# Voltage across Run cap in fan motor

• posted

While replacing the run/start capacitor in my AC unit, I wondered how necessary was it to use 440V caps in a circuit being fed by only 240V. The cap is a dual 60uf and 5uF in one package. The 5uF feed a 1/4HP fan and the

60of feeds the compressor (herm).

I measured the voltage across the input with my meter and it read 230VAC at

60Hz. I measured the voltage on the capacitor from C to Herm and was surprised to see 340VAC. I measured for C to Fan and saw a similar but lower 320VAC.

Grainger and other supply houses were selling 370V caps but that looks too close for my comfort. 440V it is.

Then I got to wondering, how is voltage getting generated across the capacitors in excess of the input voltage?

I'm sure it has something to do with motor EMF but I'm not sure how to model it in Spice.

Does someone have a good article or site that covers how these AC motors with start caps work and discusses the equations involved for calculating the voltage in currents?