Download Low Voltage and Medium Voltage Variable Frequency Drives

Low Voltage and Medium Voltage
Variable Frequency Drives:
Application Considerations
PREPARED FOR
5th Eastern Canadian Electrical and
Instrumentation Conference for the Pulp, Paper
And Sawmill Industry
September 16 - 18, 1997
Sudbury
Robert A. Hanna Ph.D., P. ENG., FIEE
1.0
INTRODUCTION
Variable Frequency Drives (VFDs) have provided the industry significant
advantages in improved process control, higher efficiencies and energy savings.
The reliability of low voltage and medium voltage VFDs have improved
significantly in the last decade due to improvements in switching devices, cooling
system, harmonic mitigation, design of converters/inverters and control systems.
VFD technology has matured and the cost of adopting it has become more
economical as is evident by the increased number of drives put in service over
the last few years. This presentation covers the application of VFD to variable
torque, constant torque and constant horsepower type load. It describes the
principles of VFD, the use of semiconductor switching devices for power
conversion and the basic types and key features of low voltage and medium
voltage VFDs. It also highlights the harmonic problems encountered when using
VFD for new or retrofit applications, mitigation techniques to suppress harmonics
and reviews IEEE 519-1992 standard for harmonic guidelines.
2.0
PRINCIPLES OF VARIABLE FRQUENCY DRIVES
Variable frequency drives used with AC induction motors are required to produce
variable frequency / variable voltage to control the speed of the motor.
Frequency is controlled in order to vary the speed of the motor according to the
following formula:
Speed = 120 x Frequency / No. Of Poles
Voltage is varied along with the frequency so that the flux density in the air gap
between the rotor and stator, and therefore, the torque produced by the motor
can be controlled.
A constant relationship between voltage and frequency
(volts/Hertz) must be maintained, see Figure 1, to optimize motor utilization and
keep constant rated flux. Higher or lower volts/Hertz ratio would result in overexcited or under-excited motor respectively, and improper motor utilization. For
example, for 575V motor the volts/Hertz ratio is 9.6 and for 4160 motor the ratio
is 69.33 across the operating speed up to 60 Hz. Above 60 Hz the voltage is
kept constant at rated value and only frequency is linearly increased, and this
results in constant horsepower. The torque, in this case, is inversely proportional
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to speed, but their product is equal to constant horsepower. Figure 2 shows the
torque speed characteristics of an induction motor at different supply frequencies
and constant volts/Hertz ratio.
3.0
SEMICONDUCTOR POWER SWITCHING DEVICES FOR VFDs
Power conversion in low and medium voltage variable frequency drive is typically
achieved by using one of the following four semiconductor power switching
devices in inverter circuit.
•
Silicon-Controlled Rectifier (SCR)
•
Gate turn-off Thyristor (GTO)
•
Bi-Polar transistor
•
Insulated gate Bi-polar transistor (IGBT)
An ideal switching device for VFDs would include the following features, all of
which will permit a simpler, more reliable, low cost VFD design at higher
horsepower:
Internal losses would be very low to provide a high operating efficiency, leading
to small size and low cost.
Switching controls would be simple. The device would be easy to switch on or
off, keeping the logic controls and associated power circuits to a minimum for low
cost and reliability.
Switching frequency capability would be fairly high.
The device could be
switched on or off at high frequencies, allowing the inverter to produce a variable
frequency sine wave with minimum harmonics to minimize motor heating.
Surge rating would be high. The device would have ample overcurrent and
overvoltage capacity to improve reliability and to simplify protection.
Current and voltage ratings would be high. The device could be manufactured
easily and reliably for high-horsepower applications.
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The choice between the above switching devices depends largely on the
manufacturer’s experiences and drive size.
For VFD application, one can
essentially classify all semiconductor devices into two broad categories:
1.
Those devices which only require an impulse on their gate to be turned
on. These are SCRs type devices (also called thyristor) and they were
the first to be introduced. Due to their high voltage and current ratings,
they are still employed particularly for large VFDs up to 20000 HP. Their
primary limitations have traditionally been their low switching frequency
and switching losses.
In addition, they may require some rather
sophisticated external control circuitry to allow SCRs to be turned off.
The gate turn-off thyristor (GTO), featuring a second gate, was introduced
in the 80s to address these issues. The second gate allows GTO to be
conveniently turned off without power commutating circuits. GTO devices
have moderate switching frequency and moderate voltage and current
ratings. GTO devices have been used in medium voltage VFDs ranging
400HP - 10000 HP.
