Why Single Phase Induction Motor Is Not A Self Starting Motor ?

Three phase induction motor rotates by rotating field produced by stator winding resulted from the three phase supply itself which is lagged 120 degree from each other.When the field rotate, it will induce emf in rotor thus the rotor will be magnetised and have its own pole. The pole N and S on rotor will start to chase the stator magnetic pole on stator which is N and S = pulled but N and N = pushed. With this push pull action at given line frequency, the rotor will rotate by itself.

Single phase motor needs different method to start and run. The single phase line itself will not produce rotating field in stator because of only one sinusoidal not three like three phase supply. To create the rotating field, we must split the phase into two different sinusoidal wave that lagged for some degree each other. There are two method that commonly in used;
1. Use smaller diameter of winding wire in starting winding, so the current will flow faster compare to Running Winding.
2. Use of capacitor in starting winding- capacitor fast charge and discharge action makes the current leading.

When the supply is applied, the current in starting winding will lead the current in running winding hence we have rotating field that will induced emf in rotor. So the push pull action will began immediately.The Rotor turns and started.

Easy Peasie....

*The single phase induction motor is not self starting bcoz in the single phase no rotating magnetic field is produced
*The rotating magnetic field produced is a pulsating one
*Since the strator has only one winding it can't produce enough magnetic field,,so it is a pulsating one or distortion

A single phase induction motor is not self starting as a few actions are needed in order for the torque to be produced such that it starts moving. The principle is explained below in brief.

There is a stator and a rotor in an induction motor. The rotor has a short circuited winding and is positioned inside the stator. The stator has a rotating magnetic field. For the motor to operate, this rotating field must generate and flux which in turn induces a flow of current in the rotor winding. This current in turn gives rise to a magnetic field in the rotor. This magnetic field spins at the frequency of the rotor current, which is at the same frequency as the stator field.

The movement in the motor is a result of the torque generated by the interaction of these two magnetic fields. The stator field and rotor field have a difference in speed and this causes the current to flow. For the motor to rotate, a resultant torque field is needed that rotates only in one direction.

Its easy. Because single phase supply can't create a rotating magnetic flux.

For a single phase motor to start there has to be a phase shift. This is because they have one set of windings. Some single phase motors have multiple windings, but they are start windings, and high or low speed windings. 3 phase motors have separate windings in them. This automatically creates a rotating field. A single phase motor has one. The field is rotating but not enough to start the rotor to spinning. There hase to be a phase shift to create this. A capicator is used to do that. In a capicative circuit current leads voltage. This shifts the sine wave and starts the rotation. Some low horsepower motors such as a shaded pole motor doesnt require this. They are a different beast all together. To learn more get a Rosenberg motor book at a book store. It is really good at explaining motors. The book can also be found on Ebay sometimes. It may cost as much as $90 though.

Single phase motor stator field does not rotate it just oscillates positive and negative, hence there is no torque to move the rotor. If you spin the rotor by hand to give it a start, it will continue to run

Due to production of pulsating magnetic field by the stator single phase supply. This type of generated magnetic field is stationary , pulsating & time variant.

Single phase induction motor is not self starting bc0z in single phase there is only one winding it will not produce rotating magnetic field