NEMA Application Guide for AC Adjustable Speed Drive Systems

NEMA Application Guide for AC Adjustable Speed Drive Systems
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NEMA Application Guide for AC Adjustable Speed Drive Systems
Copyright Material IEEE
Paper No. PCIC-2001-7
David M. Bezesky
Siemens Energy & Automation
Scott Kreitzer
Siemens Energy & Automation
4620 Forest Avenue
4620 Forest Avenue
Norwood, OH 45212-3396
Norwood, OH 45212-3396
Abstract - The application of AC adjustable speed drive
systems has presented many unique challenges not only to
electrical equipment manufacturers, but also to electrical
equipment end users as well. In an effort to aid the user in
proper selection and application of AC adjustable speed drive
systems, the National Electrical Manufacturers Association
(NEMA), through combined efforts of its Motor and Generator
Section and its Industrial Control Products and Systems
Section, has recently published an Application Guide for AC
Adjustable Speed Drive Systems. This paper summarizes key
topics addressed by the guide that will enable the user to avoid
common application pitfalls and assist the user in clearly
communicating critical application information and requirements
to the electrical equipment manufacturer.
A three-phase AC induction motor has for over 100 years
proven to be an extremely reliable electromechanical conversion
device. For the vast majority of that time period it has evolved
as a constant speed device that operates from a constant
frequency, constant voltage sinusoidal utility power source. Its
characteristics have been well defined and standardized by the
National Electrical Manufacturers Association (NEMA).
In industrial processes there has always been a need for
speed variation to meet the needs of flow or torque control.
However, flow control has historically been handled
mechanically by throttles, valves, and dampers. In some
instances, variable speed was handled electrically with DC
equipment, multi-speed induction motors, or with single speed
motors used in conjunction with slip producing devices. In
others it was accomplished with mechanical drivers such as
turbines or gas engines. Except for the use of DC equipment,
all of these other methodologies incurred a severe penalty in
system efficiency.
Advances in power electronics over the last couple of
decades have enabled a change in the approach to process
control. Speed and/or torque control are now commonly
accomplished by supplying variable voltage and frequency via
an adjustable frequency control (AFC) to an induction motor.
This change in approach has enabled the elimination of gears,
clutches, valves, throttles, dampers, and other equipment from
industrial systems.
These changes have largely been implemented very
successfully and have had a very positive impact on costs and
efficiencies in the process industries. On the other hand, the
electromechanical industry has endured many growing pains
throughout the maturation and application of new and constantly
improving AFC technologies. A plethora of excellent papers
and publications over the last several years have very well
documented many of the now well known challenges and pitfalls
that may be encountered.
Proliferation of this technology is only expected to accelerate.
It is expected to find its way into many yet to be considered
applications in the future. Because of this, NEMA formed a
committee of industry experts from both its Motor and Generator
section and its Industrial Control Products and Systems Section.
The committee has undertaken the task of assembling into a
single document information and industry accepted approaches
necessary for problem free implementation of an adjustable
speed drive (ASD) system.
The guide covers AC electrical drive systems, rated 600 volts
of less, consisting of three-phase induction motors, voltage
source pulse width modulated adjustable frequency controls,
and associated components. The guide addresses common
issues that should be considered in the selection of drive system
components and the installation and application of the drive
system. Its text exceeds 75 pages, so it is therefore an
impossible task to reasonably summarize its sections here in
this short paper. Instead, the authors have attempted to select
common issues that may be helpful to the application engineer
in communicating with his system components suppliers. All
tables and figures were taken from the NEMA documents listed
in the references.
The proper selection and installation of the drive motor is
absolutely essential to the successful operation of any variable
frequency drive system.
For this reason, various motor
parameters, such as horsepower and torque requirements, the
speed range of the motor, the acceleration and deceleration
Page 1 of 10
requirements, and duty cycle should be acquired and specified
during the initial planning phases of the ASD application.
Motor Enclosures
One of the fundamental specifications of an AC induction motor
includes the type of enclosure. The environment in which the
motor may operate and the risk of the motor's internal
components being exposed to airborne particles often dictates
the type of enclosure required. Examples of enclosures for nonhazardous locations include open drip proof, totally enclosed fan
cooled, totally enclosed non-ventilated, totally enclosed blower
ventilated, and open blower-ventilated. Each of these
enclosures provides varying degrees of protection and should
be carefully evaluated based on the motor's operating
environment. Motors used in Division I (hazardous) locations
should be certified for these environments and clearly identified
as such on the motor nameplate. Before using a motor in a
Division II location, the motor manufacturer should be notified.
Interaction between Motor & Load
A second important consideration in the proper selection of a
drive motor is the characteristic of the driven load. The loads of
many applications can be defined by one of three primary types:
variable torque, constant torque, and constant horsepower. The
first load type, variable torque, is typical for processes such as
centrifugal pumps, centrifugal fans, centrifugal blowers, and
centrifugal compressors. The torque load in these applications
usually varies linearly with speed or with the square of the speed
as illustrated in Fig. 1 below.
