[Boatanchors] GB> Using ac motor start capacitors in dc power supplies - wvdc question

[Sheldon Daitch]

Is there a duty factor consideration for motor start capacitors?   Or 
are they
the same construction/specifications as motor run caps?

A motor start capacitor normally is in the circuit only in the spin up 
phase of
the motor start and is switched out of the motor circuit when the motor
reaches a specific speed.

73
Sheldon

Marvin Match wrote:
> On 29 Sep 2009 at 22:31, Greg Mijal wrote:
>
>   
>
> Hmmm... I've used them with success. This is my analysis. You can 
> decide if I'm right or wrong:
>
> On each half cycle, the AC capacitor will see a peak that is 1.414 
> times the RMS voltage applied. So, in the first half-cycle the 370v 
> cap will see about 520 volts peak.
>
> BUT, on the next half cycle, the cap will see minus 520 volts in 
> addition to the plus 520 volts from the previous half cycle, so the 
> DC breakdown voltage has to be at least 1040 volts for a cap rated 
> 370 volts AC.
>
> So, an AC cap should be safe to 2.8 times it's AC voltage rating when 
> used in a DC supply.
>
> Gotchas? The only one I know of is that some AC caps are not just 
> caps, but have a series resistor integral to the unit. These will be 
> labeled "protected" or something similar, and I'm not sure that a 
> series resistor is the only way to "protect" a motor cap.
>
> Does this help?
>
> Marvin
> KA7TPH
>
>