Question

Describe the construction and working principle of single phase AC motor.

Answer 1

construction and working principle of single phase AC motor :

Single phase induction motors have a phase distributed winding on the stator and a squirrel cage short-circuited winding on the rotor. When this single phase winding is connected to an alternating voltage source, an alternating field is produced varying only with time. Such an alternating field acting on a stationery squirrel cage rotor cannot produce rotation. But, if once the rotor is moved, the rotor produces a cross flux that is in both space and time quadrature with the stator flux.

There are two necessary conditions to produce a rotating field. Hence, once the motor is started, the single phase motor will continue to rotate as long as the load torque is not excessive.

 

This particulate behavior of motor has been explained in two ways i) by two field or double field revolving theory and ii) by cross field theory

Single phase induction motor is not inherently self-starting. However, if rotor is given an initial rotation in any direction, the single phase induction motor develops torque and rotor continues to pick up speed in that particular direction.

However, they are made self starting providing the various special arrangements such as splitphasing (with the help of resistance or capacitor) or using shaded poles which enable them to have a rotating magnetic field atleast at starting.

OR

Alternating flux is produced around the stator winding due to AC supply. This alternating flux revolves with synchronous speed. The revolving flux is called as "Rotating Magnetic Field" (RMF).

The relative speed between stator RMF and rotor conductors causes an induced emf in the rotor conductors, according to the Faraday's law of electromagnetic induction. The rotor conductors are short circuited, and hence rotor current is produced due to induced emf. That is why such motors are called as induction motors.

(This action is same as that occurs in transformers, hence induction motors can be called as rotating transformers.)

Now, induced current in rotor will also produce alternating flux around it. This rotor flux lags behind the stator flux. The direction of induced rotor current, according to Lenz's law, is such that it will tend to oppose the cause of its production.

As the cause of production of rotor current is the relative velocity between rotating stator flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator flux to minimize the relative velocity. However, the rotor never succeeds in catching up the synchronous speed. This is the basic working principle of induction motor of either type, single phase of 3 phase.