[Boatanchors] Using ac motor start capacitors...

[Drew P.]

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.  Greg's question concerned translating the AC voltage 
withstand rating of a capacitor to a DC voltage withstand
rating.  The peak voltage that the cap would see in sine wave
AC service would be 1.414 times the RMS value of the applied
sinusoidal AC voltage.  Therefore, in 370 VAC sinusoidal 
service, the cap would see 1.414*370 or about 520V. That 
520 volts would be the withstand rating for the cap expressed
either as the peak of an AC waveform or as a steady DC voltage. 

Marvin Match wrote further:

"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."

Some motor run caps do indeed have a resistor, either internal
or external.  This is a parallel bleeder resistor to protect a
hapless technician from a real Gotcha!  In DC filter cap service
you therefore get a bleeder resistor for free, but its power 
rating is based upon the aforementioned RMS case, not upon the
actual withstand rating. If the resistor were at rated 
dissipation at the cap's rated RMS voltage, then it would be 
underrated by a factor of sqrt(2)^2 or 2 when continuously 
subjected to the cap's withstand voltage.  So, in cases where 
the supplied resistor is underrated (above 370VDC in this case)
one might desire to yank if possible the supplied resistor and
then substitute one of adequate power rating. 

One responder wrote of avoiding use of electrolytic AC motor
start capacitors for DC power supply filter service.  A friend
of mine (note that I evade responsibility here) has successfully
used scrounged caps of that type in that manner.  In DC supply
service one sometimes might be concerned with ripple current
rating; the ripple current might cause excessive heating of 
some electrolytic caps.  In motor start or run service, however,
"ripple current" is all there is and there is lots of it! So
in addition to being voltage rated, these caps are also designed
to handle AC current.  One would not see any problems in normal
DC power supply service from the use of these caps.  The one 
"problem" might be the increased bulk and cost of these capacitors.
The former is of less concern to us because we often have room to
spare in our beloved boatanchors, and the latter is of no concern
to the scrounger (that's us).

Drew