VFD types
All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them
only on or o! "sing a linear device such as a transistor in its linear mode is impractical or a VFD drive, since the power dissipated in the drive devices would #e a#out as much as the power
delivered to the load!
• %onstant voltage
• %onstant current
• %ycloconverter 
In a constant voltage converter, the intermediate D% lin& voltage remains appro'imately constant during each output cycle! In constant current drives, a large inductor is placed #etween the input
rectiier and the output #ridge, so the current delivered is nearly constant! A cycloconverter has
no input rectiier or D% lin& and instead connects each output terminal to the appropriate input
 phase!
The most common type o pac&aged VF drive is the constantvoltage type, using pulse width
modulation to control #oth the reuency and eective voltage applied to the motor load!
[edit] VFD system description
VFD system
A varia#le reuency drive system generally consists o an A% motor, a controller and an
operator interace!*+*-
[edit] VFD motor
The motor used in a VFD system is usually a threephase induction motor ! .ome types o single
 phase motors can #e used, #ut threephase motors are usually preerred! Various types o
synchronous motors oer advantages in some situations, #ut induction motors are suita#le or most purposes and are generally the most economical choice! /otors that are designed or i'ed
speed operation are oten used! %ertain enhancements to the standard motor designs oer higher
relia#ility and #etter VFD perormance, such as /G01 rated motors!*2
[edit] VFD controller
Varia#le reuency drive controllers are solid state electronic  power conversion devices! The
usual design irst converts A% input power to D% intermediate power using a rectiier  or converter #ridge! The rectiier is usually a threephase, ullwavediode  #ridge! The D%
intermediate power is then converted to uasisinusoidal A% power using an inverter switching
circuit! The inverter circuit is pro#a#ly the most important section o the VFD, changing D%
energy into three channels o A% energy that can #e used #y an A% motor! These units provide improved power actor, less harmonic distortion, and low sensitivity to the incoming phase
seuencing than older phase controlled converter VFD3s! .ince incoming power is converted to
D%, many units will accept singlephase as well as threephase input power (acting as a phase converter  as well as a speed controller)4 however the unit must #e derated when using single
 phase input as only part o the rectiier #ridge is carrying the connected load!*5
As new types o semiconductor  switches have #een introduced, these have promptly #een
applied to inverter circuits at all voltage and current ratings or which suita#le devices are availa#le! Introduced in the 1678s, the insulatedgate #ipolar transistor  (IGBT) #ecame the
device used in most VFD inverter circuits in the irst decade o the 91st century!*7*6*18
A% motor characteristics reuire the applied voltage to #e proportionally ad:usted whenever the
reuency is changed in order to deliver the rated torue! For e'ample, i a motor is designed to operate at +28 volts at 28 ;<, the applied voltage must #e reduced to 908 volts when the
reuency is reduced to 08 ;<! Thus the ratio o volts per hert< must #e regulated to a constant
value (+28=28 > 5!25 V=;< in this case)! For optimum perormance, some urther voltage ad:ustment may #e necessary especially at low speeds, #ut constant volts per hert< is the general
rule! This ratio can #e changed in order to change the torue delivered #y the motor !*11
In addition to this simple volts per hert< control more advanced control methods such as vector
control and direct torue control (DT%) e'ist! These methods ad:ust the motor voltage in such a way that the magnetic lu' and mechanical torue o the motor can #e precisely controlled!
The usual method used to achieve varia#le motor voltage is  pulsewidth modulation (?@/)!
@ith ?@/ voltage control, the inverter switches are used to construct a uasisinusoidal output
 
waveorm #y a series o narrow voltage pulses with pseudosinusoidal varying pulse durations!*7
*19
peration o the motors a#ove rated name plate speed (#ase speed) is possi#le, #ut is limited to conditions that do not reuire more power than nameplate rating o the motor! This is sometimes
called ield wea&ening and, or A% motors, means operating at less than rated volts=hert< and
a#ove rated name plate speed! ?ermanent magnet synchronous motors have uite limited ield
wea&ening speed range due to the constant magnet lu' lin&age! @ound rotor synchronous motors and induction motors have much wider speed range! For e'ample, a 188 hp, +28 V,
28 ;<, 155- C?/ (+ pole) induction motor supplied with +28 V, 5- ;< (2!10+ V=;<), would #e
limited to 28=5- > 78 torue at 19- speed (9917!5- C?/) > 188 power !*10 At higher speeds the induction motor torue has to #e limited urther due to the lowering o the #rea&away torue
o the motor! Thus rated power can #e typically produced only up to 108!!!1-8 o the rated
name plate speed! @ound rotor synchronous motors can #e run even higher speeds! In rolling mill drives oten 988!!!088 o the #ase speed is used! Eaturally the mechanical strength o the
rotor and lietime o the #earings is also limiting the ma'imum speed o the motor! It is
recommended to consult the motor manuacturer i more than 1-8 speed is reuired #y the application!
