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I need help with capacitors for a single-phase compressor motor please

DavidScott

Diamond
Joined
Jul 11, 2012
Location
Washington
My 5hp compressor isn't running right and I thought it was the run capacitor as it starts fine but then slows down after about 5 seconds. This is out of my wheelhouse so I need some help with some probably dumb questions.

Full story: The compressor is a cheap Home Depot 4hp 2-stage with a decent 5hp motor after the original died a few years ago, wired 240V. I have since slowed the rpm down by about 40% to reduce noise so there isn't much load on it making it hard to tell if something is wrong with the motor. I can't measure the capacitors, my multimeter only goes to 20uf, so I purchased what I think are correct replacements and replaced both. After turning the compressor on the magnetic contact block was humming so I replaced it with a spare on hand, the old contacts looked a bit burned. Hum is gone but it still slows down about 20% 5 seconds after start-up. I am sure I connected all the wires as they were connected. Both 120V legs are good going to the motor.

Questions are:
1) Are these capacitors basically the same or did I screw up?
2) Is this a sign of a bad capacitor in photo #1?
3) How important and what does the thing do that bridges the contacts in photo #1?
4) Which is the start capacitor?
 

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My 5hp compressor isn't running right and I thought it was the run capacitor as it starts fine but then slows down after about 5 seconds. This is out of my wheelhouse so I need some help with some probably dumb questions.

Full story: The compressor is a cheap Home Depot 4hp 2-stage with a decent 5hp motor after the original died a few years ago, wired 240V. I have since slowed the rpm down by about 40% to reduce noise so there isn't much load on it making it hard to tell if something is wrong with the motor. I can't measure the capacitors, my multimeter only goes to 20uf, so I purchased what I think are correct replacements and replaced both. After turning the compressor on the magnetic contact block was humming so I replaced it with a spare on hand, the old contacts looked a bit burned. Hum is gone but it still slows down about 20% 5 seconds after start-up. I am sure I connected all the wires as they were connected. Both 120V legs are good going to the motor.

Questions are:
1) Are these capacitors basically the same or did I screw up?
2) Is this a sign of a bad capacitor in photo #1?
3) How important and what does the thing do that bridges the contacts in photo #1?
4) Which is the start capacitor?
1. Yes, the capacitors have same values.
2. Possibly, it is not always obvious when they go bad.
3. That is a bleed resistor, it bleeds off voltage when not powered so you don't get shocked when servicing unit. I think its "optional", but I'm just a hack and don't know for sure. Edit: I have replaced caps that had the resistor with non-resistor caps and motor worked just fine, lack of that resistor on new cap is not your problem.
4. The black plastic cased caps are the start caps, the metal cased caps are the run caps.
 
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Thank you for answering my questions, I was afraid the difference in size and missing bleed resistor would be a problem. And yes I know to bleed the caps before touching them. Part of the problem was the belt was loose so when the pressure started to build the pump would slow down. I took the back of the motor off to inspect the centrifugal switch and while it works fine it looks like the motor has been running a very long time with a dead run capacitor, as the contacts are worn down. Sorry for the photo being too close up.
 

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