2.
Those devices that require a permanent current at their gate to keep them
in the conducting state. These are IGBT and a bi-polar transistor type
devices and require no external circuitry for turn on or turn off purposes.
However, they have limited voltage and current ratings when compared to
SCR to be widely used in medium voltage VFDs for high HP ratings.
The IGBT is currently the most popular switching device in low voltage 480V and
575V VFD technology and for rating up to 1000 HP. This is because of its higher
gain, lower losses and fast switching up to 20 kHz. In the last two years, IGBT
devices have been employed in 2300V and 4160V medium voltage VFDs up to
5000 HP ratings and their acceptance is growing.
IGBT produces very steep
voltage waveform due to its fast switching and extra precaution should be taken
for motor insulation protection particularly in retrofit applications and long run
cables.
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The voltage and current ratings for the switching devices described here should
only be used as a guideline since this technology is rapidly changing. The drive
manufacturer must always be consulted for the most up-to-date drive current and
voltage ratings.
4.0
BASIC TYPES OF LOW VOLTAGE VARIABLE FREQUENCY DRIVES
The basic components of a VFD are a line converter (rectifier), DC link and load
converter (inverter). The rectifier changes constant AC input voltage to constant
or variable DC output voltage.
The inverter alters the DC to variable
voltage/frequency AC power fed to motor.
The rectifier performs the power
conversion using diodes (uncontrolled DC voltage) or SCR (controlled DC
voltage). The switching power devices typically used in an inverter circuit to
achieve variable voltage/frequency are those described in the previous section.
It should be noted that all types of VFDs described here have soft start capability
to control motor starting current and acceleration time.
The basic topologies used to produce low voltage VFD are voltage source
inverter (VSI, also called variable voltage inverter VVI), current source inverter
(CSI) and pulse width modulated (PWM). When low voltage VFD is applied to
2300V or 4160V motor an output step-up transformer is used to match the
inverter voltage to motor voltage.
4.1
VSI Drives
The VSI drive, Figure 3, consists of a controlled rectifier (SCRs) to
convert the incoming AC voltage to variable DC voltage. The frequency
of the output is controlled by sequentially switching the transistors or
SCRs in the inverter section in six discrete steps to produce the output
voltage shown in Figure 3. This inverter concept was one of the firsts to
be introduced in the 70s in low voltage VFDs and is not widely used at
present.
The six-step inverter circuit is characterized by the use of
capacitors in the DC link.
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The inverter output is controlled independent of load, so any squirrel cage
induction motor, or even a group of motors, within the rating of the
inverter may be driven by a VSI drive. Harmonic currents are proportional
to the imposed harmonic voltages in the output wave and inversely
proportional to motor leakage reactance.
Motors designed with large
leakage reactance reduce the harmonic currents, motor heating and
pulsation torque. Regeneration, if required, is possible but with additional
converter section. The main features of VSI drives are:
•
Power rating up to 1,000 HP
•
Speed reduction 10 : 1
•
Suitable for variable torque and constant torque load applications.
•
Efficiency range
•
Poor input power factor at low operating speeds
•
Inverter power and control circuit is reasonably simple
•
For retrofit application, motor derating might be necessary to account
88 – 93 %
for harmonics and reduced cooling.
4.2
CSI Drives
Current source inverter drives can be identified by the large reactor in the
DC bus, see Figure 4. Most current source drives require that a motor be
connected before the drive has the capability to commutate.
Motor
inductance characteristics, in addition to capacitors in the drive, are part
of the commutation circuit. This makes it difficult to retrofit these drives to
existing fixed speed motor. The most common designs of CSI drive do
create high voltage spikes during commutation. This could be a factor in
selecting the drive in the higher voltage drives (2300 V and above) to
assure that the insulation on the motor will not be damaged by the voltage
spikes. An alternative design to this approach is one, which includes
capacitors on the output to minimize the voltage spikes.
The harmonic currents are determined by the harmonics in the output
wave while the harmonic voltages generated in the motor are proportional
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to leakage reactance. Motor heating is less in motors with low leakage
reactance. The main advantage of the CSI drive is its ability to have
complete control of motor current, which results in complete torque
control. However, this current controlling characteristic necessitates a
large filter inductor and a semi-complex regulator due to the difficulty of
controlling the motor solely by current. The main features of low voltage
CSI drives are:
•
Power rating 50 – 1,500 HP
•
Speed reduction 10 : 1
•
Suitable for variable torque, constant torque load applications.
