A client's synchronous motor failed recently due to stator earth fault. This is a 1500 KW, 3.3 KV, 750 RPM motor driving a Cement Mill through a gearbox. Two such identical motors were in operation.

We had tested these motors in 2006 & 2008, basically ESA & vibration data. The failed motor (Cement Mill 1) showed hunting/modulation in the load current back in 2006 also, this increased in 2008. The hunting at this point was significant enough to affect the captive power plant, with momentary power draw increasing drastically.

We could not diagnose any fault based on the data. The motor failed before we could perform any off-line tests to look at the insulation. The relevant data is attached.

Any opinions for causes of synchronous motor hunting &/or motor failure appreciated.

Regards,

Aditya

Cement_Mills.ppt (709 Kb, 56 downloads)

Here are my rambling thoughts (guesses)

I would not expect shorted turns to cause oscillation. I would perhaps expect high 1x vibration from shorted rotor turns (that's what we saw on our large sync turbine generator). I guess it's remotely possible that a large number of shorted turns could reduce the torque capability and lead to pole slipping, but I wouldn't picture that to be a regularly repeating event over a period of time. Of course if you get a chance to test the rotor, it couldn't hurt to double-check.

We had slow oscillations on a motor/sync-generator that was traced to oscillation of the generator field supply. We any measurements made of field current and voltage to check for oscillation?

You mention mechanical fault in gearbox/driven equipment. Certainly seems plausible as well, but I don't have much knowledge of gearboxes or mills.

The vibration signature was interesting. Looks like the healthy one (mill 2) has mostly running speed harmonics. The failed on (mill 2) seems to have a lot of non-sync stuff and increasing over time. Was it just on the motor, or the gearbox and mill also? What type of bearings and coupling? During investigation, it would be helpful to do mechanical checks of airgap, endplay, clearances etc in addition to electrical checks. Also gear backlash.

The frequency of the modulation has increased from 5.7 Hz in 2006 to 6.1 Hz in 2008. (At least those are the numbers I could read from the current waves). The Mill #2 has also modulation at ~ 2 Hz. I cannot imagine those numbers related to any possible motor or gearbox problem. Those numbers are related to the Mill itself, in my opinion.
I also think that the failure has nothing to do with the modulation. I did not notice if the mills are roller or ball mills.
jank

EPete,

You are correct about the vibration. Mill 2 motor is all running speed harmonics.

Mill 1 motor has strong peaks at the Gear Mesh Frequency & ½ the Gear Mesh Frequency. Both ½ GMF & GMF have strong sidebands at 6.15 Hz & harmonics. Now, 6.15 Hz is the modulation that we are seeing in the current, it also happens to equal ½ the running speed of the motor.

The vibration levels increase as we move from the motor towards the mill, going up to 25 mm/sec RMS on the plain bearing supporting the mill. On this bearing (SB3), the vibration is occurring at 32X (running speed is 2.05 Hz, 123 RPM) with sidebands of 3X (3X = 6.15 Hz), again the modulation we are seeing in the current.

The numbers must be telling something, I just can’t figure out what. The gearbox itself has 24 & 125 teeth; where could the 3X (or 1/2X, depending on where you look) & 32X components be coming from?

Jank,

The 2 Hz modulation on Mill 2 matches the output speed of the gearbox, must be coming from there (very minor though). I think the modulation has played a role in the motor failure. The modulation was causing the power requirement to shoot up (see attached document, Page 2), instantaneous requirements were touching 4000 kW, as compared to 2500 kW on Mill 2. This has to cause thermal degradation of the insulation. The motor was already more than 20 years old.

Regards,

Aditya

Cement_Mill_1_Vibration___Power_Curves.doc (376 Kb, 22 downloads)

Updated status:

The Cement Mill gearbox is connected to a counter-rotating shaft through a gear coupling. Seems the coupling gears were worn out (sharpened) & are being replaced.

For some reason, they are also replacing the plain bearings supporting the counter-rotating shaft & the mill with anti-friction bearings.

Still don't know how the numbers add up. If we get to go there, will take data & see what changes.

Regards,

Aditya