance. The
table below shows typical values of transfer impedance for different kinds of motor
cable. The most common type of motor cable is the single layer braided copper wire as
it offers a good shielding performance at a reasonable price.
Aluminium foil with copper drain wir
Twisted copper wires or steel wire armoured cable
Single layer braided copper wire –
with various percantages of screen coverage
Dual layer braided copper wire
Dual layer braided copper wire
with high permeable middle layer
Cable runs in rigid copper or steel conduit
Fig. 5.7 Shielding performance of different cable types
128
Electromagnetic Compatibility
Transfer impedance can be drastically increased by incorrect shield termination. The
shield of a cable needs to be connected to the chassis of the equipment through a 360
degree connection. Using “pigtails” to connect the shield increases transfer impedance
and ruins the shielding effect of the cable.
Fig. 5.8 Installation of cable shield
The question about terminating both ends or only one end of a shielded cable often
occurs. It is important to realise that the effect of a shielded cable is reduced when
only one end is terminated. It is very important to terminate correctly both ends of the
motor cable, otherwise interference problems may occur.
The reason why in some situations only one end is terminated is to do with ground
loops in signal cables. This means that there is a voltage potential difference between
the chassis of the two pieces of equipment that are connected (for example frequency
converter and PLC) and if the shield connects the two chassis a ground current will
occur (with the frequency of 50 Hz/60 Hz). This current then couples into the useful
signal disturbing it – in audio applications this is commonly known as “hum”. The best
solution is to use an equalising connection in parallel with the shielded cable. If this
is not possible then one end of the shielded cable can be terminated via a 100 nF
capacitor. This breaks the ground loop at low frequency (50 Hz) while maintaining
the shield connection in the high frequency range. In some equipment this capacitor
is already built in. For example in the case of Danfoss VLT® frequency converters the
shield connection for signal cables is provided at terminal 61.
Electromagnetic Compatibility
129
Control cables and serial communication
cables should normally be grounded at
both ends.
Never terminate shield through pigtail.
Earth potential between PLC and drive:
Disconnect cables and measure voltage
with voltmeter to check.
Use equalizing cable or make sure units are
bolted together.
50/60 Hz ground loop: Use current clamp
meter to check.
=> Ground one end through 100 nF
capacitor with short leads.
Potential equalizing currents in serial
communication cable shield between two
drives:
=> Connect one end of the shield to the
special shield connection terminal with RC
decoupling. Remember ”correct” pigtail
installation!
Fig. 5. 9 Grounding of cable shield
130
Electromagnetic Compatibility
5.4 Installations with Frequency Converters
It is important to follow good engineering practice when installing frequency
converters for ensuring electromagnetic compatibility. When designing an installation,
an EMC plan can be made following these steps:
• List components, equipment and areas
• Divide into potential noise sources and potentially sensitive equipment
• Classify the cables connecting the equipment (potentially noisy or potentially
sensitive)
• Set requirements and select the equipment
• Separate potential noise sources from potentially sensitive equipment
• Control interfaces between noise sources and sensitive equipment
• Route cables according to the classification
Fig. 5.10 Typical measures in practice in a simple frequency converter installation
Electromagnetic Compatibility
131
5.5 Legislation and Standards
Legislation vs. standards
Legislation is issued by the legislative branch of national or local government and is
mandatory to comply with – it is law. It is a political document, free of specific technical
details – these details can be found in standards. Standards are written by experts in
relevant standardisation bodies (such as the International Electrotechnical Commission
IEC or the European Committee for Electrotechnical Standardisation CENELEC) and
reflect the technical state of the art. Their role is to establish a technical common
ground for cooperation between market players.
European EMC Directive
The latest EMC Directive is 2014/30/EU and comes into force on the 20th of April 2016
replacing the previous directive 2004/108/EC. This directive is a legal requirement in
the European Union. In essence the requirements are simple:
• Products must not emit unwanted electromagnetic interference (limits emission)
• Products must be immune to a reasonable amount of interference (sets immunity
requirements)
The directive itself is a political document and gives no specific technical requirements.
A producer has the possibility of using harmonised standards to demonstrate
compliance with the directive. Compliance with the EMC Directive (and also with other
relevant directive such as the Low Voltage Directive – LVD) is stated in the product’s
Declaration of Conformity and the “CE” mark is affixed to the product.
The scope of the EMC Directive consists of following two categories:
• Apparatus: a finished appliance made commercially available as a single-function
unit and intended for the end user. Apparatus complying with the requirements of
the Directive are marked with the CE mark
• Fixed installations: a combination of apparatus or other devices which is permanently
installed at a predefined location. Fixed installations are built following “good
engineering practices” and respecting the information on the intended use of its
components. Fixed installations are not CE marked
EMC Standards
There are different categories of standards, as follows:
• Basic standards deal with general aspects such as test set-up, measurement
technique and emission lines. For adjustable speed drives the emission limits
specified in EN55011 are commonly used
Electromagnetic Compatibility
132
• Generic standards deal with specific environments and have been mainly developed
to fill in the lack of specific product standards. For residential, commercial and light
industry environments the generic immunity standard is EN61000-6-1 and the
generic emission standard is EN61000-6-3. For industrial environments the generic
immunity standard is EN61000-6-2 and the generic emission standard is EN61000-6-4
• Product standards apply for a specific product family. For frequency converters the
standard is EN/IEC61800-3
The product standard for frequency converters sets both immunity and emission limits
depending on the environment where the FC is used: residential environment (more
strict emission limits, not so high immunity levels) or industrial environment (less strict
emission limits, higher immunity levels).
EN/IEC 61800-3 Category
EN55011 Class
Generic Standard
C1
Class B
Residential area EN61000-6-3
C2
Class A, Group 1
Residential area
C3
Class A, Group 2
Industrial area EN61000-6-4
C3 (I > 100A)
Class A, Group 2, I > 100A
Industrial area
C4
No limits. Make an EMC plan
Industrial area
Table 5.1 Overview of different EMC standards
Protection against Electric Shock and Energy Hazards
133
6 Protection against Electric Shock and Energy Hazards
6.1 General
Electrical products are often operated with voltages and currents that are potentially
hazardous to people, animals and systems. These hazards can result from physical
contact, overloading, short-circuiting, destruction of components or the influence of
heat or moisture.
The resulting potential hazards must be avoided, or at least reduced to an acceptable
minimum, by means of precautionary planning and design combined with fault
analysis and estimation of the residual risk.
Considerations to ensure safety of Frequency Converters during installation, normal
operating conditions and maintenance needs to be addressed during the design and
construction of the FC. Also consideration shall be given to minimise hazards resulting
from reasonably foreseeable misuse of the FC which might occur during its lifetime.
The protection against electrical shock is basically obtained by two levels of protection.
• Basic protection which protects the user against electrical shock under normal
operating conditions. The basic protection is normally obtained by physical
enclosure or barriers, or clearance /creepage distances
• Fault protection which protects the user against electrical shock under a single
fault condition. The fault protection in FC’s is normally obtained by use of plastic
enclosures or appropriate protective earth connection
Additionally, a protective galvanic isolation is provided between the accessible
control components/circuits and power components of FC’s. This is to ensure that no
dangerous voltage (e.g. mains voltage, DC-voltage and motor voltage) can appear on
the control lines. This would make contact with the control lines potentially lethal, as
well as creating a risk of damage to the equipment.
The international/European standard IEC/EN61800-5-1 describes in detail the
requirement for protection against electrical shock as well as protection against other
hazards applicable to FC’s.
The enclosure rating of the FC provides protection against injury or damage from
contact. An enclosure rating better than IP 21 prevents personal injury due to contact.
Compliance with national accident prevention regulations (such as BGV-A3, which is
134
Protection against Electric Shock and Energy Hazards
mandatory for electrical equipment in Germany) is also necessary to ensure protection
against contact hazards.
Temperature and fire hazards
FC’s can pose a fire hazard as a result of overheating. For this reason, they should be
provided with a built-in temperature sensor that stops the operation of the FC if the
cooling arrangement fails.