?@/ VFD utput Voltage @aveorm
An em#edded microprocessor  governs the overall operation o the VFD controller! The main
microprocessor programming is in irmware that is inaccessi#le to the VFD user! ;owever, some
degree o coniguration programming and parameter ad:ustment is usually provided so that the user can customi<e the VFD controller to suit speciic motor and driven euipment reuirements! *7
[edit] VFD operator interface
The operator interace provides a means or an operator to start and stop the motor and ad:ust the
operating speed! Additional operator control unctions might include reversing and switching
 #etween manual speed ad:ustment and automatic control rom an e'ternal process control signal! The operator interace oten includes an alphanumeric display and=or indication lights and meters
to provide inormation a#out the operation o the drive! An operator interace &eypad and display
unit is oten provided on the ront o the VFD controller as shown in the photograph a#ove! The
&eypad display can oten #e ca#leconnected and mounted a short distance rom the VFD controller! /ost are also provided with input and output (I=) terminals or connecting
 
 push#uttons, switches and other operator interace devices or control signals! Aserial
communications  port is also oten availa#le to allow the VFD to #e conigured, ad:usted,
monitored and controlled using a computer !*7*1+*1-
[edit] VFD operation
@hen an induction motor is connected to a ull voltage supply, it draws several times (up to
a#out 2 times) its rated current! As the load accelerates, the availa#le torue usually drops a little
and then rises to a pea& while the current remains very high until the motor approaches ull speed!
By contrast, when a VFD starts a motor, it initially applies a low reuency and voltage to the
motor! The starting reuency is typically 9 ;< or less! Thus starting at such a low reuency avoids the high inrush current that occurs when a motor is started #y simply applying the utility
(mains) voltage #y turning on a switch! Ater the start o the VFD, the applied reuency and
voltage are increased at a controlled rate or ramped up to accelerate the load without drawing
e'cessive current! This starting method typically allows a motor to develop 1-8 o its rated torue while the VFD is drawing less than -8 o its rated current rom the mains in the low
speed range! A VFD can #e ad:usted to produce a steady 1-8 starting torue rom standstill
right up to ull speed!*12 Eote, however, that cooling o the motor is usually not good in the low speed range! Thus running at low speeds even with rated torue or long periods is not possi#le
due to overheating o the motor! I continuous operation with high torue is reuired in low
speeds an e'ternal an is usually needed! The manuacturer o the motor and=or the VFD should speciy the cooling reuirements or this mode o operation!
In principle, the current on the motor side is in direct proportion o the torue that is generated
and the voltage on the motor is in direct proportion o the actual speed, while on the networ&
side, the voltage is constant, thus the current on line side is in direct proportion o the power drawn #y the motor, that is "!I or %!E where % is torue and E the speed o the motor (we shall
consider losses as well, neglected in this e'planation)!
(1) n stands or networ& (grid) and m or motor 
(9) % stands or torue *Em, " or voltage *V, I or current *A, and E or speed *rad=s @e neglect losses or the moment $
"n!In > "m!Im (same power drawn rom networ& and rom motor)
"m!Im > %m!Em (motor mechanical power > motor electrical power) Given "n is a constant (networ& voltage) we conclude $ In > %m!Em="n That is line current
(networ&) is in direct proportion o motor power!
@ith a VFD, the stopping seuence is :ust the opposite as the starting seuence! The reuency
and voltage applied to the motor are ramped down at a controlled rate! @hen the reuency approaches <ero, the motor is shut o! A small amount o #ra&ing torue is availa#le to help
decelerate the load a little aster than it would stop i the motor were simply switched o and
allowed to coast! Additional #ra&ing torue can #e o#tained #y adding a #ra&ing circuit (resistor controlled #y a transistor) to dissipate the #ra&ing energy! @ith +uadrants rectiiers (active
 
rontend), the VFD is a#le to #ra&e the load #y applying a reverse torue and reverting the
energy #ac& to the networ&!