•
Efficiency range 88 – 93 %
•
Poor input power factor at low operating speeds
•
Inverter power circuit is simple, but control circuit is semi-complex.
•
Not suitable for retrofit applications
•
Regeneration capability
It should be noted that CSI design concept is hardly used at present for
low voltage VFDs but it is employed with some modifications for medium
voltage VFDs.
4.3
PWM Drives
PWM inverter is the most popular topology used today in the 480V and
600 V levels and has become almost the standard in the low voltage VFD
industry.
The PWM drive, see Figure 5, utilizes a diode rectifier to
provide a constant DC voltage. The inverter section in this type of drive is
required to control both voltage and frequency. This is done by varying
the width of the output pulses as well as the frequency in such a way that
the effective voltage is approximately sinusoidal. Almost universally, all
low voltage VFD manufacturers employ IGBTs switching devices in the
inverter circuit. The main feature of IGBT device is its capability to be
switched on or off at high frequencies, allowing the inverter to produce a
variable frequency sine wave with minimum harmonics to minimize motor
heating. Typically the switching frequency ranges between 2 kHz to
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12
20 kHz and voltage rise time is less than 0.2 microseconds. The higher
the switching frequency the higher is the output voltage rate of rise and
this could provide problems to the motor insulation. A steep wave front
pulse does not allow the voltage to distribute itself evenly across all the
mesh stator windings. The first few turns experience most of the initial
voltage rise and this repeatedly exert voltage stress and ultimately could
result in winding failure. Experience has shown that this problem is more
profound when the motor is located over 50 ft from the VFD.
The
combination of motor inductance and cable capacitance could cause the
pulse voltage to be as high as 2 times the DC bus voltage, namely, for
575 V VFD the DC voltage is 800 V and the spike voltage across the
motor to be 1600 V.
In the last 2-3 years, there have been several
incidences reported for motor winding experiencing insulation failures in
the first or last few turns when operating with IGBTs inverters. Motor
manufacturers are now offering inverter duty motors that have improved
insulation system. NEMA MG1 Standard has added a new section part
31 that specifies limitations for “ definite purpose inverter-fed motors”. It
calls for motors rated 600 V to withstand voltage spike up to 1.6 KV with a
rise time greater than 0.1 microseconds. Many motor manufacturers are
now building an inverter duty motor that complies with this new standard.
In addition, it is become fairly common to install 3%-5% reactor between
the inverter and motor when feeder cable is greater than 50 ft to mitigate
the voltage spikes imposed on motor.
It should be noted that this kind of drives are also sensitive to input line
disturbances and this result in nuisance tripping particularly during utility
capacitors switching.
Input line reactors with 3% value are typically
installed to mitigate this problem.
The main features of low voltage PWM drive are:
•
Power rating
•
Speed reduction 30 : 1
•
Suitable for variable torque and constant torque load applications.
5 – 1,500 HP
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•
Efficiency range 85 – 95 %
•
Input power factor is near unity
•
Inverter power circuit is simple but control circuit is complex
•
Motor voltage stress is increased at higher switching frequency
•
Susceptible to incoming line disturbances mainly due to capacitor
switching
•
Noisy when operating at lower carrier frequency
•
Suitable for retrofit application and less motor derating is required,
because harmonic currents are reduced.
In certain applications where high starting torque (above 150 %) is
required at 150 % starting current, conventional PWM drive could be
modified to include flux vector control.
In this case, the magnetizing
current component and torque-producing component of the stator current
are both independently controlled. This type of drive controller requires
and encoder mounted on motor shaft to measure the actual speed. The
flux vector controllers are also used when automatic reacceleration (after
a momentary loss of power) is specified for high inertia load such as
induced draft fan. The controller is capable to almost instantaneously
acquire synchronism with motor (catch on fly) upon power restoration
thus avoiding process upset. Flux vector control can be retrofitted on
existing PWM drives.
