Three Phase Motor Potential Single Phasing

[Nardie]

I have a three phase 10hp motor, that when we restarted today tripped the heaters within 10 seconds. The voltage system is a B-phase corner grounded delta system, which means that B-phase is grounded so there is no fuse installed for B-phase. We reset the heaters a few times, and tried to restart the fan. Each time the heaters tripped. We then checked continuity between the phases at the starter, and B-phase was open between the other phases. We also, megger tested at 500 VDC between each phase at the starter with the same results. We also checked B-phase to ground and determined there was no short. The plan tomorrow is to determ the conductors at the motor, and perform continuity and megger testing to determine if it the power conductors or motor is open. My initial evaluation based on the testing results, is that I have an open winding on the motor creating a single phasing situation. So does anybody have any other thoughts or testing recommendations. Thanks

 

[LarryFine]

Does the motor rotate?

 

[BMANN06]

Does the motor rotate?

Thats my first question too?

Go to the motor junction box, disconnect feeders and meg the windings from there. You may find its your feeders, very rare that you would burn open a winding without it going to ground. BTW did you meg the windings to ground? I would expect your ACB/Fuses to trip on start but its worth the meg test.

 

[Nardie]

So the motor starts for about 5 seconds and then the heaters tripped. We determed the conductors from the motor and checked them from the MCC cubicle. We meg them at 500 VDC and performed a continuity test between the phase conductors and ground. The results were that all the conductors maintained continuity and were not shorted to ground. We did see some degradation of the wiring insulation, however again all the testing passed. We then checked the motor leads megger and continuity test, and believe we have an open winding. Please understand that the motor is in an environment that requires a lot of protective equipment, such as respirators which limit visibility and is very noisy, so communication is difficult. We plan to come back in tomorrow to double check our testing results on the motor leads. So any other ideas would be appreciated.

 

[zbang]

We did see some degradation of the wiring insulation, however again all the testing passed. We then checked the motor leads megger and continuity test, and believe we have an open winding.

All phase-to-ground show no continuity. Phase-to-phase fails continuity on one lead. Yes? Did you check at the motor's terminal box? Otherwise, it could be a bad connection anywhere between MCC and the motor itself.

You can also check the current on the suspected bad lead to see if it's drawing anything (actually, check the currents on all 3 leads).

While you're at it, make sure the motor rotates and that the load isn't bound up.

 

[BMANN06]

This is tricky to troubleshoot with the information we are given. A meg test isn't a continuity test ... posting actual values of your readings would be useful. I'm going to assume the following from the information you've posted and try to help as best i can. If i'm wrong let me know.

1. The motor does start and run - (this will rule out single phasing, contractor, feeds,)
2. Assuming your overloads are set up correctly, your saying it trips after a few minutes. This could be motor bearings that are on the way out, or bearings in the fan that are failing. Since this is a fan i'll assume its a belt drive. Shut off the ACB and remove the belts, turn the motor rotor by hand and feel for any bearing physical resistance. If that checks out ok. Do the same test on the rotor of the fan.
3. If the bearings feel ok, check the inlet/intake of the fan ductwork and insure any dampers are open (this is a fan not a compressor), also check the ductwork on the discharge (again this is a fan not a compressor) the air you are pulling in from the fan needs a place to discharge.
4. If you still haven't found anything after those tests, leave the drive belts off and start the motor uncoupled from fan, if the overloads still trip you have an electrical problem, if if runs fine, you have a mechanical problem.

Hope this helps.

 

[LarryFine]

1. The motor does start and run - (this will rule out single phasing, contractor, feeds,)
2. Assuming your overloads are set up correctly, your saying it trips after a few minutes.

He's saying 5 to 10 seconds.

 

[BMANN06]

He's saying 5 to 10 seconds.

Oh okay. I would think he should try starting it un-coupled from the fan first.

 

[Jraef]

If the motor was single phasing, it would not rotate at startup, unless there is something mechanically rotation it already, such as wind on a fan. But if it is a fan, it would likely run longer under single phasing because on startup the load is lower until the air actually starts moving. 5-10 seconds is basically locked rotor tripping.

My guess is a high resistance fault on B phase inside of the motor so that rotation is established, but with no OCPD on that phase because of it being a corner grounded delta, it only shows up as a current increase on the other two phases, tripping the OL relay.

 

[retirede]

If the motor was single phasing, it would not rotate at startup, unless there is something mechanically rotation it already, such as wind on a fan. But if it is a fan, it would likely run longer under single phasing because on startup the load is lower until the air actually starts moving. 5-10 seconds is basically locked rotor tripping.

My guess is a high resistance fault on B phase inside of the motor so that rotation is established, but with no OCPD on that phase because of it being a corner grounded delta, it only shows up as a current increase on the other two phases, tripping the OL relay.

What would happen if the motor was dual voltage wired for low and only one winding on a phase was open?

 

[kwired]

No continuity on B phase lead to motor means it could be open between motor and controller. If it were faulted to ground on motor end of this run it would still run normally, though it would trip motor overload if it has phase loss detection. If it were faulted to ground on motor end of that grounded conductor and windings are good you would still fail ground fault megger test.

Exact details of how some of your testing was done is important to know what actually was measured in those tests. Tests you done so far appear include the conductors between controller and motor, so is possible motor is good and there is an issue with conductors or a termination.

 

[Besoeker3]

(actually, check the currents on all 3 leads).

Yep. That's the first thing I would do. That'll give a quick and easy test to see if the motor is single phasing.

 

[Nardie]

So we went back in today with different mechanics, and they noticed that the motor leads were degraded. The connection was made with pink lady's, old wire nuts from the 80's. They stripped back the motor leads, and performed the megger test and continuity test again. All testing results passed, so the problem was more likely a bad connection at the motor. This explains the single-phasing causing the heaters to trip. I had a hard time being an open winding on the motor as ya'll believe. So I'm glad they found a bad connection. We plan to fix the motor leads, and repair some other issues and restart the motor.

 

[Nardie]

Today we completed the repairs, and restarted the motor. It has now been running for over three hours, I don't expect any other problems. I do appreciate the replies to the post. Thanks.

 

[LarryFine]

That's why we're here!