6077SANUE 046

3 Phase Induction MotorCharacteristics

Interpret information from torque/slip curves of AC induction motors

List the operating characteristics of squirrel cage induction motors

List the operating characteristics of slip ring induction motors

Identify details from motor nameplates Calculate the efficiency of induction motors

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

RevisionWhat determines the synchronous speed?

Why can’t the rotor operate at synchronous speed?

What do we call the difference between nR and n?

How do we work out the percentage slip of a motor?

Motor Starting Greatest relative speed between RMF & Rotor conductors

Induced voltage greatest

Equivalent to a transformer with a shorted secondary

Frequency is highest

Rotor torque must be higher than the apposing torque for the rotor to accelerate.

As load torque increases

Rotor RPM drops

Relative speed between RMF & Rotor conductors increases

Rotor current increases

Rotor torque increases

Rotor RPM stabilises

Motor Running

rFT RR CosLIBF

2VT

Rotor Current Torque caused by induced current in rotor

conductors Rotor current is a result of:-

1. Rotor speed

2. Rotor impedance

Frequency

Resistance

Inductive reactance

Rotor Current High frequency in rotor conductors More metal surrounding conductor the deeper

in the rotor. L is highest in that part of the conductor XL is highest I is lowest Effective CSA is reduced Current is limited

fLX L 2

Torque Speed Curve

Rotor Speed

To

rqu

e

Synchronous speed

Rated torque

Rated speed

Torque Slip Curve

Slip %

To

rqu

e

Synchronous speed

Rated torque

Rated speed

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