Need a second opinion on this 8-pole motor from you gurus out there. The motor has a pulsating noise and slight speed variations (pulse frequency close to polepassfrequency). See attached PowerPoint.

Thanks in advance.
Arild F.

G30002__MILL_1_26FEB07.ppt (748 Kb, 193 downloads)

I'm no guru, but that never stopped me from chiming in before.

It sure has the appearance of rotor bar defect.

How did you conclude there were speed oscillations... strobe?

What does this motor drive... any chance the load induces oscillations?

The best verification of the rotor bar defect conclusion would be a current signature analysis looking at the magnitude of pole pass sidebands around line frequendy

Failing that, you might check some other things:
1 - is there a permanently installed ammeter and is it oscilating. Or if you can at least get an electrician's clamp-on... does it oscillate?
2 - check if your slip is higher than expected for the load level (sign of rotor bar defect)
3 - check if if the sideband spacing changes with load in the manner that pole-pass would. (doubling the load would double the spacing if it's pole pass).
4 - do the sidebands increase over time after starting indicating thermal dependence?
I think answer yes to any of these helps points even more towards rotor bar but current signature would be the most definitive.

This message has been edited. Last edited by: electricpete,

No guru? In this case I think you are
The measurements is from a papermill I visited last monday and the electrician there wanted me to look at the spectrums.
Speed oscillations were verified by the local electrician with a strobe.
I will forward your recomendations to them, they will be able to check this without my help I think (except maybe the current signature analysis)
If we get a conclusion I will post it here

Thanks!!!

Arild F.

A little more info:
First measurement without load, speed ≈736 rpm. Sidebandspacing 1,8hz.
Last measurement with load, speed ≈ 731 rpm. Sidebandspacing 2,75hz.
(Speed calculated from harmonic cursor in spectrum)

That answers question #3. The sideband is moving in the manner that would be expected for pole pass sideand. That rules out a load torque oscillation with high confidence.

I got a call from the mill today, the noise has gotten worse and the motor will be taken out and sent to a motor shop tomorrow. Asked them to send report and pictures from the overhaul if possible.

Arild F.

The motor shop has verified that it actually was a broken rotor bar

I will post more details if I get the report

Arild F.

This message has been edited. Last edited by: ArildF,

For the sake of discussion I believe speed variations at PPF is sufficient as supplemental to vibration spectrum evidence of broken bars. I wonder how many?

David

This message has been edited. Last edited by: David_G,

THANKS FOR YOUR INFORMATION

So what broke the motor rotor bar?

Still no report, but I visited the motor shop yesterday and took some pictures. The soldering on 5-10 rotor bars has melted down.
Pictures here:
Jylhavaaramill #1
Rotor
Rotor, close-up
Rotor bars, melted soldering

This message has been edited. Last edited by: ArildF,

Thanks for sharing the results of the rotor inspection.

That rotor sure is in bad shape.

Do you have any thought as to why the fan blades are discolored? Heat conducted from the rotor bar joints?

quote:

Originally posted by electricpete:
...Do you have any thought as to why the fan blades are discolored? Heat conducted from the rotor bar joints?

The local electricians told me that the motor "burned" late last year and was taken out for rewinding. It was not tested under load after repair. The noise started just after reinstallation (february-07?) and got worse over time.