Induction machine speed control_ Stator Voltage control2.pdf

Induction machine speed control_ Stator Voltage control2.pdf

1. Induction motor Drives
Stator Voltage Control

2. Stator Voltage Control of Induction Motors
•The synchronous speed and hence the rotor speed of a given
induction motor depends solely on the stator voltage and stator
frequency for specific load torque.
•When an ac voltage regulator is introduced between an ac source and
motor, the motor terminal voltage can be conveniently changed to
allow variable-speed operation.
•This scheme is simple and economical, and it is used to control the
speed of fan and pump drives and to allow smooth starting of
induction motors.

3. Stator Voltage Control: Theoretical Investigations
•Stator voltage control is one of the cheapest and most reliable methods of
induction motor speed control.
•While this scheme has a few disadvantages, such as a limited range of speed
control, reduced efficiency, and increased line current harmonics, this scheme
is widely used for fan and pump drives and the starting of large induction
motors.
•The input power to the rotor is divided into rotor copper loss and mechanical
power,

4. •Therefore, neglecting stator copper loss, iron loss, and friction and windage losses,
motor efficiency can be written as
•The above shows that, for fixed frequency stator excitation, motor efficiency
decreases as slip, S, increases.
•Thus, reduced speed operation at rated frequency operation results in poor operating
efficiency; furthermore, at low speed, S is high, making rotor copper loss higher
•Therefore, neglecting stator copper loss, iron loss, and friction and
•Neglecting magnetizing current, Is ≈ Ir.
•Thus at a given slip,

5. Speed control by changing applied voltage
•Thetorquedependsonthesquareofthe
appliedvoltage.
•Thevariationofspeedtorquecurveswith
respecttotheappliedvoltageisshownin
fig.
•Thesecurvesshowthattheslipat
maximumtorquesmaxTremainssame,
whilethevalueofstalltorque/Maxtorque
comesdownwithdecreaseinapplied
voltage.
•Thespeedrangeforstableoperation
remainsthesame.
•thestartingtorqueisalsoloweratlower
voltages.
•Thus,evenifagivenvoltagelevelis
sufficientforachievingtherunning
torque,themachinemaynotstart.
•Thismethodoftryingtocontrolthe
speedisbestsuitedforloadsthatrequire
verylittlestartingtorque,buttheir
torquerequirementmayincreasewith
speed

6. Speed control by changing applied voltage
•Figurealsoshowsaloadtorque
characteristic—onethatistypical
ofafantypeofload.Inafan
(blower)typeofload,thevariation
oftorquewithspeedissuchthatT
∝ω2
•Oncevoltageisreducedthe
torquealsoreduces
•Thevariationofspeedcanbe
achievedoversmallrangeonly
•Voltagecontrolmaybeachievedby
addingseriesresistors(alossy,
inefficientproposition),oraseries
inductor/autotransformer(abulky
solution)oramoremodern
solutionusingsemiconductor
devices.
•Atypicalsolidstatecircuitusedfor
thispurposeistheACvoltage
controllerorACchopper.

8. Effect of rotor resistance on variable voltage speed control
•AsindicatedinFig.1,evenwitha
fan-typeload,statorvoltage
controlallowsalimitedspeed
range.
•Becausefanandpumpdrives
requirelargevariationsinspeed,
thecharacteristiccurveinFig
primarilyindicatesthatstator
voltagecontrolisalsounsuitableas
amethodforspeedcontrolofthis
kind.
•Tomakestatorvoltagecontrol
suitableforthespeedcontrolof
fanorpumpdrives,therotor
resistanceisnowincreasedsothat
thestableregionofthespeed
torquespeedcharacteristiccurve
ofthemotoriswidernow(fig.2)
Fig 1
Fig 2

9. Power Converters for Stator Voltage Control
•PowerConvertersorsometimescalledacvoltage
regulators,acvoltagecontrollersemployingtriacsor
SCRsaregenerallyusedforstatorvoltagecontrolof
inductionmotordriveswithfan-orpump-typeloads.
•Forlow-powerapplicationssuchasdomesticfanspeed
control,triacsareused,whereasSCRsareusedfor
medium-tohigh-powerfanorpumploads.
•Theacvoltagecontroller-feddomesticfanmotorusesa
singletriacwithasimpleR-Cfiringcircuit.
•Commerciallyitisknownasanelectronicfanregulator
•Herethevariableresistanceorthepotentiometerissuitably
projectedoutsideforconvenientvariationofresistance,which
resultsinachangeofthetriacfiringangle.
•Thus,Rcanbeconvenientlyvaried,changingthe
statorvoltageandhenceresultinginfanspeedcontrol.

10. Three-phase ac voltage controller
•Athree-phaseacvoltagecontrollerfeeding
athree-phaseinductionmotorisshown
•Here,therearethreepairsofSCRs
connectedback-to-back,namelyT1−T1’,
T2−T2’,andT3−T3’
•ThefiringangleofT1isfixedatαsuchthat
T1’istriggeredat180°+α.
•ThepairT2–T2’receivesfiringpulsesat
120°+αand120°+180°+α,respectively.
•ForthepairT3−T3’,thefiringpulsesare
generatedat240°+αand240°+180°+α,
respectively.

11. AC Voltage Controller for Starting Induction Motor Drives
•Another application of an ac voltage controller is to start
three-phase induction motor drives.
•The thyristorizedvoltage regulator is called a soft starter
or solid-state starter.
•Compared to the conventional methods of soft starting,
such as a star-delta switch, series resistance starting, and
autotransformer-based starting, the solid-state starter
possesses several advantages such as
•neatness, compactness, ease of implementation of the closed
loop control, lower cost, and near-zero maintenance cost.
•The motor current is sensed with a current transformer
and fed to a digital controller.
•The digital processor finds the difference between the
measured current and the rated current (Is rated), and the
error is processed by the digital controller.
•Subsequently, the SCR firing angle α is varied to achieve smooth
starting with rated current.
•Once the starting process is over, the thyristor units are bypassed
through mechanical switches indicated as S1, S2, and S3

12. Energy-Efficient Operation of an ac Voltage
Controller-Fed Induction Motor Drive
•Theconceptofenhancingtheenergyefficiencyofaninductionmotordriveisbased
onthefactthatthestatorvoltagecanbeadjusteddependingontheloadonthe
shaft
•i.e.,thestatorvoltagecanbelowforalightload,mediumformediumloads,andatratedvoltage
forratedloadconditions.
•Whentheloadislessontheshaftandalowvoltageisappliedtothestatorwiththe
frequencyremainingthesame,theV/fratioislessthantheratedvalue.
•Thisreducestheairgapfluxandtheamplitudeofmagnetizationcurrent
•Thislowersstatorcopperlossaswellasflux-relatedcorelosses,leadingtoimprovedmotor
efficiencyandpowerfactor.
•Withanacvoltagecontrollernowsupplyingpowertotheinductionmotor,the
converterfiringanglecanbesuitablyadjustedfortherequiredstatorvoltagetomeet
theloadtorquedemand
•Inaclosed-loopmode,theloadiscontinuouslysensedandthestatorvoltageis
controlledaccordinglybyadjustingtheconverterfiringangletoobtainhigher
efficiencyandpowerfactor.