3 Phase Induction MotorPrinciples

List the characteristics of the magnetic fields produced by a 3Ø windings

Calculate the speed of rotation of a rotating magnetic field

Describe the basic principles of operation of an induction motor

Reverse the direction of rotation of a 3 phase induction motor

At the end of this section you will be able to:

So What Happens in the Stator?

N

NN

A

BC

B = 87%

C = – 87%

B = 87%

C = – 87%

60

= 150%

A = 87%

C = – 87%

Rotating Magnetic Field The speed of the RMF is dependant on two factors:-

1. Frequency of the supply2. Number of poles in the stator

There are always a multiple of 2 poles (North & South) per phase

Synchronous Speed

seconds

minutes

Rotating Magnetic Field

Nsync =

f=

P=

P

f602N sync

P

f120N sync

Synchronous Speed (RPM)Frequency of supplyNumber of poles per phase

Converts Seconds to minutes

Divides the poles by 2

Rotating Magnetic Field

P

f120N sync

What happens if we increase the frequency of supply?

Nsync What happens if we increase the number of poles?

Nsync

So What Happens in the Rotor?

Flemings Left Hand Rule

Motion

Field

Current

Repulsion Attraction

Magnetic field cuts conductor

Current is induced in conductor

A force acts on conductor

Conductor moves

SlipWhat happens if the conductor is the same speed as the RMF?

Magnetic field does not cut conductor

No current is induced in conductor

Friction slows conductor

Magnetic field then cuts the conductor

SlipSlip Speed

DIFFERENCE between SYNCHRONOUS speed and ROTOR speed

Normally expressed as a percentage

100n

nn%s

sync

sync

s% =

n=nsync=

Slip in %Rotor speed (RPM)Synchronous Speed (RPM)

SlipSlip Speed

At start up

Running

Highest

Lowest

Slip

Stator’s Magnetic Field

Rotor

SLIP is the DIFFERENCE between SYNCHRONOUS speed and ROTOR Speed

Rotor FrequencyRotor Stationary

Locked

Stator RMF Cuts rotor a fast rate

Frequency in rotor highest

Rotor FrequencyRotor At normal speed

Stator RMF Cuts rotor a slow rate

Frequency in rotor lowest

With some slip

Rotor Frequency

100

sff r fr =

s =f =

Frequency in rotor% slipSupply frequency

Rotor RPM

Fre

quen

cy in

Rot

or%

Slip

Can we reverse a 3Can we reverse a 3 Ø Ø Motor?Motor?

By changing the “firing order “ of the poles

N

NN

A

BC CBPrevious

Can we reverse a 3Can we reverse a 3 Ø Ø Motor?Motor?

By changing the “firing order “ of the poles

By reversing any two phase connections

U1 V1 W1

V2 W2 U2

U1 V1 W1

V2 W2 U2