Induction Motor

Induction Machine

Induction Machine (N. Tesla 1888) ➤ Principle of operation ➤ Construction ➤ Characteristics: ➤ slip ➤ rotor current ➤ power ➤ torque

➤ Classification ➤ Control

Principle of operation o squirrel-cage rotor

o o

Three-Phase Supply Voltage A

B

C

N

60 120

S A 120 C

B

Resultant Stator Flux Rotating Magnetic Flux A

A

C

B 0

C

B 60

It’s electromagnetic induction !!!

Induction Motor ➤ electromagnetic induction ➤ rotating magnetic flux induces an alternating ➤ current in the rotor

➤ motor action ➤ interaction of the induced rotor-current with ➤ the rotating field produces motor torque

Direction of Rotation

N

i

The rotor follows the rotating magnetic flux !

How to change the direction of rotation? ➤ change the direction of rotation of the

rotating magnetic flux ➤ change the phase sequence of the applied voltage

Synchronous Speed ➤ speed of the rotating magnetic flux ➤ ns

= fs / (P/2)

Slip Speed n, Slip s ➤ n = ns ➤s = ➤ if ➤ for

- nr

( ns - nr ) / ns nr then n and s nr = ns

s=0

fr : frequency of the induced voltage in the rotor ➤ fr = P n / 120

n = ns -nr

Rotor Current

1 -------- slip ---.2 - 0 0 ------ speed ---- 100%

Rotor Current Phasor

}

Ir cos (theta): horizontal component

s = .05

s = .2

SGAP = EBRIr (cos 0 + j sin 0) s=1

Interpreting the Circle Diagram

s = .05 Assume s = .05; If Tload s = .2

nr

s

Ir

Ir cos0

sufficient to carry the load s=1

Power-Flow Diagram Pgap Pmech

Pin

Pshaft Pscl

Pcore

Prcl

Pf,w Pstray

Mechanical Power vs Torque Pmech = TD

nr / 5252

(hp) shaft speed r/min

developed torque lb-ft mechanical power developed in rotor hp

TD Torque

Torque vs Speed o

o

1

breakdown

o rated value

locked rotor

slip

0

Torque-Speed Characteristics T

D C

2

T

D

A-B

Speed

=

2112 . Rr E BR 2

s nr [( Rr / s ) +

X

2 BR

]

Can slip at maximum torque be changed? Elevator Tmax

Speed

s

T max

≈

R R +(X 2

1

2

+ 1

2 ) X2

Induction Generator

Induction Motor

Turbine

Increasing rotor speed above ns will reverse direction of PGAP ==> power is transferred in reverse direction, from rotor to stator

Induction Motor Starting ➤ Autotransformer Starting ➤ Y-Delta Starting ➤ Series Impedance ➤ Solid State Starting

Breaking (rotating rotor induces currents to be dissipated in stator circuit)

➤ DC Injection ➤ Capacitor Breaking

Single-Phase Induction Motor ➤ There is no rotating magnetic flux ➤ How to produce needed rotating flux? ➤ Quadrature Field

Quadrature-Axis Flux (flux induced by rotor current) Stator or direct-axis flux

Quadrature-axis or rotor flux

Phase Splitting Single phase

Phase Splitter

Split-Phase Motors ➤ Resistance-Start Split-Phase Motors ➤ Capacitor-Start Split-Phase Motors