Capacitor size for motors

[justint]

Hi

Is there a calculator or rule of thumb to use for selecting start and run capacitors for single phase inductance motors if i know the supply voltage and current ratings for the motor.

I normally refer back to the manufacturer but was hoping for an alternative method.

Justin

 

[electricpete]

The EASA Tech Manual section 3.10 has some guideance for a motor that has a single cap in series with the start winding. They start with a thumbrule 400 mfd per horsepower at 115 volts, and 100 mfd per horsepower at 230 volts.

Then they suggest checking the value as follows:
block the rotor apply voltage to the series start winding / cap and measure voltage accross each. Cap voltage should be 5% - 10% higher than starting winding voltage.

I am not sure how this procedure changes if a run cap is also included. I'm pretty sure this has been discussed extensively on the forum before although I didn't pay much attention. Maybe someone else will chime in with more info.

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