Was wondering how many here take high res readings to monitor pole pass in their standard route readings?  If so have you found issues?

Original Post

I usually make two measurements on the motor case near the terminal box. The first measurement is a high-resolution low frequency velocity spectrum for confirming motor speed and to detect rotor pole-pass sidebands. The second measurement is high-frequency acceleration to detect rotor bar and stator slot passing frequencies along with modulation sidebands. I have detected both electrical and mechanical faults with these type of measurements. There may be other ways to accomplish the measurements for the same purpose. I also make the same type of measurements on induction-type generators on hydro-turbines.

Walt

 

I do not generally take a high res reading to look specifically for PP symptoms. I will if other issues such as audible modulation make me suspect there may be a developing rotor bar issue or if rotor dynamic eccentricity is suspected.

Be interested to know what problems you have detected with the high frequency acceleration reading Walt? I don't doubt you have! Textbook states that RBF with 2x line frequency sidebands = rotor bar issues but I have not had much success with it. I quite often see this symptom in perfectly healthy motors (confirmed by motor current analysis) so tend to ignore it? This is something i have discussed with Electricpete and we both reached the same conclusion

 

Gary

Agree with Gary, we have low frequency hi resolution reading on ALL of our machines, only take it when we have driver and driven speeds that are 100 rpm or less speed difference, hear an audible noise or higher than usual vibration values.

Keep in mind, IF you have broken/loose rotor bars, the motor will have poor performance, lose HP, draw higher than normal amps, have a modulating noise coming from the motor, will run hotter than a normal motor, time waveform will have modulation and will show a higher 1x rpm. I find very few motors that DO NOT have RBPF with 2xLF sidebands, it is a common frequency in most motors that we monitor (thousand + motors) both in velocity and peakvue. Just because these frequency exists in a spectrum does not mean there is truly a problem.

PP sidebands around 1 2 3 4x rpm is a solid indication you have rotor bar problems, just have to have enough resolution to see them.

25+ years of collecting and analyzing vibe data, I have personally seen 10 or less motors with actual rotor bar problems. When we did find suspected problem we also did online/offline motor testing and monitored motor amps while under a load to help confirm an issue exists.

Below is a spectrum and waveform of vertical motor with rotor bar faults, data was collected a year ago and it is still running today.....

Dave

 "I will if other issues such as audible modulation make me suspect there may be a developing rotor bar issue or if rotor dynamic eccentricity is suspected."

Gary, if you want to rely upon your hearing and some other means of detecting rotor dynamic eccentricity that is fine. If it is convenient for you to go back to the machine and make additional measurements that is fine. If you're making a decision not to take certain data because some faults are very unlikely that is fine.
Most of the time spent and measurements made for routine condition monitoring is uneventful. It is human nature to question whether to make any measurements at all or to reduce the number of measurements and time spent. As I indicated in my previous response, there are other ways to achieve the same goal; or perhaps there are different goals!
Walt

Sorry should have added that I do take high res readings if there has been any change in the routine data such as harmonics, increasing overall etc. With my standard data set we will see 'skirts' around 1x 2x etc which would also prompt investigation. 

As with Dave I have seen very little by way of rotor bar problems in my 20 years of collecting data. I am just traking one now on a smallish motor - previously we had only ever seen the issue on large FD/ID fan motors and we collected routine motor current readings to monitor this - it was a known issue with these motors.

 

Gary

 

I agree with above. One thing to add is "dynamic eccentricity".  

We've recently tossed around the idea of whether dynamic eccentricity can cause increased pole pass sidebands around FL in current under load. We had lots of discussion on that but a main point for Gary and I was that we've never seen that.

On the other hand, I have seen pole pass sidebands around 1x in vibration on vertical motor at least twice where subsequent refurbishment showed loose housing fit on lower bearing, and the symtpom went away after refurbishment. I attributed that to dynamic eccentricity. (btw nothing ever showed in current sidebands)

Do we take high-res routinely?  I'd like to. Not only to look for pole pass sidebands but also to help identify bearing fault frequencies... something like 6400 lines Fmax=10x if it catches pole pass also (yes I know there are other bearing techniques but I like to have everything I can get). But It's been and back and forth struggle. Data is taken by others.  They tend to hate the extra time for high res spectrum.  The compromise is that we do take it on one point only for particular machines that I request.

Last edited by electricpete

"On the other hand, I have seen pole pass sidebands around 1x in vibration on vertical motor at least twice where subsequent refurbishment showed loose housing fit on lower bearing, and the symtpom went away after refurbishment. I attributed that to dynamic eccentricity. (btw nothing ever showed in current sidebands)"

I have made similar diagnosis on two different motors, one with ball bearings and the other with journal bearings. The diagnosis was confirmed by bearing repairs, and there was a considerable savings in maintenance cost and operation downtime.

Walt

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