Per Unit of Base Speed
Fig. 2. Constant Torque Load Curve
The third load type, constant horsepower, is shown below in Fig.
3. Center driven winders and machine tools normally exhibit
this type of load. These applications maintain a constant
horsepower or a given speed, while the torques vary with the
inverse of the speed.
Per Unit of Base Speed
Fig. 3. Constant Horsepower Load Curve
Per Unit Torque or HP of Driven Load
Per Unit Torque and HP of Driven Load
Per Unit of Base Speed
Fig. 1. Variable Torque Load Curve
The second typical motor load type is constant torque, in which
the load torque remains constant over a given speed range
(See Fig. 2). This loading is typically representative of
applications with high impact loads or duty cycles. Examples
include conveyors, augers, reciprocating compressors, crushers
and positive placement pumps.
In addition to the load variation of a machine with respect to
speed, some applications experience transient loads, or
changes in load over time. The first type of transient loading,
duty cycle, refers to applications where a machine may
experience periodic, discrete load changes over a certain
interval of time. A duty cycle load may or may not be dependent
on speed. (Fig. 4) A second type of transient load, impact
loading, usually involves sharp, abrupt changes in load which
are not dependent on speed. Impact loads may be non-cyclic in
nature. When a motor operates under these variable loads,
three important issues may need to be considered.
1) Will the motor be shut down during certain portions of
the duty cycle? Motors with shaft-mounted fans have greater
cooling potential at increased speeds. During periods of brief
shutdown, the heat from the motor operation may take time to
2) Does the duty cycle or impact loading require torques
above the rated motor full load torque? Increased torques may
heat the motor beyond its designed limits and may require a
higher class of insulation. Momentary loads that require torque
greater than 140 percent of the motor full load torque at base
speed or below should be stated in the motor specification.
Page 2 of 10
Additionally, for periodic torque loads greater than 110% of the
rated torque at the maximum speed, consult the motor
manufacturer. NOTE: Torque overloads may also be needed to
accelerate a given inertia over a small period of time.
3) Will regenerative braking be required to stop or slow the
motor during the duty cycle? Regenerative braking causes
increased motor heating, which cannot be neglected when
selecting the motor for the application.
Fig. 4. Duty Cycle
The various torque capabilities of standard induction motors,
denoted by Designs A, B, C, and D in NEMA MG1, were
originally developed for motors driven by sinewave power but
can be applied to motors operating on variable frequency drives.
Fig. 5 below illustrates the torque speed relationship between
the various motor designs.
than design B motors. Design D motors are generally used in
applications that require high starting torque or high inertia
loads. Additional characteristics include high slip and lower
C. Interaction between Motor & Control:
Safe operating speeds are usually pre-defined in the ratings of
inverter fed motors. When operating a general-purpose motor
on an adjustable frequency drive, however, several motor and
control speed characteristics need to be considered.
1) Continuous Maximum Motor Speed: A motor's speed
capability is most often limited by the mechanical stress limits of
the rotating structure. For continuous operation, motors
operating above 90 Hz with constant voltage above 60 Hz may
not have the required torque to sustain a constant horsepower
load. Maximum, safe operating speeds for Design A and B
motors are given in NEMA MG 1, Part 30. The control's
maximum speed should be set such that the motor is not
unintentionally operated beyond the recommended speeds. If a
continuous speed greater than the operating speed listed in
NEMA MG 1 is required, the motor manufacturer should be
2) Maximum Motor Overspeed: Motors typically may be
required to operate for a short period of time beyond the
maximum speeds listed in NEMA MG 1. A motor with a
maximum speed greater than its rated synchronous speed can
typically be over-speed 10% beyond its maximum speed for a
period of two minutes. Motors with maximum speeds equal to
their synchronous speeds should follow the guidelines listed in
table 1 for general-purpose motors and table 2 for inverter fed
Synchronous Speed,
Overspeed Percent of
Synchronous Speed
1801 and over
1201 to 1800
1200 and below
250-500, incl.
1801 and over
1800 and below
Table 1. Over-speed Capability of General-Purpose Motors
200 &
A or B
Maximum Operating Speed,
Overspeed, Percent of
Maximum Operating Speed
3601 and over
1801 to 3600
1800 and under
Table 2. Over-speed Capability of Definite Purpose InverterFed Motors
Fig. 5. NEMA MG1 Load Designs for Induction Motors
Operating on Sine wave power
Design A motors generally have low slip and high efficiencies,
but may not be the best choice for applications requiring
bypass or across the line starting. Design B motors are
generally characterized by their high efficiencies and low slip.
