Induction motors fed by PWM frequency inverters Technical guide

Induction motors fed by PWM frequency inverters Technical  guide
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Induction motors fed by
PWM frequency inverters
Technical guide
g
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Table of contents
1
Introduction...........................................................................................................................................................................................4
2
Normative Aspects..............................................................................................................................................................................5
2.1
NEMA MG1 - Motors and generators / “United States”...............................................................................................................5
2.2
NEMA - Application Guide for AC Adjustable Speed Drive Systems........................................................................................5
2.3
IEC 60034 - Rotating Electrical Machines / “Internacional”........................................................................................................5
2.4
Other technical documents of reference.........................................................................................................................................5
3
Induction machines speed variation.................................................................................................................................................5
4
Characteristics of PWM frequency inverters............................................................................................................................... 7
4.1
General...................................................................................................................................................................................................7
4.2
Control Types.......................................................................................................................................................................................8
5
Interaction between inverter and AC power line............................................................................................................................8
5.1
Harmonics.............................................................................................................................................................................................8
5.1.1 Normative considerations about the harmonics............................................................................................................................9
5.2
Line reactor / DC bus choke..............................................................................................................................................................9
6
Interaction between inverter and motor...................................................................................................................................... 10
6.1
Harmonics influencing motor performance...................................................................................................................................10
6.1.1 Normative considerations about the inverter output harmonics.......................................................................................... 10
6.2
Considerations regarding energy efficiency..................................................................................................................................11
6.2.1 The influence of the speed variation on the motor efficiency....................................................................................................12
6.2.2 Normative considerations about the efficiency of inverter fed motors................................................................................ 12
6.3
Influence of the inverter on the temperature rise of the windings......................................................................................... 13
6.4
Criteria regarding the temperature rise of WEG motors on VSD applications.......................................................................13
6.4.1 Torque derating...................................................................................................................................................................................13
6.4.2 Breakaway torque..............................................................................................................................................................................14
6.4.3 Breakdown torque.............................................................................................................................................................................15
6.5
Influence of the inverter on the insulation system.................................................................................................................... 15
6.5.1 Rise Time............................................................................................................................................................................................15
6.5.2 Cable length........................................................................................................................................................................................16
6.5.3 Minimum time between successive pulses (MTBP)....................................................................................................................17
6.5.4 Switching frequency (fs)....................................................................................................................................................................18
6.5.5 Multiple motors...................................................................................................................................................................................18
6.6
Criteria regarding the insulation system of WEG motors on VSD applications......................................................................18
6.7
Normative considerations about the insulation system of inverter fed motors.................................................................. 18
6.8
Recommendations for the cables connecting WEG motors to inverters............................................................................ 19
6.8.1 Cable types and installation recommendations..........................................................................................................................20
6.9
Influence of the inverter on the motor shaft voltage and bearing currents......................................................................... 20
6.9.1 Common mode voltage.....................................................................................................................................................................21
6.9.2 Equivalent circuit of the motor for the high frequency capacitive currents............................................................................21
6.9.3 Methods to reduce (or mitigate) the bearings currents in inverter fed motors................................................................... 22
6.10 Criteria regarding protection against bearing currents (shaft voltage) of WEG motors on VSD applications.................23
6.11 Normative considerations about the current flowing through the bearings of inverter fed motors............................... 23
6.12 Influence of the inverter on the motor acoustic noise............................................................................................................. 23
6.13 Criteria regarding the noise emitted by WEG motors on VSD applications..........................................................................23
6.14 Normative considerations about the noise of inverter fed motors....................................................................................... 24
2
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6.15 Influence of the inverter on the mechanical vibration of the motor....................................................................................... 24
6.16 Criteria regarding the vibration levels presented by WEG motors on VSD applications......................................................24
6.17 Normative considerations about mechanical vibration of inverter fed motors................................................................... 24
7
Interaction between motor and driven load...................................................................................................................................25
7.1
Load types......................................................................................................................................................................................... 25
7.1.1 Variable torque loads........................................................................................................................................................................25
7.1.2 Constant torque loads......................................................................................................................................................................25
7.1.3 Constant horsepower loads............................................................................................................................................................26
7.2
Speed duties.......................................................................................................................................................................................26
7.2.1 Variable speed duty...........................................................................................................................................................................26
7.2.2 Continuous speed duty....................................................................................................................................................................26
8
Dimensioning and analysis of actual drive system applications – Practical examples........................................................26
8.1
Constant torque application - compressor...................................................................................................................................26
8.1.1 Example..............................................................................................................................................................................................26
8.1.2 Solution................................................................................................................................................................................................26
8.2
Squared torque application - centrifugal pump..........................................................................................................................27
8.2.1 Example...............................................................................................................................................................................................27
8.2.2 Solution................................................................................................................................................................................................27
8.3
Special application - long cable......................................................................................................................................................29
8.3.1 Example..............................................................................................................................................................................................29
8.3.2 Solution................................................................................................................................................................................................29
8.4
Variable torque / variable speed application - textile industry..................................................................................................30
8.4.1 Example...............................................................................................................................................................................................30
8.4.2 Solution................................................................................................................................................................................................31
8.5
Example considering the use of WEG Optimal Flux...................................................................................................................32
8.5.1 Example...............................................................................................................................................................................................32
8.5.2 Solution................................................................................................................................................................................................32
9
Recommendations for the measurement of PWM waveforms................................................................................................32
9.1
Warning................................................................................................................................................................................................32
9.2
Instrumentation..................................................................................................................................................................................32
9.3
Parameter measurements.............................................................................................................................................................. 33
9.4
Grounding considerations................................................................................................................................................................33
9.4.1 Grounding of control.........................................................................................................................................................................33
9.4.2 Grounding of motor...........................................................................................................................................................................33
9.5
Measurement procedures...............................................................................................................................................................33
9.5.1 Waveform visualization.....................................................................................................................................................................33
9.5.2 Oscilloscope scale setting.............................................................................................................................................................. 33
9.5.3 Triggering............................................................................................................................................................................................33
10
Conclusion..........................................................................................................................................................................................34
11Bibliography.........................................................................................................................................................................................35
Technical guide – Induction motors fed by PWM frequency inverters
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1 Introduction
The number of industry applications in which induction
motors are fed by static frequency inverters is growing fast
and, although much has already been done within this field,
there is still a lot to be studied/understood regarding such
applications. The advance of variable speed drives systems
engineering increasingly leads to the need of specific
technical guidance provision by electrical machines and
drives manufacturers, so that such applications can be
suitably designed in order to present actual advantages in
terms of both energy efficiency and costs.
This technical guide aims to clarify the main aspects
concerning applications of low voltage (≤ 690 V) induction
motors with static frequency inverters supply, for frames ≤
IEC 355 (NEMA 587), in a didactic and concise approach.
First of all the principal and most broadly followed
international standards about the subject are mentioned.
Then the theoretical basis of speed variation on induction
machines by means of indirect static inverters is presented,
as well as the fundamental characteristics of electronic
inverters.
Once the basics of adjustable speed drives are known, the
behavior of the whole power system is analyzed. Each
component of the power system (AC power line - frequency
inverter - induction motor - load) is focused, as well as the
overall interactions between them, resulting from speed
variation. In this manner the whole drive system can be well
understood.
At last examples of VSD systems designs are presented, for a
better understanding of the matters exposed throughout the
document.
Always looking out for a technical elucidation as complete as
possible along this guide, some controversial points are
emphasized. Divergences existing among distinct
standardization organisms are discussed and WEG’s point of
view is explained.
4
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2 Normative Aspects
2.1 NEMA MG1 - Motors and generators / “United
States”
2.4 Other technical documents of reference
GAMBICA/REMA Technical Guides for Variable Speed
Drives and Motors
g
GAMBICA/REMA Technical Reports for Variable Speed
Drives and Motors
gCSA C22.2 No.100-2004 Item 12 (Canada) “Motors and
Generators – Industrial Products”
gJEM-TR 148-1986 (Japan) “Application guide for inverter
drive (general-purpose inverter)”
g
IEC 60034-18-41 – Qualification and design tests for Type I
electrical insulation systems used in rotating electrical
machines fed from voltage inverters
g
Papers and books related to this subject
g
Parte 30 - Application considerations for constant speed
motors used on a sinusoidal bus with harmonic content
and general purpose motors used with adjustablefrequency controls or both (2006)
g
Parte 31 - Definite-purpose inverter-fed polyphase motor
(2006)
g
2.2 NEMA - Application Guide for AC Adjustable
Speed Drive Systems (2001)
2.3 IEC 60034 - Rotating Electrical Machines /
“International”
Parte 17 - Cage induction motors when fed from inverters
– application guide (2006)
g
Parte 25 - Guide for the design and performance of cage
induction motors specifically designed for inverter supply
(2007)
g
3 Induction machines speed variation
For an induction motor, rotor speed, frequency of the voltage
source, number of poles and slip are interrelated according
to the following equation:
n = 120 f1 (1-s)
p
Speed variation
Parameter
Number of poles
Application characteristics
Discrete variation
Oversizing
Continuous variation
Slip
Rotor losses
Limited frequency range
where:
n : mechanical speed (rpm)
ƒ1 : fundamental frequency of the input voltage (Hz)
p : number of poles
s : slip
Voltage frequency
Continuous variation
Utilization of STATIC FREQUENCY Inverters!
The analysis of the formula above shows that the mechanical
speed of an induction motor is a function of three
parameters. Thus the change of any of those parameters will
cause the motor speed to vary as per the table below.
Technical guide – Induction motors fed by PWM frequency inverters
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The utilization of static frequency inverters comprehends
currently the most efficient method to control the speed of
induction motors. Inverters transform a constant frequencyconstant amplitude voltage into a variable (controllable)
frequency-variable (controllable) amplitude voltage. The
variation of the power frequency supplied to the motor leads
to the variation of the rotating field speed, which modifies the
mechanical speed of the machine.
inverter fed induction motor is illustrated below.
The torque developed by the induction motor follows the
equation below:
T = k1 . fm . I2
Despising the voltage drop caused by the stator impedance,
the magnetizing flux is found to be:
f m= k 2 .
V1
f1
where:
T : torque available on the shaft (N.m)
fm : magnetizing flux (Wb)
I2 : rotor current (A) à depends on the load!
V1 : stator voltage (V)
k1 e k2 : constants à depend on the material and on the
machine design!
Considering a constant torque load and admitting that the
current depends on load (therefore practically constant
current), then varying proportionally amplitude and frequency
of the voltage supplied to the motor results in constant flux
and therefore constant torque while the current remains
unchanged. So the motor provides continuous adjustments
of speed and torque with regard to the mechanical load.
Losses can be thus minimized in accordance with the load
conditions by keeping the slip constant at any speed, for a
given load.
The curves below are obtained from the equations above.
The ratio V1/f1 is kept constant up to the motor base (rated)
frequency. From this frequency upwards the voltage is kept
constant at its base (rated) value, while the frequency applied
on the stator windings keeps growing, as shown next.
Thereby the region above the base frequency is referred to as
field weakening, in which the flux decreases as a result of
frequency increase, causing the motor torque to decrease
gradually. The typical torque versus speed curve of an
6
It comes out that torque is kept constant up to the base
frequency and beyond this point it falls down (weakening
field). Since the output is proportional to torque times
speed, it grows linearly up to the base frequency and from
that point upwards it is kept constant. This is summarized
by the graph beside.
The number of variable speed applications controlled by
means of a frequency inverter has increased significantly over
the recent years. This may be explained by the many benefits
provided by such applications:
g
Aloof control – the control can be installed remotely at a
suitable location, keeping just the motor in the processing
area – on the contrary of hydraulic and mechanical varying
speed systems.
g
Aloof control – the control can be installed remotely at a
suitable location, keeping just the motor in the processing
area – on the contrary of hydraulic and mechanical varying
speed systems.
g
Cost reduction – direct on line startings of induction motors
cause current peaks that harm the motor as well as other
electric equipments linked to the electrical system. Static
frequency inverters provide softer startings, resulting in
cost reduction with regard to maintenance.
g
Gain of productivity – industrial systems are often oversized
due to an expectation of future production increase. Static
inverters allow the proper regulation of the operational
speed according to the equipments available and the
production needs.
g
Energy Efficiency – the power system global efficiency
depends not only on the motor, but also on the control.