Under certain conditions, a motor connected to a FC can restart unexpectedly. For
example, this can occur if timers are enabled in the FC or temperature limits are
monitored.
Emergency stops
Depending on system-specific regulations, it may be necessary to fit an emergency
stop switch near the motor. This switch can be incorporated in the mains supply line or
the motor cable without damaging either the FC or the motor.
6.2 Mains Supply Systems
There are different ways of connecting the mains supply to earth, each with advantages
and disadvantages. There are three main earth arrangements, as defined in IEC 60364:
TN, TT and IT. The letters stand for
T – Terra (lat.) = connection to earth
N – Neutral
= direct connection to the neutral
I – Isolated = no connection/floating
TN-S system
Distribution inside a building must fulfill the requirements of a TN-S system, so no
combined PEN conductors may be used.
The TN-S system has the best EMC performance because the neutral and PE conductors
are separated. Thus a current through the N does not produce any effects on the
voltage potential of the PE. This is the preferred system for frequency converter
applications.
The disadvantage of the TN-S system, which is in general the disadvantage of both TN
and TT systems, is that in the case of an earth fault on the line, the protection fuses will
stop the operation.
Protection against Electric Shock and Energy Hazards
135
Generator or
transformer
L1
L2
L3
N
PE
Earth
Consumer
Fig. 6.1 TN-S system: Separate neutral and PE conductors.
TN-C system
In the TN-C system the PE and N conductors are combined in a PEN conductor. The
disadvantage is that a current through the N conductor is also a current through
the PE, thus a voltage potential between earth and the chassis of the connected
equipment occurs. In a 50 Hz/60 Hz world, with linear loads, this system does not
pose any special issues. But when electronic loads are present, includingFC’s, the
high frequency currents that occur can cause malfunctions. Although this system is
compatible with FC’s it should be avoided because of the associated risks. From an EMC
perspective the TN-C system is not optimal.
Generator or
transformer
L1
L2
L3
PEN
Earth
Consumer
Fig. 6.2 TN-C system: In the entire system, the neutral conductor and the PE conductor are combined in the PEN
conductor.
TN-C-S system
The TN-C-S system is a hybrid between TN-C and TN-S. From the transformer to the
building distribution point the PE and N are common (PEN) – just like in the TN-C
system. In the building the PE and N are separated, like in the TN-S. As the impedance
of the PEN conductor between the transformer and the building distribution point is
typically low, it reduces the negative effects that occur on the TN-C mains.
Protection against Electric Shock and Energy Hazards
136
Generator or
transformer
L1
L2
L3
N
PE
Earth
Consumer
Fig. 6.3 TN-C system: In the entire system, the neutral conductor and the PE conductor are combined in the PEN
conductor
TT system
In the TT system the PE at the consumer is provided by a local earth electrode. The
main advantage of the TT system is that the high frequency currents in the PE circuit of
the consumer are separated from the low frequency currents in the N conductor. From
an EMC perspective this is the ideal system.
However, because of the unknown impedance of the earth connection between the
earth of the transformer and the earth of the consumer, it cannot be guaranteed that a
line to PE short circuit at the consumer will blow the fuses quickly enough and protect
against electrical chock. This disadvantage can be mitigated by using residual current
devices (RCD).
Generator or
transformer
L1
L2
L3
N
Earth
Consumer
Earth
Fig. 6.4 TT system: Earthed neutral conductor and individual equipment/installation earthing
IT system
In the IT mains the transformer is unearthed and the three phases are floating.
The rationale for such a system is the ability of continuing operation after a line to
earth fault occurs. Isolation monitoring devices are used for observing the integrity
of the isolation between phases and earth. If the isolation is degraded, corrective
maintenance can be carried out.
Protection against Electric Shock and Energy Hazards
137
The disadvantage of this system is its poor EMC performance. Indeed, any earth
noise current will cause the entire system to float with the noise, possibly causing
malfunction of electronic equipment. When FC’s are used on IT mains special
considerations have to be taken, for example by disconnecting all capacitors to earth
(such as the common-mode capacitors in the RFI filter). Consequently, conducted
emissions will be unfiltered and a lot of high frequency noise can be found on IT mains.
Generator or
transformer
L1
L2
L3
Earth
Consumer
Earth
Fig. 6.5 IT system: Isolated mains; the neutral conductor may be earthed via an impedance or unearthed
6.3 Additional Protection
The degradation of the isolation between live parts and chassis leads to earth
leakage currents and can compromise both personal safety (risk of electric shock) and
equipment safety (the risk of over-heating components that can eventually lead to a
fire). The use of additional protective devices depends on local, industry-specific or
statutory regulations.
There are two types of protection relays for additional protection. One type uses a
fault voltage relay, while the other uses a residual current relay. Additional protection
with a fault voltage relay (FU relay) can be provided in most installations. Protection is
achieved by connecting the relay coil between the earthing terminal of the FC and the
system earthing point. A fault voltage trips the relay and disconnects the FC from the
mains.
In practice, FU relays are advantageous in situations where earthing is not allowed.
Whether or not they are allowed to be used depends on the regulations of the
electricity supply company. This form of protection is very rarely used.
Earth Leakage protection with a residual current operated circuit breaker (RCCB) is
allowable under certain conditions. Residual current operated circuit breakers contain
138
Protection against Electric Shock and Energy Hazards
a sum-current transformer. All of the supply conductors for the FC pass through this
transformer. The sum-current transformer senses the sum of the currents through these
conductors.
The sum is zero if there is no leakage current in the installation. If there is a leakage
current, the sum is not zero and a current is induced in the secondary winding of the
transformer. This current switches off the relay and disconnects the FC from the mains.
Conventional RCCBs use inductive sensing and are therefore only suitable for sensing
AC currents.
FC’s with B6 input bridge rectifiers can cause a pure DC current to flow in the supply
cable in the event of a fault. It is recommended to check whether DC current can be
present at the input to the FC. If it can, a Type B RCD (sensitive to both AC and DC)
must be used to obtain reliable protection. This type of RCD has additional integrated
circuitry that allows it to detect both AC and DC residual current.
Fig. 6.6 Fault voltage relay
These devices are commonly known as residual current operated circuit breakers
(RCCBs). The higher-level term is ”residual current operated device” (RCD) in accordance
with EN 61008-1.
Filters and components for RFI suppression (common-mode capacitors) always cause
a certain amount of leakage current. The leakage current produced by a single RFI
suppression filter is usually just a few milliamperes. However, if several filters or large
filters are used, the resulting leakage current may reach the trip level of the RCD.
The interference suppression components used with FC’s generate leakage currents.
For this reason, the earth connection must be made as follows:
• If the leakage current is greater than 3.5 mA, the cross-section of the PE conductor
must be at least 10 mm²
Protection against Electric Shock and Energy Hazards
139
• Otherwise, the equipment must be earthed using two separate PE conductors. This is
often called ”reinforced earthing”
Alternating fault currents
Pulsating DCs (pos. and neg. half-wave)
Sloping half-wave currents
Angle of slope 90° el.
135° el.
Half-wave current with overlay of smooth fault DCs of 6 mA
Smooth fault DCs
Fig. 6.7 Waveforms and designations of residual currents
Fig. 6.8 Universal RCCB
6.4 Fuses and Circuit Breakers
For protecting FC’s and the installation against electrical and fire hazard they need
to be protected against short-circuit and over-current by means of an over-current
protective device (e.g. fuse or circuit breaker). The protection needs to comply with
relevant local, national and international regulations.
A fuse interrupts excessive current, to prevent further damage to the protected
equipment. It is characterised by a rated current (the current that a fuse can
140
Protection against Electric Shock and Energy Hazards
continuously conduct) and speed (which means how long it takes to blow the fuse at
a given overcurrent). The higher the current the shorter time it takes to blow the fuse.
This is expressed by time current characteristics, as shown in Fig. 6.9 Time-current
characteristics of fuses:
Fig. 6.9 Time-current characteristics of fuses
There are different standardised time-current characteristics depending on the
intended application. For protecting FC’s typically aR fuses for semiconductor
protection are used to limit the damage in case of a short-circuit or internal component
breakdown. In some situations gG type general purpose fuses can be used. For the
specific fuse selection it is important to consult the documentation of the FC and
strictly follow those recommendations, since the recommended fuses are tested
together with the FC.
