Vfd Important

8/13/2019 Vfd Important

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Not necessarily a speed variation is must for such an application, If the speed isother than top rated speed, it is going to give cost benefits. Lower the speedmore the benefits.

Nevertheless, Simpler less expensive solutions such as changing pulleys,modifications of the fan blades or a new fan, may be more effective in

some cases, where Variable Speed is not required to be adjusted.

Ie . If a fan of 1000 RPM, has to be made to run at 500 RPM Constantly, then in that case , one

can adopt the cost effective methods.

But, if a fan of 1000 RPM, is reuire to run at any speed set!point bet"een 10# to 100# speed,

one has to $o for %&'(s )nly. *he follo"in$ topics illustrate the +ner$y &avin$ Phenomenon,

"hile usin$ the %&'(s.

Concept of variable speed drivesDC motors & AC induction motors

Any variable speed electrical drive system comprises of the following components

! An electronic actuator " the controller.

! A driving electrical machines " motor.

! A driven machine #load$ " pump, fan, blower, compressor%

&he tas' of a variable speed electrical drive is to convert the electrical power supplied by the mains into mechanicalpower with a minimum loss. &o achieve an optimum technological process, the drive must be variable in speed. &hiswill steplessly adjust the speed of the driven machine. &his is ensured by the low loss control using solid state

Technologyin electronic controllers. &he controllers are connected to mains supply and the electrical machine asshown in figure

&he solid " state devices, which convert the A( supply to )( supply were first used as variable speed devices, in )(technology. *sing these devices the armature voltage of a )( motor and therefore the speed can be adjusted,almost without losses and over a wide range of speed . *sing these features the drive can be designed which startsmoothly and jer'"free. &his helps to maintain the desired selected speed, independently of the load and operate withgood dynamic response

*he 'C drive needs special consideration in some applications. or e-ample in haardousatmosphere, vibrations and hi$her speeds the usa$e of /C motor "ith suirrel!Ca$e rotor is

advanta$eous. *he use of freuency inverters VFD's to supply to /C Motors resulted in a ne"orientation of electrical po"er for handlin$ variable speeds operation is sho"n in fi$ure

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+very standard /C motor can be fitted "ith a variable speed drive usin$ a freuency inverter.reuency and volta$e of the sin$le ! phase or three ! phase mains are varied by the freuency

inverter, such that the motor can be operated "ith varyin$ speeds over lar$e ran$e settin$s. *he

operatin$ mode of any motor connected to these variable speed drives can be classified in our2uadrants, dependin$ upon the *orue and &peed of the drive in fi$ure above

Four Quadrant Operation :

A +our"quadrant diagram can represent mode of operation of variable speed drive. n -uadrant the speed andtorque can be represented positive or forward direction. &his is consistent with a motor driving a load ta'ing powerfrom the mains. Similarly in -uadrant /, both speed and torque are in negative or reverse direction.

*his Corresponds to a motor turnin$ in the reverse direction, drivin$ a load and a$ain ta3in$po"er from the mains. In 2uadrants 4 and , the speed and torue are in mutually ! opposed

directions, that is to say, the torue of the motor is opposin$ its rotation, $ivin$ a bra3in$ effect.

It follo"s,then, that mechanical and 3inetic ener$y of the load is bein$ converted into electricalener$y. *he motor is behavin$ as a $enerator and the system as a "hole is deliverin$ po"er into

the mains.

&his behavior is 'nown as Regeneration. Aftergoing, through the mode of operation of VS)s, let us briefly discussabout the various loading patterns. &he characteristics of the load are particularly important in the trouble "+reeoperation of VS)s. 0oad refers essentially to the torque output and the corresponding speed required. 0oads can bebroadly classified as follows

(onstant torque

Variable torque

(onstant power

(1NS&AN& &12-*3 01A)

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(onstant torque load are those for which the output power requirement may vary with speed of operation, but thetorque does not vary. (onveyors, rotary 'ilns and constant " displacement pumps are typical examples of constanttorque loads.

VA2A403 &12-*3 01A)

Variable torque loads are those for which the torque required varies with speed of operation. (entrifugal pumps andfans are typical examples of variable torque loads # torque varies as the square of the speeds $.

(1NS&AN& 51632 01A)

(onstant power loads are those for which the torque requirements are typically changed inversely with speed.6inders, coilers are typically the examples of constant power loads.

*he lar$est potential for ener$y savin$s "ith variable speeds drive are $enerally in variabletorue applications. or e-ample, centrifu$al pumps and fans, "here the po"er reuirements

chan$es as the cubes of speed. Constant torue loads are suitable for %&' application.