These types of motors are typically used in variable torque,
constant torque and constant horsepower applications. Design
C motors were originally designed to accommodate the high
starting torque requirements of across the line starting. These
motors typically exhibit higher motor losses and lower efficiency
3) Operating Speed Range: Motor and drive applications
typically operate on a wide variety of speed ranges. Examples of
typical speed ratios for constant and variable torque applications
are listed below in table 3. Speed ratios for constant and
variable torque motors are expressed in terms of motor base
speed to motor minimum speed.
Applications that require
extremely slow (below 6 Hz) or extremely high speeds may
require a custom motor design.
Page 3 of 10
(Base Speed = 2500 RPM)
Minimum Speed
% Motor Base
Speed Range
Table 3. Constant and Variable Torque Speed Range Examples
Typical speed ranges for constant horsepower motors are listed
in table 4 below. Speed ranges for these applications are
expressed as motor maximum speed to motor base speed.
(Base Speed = 2500 RPM)
Maximum Speed
% Motor Base
Speed Range
Table 4. Constant Horsepower Speed Range Examples
4) Acceleration: Several items need to be considered when
using an adjustable frequency drive to accelerate a load. The
amount of torque and therefore current required to accelerate a
given load increases as the acceleration time is reduced. The
total amount of necessary accelerating torque is the sum of two
components: the torque required by the load plus the torque
required to overcome the inertia of the rotating assembly. This
total torque requirement can be calculated by the following
Trequired =
Wk 2 × ∆RPM
+ Tload
308 × t
= Torque to accelerate load (lb-ft)
= Load torque during acceleration. Use average
torque for variable torque loads.
= Inertia of the load reflected to the motor plus the
inertia of the motor (lb-ft2)
= Change in motor speed desired
= Time (seconds) required to accelerate motor
Applications requiring greater acceleration times may require an
oversize control to meet the increased current demands. If a
motor is required to produce more than 140% of the motor full
load torque during acceleration, the motor manufacturer should
be consulted.
5) Deceleration and Braking: Deceleration is often used to
save production time, prevent damage to the attached
equipment, or meet specific duty cycle requirements. Because
of the friction and windage of the rotor assembly, deceleration
time is at least equal to or less than the acceleration time. For
duty cycles that require significant deceleration over a short
period of time, dynamic or regenerative braking may be
required. Dynamic braking is typically used on applications with
high inertia loads; however, dynamic braking cannot produce
holding torque at zero speed. In this instance a mechanical
brake is required. When using dynamic braking, the following
equation can be used to estimate the rotating inertia of a
medium ac induction motor:
Poles  
 Poles 
 2 
1.35 − 0.05 x 2 
Wk = 0.02 x 2
x HP
Regenerative brakes return some of the energy generated from
the braking process to the motor and control power source.
However, like dynamic braking, regenerative braking also does
not posses holding torque capability.
6) Starting Requirements: For applications with high
starting torques, it is important that the motor has enough
breakaway torque to start the load. Applications with static
starting torques that are 140% above the motor full load torque
may require an oversized control and a motor with a large
torque capacity. Voltage boosts may also be used to obtain
high starting torques and low frequency operation (below 20 Hz)
without the associated effects of high starting current. When
using voltage boosts, care should be taken to ensure that the
motor does not operate at no-load conditions below 10 Hz for
more than one minute.
7) Bypass (Across the Line Starting): Applications that
require bypass or across the line starting may need special
consideration. Firstly, the motor must be capable of both across
the line starting and across the line operation. Some inverter fed
motors are designed specifically for connection to a control and
are incapable of across the line starting. Secondly, the motor
wiring must be sized for both AFD and bypass starting. Thirdly,
the control may be set to limit the maximum speed of the motor.
On bypass power, the motor could exceed the maximum speed
recommendations. Fourthly, in order to avoid damage to the
motor during power transfer, a time delay of one and one half
AC time constants should be observed before switching from
control to line power, while a time delay of three AC time
constants is necessary for the reverse procedure. Lastly, a
special control may be needed if it is necessary to switch from
line power to control power while the motor is rotating.
8) Motor Terminal Voltage Transients: High switching rates
of the transistors in today's controls can cause voltage
overshoots at the motor terminals of squirrel cage AC induction
motors. These overshoots are illustrated in Fig. 6 below.
Significant damage to the motor insulation can occur if these
overshoots are greater than the maximum rated voltage of the
motor. As defined by NEMA MG1, Part 30, general-purpose
motors should be limited to voltage overshoots of less than
1000 volts. General-purpose motors that operate at less than
460 volts generally due not produce overshoots of this
magnitude. Motors with voltages of 460 or greater may require
filters or reactors to reduce the amount of overshoot to an
acceptable level. According to NEMA MG1, Part 31, inverter fed
motors are designed to withstand "3.1 times the motor's rms
voltage with a rise of not less than 0.1 µs" without filters or
reactors. There are five primary factors that cause increased
voltage overshoots.