Protection against Electric Shock and Energy Hazards
141
Circuit breakers
Unlike fuses which are sacrificial devices that need to be exchanged after being blown,
circuit breakers are electromechanical devices that can be simply reset after being
activated. Because the speed of circuit breakers can be slower than fuses, their use
needs to be carefully considered. The slow speed can lead to extensive damage in the
protected device, subsequent overheating and even a fire risk. Not all FC’s are suitablydesigned to be protected with circuit breakers. Special considerations are taken in
the design phase of FC’s to limit the damage in the case of a component breakdown
inside the FC. Such measures are, for example, special internal mechanical features in
the enclosure, use of shields, use of deflecting foils, etc. to limit the consequences of
internal failures.
It is essential to consult and strictly follow the recommendations found in the
documentation of the specific FC regarding the use of circuit breakers, including the
type and manufacturer of circuit breaker to be used, since the recommended devices
have been tested with that FC.
Mains Interference
142
7 Mains Interference
7.1 What are Harmonics?
7.1.1 Linear Loads
On a sinusoidal AC supply a purely resistive load (for example an incandescent light
bulb) will draw a sinusoidal current, in phase with the supply voltage.
The power dissipated by the load is: P = U × I
For reactive loads (such as an induction motor) the current will no longer be in phase
with the voltage, but will lag the voltage creating a lagging true power factor with a
value less than 1. In the case of capacitive loads the current is in advance of the voltage,
creating a leading true power factor with a value less than 1.
Voltage
Current
Displacement angle, Fig. 7.1 Linear Load
In this case, the AC power has three components: real power (P), reactive power (Q) and
apparent power (S).
The apparent power is: S = U × I
In the case of a perfectly sinusoidal waveform P, Q and S can be expressed as vectors
that form a triangle:
S2 = P2+ Q2
Units: S in [kVA], P in [kW] and Q in [kVAR].
Mains Interference
143
S
Q
P
Fig. 7.2 Components of AC Power: Real Power (P), Reactive Power (Q) and Apparent Power (S)
The displacement angle between current and voltage is φ.
The displacement power factor is the ratio between the active power (P) and apparent
power (S):
DPF = P = cos ()
S
7.1.2 Non-linear Loads
Non-linear loads (such as diode rectifiers) draw a non-sinusoidal current. Fig. 7.3 shows
the current drawn by a 6-pulse rectifier on a three phase supply.
A non-sinusoidal waveform can be decomposed in a sum of sinusoidal waveforms with
periods equal to integer multiples of the fundamental waveform.
f(t ) = ∑ ah × sin(h ω1t )
Fig. 7.3 Non-linear Load: Current drawn by a 6-pulse rectifier on a 3-phase supply
Mains Interference
144
The integer multiples of the fundamental frequency ω1 are called harmonics. The RMS
value of a non-sinusoidal waveform (current or voltage) is expressed as:
IRMS =
hmax
Σ I2
h1 (h)
The amount of harmonics in a waveform gives the distortion factor, or total harmonic
distortion (THD), represented by the ratio of RMS of the harmonic content to the RMS
value of the fundamental quantity, expressed as a percentage of the fundamental:
THD =
hmax
Σ
h2
Ih
I1
2
u 100 %
Using the THD, the relationship between the RMS current IRMS and the fundamental
current I1 can be expressed as:
IRMS = I1 u — 1 + THD2
The same applies for voltage.
The true power factor PF (λ) is:
PF = P
S
In a linear system the true power factor is equal to the displacement power factor:
PF = DPF = cos(M)
In non-linear systems the relationship between true power factor and displacement
power factor is:
PF =
DPF
—1 + THD2
The power factor is decreased by reactive power and harmonic loads. Low power factor
results in a high RMS current that produces higher losses in the supply cables and
transformers.
In the power quality context, the total demand distortion (TDD) term is often
encountered. The TDD does not characterise the load, but it is a system parameter.
Mains Interference
145
TDD expresses the current harmonic distortion in percentage of the maximum demand
current IL.
THD =
hmax
Σ
h2
2
Ih
IL
u 100 %
Another term often encountered in literature is the partial weighted harmonic
distortion (PWHD). PWHD represents a weighted harmonic distortion that contains only
the harmonics between the 14th and the 40th, as shown in the following definition.
PWHD =
Σ
40
h
h=14
Ih
I1
2
u 100 %
7.1.3 The Effect of Harmonics in a Power Distribution System
The picture below shows an example of a small distribution system. A transformer is
connected on the primary side to a point of common coupling PCC1, on the medium
voltage supply. The transformer has impedance Zxfr and feeds a number of loads. The
point of common coupling where all loads are connected together is PCC2. Each load is
connected through cables that have respective impedance Z1, Z2, Z3.
Fig. 7.4 Example of Distribution System
Mains Interference
146
Harmonic currents drawn by non-linear loads cause distortion of the voltage, due to
the voltage drop on the impedances of the distribution system. Higher impedances
result in higher levels of voltage distortion.
Current distortion relates to apparatus performance and it relates to the individual
load. Voltage distortion relates to system performance. It is not possible to determine
the voltage distortion in the PCC knowing only the harmonic performance of the load.
In order to predict the distortion in the PCC the configuration of the distribution system
and relevant impedances must be known.
A commonly used term for describing the impedance of a grid is the short circuit ratio
Rsce, defined as the ratio between the short circuit apparent power of the supply at the
PCC (Ssc) and the rated apparent power of the load (Sequ).
Rsce = Sce
Sequ
where Ssc =
U2
Zsupply
and Sequ = U × Iequ
The negative effect of harmonics is twofold
• Harmonic currents contribute to system losses (in cabling, transformer)
• Harmonic voltage distortion causes disturbance to other loads and increase losses in
other loads
Non-linear
Current
System
Impedance
Contribution to
system losses
Fig. 7.5 Negative Effects of Harmonics: System Losses and Disturbance
Voltage
Disturbance to
other users
Mains Interference
147
7.2 Harmonic Limitation Standards and Requirements
The requirements for harmonic limitation can arise from:
• Application-specific requirements
• Requirements for compliance with standards
The application-specific requirements are related to a specific installation where there
are technical reasons for limiting the harmonics.
Example: 250 kVA transformer with two 110 kW motors connected.
Motor A is connected directly to mains supply, and Motor B is supplied through
Frequency Converter B. There is a need to retrofit FC A, so that Motor A is supplied
through its own FC, but the transformer will, in this case, be undersized. Solution: In
order to retrofit without changing the transformer, mitigate the harmonic distortion
from FC’s A and B using harmonic filters.
There are various harmonic mitigation standards, regulations and recommendations.
Different standards apply in different geographical areas and industries. The most
common are the following:
•
•
•
•
IEC/EN 61000-3-2, Limits for harmonic current emissions (≤ 16A per phase)
IEC/EN 61000-3-12, Limits for harmonic currents (>16A and ≤75A)
IEC/EN 61000-3-4, Limitation of emission of harmonic currents (> 16A)
IEC/EN 61000-2-2 and IEC/EN 61000-2-4 Compatibility levels for low frequency
conducted disturbances
• IEEE519, IEEE recommended practices and requirements for harmonic control in
electrical power systems
• G5/4, Engineering recommendation, planning levels for harmonic voltage distortion
and the connection of nonlinear equipment to transmission systems and distribution
networks in the United Kingdom
7.3 Harmonic Reduction Methods in Frequency Converters
The line current of unmitigated diode rectifiers has a total harmonic distortion (THD)
of at least 80%. This high distortion value is unacceptable in most applications with
FC’s. Therefore it is necessary to have some harmonic mitigation. The level of harmonic
mitigation depends, as explained earlier, on the specific installation and the harmonic
standards the installation needs to comply with.
Mains Interference
148
An overview of the various harmonic mitigation methods is shown in Table 7.1 –
Harmonic Mitigation Methods.
