Troubleshooting 3 phase motor

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Electron_Sam78

Senior Member
Location
Palm Bay, FL
Hey I started troubleshooting a 3 phase, 2 HP, 200V motor on a cooling tower that had been tripping a CB. Clamp on revealed A- 58 amps, B- 58 amps, C- 2 to 4 amps. This is a 20 amp circuit. Initial resistance test phase to phase with multimeter are bouncing from single digits to the 50s. Nothing to ground. With megger it is 0.0 ohms phase to phase with 277 VDC applied on the insulation test setting. Infinity to ground using the Resistance setting. There is another motor just like it in the same tower running fine. Thoughts?
 

growler

Senior Member
Location
Atlanta,GA
Hey I started troubleshooting a 3 phase, 2 HP, 200V motor on a cooling tower that had been tripping a CB. Clamp on revealed A- 58 amps, B- 58 amps, C- 2 to 4 amps. This is a 20 amp circuit. Initial resistance test phase to phase with multimeter are bouncing from single digits to the 50s. Nothing to ground. With megger it is 0.0 ohms phase to phase with 277 VDC applied on the insulation test setting. Infinity to ground using the Resistance setting. There is another motor just like it in the same tower running fine. Thoughts?

A motor doesn't have to be short to ground to be bad it can short phase to phase.

You can use your multimeter to measure resistance from phase to phase A-B, B-C, C-A and there should be a certain resistance in the windings ( just because of the winding length). These should be pretty close to the same thing. You could measure a known good motor to get a resistance value ( it's not going to be high).

For it to draw so much current on two phases you probably have an internal short of the windings ( this lets them connect before they are supposed to).
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Motor Single Phasing

Motor Single Phasing

Growler, I believe the OP answered his own question.
He stated he lost C phase (voltage.) This single phase condition caused the other 2 phases to
draw the high currents and trip the 20A breaker. Had the breaker not tripped then the overcurrent on A-B would have destroyed the winding insulation and, as you mentioned, would have developed into shorted turns in the winding, destroying the motor.
I'm not a motor guy but with this small of a HP motor I think the thermal OL protection is built inside the motor.
Once the proper 3 phase voltage is applied and the motor runs fine then it may be safe to say that no winding damage occurred; although the OL may have shortened the life of the motor. Any motor guys out there??
 

growler

Senior Member
Location
Atlanta,GA
Ok took a break and realized I forgot to check voltage. C phase is out! I blame Friday for my brain fart.

Growler, I believe the OP answered his own question.
He stated he lost C phase (voltage.) This single phase condition caused the other 2 phases to
draw the high currents and trip the 20A breaker.


You are correct.

The OP didn't have this additional information posted when I typed my response. I would have thought he would have checked for power first thing off. It's hard to make a decision based on limited information.
 

just the cowboy

Inactive, Email Never Verified
Location
newburgh,ny
readings jumping

readings jumping

You said it was a fan, if your readings phase to phase were jumping up and down the blade might of been turning in the breeze causing voltage on leads that was causing bad ohm reading. We have some big blowers like that and it is hard to get them to stop.
 

under8ed

Senior Member
I would expect a 3 ph. motor to have end user sized & installed overloads,
or an overload module with adjustable settings.
You should probably verify the sizing/settings and inspect the mechanism.
The motor may not survive single phasing a second time.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
You said it was a fan, if your readings phase to phase were jumping up and down the blade might of been turning in the breeze causing voltage on leads that was causing bad ohm reading. We have some big blowers like that and it is hard to get them to stop.

Cowboy
Thanks for this tidbit of info and attention to detail.
 

kwired

Electron manager
Location
NE Nebraska
Growler, I believe the OP answered his own question.
He stated he lost C phase (voltage.) This single phase condition caused the other 2 phases to
draw the high currents and trip the 20A breaker. Had the breaker not tripped then the overcurrent on A-B would have destroyed the winding insulation and, as you mentioned, would have developed into shorted turns in the winding, destroying the motor.
I'm not a motor guy but with this small of a HP motor I think the thermal OL protection is built inside the motor.
Once the proper 3 phase voltage is applied and the motor runs fine then it may be safe to say that no winding damage occurred; although the OL may have shortened the life of the motor. Any motor guys out there??
Very rare to see a three phase motor with internal overload protection. Have seen it a couple times on farm duty fractional hp motors but that is about it.

Proper size & operation should not have allowed the motor to run long enough to obtain those amperage readings.
I would suspect a problem in the starter, which may have also been either cause or effect of the lost phase.
58 amps is a little high but not so significantly high for a 2 hp 200 volt motor that it surprises me either. If it is a blower and is "windmilling" in reverse rotation when you hit it with voltage it may be within expected range. I have seen branch circuit devices trip on locked rotor current faster then a properly selected motor overload responds sometimes and this may depend on what trip class the overload is as well. Motors can take LRC for surprisingly long period of time. 30 sec to maybe a minute in some cases isn't too long, as long as it has time to cool before doing it again, but repeated resetting and immediate restarting or automatic resetting overload that allows immediate restarting doesn't give enough cool down time and will be harmful, and in that case it is helpful if the branch circuit device is set low enough to catch such a situation.
 

growler

Senior Member
Location
Atlanta,GA
Ok took a break and realized I forgot to check voltage. C phase is out! I blame Friday for my brain fart.

It has overload protection and I checked the Square D charts and it looks to be sized correctly.


I hate to ask this but it's another one of those simple things? You have a motor that's single phaseing and tripped breaker. Where did you lose that phase? Did you check the motor starter contacts?
 

Electron_Sam78

Senior Member
Location
Palm Bay, FL
Well it turned out that the buss finger was not secured to the main buss bar. The screw holding it on was so loose that the finger wasn't making any contact. In fact it was so loose that there was no arcing significant enough to pit the bars - just some light soot. Status: REPAIRED!
 
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