Recently, I've had a two-speed motor tripping motor feeder breaker (instantly) on high speed only.
First off, shown below are the motor specs
low speed: 480V, 100HP, FLA = 161amps, SF=1.0
high speed: 480V, 200HP, FLA = 225amps, SF=1.0
Motor starter is using a three contactor configuration: one for low speed, and one for high speed, and the third contactor is used to bring leads T1, T2, and T3 together during high-speed operation.
Normal opeartion use: blower
Operation procedure: start motor at low-speed first; after motor speed is established (and process conditions satisfied), hit "stop" button and then hit fast-speed start button to bring the motor to hi-speed.
Now, back to the question and how we troubleshoot: We disconnected the motor at the service head, and then energized the feeder and tried both low and hi speed, breaker did not trip. So we eliminated the potential problem of a feeder fault; then we de-coupled the motor from load and connected the leads back to the motor and solo run the motor-----it turned out that low speed is running fine, but as soong as we switch to hi-speed, it trips, and this is the same symptom with the motor having load on.
Interesting thing that we observed was that, during solo motor run, the motor was already pulling 175amps on all three phases,while its FLA for low-speed is 161.
We suspect that the motor is bad and possibly has a internal winding fault, however insulation resistance test didn't show any obvious results--we megged all six leads of the motor to ground, but each lead is reading above 6 Mohms.
So my question is, what could possiblly cause the trip? What kinda of internal motor internal fault could cause the motor to operate on low-speed only but not on hi-speed?
First off, shown below are the motor specs
low speed: 480V, 100HP, FLA = 161amps, SF=1.0
high speed: 480V, 200HP, FLA = 225amps, SF=1.0
Motor starter is using a three contactor configuration: one for low speed, and one for high speed, and the third contactor is used to bring leads T1, T2, and T3 together during high-speed operation.
Normal opeartion use: blower
Operation procedure: start motor at low-speed first; after motor speed is established (and process conditions satisfied), hit "stop" button and then hit fast-speed start button to bring the motor to hi-speed.
Now, back to the question and how we troubleshoot: We disconnected the motor at the service head, and then energized the feeder and tried both low and hi speed, breaker did not trip. So we eliminated the potential problem of a feeder fault; then we de-coupled the motor from load and connected the leads back to the motor and solo run the motor-----it turned out that low speed is running fine, but as soong as we switch to hi-speed, it trips, and this is the same symptom with the motor having load on.
Interesting thing that we observed was that, during solo motor run, the motor was already pulling 175amps on all three phases,while its FLA for low-speed is 161.
We suspect that the motor is bad and possibly has a internal winding fault, however insulation resistance test didn't show any obvious results--we megged all six leads of the motor to ground, but each lead is reading above 6 Mohms.
So my question is, what could possiblly cause the trip? What kinda of internal motor internal fault could cause the motor to operate on low-speed only but not on hi-speed?