Mitigation method
Circuit diagram
No mitigation
THD > 80%
Typical current waveform
///
DC inductors
THD < 40%
AC inductors
THD < 40%
///
Passive harmonic filter
THD < 10%
Multi-pulse rectifier (12/18)
THD < 10%
Active front end
THD < 5%
///
Active filter
THD < 5%
///
Waveform similar to AFE
Table 7.1 Harmonic Mitigation Methods
Harmonic mitigation can be achieved by using either passive or active circuits.
Mains Interference
149
7.3.1 Passive Harmonic Mitigation
DC inductors
DC inductors are placed in the DC link between the rectifier and the bulk DC capacitor.
It is possible to use a single inductor in either the + or the – side or use two inductors.
This solution reduces THDi to values between 35 and 45%.
AC inductors
AC inductors are placed on the line side of the rectifier. Their harmonic performance
is similar to DC inductors and reduce THDi to typical values of between 35 and 45%,
depending on the size of the inductor.
DC vs. AC inductors
Since DC and AC inductors have similar harmonic performance levels the question
about the differences between the two solutions often arises. First of all, even if the
THD value is similar, the effect of the two solutions on the components of the harmonic
spectrum is different. DC inductors attenuate more the low frequency components
(5th, 7th, 11th harmonic) while the AC inductors have a better performance for higher
harmonic orders.
Across inductors an AC voltage drop occurs. In the case of AC inductors, a voltage
drop will occur, typically around 4%. In the case of DC inductors, the DC current does
not cause a voltage drop. The only voltage drop across DC inductors results from the
current ripple of the rectifier. Consequently, using DC inductors will result in a higher
DC link voltage, thus the ability to provide more torque at the motor shaft. This is the
major advantage of DC inductors. The main advantage of AC inductors is that they
protect the rectifier against transients from the mains.
Passive harmonic filters
Passive harmonic filters are connected in series with the mains supply. They can
be realised with various circuit topologies that typically consist of combinations of
inductors (L) and capacitors (C), sometimes also damping resistors R. The filter circuit
can be a low-pass circuit, tuned to specific harmonics (5th, 7th, etc.) or slightly detuned, to avoid the risk of resonances. The performance of passive filters depends on
the specific frequency converter’s DC link configuration (with/without DC chokes, value
of capacitance) and a performance level can be assured for a specific configuration.
Danfoss Advanced Harmonic Filters (AHF) are designed specifically for Danfoss VLT®
frequency converters and can reduce THD to 10 % (AHF10 series) or even 5% (AHF
5 series). These filters use a proprietary topology with a two-stage de-tuned LC
harmonic- absorbing circuit.
150
Mains Interference
Passive filters have the disadvantage of being quite bulky (comparable in size with the
FC). They have a capacitive power factor that needs to be considered during system
level design for avoiding resonances.
Multi-pulse rectifiers
Multi-pulse rectifiers are fed from phase- shift transformers. The most common
solutions are with 12 pulses (2 x 3 phases) or 18 pulses (3 x 3 phases). Through phaseshifting, low order harmonics are in 180° opposition, cancelling each other. For
example, in the case of 12 pulse rectification the phases of the secondary have a 30°
phase offset (the offset between the D and Y windings). In this configuration the 5th
and 7th harmonics are cancelled and the largest harmonics will be the 11th and 13th.
Multi-pulse harmonic mitigation requires large transformers – larger than the FC.
Another disadvantage is that the performance is reduced in non-ideal conditions such
as voltage imbalance.
7.3.2 Active Harmonic Mitigation
Active Front End (AFE)
The diode rectifier can be replaced with an inverter with active switches (usually
IGBT transistors), similar to the inverter at the motor side. The grid-side inverter is
pulse-width modulated and the input current is nearly sinusoidal. The harmonics
of the mains frequency are not present. On the other hand the switching frequency
components are injected to the mains grid. In order to reduce the switching noise a
passive filter is used, usually in a low-pass L-C-L topology (two inductors and capacitors
between the inductors).
The main advantage of the AFE is that it allows four-quadrant operation: that means
that the energy flow is bi-directional and in the case of regenerative braking the energy
can be injected back to the grid. This is advantageous in applications with frequent
braking or long-time braking such as cranes or centrifuges.
The disadvantage of the AFE solution is a relatively low efficiency and a high
complexity. When the application does not require bi-directional energy flow the
energy efficiency of the AFE is inferior to an active filter solution.
Active filters
Active filters (AF) consist of an inverter that generates harmonic currents in anti-phase
with the harmonic distortions on the grid thus achieving a 180° cancellation effect.
The operation principle is illustrated in the illustration below, where the AF cancels the
harmonic currents from a diode rectifier.
Mains Interference
151
~
~
~
Load
Control
Fig. 7.6 Operation Principle of an Active Filter
As in the case of AFE, an LCL filter is needed to eliminate the noise at the switching
frequency.
Active filters are connected in parallel with the non-linear (harmonic generating) load.
This allows for several harmonic mitigation possibilities:
• Individual compensation of non-linear loads: an active filter compensates harmonics
from a single load. Danfoss offers an optimised filter + FC package called “Low
Harmonic Drive (LHD)”
• Group compensation: harmonics from a group of several loads (for example FC’s) are
compensated by a single filter
• Central compensation: harmonics are compensated directly at the point of commoncoupling of the main transformer
Supply
Central
compensation
M
3-
M
3-
M
3-
Group
compensation
M
3-
Individual
compensation
M
3-
Fig. 7.7 Harmonic compensation can take place in different
areas of the network
Mains Interference
152
7.4 Harmonic Analysis Tools
Harmonic analysis tools can be used to calculate harmonics in a system and design the
optimal harmonic mitigation solution to meet specific requirements. The advantage of
software tools is that different solutions can be compared, allowing the selection of the
best solution.
There are a variety of commercially available software tools ranging from simple
calculation tools for a non-linear load to complex software packages that allow the
design of an entire power system.
Vo
VTHD
Lsc
Ltr
Rtr
Vtr
Itr A
V
LEVEL 1
IL
A
IFC1
A
IB2
A
IFC2
A
IFC3
LEVEL 2
LEVEL 3
A
LEVEL 4
Lc1
LcB2
Lc2
Lc3
Rc1
RcB2
Rc2
Rc3
VFC1
VB2
VFC2
VFC3
V
V
V
V
LEVEL 5
AHF1
FC1
B2
M
LC
R
CL
AHF2
AHF3
FC2
FC3
Fig. 7.8 Calculation model with current and voltage measurement points
Danfoss offers two software tools:
• the offline tool VLT® Motion Control Tool MCT 31 and
• the on-line tool HCS (Harmonic Calculation Software)
C
Mains Interference
153
7.4.1 VLT® Motion Control Tool MCT 31
MCT 31 is an off-line software package used to calculate harmonics based on
polynomial interpolation between pre-defined simulation results. The advantage
of this method is speed and the disadvantage is that it is less precise compared to a
simulation.
MCT 31 enables simulations with all Danfoss products, including mitigation solutions
such as AHF passive filters and AAF active filters. Generic, non-Danfoss frequency
converters can be simulated as well. MCT 31 can generate harmonic reports.
7.4.2 Harmonic Calculation Software (HCS)
The HCS tool can be accessed on-line at www.danfoss-hcs.com. It is available in
two levels: basic for simple calculations and expert for more complex system level
calculations.
Behind the web interface of the HCS tool there is a powerful circuit simulator that
performs a simulation of the specific system designed by the user. Therefore it is more
precise than the interpolation-based MCT 31.
HCS has a vast library containing Danfoss FC’s, AHF passive filters, AAF active filters.
It also features the time-domain and frequency domain graphical visualisation of the
voltages and currents in a system and comparing the harmonics to different limit lines.
HCS can also generate reports in HTML or PDF format.
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Interfaces
8 Interfaces
8.1 Human Machine Interface (HMI)
The Human Machine Interface (HMI) is an important and vital part of Frequency
Converters today. The HMI interface can vary from a basic LED status indicator to a
sophisticated field bus system with detailed FC information. The HMI will set up an
interface between a human and an application that allow the user to control, monitor
and diagnose the application.