Page 4 of 10
Fig. 7. Anti-Friction Bearing Inner Race With Severe Fluting
Fig. 6. Voltage Overshoots
These include:
a) Short rise times in the transition of high to low voltage
at the motor terminals.
b) Long motor leads between the motor and control.
c) Small time periods between ASD voltage pulses.
d) Double transition conditions where the control switches
two phases simultaneously.
e) Voltage reflections due to multiple motors connected to
the same control.
In order to avoid voltage overshoots, the following precautions
should be implemented as required.
a) Use an inverter fed motor with a voltage of 230 or less
when possible.
b) Use the lowest carrier frequency that satisfies the
motor requirements. A lower switching frequency
results in less overshoots per second.
c) Avoid connecting multiple motors in parallel to the
same control.
d) If the peak voltage is over the recommended limit, use
a filter or reactor between the control and motor.
9) Shaft Voltages and Bearing Currents: In many motors
operating on adjustable frequency drives, shaft currents have
been found to discharge through the motor bearings, breaking
down the bearing grease and causing a severe wear pattern in
the bearing called "fluting." If allowed to continue, these shaft
currents will lead to high motor vibration levels and eventual
bearing failure. Fig. 7 illustrates the detrimental effects of a
continuous flow of shaft currents through an anti-friction bearing.
There are four primary causes of bearing currents, each of
which may create voltages high enough to discharge through
the bearings:
a) An unbalanced magnetic circuit can cause current to
flow in a closed loop through the shaft, bearings, end
brackets, and the motor frame.
b) Applications such as paper rollers may emit an
electrostatic charge due to the friction between the
motor shaft and the driven load.
Common mode voltages or voltage fluctuations
resulting from the switching frequency of the control
may cause unwanted bearing currents.
Common mode voltages may also cause a capacitive
coupling between the stator and rotor, creating a path
to ground between the above mentioned capacitance,
shaft, bearings, and grounded end bracket. See Fig. 8
Fig. 8. Path of Common Mode Voltage the Stator Windings to
Several safeguards against the detrimental effects of shaft
currents include the following:
a) Use an inverter fed motor with a voltage of 230 or less
when possible.
b) Use the lowest carrier frequency that satisfies the
motor requirements.
c) Install a shaft grounding brush to the motor, which will
offer an alternative, less restrictive current path around
the ball bearings.
d) Insulate the bearings on both ends of the motor.
e) Use a non-conductive coupling.
f) Ground the motor and control as required by the
g) Reduce common mode voltage at the control by
installing a drive filter.
Page 5 of 10
10) Sound & Vibration Considerations: The motor and
control system may posses several natural frequencies that may
be excited by the control when operating within a particular
frequency range. These natural frequencies may exist in the
horizontal, vertical, axial, or torsional directions and are affected
by factors such as the mass and stiffness of the motor base, the
type of control, the natural frequencies on the motor structure,
the electromagnetic design of the motor, coupling vibration, and
air flow. Once these natural frequencies have been identified,
the control should be programmed to avoid prolonged operation
at these speeds.
11) Thermal Considerations: Unlike motors operating on an
AC power supply, motor and drive systems have several
sources of heat generation resulting from the interaction
between the motor and control. In certain instances these
thermal effects may be high enough to justify derating or over
sizing of a motor or control.
a) Inverter losses are a combination of both the control
switching frequency losses and the conduction losses
due to the voltage gradient across the device. The
switching losses increase with higher frequencies, forcing
most of the higher horsepower controls to use lower
switching frequencies.
b) Current distortion is another source of control related
heat generation and is inversely proportional to switching
frequency. The motor losses decrease with increasing
switching frequency up to the point where switch dead
band becomes significant.
c) For motors with shaft mounted fans (speed dependent),
the amount of cooling decreases at slower speeds. For
variable torque applications, where the load also
decreases with speed, a speed dependent motor may be
adequate. Applications that require high torque at low
frequencies may require a motor with an auxiliary blower
(speed independent) to provide constant cooling at all
d) General-purpose motors have published maximum
allowable temperature rises for operation on continuous
sine wave power. When using these motors on an ASD,
the motor may need to be derated to meet these
requirements. Other factors affecting temperature rise,
such as duty cycle, high ambient temperatures, and high
altitude operation may also influence the decision to
derate the motor.
Control Types
Adjustable frequency controls are typically rated by the
amount of output current that they can supply on a continuous
basis for a defined maximum ambient temperature. Their
nameplates may be marked with a horsepower, but this should
be used for reference purposes only. For example, a control
that is capable of supplying a 10 horsepower 2 pole motor may
not be capable of supplying a 10 horsepower 24 pole motor
because of its significantly lower power factor, the efficiency of
the slower speed motor, and its correspondingly higher full load
current requirement. Controls are generally identified as two
basic types as distinguished by their short-time overload current
1) Variable Torque: A variable torque control is typically
capable of a 110 percent to 125 percent over current relative to
its nameplate rating for 1 minute. This overload capability is
normally sufficient for variable torque loads. It should be noted,
however, that a variable torque control is not limited to variable
torque applications.
