VARIABLE FREQUENCY DRIVES

ABSTRACT

A Variable-frequency drive (VFD) is a system for controlling the rotational speed of an

alternating current (AC) electric motor by controlling the frequency of the electrical power
supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive.
Variable frequency drives are also known as adjustable-frequency drives (AFD), Variable-speed
drives (VSD), AC drives, Micro drives or Inverter drives. Since the voltage is varied according
to the frequency these are sometimes called as VVVF (Variable voltage variable frequency)
drives. Variable-frequency drives are widely used in ventilation systems for large buildings,
variable-frequency motors on fans save energy by allowing the volume of air moved to match
the system demand. All VFDs use their output devices (IGBTs, transistors, thyristors) only as
switches, turning them only on or off. Using a linear device such as a transistor in its linear mode
is impractical for a VFD drive, since the power dissipated in the drive devices would be about as
much as the power delivered to the load. Hence they are also used on pumps, elevator, conveyor
and machine tool drives.

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CONTENTS

TITLE PAGE NO.

1. INTRODUCTION ……………………………………… 3

2. VFD system description………………………………….. 6

3. VFD operation……………………………………………. 8
4. PURPOSE OF VFD’s………………………………….... 10
5. ADVANTAGES & DISADVANTAGES………………. 11
6. VFD POWER RATINGS……………………………….. 13
7. APPLICATIONS……………………………………….... 14
8. CONCLUSION…………………………………………... 16
9. REFERRENCES………………………………………… 17

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INTRODUCTION TO VARIABLE FREQUENCY DRIVES

Induction motors have been used in the past mainly in applications requiring a constant speed
because conventional methods of speed control have been either expensive or inefficient.
Variable speed applications have been dominated by dc drives. Availability of Thyristors, Power
Transistors, IGBT have allowed that development of variable speed induction motor drives .
The main drawback of dc motors is a presence of commutator and brushes, which require
frequent maintenance and make them unsuitable for explosive and dirty environment.
On the other hand, induction motors, particularly squirrel-cage are rugged, cheaper, lighter,
smaller, more efficient, require lower maintenance and can operate in dirty and explosive
enAlthough varIable speed induction motor drives are generally expensive than dc drives.
They are used in number of applications like cranes, conveyers etc. because of the advantages of
Induction motors.

1.1 What Is a Variable Frequency Drive?

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an

alternating current (AC) electric motor by controlling the frequency of the electrical power
supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive.

Adding a variable frequency drive (VFD) to a motor-driven system can offer potential energy
savings in a system in which the loads vary with time. VFDs belong to a group of equipment
called adjustable speed drives or variable speed drives. (Variable speed drives can be electrical
or mechanical, whereas VFDs are electrical.) The operating speed of a motor connected to a
VFD is varied by changing the frequency of the motor supply voltage. This allows continuous
process speed control.

Motor-driven systems are often designed to handle peak loads that have a safety factor. This
often leads to energy inefficiency in systems that operate for extended periods at reduced load.
The ability to adjust motor speed enables closer matching of motor output to load and often
results in energy savings.

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1.2 BLOCK DIAGRAM OF VARIABLE FREQUENCY DRIVE

1.3 TYPES OF VARIABLE FREQUENCY DRIVES

All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them
only on or off. Using a linear device such as a transistor in its linear mode is impractical for a
VFD drive, since the power dissipated in the drive devices would be about as much as the power
delivered to the load.

Drives can be classified as:

 Constant voltage
 Constant current
 Cycloconverter

 Constant voltage:

In a constant voltage converter, the intermediate DC link voltage remains approximately constant
during each output cycle.

 Constant current:

In constant current drives, a large inductor is placed between the input rectifier and the output
bridge, so the current delivered is nearly constant.

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 Cycloconverter:

A Cycloconverter has no input rectifier or DC link and instead connects each output terminal to
the appropriate input phase.The most common type of packaged VF drive is the constant-voltage
type, using pulse width modulation to control both the frequency and effective voltage applied to
the motor load.

