I have a customer that has a 240 V three phase motor that has been running fine up until yesterday. Suddenly, it starts really slowly and will trip the breaker most of the time. Once in a while it’ll start up and get to full speed and it runs fine. I don’t recall the horse power offhand but the nameplate said it was around 18 A. When it starts, it’s pulling 115 A and is slow to get up to speed. I pulled the wiring cover off the motor, thinking maybe there was a cap inside that I could replace but there are only wires. Any suggestions on what might be the problem? Nothing has changed with the wiring and the voltage on all three phases is correct.....
3 phase motor problem
- Thread starter ammklq143
- Start date
- Status
- Occupation
- Licensed Electrician
Bad connection somewhere in the circuit, maybe a bad bearing.
Did you check voltage and current on all three phases?
Has anything else in the building changed?
Did you check voltage and current on all three phases?
Has anything else in the building changed?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Electrically, on a 3 phase motor the only thing that could cause this is a bad connection, i.e. a partial single phasing condition. If one phase was completely lost the motor would not start at all, but if one phase is connected but not well, i.e. high resistance, it might try to spin, but is not creating enough torque. The fact that it SOMETIMES works fine would make me look at the contacts in the motor starter.
That's either a really inefficient 5HP motor or a really efficient 7-1/2HP motor.
that's basically Locked Rotor Amps, normal, other than the fact that it doesn't get to speed.
There would not be any caps inside of a 3 phase motor...
If it's not a bad contact in the motor starter, it's going to be a mechanical problem; either a bad bearing somewhere as previously mentioned or something in the load that is causing the motor to stall, in which case it is not an electrical issue.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You can differentiate between a mechanical problem and partial single phasing by measuring current in all three phase lines to the motor.
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Sent from my XT1585 using Tapatalk
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
5:1 odds it's an air compressor and another 5:1 that the unloader is stuck closed so it's trying to start the compressor loaded.
5:1 odds it's an air compressor and another 5:1 that the unloader is stuck closed so it's trying to start the compressor loaded.
I spend a good part of my career in the compressor industry. That’s where I was headed!
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yeah, after the 50th time seeing the same tham ding over and over again, you learn to expect it...
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I've started a compressor while loaded.. . . it's trying to start the compressor loaded.
Yes, that would be my starting point. That and the voltage. It could be a bad connection somewhere/anywhere on the supply side.
Or just a bad motor. How much would it cost to replace it? Probably less than several site visits.
Good point. I did the calcs and they don't equate to 5HP (3.7kW) motor.
Thank you for all of the input. A little more info on this. It's a 7.5 hp motor that has a fan wheel on it for a filtering system so it's not under any real load at start up. I can spin the fan wheel and it spins easily and for quite a long time before it comes to a stop.
I checked the voltage on the line side of the motor contactor and it's good. I'll try to stop there again today and see if I can get some voltage readings while it's starting up.
Thanks again.
I checked the voltage on the line side of the motor contactor and it's good. I'll try to stop there again today and see if I can get some voltage readings while it's starting up.
Thanks again.
Nothing else has changed in the building. I'll try to stop there today and do some more checking.
My first thought was single phasing too. I checked the voltages on each phase and they were good but haven't done so under a start up condition to see if there's a bad connection. I'll try to stop there again today to do some more testing. Thank you.
I went back this morning. The load side of the motor contactor had one phase that wasn’t making a good connection. I took it out from under the screw, stripped it, and reinstalled it and it works again. Thanks for all of the help and input. Greatly appreciated.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Glad it worked out. :thumbsup:
- Status