[Boatanchors] Using ac motor start capacitors...

[match at ece.utah.edu]

On 1 Oct 2009 at 12:00, boatanchors-request at mailman.qth.net wrote:

> In answering a question posted by Greg Mijal, Marvin Match wrote:
> 
> <snipped>
> 
> "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."
> 
> This would be for a cap used in voltage doubler power supply
> service.  

No It's not. Drew, you missed my meaning completely.

Look at it this way: Take a cap and charge it to 100 volts. Now while it's 
charged, reverse the terminals and connect it again to a 100 volt supply. In 
this case, what voltage would the dielectric in the capacitor see? I contend 
that it "sees" 200 volts, therefore the cap must be able to withstand 200 volts 
DC. A capacitor rated for 200 volts DC would be safe in an AC application for 
only 71 volts RMS.

Or, in rating a cap for AC service, the dielectric must be able to withstand the 
peak-to-peak value of the voltage applied, which is 2.8 times the AC RMS 
voltage, not the peak value.

Marvin
KA7TPH