"James Sweet" wrote in message
news:EP4Oj.13$wO1.1@trndny04...


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

"Smitty Two" wrote in message
news 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.
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