2) Constant torque: A constant torque control is typically
capable of a 150 percent over current relative to its nameplate
rating for 1 minute.
Control Techniques
There are many types of controls and control techniques
available in the marketplace today. However, the only control
techniques that are applicable to three phase AC induction
motors within the scope of the NEMA Application Guide are
volts/Hertz control and vector control, which can be subdivided
into two schemes: sensorless vector and feedback vector
1) Volts per Hertz Control: A volts per hertz control
maintains a fixed volts to hertz ratio over its prescribed
operating range. Motors with respective base ratings of 230
volts or 460 volts 60 Hz have volts per hertz ratios of 3.83
(230/60) and 7.67 (460/60). Once established by the control set
up, the voltage supplied to the motor by the control at various
operating frequencies is strictly governed by this ratio unless
voltage boost or IR compensation is activated, or the frequency
is increased beyond a value for which system voltage is
sufficient to maintain it. Voltage boost is a fixed voltage that is
added to the voltage prescribed by the volts per hertz ratio.
Voltage boost has much more effect at lower frequencies where
the prescribed voltage is low.
Voltage boost has the
disadvantage that it may cause saturation and overheating of a
motor that is lightly loaded at low speeds.
With IR
compensation, the amount of boost applied is proportional to the
amount of current drawn by the motor. Consequently, at light
loads, a voltage that is high enough to saturate the motor will
not be applied. The operating region where the frequency
increases beyond the point at which the available system
voltage can maintain the voltage prescribed by the volts per
hertz ratio is known as the field-weakening region. It is referred
to as the field-weakening region because the motor magnetic
flux is decreased in proportion to the volts per hertz ratio. Load
torque must be reduced in this area of operation because motor
torque decreases with this decrease in motor flux.
2) Vector Control: A vector control decouples the
magnetizing flux producing and torque producing currents
supplied to a motor and controls them separately. An ASD that
utilizes this control technique exhibits very good steady state
and dynamic performance and very accurate speed and torque
control, comparable performance to that obtainable from a DC
motor. As mentioned earlier, vector control can be subdivided
into two different schemes as identified below.
Page 6 of 10
Direct vector control: A direct field orientated control
scheme makes use of Hall effect transducers or air
gap flux-sensing windings to measure the air gap flux
with the intent of regulating it in order to produce
controllable motor torque.
Indirect vector control: Indirect field orientated control
interprets the motor flux from parameters such as
speed or current. A closed loop vector control makes
use of a speed feedback sensor and can provide
precise speed control and maximum torque throughout
a speed range that extends anywhere from zero speed
to base speed. An open loop vector control monitors
motor current instead of motor speed and cannot
produce holding torque at zero speed. It also has a
narrower operating speed range than a closed loop
vector control.
To ensure successful operation of an adjustable speed drive
system, when selecting the control, each performance
consideration must be reviewed.
There are specific
considerations that point to a given technique. Common
performance considerations are given below in table 5.
Speed regulation < 1%
Low speed torque
capability < 6 Hz
Torque regulation
Speed range >20:1
High breakaway
torque >150%
Table 5
rated for 95% humidity, non-condensing. Condensation may
occur if the equipment becomes cooler than the surrounding air
temperature because of varying ambient temperatures.
Provided that the ambient temperature is above 0 °C, leaving
the control energized continuously will provide enough heat to
minimize condensation. When temperatures are likely to drop
below 0 °C, the control enclosure should be fitted for space
4) Outdoor Mounting: An adjustable frequency control may
be located outdoors when adequate protection against falling
rain, ambient temperature, including the expected sun load, dust
and dirt, and the watt-losses dissipated from the control are
provided, preventing temperature rise conditions beyond the
component specifications. This usually requires a NEMA 4
enclosure with an adequately sized air conditioner mounted on
the enclosure to maintain the temperature rise within
5) Vibration Conditions: Most adjustable frequency controls
have the ability to operate in an environment of continuous
vibration of .3 mm peak from 2 to 9 Hz, or acceleration of 1 m/s2
from 9 to 200 Hz. If an installation has vibration levels that
exceed manufacturer specification, the control must either be
located in a lower vibration area or it must be mounted on a
vibration absorbing assembly.
The following is a summary of NEMA designated enclosures
for electrical equipment.
NEMA Standard 250 contains
additional information on control enclosure classifications.
NEMA 1 -
NEMA 12 -
Control Enclosure and Environmental Considerations
The environmental conditions of the control installation are
also an important consideration.
