Operation of 3 Phase Induction Motors

Question 1

Rotating magentic field

Answer 1

When a three-phase supply is applied to the stator windings, three alternating currents produce a magnetic field that rotates at synchronous speed.

Synchronous speed is determined by Ns equation.

Question 2

Induced current in the rotor

Answer 2

The rotating magnetic field cuts across the rotor bars (squirrel cage) or windings (wound rotor), inducing a current due to Faraday’s law of electromagnetic induction.

This current creates its own magnetic field that interacts with the stator’s field to produce torque.

Question 3

Torque production

Answer 3

The rotor lags behind the synchronous speed, creating slip (SSS), to sustain the induced current and torque.

Torque depends on slip, rotor resistance, and the magnitude of the magnetic field.

Question 4

Information typically found on the faceplate of a 3 ph induction motor

Answer 4

Voltage, frequency, power factor.

Rated speed, efficiency.

Connection type (star or delta).

Question 5

How does a rotating magentic field develop in a 3 ph system

Answer 5

The three-phase currents, 120° out of phase, produce a magnetic field that appears to rotate.
(diagram)

Question 6

How do different types of loads affect the torque slip curve?

Answer 6

Constant load: Steady torque.

Variable load: Varies with slip.

Question 7

Compare the applications of 3ph induction motors to DC shunt motors

Answer 7

Induction motors: High power, constant speed (pumps, compressors).

DC shunt motors: Variable speed (machines).