Single phase induction motor capacitor value calculation

[sam781]

I know single phase motor cannot start alone as rotating magnetic flux is not generated with single phase supply. To make rotating magnetic field in stator single phase supply is made like double phase supply with the help of a capacitor and double phase stator winding. Now, I need to know how to calculate the required capacitor value to be used in the motor supply to get higher efficiency?
For example, we can consider a ceiling fan to calculate the value which is readily available induction motor.

 

[FvM]

The optimal value depends on various motor parameters and is usually specified by the motor manufacturer.

If you know the windings ratio of main and auxilary winding, rated motor current and cos φ, you can calculate the optimal capacitor value.

 

[chuckey]

For a 230V 50 HZ motor 50 MF / HP is a rough and ready starting point. i.e. for a 60W motor use 60/750 X 50 = 4 MF.
Frank

 

[sam781]

What is the reason of assuming 50uF capacitor per HP rated motor? What will happen it I increase and decrease capacitor value for a particular motor?

 

[chuckey]

See FvM's reply. Alter the value and see what happens, if the capacitor is too large the current will burn out the starting winding.
Frank

 

[FvM]

In addition, if the value capacitor is too small, the capacitor or the winding might be damaged too, due to resonance effects.

I append a phasor diagram from my German textbook Elektrische Maschinen by Rolf Fischer


You see that capacitor voltage Uc and auxilary windings voltage Uh are almost in resonance. Main and auxilary windings current Ia and Ih are 90° phase shifted.

 

[sam781]

If the inductance of the two windings are found, we can be able to find the right capacitor value. Right? I read something about single phase IM from a book writer Rosenblatt) few years ago. But, now cannot remember all the things. Could anyone please refer a good English book where such things are explained in detail?