Pulse Width Modulation

 Pulse width modulation takes alternating current (AC) as input and replaces it with a
sequence of pulses that simulate the sinusoidal shape of AC. The width of the pulses may
be reduced if this keeps the motors running at full speed, saving energy, and the
frequencies can be electronically manipulated to simulate frequencies other than the
frequency of the input AC. For some motors and conditions, 400 volts at 60 hertz can
produce the same effect as 200 volts at 30 hertz--saving more energy. Some motors start
better at low frequencies, so the pulse width modulator will start as low as 2 hertz and
increase the frequency as the motor speeds up. Shutting down is a similar process. This
saves wear and tear on the motor.

Source Inverters

 Inverters convert direct current (DC) to AC--the exact inverse of the more familiar
rectifiers that convert AC to DC. They are an important part of VFD in the green
economy. Most green power sources produce DC and most big motors want either AC or
something like it--such as pulse width modulated AC. Source inverters for VFD come in
two types: current source inverters and voltage source inverters. Current source inverters
keep the current fixed no matter how the voltage changes--required for some motors and
for sources like solar panels that deliver high voltages and low currents. Voltage source
inverters keep the voltage fixed no matter how the current changes--required for some
motors and for sources like fuel cells that deliver high currents and low voltages.

Flux Vector Drive

 Flux vector drives produce pulses like pulse width modulators, but the pulses are not
always shaped like the sinusoidal waves characteristic of AC currents. Flux vector drives
combine inputs that sample load levels, motor shaft speed and instructions from the
control panel to produce pulses outside of the sinusoidal envelope to control motor
speed--especially during startup and slowdown. The chief use of flux vector drives, that
justifies their high cost, is for motors that must change speeds quickly, like elevators.
You will never see flux vector drives used with motors that do not need to change speeds
except for starting up and shutting down--like motors used for ventilation systems and
pumps.

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VFD SYSTEM DESCRIPTION

A Variable frequency drives system generally consists of three types. They are as follows:

 VFD motor
 VFD controller
 VFD operator interface

VFD motor

The motor used in a VFD system is usually a three-phase induction motor. Some types of single-
phase motors can be used, but three-phase motors are usually preferred. Various types of
synchronous motors offer advantages in some situations, but induction motors are suitable for
most purposes and are generally the most economical choice. Motors that are designed for fixed-
speed operation are often used. Certain enhancements to the standard motor designs offer higher
reliability and better VFD performance, such as MG-31 rated motors.[6]

VFD controller

Variable frequency drive controllers are solid state electronic power conversion devices. The
usual design first converts AC input power to DC intermediate power using a rectifier or
converter bridge. The rectifier is usually a three-phase, full-wave diode bridge. The DC
intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching
circuit. The inverter circuit is probably the most important section of the VFD, changing DC
energy into three channels of AC energy that can be used by an AC motor. These units provide
improved power factor, less harmonic distortion, and low sensitivity to the incoming phase
sequencing than older phase controlled converter VFD's. Since incoming power is converted to
DC, many units will accept single-phase as well as three-phase input power (acting as a phase
converter as well as a speed controller); however the unit must be derated when using single
phase input as only part of the rectifier bridge is carrying the connected load. As new types of
semiconductor switches have been introduced, these have promptly been applied to inverter
circuits at all voltage and current ratings for which suitable devices are available. Introduced in
the 1980s, the insulated-gate bipolar transistor (IGBT) became the device used in most VFD
inverter circuits in the first decade of the 21st century.

AC motor characteristics require the applied voltage to be proportionally adjusted whenever the
frequency is changed in order to deliver the rated torque. For example, if a motor is designed to
operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the
frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant
value (460/60 = 7.67 V/Hz in this case). For optimum performance, some further voltage

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adjustment may be necessary especially at low speeds, but constant volts per hertz is the general
rule. This ratio can be changed in order to change the torque delivered by the motor.