Modern FC’s today often have these HMI interfaces:
LED
Fig. 8.1 LED indication
An LED to indicate that power is applied to the FC
An LED to indicate that a warning is present
An LED to indicate an alarm on the FC
Numerical and/or alphanumerical panels
Fig 8.2 Numerical and alphanumerical panels
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155
These devices provide an easy possibility to control the FC, monitor its actual status
and for easy commissioning of the application.
Input and output terminals
Fig. 8.3 Input and output terminals
Dedicated input and output control terminals are available in order to build an
interface between a PLC control and the FC.
Input control signals like start/stop, coast or reverse control will ensure that the user
has functions to control the FC according to the application. For controlling the speed,
and feedback singles from the application analogue input signals like 0-10 V or
0/4-20 mA can be applied.
Feedback signals from the FC to the PLC are digital output or relay output which
can be configured to indicate status like “motor running” or “alarm”. Also analogue
output signals from the FC can be configured to monitor, for instance, the actual load
conditions.
Software tools
Fig. 8.4 Software tools
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Interfaces
Integration of the FC into PC software gives the user full system configuration and
control. With PC Software it is possible to monitor the entire system more effectively for
faster diagnosis, and better preventive maintenance.
A modern PC Software tool can be used as follows:
• For planning a new communication network offline. PC Software tools contain a
complete database with supported FC products
• For commissioning FC’s online
• For easy replacement of a FC, in the event of failure
• For easy expansion of the network with more FC’s
• For back-up of parameter settings of FC’s in a communication network
• Software supports fieldbus protocol. This will eliminate the need for an extra
communication network
Fieldbus
Use a standardised fieldbus interface between the PLC and FC for commissioning,
control and monitoring of the application.
Fig. 8.5 Fieldbus connection
8.2 Operating Principles of Serial Interfaces
In serial data transmission, the bits (with a state of 0 or 1) are transmitted individually,
sequentially. A logical 0 or a logical 1 is defined by specified voltage levels. Various
methods and standards have been developed to ensure fast, error-free data transfer.
The method used depends on the specification of the interface. If we look at the lowest
level of data transfer, a distinction can be made between how the bits are transmitted
electrically (current or voltage signal) and the system used (line coding). If the bits are
transmitted via a voltage signal, the focus is less on the voltage level than the reference
potential of the level.
Interfaces
Principle
157
Standard
(application)
Devices
connected
per trunk
circuit
Max.
distance
in mm
RS 232
(point to
point)
1 sender
1 receiver
15
RS422
(point to
point)
1 sender
10 receivers
1200
Duplex: 4
± 2 V min.
RS485
(Bus)
32 senders
32 receivers
1200
Semi
duplex: 2
± 1.5 V min.
Number of
lines
Signal
level
Duplex min.
± 5 V min.
3+ various
± 15 V max.
status signals
Table 5.1 3 fieldbus principles and typical specifications
RS-232/ EIA-232 interface
The RS-232 interface, launched as early as 1962, was for a long time the serial interface
par excellence. When a serial interface was mentioned in relation to PCs, it referred to
RS-232 RS-232 was conceived for communication between two devices (point-to-point
connection) at low transmission speeds.
RS-422/ EIA-422 interface
RS-422 allows both point-to-point and multi-drop networks to be built. In multi- drop
networks, it is possible to connect multiple receivers to one transmitter.
The data is transmitted differentially via twisted data cables. One pair of lines is needed
for each transmission direction for full duplex operation.
RS-485/ EIA-485 interface
RS-485 is regarded as a higher-level version of the RS-422 standard and accordingly has
similar electrical properties.
In contrast to RS-422, however, RS-485 is designed as a multi-point (bus-capable)
interface over which up to 32 devices can communicate. There are now also transceiver
modules (combined transmitter and receiver module) with which networks of up to
256 devices can be implemented. The actual maximum possible network size depends
on both the transmission rate (line length) and the structure of the network (network
topology).
USB interface
The Universal Serial Bus (USB) standard was developed in 1995 by Intel in conjunction
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Interfaces
with companies in the IT industry. The USB 2.0 extension of the standard in 2000
increased the transmission speed from 12 Mbps to 480 Mbps. Additionally, in 2008
USB 3.0 was introduced, allowing transmission speeds of up to 5 Gbitps. The data is
transmitted differentially via a twisted pair. The maximum cable length between two
devices must not exceed 5 m.
Despite its name, USB is not a physical data bus, but rather a point-to-point interface.
The term “bus” in the name USB refers only to the structure with which a network can
be built. The USB specification provides for a central host (master) to which up to 127
different devices can be connected. Only one device can be connected directly to a
port. An additional hub is required to connect more than one device to a port.
Ethernet interface
The Ethernet standard was developed back in the early 1970’s. Since then Ethernet
has become more and more present in all kinds of products. In the 90’s Ethernet found
its way to the automation field via protocols like: MAP, Modbus TCP and EtherNet/IP.
Ethernet typically runs on 100 Mbps, over STP cables (Shielded Twisted Pair), but is also
available in wireless, fibre optic and other media. The benefit of using Ethernet is not
only the fast speed and standardised cables & connectors, but the ability to access data
inside automation equipment from the office network. This allows status to be read
from all over the plant, even from another continent.
Despite the fact that all Ethernet protocols runs on Ethernet, it does not mean that it
is possible to run different Ethernet technologies in the same network. Technologies
that change the arbitration or have strict demands towards timing make a mix of
technologies impossible. The mainstream Ethernet technologies today are PROFINET,
EtherNet/IP, Modbus TCP, POWERLINK and EtherCAT. Today, these technologies have
more than 90% of the market share in new installations.
8.3 Standard Serial Interfaces in Frequency Converters
Today, most FC’s today are fitted as standard with a serial system interface that can be
used for connection to a network.
Various standardised protocols are generally supported, in addition to unpublished,
manufacturer-specific (proprietary) protocols. Physically, the interfaces are very often
based on the specification of the RS-485 interface.
Interfaces
159
Since FC’s usually only have a serial RS-485 interface available, interface converters are
required for implementation. Manufacturer-specific solutions in which a particular FC
is required are widespread. If the interface specification is published, simple industrystandard converters (such as USB to RS-485) can be used.
USB
Fig. 8.6
USB
to
RS485
converter
RS485
USB to RS 485 communication
FC’s are increasingly being fitted with USB interfaces for simple data exchange with a
PC. Since many PCs have USB interfaces, the use of interface converters is becoming
obsolete.
8.4 Fieldbus Interfaces in Frequency Converters
The use of modern FC’s without a serial communication interface is almost
inconceivable today. In the simplest case, the interface consists of two data lines
through which the FC can be controlled, monitored, configured and documented.
Almost all bus systems enable multiple devices to be on the same network.
Compared with conventional FC control via digital and analogue inputs and outputs,
there is less cabling involved in serial bus systems, which reduces installation cost. On
the other hand, costs are incurred for the interfaces and additional components are
required to control the bus system. Depending on the bus system used, only a few
networked devices are necessary to generate considerable cost benefits compared
with conventional control.
Traditional wiring. No fieldbus.
In this type of network, communication between the drive and PLC requires one cable
for each parameter that needs to be controlled. The advantage of such a system is that
the individual components themselves are relatively cheap, and the system itself is not
among the most complex.
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Interfaces
This, however, comes at a price, as such systems are relatively expensive both to
install and extend, as each additional parameter or drive requires new cabling, PLC
programming and often more I/O hardware. For owners this means higher capital costs
and restricted flexibility. At the same time the risk of error is high, as the risk of a faulty
connection to the PLC increases with the number of cables.
Fig. 8.7 Traditional wiring. No fieldbus
Fieldbus wiring
A typical fieldbus system only uses twisted pair cables to connect the drive to the PLC.
Despite the higher cost of components, fieldbus systems offer several advantages over
older, hardwired systems: fewer cables, faster commissioning and a reduced risk of
faults.
Additional drives are connected in a serial Ethernet-based network that can be
extended easily. New parameters only need to be coded into the PLC, which is both
faster, safer and at significantly lower cost than a hardwired system.
