single phase ac motor question

My furnace is randomly throwing the circuit breaker. It’s an old furnace with a new starter relay and a 10 year old 1/2 horse a/c motor.

The motor spins freely as does the blower. It acts like there’s a bad spot for starting that draws excessive power. I indexed the motor with some paint and it tripped at the same point as before. haven’t had it repeat yet.

My house was exposed to an over-voltage event from bare primary wires. pretty much anything with a transformer took a hit and the relay was one of the items. It’s been replaced. A year later I’m having problems with either the new relay or the blower motor.

Is there a test I can do on the motor that would indicate a bad spot in it’s rotation?

Disconnect the motor leads. With an Ohm meter clip on side to a motor lead and the other to a ground. The reading should infinite. Spin the armature and see if it changes as it goes around.

The motor may list it’s resistance, if you clip the ohm meter on each side of the lead it should match that resistance. Once again spin the armature and see if it changes.

In either case if it doesn’t remain consistent it’s a lost cause. If a motor is under 3HP it’s almost always cheaper to replace than it is to rebuild.

Also, while you are checking the winding resistance, use your handy dandy meter to check for a short between the windings and the case as you spin it around.

It could be just a bad motor capacitor. Or if it has a starting winding and switch, the switch may not be releasing at the proper speed.

If it doesn’t start and just buzzes then blows the breaker, then the internal switch that connects the starting winding is likely intermittent. There is a mechanism that opens this switch as the motor comes up to speed.

Dust in the mechanism, or burning of the contacts may be the issue. If it were me, I would open up the motor, clean everything, dress the contacts (emery paper) and put it all back together. Then when it keeps doing the same thing, I would go online and order a new motor like I should have done in the first place. The upside is that I will be really good at getting the motor out of there the second time.

Check the start capacitor for swelling or leakage, and with a vom. The starter switch is another place. Also, fan motors are exposed to a lot of dust, even with regular filter changes. This tends to wear the bearings. Even if it spins freely when unpowered, upon power up the armature can move enough to rub the field windings. Find your local furnace supply store and ask if they can check the motor. If you get a new one get a new cap too.

thanks for all the responses. I will check it with a meter this weekend. I didn’t know these had starting capacitors inside them. I thought those were external devices.

this is exactly what I’ve experienced. It buzzes and then trips the breaker. the relay go hot enough that the plug worked itself out of the socket over time. It snapped back in place so I think that’s OK. I also didn’t know they have internal switches but the motor did sound like there was a mechanism involved in spool up. Or more precisely spool down. Almost like there was a clutch in it.

I suspect it’s going to call for replacement but I’ll definitely want to tear it down to see all the internal workings. I thought it was just a bunch of windings.