Capacitor Question

I replaced the capacitor on my coolant pump motor. I found a film type capacitor in my junk box that I tried and it worked, even though the capacitance didn't match the OEM value ( I think it was something like an 8uFd for a 10 uFd). It has been working for four years now with no problem.
 
TOOLMASTER said:
GOOD INVESTMENT

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Not really, often what changes in a capacitor is its ESR value (equivalent series resistance) so while the capacitance had not changed, the capacitor has developed an unnecessarily large parasitic resistance. This results in more voltage across your capacitor and less across your other components.

You need something that measures capacitance and esr to check if a capacitor is still good.

I recently fixed a ups, problem was one small capacitor, capacitance was fine but esr was 48ohms (should be like 0.4)

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You can get a replacement at any electronics store. 15uF and voltage greater than 200v. Make sure though that it is not an electrolytic capacitor, they don't like AC. film style will work,maybe a polyster cap.

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atunguyd said:
Make sure though that it is not an electrolytic capacitor, they don't like AC
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Not really correct, there are different rating for different capacitors types, some work better then others in certain applications. CDE and other capacitor manufactures make AC motor start specific non-polar or bi-polar electrolytic capacitors.
http://www.cde.com/catalogs/PSU.pdf
http://www.elcomp.net/conis.pdf
The advantage of electrolytic is size, comparable film and oil types are much larger. Most likely the capacitor that went bad is an electrolytic, possibly film based on its size and VAC rating. The negatives of electrolytic types are heat leads to rapid degradation and they have a shelf life, they will not last as long as a MKP film or oil type. They are usually only used for motor starting capacitors and not run. My experience in having them go bad in motors, is when they go, you measure near 0 capacitance, if the ESR increases, they cook and go bad very quickly in these applications. Also rapid on off cycling of single phase motors, causes heat buildup in the capacitors which leads to rapid failure. Capacitors in circuits, is a different story, ESR, leakage etc. are important to measure based on the circuit application.

In this application I would look for a encapsulated film type (MKP) 15-16uF and a minimum voltage rating of 250VAC, oil will probably be too large.
 
Agreed, you do get non polarised electrolytics but they are not common and as per my advise if you go to an electronics store and buy one the chances are very high you will get a polarised version.

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atunguyd said:
Agreed, you do get non polarised electrolytics but they are not common and as per my advise if you go to an electronics store and buy one the chances are very high you will get a polarised version.

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Non-polarized (actually back-to-back) electrolytics are almost always what are used as motor start capacitors.
 
I have a HVAC company and I have maybe 5 of those on my truck. But this is a useless reply only to make you feel better. :)

If you see a HVAC truck or company or supply house in your area, stop and ask for one, they are very common.
 
Concerning capacitors, and I am dumber than a bag of hammers on electrical issues, Do they make any noise when they [may] go bad? On my Jet drill the motor is beginning to emit a rattle. I used my stethoscope and it sounds like it is coming from the capacitor. I removed the two screws and looked at it, no leaks or burnt signs so its back up and running. The motor makes no sound with the belt off but kinda growl/rattles when the belt is looser or tighter.
Any thoughts?
thanks in advance
toolroom
 
toolroom said:
Concerning capacitors, and I am dumber than a bag of hammers on electrical issues, Do they make any noise when they [may] go bad? On my Jet drill the motor is beginning to emit a rattle. I used my stethoscope and it sounds like it is coming from the capacitor. I removed the two screws and looked at it, no leaks or burnt signs so its back up and running. The motor makes no sound with the belt off but kinda growl/rattles when the belt is looser or tighter.
Any thoughts?
thanks in advance
toolroom
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That sounds more like a bearing issue. Normally when caps fail with noise, they either go BANG or BBBzzzzzzzzT Sometimes accompanied by clouds of smoke.
 
Jim,
Exactly what I thought, but again, listening through the stethoscope ... Guess I'll run it till the bearing fails.
Thanks again
 
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