"Smitty Two" wrote in message
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Smitty Two wrote:

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.

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available, even though the motor is obsolete.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?
http://www.surpluscenter.com/item.as...tname=electric

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.