Capacitor size for Permanent Split-Capacitor (PSC) motor

The capacitor causes the starting winding current to lead the applied voltage by about 40*. The start and running windings are already 90* out of phase.The motor characteristics improve. A higher starting torque with better power factor at lower current. If you deviate from the design, you lower the torque, raise the current draw and lower the power factor. At some point you damage the motor.

Good luck,

James

Hello nike123

If your motor is a standard "split phase" AC "squirrel cage" = synchronous motor, then please remember that the starting capacitors which are usually fitted to improve starting torque are not precision capacitors.

These "starting" capacitors are normally non-polarised electrolytic capacitors, which have ratings sometimes +40% to -20% of the actual printed Rating in μF (Microfarads).

Because as hastingselectric explained above, these are usually on for a couple of seconds at each start, and a percentage difference in μF does not need to be perfectly accurate, it should not usually matter.

If these "starting" capacitors are energised for more than a few seconds, because the centrifugal switch is faulty, the capacitor will overheat, but the motor start windings would be damaged by this time anyway, and perhaps require a visit to a motor rewinding shop.

Example: Motors requiring 30μF may safely have a marked 40μF starting capacitor fitted, without any trouble.

Trust that assists you. Happy motoring.

Kind Regards....

GA.

My opinion exactly as in my experience (limited), these were the only capacitors that I have ever seen that were often right on the maximum "-" edge of the marked value. It did in fact give us a problem or two from time to time that the motors failed to start if there was a tiny bit more friction in the system than there should have been....changing to a cap that had been checked to be on the button or slightly higher in actual value, fixed the problems each and every time.....

It pointed out really that the design was a little too near the reality torque needed....and thus was not 100% reliable with the standard caps and their allowed tolerances.....

Sparky,

A couple of details you seem to have missed.

1. Where you have typed =synchronous, most likely you'd mean asynchronous ie induction motor.
2. The original poster specified permanent split capacitor. This means the cap stays in circuit (in series with the "start" winding) and switch gear is not required. The capacitors utilised are more likely to be mylar film or similar

I have seen start capacitance increased to get difficult to start Aircon compressors away, with mixed results. Get it right and both the current and run up time reduce. If you do make a mod measure and record. The consequences of getting if wrong are already stated.

On a similar vein I have a motor on my wood lathe that is noisy (not totally unusual for single phase induction motors) and I plan, when time permits, to investigate the cause and if needed fiddle with cap values (cap run motor). I'm not making any predictions but will look at phase angle and waveform in both windings along with supply current and motor temp when I do it. I have the instrumentation and the interest, just the time is lacking. The lathe works fine, just the noise bugs me.

Thank you guys for yours insights! It was helpful.