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compressor start capacitor sizing

1carwash1

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My 5hp air compressor‘s start capacitor seems to last for about a year before needing replacement (open circuit). The compressor was purchased used and I wonder if somewhere along the line someone replaced the start capacitor with the wrong size. The current start capacitor’s size is 233-280MFD with a voltage rating of 220-250VAC. I’m beginning to think that the start capacitor is too big thereby causing the startup winding’s current to spike too high and eventually causing the capacitor to fail. I used an online start capacitor calculator and it indicated around 122uf which is way different than what I have. Since the motor had no efficiency rating on the data plate, I assumed 80%.

it’s an IR brand air compressor with Dayton 5HP motor. Input Voltage 208.

Any ideas or suggestions?
 

JGinther

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Compressors have to start heavily loaded, so I would expect the large capacitor was needed. It's important that the tubing between the compressor head and the tank check are unloading after a run, so that the next start will not be too hard on the motor and capacitor. Is it unloading properly? If the cap is blowing, you could have a problem with the contacts/switch in the motor causing it.
 

Randy

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The best advise I can give you is to take the motor to good motor shop and have them load test the motor. Another option is to get rid of the Dayton motor, back in the day they made a pretty good motor, but not anymore. My motor guy is big on Leeson motors. The centrifugal start switch in the back of the motor could be going bad. You shouldn't have to replace capacitors once a year.
 

MEP001

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You can get a clamp-on amp meter for $30 and load test the motor in place.

Make sure the tank is being drained. Frequent starts and short runs will cause early failures.

You should hear a brief release of air every time it shuts off. If you don't, it's starting under load as JGinther mentioned.
 
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