Electrical Engineering ⇒ Topic : Principle of Operation
SINGLE-PHASE INDUCTION MOTOR
Principle of Operation
When a single-phase supply is given to the stator winding of a single-phase induction motor, a pulsating magnetic field is produced shown in Figure (a).
Figure (a) Pulsating magnetic field produced by stator.
It is seen from Figure (a) that due to an AC voltage applied to the single-phase winding of the stator, the flux becomes a pulsating one with a maximum value equal to Om. By Ferraris principle, a single-phase sinusoidal field, varying or pulsating sinusoidally with time along the field axis, can be divided into two equal sinusoidal fields rotating in opposite directions, each having a maximum value equal to one half that of the initial field.
Figure (b) shows the two revolving fields, each having a magnitude Φm/2, acting independently upon the rotor
figure (b) Two rotating fields each having a magnitude (Φm/2) revolving in opposite direction.
These two revolving fields (one revolving in clockwise direction and the other revolving in anticlockwise direction) are similar to the rotating field of the polyphase induction motor. One field tends to cause rotation in a clockwise direction, and the other field tends to cause rotation in a counter-clockwise direction.
The slip torque curves for both the side fields, T1 and T2 corresponding to rotating fields Φ1 and Φ2 are shown in Figure (c).
Figure (c) Two opposing torques in a single-phase induction motor
The two torques act in opposite directions shown in Figure (c). At standstill (s = 1), the two torques are opposite and equal, and the rotor has no tendency to start. If the rotor in some manner be caused to rotate in the direction in which T1 is acting, T1 will immediately exceed the counter-clockwise torque T2 and the armature will begin to accelerate in the direction of T1. As the armature speeds up, T1 predominates over T2, and the armature approaches synchronous speed without difficulty. The counter torque T2 always exists, however, it has little effect near synchronous speed of the field that produces T1.
When the rotor operates near synchronous speed in the direction of T1, its slip is nearly equal to 2 with reference to T2. The rotating field that produces T2, therefore, induces double frequency currents, however, it produces little torque because of this high frequency. Because of double frequency with respect to stator, the reactance is high, the current becomes less and makes considerable space angle with air-gap flux, developing little counter torque. Whatever is said to T1 and T2 so far is applicable for T2 and T1 respectively, if the motor is given a slight torque in the direction of T2.
It is obvious that the single-phase induction motor rotates in the direction in which it is started.
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