[email protected] (R.Legg) wrote in message

snip---

Heheh. Must be the most difficult question to answer or calculation

to perform. I for one would like to hear what your application is.

In a nutshell it boils down to hottest temperature inside of cap. Not

as difficult to predict if load and conditions are known.

case 1 - Set top box for cable tv receiver. Ambient temperature is

25c, load is well defined, and can assume some duty cycle. On at 6am,

off at midnight if you wish. Line impedance is reasonablty well

defined as most residential installations closer to 10kVA than 100kVA.

case 2 - 3 phase AC motor speed control bus caps. Ambient temperature

0-50c, load unknown, duty cycle unknown, phase imbalance unknown, line

impedance unknown...yada yada yada..

If case one then you measure inner capacitor temperature (or outer if

you can then calculate inner temp from that) and use this to predict

lifetime. Can also get frequency spectrum of current waveform and

apply each harmonic to esr at that harmonic to figure out power

dissapated. That and can geometry will tell you internal temperature

and then get lifetime from there.

If case two then it's basically pull a number out of your ass. In

other words experience will be your guide. To attempt an analytical

approach to case two would be an excerise in futility.

Hope this helps. Again, what is application?

regards,

Bob