Scalar Control of Ac Drives

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Topic 12: Scalar Control of AC Induction

Motor Drives

Spring 2004

ECE 8830 - Electric Dr

ives


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Introduction

Scalar controlof an ac motor drive is onlydue to variation in the magnitudeof thecontrol variables. By contrast vector control

involves the variation of both themagnitude and phaseof the controlvariables.

Voltage can be used to control the air gapflux and frequency or slip can be used tocontrol the torque. However, flux andtorque are functions of frequency and

voltage, respectively but this coupling isdisregarded in scalar control.


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Introduction (contd)

Scalar control produces inferior dynamicperformance of an ac motor compared tovector control but is simpler to implement.In variable-speed applications in which asmall variation of motor speed with loadingis tolerable, a scalar control system can

produce adequate performance. However, ifprecision control is required, then a vectorcontrol system must be used.


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Speed Control

Three simple means of limited speedcontrol for an induction motor are:

1) Reduced applied voltage magnitude

2) Adjusting rotor circuit resistance

(suitable for a wound rotor machine

and discussed earlier)

3) Adjusting stator voltage and frequency

These are discussed in section 9.2 Ongtext and are not presented further here.


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Constant Air Gap Flux

Generally, an induction motor requires anearly constant amplitude of air gap fluxfor satisfactory working of the motor.

Since the air gap flux is the integral of thevoltage impressed across the magnetizinginductance, and assuming that the air gapvoltage is sinusoidal,

Thus a constant volts/Hz ratioresults in a

constant air gap flux.

sin cosag

ag ag ag

Vv dt V tdt t


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Constant Air Gap Flux (contd)

The torque-speed curves with a constantair gap flux at different excitationfrequencies are shown below:


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From the curves on the previous slide, itcan be seen that we will obtain the sametorque at the same value of slip speed if

we operate at a constant air gap flux. Thisis the basis for constant volts/Hz controlof an induction motor. This type of controlmay be implemented either in open loopor in closed loop.


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A set of 6-step voltage waveformsillustrating constant volts/Hz is shown below:

Ref: D.W. Novotny and

T.A. Lipo, Vector

Control and Dynamics

of AC Drives


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Open Loop Volts/Hz Control of a

Voltage-Fed Inverter

Three regions of operation for the induction

motor are possible:

1) Holds slip speed constant and regulatesstator current to obtain constant torque.

2) Holds stator voltages at its rated valueand regulates stator current to obtain

constant power.3) Holds stator voltage at its rated value

and regulates slip speed just below itspull-out torque value.


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Voltage-Fed Inverter (contd)


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The open loop volts/Hz control of an inductionmotor is very popular because of its simplicity.A block diagram of such a control system isshown below:


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Voltage-Fed Inverter

The power circuit comprises:

1) A diode rectifier supplied by either a

single-phase or three-phase supply

2) An LC filter

3) A PWM voltage-fed inverter.

The primary control variable is the frequencyer. The commanded phase voltage Vs* isgenerated by a gain stage based on thespeed

e

to maintain a constant air gap flux.


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As the frequency becomes small at lowspeed, the voltage drop across the statorresistance can no longer be neglected

and so a boost voltage V0needs to besupplied allowing the rated flux (and thusthe full torque) to be available down tozero speed. The effect of the boost

voltage is negligible at higherfrequencies.


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The drives steady state performance for afan or pump-type load (TL=Kr

2) is shownbelow:


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As the frequency is increased the speedincreases almost proportionally and wemove along the load torque curve from

points 1->2->3 etc. moving smoothlythrough the different operating modes ofthe induction motor.


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Let us now look at the effects of dynamicvariations in load torque and line voltage.

Suppose the load torque is changed from

TLto TL for the same frequencycommand, the speed will drop slightlyfrom rto r. This type of speed variationcan easily be tolerated by a fan or pump.

Now suppose the operating point is a andthe line voltage drops so that theoperating point moves to b. Again the

speed is tolerable for some applications.


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The safe acceleration/decelerationcharacteristics are shown below:


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Assume a pure inertia type load and themotor initially operating at point 1. A smallstep increase in command frequency willinitially move the operating point to point 2(the rated torque) and then steadily increaseto point 3. The frequency can then bedecreased slightly to achieve the steady stateoperating point 4. All of these transitions are

done in a gradual manner to prevent themachine from becoming unstable.Decrementing the frequency command in astep will shift the operating point from 1 to 5

due to a negatively developed torque.


