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SCALAR CONTROLByB.SakthisudhursunB.V.S.RaghavaCh.Balakishan

Scalar Control DefinitionSpeed control techniques of Induction MotorOperating regions of VVVF DriveOpen loop V/f ControlClosed loop V/f ControlCurrent Fed Inverter controlEnergy Savings with Variable Frequency DrivesApplication, Advantages & LimitationReferencesVVVF-Variable voltage and Variable frequency7)Dictionary Definition:Quantity which only has magnitude and no directionA physical quantity that is not changed by rotation or translationA variable quantity that cant be resolved into components Scalar control means controlling the speed of the motor by varying only magnitude of the control variables.The torque and flux parameter are not decoupled hereBy contrast vector control involves the variation of both the magnitude and phase of the control variables. 1) Scalar Control- An IntroductionStator: voltageCurrentFrequency

Rotor: voltageCurrentFrequency

Control Variables available in Induction MotorBoth in SQIM & SRIMOnly in SRIM*SQIM-Squirrel cage Induction motor*SRIM-Slip Ring Induction motorTypical Load & Motor TorquersTratedPull out Torque(Tmax)TessmratedrotorTLTeIntersection point (Te=TL) determines the steady state speed5Given a load T characteristic, the steady-state speed can be changed by altering the T of the motor:Pole changing Synchronous speed change with no. of polesDiscrete step change in speedVariable voltage (amplitude), frequency fixedE.g. using transformer or triacSlip becomes high as voltage reduced low efficiencyVariable voltage (amplitude), variable frequencyUsing power electronics converter Operated at low slip frequency 2) Speed control techniques ofSQIMVariable voltage, fixed frequency

Decreasing VMaximum Torque ReducedMachine rating is also reducedMotor TorqueLoad Torque

Limitations:Distorted line currentMore Current for same torque hence result in high copper LossesPoor power factor at low speed

A way to implement Variable voltage and fixed frequencyVariable voltage, variable frequency Constant V/f

Maximum Torque sameUniversally adopted for variable speed driveTwo option of variation V & f possible: Below the rated

Above the ratedVoltage cant than its rated voltage Constant Torque operationConstant Power region

3) Operating Regions of VVVF DrivesVSIRectifier3-phase supplyIMPulse Width Modulators*+RampfCVrate limiter is needed to ensure the slip change within allowable range (e.g. rated value)VVVF open-loopBlock Diagram:11Soft starting using VVVF drive The drives steady state performance for a fan or pump-type load (TL=Kr2) is shown below:

1Limitation of Open Loop

At low speed, voltage drop across stator impedance is significant compared to airgap voltage - poor torque capability at low speedSolution:Boost voltage at low speedMaintain Im constant constant ag13

A better solution : maintain ag constant. How?ag, constant Eag/f , constant Im, constant (rated)Rr/s+

Vs

RsLlsLlr+

Eag

IsIrImLmmaintain at ratedControlled to maintain Im at ratedContVratedfratedLinear offsetNon-linear offset varies with IsBoostVVVF with Voltage boost at Low frequencyCharacteristics with voltage boost to compensate Stator Impedance Drop:With out compensation15

With voltage boost of Irated*RsEffect of Maintaining E/f ConstantPoor speed regulation

Solution:Compesate slipClosed-loop control

Problems with open-loop constant V/f2Limitation of Open Loop Cont..

Poor speed regulationRequire speed encoderIncrease complexity due to addition of controllers5) Closed Loop VVVF Drive

Block Diagram of Closed Loop VVVF Drive:Closed Loop VVVF Drive ContFunctions of different components:Error Detector: To find the error between set and actual speedSpeed controller: Normal PI controllerSlip limiter: To limit the slip below the critical slip at which motor goes to unstable at given torqueOperation:With the help of speed controller , slip limiter and actual speed the reference synchronous speed is determined.Above is given as frequency input command for the inverterThe voltage input command for the inverter obtained from V/f function generatorV/f Function Generator: To generate the reference voltage for inverter by considering saturation effect and to limit the controlClosed Loop VVVF Drive ContEffect of load and voltage disturbance on closed loop

Torque increased from TL to TL Speed tend to fall to 2 but speed controller will increase the frequency input & Correct the speedVoltage variation input causes operating point will shift from 1 to 2But speed controller will increase the frequency input & correct the speedThe simplest implementation of a closed loop control system for a current-fed inverter, allowing independent control of dc link current Id and slip sl, is shown below.

Current-fed inverter cannot be operated open loop

6) Current Fed Inverter Control In this implementation, the fed back rotor speed r and command slip sl* are added to give the command frequency e*. The dc link current Id is controlled by a feedback loop that controls the output voltage of the rectifier, Vd.

With +ve slip, acceleration occurs; with -ve slip, Vd and VI both become -ve and power is fed back to the source.

The torque can be controlled either by Id or sl. However, no flux control is possible with this control scheme.

Current Fed Inverter Control Cont7) Energy Savings with Variable Frequency DrivesTypically in an industry almost 2/5 loads are used for applications like compressed air , fluid flow. This motors will be operated in light load condition in most of the times, hence it operated at low efficiency.By having Variable speed drive an increased in efficiency about 25-35% can be achieved

8) Applications, Advantages & Limitation of Scalar Control Applications: CompressorsBlowersPumpFanAdvantages:SimpleEasy to implement as number of components is lessLow cost Limitations:Poor dynamic response compared to vector control

9) ReferencesModern Power Electronics and AC Drives by Bimal K.BoseElectric Drives Concepts & Applications by Vedam SubramaniumAN 887, article on Induction motor Fundamentals by MicrochipProject report of Analysis and comparison of Scalar & Vector control techniques for variable speed drivesProject report of Scalar control of Voltage Fed Induction Machine