Shaft rotational speed harmonics and cogging torque. 1

[TN1981]

To whom it may concern:

I am investigating potential causes of some shaft speed related tones in a 3-phase 6 pole electric motor. Using a laser vibrometer, one of the recent motors tested showed signs of elevated 3X, 5X, and 11X shaft speed tones (1X = 240 Hz). This blower was further analyzed on a crude cogging torque test fixture (I have atached the results of the cog torque test and laser vibrometer test for review).

The cog torque test showed a sharp transition in the 6th wave of 1 cycle compared to the other 5 waves. Could this sharp transition be the root cause of my 3X, 5X, and 11X tones? What would cause this transition? Variation in the magnitization process? Stator design? Magnet to stator concentricity?

I tried ot search the web but could not find anything specific to the motor design I am working with.

 

[GregLocock]

I suspect not. An event that happens once per rev is most likely to affect first order.

Have you tried a run up to see if the 5th and 11th harmonics are due to resonances? It is tempting to say that they might be 6-1 and 12-1 excitation respectively, but I'm not sure if electric motors do the -1 thing.

I'm not surprised to see 3rd order in a 6 pole machine.



Cheers

Greg Locock


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[TN1981]

Thanks for the input Greg.

I would have thought that the 1X would have been affected the most also but I didn't see much of an event at 240 Hz (1X)

We did a 3rd party analysis several years ago on 5th and 10th order and were able to trace it back to a magnitization issue but I think those 5th and 10th orders were high across the entire shaft speed range 6000-22000 rpm. So it was clear resonance was not the case there.

With these 6 pole motors, you are correct. We typically see varying 6X and 12X...sometimes 3X.

I will have to try a run up with this motor to see if the 5th and 11th are present in the entire RPM range. If so I guess I can rule out resonance at that point. If not...from your experience at a motor level, where should I look for resonance root cause? At a system level these motors are constrained by means of basically 3 rubber "trampolines" with a durometer of approx 30 shore A. However, this material can vary ±6 shore per the manufacturers specs. Could this variation be the cause?

 

[Strong]

The most common dynamic force for vibrations at 1x shaft speed is rotor unbalance. A resonant rotor or support structure can amplify vibration response. The unbalance force can act both radial to shaft axis and torsionally. Dynamic torque can be measured with a strain gage telemetry system and torsional vibrations can be measured with acceleroemters and telemetry or by the encoder voltage to frequency converter method. The whine you hear from a high speed dental drill is produced by rotor unbalance!

Walt