Static inverters are high efficiency apparatuses, reaching
typically 97% or more. Induction motors also present high
efficiency levels, reaching up to 95% or even more in larger
Technical guide – Induction motors fed by PWM frequency inverters
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machines operating at rated conditions. When speed
variation is required, the output changes in an optimized
way, directly affecting the energy consumption and leading
to high efficiency levels performed by the system (inverter +
motor).
g
Versatility – static frequency inverters suit both variable and
constant torque loads. With variable torque loads (low
torque demand at low speeds) the motor voltage is
decreased to compensate for the efficiency reduction
normally resultant from load reduction. With constant
torque (or constant power) loads the system efficiency
improvement comes from the feasibility of continuous
adjustment of speed, with no need to use multiple motors
or mechanical variable speed systems (such as pulleys and
gears), which introduce additional losses.
High quality – the accurate speed control obtained with
inverters results in process optimization, providing a final
product of better quality.
Aloof control – the control can be installed remotely at a suitable location, keeping just the motor in the processing area – on the contrary of hydraulic and mechanical varying speed systems.
g
4 Characteristics of PWM frequency inverters
4.1 General
PWM voltage source static frequency inverters presently
comprehend the most used equipments to feed low voltage
industrial motors in applications that involve speed variation.
They work as an interface between the energy source (AC
power line) and the induction motor.
In order to obtain an output signal of desired voltage and
frequency, the input signal must accomplish three stages
within a frequency inverter:
Diode bridge - Rectification of the AC input voltage -
g
constant amplitude and frequency - coming from the
power grid;
DC link or filter - Regulation/smoothing of the rectified
signal with energy storage through a capacitor bank;
g
IGBT power transistors – Inversion of the voltage coming
from the link DC into an alternate signal of variable
amplitude and frequency.
g
The following diagram depicts the three stages of an indirect
frequency inverter.
V PWM
Vin
DC
AC
AC
Rectifier
Filter
Input:
50/60 Hz (1 f or 3 f)
VDC  1.35 Vin or 1.41 Vin
Inverter
3f
Motor
| motor
Output:
Variable voltage
and frequency
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NOTES:
g
Under light load (or at no load) conditions, the DC link
voltage tends to stabilize at
2 Vrede @ 1,41 Vrede
However, when the motor drives heavier loads (for instance, at full load), the DC link voltage tends to the value
(3/P )
g
2 Vrede @ 1,35 Vrede
The criteria used to define the insulation system of WEG
motors fed by inverters, presented further on, consider the
highest of those values (1.41Vin), which is more critical to
the motor. In this way WEG motors attend both situations
satisfactorily.
compensate for the voltage drop due to the stator
resistance), so that the torque capacity of the motor is
maintained. This is the most used control type owing to its
simplicity and also to the fact that the majority of applications
do not require high precision or fast responses of the speed
control.
The vector control enables fast responses and high level of
precision on the motor speed and torque control. Essentially
the motor current is decoupled into two vectors, one to
produce the magnetizing flux and the other to produce
torque, each of them regulated separately. It can be open
loop (sensorless) or closed loop (feedback).
Speed feedback – a speed sensor (for instance, an
incremental encoder) is required on the motor. This control
mode provides great accuracy on both torque and speed
of the motor even at very low (and zero) speeds.
g
4.2 Control Types
There are basically two inverter control types: scalar (open
loop) and vector (open or closed loop).
The scalar control is based on the original concept of a
frequency inverter: a signal of certain voltage/frequency ratio
is imposed onto the motor terminals and this ratio is kept
constant throughout a frequency range, in order to keep the
magnetizing flux of the motor practically unchanged. It is
generally applied when there is no need of fast responses to
torque and speed commands and is particularly interesting
when there are multiple motors connected to a single drive.
The control is open loop and the speed precision obtained is
a function of the motor slip, which depends on the load,
since the frequency is imposed on the stator windings. In
order to improve the performance of the motor at low
speeds, some drives make use of special functions such as
slip compensation (attenuation of the speed variation as
function of load) and torque boost (increase of the V/f ratio to
Sensorless – simpler than the closed loop control, but its
action is limited particularly at very low speeds. At higher
speeds this control mode is practically as good as the
feedback vector control.
g
The main difference between the two control types is that the
scalar control considers only the magnitudes of the
instantaneous electrical quantities (magnetic flux, current and
voltage) referred to the stator, with equations based on the
equivalent electrical circuit of the motor, that is, steady state
equations. On the other hand, the vector control considers
the instantaneous electrical quantities referred to the rotor
linkage flux as vectors and its equations are based on the
spatial dynamic model of the motor. The induction motor is
seen by the vector control as a DC motor, with torque and
flux separately controlled.
5 Interaction between inverter and AC power line
8
THD =
8
The power system harmonic distortion can be quantified by
the THD (Total Harmonic Distortion), which is informed by the
inverter manufacturer and is defined as:
å
h=2
æ An æ2
æ A
1
æ
5.1 Harmonics
For the AC power line, the system (frequency inverter +
motor) is a non-linear load whose current include harmonics
(frequency components multiples of the power line
frequency). The characteristic harmonics generally produced
by the rectifier are considered to be of order h = np1 on the
AC side, that is, on the power line (p is the number of pulses
of the inverter and n =1,2,3). Thus, in the case of a 6 diode (6
pulses) bridge, the most pronounced generated harmonics
are the 5th and the 7th ones, whose magnitudes may vary
from 10% to 40% of the fundamental component, depending
on the power line impedance. In the case of rectifying
bridges of 12 pulses (12 diodes), the most harmful harmonics
generated are the 11th and the 13th ones. The higher the
order of the harmonic, the lower can be considered its
magnitude, so higher order harmonics can be filtered more
easily. As the majority of drives manufacturers, WEG
produces its low voltage standard inverters with 6-pulse
rectifiers.
where:
Ah are the rms values of the non-fundamental harmonic
components
A1 is the rms value of the fundamental component
The waveform above is the input measured current of a
6-pulse PWM inverter connected to a low impedance power
grid.
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5.1.1 Normative considerations about the harmonics
The NEMA Application Guide for AC ASD Systems refers to
IEEE Std.519 (1992), which recommends maximum THD
levels for power systems  69 kV as per the tables presented
next. This standard defines final installation values, so that
each case deserves a particular evaluation. Data like the
power line short-circuit impedance, points of common
connection (PCC) of inverter and other loads, among others,
influence on the recommended values.
a result of the decrease of both the rms current of the
rectifying diodes and the current ripple through the middle
circuit capacitors.
Converter input current
Voltage harmonics
Even components
3,0%
Odd components
3,0%
THD voltage
5,0%
(a)
Converter input voltage
The maximum harmonic current distortion recommended by
IEEE-519 is given in terms of TDD (Total Demand Distortion)
and depends on the ratio (ISC / IL), where:
ISC = maximum short-current current at PCC.
IL = maximum demand load current (fundamental frequency
component) at PCC.
Individual Odd Harmonics
(Even harmonics are limited to 25% of the odd harmonic limits)
Maximum harmonic current distortion in percent of IL
ISC / IL
< 11
11 ≤ h ≤
17 ≤ h ≤
23 ≤ h ≤
17
23
35
(b)
35 ≤ h
TDD
< 20*
4.0
2.0
1.5
0.6
0.3
5.0
20 < 50
7.0
3.5
2.5
1.0
0.5
8.0
50 < 100
10.0
4.5
4.0
1.5
0.7
12.0
100 < 1000
12.0
5.5
5.0
2.0
1.0
15.0
> 1000
15.0
7.0
6.0
2.5
1.4
20.0
* All power generation equipment is limited to these values of current distortion,
regardless of actual ISC / IL.
The documents mentioned from IEC, however, do not set
limits for the harmonic distortion injected by inverters into the
power line.
5.2 Line reactor / DC bus choke
Harmonic currents, which circulate through the power line
impedances and depend on the rectifier input/output
impedance values, cause harmonic voltage drops that distort
the power supply voltage of the inverter and other loads
connected to this line. These harmonic current and voltage
distortions may increase the electrical losses in the
installation, lowering the power factor and overheating
components such as cables, transformers, capacitor banks,
motors, etc.
The addition of a line reactor and/or a DC bus choke reduces
the harmonic content of the current and increase the power
factor. The DC bus choke has the advantage of not
introducing a motor voltage drop but depending on the
combination of its value with the power line impedance and
the DC link capacitance values it may result in undesirable
resonances within the overall system. On the other hand, the
line reactor decreases the medium voltage of the
intermediate circuit but attenuates more effectively power
supply voltage transients. Besides that, it extends the
semiconductors and the DC link capacitor bank lifetimes, as
(a)
(b)
Current and voltage waveforms with (b) and without (a) line
reactor. It can be seen that line reactors soften the peaks,
thus reducing the harmonic content and the rms value of the
input current. Additionally, diminution of the supply voltage
waveform distortion is thereby caused.
A minimum line impedance that introduces a voltage drop
from 1 to 2%, depending on the inverter size, is
recommended in order to ensure the inverter lifetime.
As rule of thumb, it is recommended to add a line reactor to
the existing power supply impedance (including transformers
and cables) so that a total voltage drop of 2 to 4% is
achieved. This practice is considered to result in a good
compromise between motor voltage drop, power factor
improvement and harmonic current distortion reduction.
The value of the line reactor needed for the desired voltage
drop to be obtained can be calculated as follows:
L=
(voltage drop)%. Vline
[ H]
3.2.p.fline .Irated
The (a) line reactor and (b) DC bus choke electrical
installations are shown next.
(a) Input line reactor connection
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There are basically the following solutions to mitigate the
harmonics generated by a PWM frequency inverter:
Methods of reduction of harmonics
Solution characteristics
Installation of output passive filters
Restrictions for vector control operation
(L, LC (sinusoidal), dV/dt)
Voltage drop (motor horsepower
Installation costs increase
reduction)
Costs increase
Use of multi-level inverters
Inverter reliability decrease
Control complexity increase
Space Vector Modulation (SVM)*
Pulse Width Modulation quality
Do not increase costs
improvement (optimization of pulse
Voltage control upgrade
patterns)
Higher system (inverter + motor)
(b) DC bus choke connection
efficiency
Inverter efficiency decrease (higher
6 Interaction between inverter and
motor
6.1 Harmonics influencing motor performance
The induction motor, when under PWM voltage coming from
the inverter, is subjected to voltage harmonics (frequency
components above the fundamental frequency). Depending
on the type of PWM employed, the switching frequency and
other peculiarities of the control, the motor may present
efficiency decrease and losses, temperature, noise and
vibration levels increase.
Furthermore other effects may appear when induction
motors are fed by inverters. Insulation system dielectric stress
and shaft voltages allied with potentially damaging bearing
currents are well known side effects. Although not produced
specifically by harmonics but by other matters that will soon
be approached, these are important effects and should not
be neglected. The motor current and voltage waveforms
when under PWM supply are illustrated below.
PWM voltage at the
inverter output
Switching frequency increase
increase
* All frequency inverters manufactured by WEG employ Space Vector Modulation.
6.1.1 Normative considerations about the inverter
output harmonics
There is no international standardization defining maximum
acceptable values for voltage and current harmonic
distortion. However, the international standards do consider
the increase of motor losses due to the non-sinusoidal
supply.
IEC 60034-17 provides an example of motor losses increase
owing to PWM supply. Motor info: 315 IEC frame, rated
torque and speed values.
Inverter fed motor current
Then the motor fed by frequency inverter sees a pulsating
(PWM) voltage and a practically sinusoidal current, so that
the voltage harmonics generally present higher magnitudes
than the current harmonics.
10
switching losses)
- Common mode leakage current flow
Technical guide – Induction motors fed by PWM frequency inverters
HVF =
å
n=5
æ Vn æ2
æ n
æ
8
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Where:
n: order of the odd harmonic, not including those divisible by
three
Vn: per unit magnitude of the voltage at the nth harmonic
frequency
Losses caused by fundamental frequency
A – Stator winding losses
B – Rotor winding losses
C – Iron losses
D – Additional load losses
E – Frictional losses
Losses caused by harmonics
F – Stator winding losses
G – Rotor winding losses
H – Iron losses
I – Additional load losses
J – Commutation losses
NOTE: frame 315 (IEC) motor operating at rated speed and
torque.
6.2 Considerations regarding energy efficiency
The lack of international standards that specify test
procedures to evaluate the system (motor + inverter)
efficiency allows such tests to be carried out in many different
and non contestable ways. Therefore, the results obtained
should not influence the acceptance (or not) of the motor,
except under mutual accordance between customer and
manufacturer. Experience shows the effectiveness of the
considerations below.
