Devilbiss motor just jumps and hums til breaker trips.

by Chris
(South carolina)

Compressor motor will just jump and hum, almost like it is locked up. Sometimes it will happen on start up, sometimes it will work fine for 20 cycles of starting, then happens again. There is no rhyme or reason that I can see. I had motor checked out by reputable motor co. I have checked the check valve and unloader switch. All working as they should.


Also put new breather on in hopes it was that. It is an old devilbiss, looks like the 447, but has 7502 on tag, with the unloaded on the side of motor.

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Jun 27, 2017
hum and trip.
by: Doug in s.d.ca.

Ah!. 3phase - so the centrifugal switch I was thinking of is not present.

The positional thing wouldn't apply unless the thing has brushes. But it's induction, yeah?

So that leaves only the starter. I think what must be happening is every now and then one phase of the power fails to get connected. If all three phases are present, the thing will self start. Miss one, and it can't decide which way to go, or hums.

Maybe you want to hook up three indicators and run it until it fails, and I'm sure one indicator won't register. Might be a loose wire somewhere, or one of the contacts. Even when it runs, there may be enough voltage difference that you can pick out which phase is deficient.

Good luck and lets us know what you find, please,

Jun 27, 2017
Devilbiss motor
by: Chris

It is a 7.5 horsepower 3-phase belt drive. It does have a starter, and seems to be working fine. Motor was checked under load. Centrifugal switch seems to be working good, it releases pressure at the end of the cycle. Someone told me there could be a bad spot in the motor that only shows itself when it tries to start in a certain position. Does this sound like a possibility?

Jun 27, 2017
"Devilbiss" motor
by: Doug in s.d.ca.

It might matter what model you have - if you can't determine that, then at least if it's belt drive and motor HP.

Sounds like you've done all the common issue checks.

If it starts strong when it does start, then there's only about three possibilities aside from mechanical.

Did the motor check include cycling the motor under load to check that the start winding and the centrifugal switch are staying good?

That's two - and three, how are the contacts in the pressure switch and magnetic starter, if one is used?

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