It is also called main and the auxiliary windings that are present in single phase ac motors like the one’s used in submersible motors.
Basic principle of an induction motor
is that when a rotating magnetic field cuts through a stationary rotor conductor.it induces emf in it which in turn generates a current in it.
We know that when a current-carrying conductor is present inside a magnetic field, it experiences a force. This force acts on the rotor and it starts rotating.
Now the most important thing here is the rotating magnetic field but a single phase cannot generate an effective rotating magnetic field so we use an auxiliary winding with a capacitor.
The auxiliary field with the capacitor has an about 90° leading current than the main winding. When the magnetic field generated by auxiliary interacts with primary winding’s a rotating magnetic field is generated.
In single-phase, the auxiliary winding can be disconnected when the motor has gained speed as only pulsating magnetic field of a single-phase can run motor, that’s why it can be called a starting winding which is not required for continuously running the single-phase motor.
We are Very much thankful Mr.Gupta from agrofab, Jaipur had given some critical and knowledgeable information about single phase motor as mention below.
Single-phase does not produce a rotating mag. Field. To do so you need at least 2 phases. By a tech. twist 1 phase is converted to 2 phases. In three-phase, the angle between phases is 120 degrees. But by using capacitor phase can be split by 90 degrees only
Using the available split by capacitor induction motor is run and a smooth rotation is achieved. But the magnitude is 1/3 as compared to a similar 3 phase motor. So core lengths are chosen accordingly.