An induction motor fed by PWM voltage presents a lower
efficiency level than when fed by purely sinusoidal voltage,
due to the losses increase caused by harmonics;
g
IEC 60034-25 illustrates the motor losses increase due to
PWM supply by means of the following curves:
Anyway, when induction motors are fed by static inverters,
the efficiency of the overall system, rather than the motor
efficiency only, should be evaluated;
g
Each case must be properly analyzed, taking into account
characteristics of both the motor and the inverter, such as:
operating frequency, switching frequency, speed range,
load conditions and motor power, THD, etc.
g
The measuring instrumentation is extremely important for
the correct evaluation of electrical quantities on systems
under PWM duty. True RMS meters must be used, in order
to permit reliable measurements of power;
g
Higher switching frequencies increase the motor efficiency
and decrease the inverter efficiency (due to the increase of
commutation losses).
g
NEMA MG1 – Part 30 considers a derating factor (torque
reduction) to avoid excessive overheating of a general
purpose motor fed by converter, compensating for the
circulation of harmonic currents due to the PWM voltage
harmonic content:
High efficiency motors keep their efficiency higher,
compared to standard motors, when both are fed by
inverters.
g
Technical guide – Induction motors fed by PWM frequency inverters
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6.2.1 The influence of the speed variation on the motor
efficiency
The effects of speed variation on the motor efficiency can be
understood from the analysis of the behavior of the inverter
fed motor output power as a function of its operation speed.
6.2.2.1 Numerical example
conv = P2 /P1
conv = P3 /P2
}
sist = Pout /Pin = P3 /P1 = conv .mot
Some practical values found by means of the input-output
measurement method are shown below for standard motors:
Motor 75 HP (55 kW) – 6 poles – 400 V – 50 Hz
Supposing, for instance, a 60 Hz frequency base for the
situations outlined above:
P60Hz = Pu
P30Hz = Pu = 0,5 Pu
60
30
Considering that the motor losses are essentially comprised
of Joule losses (PJ) and iron losses (PI) and assuming that the
Joule losses prevail, then the motor efficiency fall at low
speeds, where the motor output power is reduced and,
despite the slight decrease of the iron losses (frequency
dependant), the Joule losses (current square dependant) are
kept nearly constant for a constant torque load, so that after
all there is no significant variation of the overall losses.
Motor 15 HP (11 kW) – 4 poles – 400 V – 50 Hz
The equations next explain that. Defining efficiency as:
h%=
Pout
Pin
=
Pout
Pout + å Losses
And, according to the exposed above,
å Losses @ PJ + Piron
(PJ > Piron )
6.2.2 Normative considerations about the efficiency of
inverter fed motors
Then the following situation results from speed reduction:
}
¯Piron + PJ @ constant (PJ >> Piron ) Þå Losses @ constant ¯ h % ¯
¯ Pout
12
NEMA MG1 Part 30 – Efficiency will be reduced when a
motor is operated on a bus with harmonic content. The
harmonics present will increase the electrical losses which,
in turn, decrease efficiency. This increase in losses will also
result in an increase in motor temperature, which further
reduces efficiency.
g Technical guide – Induction motors fed by PWM frequency inverters
www.weg.net
NEMA MG1 Part 31 – Performance tests, when required,
shall be conducted on a sinusoidal power supply unless
otherwise specified by mutual agreement between the
manufacturer and the user.
g NEMA Application Guide for AC ASD Systems – The overall
efficiency of an ASD is based on the total losses of the
control, the motor, and any auxiliary equipment. (...) The
motor efficiency when operated on a control is slightly less
than when operated on sinewave power. Overall system
efficiency is often increased when used an ASD. Traditional
methods of changing speed such as gears or belts
introduce additional losses which reduce efficiency.
g IEC 60034-17 – The performance characteristics and
operating data for drives with inverter-fed cage induction
motors are influenced by the complete system, comprising
supply system, inverter, induction motor, mechanical
shafting and control equipment. Each of these components
exists in numerous technical types. Any values quoted in
this technical specification are thus indicative only. (...)
There is no simple method to calculate the additional
losses and no general statement can be made about their
value. Their dependence upon the different physical
quantities is very complex. Also there is a great variety both
of inverters and of motors.
Therefore, when operating with frequency inverters, both the
effects mentioned above must be considered. There are
basically the following solutions to avoid excessive
overheating of the inverter fed motor:
Torque derating (oversizing of the self ventilated motor
frame);
g
Utilization of independent cooling system (separate
ventilation);
g
Utilization of the “Optimal Flux Solution” (exclusive to
applications using WEG drives and motors).
g
g IEC 60034-25 – The recommended methods to determine
the motor efficiency are given in IEC 60034-2 (summationof-losses method for motors > 150 kW and input-output
measurement for motors ≤ 150 kW). The no-load losses
(including the additional losses) should be measured at the
same pulse pattern and pulse frequency that the inverter
will produce at rated load. The determination of the overall
efficiency of the system (motor + inverter) by means of
input-output measurement for motors > 150 kW is also
applicable under agreement between manufacturer and
user. In this case, however, the motor efficiency shall not be
determined separately.
g 6.4 Criteria regarding the temperature rise of WEG
motors on VSD applications
6.4.1 Torque derating
In order to keep the temperature rise of WEG motors, when
under PWM supply, within acceptable levels, the following
loadability limits must be attended (observe the motor line
and the flux condition).
NOTE: Applications with motors rated for use in hazardous
areas must be particularly evaluated - in such case please
contact WEG.
6.4.1.1 NEMA market
TEFC W21 and W22 (High Efficiency) motors
Frame Size
143 –
587(***)
6.3 Influence of the inverter on the temperature rise of
the windings
Induction motors may heat up more when fed by frequency
inverter than when fed by sinusoidal supply. This higher
temperature rise results from the motor losses growth owing
to the high frequency components of the PWM signal and the
often reduced heat transfer resulting from speed variation.
587(****)
Variable
Constant
Torque
Torque
Power
12:1
1000:1
60 – 120 Hz
Any
100:1(*)
-
60 – 120 Hz
WEG(**)
4:1
1000:1
60 – 120 Hz
Any
10:1
-
60 – 120 Hz
WEG(**)
Drive
Comments
Constant
flux
Optimal flux
Constant
flux
Optimal flux
TEFC NEMA PREMIUM EFFICIENCY motors
Frame Size
143 –
587(***)
The voltage harmonic distortion contributes to increase the
motor losses, once that creates minor hysteretic loops in the
lamination steel, increasing the effective saturation of the
magnetic core and giving rise to high frequency harmonic
currents, which bring about additional Joule losses.
Nevertheless, these high frequency components do not
contribute to the production of torque at steady operation of
the motor, since they do not increase the airgap fundamental
flux, which rotates at synchronous speed. The operation at
low speeds causes the ventilation over the (self-ventilated)
motor frame to decrease, consequently lowering the motor
cooling and raising in this way the thermal stabilization
temperature.
Constant
587(****)
Constant
Variable
Constant
Torque
Torque
Power
20:1
1000:1
60 – 120 Hz
Any
1000:1(*)
-
60 – 120 Hz
WEG(**)
6:1
1000:1
60 – 120 Hz
Any
12:1
-
60 – 120 Hz
WEG(**)
Drive
Comments
Constant
flux
Optimal flux
Constant
flux
Optimal flux
(*)Satisfactory motor performance depends on proper drive setup – please contact WEG
(**)WEG drive CFW-09 version 2.40 or higher, operating in sensorless (open loop) vector
mode
(***)Motors with rated power  250 hp. Criteria also valid for motors of the frame sizes 447
and 449
(****)Motors with rated power > 250 hp. Criteria also valid for motors of the frame sizes 447
and 449
Technical guide – Induction motors fed by PWM frequency inverters
13
www.weg.net
NOTE:
1.The speed ranges stated above are related to the motor
thermal capability only. Speed regulation will depend on VFD
mode of operation and proper adjustment.
2.W21 and NEMA PREMIUM EFFICIENCY WEG MOTORS of
all frame sizes can also be blower cooled under request. In
such case, the motor will be suitable for variable and
constant torque applications rated up to 1000:1 with any
drive.
3.W21 and NEMA PREMIUM EFFICIENCY WEG MOTORS
comply with those maximum safe operating speeds
established in NEMA MG1 Parts 30 and 31 (2003).
The relations set above describe operation speed ranges.
Supposing for instance a 60 Hz base frequency, the following
equivalence is valid:
Relation
Frequency range
4:1
15 – 60 Hz
10:1
6 – 60 Hz
12:1
5 – 60 Hz
20:1
3 – 60 Hz
100:1
0,6 – 60 Hz
1000:1
0,06 – 60 Hz
6.4.1.2 IEC market
Constant flux condition:
Encompassed motor lines: Totally enclosed off-the-shelf
motors attending IE1 (as per IEC 60034-30) or higher
efficiency levels.
losses (heat sources) by means of the optimization of its
magnetic flux, parameter controlled by the CFW09. From the
study of the composition of the overall motor losses and their
relation with the frequency, the magnetic flux and the current,
as well as the influence of the ventilation system on the motor
temperature rise, it was found an optimal flux value for each
frequency, allowing for a continuous minimization of the
overall motor losses through the whole speed range. The
solution obtained was implemented within the CFW09, in
order that the motor magnetic flux optimal condition can be
achieved automatically by the drive, sufficing for that a simple
adjustment of the inverter properly made.
The motor iron losses strongly depend on the frequency. As
the operation frequency is varied downwards, the iron losses
are gradually reduced. Therefore it is interesting at low speed
operation to increase the magnetic induction (flux density) of
the motor, so that the torque can be kept constant with a
reduced current, which causes reduced Joule losses. Thus
as the speed falls, it is possible to reduce the voltage
proportionally less than the frequency, resulting in an optimal
V/Hz ratio (greater than the rated value), which minimizes the
motor losses altogether. It is considered thereby that the
major motor losses occur due to Joule effect on the
windings.
This solution was especially conceived for low speed
applications with constant torque loads and must be used in
no way with variable torque loads or above the motor base
frequency. Besides, the Optimal Flux WEG solution is
applicable only when:
the motor is fed by WEG inverter (CFW09) version 2.40 or
higher;
g
sensorless vector type control is used.
g
Optimal flux condition:
Encompassed motor lines: Totally enclosed off-the-shelf
motors attending IE2 (as per IEC 60034-30) or higher
efficiency levels.
The patented WEG “Optimal Flux” solution was developed for
the purpose of making WEG induction motors able to operate
at low speeds with constant torque loads still keeping an
acceptable temperature rise level, without the need of neither
oversizing the machine nor blower cooling it.
6.4.2 Breakaway torque
According to NEMA MG1 Parts 30 and 31, the motor should
be capable of producing a breakaway torque of at least
140% of rated torque requiring not more than 150% rated
current. WEG motors when fed by inverters attend such
recommendation.
It is based on the continuous minimization of the motor
14
Technical guide – Induction motors fed by PWM frequency inverters
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6.4.3 Breakdown torque
Above base speed the motor voltage must be kept constant
for constant power operation, as already shown. NEMA MG1
Part 31 prescribes that the breakdown torque at any frequency within the defined frequency range shall be not less than
150% of the rated torque at that frequency when rated voltage for that frequency is applied. WEG motors when fed by
inverters satisfy such criterion up to 90 Hz.
Then the inverter fed motor is subjected to extremely high dV/
dt rates, so that the first turn of the first coil of a single phase
is submitted to a high voltage level. Therefore variable speed
drives can considerably increase the voltage stress within a
motor coil, though owing to the inductive and capacitive
characteristics of the windings, the pulses are damped on
the subsequent coils.
The maximum torque capability of the motor (breakdown
torque) limits the maximum operating speed in which constant power operation is possible. Attending NEMA recommendations, one can approximately find this limit from the following equation:
æ
æ
RPMmax = 2 Tmax RPMbase
3 æ Tbase
æ
6.5 Influence of the inverter on the insulation system
The evolution of the power semiconductors have led to the
creation of more efficient, but also faster, electronic switches.
The high switching frequencies of the IGBT transistors
employed in modern frequency inverters bring about some
undesirable effects, such as the increase of electromagnetic
emission and the possibility of voltage peaks, as well as high
dV/dt ratios (time derivative of the voltage, that is, rate of
electrical potential rise), occurrence at the inverter fed motor
terminals. Depending on the control characteristics (gate
resistors, capacitors, command voltages, etc.) and the PWM
adopted, when squirrel cage induction motors are fed by
frequency inverters, those pulses combined with the
impedances of both the cable and the motor may cause
repetitive overvoltages on the motor terminals. This pulse
train may degrade the motor insulation system and may
hence reduce the motor lifetime.
The cable and the motor can be considered a resonant
circuit, which is excited by the inverter rectangular pulses.
