Single Phase Motor: Questions Answered

[NT DANG KHOA]

Dear expert, dear all.
As my knowledge, the single phase motor is not a seft-starting motor. i suppose that there is only main winding in single phase motor.
as you know, the rotor can not start because of the 2 opposites force cause by 2 oppositely rotating magnetic fields.
the question is that :
1. Why we give the 1 phase motor an external force in either direction , the motor will start to run ? ( i think the motor only run for short time until this force run out , because it always exist 2 opposites force when the main winding of stator have power supply. like we turn the wheel by hand )
2. with a normal single phase motor ( starting winding with a capacitor and main winding ) what is the cause can make the speed of the motor increase because the capacitor and the starting winding only give it the constant force in the starting period, and why we cut out the starting winding , the motor continue increase speed to the rated speed while there is only 1 main winding which have cause the 2 same opposites force ?
Forgive me if my eng is not good and it is a stupid question,
thank in advance.

 

[Baluncore]

Welcome to PF.

A single phase induction motor has a start winding that is wound at 90° phase to main winding.
The start capacitor provides a phase shifted current that excites the start winding.
The sum of the magnetic field from main and start winding is a true rotating vector field.

Once the motor is running the current induced in the armature is sufficient to maintain rotation.

 

[NT DANG KHOA]

Welcome to PF.

A single phase induction motor has a start winding that is wound at 90° phase to main winding.
The start capacitor provides a phase shifted current that excites the start winding.
The sum of the magnetic field from main and start winding is a true rotating vector field.

Once the motor is running the current induced in the armature is sufficient to maintain rotation.

SORRY BUT COULD YOU EXPLAIN MORE DETAIL ABOUT THIS SENTENCE "Once the motor is running the current induced in the armature is sufficient to maintain rotation"

 

[Baluncore]

The current induced in the rotating armature has a phase delay that makes it not parallel with the field windings.
Because there is a phase difference between armature and main winding there is a directional torque.
http://en.wikipedia.org/wiki/AC_motor#Slip