&he latest industrial trend is to use A( drives for variable speed application. As already discussed, to vary the speedof an A( motor and at the same time retain its torque producing capability a power source is required. &his powersource has to provide variable voltage and frequency output in such a way that, in most of the operating area the V 7 fratio is maintained constant. &his can be achieved through an A( drive which gives variable frequency and variablevoltage as out put by ta'ing fixed voltage as input.

&he principle involved in this technique is first to convert the fixed frequency, fixed voltage A( supply into a variableor constant )( voltage. &his is then into the A( supply of desired frequency 8 amplitude. &he criteria for theselection of A( inverter drive are essentially the same as for a )( variable speed drive. &he latest developments intechnology and successful development of electronic drives #A( drives$ for cage motors have resulted in thefollowing benefits

$ Availability of full load torque from standstill9$ Absence of torque fluctuations at low speed./$ Ability to hold a set speed, regardless of load torque variation:$ Ability to control the rate of increase 8 decrease of speed;$ )ynamic response.

Induction Motor :

An A( induction motor essentially consists of two parts namely a stationary part called the


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rotating. &he speed of the rotor however is less than synchronous speed Ns #the speed of rotating magnetic fielddeveloped by the stator$. f the rotor runs exactly at the synchronous speed induced emf in the rotor will be =ero.>ence there will be no rotor current and rotor torque.

&he synchronous speed is a function of the no of poles of the motor and supply frequency. &his is given by

Ns ? 9@ frequency #f$ 7 number of poles #5$

>ence the speed of an A( motor is a function of frequency and the number of motor poles. &he speed of the rotorrelative to that of the stator"rotating field is called as


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6ide speed control rangemproved efficiency compared to traditional flowcontrol methods e.g. damper control, throttling

lower maintenance

>igh reliability and availability2educed downtime

mproved process availability

0ow audible noise mproved wor'ing environment foroperators

(apability for speed reversal 7 regenerative bra'ing)esired torque during bra'ing,therefore better product quality

improved bra'ing characteristics

>igher efficiency

+lux optimisation #motor flux automatically adapted to load$mproved motor efficiency

2educed motor noise

5ower loss ride through 2educed number of drive trips

4etter process availability

Automatic start #drive can catch a spinning load$2educed waiting time

2educed downtime

3nergy saving A( drives can be retroffied tostandard induction motors, toprovide substantial energy savings

$peed control o# Induction Motor :&he power supply to the induction motor is through the stator winding terminal. &he speed control of the inductionmotor is possible at the stator winding terminal, by appropriately changing the electrical supply voltage, frequency orthe internal winding. &he rotor circuit windings available in a slipring induction motor, allows an additional means tocontrol the speed. &his method of varying the motor speed by adding resistance in the rotor circuit is 'nown as rotorresistance control # 22( $. &he operating principle of 22( is explained as follows

n the rotor resistance control method, the speed variation in a motor can be achieved by altering the slip the motorcan operate. &his method is applicable for slipring induction motors, as it involves addition of the external resistancein the rotor circuit of the motor #as shown in fig $.&he principle employed in the rotor resistance control is changing theinternal motor circuit parameters, by adding external rotor resistance. &his in turn changes the torque"speedcharacteristics of the motor.

+igure" Slipring induction motor " with external rotor resistors

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7ith increasin$ resistance, the slope of the motor curve decreases, shiftin$ the stable operatin$

point for the $iven load curve to a point "ith hi$her slip. *hus the speed control is achieved inthe rotor resistance control. *his is represented in the fi$. belo"

+igure" Slipring induction motor " with external rotor resistors

&he above graph shows the variations of the torque with slip, the other factors remaining constant. &he change in slipis attained by changing the value of rotor resistances. n the graph, the curves A, 4, ( 8 ) have rotor resistances 2a,

2b, 2c 8 2d respectively. &he relative values of resistanceBs is as follows Rd(Rc(Rb(Ra. t is observed that a

significant amount of input power has to be dissipated in the external resistors. &his power lost due to the increase inslipis called as slip power. &he ratio of slip power to total power input changes with speed.

2otor 2esistance (ontroller #22($ 2otor 2esistance (ontroller #22($ is a method of speed control applicable tothe slip"ring induction motor only.

Advantages of RRC#

No harmonic generation#*nli'e A( drives 8 S52S, 22( has no adverse effect, such as harmonics generationwhich affects the distribution networ'.

Ambient conditions#22( has no electronic components li'e that of other electronic variable speed drives. >ence,they can be installed in even adverse environments.

)isadvantages of RRC#

%ternal coolin":/ portion of the input po"er has to be dissipated in the e-ternal rotor

resistors. *hese resistors reuire coolin$ fans to dissipate the heat $enerated by them. *he

coolin$ fans form an additional load.

$peed adustment:In this methods the speed ad8ustment is in steps or "ith very poor

re$ulations

Maintenance:*his method of control has lot of contractors 9 orther movin$ parts, "hich

reuires re$ular maintenance.