When the values of R, L and C are such that the peak voltage
exceeds the supply voltage (VDC  1.41 Vin), the circuit
response to this excitation is a so called “overshoot”. The
overshoots affect especially the interturn insulation of random
windings and depend on several factors: rise time of the
voltage pulse, cable length and type, minimum time
between successive pulses, switching frequency and
multimotor operation.
6.5.1 Rise Time
The PWM voltage takes some time to rise from its minimum
to its maximum value. This period is often called “rise time”.
Due to the great rapidity of switching on the inverter stage,
the growth of the voltage wavefront takes place too fast and,
with the power electronics advance, these transition times
tend to be more and more reduced.
So the rise time (tr) has a direct influence on the insulation life,
because the faster the pulse wavefront grows, the greater the
dV/dt ratio over the first coil and the higher the levels of
voltage between turns, causing the insulation system to wear
more quickly away. Thus the motor insulation system should
present superior dielectric characteristics in order to stand
the elevated voltage gradients occurring on PWM
environment.
6.5.1.1 Normative considerations about rise time
The definitions of rise time (tr) according to NEMA and to IEC
Standards differ, as shown below, allowing for interpretation
divergences and conflicts between manufacturers and users
of motors and drives.
NEMA MG1 Part 30
tr: time needed for the voltage to rise from 10 to 90% of the
DC link voltage (1.41Vrated)
Technical guide – Induction motors fed by PWM frequency inverters
15
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NEMA definition of dV/dt
Supposing the motor voltage Vrated = 460 V
Vlink DC  1,41 x 460 = 648,6 V
V = 0,8 x 648,6 = 518,9 V
Assuming that rise time = 0,1s
t = 0,1s
[ [
dV
V
= DV = 518,9 = 5189
dt
Dt
0,1
s
IEC 60034-25
tr: time needed for the voltage to rise from 10% to 90% of the
peak voltage at motor terminals
IEC definition of dV/dt
According to NEMA criterion the DC link voltage ( 1.41 Vin)
must be taken as 100% voltage reference for the
determination of rise time and the calculation of dV/dt.
According to IEC criterion, however, the peak voltage arriving
at the motor terminals is what must be taken as 100%
voltage reference. Due to the cable, the rise time to be
considered in IEC criterion will be normally higher than the
one considered in NEMA criterion (which is the value
informed by the inverter manufacturer). Thus depending on
the criteria considered throughout the calculations, pretty
different values of dV/dt are likely to be attributed to the same
situation.
The insulation criteria defined for WEG motors are based on
NEMA, in order not to depend on the final customer
installation. Furthermore the NEMA criterion seems
appropriate for considering just the linear stretch of the curve
to approximate the derivative (dV/dt  V/t). The IEC
criterion considers the peak voltage at the motor terminals,
something extremely complicated to be predicted or
estimated a priori. The rise time at the motor terminals is
increased by the cable high frequency impedance. The dV/dt
ratio at the motor terminals (milder than at the drive terminals)
can be also calculated, but it requires a reliable measurement
of the voltage pulses at the motor leads and most of times
this is not easily accomplished or not even feasible,
demanding a technician familiar with such applications
equipped with a good oscilloscope.
6.5.2 Cable length
Beside the rise time, the cable length is a predominant factor
influencing the voltage peaks occurrence at the inverter fed
motor terminals. The cable can be considered a transmission
line with impedances distributed in sections of inductances/
capacitances series/parallel connected. At each pulse, the
inverter delivers energy to the cable, charging those reactive
elements.
Supposing the motor voltage Vrated = 460 V
with incidence of 1200 V peaks
V = 0,8 x 1200 = 960 V
Assuming tr = 0,25s:
[ [
dV
V
= DV = 960 = 3840
dt
Dt
0,25
s
NOTE: Due to the cable, the rise time is higher at the motor
terminals than at the inverter terminals. However, a very
common mistake in the dV/dt calculation is to consider the
rise time at the inverter terminals and the voltage peak at the
motor terminals, resulting in an unlikely dV/dt value. For
instance, considering tr = 0.1 s (typical value found at the
inverter) in the case above it would result dV/dt = 9600 V/s!
Owing to the differences existing between the rise time
definitions given by NEMA and IEC, misunderstandings often
happen when calculating the voltage gradient (dV/dt).
16
The signal arriving at the motor through the cable is partially
reflected, causing overvoltage, because the motor high
frequency impedance is greater than the cable impedance.
Excessively long leads increase the overshoots at the motor
terminals. According to the NEMA Application Guide for AC
ASD Systems, with the modern IGBT controls overshoots
begin to occur with a cable length of a few feet and can
reach 2 times the control DC bus voltage at a length less
than 50 feet. In some cases, however, very long cables (in
excess of 400 feet, for example) can result in a situation
where the overshoot does not decay quickly enough. In this
case the voltage peak at the motor terminals can ring up well
beyond 2 times the inverter DC link voltage. This behavior is a
function of the PWM pulse pattern, the rise time and the very
Technical guide – Induction motors fed by PWM frequency inverters
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cable type. Voltage measurements realized at the inverter
terminals (0 ft cable) and at the motor (Vrated = 400 V)
terminals with different cable lengths are presented next. The
overshoots also depend on the type of cable used in the
installation; therefore the waveforms shown below are
illustrative only.
Converter terminals
65.5 ft cable
Partial discharge effect on the motor insulation system
Vpeak = 560 V
98.5 ft cable
Vpeak = 630 V
Damaged insulation due to PD activity
328 ft cable
PD is thus a low energy discharge which, after long term
activity, prematurely degrades the motor insulation. The
erosion reduces the thickness of the insulating material,
resulting in a progressive reduction of its dielectric properties,
until its breakdown voltage capability falls below the level of
the applied voltage peak, then the insulation breakdown
occurs.
Vpeak = 750 V
Vpeak = 990 V
6.5.2.1 Corona effect
Depending on the quality/homogeneity of the impregnation
the impregnating material may contain voids (cavities), in
which the failure mechanism of the interturn insulation
develops. The deterioration of the motor insulating system
due to the voltage overshoots occurs by means of Partial
Discharges (PD), a complex phenomenon resulting from
Corona.
Between adjacent charged conductors there is relative
voltage, which gives rise to an electric field. If the established
electric field is high enough (but below the breakdown
voltage of the insulating material), the dielectric strength of
the air is disrupted, that is, if there is sufficient energy, oxygen
(O2) is ionized in ozone (O3). The ozone is highly aggressive
and attacks the organic components of the insulation system
damaging it. For this to happen though the voltage on the
conductors must exceed a threshold value, the so called
“Corona Inception Voltage”, that is the local breakdown
strength in air (within the void). The CIV depends on the
windings design, insulation type, temperature, superficial
characteristics and moisture.
6.5.3 Minimum time between successive pulses
(MTBP)
The voltage measurements presented above show that there
is a succession of peaks in the voltage waveform delivered by
the drive and arriving at the motor terminals. This signal
propagates trough the cable at a determined velocity.
Depending on the winding characteristics and, with respect
to the waveform, on the minimum time between successive
pulses, the voltage appearing between turns may vary
sensibly.
The average voltage applied at the motor terminals is
controlled by the width of the pulses and by the time
between them. The overshoots get worse with shorter times
between pulses. This condition is most likely to occur at high
peak or high output voltages and during transient conditions,
such as acceleration or deceleration. If the time between
pulses is less than three times the resonant period of the
cable (typically 0.2 to 2 s for industrial cable), then additional
overshoot will occur. The only way to be sure that this
condition does not exist is by measuring the pulses directly
or by contacting the control manufacturer.
Technical guide – Induction motors fed by PWM frequency inverters
17
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When the time between successive pulses is less than 6 s,
particularly when the first and the last turns of a single coil of
a random winding are side by side, it may be assumed that
the voltage between adjacent conductors is the peak to peak
value between pulses. This fact results from the rapidity of
the pulse propagation within a coil, because while the first
turn stands a peak to peak voltage value, the voltage on the
last turn is very low, probably zero.
In the case of the example shown above the MTBP was
below 6 s and there were actually motor failures due to
short circuit between turns.
INVERTER
When connecting multiple motors to a single inverter, L must
be as short as possible.
6.6 Criteria regarding the insulation system of WEG
motors on VSD applications
When WEG low voltage induction motors are used with
inverters, the following criteria must be attended in order to
protect the insulation system of the motor. If any of the
conditions below are not satisfied, filters must be used.
6.5.4 Switching frequency (fs)
Beside the effects caused by the rise time and the MTBP,
there is also the frequency at which they are generated.
Differently from eventual impulses caused by line handles, it
is about a pulse train supported at a certain frequency.
Owing to the fast developments on power electronics,
presently this frequency reaches easily values such as 20
kHz. The higher the switching frequency, the faster the
degradation of the motor insulation takes place. Studies bear
out that there is no simple interrelation between the insulation
life and the switching frequency, in spite of that experiences
have shown interesting data:
g
g
NOTE: Applications with motors rated for use in hazardous
areas must be particularly evaluated - in such case please
contact WEG.
Voltage
Motor rated voltage
Spikes
motor
terminals
dV/dt
Rise
inverter
Time do
terminals
conversor*
Vrated < 460 V
 1600 V
 5200 V/s
460 V ≤ Vrated < 575 V
 2000 V
 6500 V/s
575 V ≤ Vrated ≤ 1000V
 2400 V
 7800 V/s
 0,1 s
MTBP*
 6 s
If fs  5 kHz the probability of insulation failure occurrence
is directly proportional to the switching frequency
* Informed by the inverter manufacturer
If fs > 5 kHz the probability of insulation failure occurrence
is quadratically proportional to the switching frequency.
The maximum recommended switching frequency is 5 kHz.
High switching frequencies can cause bearing damages. On
the other hand, switching frequency increase results in the
motor voltage FFT improvement and so tends to improve the
motor thermal performance besides reducing noise.
6.5.5 Multiple motors
If more than one motor is connected to a control, there can
be additional overshoot due to reflections from each motor.
The situation is made worse when there is a long length of
lead between the control and the common connection of
motors. This length of lead acts to decouple the motor from
the control. As a result, reflections which would normally be
absorbed by the low impedance of the control can be carried
to another motor and add to the overshoot at its terminals.
18
Moisture is detrimental to insulating materials and therefore
must be avoided for a longer motor life to be guaranteed. In
order to keep the motor windings dry, it is recommended the
use of heating resistors.
The insulation system to be used in each case depends on
the motor rated voltage range and on the frame size.
6.7 Normative considerations about the insulation
system of inverter fed motors
NEMA MG1 – if the voltage at the inverter input does no
exceed the motor rated voltage and if the voltage observed
at the motor terminals does not exceed the limits shown
below, it may be assumed that there will be no voltage
stress reducing significantly the life of the insulation system.
g
Technical guide – Induction motors fed by PWM frequency inverters
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Nema MG1 - Part 30
Nema MG1 - Part 31
General purpose motors
Definite purpose inverter fed motors
Vrated  600 V : Vpeak  1kV
Vrated > 600 V : Vpeak  3,1 Vrated
Rise time  0,1s
Rise time  2s
Vrated  600 V : Vpeak  2,04 Vnom
Vrated  600 V : Vpeak  2,04 Vrated
Rise time  1s
Rise time  1s
WEG motors fully attend NEMA MG1 Parts 30 and 31.
IEC 60034 – for motors up to 500 V the insulation system
must stand voltage peak levels as shown below. For
motors above 500 V, reinforced insulation systems must be
applied or filters shall be installed at the inverter output,
aiming to increase the rise time and to limit voltage peaks.
g
It is remarkable the similarities existing between IEC and
GAMBICA criteria, as well as their disparity with respect to
NEMA criteria. This results from the particular definitions of
rise time and dV/dt according to each institution. One can
notice that the insulation criteria from both IEC and GAMBICA
take into account the cable length, information which WEG
also considers relevant.
IEC 60034-17
General purpose motors
6.8 Recommendations for the cables connecting WEG
motors to inverters
As already mentioned the maximum peak voltage appearing
at the terminals of the inverter fed motor depends on many
factors, predominantly the cable length.
When supplying WEG motors with inverters, the following
practical rules are suggested for the evaluation of the need of
using filters between motor and inverter.
Valid for standard motors.
Cable length L
L  100 m
100 m < L  300 m
IEC 60034-25
Definite purpose motors
L > 300 m
Output filters
Not needed
Output reactor needed
(at least 2% voltage drop)
Special filters needed (contact WEG)
The output reactor is necessary for the eddy current that
flows from inverter to earth to be limited. The input (line)
reactor prevents the inverter ground fault from tripping.
