Increasing Vibration Peaks at 21,600 CPM

[JAF8871]

I have two 1500 horsepower, 4160 volt, 900 rpm motors running from the same 60 hertz electrical supply. Both are exhibiting increasing peaks at 21,600 cpm. Line frequency and other harmonics of line frequency are present also but not as high and not increasing. Rotor bar peaks and two times rotor bar peaks are showing up as well in both motors, but at less than .01 inches per second. The worst of the 21,600 peaks in the horizontal plane in 7 days has gone from .03762 inches per second to .05908 inches per second at the uncoupled end of one of these motors. The motors are operating at 897 CPM. While these levels are not that high, I am at a loss as to what might be causing them. Does anyone have any ideas or thoughts or experience with this particular problem?

Thanks.

Randy Fizer
Technical Services Engineer
Arch Coal Inc.

 

[edison123]

pete,

you mentioned about detecting ozone (which is persumably coming due to slot discharges which in turn are due to loose coils). I always thought slot discharges start at 6 KV and above. This is a 4 KV motor. Am I wrong ?

Another thing. If loose coils produce 2*LF and mutiples thereof, how come we missed 4*LF and went to 6*LF ? Just trying to understand.

 

[electricpete]

I forgot this was a 4kv motor. You are probably right that ozone may not be an effective check for loose windings in this motor. There really are no other easy checks that I know of short of disassembly and tap test. One possibility is that there may be limited access to the end-turn area and slot ends with motor in place. In that case you could do a little inspection but may not be conclusive.

Once again I am not recommending to disassemble the motor, just throwing out ideas.

If the loose coil scenario were true I would expect to see 2LF, 4LF, 6LF etc visible on log scale. 6LF certainly could jump far above the others due to resonance. I see very often an analogous situation in the presence of looseness: sometimes 2x, 3x or 4x will jump far above the others and machine bump test shows that is a resonant frequency. For such a high frequency as 21,600cpm it is not likely a machine resonance that could be confirmed by resonance but still can be resonance associated with transmission path from slot to frame.

If you put it on log scale and saw no peak at 4LF (blends in with noise floor), then definitely reject the loose coil scenario.

 

[JAF8871]

electricpete, edison123, Franko,

I just set up the spectrum with log scale and 3600cpm fundamental plus harmonics 2 through 8 are present in the spectrum with the 21,600 being the highest. The ninth harmonic is very low and buried in the noise. 10th and 11th harmonics of Fl are present too and large enough to be above the noise floor. Perhaps this will aide you further in aiding me. I really appreciate your help.

Thanks.

Randy Fizer

 

[electricpete]

Are the 2/4/6/8 a little higher than the 1/3/5/7?

 

[JAF8871]

electricpete, edison123, Franko,

Looked again at spectrum with linear scale and the 21,600 is sidebanded with line frequency peaks at very low amplitude. Two on each side of the 21,600 peak are very distinctive then several more that more or less fade into the noise.

Thanks.

Randy Fizer

 

[ronfrend]

Randy,
First of all - the 21,600 vibration is definitely electrical - I have seen that on a heck of a lot of motors. They are are several possible causes and they are all related to some form of distortion of the magnetic field(s).

Imbalance of the mains electrical supply. The worst case of this I observed was when one bank of the power correction capacitors went out. Get your electrician to put a scope on the supply and make sure it is clean.

Rotor problems including eccentric air gap. These will usually have sidebands at 1x.

Stator winding problems. These will usually have sidebands at pole pass frequency.

I would suggest you check out the frequency of the sidebands first and then make a plan for further analysis.

Ron Frend
tel: 011 44 1253 400541
ron.frend@predicon.net

http://www.predicon.net

 

[Chaotic]

What's your history like? Did you experience any restart issues after a recent maintenance job? I would stop looking at the motors (my I&E guys would flog me but..)and start looking at the power source(s). If you are in an area that was effected by the black-out, perhaps it effected the control curcuits. Especially since your experiencing it on two units at the same time.

 

[pickersa]

Hello Everyone;
This is a very interesting thread. We have the same problem here at the mine. A 6 times line frequency vibration that was first noticed about 7 weeks ago. Now we did confirm it was electrical by shutting down the fan for a few seconds and restarting it. Provided you don't have a complicated interlocking scheme you should be able to do the same. A few seconds of downtime is all that is required. We have significant peaks at 1x,and 6x LF. A smaller peak at 2x LF. Looking forward to hearing what you find out!
Al

 

[vanstoja]

Multiple harmonics of 6 times line frequency (6E) measured on motors and other electrical machinery is generally due to harmonic contamination of the motor's power supply and is not indicative of any malfunction of the motor. Electrical power supplies connected to variable speed controllers that rectify AC current then invert it to simulate sinewaves of different frequencies (for speed control)when connected to a power supply can input distorted sine waveforms to every other component on a common electrical bus. The worst distortion comes from 6-step inverters, followed by 12-step inverters which generate low order 6E harmonics (ie. 6E,12E,18E,24E). For pulse-width modulated (PWM) inversion, which provides a less-distorted quasi-sinewave, the low order 6E harmonics are generallly absent but the higher 6E harmonics are usually present (eg., 54E,60E,66E,72E, 78E,84E, etc.). These higher 6E harmonics, though present, may not be seen unless frequency spectra are obtained above 2000 Hz. For slower motor speeds like 900 and 1200 RPM, PWM-generated waveforms may show higher-order 6E harmonics as low as 30E.
The 6E harmonics in the noise/vibration signature are the result of interactions between odd-order forward and backward rotating harmonics (ie, 5th-7th, 11th-13th...etc.)in the motor's magnetic flux field.


motor's magnetic fluxfield.

 

[electricpete]

I agree 6LF can definitely be associated with power quality.

It's what I said above when I mentioned electronics... although the follow-on discussion focused on questions about loose coils which is another (remote) possibility.

At the levels you're seeing I wouldn't worry about it plain and simple.