"Smitty Two" wrote in message
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In article wf4Oj.7$kt1.0@trndny06,
"James Sweet" wrote:

"Smitty Two" wrote in message
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Our air compressor at work is powered by a 5hp, single phase, 230 volt
motor. Often when the compressor tries to start again (triggered by
falling tank pressure) the breaker trips. By rotating the motor by hand
prior to resetting the breaker, the motor will then start. Since this
happens between two and five times per day, we're getting tired of it.

The nameplate says the motor draws 24 full load amps, so I had the
electrician (who was there for another job) pull the motor off the 20
amp breaker and put it on a 30 amp. (wire gauge sufficiency verified.)

Still trips, and not knowing what else to do, I replaced the motor
starting capacitors.

That didn't fix it. So, before taking this to a motor repair shop, or
replacing it, is there anything else an idiot could look at and
possibly
fix? TIA.


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.