Based upon these
considerations, suitability of the designated NEMA enclosure
must be evaluated. Conditions that should receive special
attention are ambient temperature, altitude, humidity, outdoor
mounting, and shock and vibration conditions.
1) Ambient Temperature: Adjustable frequency controls are
typically suitable for operation in a temperature range of 0 °C to
40 °C. When ambient temperatures are expected to dip below 0
°C, the enclosure should be fitted with space heaters. Ambient
temperatures that exceed 40 °C require derating of the control
output per recommendations of the manufacturer.
2) Altitude: Beyond 3300 feet above sea level, thinner air
negatively impacts the ability of the heat sink to adequately cool
the electronic equipment. As a result, the adjustable frequency
control must be derated according to the recommendations of
the manufacturer.
3) Humidity: Adjustable frequency controls are typically
NEMA 4 -
Designates enclosures that are designed for
indoor use. These enclosures protect the
components they contain from physical contact
with operating and maintenance personnel.
Designates enclosures that are designed for
outdoor use. These enclosures protect the
components they contain from falling rain, sleet
and external ice formation.
Designates enclosures that are designed for
indoor use. These enclosures protect the
components they contain from dust and dripping
liquids. This includes protection against fibers,
flyings, lint, dust, dirt, and non-corrosive dripping
Designates enclosures that are designed for
indoor and outdoor use. These enclosures protect
the components they contain against dust, dirt,
splashing water, falling water, seepage, hosedirected water, and external condensation.
Designates enclosures that are designed for
indoor and outdoor industrial use. NEMA 4X
enclosures protect the components they contain
from the same elements as systems designated
NEMA 4, but they are also corrosion resistant.
Control Input Voltage
Typical Control voltage ratings are 200, 208, 230, 460, and
575 volt. Depending upon the magnitude of line voltage
transients and the type of control design, surge protection may
Page 7 of 10
or may not be required for the control. If line voltage transients
are known to be a problem in the installation, a line reactor or
isolation transformer may be used for transient attenuation.
Control HP/Current Rating Considerations
Most controls are horsepower rated based upon full load
amperes listed in the National Electric Code Table 430-152.
These current ratings are typically for 2 and 4 pole motor
The control may need to be oversized to
accommodate higher pole count motors. Consequently, when
selecting an adjustable frequency control, the output current
rating should be based upon the connected motor nameplate
rated full load current and not on its horsepower rating.
Additionally, the control may need to be oversized to
accommodate application requirements such as high breakaway
torque, overload, or accelerating torque. Another reason to
upsize the control is multi-motor operation.
(Short time over current capability has been addressed in
section A, control types.)
XLPE should have a longer life relative to reflected wave voltage
Further tests were performed on 30 mil XLPE and 15-mil
PVC wire under wet conditions of 90 percent relative humidity
for 48 hours. The tests showed that under these conditions the
CIV of XLPE only decreased by 5 percent while the CIV for the
PVC decreased by 50 percent. Looking at table 6, it is obvious
that PVC wire with insulation thickness 20 mils or less is of
concern when used in moisture-laden environments. It is clear
from the data that 600 volt XLPE and 600 volt PVC of thickness
30 mils are adequate for applications whose terminal voltage
conditions are within the guidelines prescribed by NEMA MG 1,
part 31. PVC wire of 15 and 20 mils should be restricted to dry
Power Cable Selection
A control nameplate may have two current ratings listed on its
nameplate: an input and an output current rating. The input AC
current rating may be higher than the output current rating
because of current harmonic distortion. Care should be taken to
size the input cables according to this current rating. The output
cables must be sized according to the motor nameplate current
rating. Other considerations include motor terminal voltage
transients and common mode voltages and their associated
common mode currents.
The most likely cable insulation failure mechanism to occur
would be degradation due to corona. Corona inception voltage
(CIV) is the minimum applied voltage between cables that will
result in partial discharges in the air spaces between cables.
No degradation of service life can be expected if the CIV
measured peak voltage is higher than the peak transient
voltages that are expected in service. The NEMA Application
Guide offers test results for XHHW XLPE and THHN PVC
insulated wire. Excerpts from that table are offered here in
table 6. The test results show that both insulation types should
achieve rated life under dry conditions. Because XLPE had
higher CIV than PVC for the same thickness of insulation,
250 to
MCM to
1 thru 2/0
1) Motor Terminal Voltage Transients: The impact of motor
terminal voltage transients due to reflections in the cables
between the motor and control must be considered. A definite
purpose inverter rated motor as defined by NEMA MG 1, Part
31 must be able to withstand terminal voltage transients of Vpeak
= 3.1xVrated. That means that terminal voltages can range from
1488 – 1860 volts for motors rated 480 – 600 volts. Although
the peak transient voltage duration is less than 1 µs, the
transients occur at the control output device carrier frequency
rate, which is typically 3 to 13 kHz for drives to 20 Hp and 1.5 to
3 kHz for larger drives. Thus, there is concern that satisfactory
life may not be achievable for 600 Vrms cable.