A: Valid for motors up to 500 Vac (without filters)
B: Valid for motors up to 690 Vac (without filters)
C: Measured results at 415 Vac supply with different cable
lengths
The output reactor design must take account of additional
losses occurring due to current ripple and current leakage to
earth, which increases as cable length rises. For long cables
and reactors designed for small currents there will be great
influence of the leakage currents on the reactor losses (and
heating). The cooling system of the inverter panel must also
take the reactors additional losses into account for a safe
temperature operation to be assured.
The output reactor must be installed near the inverter, as
shown below.
GAMBICA/REMA – the European association of motors
g
(REMA) and inverters (GAMBICA) manufacturers set the
criteria shown next based on its members’ experience.
Technical guide – Induction motors fed by PWM frequency inverters
19
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The basic given recommendations are summarized in the
table below. For more details and updated information the
current standard version shall be consulted.
L1 = Line reactor – selection criteria according to clause 5.2
L2 = Output reactor – must be installed next to the inverter.
6.8.1 Cable types and installation recommendations
The characteristics of the cable connecting motor and
frequency inverter, as well as its interconnection and physical
location, are extremely important to avoid electromagnetic
interference in other devices.
The grounding system must be capable to provide good
connections among equipments, for example, between
motor and inverter frame. Voltage or impedance differences
between earthing points can cause the flow of leakage
currents (common mode currents) and electromagnetic
interference.
Examples of shielded cables recommended by IEC
60034-25
PE
PE
L3
6.8.1.1 Unshielded cables
g
g
g
g
Three-core unshielded motor cables can be used when
there is no need to fulfill the requirements of the European
EMC Directives (89/336/EEC).
Certain minimum distances between motor cables and
other electrical cables must be observed in the final
installation. These are defined in the table below.
Emission from cables can be reduced if they are installed
together on a metallic cable bridge which is bonded to the
earthing system at least at both ends of the cable run. The
magnetic fields from these cables may induce currents in
nearby metalwork leading to heating and increasing losses.
g
They help to reduce the radiated emission through the
motor cables in the Radio Frequency range (RF).
They are necessary when the installation must comply with
They are also necessary when using Radio Frequency
Interference Filter (whether built-in or external) at inverter
input.
Minimum distances between motor cables and other
electrical cables (for instance, signal cables, sensor cables,
etc.) must be observed in the final installation, as per table
below.
Recommended separation distances between motor cable
(shielded or not) and other cables of the installation
Cable Length
Minimum separation distance
 30 m
 10 cm
> 30 m
 25 cm
6.8.1.3 Installation recommendations
IEC 60034-25 presents cable types and construction details.
20
PE
L2
Scu
Alternate motor cables for conductors up to 10 mm2
L3
L1
L2
L1
L3 L2
PE
Scu
PEs
AFe
Afe = steel or galvanized iron
Symmetrical Shielded Cables: three-core cable (with or
without conductors for protective earth) symmetrically
constructed + a concentric copper or aluminum protective
shield/armour
the EMC Directive 89/336/EEC as per EN 61800-3.
g
L3
L2
Scu
6.8.1.2 Shielded cables
g
L1
L1
PE = protective earth conductor
SCU = concentric copper (or aluminum) screen
Cable shield must be grounded at both ends, motor and
inverter. Good EMC practices such as 360° bonding of the
shields are recommended, in order for low impedance for
high frequency to be provided.
For the shield to operate also as protective conductor, it
should have at least 50% of the phase conductors’
conductance. If the shield does not have enough crosssection for that, then a separate earth conductor is needed
and the shield provides EMC and physical protection only.
The shield high-frequency conductance should be at least
10% of that of the phase conductors.
6.9 Influence of the inverter on the motor shaft voltage
and bearing currents
The advent of static inverters aggravated the phenomenon of
induced shaft voltage/current, due to the unbalanced
waveform and the high frequency components of the voltage
supplied to the motor. The causes of shaft induced voltage
owing to the PWM supply is thus added to those intrinsic to
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the motor (for instance, electromagnetic unbalance caused
by asymmetries), which as well provoke current circulation
through the bearings. The basic reason for bearing currents
to occur within an inverter fed motor is the so called common
mode voltage. The motor capacitive impedances become
low in face of the high frequencies produced within the
inverter stage of the inverter, causing current circulation
through the path formed by rotor, shaft and bearings back to
earth.
6.9.1 Common mode voltage
The three phase voltage supplied by the PWM inverter,
differently from a purely sinusoidal voltage, is not balanced.
That is, owing to the inverter stage topology, the vector sum
of the instantaneous voltages of the three phases at the
inverter output does not cancel out, but results in a high
frequency electric potential relative to a common reference
value (usually the earth or the negative bus of the DC link),
hence the denomination “common mode”.
The sum of the instantaneous voltage values at the (three
phase) inverter output does not equal to zero
Cer : Capacitor formed by the stator winding and the rotor
lamination (Dielectric = airgap + slot insulation + wire
insulation)
This high frequency common mode voltage may result in
undesirable common mode currents. Existing stray
capacitances between motor and earth thus may allow
current flowing to the earth, passing through rotor, shaft and
bearings and reaching the end shield (earthed).
Crc : Capacitor formed by the rotor and the stator cores
(Dielectric = airgap)
Practical experience shows that higher switching frequencies
tend to increase common mode voltages and currents.
Cmd e Cmt : Capacitances of the DE (drive end) and the NDE
(non-drive end) bearings, formed by the inner and
the outer bearing raceways, with the metallic
rolling elements in the inside. (Dielectric = gaps
between the raceways and the rolling elements +
bearing grease)
6.9.2 Equivalent circuit of the motor for the high
frequency capacitive currents
The high frequency model of the motor equivalent circuit, in
which the bearings are represented by capacitances, shows
the paths through which the common mode currents flow.
The rotor is supported by the bearings under a layer of nonconductive grease. At high speed operation there is no
contact between the rotor and the (earthed) outer bearing
raceway, due to the plain distribution of the grease. The
electric potential of the rotor may then rise with respect to the
earth until the dielectric strength of the grease film is
disrupted, occurring voltage sparking and flow of discharge
current through the bearings. This current that circulates
whenever the grease film is momentarily broken down is
often referred to as the capacitive discharge component.
There is still another current component, which is induced by
a ring flux in the stator yoke and circulates permanently
through the characteristic conducting loop comprising the
shaft, the end shields and the housing/frame, that is often
called the conduction component.
apacitor formed by the stator winding and the frame
Cec : C
(Dielectric = slot insulation + wire insulation)
ICM : Total common mode current
apacitive discharge current flowing from the stator to
Ier : C
the rotor
Ic : Capacitive discharge current flowing through the bearings
These discontinuous electric discharges wear the raceways
and erode the rolling elements of the bearings, causing small
superimposing punctures. Long term flowing discharge
currents result in furrows (fluting), which reduce bearings life
and may cause the machine to fail precociously.
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Motor with one drive end
Crater occasioned by electroerosion on the
inner raceway of the bearing
Without bearing protection:
Bearing raceway damaged by bearing
currents flow
With protected bearing:
Fluting caused by electric discharges within
the bearing
Motor with two drive ends
6.9.3 Methods to reduce (or mitigate) the bearings
currents in inverter fed motors
For the motor bearing currents to be impeded to circulate,
both the conduction (induced on the shaft) and the capacitive
discharge (resultant from common mode voltage)
components must be taken into account. In order to eliminate
the current flowing through the characteristic conducting
loop it is enough to isolate the motor bearings (only one of
them, in the case of a single drive end, or the both of them, in
the case of two drive ends). However, for the capacitive
components to be withdrawn it would be also necessary to
isolate the bearings of the driven machine, in order to avoid
the migration of electric charges from the motor to the rotor
of the driven machine through their shafts, which are
electrically connected in the case of direct coupling. Another
way of extinguishing the capacitive discharge current
component consists of short circuiting the rotor and the
motor frame by means of a sliding graphite brush. This way,
the inductive current component flowing through the
characteristic conducting loop can be eliminated by
insulating just a single bearing of the motor, while the
capacitive current component, as well as the transfer of
capacitive charges to the driven machine, can be eliminated
by use of a short circuiting brush.
22
Without bearing protection:
With bearing protection:
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6.10 Criteria regarding protection against bearing
currents (shaft voltage) of WEG motors on VSD
applications
Platform
Frame Size
W21
mod < 315 IEC
W22
mod < 504 NEMA
Standard
Optional
No protection
Please contact WEG
IEC 60034-25 – do not specify a minimum frame size on
which bearing protection must be applied. Within the
clause broaching the effects of magnetic asymmetries as
shaft voltages/bearing currents cause, it is mentioned that
bearing currents commonly occur in motors above 440
kW. For other causes, no mention is made concerning
frame sizes. According to the document, the solution
adopted to avoid bearing currents depends on which
current component is to be avoided. It may be made either
by means of insulated bearings or shaft grounding system
though.
g
Insulated bearing in
any or both motor
W21
315 and 355 IEC
W22
504/5 and 586/7 NEMA
ends
No protection *
Earthing system with
slip ring and graphite
brush between frame
CSA 22.2 Nº100 Item 12 – shaft earthing brushes must be
used in motors of frame above IEC 280 (NEMA 440).
g
and shaft
HGF
315 ≤ mod ≤ 630 (IEC)
Earthing system with
500 ≤ mod ≤ 1040
slip ring and graphite
(NEMA)
brush between frame
Insulated DE bearing
and shaft
Insulated NDE bearing
M
280 ≤ mod ≤ 1800 (IEC)
Earthing system with
440 ≤ mod ≤ 2800
slip ring and graphite
(NEMA)
brush between frame
Gambica/REMA Technical Guide – for motors of frames
below IEC 280 the effects of bearing currents are seldom
appreciable and therefore no extra protection is needed. In
such cases, adhering strictly to the motor and drive
manufacturers’ recommendations regarding the
installation, cabling and grounding is enough. For frames
above IEC 280, the effects of bearing currents may be
significant and for security special protection is advisable.
This may be obtained by means of insulated NDE bearing
and shaft grounding system use. In such case, care must
be taken not to bypass the bearing insulation.
g
Insulated NDE bearing
Insulated DE bearing
and shaft
* For Inverter Duty line motors, the earthing system is standard.
NOTE: Applications with motors rated for use in hazardous
areas must be particularly evaluated - in such case please
contact WEG.
6.11 Normative considerations about the current
flowing through the bearings of inverter fed motors
NEMA MG1 Part 31 – with sinusoidal supply shaft voltages
may be present usually in motors of frame 500 and larger.
(...) More recently, for some inverter types and application
methods, potentially destructive bearing currents have
occasionally occurred in much smaller motors. (...) The
current path could be through either or both bearings to
ground. Interruption of this current therefore requires
insulating both bearings. Alternately, shaft grounding
brushes may be used o divert the current around the
bearing. It should be noted that insulating the motor
bearings will not prevent the damage of other shaft
connected equipment.
6.12 Influence of the inverter on the motor acoustic
noise
The rotating electrical machines have basically three noise
sources:
g The ventilation system
g The rolling bearings
g Electromagnetic excitation
g
NEMA Application Guide for AC ASD Systems – the
circulating currents caused by common mode voltage may
cause bearing problems in frame sizes smaller than 500
(most likely in the 400 and larger frames).
g
IEC 60034-17 – for machines with frame numbers above
315 it is recommended either to use an inverter with a filter
designed to reduce the zero-sequence component of the
phase voltages (so called common mode voltages) or to
reduce the dV/dt of the voltage or to insulate the motor
bearing(s). The need to insulate both motor bearings is
seldom necessary. In such a case, the examination of the
whole drive system by an expert is highly recommended
and should include the driven machine (insulation of the
coupling) and the grounding system (possibly use of an
earthing brush).
g
Bearings in perfect conditions produce practically despicable
noise, in comparison with other sources of the noise emitted
by the motor.
In motors fed by sinusoidal supply, especially those with
reduced pole numbers (higher speeds), the main source of
noise is the ventilation system. On the other hand, in motors
of higher polarities and lower operation speeds often stands
out the electromagnetic noise.
However, in variable speed drive systems, especially at low
operating speeds when ventilation is reduced, the
electromagnetically excited noise can be the main source of
noise whatever the motor polarity, owing to the harmonic
content of the voltage.
Higher switching frequencies tend to reduce the magnetically
excited noise of the motor.
6.13 Criteria regarding the noise emitted by WEG
motors on VSD applications
Results of laboratory tests (4 point measurements
accomplished in semi-anechoic acoustic chamber with the
inverter out of the room) realized with several motors and
inverters using different switching frequencies have shown
that the three phase induction WEG motors, when fed by
frequency inverters and operating at base speed (typically 50
or 60 Hz), present and increment on the sound pressure level
of 11 dB(A) at most.
