oil filled motor start caps

[N3DRB The Derb]

I gotz 4 oil filled PCB LADEN caps rated at 444volts ac. Abt how much dc i safe on these?

[W7TFO]

Those are good to have for their staying power.  I'd run them up to 600DC with no worries.

[N4LTA]

Theoretically about 1.4 x the AC rating - I stay a little below that for safety. 600 volts is about perfect. The 440 volt rating is likely very conservative as most are used in a 480 volt system.

[W2PFY]

This will tell you how numb my brain is. Someone once told me the capacity in UF is different when when using an AC capacitor for a DC application?? Can someone shed some sunshine on this comment? Can't remember if it was less or more per a casual comment along time ago. PS, I'm am not an engineer.

[N4LTA]

In a true perfect capacitor - there is no difference - but then there is no such thing.

Most capacitors that are designed for AC work fine for DC - The opposite is not often true.

I have heard many old timers say that an oil filled power supply cap is good for twice the uF value of an electrolylic cap. It probably has to do with the ability to handle AC current. Capacitors come in many types and they are not equal.

It popped into my mind the other day - what would happen if all of a sudden we had real perfect lossless capacitors and inductors. Infinite current and voltage might be dangerous!

[W7TFO]

The following does NOT apply to electrolytic caps.....

The frequency it operates at will dictate the choice of dielectric material used.  We commonly see paper & oil, and plastics like mylar and polystyrene used below 30 kHz. 

Mica and certain ceramics still rule supreme at RF, tho teflon is making some inroads.

The choice of dielectric material in a cap will have a direct effect on it's power factor, which is strictly an AC proposition.  That in turn will dictate the ability to handle the relationship between current and voltage impressed upon it.  It is easy to see the result of exceeding ratings.   

Caps used on DC (that is without any AC ripple included) rely only on breakdown voltage across the dielectric.  AC ratings are usually lower than DC ratings for any given cap.

A uF is a uF in ratings terms.  Doesn't change unless it is obviously a variable capacitor.  The manufacturers tolerance rating also comes into play, but usually that is a not a point of order. 

Excessively high operating temps also will cause a good cap to fail.  To wit, I have a big paper-n-oil cap that has a water cooling pipe wound thru it.  How much larger would it be if dry?

What matters is the use category as stated above.