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Old 08-03-2010, 03:00 PM   #1
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Compressor starter capacitor info please.

I have a 13.5 Duo Therm. The last time we took our trailer out the start capacitor burned out. I took one off a bad unit we had laying around at work and it seemed to fix it at first. Everything was working great, however, after a few hours when it began its normal thermostat cycling, the compressor is very slow to kick in. In fact, sometimes it kicks the breaker on my 7500 watt generator. The size of the capacitor that burned up as well as the used replacement is 47-56mfd. I went to the local supply store to get info on these but I'm not sure they knew the answers. Here's my questions; Could this used replacement be operating at less than 100% (could it be half wore out)? Would a new one the same size be stronger? Also, the supply store sold me the step up stronger one they had in stock, 72-88uf, and warned me it was more than the recommended +/- 10%. I have not installed it yet, Going to wait and see what you guys have to say about it. I also realize my unit may be going out, just want to make sure its dead before I replace it. Thanks in advance for all comments. John
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Old 08-03-2010, 03:05 PM   #2
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Originally Posted by driverjohnlh View Post
I have a 13.5 Duo Therm. The last time we took our trailer out the start capacitor burned out. I took one off a bad unit we had laying around at work and it seemed to fix it at first. Everything was working great, however, after a few hours when it began its normal thermostat cycling, the compressor is very slow to kick in. In fact, sometimes it kicks the breaker on my 7500 watt generator. The size of the capacitor that burned up as well as the used replacement is 47-56mfd. I went to the local supply store to get info on these but I'm not sure they knew the answers. Here's my questions; Could this used replacement be operating at less than 100% (could it be half wore out)? Would a new one the same size be stronger? Also, the supply store sold me the step up stronger one they had in stock, 72-88uf, and warned me it was more than the recommended +/- 10%. I have not installed it yet, Going to wait and see what you guys have to say about it. I also realize my unit may be going out, just want to make sure its dead before I replace it. Thanks in advance for all comments. John
Oil filled capacitors operate at 100 percent or zero percent.

No such thing as one "getting weak".

Andy
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Old 08-03-2010, 03:39 PM   #3
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Start capacitors need to be the same MFD & voltage rating as the original.
A slight +/- in MFD or a higher voltage will work but no more than 10%.

A bad start or run capacitor can indeed burn up the compressor it is only used on start up and generally a relay will kick it out once the compressor is running.

One sure way to tell a bad capacitor is if it is swollen or buldging it is bad otherwise you need a capacitor tester.

WARNING capacitors are electrical storage devices and can have an electrical charge even with power off so be careful working with them.

72 to 88 MFD is not a good substitute.
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Old 08-03-2010, 03:53 PM   #4
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Sorry to answer your question YES the used capacitor could be failing.

They are made up of hundreds of plates and they don't wear out but usually will open or short internally.
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Old 08-03-2010, 04:06 PM   #5
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While it is true that oil-filled capacitors fail rarely and nearly always all at once, most start capacitors are electrolytic, not oil-filled. The more costly and larger oil-filled capacitors are ordinarily used for run capacitors.

Electrolytic capacitors generally do lose capacitance over time. They also lose their capacitance as they heat up, which may explain why it worked for a while. I would not install a used one except as a last resort.

Whether the larger cap will create problems is hard to say. With some kinds of potential relays an oversize cap will cause the relay to stay closed which can damage the cap itself and also the start windings in the compressor. With time delay relays this is less of a problem.
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Old 08-03-2010, 04:08 PM   #6
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Sorry to answer your question YES the used capacitor could be failing.

They are made up of hundreds of plates and they don't wear out but usually will open or short internally.
Actually the typical construction is a single pair of flexible foil plates separated by an absorbent mat, wound into a spiral shape.

The loss of capacitance over time is caused by decomposition of the electrolyte and plates, or by loss of electrolyte via evaporation if the case seal is compromised.
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Old 08-03-2010, 05:21 PM   #7
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Thanks for the responses! The original was bad as the top was off and we could smell it burning. It was in fact the cylinder shaped spiral wound type and I do not believe it was oil filled. One more detail, when it does kick on (compressor), it runs flawless and cools great.
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