Electric motor question

Hi all,

I have a 3hp air compressor that I just inherited. When I got it, the starter cap had clearly blown (guts were oozing out). I went through all the precursor stuff (hydro tested the tank, replaced the cut in/out switch, ensured the blowdown valves worked, etc.) but when I finally started it up, it ran perfectly… until about 30 second in when the started cap blew again.

My initial thought was that the centrifugal switch in the motor was bad. so I tore into is… only to find that there isn’t one!!!

I also saw that the run capacitor may have been bad as well, so have now ordered both a start and run capacitor.

But I’m REALLY confused as to the fact that this motor doesn’t seem to have any circuitry to knock out the started capacitor. Is this a thing in compressor motors? It looked like the wiring just went straight into the windings when I took it all apart. Other than the fact that BOTH caps were bad, is there anything else I need to look at before I put another $30 worth of caps on it?

Thanks,
Ian

Im not an expert on motors but have replaced bearings and capacitors on quite a few old motors.
Make sure the motor is wired correctly, many motors can run on 120 or 240 volt, they usually have a wiring diagram on the motor that shows how to wire it.

If its an actual 3hp motor it would be a 220v motor. If the fine print says develops 3hp they are talking about locked rotor amps when the motor starts, and its meaningless. Sears started doing this many years ago to make people think they were getting a bigger motor than was actually used. Motor amps is a better measure.

You might have a short to ground or a short between windings. I’m pretty sure when there is a centrifical switch they call the capacitor a start capacitor and with no switch they csll it a run capacitor because its in the circuit all the time. I might have a book on motors, I can check in the morning.

Wiring looked ok, not sure how to check a motor for a short though as all I can really do is check continuity. I’ll throw new caps on it when they get here tomorrow and hope for the best!

Possibly you have a motor with a potential relay if you can’t find the centrifugal switch. It has been years since I had to mess with one. If that relay is sticking, it will cause the capacitor to get rid of the internal smoke. Another thing is, potential relays are position sensitive.

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As in orientation relative to gravity, or rotor position? Not familiar with these.

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Gravity positional, Ie you cant put it 90 degrees & expect it to work right.

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What would that look like?

You’ll need to register to read the whole article, but the picture may be enough.

Ah, yes, that component I recognize from my AC… doesn’t seem to be one on this motor though. Very interesting!