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The motor torque and speed are related by:

where J = moment of inertia,

Te= torque developed by motor,

and TL= load torque

With the rated Tethe slope of theacceleration curve dr/dt is determined by J.

The higher J, the smaller the slope.

e Lr

T Tdt

J


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Typical Volts/Hz drive performance is shownbelow:


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Energy Savings with Variable

Frequency Drives

Considerable energy savings can beachieved with variable frequency drivescompared to constant frequency drives

(see figure below and text).


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Closed Loop Volts/Hz Control with

Slip Regulation

An improvement over open loop Volts/Hzcontrol is closed loop Volts/Hz control withslip regulation (see block diagram below).


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Slip Regulation (contd)

Here the speed loop error generates aslip command sl

*via a proportional-integral controller and limiter. This slip

command is added to the feedback speedsignal rto get the frequency commande

*which, in turn, generates the voltagecommand through a volts/Hz function

generator. Since slip is proportional totorque at constant flux, this approachmay be considered as open loop torquecontrol within a speed control loop.


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If a step-up speed command is provided,the motor accelerates freely until a slip limit(corresponding to the motors torque limit)

is achieved and then settles down to thesteady state load-limited torque.

If r*is stepped down, the drive behaves as

a generator and decelerates with constantnegative slip - sl*. However, the value of

- sl*must be limited to a safe margin

below the slip speed corresponding to the

pull-out torque point.


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Since the slip speed is relatively smallcompared to the rotor speed, this mode ofoperation requires precise measurement

of the rotor speed. Also, in negative slipmode of operation, the regenerated powermust either be dissipated in a brakingresistor or fed back to the ac mains.

One disadvantage of this approach is thatthe flux may drift due to load torque orsupply voltage variations.


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A speed control system with closed looptorque and flux control is shown below.However, additional feedback control loopsincreases system complexity and potentialstability problems.


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Current-Regulated Voltage-Fed

Inverter Drive

Instead of controlling inverter voltage bythe flux loop, the stator current can becontrolled which has the benefit of

providing inherent overcurrent protectionto the switching devices as well asachieving direct control of the motortorque and air gap flux.

A current-regulated VSI drive, with torqueand flux control in an outer loop andhysteresis-band current control in the inner

loop, is shown on the next slide.


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Inverter Drive (contd)

Flux control loop -> stator current amplitude

Torque control loop -> frequency command

Only need 2 current sensors since ia+ib+ic=0

(for an isolated motor neutral).


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Inverter Drive (contd)

The performance of the drive for subwaytraction application is shown below:


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Traction Drives with Parallel

Machines

Multiple voltage-fed inverters can beoperated in parallel. An example of such a

system for a locomotive drive is discussedin the Bose text, pp. 348-349.


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Current-Fed Inverter Control

Some of the same principles for controlof voltage-fed inverters can be applied tocurrent-fed inverters. However, the

current-fed inverter cannot be operatedopen loop.

The simplest implementation of a closed

loop control system for a current-fedinverter, allowing independent control ofdc link current Idand slip sl, is shown onthe next slide.


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Current-Fed Inverter Control (contd)


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In this implementation, the fed back rotorspeed rand command slip sl

*are added togive the command frequency e

*. The dc linkcurrent Idis controlled by a feedback loop

that controls the output voltage of therectifier, Vd.

With +ve slip, acceleration occurs; with -ve

slip, Vdand VIboth become -ve and power isfed back to the source.

The torque can be controlled either by Idor

sl

. However, no flux control is possible with

this control scheme.


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Speed and flux control can be achieved ina current-fed inverter using the belowcontrol system.


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In this case the speed control loopcontrols the torque by slip control (asbefore) but also controls the current Id

*

by a pre-computed function generator tomaintain a constant flux. This open loopapproach is satisfactory but the machineflux may still vary with parameter

variations. An independent flux controlloop (as shown earlier for the voltage-fedinverter) can be implemented for tighterflux control if desired.


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A volts/Hz implementation for a current-fedinverter is shown below:

A particular advantage of this approach isthat the motor flux is unaffected by line

volta e variation.


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Efficiency Optimization Control by

Flux Program

Normally a motor is operated at its ratedflux because the developed torque is highand the transient response is fast. Under

light loads, this can lead to poor efficiencyof the drive. The rotor flux can be loweredat light loads so that the motor losses arereduced and the conversion efficiency of

the drive optimized. See text pp. 352-254(Bose) to see how this may be achieved.

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Scalar Control of Ac Drives