Single phase induction motor has single phase distributed winding in stator and a squirrel-cage rotor.
When Single phase supply is applied to single phase stator winding an alternating flux (field) is produced in stator.
Alternating flux varied with space (single) axis only and it’s not a constant magnitude synchronously revolving (rotating) flux as in the case of a two or a three phase stator winding fed from a 2 or 3 phase supply.
Now, an alternating or pulsating flux acting on a stationary squirrel-cage rotor cannot produce rotation due to rotor inertia (only a revolving flux can produce rotation).
That is why a single phase motor is not self-starting.
Why synchronous motors are not self-started?
Synchronous motor start rotating only if Magnetic locking is ensuring between stator poles & rotor poles (Ns –Sr & Ss- Nr).
Magnetic locking is not possible as stator poles are rotating at very high synchronous speed (3000 rpm, for 50 Hz & 2 pole) and rotor pole has zero speed and high inertia at starting.
Due to that motor fail to start which can be explain by following figures.
Fig. A show one particular instant of stator and rotor pole. At this particular instant rotor and stator poles might be of the same polarity (Ns-Nr or Ss-Sr) causing a repulsive force on the rotor. Due to this repulsive force rotor try to rotate in anticlockwise direction.
After a fraction of seconds (10 ms), the stator polarities will change to that as shown in Fig. B. Here the instantaneous torque produced on the rotor is in clockwise direction due to the attraction of unlike poles (Ns-Sr or Ss-Nr).
As it tries to rotate in clockwise direction but again stator pole change (Fig. A) and repulsion torque will produced again.
Due to the inertia of the rotor, it cannot respond to such quickly reversing torques. So the motor is subjected with a pulsating torque. Hence synchronous motor is not self-starting.
Methods of Starting the Synchronous Motor
To make SM self-starting, some techniques will use which initially rotates the rotor very close to synchronous speed in the direction of the magnetic field.
On achieving this speed, rotor pulled into synchronism with stator pole and due to that magnetic locking occurs.
After magnetic locking, the synchronous motor continues to rotate even after removal of external mechanism. Different Starting methods used in synchronous motor, like
1.Starting as Induction motor with damper winding and slip ring
- Starting with Low Supply Frequency
3.Starting with Prime Mover with DC motor or excitor and small induction motor.
Application of Single-Phase Induction Motors
The most common applications are table fans, exhaust fans, hair driers, fans for refrigeration and air-conditioning equipment’s, electronic equipment, cooling fans etc.
They are also used in record players, tape recorders, slide projectors, photocopying machines, in starting electric clocks and other single-phase synchronous timing motors.
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