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Let me see if I can confuse you some more....

I believe that the answer you seek is even more obscure.

I looked through datasheets for a number of "dual-rated" capacitors to see if there's a correlation bvetween the AC rating and the DC rating.

They're all over the map, from a low of VDC = 1.4x VAC, up to VDC = 3.0 x VAC Most seemed to be either close to 1.4 (especially smaller film caps) or closer to 2.2 (especially big motor-run caps).

I know I have some old GE paper/oil caps that are rated 660VAC/1500VDC.

I think part of the discrepency is that AC motor-run caps may be rated for the AC line voltage, but in operation the cap can get more than the AC line voltage imposed across them (due to the inductive nature of the motor) - one thing I read said up to 1.5x the line voltage. That might explain why a 660V motor run cap might actually be good for 990VAC, or 1500VDC. Maybe....

Also, one set of datasheets I was looking at showed a DC rating of 1.7x the AC rating, but also listed a DC "transient" rating of 2.3x the AC rating.

In general, I suspect that caps designed for motor-run applications are very rugged and over-rated. They have to withstand line surges and wicked inductive spikes when a motor is turned on and off. I'd guess that the 660V caps will be OK at 950V.

As someone already mentioned, I would be a little worried about old-stock caps, possibly containing PCB's and likely without a safety mechanism (newer caps are all marked "protected", which I think means that they fail in a less-than-explosive manner (open circuit)!).

I bought some 12uF/660VAC motor run caps from All Electronics (www. allelectronics.com) for $3.50 each - seems like a great price. These are modern polypropylene-in-oil caps, rated 660VAC, protected. I haven't run them over 750VDC yet (I'm planning on using them for an 845 amp someday at 800V-1kV). At least if they fail they won't hurt you.

Hope this helps....

Pete


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  • Let me see if I can confuse you some more.... - Pete Millett 14:19:44 11/01/01 (0)


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