In addition to these simple volts per hertz control more advanced control methods such as vector
control and direct torque control (DTC) exist. These methods adjust the motor voltage in such a
way that the magnetic flux and mechanical torque of the motor can be precisely controlled.

The usual method used to achieve variable motor voltage is pulse-width modulation (PWM).
With PWM voltage control, the inverter switches are used to construct a quasi-sinusoidal output
waveform by a series of narrow voltage pulses with pseudo sinusoidal varying pulse durations.

Operation of the motors above rated name plate speed (base speed) is possible, but is limited to
conditions that do not require more power than nameplate rating of the motor. This is sometimes
called "field weakening" and, for AC motors; means operating at less than rated volts/hertz and
above rated name plate speed. Permanent magnet synchronous motors have quite limited field
weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous
motors and induction motors have much wider speed range. For example, a 100 hp, 460 V,
60 Hz, 1775 RPM (4 pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be
limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power. At higher speeds
the induction motor torque has to be limited further due to the lowering of the breakaway torque
of the motor. Thus rated power can be typically produced only up to 130...150 % of the rated
name plate speed. Wound rotor synchronous motors can be run even higher speeds. In rolling
mill drives often 200...300 % of the base speed is used. Naturally the mechanical strength of the
rotor and lifetime of the bearings is also limiting the maximum speed of the motor. It is
recommended to consult the motor manufacturer if more than 150 % speed is required by the
application.

VFD operator interface

The operator interface provides a means for an operator to start and stop the motor and adjust the
operating speed. Additional operator control functions might include reversing and switching
between manual speed adjustment and automatic control from an external process control signal.
The operator interface often includes an alphanumeric display and/or indication lights and meters
to provide information about the operation of the drive. An operator interface keypad and display
unit is often provided on the front of the VFD controller as shown in the photograph above. The
keypad display can often be cable-connected and mounted a short distance from the VFD
controller. Most are also provided with input and output (I/O) terminals for connecting
pushbuttons, switches and other operator interface devices or control signals. A serial
communications port is also often available to allow the VFD to be configured, adjusted,
monitored and controlled using a computer.

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VFD OPERATION

Induction motors, the workhorses of industry, rotate at a fixed speed that is determined by the
frequency of the supply voltage. Alternating current applied to the stator windings produces a
magnetic field that rotates at synchronous speed. This speed may be calculated by dividing line
frequency by the number of magnetic pole pairs in the motor winding. A four-pole motor, for
example, has two pole pairs, and therefore the magnetic field will rotate 60 Hz / 2 = 30
revolutions per second, or 1800 rpm. The rotor of an induction motor will attempt to follow this
rotating magnetic field, and, under load, the rotor speed "slips" slightly behind the rotating field.
This small slip speed generates an induced current, and the resulting magnetic field in the rotor
produces torque.

Since an induction motor rotates near synchronous speed, the most effective and energy-efficient
way to change the motor speed is to change the frequency of the applied voltage. VFDs convert
the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing
adjustable motor speed.

When an induction motor is connected to a full voltage supply, it draws several times (up to
about 6 times) its rated current. As the load accelerates, the available torque usually drops a little
and then rises to a peak while the current remains very high until the motor approaches full
speed.

By contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the
motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency
avoids the high inrush current that occurs when a motor is started by simply applying the utility
(mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and
voltage are increased at a controlled rate or ramped up to accelerate the load without drawing
excessive current. This starting method typically allows a motor to develop 150% of its rated
torque while the VFD is drawing less than 50% of its rated current from the mains in the low
speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill
right up to full speed. Note, however, that cooling of the motor is usually not good in the low
speed range. Thus running at low speeds even with rated torque for long periods is not possible
due to overheating of the motor. If continuous operation with high torque is required in low
speeds an external fan is usually needed. The manufacturer of the motor and/or the VFD should
specify the cooling requirements for this mode of operation.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency
and voltage applied to the motor are ramped down at a controlled rate. When the frequency
approaches zero, the motor is shut off. A small amount of braking torque is available to help

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decelerate the load a little faster than it would stop if the motor were simply switched off and
allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor
controlled by a transistor) to dissipate the braking energy. With 4-quadrants rectifiers (active-
front-end), the VFD is able to brake the load by applying a reverse torque and reverting the
energy back to the network.

A VFD converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage
and the inverter stage. The conversion process incorporates three functions:

 Rectifier stage: A full-wave, solid-state rectifier converts three-phase 60 Hz power from

a standard 208, 460, 575 or higher utility supply to either fixed or adjustable DC voltage.
The system may include transformers if higher supply voltages are used.

 Inverter stage: Electronic switches - power transistors or thyristors - switch the rectified
DC on and off, and produce a current or voltage waveform at the desired new frequency.
The amount of distortion depends on the design of the inverter and filter.

 Control system: An electronic circuit receives feedback information from the driven
motor and adjusts the output voltage or frequency to the selected values. Usually the
output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz).
Controllers may incorporate many complex control functions.

Converting DC to variable frequency AC is accomplished using an inverter. Most currently

available inverters use pulse width modulation (PWM) because the output current waveform
closely approximates a sine wave. Power semiconductors switch DC voltage at high speed,
producing a series of short-duration pulses of constant amplitude. Output voltage is varied by
changing the width and polarity of the switched pulses. Output frequency is adjusted by
changing the switching cycle time. The resulting current in an inductive motor simulates a sine
wave of the desired output frequency. The high-speed switching of a PWM inverter results in
less waveform distortion and, therefore, lowers harmonic losses.

The availability of low-cost, high-speed switching power transistors has made PWM the
dominant inverter type.

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PURPOSE OF VFD’s

A VFD (Variable Frequency Drive), or rather a VVFD (Variable Voltage and Frequency Drive)
is a precision electronic device specifically designed and used to control the speed of AC
induction motors (single as well as three phase) without affecting the electric consumption,
torque, impedance, magnetic flux, etc. of the motor. It is integrated to an operator interface for
receiving the required speed control commands (using keypads). Why can’t VFDs be replaced by
other straightforward means? The following discussion will provide the exact purpose of using
VFDs to control AC motor speed.

The fundamental speed of any AC motor is inversely proportional to its number of stator poles
and directly proportional to the supply voltage's frequency. Therefore, to alter the speed of an
AC motor, we need to either change the frequency or the number of stator poles. Since the
number of stator poles for every motor is fixed, obviously we cannot change them. By varying
the frequency of the supply voltage through some simpler means, the speed of the motor can be
changed.

However, changing only the frequency at a constant voltage (120 or 230) causes the equivalent
impedance of the motor to decrease, resulting in greater magnetic flux and causing the motor to
start drawing dangerously huge currents. Therefore it becomes imperative that the supply voltage
is also proportionately reduced along with the frequency at a particular fixed ratio. Failing to do
this would cause the magnetic flux of the motor to saturate and the motor to become damaged.
Varying the frequency and voltage proportionately also ensures a constant torque since the
magnetic field in the air gaps is constant.

The purpose of a VFD is specifically intended to control the speed of an AC motor by strictly
observing the above parameters. Here, the speed of the motor is varied by changing the
magnitude of the input voltage as well the frequency at a constant ratio and thus the motor is able
to maintain a constant torque even at lower speeds.

Main purposes of Variable Frequency Drives are as follows:

1. Energy savings on most pump and fan applications.

2. Better process control and regulation.

3. Inherent power-factor correction.

4. Emergency bypass capability.

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5. Protection from overload currents.