Fig. 8.8 Fieldbus wiring
Interfaces
161
Fieldbus over Ethernet
The Ethernet interface enables the possibility to access drive parameters and
information from locations outside the production facility. This method bypasses the
traditional control hierarchy, as communication with the fieldbus-fitted drives and
other equipment does not necessarily need to pass through the PLC.
External access is routed through a firewall, enabling communication with the fieldbus
option’s built-in webserver.
Not only does this provide a high degree of flexibility during commissioning, it also
provides advantages such as external monitoring and application support.
Fig. 8.9 Fieldbus over Ethernet
8.5 Fieldbuses Standardisation
The development of fieldbuses began in the 1980s so that the benefits of serial
communication could also be used in the field. The driving force for the development
included not only the potential to save cost and time during planning and installation
but also the ease of expansion and increased interference immunity when transmitting
analogue signals.
In the years that followed, it became clear that the success of a system depends not
only on industrial capability in a demanding environment but also on “openness”.
In open-bus systems, the installation and control are the same, irrespective of the
manufacturer of the bus components. The end user can therefore replace a (defective)
device from one manufacturer with a device from another manufacturer without
having to make major changes to the system.
Interfaces
162
The principal difference between the interfaces and bus systems available on the
market are the physical design and the protocols used. Which system is used depends
on the requirements of the application in question.
Fast processes such as packaging machines may need bus cycle times of just a few
milliseconds, whilst response times of seconds may suffice for climate control systems.
For the purposes of better classification, communication systems can be considered
in terms of data volume, transfer time and transmission frequency. The diagram below
shows the basic division into three different levels.
Company level
Ethernet/TCP/IP
Area
controller
TCP//IP/Ethernet
Cell level
“Process” bus
Field level
Fieldbus
EX Bus
Fig. 8.10 Production pyramid
• At the company level, large data volumes in the megabyte range are exchanged. The
transfer times can extend to hours.
• At the cell level, the data volume decreases to the kilobyte range. At the same time,
the transfer time shortens (seconds) and frequency of data exchange (minutes/hours)
rises.
• At the field level, very small data volumes of a few bytes or even bits are ex- changed.
The transfer time and transmission frequency are a matter of milliseconds.
The world’s most important field buses have been standardised since 1999 in IEC
61158.
Interfaces
163
Fig. 8.11 Typical field buses
Different Bus systems have more or less significance depending on the region and
application. If you look at drive technology, Profibus and its Ethernet based successor
(PROFINET) can be considered to have a larger market share in Europe. In contrast,
DeviceNet and EtherNet/IP are frequently used in North and South America as well as
in Asia. This defines the precondition for the high market acceptance of the respective
Ethernet based successors PROFINET in Europe and EtherNet/IP in North America.
164
Sizing ad Selection of Frequency Converters
9 Sizing and Selection of Frequency Converters
9.1 Get the Drive Rating Right
Selecting the right Frequency Converter is a key aspect of designing a variable
speed drive system. If the selected unit is too small, it will not be able to control the
connected motor optimally at all necessary operating points. If, on the other hand it is
too large, there is a risk that the motor will not always be controlled properly, and the
design may not be cost-effective.
For the design of most FC’s, knowledge of the following basic parameters is sufficient:
• Rating of FC from motor specifications
• Current distribution in the FC (cos φ of the motor)
• Overload capacity
• Control range and field weakening
• Derating of FC
• Regenerative energy
• Motor cable length
• Environment
• Central versus de-centralised installation
After clarification of the basic design parameters for an application, design and
analysis of the mechanical components is carried out. The motor to be used must
be determined before a suitable FC can be selected. In facility service systems, for
example, final selection often takes place only shortly before the building is completed.
Only at this time are most of the components to be used defined, so that an optimised
analysis of flow conditions can be carried out reliably.
The more dynamic and challenging the application, the greater the number of factors
that must be taken into account in the design. Since FC manufacturers can save costs
by restricting the technical features, for each particular case it is necessary to confirm
that the features needed for the drive are actually available.
Sizing ad Selection of Frequency Converters
165
9.2 Rating of the Frequency Converter from Motor Specifications
A widely used method for selecting FC’s is simply based on the rated power of the
motor to be used. Although manufacturers specify the power ratings of their FC’s, this
data normally relates to standard four-pole motors. Since the rated currents of motors
differ significantly at the same power depending on the construction of the motor (e.g.
standard motor and geared motor) and its number of pole pairs, this method is only
suitable for providing a rough estimate of the proper FC size. Fig. 9.1 – Nominal current
for 1.50kW motors of different poles and manufacturer – shows examples of the rated
currents of various 1.5 kW motors.
5.0
4.5
4.0
3.5
3.0
2.5
5.0
1.5
1.0
0.5
0.0
680
700
930
930
1410
1425
1440
2850
2900
RPM [min-1]
Fig. 9.1 Nominal current for 1.50kW motors of different poles and manufacturer
Furthermore, it should be noted that the current drawn by a motor depends on
whether it is connected in star or delta configuration. For this reason FC’s should be
selected based on the rated current for the type of configuration selected (star or
delta).
In addition to the motor current, the required motor voltage must be taken into
account. Many FC’s can operate over a wide mains supply voltage range (e.g. 3 x 380
– 500V ) and thus provide a wide output voltage range. However, the apparent power
that the unit can supply is constant over the whole voltage range. Thus the maximum
output current is higher at a lower mains voltage and, correspondingly, lower at a
higher voltage.
166
Sizing ad Selection of Frequency Converters
Fig 9.2 Identification data of a Danfoss frequency converter
The nameplate in Fig. 9.2 Identification data of a Danfoss frequency converter comes
from a 0.75 kW FC. The specified current values apply to two different voltage ranges.
The FC can deliver 2.4 A with a mains voltage of 380 – 440 V. If the unit is supplied
with a mains voltage of 441 – 500 V, it can deliver 2.1 A. However, the apparent power
available with both voltage ranges is always 1.70 kVA.
9.3 Overload Capacity
When selecting a FC, the load conditions of the application should always be taken into
account first. A fundamental distinction is made between quadratic and constant load
characteristics, which are the most common in practice.
When a FC controls a motor, torque limits can be set for that motor. Selecting a FC
with an apparent power rating that matches the rated current or power of the motor
ensures that the required load can be driven reliably. However, an additional reserve
is necessary in order to enable smooth acceleration of the load and also cater for
occasional peak loads.
Below are examples of a constant load torque characteristic. If a load is placed on a
conveyor belt, the torque that must be applied to transport the load is constant over
the entire speed range.
Sizing ad Selection of Frequency Converters
Application
167
Excess load
Lifting equipment
160%
Conveyor belt
160%
Stirrer / Mixer / centrifuge
160%
Rotary piston compressor / piston compressor
150%
Spiral pump (thick sludge)
150%
Sludge dehydration press
150%
Piston pump
150%
Rotary gate valve
150%
Rotary piston blower
110%
Surface aerator
110%
Metering pump
110%
Booster pumps (2-stage)
110%
Recirculation pump
110%
Side channel blower for pool aeration
110%
Table 9.1 Typical overloads in constant torque applications
With a constant load, an over-load reserve of approximately 50 to 60% for 60 seconds
is typically used. If the maximum over-load limit is reached, the response depends on
the FC used. Some types switch off their output and lose control of the load. Others are
able to control the motor at the maximum over-load limit until they trip for thermal
reasons.
A quadratic load characteristic usually occurs in applications where increasing speed
leads to an increasing quadratic load torque. Fans and centrifugal pumps are amongst
the types of equipment that display behaviour of this kind. Furthermore, most
applications with a quadratic torque characteristic, such as centrifugal pumps or fans,
do not require rapid acceleration phases. For this reason excess load reserves of 10 %
are usually chosen for quadratic torques.
See next page with examples of a quadratic load torque characteristic.
