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Capacitor size for motors

Capacitor size for motors

Capacitor size for motors

(OP)
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

RE: Capacitor size for motors

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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