6. Safe Acceleration.

ADVANTAGES & DISADVANTAGES

In addition to energy savings and better process control, VFDs can provide other benefits:

ADVANTAGES

 A VFD may be used for control of process temperature, pressure or flow without the use
of a separate controller. Suitable sensors and electronics are used to interface the driven
equipment with the VFD.
 Maintenance costs can be lower, since lower operating speeds result in longer life for
bearings and motors.
 Eliminating the throttling valves and dampers also does away with maintaining these
devices and all associated controls.
 A soft starter for the motor is no longer required.
 Controlled ramp-up speed in a liquid system can eliminate water hammer problems.
 The ability of a VFD to limit torque to a user-selected level can protect driven equipment
that cannot tolerate excessive torque.
 The VFD can run 10-20% higher in speed and make up for lost of capacity in a flow and
demand type of system.

 When there is capacity in the motor and the VFD can be programmed to do this, a new,
larger motor does not have to be purchased and installed.
 Efficiency is up to 97%. VFDs are more efficient because they regulate motor speed by
varying the frequency of the electricity to the pump so system demand is satisfied without
running the motors at full speed.
 Possible catch of already operating motor.
 Automatic restart after short-time power supply interruption.
 The control for vibration activity of the motor.
 Productivity is not influenced by temperature of external environment.
 The precious speed control and the wide range of regulation.
 Communication possibility with SCADA
 The automated electric control can eliminate necessity for operators at station

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DISADVANTAGES

 Cost and size of the drive. These issues have been minimized over time due to production
improvements and technology enhancements and VFD prices and sizes have come down
dramatically. Typically, a two-pump VFD system does not cost more than a three-pump
constant pressure system and the VFD system size is smaller.
 Complexity of the drive. PRVs, although complex, can be repaired or replaced quickly by
field technicians. In the past, VFDs, being controlled by computers, required specialized
test equipment and trained personnel to service them. This added up to costly service
charges. With recent innovations in computer technology and training, such service
professionals have become commonplace and knowledgeable in VFD equipment.

 These drives are appropriate on large scale HVAC (Heating ventilation Air
Conditioning).

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VFD POWER RATINGS

Variable frequency drives are available with voltage and current ratings to match the majority of
3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers
designed to operate at 111 V to 690 V are often classified as low voltage units. Low voltage units
are typically designed for use with motors rated to deliver 0.2 kW or 1/4 horsepower (hp) up to
several megawatts. For example, the largest ABB ACS800 single drives are rated for
5.6MW.Medium voltage VFD controllers are designed to operate at 2,400/4,162 V (60 Hz),
3,000 V (50 Hz) or up to 10 kV. In some applications a step up transformer is placed between a
low voltage drive and a medium voltage load. Medium voltage units are typically designed for
use with motors rated to deliver 375 kW or 500 hp and above. Medium voltage drives rated
above 7 kV and 5,000 or 10,000 hp should probably be considered to be one-of-a-kind (one-off)
designs.

Medium voltage drives are generally rated amongst the following voltages: 2, 3 KV - 3, 3 Kv - 4
Kv - 6 Kv - 11 Kv.

The in-between voltages are generally possible as well. The power of MV drives is generally in
the range of 0, 3 to 100 MW however involving a range at several different type of drives with
different technologies.

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APPLICATIONS

Variable speed drives are used for two main reasons:

 to improve the efficiency of motor-driven equipment by matching speed to changing load

requirements; or
 To allow accurate and continuous process control over a wide range of speeds.

Motor-driven centrifugal pumps, fans and blowers offer the most dramatic energy-saving
opportunities. Many of these operate for extended periods at reduced load with flow restricted or
throttled. In these centrifugal machines, energy consumption is proportional to the cube of the
flow rate. Even small reductions in speed and flow can result in significant energy savings. In
these applications, significant energy and cost savings can be achieved by reducing the operating
speed when the process flow requirements are lower.

