Principle of operation of SINGLE-PHASE INDUCTION MOTORS
Principle of Operation of Single-Phase Induction Motors:
Fig: Single-phase capacitor-run induction motor.
- If one of the leads of a 3-phase motor is disconnected while it is running light, it will continue to run with a barely perceptible drop in speed, and a somewhat louder hum. With only two leads remaining there can only be one current, so the motor must be operating as a single-phase machine. If load is applied the slip increases more quickly than under 3-phase operation, and the stall torque is much less, perhaps one third.
- When the motor stalls and comes to rest it will not restart if the load is removed, but remains at rest drawing a heavy current and emitting an angry hum. It will burn out if not disconnected rapidly. It is not surprising that a truly single-phase cage induction motor will not start from rest, because the single winding, fed with a.c., simply produces a pulsating flux in the air-gap, without any suggestion of rotation. It is, however, surprising to find that if the motor is given a push in either direction it will pick up speed, slowly at first but then with more vigour, until it settles with a small slip, ready to take-up load. Once turning, a rotating Weld is evidently brought into play to continue propelling the rotor.
- We can understand how this comes about by first picturing the pulsating MMF set up by the current in the stator winding as being the resultant of two identical travelling waves of MMF, one in the forward direction and the other in reverse. When the rotor is stationary, it reacts equally to both travelling waves, and no torque is developed. When the rotor is turning, however, the induced rotor currents are such that their MMF opposes the reverse stator MMF to a greater extent than they oppose the forward stator MMF.
- The result is that the forward flux wave (which is what develops the forward torque) is bigger than the reverse flux wave (which exerts a drag). The difference widens as the speed increases, the forward flux wave becoming progressively bigger as the speed rises while the reverse flux wave simultaneously reduces. This ‘positive feedback’ effect explains why the speed builds slowly at first, but later zooms up to just below synchronous speed. At the normal running speed (i.e. small slip), the forward flux is many times larger than the backward flux, and the drag torque is only a small percentage of the forward torque. As far as normal running is concerned, a single winding is therefore sufficient. But all motors must be able to self-start, so some mechanism has to be provided to produce a rotating Weld even when the rotor is at rest.
- Several methods are employed, all of them using an additional winding. The second winding usually has less copper than the main winding, and is located in the slots which are not occupied by the main winding, so that its MMF is displaced in space relative to that of the main winding. The current in the second winding is supplied from the same single-phase source as the main winding current, but is caused to have a phase-lag, by various means which are discussed later.
- The combination of a space displacement between the two windings together with a time displacement between the currents produces a 2-phase machine. If the two windings were identical, displaced by 900, and fed with currents with 900phase-shift, an ideal rotating field would be produced. In practice we can never achieve a 900phase-shift between the currents, and it turns out to be more economic not to make the windings identical.
- Nevertheless, a decent rotating field is set up, and entirely satisfactory starting torque can be obtained. Reversal is simply a matter of reversing the polarity of one of the windings, and performance is identical in both directions. The most widely used methods are described below. At one time it was common practice for the second or auxiliary winding to be energised only during start and run-up, and for it to be disconnected by means of a centrifugal switch mounted on the rotor, or sometimes by a time switch. This practice gave rise to the term ‘starting winding’. Nowadays it is more common to find both windings in use all the time.