Voltage across Run cap in fan motor

[mook johnson]

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?

[Michael A. Terrell]

mook johnson wrote:
>
> 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?

Think: It's a low Q resonant circuit, to cause a phase shift.
Without a phase shift, the motor doesn't run.