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6.14 Normative considerations about the noise of
inverter fed motors
g
g
g
NEMA MG1 Part 30 – the sound level is dependent upon
the construction of the motor, the number of poles, the
pulse pattern and pulse frequency, and the fundamental
frequency and resulting speed of the motor. The response
frequencies of the driven equipment should also be
considered. Sound levels produced thus will be higher than
published values when operated above rated speed. At
certain frequencies mechanical resonance or magnetic
noise may cause a significant increase in sound levels,
while a change in frequency and/or voltage may reduce the
sound level. Experience has shown that (...) an increase of
up to 5 to 15 dB(A) can occur at rated frequency in the
case when motors are used with PWM controls. For other
frequencies the noise levels may be higher.
IEC 60034-17 – due to harmonics the excitation
mechanism for magnetic noise becomes more complex
than for operation on a sinusoidal supply. (...) In particular,
resonance may occur at some points in the speed range.
(...) According to experience the increase at constant flux is
likely to be in the range 1 to 15 dB(A).
IEC 60034-25 – the inverter and its function creates three
variables which directly affect emitted noise: changes in
rotational speed, which influence bearings and lubrication,
ventilation and any other features that are affected by
temperature changes; motor power supply frequency and
harmonic content which have a large effect on the
magnetic noise excited in the stator core and, to a lesser
extent, on the bearing noise; and torsional oscillations due
to the interaction of waves of different frequencies of the
magnetic field in the motor airgap. (...) The increment of
noise of motors supplied from PWM controlled inverters
compared with the same motor supplied from a sinusoidal
supply is relatively small (a few dB(A) only) when the
switching frequency is above about 3 kHz. For lower
switching frequencies, the noise increase may be
tremendous (up to 15 dB(A) by experience). In some
circumstances, it may be necessary to create “skip bands”
in the operating speed range in order to avoid specific
get those problems around, so that for instance specific
frequencies within the operating range can be skipped and
the acceleration/deceleration times can be conveniently
adjusted.
6.16 Criteria regarding the vibration levels presented
by WEG motors on VSD applications
Tests realized with several motors and inverters following the
procedures recommended by IEC 60034-14 confirmed that
the vibration levels of induction motors increase when these
are fed by frequency inverters.
Furthermore, the observed increment on vibration speeds
generally were lower with higher switching frequencies, that
is, switching frequency increases tend to reduce the
mechanical vibration of the inverter fed motor.
In any case, even when operating above the base speed,
WEG motors presented RMS vibration velocity values (mm/s)
below the maximum limits established by both the IEC
60034-14 and the NEMA MG1 Part 7 standards, thus
attending the criteria required.
6.17 Normative considerations about mechanical
vibration of inverter fed motors
NEMA MG1 Part 30 – When an induction motor is operated
from a control, torque ripple at various frequencies may
exist over the operating speed range. (…) It is of particular
importance that the equipment not be operated longer
than momentarily at a speed where a resonant condition
exists between the torsional system and the electrical
system (i.e., the motor electrical torque). (…) It also is
possible that some speeds within the operating range may
correspond to the natural mechanical frequencies of the
load or support structure and operation other than
momentarily could be damaging to the motor and or load
and should be avoided at those speeds.
g
NEMA MG1 Part 31 – Machine sound and vibration are
influenced by the following parameters: electromagnetic
design; type of inverter; resonance of frame structure and
enclosure; integrity, mass and configuration of the base
mounting structure; reflection of sound and vibration
originating in or at the load and shaft coupling; windage. It
is recognized that it is a goal that motors applied on
inverter type supply systems for variable speed service
should be designed and applied to optimize the reduction
of sound and vibration in accordance with the precepts
explained above. However, since many of these influencing
factors are outside of the motor itself, it is not possible to
address all sound and vibration concerns through the
design of the motor alone.
g
resonance conditions due to the fundamental frequency.
6.15 Influence of the inverter on the mechanical
vibration of the motor
Interactions between currents and flux harmonics may result
in stray forces actuating over the motor causing mechanical
vibration and further contributing to increase the overall noise
levels. This mechanism gains importance especially when
amplified by mechanical resonances within the motor or the
driven machine. If any of the non-fundamental harmonics is
near the natural frequencies of the motor, the forces
produced can excite vibration modes.
Such effects can be attenuated with a careful design of the
motor with respect to the stator and rotor slots, lamination
and frame, always looking out for simplifying the mechanical
system thus reducing the possibility of exciting natural
frequencies that develops modes of vibration within the
motor.
Modern frequency inverters are also provided with tools to
24
IEC 60034-17 – The asynchronous (time-constant) torques
generated by harmonics have little effect on the operation
of the drive. However, this does not apply to the oscillating
torques, which produce torsional vibrations in the
mechanical system. (...) In drives with pulse-controlled
inverters, the frequencies of the dominant oscillating
torques are determined by the pulse frequency while their
amplitudes depend on the pulse width. (...) With higher
g
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pulse frequencies (in the order of 21 times the fundamental
frequency) the oscillating torques of frequencies 6 x f1 and
12 x f1 are practically negligible, provided a suitable pulse
pattern is applied (e.g. modulation with a sinusoidal
reference wave or space-phasor modulation). Additionally,
oscillating torque of twice the pulse frequency are
generated. These, however, do not exert detrimental
effects on the drive system since their frequency is far
above the critical mechanical frequencies.
Torque varies at a rate proportional to the square of the
speed
g Horsepower varies as the cube of the speed
g 100% load torque and horsepower at base speed
g
Linear torque variation
IEC 60034-25 – If the inverter have appropriate output
characteristics and if due care is taken with respect to the
mechanical characteristics and the mounting of the motor,
vibration levels similar to those resulting from sinusoidal
environment will be produced. Therefore, there is no need
for defining vibration criteria different from those already
established in IEC 60034-14 for sinusoidal supply. Vibration
levels measured with decoupled motors are indicative of
the motor quality only, but in measurements accomplished
at the actual application (with the motor finally installed)
rather different values of vibration levels may be obtained.
g
Torque varies linearly with speed
Horsepower varies as the square of the speed
g
100% load torque and horsepower at base speed
g
7 Interaction between motor and
driven load
7.1 Load types
The correct dimensioning of the variable speed drive system
depends on the knowledge of the behavior of the load, that
is, how the load is related with speed and consequently how
much torque is demanded on the motor shaft. In most
processes the load may be described by one of the following
terms: variable torque, constant torque and constant
horsepower.
7.1.1 Typical examples:
Typical examples:
gCentrifugal pumps
gCentrifugal fans
gCentrifugal blowers
gCentrifugal compressors
Variable torque loads are good candidates to apply VSDs for
energy savings, once that the mechanical power available at
the motor output will not be constant - it will actually vary
suitably in accordance with the load demand, as shown
before in Clause 3 of this technical guide.
g
7.1.2 Constant torque loads
Typical examples:
g Screw compressors
g Reciprocating compressors
g Positive displacement pumps
gExtruders
gCrushers
g Ball mills
gConveyors
g Augers
g Process lines (strip, web, sheet)
Machines that are high impact loads (intermittent torque
loading not as function of speed, requiring that the motor and
control combination produce sufficient accelerating torque to
return the load to the required speed prior to the beginning of
the next work stroke) or duty cycle loads (discrete loads - at
changing or constant speeds - applied for defined periods of
time repeated periodically) typically fall into the constant
torque classification.
Squared torque variation
Load torque remains constant throughout the speed range
Horsepower changes linearly with operation speed
g
Rated load torque and horsepower at base speed
g
g
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7.1.3 Constant horsepower loads
Typical examples:
g
Machine tools (where heavier cuts are taken at lower
speeds and lighter cuts at higher speeds)
g
Center driven winders
8.1.2 Solution
8.1.2.1 Regarding the temperature rise on the windings
(derating torque)
Compressors are loads that feature a constant torque
demand along the whole speed range. The motor must be
dimensioned to cope with the most critical operation
condition, in this example the lowest speed within the
operating range, in which the ventilation is reduced to its
minimum while the torque demand remains constant.
Considering that the operation speed may change from 180
to 1800 rpm and that the base frequency is 60 Hz, then a
4-pole motor must be chosen.
Neglecting the slip, the demanded horsepower at the base
point of operation is:
æ
æ
TL (kgfm) = 960P(kW) Þ P = 34 1800 = 6.5 kW
æ 9.81 960
n(rpm)
æ
7.2 Speed duties
Nevertheless, from the thermal point of view the worst
working point of this self-ventilated motor is 180 rpm (6 Hz),
which means the lowest speed and therefore the lowest
effectiveness of the cooling system of the motor within the
defined speed range. For this reason the torque derating
must be calculated for this very condition.
7.2.1 Variable speed duty
Motors designated for variable speed duty are intended for
varied operation over the defined speed range marked on the
motor and is not intended for continuous operation at a single
or limited number of speeds. The motor design takes the
advantage of the fact that it will operate at a lower
temperature at the load levels for some speeds than at other
over the duty cycle.
According to the WEG derating criteria (subclause 6.4.1.2),
when operating at 6 Hz a torque reduction of 40% results in a
temperature rise of 80 oC on the motor windings.
Furthermore it must be assumed constant V/f condition,
because it is asked that the motor be able to operate with
any WEG drive (for the optimal flux solution to be applicable,
a WEG high efficiency motor must be driven by a WEG
inverter model CFW-09 version 2.40 or higher).
7.2.2 Continuous speed duty
Motors designated for continuous speed duty can be
operated continuously at any speed within the defined speed
range. The motor is designed on the principle that it may be
operated at its load level at the speed which results in the
highest temperature rise for an indefinite period of time.
8 Dimensioning and analysis of
actual drive system applications –
Practical examples
8.1 Constant torque application - compressor
8.1.1 Example
Please dimension a WEG standard squirrel cage induction
motor (TEFC) to operate with any WEG frequency inverter
from 180 to 1800 rpm, driving a compressor demanding 34
Nm of torque. Temperature rise of thermal class B (80 K)
wanted.
General data:
g Mains: 3-phase / 400 V / 60 Hz
g Environment: maximum temperature 40C; altitude 1000 m;
normal atmosphere
g Frequency inverter CFW-09: tr = 0,1 μs; fchav = 5 kHz
26
f = 6 Hz  f/fn = 6/60 = 0,10 per unit
f/fn = 0,10 p.u.  Tr = 0,6 per unit
That is, at 180 rpm the motor will be able to supply only 60%
of its rated torque. Once the load demands constant torque
(equal to the torque demanded at base speed) throughout
the operating range, the motor must be oversized in
accordance with the derating calculated.
T=
TL
Tr
=
34
= 56.7 Nm
0,6
Thus the motor rated horsepower will be:
æ
æ
1800
P = 56.7
= 10.83 kW
æ 9.81 960
æ
Load torque drops as speed increases
g
Horsepower results constant throughout the speed range
g
Rated load torque and horsepower at base speed
g
Consulting the WEG motors catalog, the ideal motor for this
application is the 11 kW (15 hp) - 4 pole - 60 Hz - frame IEC
132M (NEMA 215 T).
The use of forced cooling system would be an alternative
option. In this case, motor oversizing is not needed and a
motor rated 7,5 kW (10 hp) – 4 pole (frame IEC 132S/NEMA
213T) would satisfactorily attend the application needs.
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This way it is assured that the temperature rise of the motor
will be equal to or less than 80 K at any operation condition.
8.1.2.2 Regarding the insulation system
According to NEMA criteria the situation is the following:
8.1.2.3 Regarding the bearings’ protection
According to WEG criteria regarding protection against
bearing currents (clause 6.9), WEG standard motors have
optional protected bearings for frames above (including) 315
IEC / 504 NEMA. The selected motor frame is 132 M IEC /
215 T NEMA, thus not needing shaft earthing system neither
special insulated bearings.
8.1.2.4 Regarding the noise
When fed by inverter, the acoustic noise produced by the
motor may increase up to 11 dB(A), considering that scalar
control type will be used.
Voltage at the motor terminals:
8.2 Squared torque application - centrifugal pump
8.2.1 Example
According to WEG insulation criteria (clause 6.6), WEG
motors rated 400 V are able to stand:
dV/dt values up to 5200 V/μs at the drive terminals, thus
satisfying the needs of this example.
g
Please dimension an induction motor WEG NEMA Premium
Efficiency (TEFC) to operate with a CFW-09 on vector control,
driving a centrifugal pump rated 10 hp (7.5 kW) at maximum
speed 2700 rpm.
