Choosing a right capacitor


Thread Starter


Guys...we use about 2.5 micro Farad capacitor for our ceiling fan motor (rated 60 W,50 Hz ). Can somebody help me out with the theoretical formula for selecting a right capacitor in accordance with a given motor rating.

Lets say that i want to choose a capacitor for a 150 W, single phase- 50 Hz motor. Then, how would i make my choice?

Thanks in advance.
Sensorless... if memory serves me, the theoretical equation you seek may be found in the Control.Com Archives. If your search is negative then let us know!

Regards, Phil Corso
Senseless… first some basics. The capacitor-start motor is actually known as the Split-Phase Capacitor-Start motor. It has two electrical coils; one called the 'start-winding' or Ws, the other the 'run-winding' or Wr.

They are physically displaced by 90 electrical degrees around the periphery of the stator, and they are connected to the same supply voltage. If the resultant currents were also displaced by 90 deg in time there would not be a need to add a capacitor. Why the capacitor?

The answer... Ws and Wr have vastly different impedances! Ws (comprised of small wire/less turns) has hi-resistance and lo-reactance (yielding current Is!) Conversely, Wr (comprised of large wire/more turns) has lo-resistance and hi-reactance (yielding current Ir!)

Thus, at start, the quadrature-component of Ir lags the quadrature-component of Is by a too small an amount, resulting in a low starting torque. While this construction is adequate for smaller Split-Phase motors (w/o a capacitor) the resultant phase-displacement for the larger motor between Is and Ir is not anywhere the ideal 90 deg, but instead, considerably less!

The capacitor, installed in series with Ws, introduces a phase-shift closer to 90 deg, thus insuring a larger starting-torque.

Sensorless... I don't want to raise the ire of readers that abhor posting of formulae, so if you want the theoretical formula in its entirety, contact me off-list!

Regards, Phil Corso
Sensorless... correction to my earlier post!

Because you identified a relatively small capacitor, the motor you have is probably not a split-phase, capacitor-start, but a permanent split-phase, capacitor-run motor.

If this is the case, please identify which type of motor you want analyzed when you contact me!

Regards, Phil Corso
Phil...Give me your e-mail id so that i can contact you off-list.

I have asked about this capacitor selection way to many people, but, couldn't get a good answer.
Thanks in advance...