5.0
BASIC TYPES OF MEDIUM VOLTAGE VARIABLE FREQUENCY DRIVES
The basic components of medium voltage VFDs are identical to low voltage
VFDs, namely rectifier front end, DC bus and inverter. Medium voltage VFDs
operating voltage is at 2300V, 4160V or 6600V but the majority are designed at
4160V. Medium voltage VFDs size ranges between 400HP to 20000HP. The
highest speed for which a VFD was built is 11000rpm for compressor application
at 3500HP rating. First medium voltage drive for induction motor was introduced
to the market in 1983. Presently, there are over 1500 medium voltage VFDs in
service in North America, and over 15 in Ontario ranging from 700 HP to 3500
HP. The use of medium voltage drives is increasing and over 400 drives were
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sold in 1996 in North America and this number is expected to reach 500 in 1997.
There are four manufacturers of medium voltage VFDs in North America and this
number is expected to increase to six by the end of 1997. All manufacturers offer
both air and liquid cooled VFDs depending on size and customer requirements.
Typically, drives rated up to 2500 HP are air-cooled and above that are liquid
cool to reduce the requirements for air-conditioning system. Heat loss produced
by air cool drives is typically 2% KW/HP rating, namely, for example, for 2500 HP
rating the heat loss is 50 KW.
All variable frequency drives manufactured today produce at least some
harmonic current and voltage, which will be impressed on the motor causing
additional heating and voltage stress. The impact of VFD on motor or power
system depends on drive topologies and switching devices. Generally, the basic
topology used to produce medium voltage VFDs are current source inverter –
pulse width modulated (CSI-PWM), three level voltage source inverter – pulse
width modulated (VSI – PWM) and multipulse pulse width modulated.
5.1
CSI – PWM Drives
This drive concept was introduced in the market in 1993 and since then
there are several hundreds of drives in service for pumps, fans and
compressors applications. The CSI – PWM drive, see Figure 6, consists
of rectifier circuit utilizing SCRs in to convert 60 Hz input supply to
variable DC supply, large reactor in the DC link and PWM inverter using
GTOs coupled to an output capacitor for high order harmonic reduction.
PWM inverter uses Selective Harmonic Elimination (SHE) techniques to
eliminate lower order harmonics mostly the dominant 5th and 7th
components. The key features of CSI – PWM drives are:
•
Power rating up to 12000 HP
•
Suitable for variable torque and constant torque application
•
Input isolating transformer is required for retrofit applications and is
optional for new applications.
•
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•
Input converter (rectifier) can be configured in 6 pulse or 12 pulse for
harmonic reduction.
•
Input power factor reduces linearly with speed and input filter could be
required for power factor correction and harmonic reduction
5.2
•
Regeneration capability for motor braking
•
Air cooled or liquid cooled with redundant pumps
•
Bypass option
•
Redundancy in switching components
•
Vector control for high torque, low speed application (extruder)
3 - Levels Voltage Source – PWM Drives
At present, this drive topology is not adopted by VFD manufacturers in
North America but mostly used in Europe. The drive, see Figure 7 has
diode rectifier input, a large DC link capacitor and PWM inverter that
normally uses GTOs arranged in three levels. Typically, these drives are
build at 3300V thus limiting their use for retrofit applications with motors
rated 4160V or for new applications having bypass. The key features of
VSI – PWM drives are:
•
Power rating up to 5000 HP
•
Suitable for variable torque and constant torque application
•
Input isolating transformer is required for new and retrofit applications
•
Input converter (rectifier) can be configured in 6 pulse or 12 pulse for
harmonic reduction.
•
Input power factor is greater than 0.95 throughout the speed range
•
Mostly liquid cooled with redundant pumps
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5.3
Multi- Level Voltage Source – PWM Drives
This drive technology was first introduced to the market in early 1995 for
use with medium voltage induction motors. The drive is characterized by
the use of multi-winding indoor input isolating transformer, multi-pulse
diode rectifier and multi-pulse IGBTs PWM inverter, see Figure 8. The
drive converter consists of several cells connected in series to produce
the required motor voltage, namely, three cells in series per phase for
2300V and five cells in series for 4160V. Each cell design is identical to
low voltage 480V PWM circuit.
The main features of this drive is as
follows:
•
Up to 5000 HP rating built and capability up to 10000 HP
•
Suitable for variable torque and constant torque application
•
Input indoor isolating transformer is required for new and retrofit
applications
•
Input converter (rectifier) and load converter (inverter) is configured in
18 pulse, or 24 pulse or 30 pulse for harmonic reduction
•
Input power factor remains above 0.95 across operating speed range
•
Meet IEEE 519-1992 harmonic requirements for current and voltage
total harmonic distortions at drive input and output without the use of
harmonic filters
•
Air cooled with optional redundant fans or liquid cooled with standard
redundant pumps.
