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 Φ_m/2, acting independently upon the rotor

                             figure (b) Two rotating fields each having a magnitude (Φ_m/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.