Re: OT - electric motor issue


"James Sweet" <jamessweet@xxxxxxxxxxx> wrote in message news:EP4Oj.13$wO1.1@xxxxxxxxxxx


"Smitty Two" <prestwhich@xxxxxxxxxxxxx> wrote in message news:prestwhich-146490.10093618042008@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
In article <wf4Oj.7$kt1.0@trndny06>,
"James Sweet" <jamessweet@xxxxxxxxxxx> wrote:

"Smitty Two" <prestwhich@xxxxxxxxxxxxx> wrote in message
news:prestwhich-349A04.08280818042008@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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

.

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