General information:
g
Power line: 3 phase / 460 V / 60 Hz
g
Environment: maximum temperature 40C; altitude 1000
m; normal atmosphere
g
WEG frequency inverter CFW-09: tr = 0.1 μs; fchav = 2.5
kHz
tr ≥ 0.1 μs at the inverter terminals, thus attending this
example’s application.
8.2.2 Solution
Vpeak ≤ 1430 V at the motor terminals. If this condition is
not attended at the definitive installation, filters must be
connected to the inverter output.
8.2.2.1 Regarding the temperature rise (derating
torque)
g
g
The switching frequency defined for this example (5 kHz) is in
agreement with WEG recommendations too. Therefore the
motor designed fully attend this application’s demands with
regard to the insulation system.
However, it will not be possible to evaluate the matter on the
point of view of IEC, because it requires the measurement of
the voltage at the motor terminals. As the VSD system is still
at the dimensioning stage and there is no actual motor at the
application, it is understood that the final motor environment
is still not defined, so that measures are made unfeasible and
the actual voltage peak and rise time values at the motor
terminals are unknown. Such values will depend on type and
length of the cable employed at the end user.
Centrifugal pumps present a torque that characteristically
varies at a rate proportional to the square of the speed, while
the horsepower varies as the cube of the speed. In this case,
the motor must be dimensioned for the highest speed
within the operation range of the pump, because the
maximum torque demand for the motor happens there.
The figure next shows that this example allows two
alternatives for the dimensioning: a 2-pole motor or else a
4-pole motor. The 2-pole motor would operate at the
constant torque region (below base speed), while the 4-pole
motor would operate at field weakening region (above base
speed).
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Owing to the operation in the weakening field region, the
motor breakdown torque must be also verified. According to
the WEG motors’ breakdown torque criteria (subclause
6.4.3), the motor is able to attend the application needs.
Consulting the WEG NEMA Motor Catalog, the most
appropriate 3 phase IP55 NEMA Premium Efficiency motor is
the 10 hp (7.5 kW) – 4 poles – 60 Hz – frame 215 T.
Therefore, after both technical and economical analyses, the
most suitable motor for this application turns out to be the
4-pole / 7.5 kW (10 hp) / 60 Hz / 460 V / frame 215T NEMA
Premium Efficiency.
8.2.2.2 Regarding the insulation system
According to NEMA criteria the situation is the following:
The torque required by the pump at maximum speed is given
below:
T (kgfm) =
716P (hp)
n(rpm)
ÞT b =
716 (10)
2700
= 2,65 kgfm = 25.99 Nm
2-pole motor
2700 rpm = 0.75 p.u.  45 Hz
According to the derating criteria of WEG NEMA Premium
Efficiency TEFC motors (subclause 6.4.1.1), any WEG NPE
motor is able to operate 1000:1 with variable torque loads,
that is, no torque derating is needed throughout the speed
range. Then the derating factor will be 1.
æ Tb æ 25.99 Nm
1
æ
T2p = æ
df
= 19.17 Ibft
Consulting the WEG NEMA Motor Catalog, the most
appropriate 3 phase IP55 motor is the NEMA Premium
Efficiency 15 hp (11 kW) – 2 poles – 60 Hz – frame 254 T.
4-pole motor
2700 rpm = 1.50 p.u.   90 Hz
According to the derating criteria of WEG NEMA Premium
Efficiency TEFC motors (subclause 6.4.1.1), any WEG NPE
motor is able to support constant horsepower from 60 to 90
Hz with variable torque loads. Then at 90 Hz the derating
factor will be (1.5)-1.
æ Tb æ
df
æ
T4p =æ
= 25.99 Nm = 38.99 Nm = 28.75 Ibft
1
1.5
28
dV
dt
=
DV
Dt
=
V
520.45 V
@ 5200 ms
0.1 ms
According to WEG insulation criteria (clause 6.6), WEG
motors rated 460 V are able to bear:
dV/dt values up to 5200 V/μs at the drive terminals, thus
satisfying the needs of the application of this example.
g
tr ≥ 0,1μs at the inverter terminals, thus attending this
example’s application.
g
Vpeak ≤ 1430 V at the motor terminals. If this condition is
not attended at the definitive installation, filters must be
connected to the inverter output.
g
WEG recommends that switching frequencies up to 5 kHz be
used. The switching frequency defined for this example (2.5
kHz) thus attends WEG’s recommendation.
Therefore the motor designed fully attend this application’s
demands with regard to the insulation system.
However, it will not be possible to evaluate the matter on the
point of view of IEC, because it requires the measurement of
the voltage at the motor terminals. As the VSD system is still
at the dimensioning stage and there is no actual motor at the
application, it is understood that the final motor environment
is not defined yet, so that measures are still made unfeasible
and the actual voltage peak and rise time values at the motor
terminals are unknown. Such values will depend on type and
length of the cable employed at the end installation.
Technical guide – Induction motors fed by PWM frequency inverters
www.weg.net
8.2.2.3 Regarding the bearings’ protection
According to WEG criteria regarding protection against
bearing currents (clause 6.9), WEG motors may optionally
have protected bearings above (including) 504 NEMA (315
IEC) frames. The selected motor frame is 215T NEMA, thus
needing neither shaft earthing system nor special insulated
bearings.
8.3 Special application - long cable
8.3.1 Example
Evaluation of the voltage peaks at the terminals of a special
WEG motor rated 9 kW – 2115 rpm – 500 V – 72 Hz. Due to
matters intrinsic to the application, the motor must be fed by
a PWM inverter through a 100 m long cable.
Upper curve: PWM voltage leaving the inverter
g
Lower curve: PWM voltage arriving at the motor
g
8.3.2 Solution
Supposing that the derating, bearing protection and noise
criteria have already been verified and that the motor in
question fully attends them, the insulation system of the
motor is still left to be evaluated. It must be assured that the
motor insulation will bear the application’s conditions.
Owing to the long length of the cable leads there is the
chance of excessive voltage peaks (overshoots) to occur at
the motor terminals and therefore special attention must be
addressed to the insulation matter. In this case, for an
appropriate evaluation of the insulation system, the highest
speed within the operating range must be considered, in
order that maximum voltage levels come to the motor
terminals, causing the voltage peaks to be as high as
possible, as well.
According to the insulation criteria of WEG motors (clause
6.6), induction machines rated 500 V are able to bear voltage
peaks up to 1780 V and dV/dt up to 6500 V/μs.
Zoom in the voltage pulse shown beside, for an analysis
of tr and Vpeak.
Magnitude of the voltage peak appearing at the motor
terminals
In this case it will be possible to analyze the voltage peaks at
the motor terminals as requires IEC, once the actual
installation exists and the factors decisively influencing the
occurrence and gravity of overshoots are well defined.
The next waveforms were obtained by means of
measurements accomplished at the inverter terminals (upper
curves – PWM signal before the cable) and at the motor
terminals (lower curves - PWM signal after the cable). It is
important to stand out, that the voltage profiles appearing at
the motor input would change if other cable were used. The
cable used herein was not shielded and comprised of 4
conductors (3 phases + earth) asymmetrically distributed.
The inverter was fed by sinusoidal 500 V / 50 Hz voltage and
scalar control with switching frequency 4 kHz was used.
Vpeak  1040 V
WEG criterion  1780 V (>1040 V)  Ok!
NEMA criterion  3,1.500 = 1550 V (< 1780 V)  Ok!
IEC criterion   1300 V (< 1780 V)  Ok!
Technical guide – Induction motors fed by PWM frequency inverters
29
www.weg.net
Rise time
MTBP (minimum time between successive pulses)
®
®
®
®
tr  0,8 . 0,315 = 0,25 s = t
WEG criterion  0,1 s (minimum) at the inverter terminals 
Ok!
NEMA criterion  0,1 s (minimum) at the inverter terminals
 Ok!
MTBP  8,6 s
(the waveform shown beside is the very waveform shown in
the other figures throughout this example, but a convenient
zoom was given to it in order to benefit the evaluation of the
minimum time between successive pulses).
tr  0,8 . 1,24 = 0,99 s = t
WEG criterion = 6 s (minimo)  Ok!
IEC criterion  do not establish minimum value for tr at motor
terminals
So all the insulation criteria of WEG motors are attended and
therefore the use of filters is not necessary. However, these
conclusions are valid strictly for the ensemble (inverter –
motor – cable leads) investigated. As mentioned before, the
utilization of other cable or inverter would cause the voltage
peaks at the motor terminals to change.
dV/dt
At inverter terminals:
V = 0,8. Vlink DC = 0,8 (500.1,414) = 565,6 V
t = 0,25 s
dV/dt  V/t = 2262,7 V/s
At motor terminals:
V = 0,8. Vpico = 0,8.1040 = 832 V
t = 0,99 s
dV/dt  V/t = 840,4 V/s
8.4 Variable torque / variable speed application textile industry
8.4.1 Example
A standard IP55 squirrel cage induction WEG motor must be
dimensioned for a textile industry application and may be
driven by a frequency inverter unknown.
Application info:
50 Nm at full load
g
WEG criterion  6500 V/s (> 2262,7 V/s)  Ok!
NEMA criterion  6500 V/s  Ok!
IEC criterion  840,4 V/s (< 6500 V/s)  Ok!
30
Speed range from 540 to 3600 rpm
g
Temperature rise of thermal class B (80 K) wanted on the
windings
g
Technical guide – Induction motors fed by PWM frequency inverters
www.weg.net
Direct coupling
Consulting the WEG Stock Products Catalog, the standard
motor which better fits the application has 22 kW and 2
poles. If the duty cycle were continuous, with full load full time
and no speed variation, the dimensioning would be well done
so and already concluded. Nevertheless the actual duty
cycle embraces speed changes and different load
percentages. Therefore, in order to achieve a suitable thermal
dimensioning, the load demand at every operating condition
must be analyzed, so that a motor equivalent torque can be
finally calculated considering the whole duty cycle. Once
obtained the equivalent torque, it must be assured that the
chosen motor will be able to provide the maximum
horsepower demanded throughout the operation duty.
Assuming that the temperature rise is directly proportional to
the losses and that the Joule losses comprise the prevailing
component of motor losses, then the losses vary as the
square of the speed and the equation below is true:
Duty cycle as shown below
g
No forced ventilation wanted
g
General data:
Mains: 3 phase / 380 V / 6 0Hz
g
Room temperature 40C, altitude 1000 m
g
7
å
Teq =
i=1
æ Ti æ2
æ df
i
t if
æ
g
7
åti
i=1
where,
Teq: equivalent torque of the motor
Ti: torque demanded by the load at each operating speed
dfi: derating factor to be applied at each operating speed,
due to the temperature rise increase occasioned by both
harmonics and ventilation reduction;
ti: period of each duty stretch, considered as below.
8.4.2 Solution
Considering the operation range (from 540 to 3600 rpm) and
the base frequency (60 Hz), a 2 pole motor must be chosen,
because higher polarities would lead to high operating
frequencies and increasing torque reduction above 60 Hz.
At base speed, neglecting the slip the load demands:
Tr (kgfm) .n (rpm)
716
æ
=
50
æ 9,81
æ
æ
P (CV) =
3600
960
= 18,72 kW
According to WEG standard motors’ torque derating criteria
valid for constant flux (constant V/f) condition (subclause
6.4.1.2), for operation at 60 Hz (1 per unit) the torque must be
reduced to 0.95 per unit in order for the temperature rise of
the machine to attend the limits of thermal class B. However,
it is not possible to reduce in 5% the load, because is
demands constant torque. Since the use of independent
ventilation is also out of question, the motor has to be
oversized. Thus the motor rated horsepower is actually higher
than the value firstly reckoned:
P=
18.72
0.95
= 19.70 kW
ti = tif + tip /kv
tif: time intervals with motor running (either loaded or not)
tip: sum of time intervals with motor stopped
kv: constant value that depends on the motor cooling.
When ventilation does not depend on motor operation (for
instance, TENV motors), then kv=1.
When ventilation is linked to motor operation (for instance,
TEFC motors), then kv=3.
It is thus necessary to calculate de derating factor (df)
suitable to each stretch of the duty cycle:
Period [min]
2
18
4
2
18
6
10
Torque p.u.
0,50
1,00
0,75
0,50
1,00
0,50
1,00
Torque [kgfm]
2,50
5,00
3,75
2,50
5,00
2,50
5,00
Speed [rpm]
540
540
1080
1080
2520
3600
3600
Frequence [Hz]
9
9
18
18
42
60
60
Frequence p.u.