[edit] Power line harmonics
@hile ?@/ allows or nearly sinusoidal currents to #e applied to a motor load, the diode
rectiier o the VFD ta&es roughly suarewave current pulses out o the A% grid, creating
harmonic distortion in the power line voltage! @hen the VFD load si<e is small and the availa#le utility power is large, the eects o VFD systems slicing small chun&s out o A% grid generally
go unnoticed! Further, in low voltage networ&s the harmonics caused #y single phase euipment
such as computers and TVs are such that they are partially cancelled #y threephase diode #ridge harmonics!
;owever, when either a large num#er o lowcurrent VFDs, or :ust a ew very largeload VFDs
are used, they can have a cumulative negative impact on the A% voltages availa#le to other
utility customers in the same grid!
@hen the utility voltage #ecomes misshaped and distorted the losses in other loads such as
normal A% motors are increased! This may in the worst case lead to overheating and shorter
operation lie! Also su#station transormers and compensation capacitors are aected, the latter
especially i resonances are aroused #y the harmonics!
In order to limit the voltage distortion the owner o the VFDs may #e reuired to install iltering
euipment to smooth out the irregular waveorm! Alternatively, the utility may choose to install
iltering euipment o its own at su#stations aected #y the large amount o VFD euipment
 #eing used! In high power installations decrease o the harmonics can #e o#tained #y supplying the V.Ds rom transormers that have dierent phase shit!*15
Further, it is possi#le to use instead o the diode rectiier a similar transistor circuit that is used to
control the motor! This &ind o rectiier is called active ineed converter in I% standards! ;owever, manuacturers call it #y several names such as active rectiier , I." (IGBT .upply
"nit), AF (Active Front nd) or our uadrant rectiier! @ith ?@/ control o the transistors
and ilter inductors in the supply lines the A% current can #e made nearly sinusoidal! ven #etter
attenuation o the harmonics can #e o#tained #y using an % (inductorcapacitorinductor) ilter  instead o single threephase ilter inductor!
Additional advantage o the active ineed converter over the diode #ridge is its a#ility to eed
 #ac& the energy rom the D% side to the A% grid! Thus no #ra&ing resistor is needed and the
eiciency o the drive is improved i the drive is reuently reuired to #ra&e the motor!
[edit] Application considerations
 
The output voltage o a ?@/ VFD consists o a train o pulses switched at the carrier reuency!
Because o the rapid rise time o these pulses, transmission line eects o the ca#le #etween the
drive and motor must #e considered! .ince the transmissionline impedance o the ca#le and motor are dierent, pulses tend to relect #ac& rom the motor terminals into the ca#le! The
resulting voltages can produce up to twice the rated line voltage or long ca#le runs, putting high
stress on the ca#le and motor winding and eventual insulation ailure! Increasing the ca#le or motor si<e=type or long runs and +78v or 288v motors will help oset the stresses imposed upon
the euipment due to the VFD (modern 908v single phase motors not eected)! At +28 V, the
ma'imum recommended ca#le distances #etween VFDs and motors can vary #y a actor o 9!-$1! The longer ca#les distances are allowed at the lower %arrier .witching Freuencies (%.F) o
9!- &;<! The lower %.F can produce audi#le noise at the motors! For applications reuiring long
motor ca#les V.D manuacturers usually oer du=dt ilters that decrease the steepness o the
 pulses! For very long ca#les or old motors with insuicient winding insulation more eicient sinus ilter is recommended! 'pect the older motor3s lie to shorten! ?urchase VFD rated motors
or the application!
[edit] Motor bearings
/ain article$ .hat voltage
Further, the rapid rise time o the pulses may cause trou#le with the motor #earings! The stray
capacitance o the windings provide paths or high reuency currents that close through the  #earings! I the voltage #etween the shat and the shield o the motor e'ceeds ew volts the stored
charge is discharged as a small spar&! Cepeated spar&ing causes erosion in the #earing surace
that can #e seen as luting pattern! In order to prevent spar&ing the motor ca#le should provide a
low impedance return path rom the motor rame #ac& to the inverter! Thus it is essential to use a ca#le designed to #e used with V.Ds!*17
In #ig motors a slip ring with #rush can #e used to provide a #ypass path or the #earing currents!
