Single Phase Induction Motor

The rotation of a conductor in rotating magnetic field is used in the construction of an induction motor. A single phase ac induction motor essentially consists of two parts (i) rotor, and (ii) stator.

Rotor. Rotor is the rotating part of the motor. It consists of an iron core with large number of copper bars placed in slots along the circumference. The bars are connected to heavy copper rings at the ends. It is called a squirrel cage rotor.

Stator. Stator is the stationary part of the motor. It consists of two pairs of coils AD and EC connected to the same ac supply through a capacitor or an inductance. The capacitor or the inductance produces the phase difference π/2 between the currents passing through the coils.

Working. When the rotor is placed in the rotating magnetic field produced by the stator, the rings and the copper bars of the rotor act like a closed loop and current is induced in it. As a result, the core of the rotor gets magnetized. The magnetic field due to the rotor reacts with the rotating magnetic field. Consequently rotor begins to rotating magnetic field in accordance with the Lenz’s law.

The starting torque is provided by using an auxiliary set of coils arranged to give a field at right-angles to that of the main stator coils. By including a capacitor in series with the auxiliary windings the current in these is mad to lead approximately 90o on the current in the main winding. The system is called phase splitting. We have now in effect a two-phase stator, which produces a rotating field. As soon as the rotor has reached sufficient speed to maintain its motion in the right direction, a centrifugal switch operates to disconnect the auxiliary coil.



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