Partial Contact and Arcing

The short answer is that I do not actually know why one intermittent phase would cause your overload circuit breaker (o.l's trip?) to trip. I do have some possible reasons that come to my mind though. Like the following:

• The two phases were capable to start motion of motor and mechanical load. The timing of the third phase connections made for large surge currents to run through that third phase currents.
• The two phases were capable to start motion but with insufficient torque to generate enough speed to disengage the starter windings. The higher starter current eventually exceeded the thermal limit of your overload trip breaker.
• The motor torque is marginal for the mechanical load. This causes a much longer start cycle current draw. This eventually pitted a contact reducing the torque even more. Now the start current circuit never opens and the breaker prevents a fire.
• The poor contact and the tripping thermal overload breaker were in the same box. So the higher heat produced by the poor contact quickly raised the breaker's temperature, causing a trip.
• The evil contact gremlin after chewing on your arcing blades, decided to mess with your head by throwing the breaker during his get away.
• The excessive current from a motor winding short caused acing in one of your three phases. This failure current exceeds the current rating of your overload breaker. It's doing its job.

Thats just the ideas that comes immediately to my warped mind. On site testing might reveal a completely different reason than the ones I've suggested.

A 3 phase motor may overload when running on 2 phases since the two phases still trying to run the motor have an increased load.

A three phase motor will eventually burn out if run on two phases long enough.

The three phase motor running on two phases is dangerous since it will build up a large potentials. If you average all three phases the sum is zero since the differences in phases negate each other. If you average just two phases, the total is greater than a single phase.

Thanks guys for your quick responses and helping me better understand this situation . By the looks of the burnt blade in the disconnect it appears that it has been going on for awhile . After megging the motor and getting 2k ohms res. to ground on two of the windings I assumed the motor was at fault and replaced it . It wasn't until after that that we found the arcing in the disconnect . So the point made about a motor eventually burning out after running on only 2 phases long enough made me think I did the right thing . It was a 15 HP motor mounted about 5 ft. off the floor so it was not an easy task . Thanks again everyone for sharing your knowledge .

Since we're on the subject , I have one more question in regards to megging a motor . I understand that while using a megger you are generating a voltage to measure resistance to ground or between phases . Why is this generated voltage needed ? Why not just use a Fluke meter to check for resistance ?

Banjo,

Any time you are making a resistance measurement, you are using a generated voltage to do so. A megger uses a larger voltage than your standard DMM. This is because it is measuring resistance into the Megaohms (the reason for the Megger name). Hope that helps.

Tom

As the others have said, it is difficult to get an accurate measurement of high resistance with the fluke meter. If you try to use the fluke meter, you will see the measurement changing. Along with the high resistance comes capacitance. A small DC test voltage (and current) will slowly charge this capacitance resulting in a changing reading. A large voltage will completely charge in a shorter time and give a more accurate reading.

As usual , great responses guys . Your replies were well put and easy to understand . For that I thank you !