Re: [TCML] Power Factor correction cap is heating up

[FIFTYGUY@xxxxxxx]

In a message dated 7/26/08 1:43:11 P.M. Eastern Daylight Time,  
owner@xxxxxxxxxxxxxxxxxx writes:

>Anyhoo, after a run of about 30 to 45 seconds, I

>shut it down, and made sure all was discharged,

>to make further adjustments, and thats when I

>found out that the power factor correction cap

>(55uf, 440vac) was very warm... almost hot.
>What could cause this?  It runs in parallel with
>the NST primary, following the line filter.






First thing I'd do would be to check the actual  value of that cap. I've fou
nd *many* caps that have failed in  service (varied types, ratings, and duty!) 
that didn't fail as a dead  short or open. Many are still within their 
tolerance range - despite the fact  that their packaging has burst and they've spewed 
hot dielectric all over the  place. Sometimes they just run very hot in 
service. Or it may be  a situation where they read fine with a low voltage tester, 
but at  service voltage they break down. 
    Metal can oil-filled caps seem to keep  pretty tight tolerances, even 
after severe abuse (if they're still OK). Some  of the better ones are even 
self-healing. But if the value is way  off from the nominal rating, I wouldn't 
hesitate to chuck it. You could always  plink at it with that little Taurus.(the 
metal can oughta reflect the laser  very nicely! ;) ). 
    Suitable caps are of course pretty easy to  scrounge, and substituting 
another cap to see if the problem remains is a  pretty easy troubleshooting 
method. Heck, brand new they run less than $20  each.
    BTW, that cap looks like a reasonable size (EIA  case code "T"?) for a 
55uF 440VAC motor run cap, so it shouldn't be having a  problem if it's in good 
shape. Keep in mind that these things have temp  ratings of 70-90 deg C, so 
they'll survive in a hot ambient environment  (strapped to a motor or inside a 
light fixture), but they shouldn't get  anywhere near that hot on their own in 
normal duty!
    You could also put a meter on the cap leads, to  see how much current is 
flowing through the thing in operation. That oughta  give you some idea of 
what's going on, and maybe you'll want to change the cap  value. Normally a cap 
of that size will flow a few amps continuously when it's  doing its job. Maybe 
if you've gotten really unlucky and hit a resonant  condition with the NST 
primary, it wouldn't take much of a change in cap value  or operating conditions 
to get away from that point.
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities




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