Hi Guys,

am attaching a spectrum of a 2 pole 100 Hp motor, antifriction bearings.Amplitudes are normal but can see what i think is slot pass freq with 2Lf sidebands which does not worry me much but TWF shows distinct impacts 2 per rev.

Ur comments on the plot would be highly appreciated.

motor_spec.doc (28 Kb, 88 downloads) Motor Plot

First, I assume you have checked there is not much beyond your Fmax (digital overall is pretty close to analogue overall).

In that case, I'm pretty sure that what looks like impacting at first glance is actually a smoothly modulated waveform associated with the RBPF +/- 2*lf pattern.

Roughly speaking, it would be a modulated waveform with the "carrier" frequency in the neighborhood of RBPF and the "envelope" frequency around 2*LF. The 2*LF envelope is what looks like 2*TurningSpeed impacting (LF very close to TurningSpeed in this case of 2-pole motor). You could zoom in and check the spacing of the carrier within that envelope to verify it is in the neighborhood of 2000hz.

Then the assessment remains the same. It is a rbpf pattern and I agree with your assessment that it is not cause for concern.

This message has been edited. Last edited by: electricpete,

I am in complete agreement with Pete.

Just would like to add that impacting and modulation look differently in the TWF with impact having sharp rise and exponential slope down vs. modulation having an envelope resembling a SIN wave.

Viki,
Pete is right; this is a rotor bar pattern. Your cursor is on the tall peak at 40 orders, but the actual rotor bar frequency is probably the peak that is 7200 cpm to the left of the cursor. I would say that 80% of the ones I see like this turn out to be eccentricity. I used to turn these in as a rotor bar problem and our motor testing guys would come back with the eccentricity diagnosis. Try checking for soft foot.

David Eason

Good point about the soft foot check. I have heard of that reducing RBPF problems on some 2-pole motors before. (Other times high RBPF pattern vib is just a characteristic of the motor design).

Thanks Guys,
Yes u are right it looks more like modulation. Checked up the reliance motor chart it says 38/40 rotor bars for this capacity motor so one of the peaks is the RBPF with 100 Hz sidebands. I raised quite a few false alarms earlier on seeing this pattern but current spectrums did not reveal anything and as pete mentioned some healthy motors also show this pattern.

Yes David i will check for a soft foot

David, just to clarify, are you saying the sideband peak of 2xlf off of RBPF is indicative of eccentricity, or an elevated peak at 2xlf, or both? Were there certain checks performed by the motor testing guys that verified eccentricity?
viki, I seen this signature on certain motors for years. Was this the first read on the motor or has this condition just developed?

Sid,
This the first time i am measuring on this motor so dont know the history but appears no problems earlier. This motor drives the fire fighting sprinkler pumps so they only run for a short period for weekly testing.

I checked with Fmax upto 10k but found nothing beyond what i posted.There is another identical machine which has a similar spectrum but more signifcant on inboard horizontal,posting that plot too.

motor_spec_2.doc (29 Kb, 24 downloads) Motor Plot 2

Sid,
Like you, I have seen this look a lot. I always thought that the tallest peak was the rotor bar frequency until we started a motor testing program. I would tell the sparkys what I thought rbpf was and they would correct me. It always ended up being 7200 cpm to the left of the tall peak and they almost never found a rotor bar problem. There is probably a better "formal" procedure, but I developed my own 3-step check. When I see a pattern like the one viki posted, I look at the trend. The tallest peak has to be .06 ips and rising. If it isn't let it go. Second, I look at my PeakVue for a tall peak at 7200 cpm. The trend has to be rising in this one also. Third, take a high resolution reading (24,000 cpm fmax, 6400 lor). Look for sidebands of pole pass around running speed or harmonics of running speed. If I meet all three of these hurdles, the sparkys usually find a rotor bar problem. If I don't find all 3 of these, they usually find eccentricity.

David Eason

Thanks David. I'm curious how eccentricity was verified by the electricians. Or, is it assumed that since no rotor bars may have been found, was eccentricity only presumed? Thanks in advance.

Sid,
I'm not a sparky, so I can't talk very intelligently very far on this, but they use the PDMA MCE tester and eccentricity is one of the things they can detect with it. Hopefully, somebody familiar with PDMA will explain it to both of us.

David Eason