AC Motor Control and EV Applications

Induction Motor Basics

Chapter 3

Squirrel Cage of an Induction Motor

End Ring

Rotor Bar

S

N

Loop 2Flux Increasing;

Current flowsso as to oppose tothe PM flux increase.

Magnet moving direction

Loop 1Flux Decreasing;

Current flowsso as to oppose tothe PM flux decrease.

Rotationaldirection

Squirrel Cage of an Induction Motor

End Ring

Rotor Bar

Stator Flux

Stator Flux

Laminated Sheets for Stator and Rotor Cores

Assembled IM Rotor

Flux Linking between Stator and Rotor Windings

as

as ’

ar

ar ’

End ring conductor

Air gap

Secondary windingPrimary winding

Steady State Model

Air gap power:

Circuit Simplification and Torque

Vs

rs Lls

Lm

Llr

rr /s

Vs

rs Lls

Lm

Llr

rr /sIrIs

Torque Speed Curve

Starting torque

Breakdowntorque

Motoring Regenerating

Linear in s

Te

s=1 s=0 m

e

Torque-Speed Curve

Starting torque

Breakdowntorque

Plugging Motoring Regenerating

Linear in s

s=1 s=0sm

s=1 s=0

Rated torque

Rated current

Locked rotor current

(a)

(b)

Breakdown Torque

Breakdowntorque

Breakdown Torque for Different Rotor Resistance

Breakdown torque is independent ofrotor resistance.

(Nm)

(b)

00.10.20.30.40.50.60.70.80.910

50

100

150

200

250

Slip

Torq

ue

(a)

(b)

Torque-Speed Curve Based on Simplified Model

Vs254V, 60Hz

rs =0.1

0.25/s

Lls =3mH Lls =3mH

Lm = 150mH

rs =0.1 Lls =3mH Lls =3mH

0.25/sLm =

150mHVs

254V, 60Hz

(a) (b)

Stable/Unstable Regions

UnstableRegion

StableRegion

Te

s=1 s=0e

Larger load

Smaller loadA

A’

A’’

sm

B

0.8571.2

Parasitic Torque Caused by 5th and 7th Harmonics

0

7th5th

Slip

Torque

Parasitic torque

Slot, Zigzag and End-Turn Leakage Flux

Zigzag leakage

Slot leakage

End turn leakage

(a) (b)Mutual flux

Shaft

Stator

Rotor

Circle Diagram

Circle Diagram for Different Slips

Min. PFangle

Ratedcurrent

Currentyieldingbreakdowntorque

Currentat locking

Rotor Slot Leakage Flux at Zero Slip

J barxx

xxx

(a) (b) (c)

Rotor Slot Leakage Flux at None Zero Slip

J bar

x x

xx

xx

x

Jbar

Isl hbar hbar

Bx

(a) (b) (c)

Current Displacement

Current Displacement of a Deep Bar

(a) (b)

Flux Lines over Double Cage Rotor

(a) (b)

High Slip : rr is large Low Slip : rr is small

Rotor Types and NEMA Classification

Doublecage

Deep wedge

Round

100

200

slip00.51

0

Class A

Class D

100

200

300

Class C

Class B

10050

Class A motors have high efficiency and high stiffness.Class D motors have high starting torque, but low efficiency.

Line Starting

Soft Starter

3 phase AC input

Induction motor

va

vb

vc

(b)

va

vb

vc

(c)

(a)

Gating signal

va

vb

vc

Speed variation range

Variable Voltage Control

VVVF Control

VVVF implies that flux level is kept constant so that it does not saturate the motor iron core.

PI Inverter

limiter

V/Ftable-

P/2

+

Motor

Speedsensor

0 Low frequency cut off

High voltage limit

Voltage boostregion

ConstantV/F

Slip Based VVVF Speed Controller