•
Bypass option
•
Cell bypass
•
Vector control for high torque, low speed application (ex: extruder)
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6.0
APPLICATION CONSIDERATION OF VFD
VFDs are extensively used in industry for various applications.
They are
normally divided into three categories depending on their torque requirement and
these are constant torque, variable torque and constant horsepower.
6.1
Constant Torque Application
A constant torque load is characterized as one in which the horsepower
requirement is directly proportional to the operating speed.
The
horsepower torque relationship is defined by the following formula:
Torque (lb.ft) = (HP x 5250) / RPM
This characteristic is shown graphically in Figure 9. Constant torque is
achieved by maintaining the motor current constant at all specified
operating speed range. Horsepower requirements is linearly proportional
to speed and hence energy saving is less when compared with variable
torque application. Therefore, in general, it is difficulty to justify the use of
these drives on energy basis alone without considering the other process
requirements.
For constant torque application, the motor experiences considerable
temperature rise when operating at minimum specified speed, because of
reduced ventilation and constant current requirement. For self ventilated
motor, temperature could rise by up to 20 ° C above its sine wave rating
when running at rated load and 50 % speed. Figure 10 shows a typical
motor continuous torque capability versus speed for self-ventilation. In
general, the motor is capable to safely operate at rated torque and current
between 100 % to 50 % speed particularly if it is designed with 1.15
service factor and class F insulation. Below 50 % speed operation, motor
performance should be carefully reviewed to avoid premature winding
failures.
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Typical examples of constant torque applications are reciprocating
compressors, reciprocating pumps, conveyors, screw type mixers,
extruders, etc.
It is recommended to carry out torsional analysis in large drive constant
torque applications especially for two poles motor to ensure that the
natural resonance of the mechanical components are not excited by the
drive harmonics within the operating speed range.
6.2
Variable Torque Application
For variable torque load, the torque varies directly with speed squared,
and horsepower varies directly with speed cube, see Figure 11.
Harmonic losses are maximum at rated speed.
Considerable energy
saving is achieved with this application even at slightly reduced speed.
For example, at 80 % speed the horsepower requirement is almost 50 %
rating. Typical applications are centrifugal pumps, fans and compressors.
Motor should be specified with 80 ° C rise above 40 ° C ambient when
connected to VFD and at rated speed to properly allow for harmonic
losses. Class F insulation is recommended. Motor derating is minimal for
retrofit variable torque application. Continuous torque capability without
motor overheating is from 25 % to 100 % speed for self ventilated
machine.
6.3
Constant Horsepower Application
For constant horsepower load, torque is a function of speed in the
constant horsepower range, such that, as speed is increased, torque will
decrease inversely, and horsepower will remain relatively constant, see
Figure 12. Typical examples of constant horsepower loads are grinders,
lathes and winders. Typically constant horsepower loads are operated
above base speed.
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7.0
HARMONICS IN VARIABLE FREQUENCY DRIVES
Harmonics currents and voltages are produced whenever VFDs are used. Their
operation, as described earlier, require converting AC to DC and DC to AC and
this results in generating harmonics both into board and power supply.
Common sources of harmonics in industrial electrical system are as follows
•
Variable Frequency Drive
•
Rectifiers
•
DC Motor Drives
•
UPS
•
Arc Furnaces
•
Static Var Generator
•
Cyclo Converter
•
Static Motor Starter
The presence of these devices in a plant does not necessarily indicate that there
is a harmonic problem.
At one extreme, these harmonics may be of low
magnitude and, therefore, harmless, or they may be high enough to cause
problems such as motor overheating, capacitor’s failure and telephone
interference. It has been suggested that when 20 % or more of the plant load
consists of harmonic producing equipment, then a harmonic study should be
considered.
