1

Vector Control of an Induction Motor Fed by a PWM Inverter with Output LC Filter

Janne Salomäki and Jorma Luomi

2

Contents

• Introduction

• Filter and motor models

• Cascade control system

• Full-order observer

• Results

• Conclusion

3

Introduction

• The output voltage of a PWM inverter consists of rectangular voltage pulses

• An LC output filter provides motor friendly voltage, but the control of the system becomes more difficult

• The paper presents a vector control method that does not need additional measurements

4

Filter and motor models

A

f

R

s

s

A

msr

R

mrs

ss

fs

f

fs

f

lf

R

s

s

A

uL

iui

R

LL

CC

LLR

iui

dtd

B

A

000

1

)j(100

j11j110

01j1

001j

xC

0001Ai

IMm

su fC

fL sidcu

Ai

5

Cascade control system

• Measurements needed: inverter current,DC link voltage and rotor speed

PI P PI PI

Observer

IM

refm,

m

sisu

refsu , refsi ,refAi ,

su fC

fL sidcu

A,refu

Ai

6

Full-order observer

• Estimated rotor flux reference frame

• The observer gain selection:– pole placement– constant gain

• Symmetric Euler discretization

)ˆ(ˆˆ AAA iiu KBxAx

T4321 ][ kkkkK

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Full-order observer (2)

• Observer poles with constant gain T1 ]000[kK

magnification

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Experimental results

• Experiments were made with 2.2-kW four-pole induction motor (400 V, 50 Hz)

• The cutoff frequency of the LC filter was 566 Hz

Freq.converter IM

PMservo

SpeedTorque

dSPACE

LC filter Freq.converter

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Experimental results (2)

Inverter voltage

Stator voltage

Inverter current

Stator current

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08-500

0

500

Vol

tage

(V)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08-10

-505

10

Cur

rent

(A)

Time (s)

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Experimental results (3)

Measured

Reference

Measured

Reference

Reference

Estimated

0 2 4 6 8 100

0.51

0 2 4 6 8 100

1

2

0 2 4 6 8 100

0.5

1

t (s)

(p.u

.)R

(p

.u.)

m

NTT

/

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Conclusion

• When the inverter output voltage is filtered by an LC filter, the vector control of the induction motor can be based on nested control loops

• As the system states are estimated, the only required measurements are the inverter current, DC link voltage and rotor speed

• Simulation and experimental results show that the proposed method works properly