INDUCTION MOTORScalar Control(squirrel cage)

MEP 1422

ELECTRIC DRIVES

Scalar control of induction machine:

Control of induction machine based on steady-state model (per phase SS equivalent circuit):

Rr’/s+

Vs

–

RsLls Llr’

+

Eag

–

Is Ir’

Im

Lm

Scalar control of induction machine

r

s

Trated

Pull out Torque(Tmax)

Te

ssmratedrotor

TL

Te

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

Given a load T– characteristic, the steady-state speed can be changed by altering the T– of the motor:

Scalar control of induction machine

Pole changing Synchronous speed change with no. of polesDiscrete step change in speed

Variable voltage (amplitude), frequency fixedE.g. using transformer or triacSlip becomes high as voltage reduced – low efficiency

Variable voltage (amplitude), variable frequencyUsing power electronics converter Operated at low slip frequency

Variable voltage, fixed frequency

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

Tor

que

w (rad/s)

Lower speed slip higher

Low efficiency at low speed

e.g. 3–phase squirrel cage IM

V = 460 V Rs= 0.25

Rr=0.2 Lr = Ls = 0.5/(2*pi*50)

Lm=30/(2*pi*50)

f = 50Hz p = 4

Variable voltage, variable frequency

Constant V/f

Approximates constant air-gap flux when Eag is large

Eag = k f ag

f

V

f

Eag ag = constant

Speed is adjusted by varying f - maintaining V/f constant to avoid flux saturation

Variable voltage, variable frequency

Constant V/f - assuming constant airgap flux

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

700

800

900

Tor

que

50Hz

30Hz

10Hz

Variable voltage, variable frequency

Constant V/f

Vrated

frated

Vs

f

Constant V/f – open-loop

VSIRectifier

3-phase supply IM

Pulse Width

Modulators*+

Rampf

C

Variable voltage, variable frequeny

V

Constant V/f – open-loop

Variable voltage, variable frequeny

Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= 0.0971 H, Lm = 0.0955,

0 20 40 60 80 100 120 140 160 180 200-100

0

100

200

300

400

500

600

Steady state T-

Constant V/f – open-loop

Variable voltage, variable frequeny

Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= 0.0971 H, Lm = 0.0955,

0 20 40 60 80 100 120 140 160 180 200-100

0

100

200

300

400

500

600

Steady state T- and transient T- characteristic – without ramp limitter

Constant V/f – open-loop

Variable voltage, variable frequeny

Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25, Rr = 0.2, Lr=Ls= 0.0971 H, Lm = 0.0955,

0 20 40 60 80 100 120 140 160 180 200-100

0

100

200

300

400

500

600

Steady state T- and transient T- characteristic – with ramp limitter

1Variable voltage, variable frequency

Constant V/f

Problems with open-loop constant V/f

At low speed, voltage drop across stator impedance is significant compared to airgap voltage - poor torque capability at low speed

Solution:Boost voltage at low speedMaintain Im constant – constant ag

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

700

Tor

que

50Hz

30Hz

10Hz

Constant V/f

Variable voltage, variable frequeny

with compensation (Is,ratedRs)

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

700

Tor

que

Constant V/f

• Torque deteriorate at low frequency – hence compensation commonly performed at low frequency

• In order to truly compensate need to measure stator current – seldom performed

Variable voltage, variable frequeny

with voltage boost at low frequency

Constant V/f

Vrated

frated

Linear offset

Non-linear offset – varies with IsBoost

Variable voltage, variable frequeny

Constant V/f

Variable voltage, variable frequeny

Poor speed regulation

Solution:Compesate slipClosed-loop control

Problems with open-loop constant V/f

2

Constant V/f – open-loop with slip compensation and voltage boost

VSIRectifier

3-phase supply IM

Pulse Width

Modulator

VboostSlip speed calculator

s*++

++ V

Vdc Idc

Rampf

C

Variable voltage, variable frequeny

Constant air-gap flux

A better solution : maintain ag constant. How?

ag, constant → Eag/f , constant → Im, constant (rated)

Rr’/s+

Vs

–

RsLls Llr’

+

Eag

–

Is Ir’

Im

Lm

maintain at rated

Controlled to maintain Im at rated

Variable voltage, variable frequeny

Constant air-gap flux

Variable voltage, variable frequeny

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

700

800

900T

orqu

e

50Hz

30Hz

10Hz

sr

mlr

rlr

m I

sR

)LL(j

sR

LjI

,I

1T1

j

1TjI

I

sR

L1

j

sR

LjI

s

rr

rslip

rslipm

s

rr

r

r

rr

m

,I1Tj

1T1

j

I mrslip

rr

rslip

s

• Current is controlled using current-controlled VSI

• Dependent on rotor parameters – sensitive to parameter variation

Constant air-gap flux

Variable voltage, variable frequeny

Constant air-gap flux

VSIRectifier

3-phase supply IM

*

+

+ |Is|slip

C

Current controller

s

PI

+

Variable voltage, variable frequeny

r

-