Harmonics are currents or voltages which have frequencies that are integer
multiples of the fundamental power frequency. For example, for 60 HZ supply
the fifth harmonic is 300 HZ. A 3-phase power converter generates harmonic
current the order and magnitude of which are given the following equations:
h
= KP ±
1
Ih = I1 / h
h : order of harmonic
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P : number of pulses of the converter system
K : any integer 1, 2, 3, ---
Ih : harmonic current
I1 : fundamental current
It can be seen that the magnitude of harmonic current is inversely proportional to
its harmonic order, and therefore, the magnitudes of high order harmonics
diminish rapidly. A six pulse converter system (P = 6), see Figure 13, would
produce harmonic currents of the order 5th, 7th, 11th, 13th, 17th, 19th, 23rd, etc. For
a 12 pulse converter configuration, as shown in Figure 14, the harmonics
generated are 11th, 13th, 23rd, 25th, etc. Therefore, a 12 pulse converter provides
a significant reduction in the voltage distortion and, equally important, it
eliminates, assuming balanced conditions, the lowest order harmonics of 5th and
7th which are typically of most troublesome.
The 12 pulse configuration is
typically used with drives rated say 2,000 HP and above.
8.0
HARMONIC GUIDELINES
In order to compare levels of harmonic distortion in a power system, the Total
Harmonic Distortion factor is used, and is defined in IEEE Standard 519-1992
(Recommended practices and requirements for harmonic control in electrical
power system) as:
½
THD
= [
sum of squares of amplitude of all harmonics ]
square of amplitude of fundamental
X
100%
This standard specifies two criteria to evaluate harmonic distortion:
1. A limitation in the harmonic current that a use can transmit into the utility
system.
2. The quality of the voltage that the utility must furnish the user.
The first criteria puts the responsibility on the user to limit harmonic current
injected back into power system and these limits are given in Table 1. These
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harmonic current limits are based on the size of the user with respect to the size
of the power system to which the user is connected. The smaller the ratio of
VFD load to available short circuit level, the larger is the harmonic current
allowed to be injected back into utility.
The second criteria put the responsibility on the utility to furnish the user with a
good quality voltage. Table 2 shows the permissible harmonic voltage level at
the incoming power supply. Ontario Hydro voltage THD requirements are, in
general, more stringent than those given in Table 2. For example at 27.6 KV
level Ontario Hydro voltage THD requirement is 3%, whereas IEEE recommends
5%.
Another harmonic limitation which is normally very difficult to meet, and is
sometimes overlooks is the IT products.
This represents the harmonics
interference with telephone lines when running in the proximity or power lines.
Table 3 shows the IT limits as specified by IEEE 519-1992. Ontario Hydro IT
products limits are more stringent than IEEE guidelines and is 5,000A at 27.6 KV
voltage level. It is extremely important to consult with the local utility as their
harmonic guidelines could have a significant impact on the filter requirement and
overall VFD cost.
9.0
CONCLUSIONS
The use of medium voltage VFDs is growing rapidly based on the number of
drives sold over the last few years. More manufacturers are entering the market
to build medium voltage VFDs and this trend is expected to continue. The users
have accepted this technology and are employing it to control critical processes.
When selecting a VFD for a particular application several factors need to be
carefully examined at the initial phase.
These include type of VFD, torque
requirements at all operating speed range, power quality issues, torsional
analysis, impact on existing motor and mechanical equipment, space availability,
HVAC requirements, bypass option and redundancy.
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For low voltage VFDs, measures should be incorporated to reduce drive
susceptibility to incoming line transients as well as impact of output voltage
spikes on motor winding.
For medium voltage VFDs, the users should become familiar with the various
inverter technologies offered by the manufacturers to make a selection that best
meets the project requirements.
An overall system evaluation should be
conducted that includes harmonic filters, motor features, VFD configuration (6 pulse or 12 - pulse or multi - pulse), bypass option, air versus liquid cool, space
requirement, torsional analysis, etc.
It is inevitable that harmonics will be generated whenever a VFD is used. The
order and magnitude of these harmonics greatly depends on the drive
arrangement and system impedance.
The principle effect of harmonics on
electrical equipment is overheating and possible excitation of the combination of
the existing power factor correction capacitors and system impedance.
For constant torque application it is recommended to specify a motor with 80 ° C
temperature rise at rated torque and minimum operating speed. For variable
torque application, 80 ° C temperature rise is recommended at rated load and
rated speed because harmonic losses are maximum. Class F insulation should
always be specified as minimum for constant or variable torque applications.
For large VFD installation, harmonic study is recommended to ensure
compliance with the utility requirements and that there is no resonance available
with power factor correction capacitors.
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