%ner" sa!in" concept & #an cur!es

7e all 3no" that lot of ener$y is "asted in fan:pump:blo"er applications if not properlydesi$ned. 7hen "e use conventional motor control system, in "hich /C motor is run at full

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speed, the flo" of $ases:air :liuid is re$ulated usin$ the damper :throttle control. In this

process , substantial ener$y is lost in the damper:throttle. *his "aste of ener$y can be as hi$h as

45 to 60 # of motor ratin$. /l"ays $o for reliable v:f , variable speed drives to control the speedof fan:pump:blo"er, "hich in turn "ill automatically control the flo". armonic )istortion is a 'ind of pollution in electrical supply. &he distortion is caused by different


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f possible use 9"pulse 2ectifier in the )rivenstall the cabling and earthing properlynstall Shunt +ilters or >armonic &raps

Formula #or calculatin" Motor Capacit

2otary motion0inear motion #>ori=ontal motion$

! 5o ?9! &l! N

C@ !

x @"/ G'6H ! 5o ?! 6 ! Vl

C9@ !

G'6H

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! 5a ?:Il! #Nl$9

/C; J @/ ! ta

G'6H ! 5a ?6 ! #Vl$9

/C@@ J @/! ta

G'6H

! &0 ?Nl

NF!! &l GN! FH ! &0 ?

E.K m ! 6 ! Vl

9! NF ! pGN! FH

! I0? Nl

Nm!Il G'g ! m

9H ! I0 ? Vl

: ! NFG'g ! m9H

! ta ?9#IF L I0$ ! NF

C@ ! #&F ! a" &0$

GsecH ! ta ?9p#IF L I0 $ !NF

C@ ! #&F !" &0$GsecH

! td ?9#IF L I0$ ! NF

C@ ! #&F !L &0$ GsecH ! td ?

9p #IF L I0$ ! NF

C@ ! #&F ! L &0$GsecH

0egend

5o @ 2unning power G'6H &l @ 0oad torque GN ! mH5a @ 2equired power for accel G'6H &0 @ 0oad torque GN ! mH

Nl @ 0oad speed Gr7mH #2eflected to motor shaft$

NF @ Fotor shaft speed Gr7mH &F @ Fotor rated torqueGN.mH

Vl @ 0oad velocity of load Gm7min.H ta @ Acceleration time GsecH

@ Fachine efficiency td @ )ecceleration tiem GsecH

@ +riction factor @ @.K " .9

6 @ 6eight of load G'gH @@. " @.9 #9@@V class$@.@; " @. #:@@ V class$

IF @ Fotor inertia G'g ! m9H

Il @ 0oad inertia G'g ! m9H

I0 @0oad inertia G'g ! m9H#2eflected to motor shaft$

Do)nsi*in"

6ith the expansion of the field of application of drives, the demands for ma'ing drives more compact and lower incost are becoming stronger.

Among the above, the issue of cooling is especially important in counteracting the increase of heating densityaccompanying downsi=ing and is becoming difficult to accommodate by normal conventional methods. 6ith regard to

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this issue, we will describe the building"in of quality and the the prior verification activities which are performed at theinitial stages of development by the effective utili=ation of (A)7(A3 tools.

7hen an induction motor is driven by a P7M /C drive, a sur$e volta$e may occur at the motor

terminals due to the characteristics of the drives output volta$e dv:dt. ar$e sur$e volta$es canbrea3 do"n the motor insulation and cause premature motor failure. *he article attempts to

discuss this phenomenon in both a theoretical and practical "ay.

. 6hat is VVV+ A( drive M

VVV+ A( drive is the power electronic controller used to control the speed of /ph A( motors#synchronous or inducution$ by varying the frequency and the voltage applied to the motor terminals.Voltage and frequency relationship is decided based on the motor name plate data and the loadcharacteristics.

9. 6hat is the typical power circuit configuration of VVV+ A( driveM

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&ypical power circuit configuration involves /5h. diode rectifier at the input, which converts the A( inputto )( voltage. 0( or ( filter reduces the ripple in the )( voltage. /5h 4& A( drive stage converts this)( voltage into variable voltage variable frequency output as per the desired pattern

/. 6hat are the different types of VVV+ A( driveM

VVV+ A( drive are generally classified into three types based on the type of control philosophy adoptedfor motor controlScalar control756F control.

Sensorless vector control.Vector control #with sensor$ or +lux vector control.

:. 6hat is scalar controlM

n scalar control, relationship between voltage and frequency of the A( voltage applied to the motorterminals is predetermined by the user. &his relationship is marginally altered in scalar drives sometimes,to improve the performance of the drive. Scalar controlled inverters can have only speed control andthese are ideal for group7multi motor drives.