In some applications, such as conveyers, machine tools and other production-line equipment, the
benefits of accurate speed control are the primary consideration. VFDs can increase productivity,
improve product quality and process control, and reduce maintenance and downtime. Decreasing
cost and increasing reliability of power semiconductor electronics are reasons that VFDs are
increasingly selected over DC motors or other adjustable speed drives for process speed control
applications.

Motors and VFDs must be compatible. Consult the manufacturers of both the VFD and the motor
to make sure that they will work together effectively. VFDs are frequently used with inverter-
duty National Electrical Manufacturers Association (NEMA) design B squirrel cage induction
motors. (Design B motors have both locked rotor torque and locked rotor current that are
normal.) De-rating may be required for other types of motors. VFDs are not usually
recommended for NEMA design D motors because of the potential for high harmonic current
losses. (Design D motors are those that have high locked rotor torque and high slip.)

VFD Application with different load types

Variable frequency drive (VFD, VVVF drive) is the most effective motor controller in today's
industry. VFD applications can be divided into the following individual load types. They are as
follows:

 Constant Torque loads

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 Constant Horse power loads

 Variable Torque loads

1. Constant torque loads

These loads represent 90% of all general industrial machines (other than pumps and fans).
Examples are general machinery, hoists, conveyors, printing presses, positive displacement
pumps, some mixers and extruders, reciprocating compressors, as well as rotary compressors.

2. Constant horsepower loads.

These loads are most often found in the machine-tool industry and center driven winder
applications. Examples of constant horsepower loads are winders, core-driven reels, wheel
grinders, large driller machines, lathes, planers, boring machines, and core extruders.

Traditionally, these loads were considered DC drive applications only. With high-performance
flux vector VFD's now available; many DC drive applications of this type can be now handled
by VFDs.

3. Variable torque loads.

Variable torque loads are often found in variable flow applications, such as fans and pumps.
Examples of applications include fans, centrifugal blowers, centrifugal pumps, propeller pumps,
turbine pumps, agitators, and axial compressors.

Variable Frequency Drives (VFDs) offer the greatest opportunity for energy conservation (also
energy saving) when driving these loads, because horsepower varies as the cube of speed and
torque varies as square of speed for these loads. For instance, if the motor speed is reduced 20%,
motor horsepower is reduced by a cubic relationship (0.8 X 0.8 X 0.8), or 51%. As such, utilities
often offer subsidies to customers investing in Variable Frequency Inverter (VFD, ac drive,
frequency inverter) technology for their applications. Many VFD (frequency converter)
manufactures have free software programs available for customers to calculate and document
potential energy savings by using these AC Drives.

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CONCLUSION

Due to many advantages offered by ac drives like automatic control, closed loop control,
economical cost etc. ac motors are being replaced in fields which were totally capitalized by dc
motors like traction, some industrial applications etc.

1. Significant energy savings.

2. Easy setup and programming.

3. Retrofits.

4. Space.

5. Better design.

6. Competitive edge.

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REFERENCES
1. http://www.ehow.com/list_7307308_types-vfd.html
2. http://www.brighthub.com/engineering/electrical/articles/76956.aspx
3. http://www.thefreelibrary.com/Keeping+New+York+water+running+may+take+a+VFD.-
a0142634119
4. http://www.seminarprojects.com/Thread-variable-voltage-and-variable-frequency-drives-full-
report#ixzz1H4IN05od
5. http://oee.nrcan.gc.ca/industrial/equipment/vfd/index.cfm?attr=24 key words:introduction to
vfd
6. http://www.inverter-china.com/blog/articles/Knowledge-of-frequency-converter-ac-motor-
drive/38.html a
7. http://www.ehow.com/list_7307308_types-vfd.html
8. http://www.brighthub.com/engineering/electrical/articles/76956.aspx
9. http://www.thefreelibrary.com/Keeping+New+York+water+running+may+take+a+VFD.-
a0142634119

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