All though in both types of Induction Motors, capacitor does the job of phase shifting to produce the rotating magnetic field, their working principle is the same. Then whats the point in disconnecting the capacitor by centrifugal switch for capacitor start, and not disconnecting for the capacitor run motors. I mean is either of the Induction Motor designated for particular power rating, plus i read somewhere that capacitor start Induction Motor has high starting torque. Also when i browsed the internet found this piece of info that capacitor run motor has improved p.f. due to start capacitor, so isn`t it always advantageous to have capacitor permanently to improve motor efficiency.
A Capacitor Start Motor not only switches out the capacitor when up to speed, but also switches out the starting winding which is in series with the start capacitor. Capacitor Start motors have very high starting torque for a single phase AC motor. The same is true for a capacitor start / capacitor run motor WITH a centrifugal switch. In this case, the run capacitor is a different value and just provides some additional phase lag especially to help the motor maintain speed under load, and improve the power factor. A Capacitor start/run motor without a centrifugal switch has a lower starting torque, but good performance under load, and again, an improved power factor.
Why not experiment with a capacitor-start motor yourself to see what happens if the start capacitor remains connected after the motor has reached operating speed.
You would notice two or three things:
1) the motor starts to get very hot.
2) the motor does NOT reach full speed until the starting capacitor / winding is disconnected.
3) the motor makes unpleasant sounds. Sort of a growling sound.
Like you, I was curious when I was a youngster. So I experimented to find out what happens.
That experience has done me well over the years - it makes diagnosing a malfunctioning motor easy.