7.5- 20
12 thru
0.5- 5
12 thru
Table 6
2) Common Mode Considerations: Common mode
voltages are a natural result of either the PWM modulation
scheme or various cable and grounding dissymmetries. Proper
selection of cable helps to mitigate these voltages and their
resultant currents.
While many installations perform
successfully with standard cable, to assure that this issue is
minimized, continuous welded aluminum armor cable may be
G. Interaction Between Control and Power Supply
The performance or protection of an adjustable frequency
control can be affected by the power distribution system. These
issues and interactions are reviewed in the following sections.
Page 8 of 10
1) Source Impedance/Short-circuit Ampere Interrupting
Capacity (AIC) Ratings: In order to assure proper operation and
protection of the equipment, the system impedance and AIC
rating of the power source that an adjustable frequency control
is connected to must be reviewed. The AIC rating is typically
listed on the control nameplate. The control AIC rating should
exceed the available short circuit current from the power source.
When this is not the case, current-limiting fuses and line
reactors should be used to provide protection. Additionally,
source impedance should be reviewed in the event that bypass
operation is desired. It must be low enough to assure that
voltage does not sag excessively at the motor terminals during
across the line starting.
2) Line Voltage Variations: Most adjustable frequency
controls will operate satisfactorily with a +/- 15 percent variation
in voltage. However, the other electromechanical devices
associated with the ASD including the motor are limited to a +/10 percent voltage variation.
3) Line Voltage Phase Unbalance: Phase voltage
unbalance results in a phase current unbalance. If this current
unbalance results in phase current that exceeds the rectifier
device rating, damage to the control may occur. Adjustable
frequency controls will typically tolerate a maximum of 3 percent
input voltage phase unbalance.
Control Protection
Due to its limited capacity, protection circuits must be utilized
to prevent control failures under certain fault conditions.
1) Short Circuit Protection: It is the responsibility of the user
to provide branch circuit protection according to the National
Electrical Code. For additional information, see part 1 of section
2) Transient Voltage Protection: Transient voltages
occurring on AC power systems likely originate from lightning,
system switching transients, and capacitor switching. A control
may be very sensitive to voltage transients compared to other
equipment connected to the power system. Metal oxide
varistors are commonly used for transient voltage protection.
When a varistor is exposed to a voltage transient, its impedance
changes from near infinity to nearly zero in order to clamp the
transient voltage to a safe value. When the varistors act, a
short circuit occurs which could result in a fuse operation and
tripping of the control.
3) Overvoltage Protection: The major causes of control
overvoltage tripping are power system transients, lightning,
regenerative loads, and poor grounding techniques. When the
control senses an overvoltage condition, an overvoltage
protective fault will occur. The control overvoltage trip is not
adjustable and is used to protect the control from component
failure. Persistent tripping requires corrective action. Line
reactors or isolation transformers may be used to protect
against line voltage transients. Dynamic braking resistors may
be useful for dissipating the regenerative energy from
regenerative or overhauling loads.
4) Undervoltage Protection: An undervoltage condition
may be the result of low line voltage of momentary power
interruption. During undervoltage conditions, the voltage output
of the control may be reduced, resulting in decreased motor
output torque and system performance. A control may include a
re-start or ride-through function to minimize the effects of a
momentary power interruption. The ability of ride-through to
maintain control of the connected motor through the event is
dependent on the duration of the undervoltage, the amount of
stored energy available from the control, and the demands of
the connected motor load.
5) Single Phase Input Protection: Single-phase operation
will result in a significant increase in input current in the
unaffected phases, causing additional heating in the electronic
rectifier devices and additional heating in the DC bus capacitors.
Most controls are equipped with single-phase protection that
either reduces the load on the equipment or shuts down the
6) Ground Fault Protection: A ground fault may be the
result of a motor phase to ground fault, a motor cable phase
shorted to ground, or parasitic capacitive coupling to ground.
The control protects itself from component failure in the event of
an output ground fault condition. The trip value is specified by
the control manufacturer. Ground faults due to parasitic
capacitive coupling to ground may be cancelled with the use of
a line reactor or an LC filter between the control and motor.
Process loads can generally be characterized by three basic
categories: variable torque, constant torque, and constant
horsepower. In general, most of the considerations covered up
to this point apply to all three of these basic load types.
However, there are a few application considerations that are
unique to each and are identified in this section.
1) Variable Torque Application:
a) Motor: Because of very low torque requirements of
variable torque loads at low speeds, low speed
operation is not normally an important consideration in
these applications. The load decrease more than
offsets the affects of reduced cooling at low speeds.
