VFD types All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches only on or o! "sing a linear device such as a transistor in its linear mode is im VFD drive, since the power dissipated in the drive devices would #e a#out as much a delivered to the load! Drives can #e classiied as$ • %onstant voltage • %onstant current • %ycloconverter In a constant voltage converter, the intermediate D% lin& voltage remains appro'ima during each output cycle! In constant current drives, a large inductor is placed #e rectiier and the output #ridge, so the current delivered is nearly constant! A cyc no input rectiier or D% lin& and instead connects each output terminal to the appr phase! The most common type o pac&aged VF drive is the constant voltage type, using pulse modulation to control #oth the re uency and eective voltage applied to the motor [edit] VFD system description VFD system A varia#le re uency drive system generally consists o an A% motor, a controller a operator interace! *+ *-
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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!
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!
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