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

[mac]

Presume the consideration here is using these as filter caps in power  
supplies. Simple answer is 1.414 times the AC rating or 523 vdc for a  
370 vac rated unit or 620 vdc for a 440 vac can.  Gotta be careful,  
though, about just what kind of a "motor" cap it is.  There are some  
types that were made up of a pair of electrolytics in series wired up  
+ - to - + .  On one half of the cycle one cap conducted and the other  
half the other unit provided the required phase shifting capacity and  
the two caps switched functions on the other half of the cycle.  Small  
and compact but only switched in for a few seconds on motor startup.   
Don't try to use these in your PS.  Other non-polarized types (oil  
filled or other modern dielectric) are in widespread use for both  
motor start and motor run (i.e., connected all the time)  
arrangements.  If it's a motor "run" type should be OK to use in your  
PS.  If a "start" type make sure just what you've got first.

Dennis D. W7QHO
Glendale, CA



On Sep 29, 2009, at 7:31 PM, Greg Mijal wrote:

> I just found a box of motor starting capacitors in my garage.  PPD  
> film capacitor 15 uf @ 370 vac 50 - 60 hz.
> My question is how do you calculate the maxium DC voltage capability  
> of an AC rated capacitor and are there any trade-offs or gotchas!  
> involved with DC service.
> Thanks!