Because the load torque requirement increases with
speed in these applications, the greatest load and
therefore temperature occurs at the highest operating
speed. As a result, the motor must be sized with
regard to the load torque at top speed. Additionally,
the motor must be oversized if it is intended to be run
above the motor base speed, since torque capability
declines in the field weakening part of the speed
Page 9 of 10
Although load inertia is quite high for some variable
torque applications, rapid acceleration is not normally
needed; therefore, these types of loads can normally
be accelerated without exceeding the rated torque of
the motor. However, bypass is most common in
variable torque applications. If bypass is required for
high inertia loads, it is important to make certain that
the motor is capable of accelerating the load without
Control: Volts per hertz controls typically meet the
requirements of variable torque loads. As mentioned
earlier, variable torque controls typically can supply
110 to 125 percent rated current for 1 minute for
overload and acceleration. If faster acceleration is
required, a constant torque control should be utilized.
The typical speed range for variable torque loads is
2:1; however, all adjustable frequency controls operate
over a minimum speed range of 6:1.
2) Constant Torque Application:
a) Motor: Motor cooling and available motor torque at low
speeds are special considerations for constant torque
applications. In addition, high breakaway torque,
stringent acceleration and deceleration requirements,
overload, and duty cycles are all common
requirements of constant torque loads. All of these
requirements must be clearly and completely
communicated to the motor manufacturer in order to
insure proper motor sizing.
b) Control: Both of the control techniques identified earlier
in this text may be used for constant torque
applications, volts per hertz controls and vector
controls. A volts per hertz control is typically used
when the minimum frequency in which the system will
operate is greater than the slip frequency (the
difference between the synchronous speed and
operating speed multiplied by the ration of operating
frequency to synchronous speed). Further, the low
speed overload requirements must be low (typically
less than 120 percent of motor torque). A vector
control may be needed for operation below slip
frequency, operation at zero speed, precise torque
control, or high peak torque at low speeds.
The basic horsepower/current rating of a control, as
shown on the nameplate, is the continuously rated
duty. For duty cycle loads, an equivalent rms current
may be calculated for sizing the control, so long as the
short term overload requirements do not exceed the
150 percent full load 1 minute rating of the control.
3) Constant Horsepower Application:
The amount of torque required at maximum
operating speed in the constant horsepower speed range
could affect the size of the motor. In the portion of the speed
range where the voltage remains constant, the motor
breakdown torque decreases at a rate proportional to the
square of the change in frequency (speed). However, the
load torque requirement decreases much more slowly at a
rate inversely proportional to the change in frequency
(speed). Therefore, within the mechanical limitations of the
motor, the maximum possible speed at which the motor can
carry a constant horsepower load is equal to the speed at
which the motor breakdown torque equals the load torque.
For reliable performance from the ASD, the motor and
control manufacturer should be consulted regarding the
margin between load torque and the motor breakdown
torque necessary for stability considerations. If short time
overload torque is required in addition to the rated load
torque, it must also be taken into account.
Control: It is common for motors designed for constant
horsepower applications to also be designed with a
lower rated voltage at base speed. This lower voltage
design results in higher full-load current. For this
reason, the control should be sized to match the motor
base speed current. In these cases, it is typical for
controls to have horsepower ratings exceeding the
motor’s horsepower rating.
This paper touched upon a large number of issues addressed
by the NEMA Application Guide for Adjustable Speed Drive
Systems that must be considered for successful application of
an adjustable speed drive system. Although the limited space
here did not allow the authors to do justice to many topics, the
material covered here should be helpful to an application
engineer attempting to create an initial specification. It is clear
that such a specification must consider the driven load, motor,
control, and utility power supply. The authors believe that all
users of ASD systems would be well served by having a
complete copy of the application guide for ready reference when
installing a new ASD installation.
[1] NEMA Publication, Application Guide for AC Adjustable
Speed Drive Systems
[2] NEMA Standards Publication No. MG 1-1998 Motors and
Scott Kreitzer graduated with a BSME degree from Wright
State University in 1993 and received a Master of Science
degree in Aerospace Engineering from the University of
Cincinnati in 1995. Scott worked for Reuland Electric in 1994
as a Design Engineer developing high-speed AC induction
motors. He has been a Mechanical Engineer in the Above
NEMA motor development group at Siemens Energy and
Automation since 1995. Scott is an associate member of
David Bezesky has been in the electric motor industry since
1984. He initially worked as a technician in a motor rewind
shop until he joined Reuland Electric as a field service
technician in 1987. In 1990 he graduated with a BSME
degree in Electrical Power Distribution and Electric Machinery
from Michigan Technological University. He was then
promoted to Electrical Design Engineer at Reuland Electric
and went to work developing high-speed AC induction inverter
driven motors. In 1994 he joined the above NEMA motor
development group at Siemens Energy & Automation where
he currently holds the position of Senior Product Engineer.
Page 10 of 10
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