;. 6hat is vector control or flux vector controlM6hat are the typical applicationsM

n Vector (ontrol motor, current is controlled with two independent components i.e., torque component

and flux component. &hese components are computed based on the rotor position, rotor speed and motorparameters. Fotor speed is controlled rather than output frequency. 2elationship between voltage andfrequency is decided by operating conditions. Vector controlled inverters invariably use encoders for rotorspeed and position feedbac'. As flux and torque components of current are decoupled, fast dynamicresponse is obtained. t is possible to get more than rated torque at =ero speed also. Vector control canbe achieved for single motor only. Vector control inverters are used for applications demanding =erospeed regulation, wide speed control range and excellent dynamic response. 3x. 5aper machine drives,film line drives.

C. 6hat is Sensorless Vector (ontrolM

n sensorless vector control, motor speed is estimated based on the measured motor terminal parametersand hence speed sensor is avoided. 4ased on motor parameters and computed rotor speed, flux and

torque component of motor current are computed. As flux and torque component of current areindependently controlled, fast dynamic response is achieved. Speed regulation is better than scalar drivesand typical value is L7@.;O. &his speed regulation is typically achieved in the range ;@. >igh startingtorque #P;@O$ is also achieved by this control.

D. 6hat is 56F (ontrolM

5ulse 6idth Fodulation #56F$ is the method of control where variable voltage #A(7)($ is achieved froma fixed )( voltage using switching devices. )( voltage is applied for sometime in the cycle and in theremaining period, no voltage is applied to the load. 4y adjusting the duty ratio, #ratio of on period to cycletime$ output voltage is adjusted between =ero and rated voltage

K. 6hat is applied motor rating as specified by inverter specificationM

Applied motor rating specifies the maximum rating of the :"pole motor that can be connected to the

inverter to obtain its rated output power at the rated speed. t is necessary that the rated input voltage ofthe inverter and motor are matched or else specified out power may not be achieved.

E. 6hat is rated QVA output capacityM

t is the apparent power that can be delivered continuously by the inverter at the rated frequency. &his iscalculated as GS-2/$ x rated output voltage x rated currentH7@@@.

@. 6hat is rated output voltageM

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2ated output voltage is the fundamental rms value of the output terminal voltage at rated input and outputconditions.

. 6hat is the output rated currentM

)utput rated current is the rms current the inverter can continuously supply irrespective of the output freuency.

9. 6hat is the rated input voltage 8 frequencyM

t is the rated supply voltage and frequency for which invertor delivers its rated output.

/. 6hat is input voltage variation and frequency variationM

nput voltage and frequency variation range specifies the range wherein the inverter can deliver the ratedcurrent without affecting the life of the equipment. 1ther specifications as output power, voltage etc., maynot be met during the variation.

:. 6hat is the power factor as claimed by manufacturersM

nput power factor can be specified in two ways i.e., displacement power factor and harmonic powerfactor. f diode rectifier is used displacement between the fundamental voltage and current is nearly =eroand hence displacement power factor is approximately .@. >armonic power factor in the ratio of inputeffective power and input apparent power. &his depends on output speed and load conditions. t is

normally specified at rated input and output conditions. 5ower factor depends on the power circuitconfiguration.

;. 6hat is input QVA capacityM

t is the input apparent power drawn by the inverter at the rated output conditions.

C. 6hat is the frequency 7 speed rangeM

+requency7speed range is the ratio of minimum and maximum frequency7speed in the defined operatingcondition. 3x.@ speed range with constant torque.

D. 6hat is frequency stabilityM

+requency stability specifies the variation in output frequency with the defined temperature variation'eeping frequency reference constant. 3x. L7"@.;O of max frequency for 9;R (. L7"@R (.

K. 6hat is inverter efficiencyM

nverter efficiency is the ratio of the output power to the input power of inverter at rated output conditionsi.e., with rated voltage, rated current 8 rated power factor at the output of inverter.

E. 6hat is A( 2eactor70ine (ho'eM

A( 2eactor is used when supply line has to be isolated from commutation notches caused by the inverterand to reduce the rectifier pea' current.

9@. 6hat is noise filterM

oise filter is the element involvin$ inductor and noise capacitor to suppress hi$h freuencyvolta$es, "hich can cause interference to sensitive electronic euipment.

%FF%C+ OF F,%Q-%.C/ VA,IA+IO. O. MO+O, C0A,AC+%,I$+IC$

Motors are normally desi$ned to operate at a freuency ran$e of

D:!5# from desi$ned freuency.

Runnin$ a motor at a lo" freuency results in reduction of the output

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po"er.But the effect of lo" freuency is not very $reat since there is

no mar3ed chan$e in po"er factor.

*he losses due to freuency variations are mainly due to hysteresis 9eddy current losses.

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Vfd Important