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.

 

[dgallagher]

Randy - do you have any VFD's operating on the same circuit? 21,600 cpm seems to be a magic number that VFD controllers have been seen to feed back through the entire system. If there is a VFD, I would turn it off to see if the 21,600 goes away. You may have to tune the VFD to a different frequency.

Regards,
Dave G.

http://www.reliabilitydirect.com

 

[JAF8871]

Dave,

No VFD's anywhere near these two motors. They are both across the line start.

Regards

Randy Fizer

 

[electricpete]

24 fan blades?

Maybe a 2*LF side-band of rotor bar pass frequency pattern?

Maybe you have rolling element bearings? (probably not on this large motor)

Maybe it would help to take a look at your time waveform in g's.

 

[JAF8871]

Electricpete,

Motors drive fans with 16 blades. Motor rotor bar count is 90. Motor does have rolling element bearings one end is a 6324 the other end is a 6234. Time wave form has major peaks that vary in frequency depending on where in the time wave form you make the measurement but all are very close to 21600 with an amplitude of .8 g's peak to peak. This frequency dominates the wave form. Each peak top and bottom has a smaller double or triple peak within it. This double peak is at about 114,000 CPM. No amplitude modulation within the time wave form. The 21,600 peak is 8.2 times 2FL's away from the rotor bar peak.

Regards

Randy Fizer

 

[electricpete]

I just figured out that 21600 is exactly 6x line frequency and roughly 24 times running speed (you guys probably already knew that). Running speed would probably be 21525-21550. If you are confident that you have pegged the exact frequency, that would certainly suggest electrical 6xLF. The fact that you have other harmonics of LF in your spectrum would also tend to confirm.

I'm stupmped.

 

[dgallagher]

If the 6324 bearing is SKF, then 21,600 is almost exactly 11 times Ball Spin frequency. Odd that BS would be excited by itself... If you can capture a coast down, use your analyzer and turn the machine off - if the fault is electrical it would immediately disappear.

 

[JAF8871]

Bearing either SKF or FAG. Agree BSF alone is odd.
I would think the 11th harmonic of BSF would be very low though I have no experince to back this opinion. Coast down good idea. Hard to shut machine down as it is a mine ventilation fan, but if opportunity comes I will do the coast down to see if peak disappears. If it does, then perhaps the future will bring another point of evidence that will lead to the root cause. Would motor current analysis be of any use?

Regards

Randy Fizer

 

[Franko]

It appears to me that you have instrumentation noise. Does six times line frequency show up on the driven unit? If your probe is a velocity probe, try putting an aluminum can around it to shield it - also covering the connector. We used to do that before switching to accelerometers for motor monitoring for proof runs on test floor of our Elliott PAP plant air compressors (compressors have shaft probes). IRD (now Entek) used to see a shield.

If not instrumentation and not electrical as proven by transient analysis at electrical cut-off, after shut down, a rap test could show if 21600 cpm is a highly responsive natural frequency of the motor case (very doubtful.

 

[JAF8871]

Franko,

The 21,600 or six time line frequency does not show up on the driven unit. Using an accelerometer for vibration data. The motor is fitted with various tranducers for temperature and vibration data. Will check with electrical department to see if there is any data transmission at this speed. Thanks for the idea.

Randy Fizer

 

[rondud]

21,600 is typical of a unit with 6 SCR's. Is this a possibility in your case?

 

[Irajak]

Randy,
The culprit may be resonace of the stator. The motor's stiffness is changing with operation and the 6 times line frequency and stator resonant frequency may be getting closer to each other. Check vibration of motor housing between bearings. If amplitude is higher, I would suspect resonance.

 

[MJP80]

Your fault of 21,600 is an electrical, not mechanical fault. You are finding the same fault in 2 different motors (60 HZ) and the vibration is not in the driven. Typically, 21,600 is an SCR fault or broken motor windings in a DC drive system, but you have AC. It is definitly electrically (not mechanically) induced. It would be worth the money to have an electrical vibration consultant test your equipment. CSI, Technical Associates, or PdMA are very good sources.

 

[JAF8871]

Irajak,

Thanks for the suggestion. It is one I had not considered.

Randy Fizer

 

[McVibe]

I agree that this is an electrical fault. Let us know when and how you resolve it.

Alan Friedman

http://www.DlIengineering.com

 

[JAF8871]

Mcvibe,MJP80,

I agree the fault is electrical. And if and when I resolve it, I'll be sure to let you know what I found. The machine drives a mine ventilation fan and must run 24/7 just like a lot of your machines and is difficult to schedule for outage so it may be some time before I can resolve.

Thanks though for your help.

Randy Fizer

 

[electricpete]

I don't think 6x line frequency would be an exciting frequency in a normal motor. Therefore it shouldn't appear even if there is a resonance.

2*LF is a normal exciting frequency. If it is sinusoidal it creates only 2*LF in the spectrum. But if there is some kind of impacting triggered by the 2*LF force, then you can get 2*LF, 4*LF, 6*LF etc. And yes the highest of those will likely be near a resonance.

What could create impacting at 2*LF? One thing is coils loose in the slots.

I have seen firsthand in our plant 4000 hp 13.2kv vertical motor. We disassembled it for other reasons (loose lower bearing indicated by "swing check&quot and found the coils extremely loose in the slot. Due to time constraints we couldn't rewind and we rewedged instead. Plan to rewind next year.

I went back and looked at the data on log scale. There was 0.02ips at 2*LF and 0.02ips at 4*LF. When we rewedged the 4*LF went away and the 2*LF remained the same.

Coils most at risk for becoming loose are those originally wound with B-stage winding.

I don't want to alarm you on this one. As was mentioned electronics is a common cause of those frequencies. Also I am always reluctant to get too excited about low magnitude frequencies that I don't understand... there are plenty of them out there.

If you have windings very loose in the slot it is commonly accompanied by detectable ozone >50ppb in the exit airstream. That can be very easily tested using very cheap equipment (Draeger pump and tubes) which I am guessing your industrial safety people have readily available in mining industry.

 

[Franko]

Before taking it apart to make sure it isn't loose coils - check accelerometer cables/grounding - did you try switching between floating and grounded if your system amplifiers have the option.

http://www.globalspec.com/wilcoxon/ref/AccelInstall.html

http://www.reliabilitydirect.com/newsletter/vol1issue3.htm

Usually we have had line frequency noise with accels - but sometimes harmonics show up - one case was due to interference from crane controls.

 

[electricpete]

I never suggested taking anything apart. I suggested checking the ozone. A simple non-intrusive check.

 

[Franko]

OK - sorry about that - I read yours and posted a bit later. I never was very good at EE - still don't know if the electrons go from plus to minus or reverse. My great retired technician said it doesn't make any difference - just keep away from power line connections.