168
Sizing ad Selection of Frequency Converters
Application
Excess load
Fan
110%
Well pump
110%
Booster pump / centrifugal pump
110%
Filter infeed pump
110%
Groundwater pump
110%
Hot water pump
110%
Non-clogging pump (solid materials)
110%
Centrifugal pump / fan
110%
Primary and secondary heating pump
110%
Primary and secondary cooling water pump
110%
Rainwater basin evacuation pump
110%
Recycling sludge pump
110%
Spiral pump (thin sludge)
110%
Submerged motor pump
110%
Excess sludge pump
110%
Table 9.2 Typical overloads in variable torque applications
Even with quadratic load and an over-load capacity of 10% modern FC’s can be set
up to have a higher break-away torque at start to ensure the proper start of the
application.
Remember to consider whether the application will always require a quadratic torque.
For example, a mixer has a quadratic torque requirement when it is used to mix a very
fluid medium, but if the medium becomes highly viscous during processing, the torque
requirement changes to constant.
9.3.1 Energy Efficiency Concerns
In chapter 4 Saving Energy with Frequency Converters we have seen different
considerations to be taken to save energy. It is important to remember, that the most
energy efficient solution is where the machine, the motor and the FC are selected for
the best system efficiency. For example fans speed will typically differ from nominal
speed, and so the motor, but many motors have their highest efficiency at a speed
between 75 and 100% of nominal speed.
Some brands of FC have a built-in software function, which secure the best motor shaft
power related to the FC input power.
Sizing ad Selection of Frequency Converters
169
9.4 Control Range
The advantage of a FC lies in its ability to regulate smoothly the speed of the motor.
However, a wide variety of limits are set for the available controlling range.
On the one hand the possible controlling range (speed range) depends on the control
algorithms available of the unit. With the simple U/f control, control ranges that can
vary within 1:15 can usually be achieved. If a control algorithm with a voltage vector
control is used, a range of 1:100 is possible. If the actual motor speed is fed back to the
FC by an encoder, adjustment ranges from 1:1,000 to 1:10,000 can be realised.
In addition to the limits of the control algorithms used, the field-weakening range
around the rated frequency of the motor and also low speed running must be taken
into account. At low speeds, the motor’s self-cooling capacity is reduced. Therefore,
in the event of continuous operation in this speed range, either a separately powered
external fan must be used to cool the motor or the shaft load must be reduced. The
speed below which the torque must be reduced can be found in the manufacturer’s
data sheets.
Torque [T]
If the motor is operated in the field-weakening range, the reduction in the available
torque with 1/f and the breakdown torque with 1/f2 must also to be taken into account
The field-weakening range begins when the FC can no longer hold the U/f ratio
constant. In Europe this point typically lies at 400 V/50 Hz and in North America at 460
V / 60 Hz.
TB
TS
TL
Constant excess load 160 %
Constant excess load 110 %
TN
Break away torque
Load torque
n
Fig. 9.3 Frequency converter with an optimised characteristic for quadratic loads and an over-load of 110%.
In order to achieve higher breakaway torque, the drive is sometimes started with a constant torque before the
quadratic characteristic is used
Sizing ad Selection of Frequency Converters
170
Sometimes motor manufacturers specify higher available torque at a lower duty cycle.
A design optimised for intermittent operation can be economical, but it requires a
more complex design as shown in Fig. 9.4 Obtaining a good match in speed selection.
% Torque
120
Quadratic load
Forced ventilation
100
80
Constant load 20 - 60 Hz
60
Constant load 5 - 70 Hz
40
20
0
0
10
20
30
40
50
60
70
Hz
Fig. 9.4 Obtaining a good match in speed selection
9.5 Derating of FC
Maximum ambient temperatures are defined for FC’s, as for all electronic units. If the
maximum ambient temperature is exceeded, it could lead to failure of the FC, but
it also reduces the life-time of the electronics. According to Arrhenius’ law, the lifetime of an electronic component is reduced by 50% for each 10°C that it is operated
above its specified temperature. If FC’s have to be operated continuously near the
maximum rated operating temperature and the specified life-time of the FC still must
be maintained, one option is to derate the power.
Load
110%
100%
80%
60%
0%
20%
0%
kHz
0
2
4
6
8
10
12
14
Fig. 9.5 Power reduction diagram for switching frequency and temperature
16
Sizing ad Selection of Frequency Converters
171
In the diagram 9.5 Power reduction diagram for switching frequency and temperature,
the switching frequency of the inverter is plotted on the X axis. The output current (in
%) of the unit is plotted on the Y axis.
Higher switching frequencies result in less irritating motor noise levels. However, the
power dissipation in the inverter increases with the switching frequency, leading to
additional heating of the unit. Reducing the switching frequency allows the switching
losses to be reduced. If the switching frequency is too low, the motor tends to run
less smoothly. The switching frequency is thus always a compromise between noise
generation, smooth running, and losses.
If, for example a unit is operated at an ambient temperature of 45°C, it can continuously
deliver 100% of its rated output current at a switching frequency of 4 kHz. If the
ambient temperature increases to 55°C, a current of only around 75% is possible in
continuous operation without a reduction of life-time. If the reduction of life-time is not
acceptable, a larger FC with sufficient power reserve must be used.
Power derating curves must be observed not only at elevated temperatures, but also at
reduced air pressures, such as when FC’s are used at elevations above 1000 metres.
9.6 Regenerative Energy
If a motor is driven by an FC that during deceleration the rotor will run faster than the
rotating magnetic field causing the motor to act as a generator.
Depending on how much energy is fed back from the motor and how often, various
measures must be taken. If the power exceeds the total power losses of the motor and
the FC, the intermediate circuit voltage will increase until, at a defined voltage, the FC
disables its output and consequently loses the control of the motor.
Motor shaft frequency
Speed
FC output frequency
Load torque
Start
Fig. 9.6 Start/Stop illustrations for regenerative principle
Stop
172
Sizing ad Selection of Frequency Converters
A simple way to avoid such an overvoltage situation is to oversize the FC which
would then be able absorb more regenerative energy and hence reduce the risk
over-voltage. However, this is often a more expensive solution compared to dynamic
braking methods, including the possibility of feeding back the energy to the supply
grid. For details please refer to the corresponding subsections in chapter 3 Frequency
Converters and Motors.
9.7 Motor Cables
The power components of FC’s are designed for specific motor cable lengths. If the
specified cable length is exceeded, malfunctions can occur and the FC could trip with
an error/alarm message. The capacitance of the cable used is partly responsible for this
behaviour. If the capacitance at the FC output exceeds a specified value, transients can
occur on the cables that can lead to a malfunction of the FC.
Most manufacturers prescribe shielded cables for their FC’s to prevent potential EMC
problems. If the user decides on other suitable measures for compliance with EMC
requirements then unshielded cables can be used. Since the unshielded cable places a
lower capacitive load on the FC, a longer cable length is possible in this case. Typically
cable lengths that can be used are 50 m / 75 m (shielded) or 150 m /300 m (unshielded).
Not using shielded motor cables can only be recommended if other measures are
taken. Even if an installation operates properly during its acceptance test without
shielded motor cables, EMC problems can occur sporadically, or as a result of
modifications or extensions to the installation. The financial expenditure then
required to eliminate such problems is usually greater than the money saved by using
unshielded cables.
When installing cables, care must be taken to avoid additional inductance resulting
from routing cables in the form of an air-core coil and additional capacitance resulting
from parallel conductors.
If several motors are connected in parallel to the output of a FC, the lengths of
the individual motor cables must be added together to determine the connected
cable length. Here it should be noted that some manufacturers specify geometrical
addition of the individual cable lengths. In such cases, daisy-chaining the motor cable
is advisable (Fig. 9.7 Total motor cable length is the sum of all connected parts).
A star formation can cause problems due to the additional capacitance between
the individual conductors.
Sizing ad Selection of Frequency Converters
173
Fig. 9.7 Total motor cable length is the sum of all connected parts
9.8 Environment
Several considerations to the environment should be taken before installing a FC. The
following factors should be checked:
• Ambient temperature
• Altitude
• Environment
• EMC
• Harmonic distortion
Minimum and maximum ambient temperature limits are specified for all FC’s. Avoiding
extreme ambient temperatures prolongs the life of the equipment and maximises
overall system reliability. If the FC is installed in environment where the ambient
temperature is higher than specified, derating of the power is needed, see Derating of
FC.
