OT - electric motor issue

The starting amps are much higher than the full load amps, for a 5HP motor you're probably looking at around 100A for a split second as it starts up. I suspect the wire run is long, or you have some resistance somewhere which is delaying the motor getting up to speed long enough for the breaker to trip. One option is to run the circuit with heavier wire, though a likely better option is to install an unloader valve on the compressor which will greatly reduce inrush. These release the pressure on the line between the compressor and the valve so that the motor isn't working against the tank pressure when it starts up. Also if you haven't changed the oil in the compressor recently that wouldn't hurt.

That all makes sense, but the reason I think it's an internal motor issue is that the motor seems to have one or more spots that create hard shorts. Rotating the motor by hand, even through 20 degrees or so, enables restart without tripping the breaker. We've observed this repeatedly: Once the motor stops in a given position and trips the breaker on attempted restart, the breaker will trip repeatedly until the motor is rotated, and then it will always start without tripping the breaker. Is my logic faulty?

That's interesting, I've never seen a fault like that in an induction motor. It really does sound like a problem with the motor itself. If a specific position causes the breaker to trip I would suspect the rotor is damaged, there's not much to these but I suppose it's possible.

The rotor of an induction motor is filled with shorted turns. This is what makes it an induction motor. If some of those turns become open, the fault you observe will be the result.

David

I think you've covered it all except if the motor was a repulsion/induction motor then it would have brushes and commutator. However this motor has a start cap so it would be my educated guess it's just an induction motor with a weakness in the motor windings.

Check your unloader valve.

Is that a problem that you would characterize as economically repairable on a 5 hp motor?

I would assume it to require a new rotor, in which case probably not, but a motor repair shop probably has some good used motors available. Depending on the nature of the fault, it might be possible to fix it, I'd at least take it in for an evaluation if you don't feel like opening it up yourself.

I'd go with that. Although very unusual for a motor like this. The rotor is just a pile of steel plates with the bars of the squirrel cage embedded in them and welded at the ends.

Is it always the same orientation of the rotor?

Another possibility is that the centrifugal or other starting switch is somehow not closing so the starting winding is not being energized. I don't know how rotating the shaft a few degrees could cure this though.

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I don't know that.

If I monitor an analog voltmeter on the starting caps while I cycle the power switch until it fails to start and the breaker trips, would that tell me whether it could be the centrifugal switch?

Yeah, if you dont hear a 'hiss' when it shuts down it's very likely the unloader valve needs attention.

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Cheers ............. Rheilly

I have had experience with a motor with a bad squirrel cage ! It ran a little rough and was way down in torque. It also got a lot hotter than it normally did.

I tried to solder the copper bars, two of them, where they had cracked at the end. A waste of time ! A new rotor cured the problem, curtesy of Brook Motors.

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Best Regards:
                     Baron.

Status report: I've traced this to a malfunctioning centrifugal switch, as suggested, but still cannot account for why a small manual turn of the motor would "fix" that, particularly 100% of the time.

The mechanical portion of the switch seems fine, so I have to suspect corroded contacts. Unfortunately, they're buried inside a metal shell and all but completely inaccessible. But, one half of one of the two contact pairs definitely shows erosion, in profile view.

I put a call into the manufacturer today to see whether parts are still available, even though the motor is obsolete. They have to do some research and get back to me. Failing that it looks like we'll be out $500 or so for a new motor. That'd be a shame, given the otherwise perfect condition of it.

Would this work?

They have a 1750 RPM model for about $100 more, not sure which your compressor uses.

The motor you have is probably better built, if you can figure out a fix for the start switch.

Thanks again to all respondents. Tracking down a replacement centrifugal switch was a bureaucratic nightmare of epic proportion. Cost about $42 plus shipping, so not counting time and frustration, about 1/10 the price of a new motor of same specifications. Compressor has been up and running fine for about a week now.