Induction Motor Scalar Control

MEP 1422 ELECTRIC DRIVES

INDUCTION MOTOR Scalar Control (squirrel cage)

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

Is

Lls

Llr’ +

+ Vs –

Ir ’

Lm Im

Eag –

Rr’/s

Scalar control of induction machine

Te Pull out Torque (Tmax)

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

Te TL

Trated

s

sm

ωω ω rated rotor

ωr s

Scalar control of induction machine

Given a load T–ω characteristic, the steady-state speed can be changed by altering the T–ω of the motor: Pole changing Synchronous speed change with no. of poles Discrete step change in speed

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

Variable voltage (amplitude), frequency fixed E.g. using transformer or triac Slip becomes high as voltage reduced – low efficiency

Variable voltage, fixed frequency e.g. 3–phase squirrel cage IM

600

V = 460 V 500

Rs= 0.25 Ω

Rr=0.2 Ω Lr = Ls = 0.5/(2*pi*50) Lm=30/(2*pi*50)

400 Torque

f = 50Hz

p=4

300

200

Lower speed → slip higher

100

Low efficiency at low speed

0

0

20

40

60

80 w (rad/s)

100

120

140

160

Variable voltage, variable frequency Constant V/f Approximates constant air-gap flux when Eag is large Eag = k f φag

φag = constant

=

E ag f

≈

V f

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 900 800 50Hz 

700 30Hz 

600 Torque

500

10Hz 

400 300 200 100 0

0

20

40

60

80

100

120

140

160

Variable voltage, variable frequency Constant V/f Vs Vrated

frated

f

Variable voltage, variable frequeny

Constant V/f – open-loop

Rectifier

3-phase supply

VSI

IM

C

f Ramp

ωs*

+

V

Pulse Width Modulator

Variable voltage, variable frequeny

Constant V/f – open-loop Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25Ω, Rr = 0.2Ω, Lr=Ls= 0.0971 H, Lm = 0.0955, 600 500

Steady state T-ω

400 300 200 100 0 ­100

0

20

40

60

80

100

120

140

160

180

200

Variable voltage, variable frequeny

Constant V/f – open-loop Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25Ω, Rr = 0.2Ω, Lr=Ls= 0.0971 H, Lm = 0.0955, 600 500

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

400 300 200 100 0 ­100

0

20

40

60

80

100

120

140

160

180

200

Variable voltage, variable frequeny

Constant V/f – open-loop Simulation example: 460V, 50Hz, 4 pole, Rs = 0.25Ω, Rr = 0.2Ω, Lr=Ls= 0.0971 H, Lm = 0.0955, 600 500

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

400 300 200 100 0 ­100

0

20

40

60

80

100

120

140

160

180

200

Variable voltage, variable frequency Constant V/f

1

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 speed Maintain Im constant – constant Φag

Variable voltage, variable frequeny

Constant V/f 700 600

50Hz 

500 400

Torque

30Hz 

300 10Hz 

200 100 0

0

20

40

60

80

100

120

140

160

Variable voltage, variable frequeny

Constant V/f with compensation (Is,ratedRs) 700

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

600 500 400

Torque

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

300 200 100 0

0

20

40

60

80

100

120

140

160

Variable voltage, variable frequeny

Constant V/f with voltage boost at low frequency

Vrated Linear offset

Boost

Non-linear offset – varies with Is

frated

Variable voltage, variable frequeny

Constant V/f

2

Problems with open-loop constant V/f Poor speed regulation

Solution: Compesate slip Closed-loop control

Variable voltage, variable frequeny Constant V/f – open-loop with slip compensation and voltage boost

Rectifier

3-phase supply

VSI

IM

C

f Ramp

ωs*

+

+ +

Slip speed calculator

Vdc

Idc

V +

Vboost

Pulse Width Modulator

Variable voltage, variable frequeny

Constant air-gap flux A better solution : maintain Φag constant. How? Φag, constant → Eag/f , constant → Im, constant (rated)

Rs

Is

Controlled to maintain Im at rated

Lls

Llr’ +

+ Lm

Vs maintain at rated

–

Ir ’

Im

Eag –

Rr’/s

Variable voltage, variable frequeny

Constant air-gap flux 900 800 50Hz 

700 30Hz 

600 Torque

500

10Hz 

400 300 200 100 0

0

20

40

60

80

100

120

140

160

Variable voltage, variable frequeny

Constant air-gap flux

Im =

Im =

Im =

jω L lr +

Rr s

Is

R jω (L lr + L m ) + r s jω L r +

Rr s

 σ  R jω  r L r + r s  1 + σr  jωslip Tr + 1  σ  jωslip  r Tr + 1  1 + σr 

Is

Is ,

 σ  jωslip  r Tr + 1 1 + σr   Is = Im , jωslip Tr + 1 • Current is controlled using currentcontrolled VSI • Dependent on rotor parameters – sensitive to parameter variation

Variable voltage, variable frequeny

Constant air-gap flux

3-phase supply

VSI

Rectifier

IM

C

Current controller

ω*

+

PI

-

ω slip +

ωr +

|Is|

ωs