The cooling capability of air is decreased at lower air pressure. Above 1000 m derating
of FC’s should be considered.
Electronic equipment is sensitive to the environment. For instance moisture, dust and
temperature can all influence the reliability of electronics. Reduced reliability causes
downtime in the application with reduced productivity as a result. Therefore it is
important to choose the right solution for the actual application.
Basically, it is important to protect the electronics from a harsh environment. The best
way to do that is to avoid the harsh environment by placing the electronics outside the
harsh environment.
In most cases you cannot directly see how critical the environment is. It depends
mainly on 4 factors, the concentration of pollutants present, dirt, the relative humidity
174
Sizing ad Selection of Frequency Converters
and temperature. Most FC manufactures offer these solutions to minimize the effect of
the environment:
• Mount the FC’s in a central cabinet with long motor cables. In this way the FC’s are
remote from the critical environment
• Install air-conditioning in the control cabinet that ensures critical environment does
not contact the FC’s and other electronics. (Positive-pressure).
• Some FC’s are fitted with a cold plate. With this solution you can place the FC inside
a cabinet and via the cold plate the heat is transmitted to the outside. Then the FC’s
electronics are kept away from the critical environment
• Use a FC which is fitted with a sealed enclosure. FC manufacturers today offer an
enclosure ingress protection up to IP66/Nema 4X which will protect the electronics
from the outside environment and eliminates the cost of a separate enclosure
• Order the FC’s with conformal coating which will significantly improve protection
against chlorine, hydrogen sulphide, ammonia and other corrosive environments
Fig. 9.8 Printed circuit board with conformal coated
The FC is mostly used by professionals of the trade as a complex component forming
part of a larger appliance, system, or installation. Therefore note that the responsibility
for the final EMC and harmonics properties of the appliance, system or installation rests
with the installer who has to ensure compliance with the local regulations.
For details about EMC and harmonics please refer to chapter 5 Electromagnetic
Compatibility and chapter 7 Mains Interference.
Sizing ad Selection of Frequency Converters
175
9.9 Centralised versus Decentralised Installation
The most common form of installation is beyond doubt centralised installation of FC’s
in control cabinets. The advantages of centralised control cabinet technology lie, above
all, in the protected installation of the units and centralised access to them for power,
control, maintenance, and fault analysis.
With installation in the control cabinet, the primary aspect that must be taken into
account is heat management, not only of the units but also of the whole installation. As
a result of the heat dissipation in the control cabinet, additional cooling of the control
cabinet may be necessary.
Depending on the FC manufacturer’s mounting regulations, minimum distances
must be maintained above and below the unit and between the unit and adjacent
components. For better heat removal, direct mounting on the rear wall of the control
cabinet is recommended. Some manufacturers also specify minimum distances
between the individual units. It is however, preferable to mount the units side-by-side
if possible in order to utilise mounting surface area effectively.
Fig. 9.9 Recommendations for mounting of converters (centralised solution)
A disadvantage of centralised installation in some cases is the long cable lengths to the
motors. While the use of shielded cables definitely reduces the RFI effects of the motor
cable, these effects are not completely eliminated.
As an alternative to centralised installation, a decentralised approach to the lay-out of a
facility can also be chosen. Here the FC is located very close to or directly on the motor.
Motor cable lengths are thereby reduced to a minimum. In addition, decentralised
installation offers advantages in fault detection, since the relationship between the
176
Sizing ad Selection of Frequency Converters
controllers and their associated motors is easy to see. In decentralised configurations,
a field-bus is usually used to control the drives.
Fig. 9.10 Two concepts – different sets of benefits
When planning a decentralised installation, factors such as ambient temperatures,
mains voltage drops, the limited motor cable lengths, etc. must be taken into account.
Important factors such as these are often overlooked in the high-level design of
engineering projects.
For example, not only the decentralised units but also the supply cables must be
suitable for the installation environment. For instance the field-bus cable must be
suitable for a harsher environment and sometimes also of the flexible type. In addition,
installation of units in inaccessible locations should be avoided in order to ensure quick
access for servicing.
Another major consideration is the segmentation of a decentralised network. For
economic reasons it is beneficial to combine units into groups or segments. Careful
consideration must be given to determining which segments require other segments
for their operation, and which segments can, must, may, or should continue to operate
autonomously. For example, if certain chemical processes cannot be interrupted, the
failure of a lower-level segment must not be allowed to disrupt important segments.
Finally the expertise that is necessary for the installation of a decentralised network
should not be underestimated. In addition to knowledge of the field-bus systems used,
the technician must be aware of the structure (what happens to the total system if an
individual unit fails) and the ambient conditions of a decentralised network and must
be able to estimate these effects.
Although decentralised units are always more expensive than centralised units, wellconceived decentralisation concepts can achieve savings of around 25% compared
Sizing ad Selection of Frequency Converters
177
to centralised systems. The potential for savings in the installation arise from reduced
cable lengths and from using equipment modules that have already been built and
tested by the machine manufacturer or supplier.
9.10 Examples
The following examples illustrate the basic procedure for selecting a FC in the design
process. Here the data sheet reproduced below is used for the selection process. The
VLT® AutomationDrive FC 302 is selected as a FC that can operate with a 150m shielded
cable.
P11K
P15K
P18K
P22K
HO
NO
HO
NO
HO
NO
HO
NO
Output Current
Continuous (380-440 V)
[A]
24
32
32
37.5
37.5
44
44
61
Intermittent (380-440V)
[A]
38.4
35.2
51.2
41.3
60
48.4
70.4
67.1
Continuous (441-500 V)
[A]
21
27
27
34
34
40
40
52
Intermittent (441-500 V)
[A]
33.6
29.7
43.2
37.4
54.4
44
64
57.2
Continuous (400 V)
[KVA]
16.6
Continuous (460 V)
[KVA]
Typical shaft output
[kW]
11
15
18.5
22.0
30.0
Continuous (380-440 V)
[A]
22
29
34
40
55
Intermittent (380-440V)
[A]
35.2
Continuous (441-500 V)
[A]
19
Intermittent (441-500 V)
[A]
30.4
27.5
40
34.1
49.6
39.6
57.6
51.7
Estimated power loss at rated
max. load
[W]
291
392
379
465
444
525
547
739
Output Power
22.2
26
21.5
30.5
27.1
42.3
31.9
41.4
Max. Input Current
31.9
46.4
25
Max. pre-fuses
54.4
44
31
Efficiency
Max. cable size (mm²)
37.4
64
36
47
0.98
([AWG2])
[A]
Table 9.3 Data for the VLT® AutomationDrive
60.5
16 (6)
35 (2)
63
80
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Sizing ad Selection of Frequency Converters
Example 1
A 15.0 kW, 3 x 400 V motor (4-pole) is installed together with a transport system
(a screw conveyor with a break-away torque of approximately 160%). The current
consumption of the motor is 30.0A in continuous operation.
Recommended solution 1
A VLT® AutomationDrive P15K (typical for a 15 kW motor with a high constant load
torque) can supply 32 A in continuous operation and has sufficient excess load reserve
(160 % / 60 s) to enable it to be used in this application.
Example 2
A 15.0 kW, 3 x 400 V motor (4-pole) is installed together with a centrifugal pump (breakaway torque of approximately 60 %).
The current consumption of the motor is 30.0 A at its rated speed.
Recommended solution 2
A VLT® AutomationDrive P11K (typical for an 11 kW motor with a high constant load
torque) can nevertheless supply 32 A with a nominal excess load torque of 110 % / 60 s
(max.) and can therefore be used in this application. The unit also has tailored functions
for additional energy savings.
Sizing ad Selection of Frequency Converters
179
Contributors:
John Bargmeyer, Michael Burghardt, Norbert Hanigovszki, Marie Louise Hansen, Anna Hildebrand Jensen,
Johnny Wahl Jensen, Hans Seekjar, Ana-Mari Tataru-Kjar, Firuz Zare, Thomas Jansen and Martin Černý.
DKDD.PM.403.A3.02
© Copyright Danfoss | Produced by PE-MSMBM/ColorSigns | December 2014
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