Too much capacitance on capacitor start motor??

[nelsonhoover]

Is there such a thing as having too much capacitance on a capacitor start motor? In other words, is it possible to have too many uF worth of start capacitors?

What I have run into is a 15 HP 1ø 3450 RPM centrifugal fan. Some of the run capacitors where a bit corroded at the terminals so I replaced them. In the process, I checked the starts too. One was bad so I replaced it with an identical start (216uF 250V). Upon starting the fan, the new capacitor promptly blew its top (the other 2 where cool to the touch yet).
I double checked the wiring and installed another. Same results. Each time the fan would start, but not without blowing the newly added capacitor. In all I toasted 3, till finally I started it on the remaining 2 and left the third off. Seemed to work great, at least for now.

Any ideas what might have been doing this?

 

[raisinbran]

Capacitors "blowing up" is typically a sign of over-voltage. Possibly, you worked yourself into a resonant condition?
Just a thought,
Raisinbran

 

[nelsonhoover]

Could you clarify "resonant condition"? Also, why don't the other 2 identical capacitors at least get warm or something? They seem to be fine. I guess I've just never run into anything quite like this.

 

[sreid]

When you say the replacement capacitor is "Identical," could you tell us about the new capacitor in detail. The suspicion is that the replacement capacitor is not correct in some way.

 

[waross]

It sounds as if the new capacitor may have an inadequate voltage rating. Check the voltage rating of the surviving capacitors.
respectfully

 

[nelsonhoover]

It's exactly the same. The 2 remaining are 216 uF 250 V. The third, which I attempted to replace, was identical. Frankly, it has me stumped.

 

[electricpete]

As far as I understand it, electrolytic capacitors by themselves cannot tolerate dc. They are often supplied two in series within a single case which somehow allows them to survive ac.

Is it possible you got a single electrolytic cap instead of a dual? It could be that the markings would look similar. But the single elctrolytic cap I think would have a + / - marked on it.

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[electricpete]

duh. First sentence should have been
"electrolytic capacitors by themselves cannot tolerate ac"
duhhh, Sorry ;-)

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[nelsonhoover]

No, I'm positive it's AC. I get them from Essex-Brownell. Its the BC-216M-250 which is a 216-250 uF 250 VAC start capacitor. Maybe bad batch of capacitors? I might try a 330 V start on it, or something. See what happens I guess. Thanks for the input.

 

[waross]

Try a higher voltage rating. The original motor starting caps may be rated for applied voltage and the new capacitor rated for maximum voltage. This may explain your problem.
You have the symptoms of over voltage so by all means try the 330 volt caps.
respectfully