capacitor

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Old 08-31-06, 02:09 AM
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capacitor

I have an old window a/c unit that seems to have trouble starting up. (the compressor) Sometimes when it trys to start the lights just dim for a while (7-8 seconds) and I shut it off for fear of blowing a fuse. After doing a little reading I think it may be a capacitor problem, or a problem that a capacitor could help. The capacitor is marked "366-430 VAC MAX" 25 mf. I don't know if it's a start or run cap. The caps I've seen advertised say 370. Is this the value I should have? Should I look for a start cap or a run cap. Is it possible to use both? Thanks to anyone who can explain.
 
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Old 08-31-06, 05:34 AM
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That's a run cap. Although it's a run cap, it's used for starting. The difference being a run cap stays in the circuit 100% of the time, a start cap is only in circuit during startup and is disconnected from the circuit through a potential relay or other device. Also, never substitute a start capacitor for a run capacitor. You may however, substitute a run capacitor for a start capacitor. Enough said.
The 370 volt unit will be fine. If unavailable, you may use a 440 volt unit. Be sure the new unit says 25 mf or uf.
Usually, bad caps have tell tale signs such as bulging, leaking oil, top blown off, etc.
Substitute the cap and check all wiring carefully, check for bad connections, burnt wires, etc. Also, check the connections at the compressor.
Good luck.
 
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Old 08-31-06, 08:06 AM
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There is nothing about a capacitor that makes it a start or a run. It is how it is connected to the motor. When you have a single cap like yours, the motor may have a centrifugal switch which moves the cap from the start winding to the run winding.

A capaictor has a capacitance value...in your case 25 microfarads, which is abbrebiated UF of sometimes MF. It also has a max voltage rating, and on a single phase 240 volt system 370 VAC is an adequate rating.
 
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Old 08-31-06, 10:59 AM
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Start capacitors are designed differently than run capacitors. The start capacitor is unable to get rid of heat as quickly and hence cannot be left in circuit. This is why start caps CANNOT be substituted for run caps. For those interested in more details, consult the manufacturer or read a good technical book such as Modern Refrigeration & Air Conditioning.
 
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Old 08-31-06, 05:43 PM
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Thank you all for explaining that. Very helpful.
 
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Old 09-01-06, 05:51 AM
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Also start cap stop being used whe the compressor reaches 70-75% of its speed. Then they get kicked out of line by the internals of the compressor.
 
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