Induction MachineInduction Machine

Induction Machine (N. Tesla 1888)Induction Machine (N. Tesla 1888)

Principle of operationPrinciple of operation ConstructionConstruction Characteristics:Characteristics:

slipslip rotor currentrotor current powerpower torquetorque

ClassificationClassification ControlControl

Principle of operationPrinciple of operation

squirrel-cage rotorsquirrel-cage rotor

oo

oooo

AA BB CC

Three-Phase Supply VoltageThree-Phase Supply Voltage

120120

AA

BBCC

60 12060 120

NN

SS

Resultant Stator FluxResultant Stator Flux

AA

BB CC BB CC

AA

00 6060

Rotating Magnetic FluxRotating Magnetic Flux

It’s electromagneticIt’s electromagneticinduction !!!induction !!!

Induction MotorInduction Motor

electromagnetic inductionelectromagnetic induction rotating magnetic flux induces an alternating rotating magnetic flux induces an alternating current in the rotorcurrent in the rotor

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

Direction of RotationDirection of Rotation

NNi

The rotor follows the rotating magnetic flux !

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

change the direction of rotation of the change the direction of rotation of the rotating magnetic fluxrotating magnetic flux

change the phase sequence of the applied change the phase sequence of the applied voltagevoltage

Synchronous SpeedSynchronous Speed

speed of the rotating magnetic fluxspeed of the rotating magnetic flux nnss = f = fs s / (P/2)/ (P/2)

Slip Speed n, Slip sSlip Speed n, Slip s

n = nn = nss - n - nrr

s = ( ns = ( nss - n - nrr ) / n ) / nss

if nif nrr then n and s then n and s

for nfor nrr = n = nss s = 0 s = 0

fr : frequency of the induced voltage in the rotorfr : frequency of the induced voltage in the rotor

ffrr = P n / 120= P n / 120

n = ns -nr

Rotor CurrentRotor Current

1 -------- slip ---.2 - 00 ------ speed ---- 100%

Rotor Current PhasorRotor Current Phasor

s = 1

s = .2

s = .05

Ir cos (theta): horizontal component}

SGAP = EBRIr (cos 0 + j sin 0)

Interpreting the Circle DiagramInterpreting the Circle Diagram

s = 1

s = .2

s = .05

Assume s = .05; If Tload

nr s Ir Ir cos0

sufficient to carry the load

Power-Flow DiagramPower-Flow Diagram

Pscl Pcore Prcl Pf,w Pstray

Pin Pshaft

Pgap

Pmech

Mechanical Power vs TorqueMechanical Power vs Torque

Pmech = TD nr / 5252 (hp)

developed torque lb-ft

shaft speed r/min

mechanical power developed in rotor hp

Torque vs SpeedTorque vs SpeedT

D T

orqu

e

slip1 0

olocked rotor

o breakdown

o rated value

Torque-Speed CharacteristicsTorque-Speed Characteristics

T

Speed

D

C

A - B

Dr BR

r r BR

T R ER Xs sn

21122

2 2

.

[( / ])

Can slip at maximum torque be changed?Can slip at maximum torque be changed?

Elevator

Tmax

Speed

TsR

R X Xmax

( )

2

1

2

1 22

Induction GeneratorInduction Generator

InductionMotor Turbine

Increasing rotor speed above ns will reverse direction of PGAP

==> power is transferred in reverse direction, from rotor to stator

Induction Motor StartingInduction Motor Starting

Autotransformer StartingAutotransformer Starting Y-Delta StartingY-Delta Starting

Series ImpedanceSeries Impedance

Solid State StartingSolid State Starting

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

DC InjectionDC Injection Capacitor BreakingCapacitor Breaking

Single-Phase Induction MotorSingle-Phase Induction Motor

There is no rotating magnetic fluxThere is no rotating magnetic flux How to produce needed rotating flux?How to produce needed rotating flux? Quadrature FieldQuadrature Field

Quadrature-Axis Flux (flux induced by rotor current) Quadrature-Axis Flux (flux induced by rotor current)

Stator or direct-axis flux

Quadrature-axis orrotor flux

Phase SplittingPhase Splitting

Singlephase

PhaseSplitter

Split-Phase MotorsSplit-Phase Motors

Resistance-Start Split-Phase MotorsResistance-Start Split-Phase Motors Capacitor-Start Split-Phase MotorsCapacitor-Start Split-Phase Motors