0,15
0,15
0,30
0,30
0,70
1,00
1,00
Derating factor* (df)
0,65
0,65
0,77
0,77
0,91
0,95
0,95
* According to WEG derating criteria for standard motors under constant flux (constant V/f)
condition (subclause 6.4.1.2)
Technical guide – Induction motors fed by PWM frequency inverters
31
www.weg.net
Thus,
æ T50% æ2 æ T100%æ2
æ T 75% æ2 æ T50% æ2 æ T100%æ2
æ T50% æ2 æ T100%æ2
.2+
.18+
.4+
.2+
.18+
.6+
.10
æ1,00
æ1,00
æ1,00
æ1,00
æ1,00
æ1,00
æ
æ
æ
æ1,00
æ
æ
æ
æ
æ
æ
æ
(2+18+4+2+18+6+10)
(2+18+4+2+18+6+10)
æ 3,75 æ2 æ 2,50æ2 æ 5,00æ2
æ 2,50æ2 æ 5,00æ2
æ 2,50æ2 æ 5,00æ2
.2+
.18+
.4+
.2+
.18+
.6+
.10
æ0,65
æ0,65
æ0,77
æ0,77
æ0,91
æ0,95
æ0,95
Teq =
æ
æ
æ
æ
æ
æ
Teq =
æ
Teq =
æ
æ0,65
æ
æ T50% æ2 æ T100%æ2
æ T 75% æ2 æ T50% æ2 æ T100%æ2
æ T50% æ2 æ T100%æ2
.2+
.18+
.4+
.2+
.18+
.6+
.10
æ0,65
æ0,77
æ0,77
æ0,91
æ0,95
æ0,95
Teq =
æ
Thus,
(2,50)2 .2+ (5,00) 2 .18+ (3,75) 2 .4+ (2,50) 2 .2+ (5,00) 2 .18+ (2,50) 2 .6+ (5,00) 2 .10 =
1268,75 = 4,60 kgfm
(2+18+4+2+18+6+10)
60
æ
(2+18+4+2+18+6+10)
(3,85) 2 .2+ (7,69) 2 .18+ (4,87) 2 .4+ (3,25) 2 .2+ (5,49) 2 .18+ (2,63) 2 .6+(5,26) 2 .10 =
2072,60 = 5,88 kgfm
(2+18+4+2+18+6+10)
60
The load demands the following horsepower then:
The load demands the following horsepower then:
æ 58,8 æ 3600
æ 9,81
æ
P=
960
= 22,48 kW = 30,14 hp
P=
Consulting the WEG Electric Motors Manual, for 3600 rpm
and 60 Hz, the ideal motor for this application is a threephase 30 kW (40 hp), 2 poles, 60 Hz, frame IEC 200M
(NEMA 324T) TEFC.
8.5 Example considering the use of WEG Optimal Flux
8.5.1 Example
Considering the same application of the last example, please
dimension a self-ventilated squirrel cage induction WEG
Premium Efficiency motor to be driven by a frequency inverter
WEG model CFW-09 (software version 2.40). It is desired
temperature rise of thermal class F (105 K).
8.5.2 Solution
Observing the motor line (Premium Efficiency) and the
inverter characteristics (CFW09 version 2.40 or higher) it is
remarkable that in this case the optimal flux can be
beneficially used. This example aims to evidence the
advantages provided by the employment of the optimal flux
solution.
It will be necessary to reckon again the derating factor (df)
applicable at each stretch of the duty cycle, but this time
according to the torque derating criteria valid for Premium
Efficiency motors at optimal flux condition (subclause 6.4.1.2),
considering the temperature rise of class “F”.
Period [min]
2
18
4
2
18
6
10
Torque p.u.
0,50
1,00
0,75
0,50
1,00
0,50
1,00
Torque [kgfm]
2,50
5,00
3,75
2,50
5,00
2,50
5,00
Speed [rpm]
540
540
1080
1080
2520
3600
3600
Frequence [Hz]
9
9
18
18
42
60
60
Frequence p.u.
0,15
0,15
0,30
0,30
0,70
1,00
1,00
Derating factor* (df)
1,00
1,00
1,00
1,00
1,00
1,00
1,00
æ 45,98 æ 3600
æ 9,81
960
= 17,58 kW = 23,58 hp
Consulting the WEG Electric Motors Manual, for 3600 rpm
and 60 Hz, the ideal motor for this application is a threephase 18,5 kW (25 hp), 2 poles, 60 Hz, frame IEC 160M
(NEMA 284T) TEFC. It was thereby shown that the optimal
flux solution provides a better utilization of the energy,
allowing for a smaller frame motor to attend the application
needs yet not using forced ventilation or oversizing.
9 Recommendations for the
measurement of PWM waveforms
9.1 Warning
The measurements dealt with in this clause involve potentially
lethal voltage and current levels. Only qualified individuals,
familiar with the construction and operation of the equipment
and hazards involved should take these measurements.
9.2 Instrumentation
Frequency inverters supply motors with PWM voltage, which
is non-sinusoidal. Measurements of such voltages must be
taken with proper equipments in order to be reliable. Modern
digital measurement instruments that are able to read true
rms values must be used. Some of them will not read the
fundamental component of a PWM waveform though.
Harmonic measurement instruments with fast enough
sampling rate are capable of reading both rms and
fundamental values of voltage, current and power. An
oscilloscope with isolated probes and proper bandwidth is
appropriate in most cases.
* According to WEG torque derating criteria valid for high efficiency motors under optimal
flux (optimal V/f) condition (subclause 6.4.1.2)
32
æ
Teq =
Technical guide – Induction motors fed by PWM frequency inverters
www.weg.net
9.3 Parameter measurements
According to the NEMA Application Guide for AC ASD
Systems, the recommended instrumentation for the
measurement of various parameters should be as described
in the table below.
Recommended instrumentation for the measurement of
various parameters
Parameter
Control input
voltage
Typical Reading
Fundamental
Transient
Instrumentation
Required
Analog or digital
Reason
Very control input
voltmeter
voltage
20 MHz or higher
Capture line voltage
storage oscilloscope
variation
A meter capable of
Fundamental
Control output
input voltage
Peak transient
and dV/dt
Control input
current
True ms
True ms
input voltage
Very motor input
of a non-sinusoidal
voltage
Oscilloscope with a
sampling rate of at least
1Ms/sec
Fundamental
Compare to the
and rise-time
withsand capability
True ms meter
Very feeder size
True ms meter
Estimate overheating
measuring fundamental
of a non-sinusoidal
Estimate torque
wave form
Input voltage
Fundamental
harmonics
plus harmonics
Input current
Fundamental
harmonics
plus harmonics
Spectrum analyzer
Spectrum analyzer
9.5 Measurement procedures
Actual operation conditions (especially concerning motor
speed, control type and switching frequency) should be
attended when taking measurements. It is worth noting that
higher speeds imply higher voltage levels and therefore
operation at the highest speed within the operation frequency
range will probably result in the highest possible voltage
peaks at the motor terminals.
motor’s peak voltage
A meter capable of
Control output
currente or motor
measuring fundamental
wave form
voltage or motor
9.4.2 Grounding of motor
The output ground conductor may be run in the same
conduit as the AC motor power leads. The grounded metal
conduit carrying the output power conductors can provide
EMI shielding, but it does not provide an adequate ground for
the motor; a separate ground conductor should be used. The
motor’s ground wire should not be connected to the metallic
conduit.
Ensure compliance
with IEEE-519
Ensure compliance
with IEEE-519
9.5.1 Waveform visualization
The correct evaluation of a VSD System strongly depends on
a proper analysis of the waveforms measured. The
visualization of one cycle (or specific parts of a cycle) of the
PWM voltage waveform at the motor terminals gives an idea
about the pulses’ quality at the motor terminals. For a better
verification of the consistency of these pulses, the
visualization of two or three cycles is recommended, once it
evidences the repetitiveness of such pulses. A detailed
analysis of a single pulse finally allows that conclusions
about the rise time and the intensity of the peak voltages be
found.
Not pratical due to
dificulty of
Drive efficiency
NA
NA
accurately
measuring motor
output in situ
9.4 Grounding considerations
Safe, reliable and interference-free measurements depend on
good grounding practices. The manufacturer’s
recommendations as well as local regulations concerning
grounding must always be followed when installing ground
wiring.
9.4.1 Grounding of control
The control must be solidly grounded to the main distribution
system ground. A ground common with electrical welding
equipment or large current electrical equipment (typically 5x
rating of the control) should not be used. If either of these
these conditions exist an isolation transformer sized for the
installed control with a wye secondary neutral solidly
grounded should be used. Where more than one control is
used, each of them should be grounded directly to the
system ground terminal - they should not be loop grounded
nor installed in series.
9.5.2 Oscilloscope scale setting
The better choice of which scale should be adopted while
taking measurements will evidently depend on the
magnitudes of the electrical quantities being measured.
However, the ranges shown in the table below are commonly
suitable for 50/60 Hz measurements and can be used as a
first orientation.
Suggestions of oscilloscope’s scale setting
Visualization
X-axis
Y-axis
1 cycle
1  2 ms / div
100  500 V / div
3 cycles
5  10 ms / div
100  500 V / div
1 pulse
0.1  10 s / div
100  500 V / div
9.5.3 Triggering
Oscilloscopes are instruments ordinarily employed for
metering and not for monitoring electrical quantities. In spite
of that, the trigger of some modern oscilloscopes can be
suitably set, so that it is enabled to hold data of particular
interest, for instance waveforms of voltage peaks taken
during transient conditions such as acceleration and
deceleration periods. Further information on this topic can be
found in the User’s Manual of the instrument.
Technical guide – Induction motors fed by PWM frequency inverters
33
www.weg.net
10 Conclusion
The fast advance of the power electronics have allowed
induction motors, the traditional solution for fixed speed
rotating power applications, to be used successfully also in
variable speed drive systems. In such cases, though, the
motor must be fed by means of a static frequency inverter,
rather than directly by the (sinusoidal) power line.
The utilization of squirrel cage induction motors with
electronic inverters presents great advantages regarding
costs and energy efficiency, compared with other industrial
solutions for varying speed applications. Nevertheless, the
inverter affects the motor performance and might introduce
disturbs into the mains power line.
The increasing number of applications with induction motors
fed by PWM inverters operating in variable speed duty thus
requires a good understanding of the whole power system as
well as the interactions among its parts one another (power
line – frequency inverter – induction motor – load).
This Technical Guide aimed to clarify the main aspects
related to the application of squirrel cage induction motors
together with static frequency inverters, presenting
theoretical basics and practical criteria for specific topics,
originated from studies and from the experience of WEG’s
technical body in this subject. The most important and
internationally recognized technical references concerned
with such matters are mentioned and also discussed.
It must be finally considered that the criteria presented here
are not permanent. Like every technology, they may change
as new materials are developed and new experiences are
accomplished. So the application criteria established so far
may be altered without previous advice and therefore it is
important that this document be periodically revised and
updated.
34
Technical guide – Induction motors fed by PWM frequency inverters
www.weg.net
11 Bibliography
NEMA MG1 Part 30 - Application considerations for
constant speed motors used on a sinusoidal bus with
harmonic content and general purpose motors used with
adjustable-frequency controls or both (2006)
g
NEMA MG1 Part 31 - Definite-purpose inverter-fed
polyphase motor (2006)
g
NEMA - Application Guide for AC Adjustable Speed Drive
Systems (2001)
g
IEC 60034-17 - Cage induction motors when fed from
inverters – application guide (2006)
g
IEC 60034-25 - Guide for the design and performance of
cage induction motors specifically designed for inverter
supply (2007)
g
GAMBICA/REMA Technical Guides for Variable Speed
Drives and Motors
g
GAMBICA/REMA Technical Reports for Variable Speed
Drives and Motors
g
Brochure of the mini-course Squirrel Cage Induction
Motors Fed by PWM Frequency Inverters – R&D of the
Product Department – WEG Equipamentos Eletricos S.A.
– Motores
g WEG Stock Products Catalog
g WEG NEMA Motors Catalog
g User’s Guide of the CFW-09
g
Technical Reports of the R&D Department – WEG
Equipamentos Eletricos S.A. – Motores
g WEG Technological Reports (TT 2000-002, TT 2003-011)
g
Technical Notes of the Development of Products
Department – WEG Equipamentos Elétricos S.A. –
Automação
g
Minimization of Losses in Inverter-Fed Induction Motors –
Optimal Flux Solution – Waldiberto L. Pires and Hugo G. G.
Mello – PCIC BR 2006
g
Low-Voltage PWM Inverter-Fed Motor Insulation Issues –
Michael J. Melfi – IEEE Transactions on Industry
Applications, vol.42
g
Technical guide – Induction motors fed by PWM frequency inverters
35
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Cod: 50029350 | Rev: 29 | Date (m/y): 01/2016
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