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Increasing Vibration Peaks at 21,600 CPM

Increasing Vibration Peaks at 21,600 CPM

Increasing Vibration Peaks at 21,600 CPM

(OP)
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.

RE: Increasing Vibration Peaks at 21,600 CPM

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.
www.reliabilitydirect.com

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
Dave,

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

Regards

Randy Fizer

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
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

RE: Increasing Vibration Peaks at 21,600 CPM

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.






RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
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

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
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

RE: Increasing Vibration Peaks at 21,600 CPM

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

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
Irajak,

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

Randy Fizer

RE: Increasing Vibration Peaks at 21,600 CPM

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

Alan Friedman
www.DlIengineering.com

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
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

RE: Increasing Vibration Peaks at 21,600 CPM

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") 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.

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

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

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
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

RE: Increasing Vibration Peaks at 21,600 CPM

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

RE: Increasing Vibration Peaks at 21,600 CPM

(OP)
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

RE: Increasing Vibration Peaks at 21,600 CPM

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

RE: Increasing Vibration Peaks at 21,600 CPM

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.

RE: Increasing Vibration Peaks at 21,600 CPM

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

RE: Increasing Vibration Peaks at 21,600 CPM

   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.

RE: Increasing Vibration Peaks at 21,600 CPM

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.

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