Alternatively isolated #earings can #e used!
The 9!- &;< and - &;< %.Fs cause ewer motor #earing pro#lems than the 98 &;< %.Fs!*16  .horter ca#les are recommended at the higher %.F o 98 &;<! The minimum %.F or
synchroni<e trac&ing o multiple conveyors is 7 &;<!
The high reuency current ripple in the motor ca#les may also cause intererence with other
ca#ling in the #uilding! This is another reason to use a motor ca#le designed or V.Ds that has a symmetrical threephase structure and good shielding! Further, it is highly recommended to route
the motor ca#les as ar away rom signal ca#les as possi#le!*98
[edit] Available VFD power ratings
Varia#le reuency drives are availa#le with voltage and current ratings to match the ma:ority o 0phase motors that are manuactured or operation rom utility (mains) power! VFD controllers
designed to operate at 111 V to 268 V are oten classiied as low voltage units! ow voltage units
are typically designed or use with motors rated to deliver 8!9 &@ or 1=+ horsepower  (hp) up to
 
several megawatts! For e'ample, the largest ABB A%.788 single drives are rated or -!2 /@*91 !
/edium voltage VFD controllers are designed to operate at 9,+88=+,129 V (28 ;<), 0,888 V
(-8 ;<) or up to 18 &V! In some applications a step up transormer  is placed #etween a low voltage drive and a medium voltage load! /edium voltage units are typically designed or use
with motors rated to deliver 05- &@ or -88 hp and a#ove! /edium voltage drives rated a#ove 5
&V and -,888 or 18,888 hp should pro#a#ly #e considered to #e oneoa&ind (oneo) designs! *99
/edium voltage drives are generally rated amongst the ollowing voltages $ 9,0 HV 0,0 Hv +
Hv 2 Hv 11 Hv
The in#etween voltages are generally possi#le as well! The power o /V drives is generally in the range o 8,0 to 188 /@ however involving a range a several dierent type o drives with
dierent technologies!
[edit] Dynamic braking
"sing the motor as a generator to a#sor# energy rom the system is called dynamic #ra&ing! Dynamic #ra&ing stops the system more uic&ly than coasting! .ince dynamic #ra&ing reuires
relative motion o the motor3s parts, it #ecomes less eective at low speed and cannot #e used to
hold a load at a stopped position! During normal #ra&ing o an electric motor the electrical energy produced #y the motor is dissipated as heat inside o the rotor, which increases the
li&elihood o damage and eventual ailure! Thereore, some systems transer this energy to an
outside #an& o resistors! %ooling ans may #e used to protect the resistors rom damage! /odern systems have thermal monitoring, so i the temperature o the #an& #ecomes e'cessive, it will #e
switched o !*90
[edit] egenerative variable!fre"#ency drives
Cegenerative A% drives have the capacity to recover the #ra&ing energy o an overhauling load and return it to the power system!*9+
ine regenerative varia#le reuency drives, showing capacitors(top cylinders)and inductors attached which ilter the regenerated power!
*9*0*9+*9-*92*95
%ycloconverters and currentsource inverters inherently allow return o energy rom the load to
the line4 voltagesource inverters reuire an additional converter to return energy to the supply!*97
Cegeneration is only useul in varia#lereuency drives where the value o the recovered energy is large compared to the e'tra cost o a regenerative system,*97 and i the system reuires
reuent #ra&ing and starting! An e'ample would #e use in conveyor #elt during manuacturing
where it should stop or every ew minutes, so that the parts can #e assem#led correctly and moves on! Another e'ample is a crane, where the hoist motor stops and reverses reuently, and
 #ra&ing is reuired to slow the load during lowering! Cegenerative varia#lereuency drives are
widely used where speed control o overhauling loads is reuired!
[edit] $r#shless D% motor drives
/uch o the same logic contained in large, powerul VFDs is also em#edded in small #rushless
D% motors such as those commonly used in computer ans! In this case, the chopper  usually
converts a low D% voltage (such as 19 volts) to the threephase current used to drive the electromagnets that turn the permanent magnet rotor!