Electrical – determining the start and run capacitors for single phase motor

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.

Yes, you need a cap. The cap must be rated for the job. A 250VAC or better metal film cap will be needed.

The phase shift is never exactly 90 degrees but it does not have to be. The capacitance needed is roughly proportional to the size of the motor. If your cap is small you will still get starting torque but not very much. If your cap is too big, you get more starting torque but you increase no load current consumption and risk burning out the motor. Running torque does not change much with cap value.

If your load is "easy" like a fan you have a wide range of capacitance to work with. If your load is "hard" like a piston compressor you will have to get the cap right be ensuring that the motor starts under load. Under no load on the bench you could start with 10uF and take things from there.