Induction Motor Drives – Scalar Control By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku

Induction Motor Drives – Scalar ControlByDr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering

Dr. Ungku Anisa, July 2008 1EEEB283 - Electrical Machines & Drives

OutlineIntroductionSpeed Control of Induction Motors

Pole ChangingVariable-Voltage, Constant FrequencyVariable Frequency

Constant Volts/Hz (V/f) ControlReferences

Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 2

IntroductionScalar Control - control of induction machine

based on steady-state model (per phase steady-state equivalent circuit)


Rr’/s+

Vs

–

RsLls Llr’

+

E1

–

Is Ir’

Im

Lm

Introduction


r

s

Trated

Pull out Torque(Tmax)

Te

ssm ratedrotor

TL

Te

Intersection point (Te=TL) determines the steady –state speed

1 0

What if the load must be operated here?

rotor’

Speed Control of IMGiven a load T– characteristic, the steady-state speed can be

changed by altering the T– curve of the motor


fPPs 42

2

2'

2'3

lrlsr

s

s

s

re

XXsR

R

V

s

RT

Pole Changing

Varying line frequency

Varying voltage (amplitude)2

3

1

Speed Control of IMPole ChangingMachines must be specially manufacturedOnly used with squirrel-cage motorsTwo methods:

Multiple stator windings – simple, expensiveConsequent poles – single winding divided into few coil groups

Consequent poles:No. of poles changed by changing connections of coil groupsChange in pole number by factor of 2:1 only

Discrete step change in speed


Speed Control of IMVariable-Voltage (amplitude), Constant FrequencyControlled using:

AC Voltage Controllers (anti-parallel thyristors) voltage control by firing angle

control also used for soft start of

motors


Speed Control of IMVariable-Voltage (amplitude), Constant FrequencyFrom torque equation, Te Vs

2

When Vs , Te and speed reduces.If terminal voltage is reduced to bVs,:

Note: b 1


2

2'

2'3

lrlsr

s

s

s

re

XXsR

R

bV

s

RT

Speed Control of IMVariable Voltage (amplitude),

Constant FrequencyDisadvantages:

limited speed range when applied to Class B (low-slip) motors

Excessive stator currents at low speeds high copper losses

Distorted phase current in machine and line

Poor line power factorHence, only used on low-power,

appliance-type motors where efficiency is not importante.g. small fan or pumps drives


Speed Control of IMVariable FrequencySpeed control above rated

(base) speed Frequency increasedStator voltage held constant at

rated valueAirgap flux and rotor current

decreases Developed torque decreases

For control below base speed – use Constant Volts/Hz method


Constant Volts/Hz (V/f) ControlAirgap flux in the motor is related to the induced stator

voltage E1 :

For below base speed operation:Frequency reduced at rated Vs - airgap flux saturates

(f ,ag ):- excessive stator currents flow- distortion of flux wave

Hence, keep ag = rated fluxstator voltage must be reduced proportionally


f

Eag

1f

Vs Assuming small voltage drop across Rs and Lls

Constant Volts/Hz (V/f) ControlMax. torque remains almost

constantFor low speed operation:

can’t ignore voltage drop across Rs and Lls

poor torque capabilitystator voltage must be

boosted – maintain constant ag

For above base speed operation (f > frated):stator voltage maintained at

rated value


s

sVT

2

max

Constant Volts/Hz (V/f) Control


Vrated

frated

Linear offset

Non-linear offset – varies with IsBoost

Vs vs. f relation in Constant Volts/Hz drives

Vs

f

Constant Volts/Hz (V/f) Control For operation at frequency times rated frequency:

fs = fs,rated s = s,rated (1)

Stator voltage: (2)

Voltage-to-frequency ratio = d = constant:

(3)


rated,

rated,

s

sVd

rated,rated,

rated,rated,

when,

when,

sss

ssss ffV

ffVV


Hence, the torque produced:

(4)

where s and Vs are calculated from (1) and (2) respectively.


22

2'

2'3

lrlsr

s

s

s

re

XXsR

R

V

s

RT


The slip for maximum torque is:

(5)

The maximum torque is then given by:

(6)

where s and Vs are calculated from (1) and (2) respectively.


222

'

max

lrlss

r

XXR

Rs

222

2

max 2

3

lrlsss

s

s XXRR

VT



Field Weakening Mode (f > frated)• Reduced flux• Torque reduces

Constant Power Area

Constant Torque Area



Constant Power Area

Constant Torque Area

Constant Volts/Hz (V/f) Control – Open-loop Implementation


PWM Voltage-Source

Inverter (VSI)

Constant Volts/Hz (V/f) Control – Open-loop ImplementationMost popular speed control methodUsed in low-performance applications

where precise speed control unnecessarySpeed command s* - primary control variable Phase voltage command Vs* generated from V/f relation

Boost voltage applied at low speedsConstant voltage applied above base speed

Sinusoidal phase voltages (vabc*)generated from Vs* & s*

vabc* employed in PWM inverter connected to motorDr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 20

ReferencesKrishnan, R., Electric Motor Drives: Modeling, Analysis and Control,

Prentice-Hall, New Jersey, 2001.Bose, B. K., Modern Power Electronics and AC drives, Prentice-Hall,

New Jersey, 2002.Trzynadlowski, A. M., Control of Induction Motors, Academic Press,

San Diego, 2001.Rashid, M.H, Power Electronics: Circuit, Devices and Applictions, 3rd

ed., Pearson, New-Jersey, 2004.Nik Idris, N. R., Short Course Notes on Electrical Drives,

UNITEN/UTM, 2008.Ahmad Azli, N., Short Course Notes on Electrical Drives,

UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008 21EEEB283 - Electrical Machines & Drives

Documents

Induction Motor Drives – Scalar Control By Dr. Ungku Anisa Ungku Amirulddin Department of Electrical Power Engineering College of Engineering Dr. Ungku