Sponsored by the Association for Maintenance Professionals Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About Motor Testing Internal Motor Problems
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I just returned from 2 days of seminars, some dealing with motor testing and identifying rotor bar, stator and other motor related problems. I have another 350 hp blower motor out there which I've been troubleshooting, but want to make sure my microlog settings are set properly to catch these rotor bar peaks etc...
This motor has pretty severe modulation going on to which leads me to believe stator, rotor, windings etc, but I'd like to pinpoint the problem if possible. You can plainly hear the humming up and down changing. Can someone suggest the settings I should use for this. They mentioned 6400 lines resolution but nothing about the FFT range, and accelerometer measuring locations. I'm assuming at the 1 & 2 Horizonal positions. Also would these problems show up more in the acceleration spectrum and or velocity? Or both. I've seen rotor bar frequencies in ACC before I believe. I'm starting to get deeper into these motor problems as you can see... Thanks R. Bell Baton Rouge, LA |
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Hi Rod,
This is how I have learned it in the field and it surely works fine: If there is a rotor bar problem then one of the prime indicators is an amplitude modulation of RBPF with PPF modulation frequency. So first make a guess or determine RBPF, 500 .. 1500 hz is a good guess for many industrial applications. Now in the FFT spectrum you will often need much more than 6400 lines to discriminate the PPF, so you need to find a trick. The trick is envelopping in Microlog, demodulation in VB Commtest, PeakVue in CSI, etc, etc. Different names, same baby. Select the filter that best captures RBPF. Set to ACC not VEL, max. no. of lines available, 4 .. 8 averaging. Frequency span 0 .. 200 hz. Show patience as this may take a minute or more. Measuring location does not matter, because modulation index is the same at any location, best results if sensor is somewhere in the middle of the motor frame close to the stator iron core. Observe the very left end of the spectrum: PPF?! Also observe 1*RPM and multiples in the spectrum: do they come with PPF side bands!? If 2 times yes then this is a good indicator for a problem. This test can also provide trend information but requires a reproducable measuring point for best results, like a stud glued on frame. You will also observe the 2*Fline, an indicator of air-gap non-uniformity. If belt worn out then you will find belt repetition frequency. The humming should also be PPF. Should work fine is USA too, give it a try!Regards Arie Mol NL |
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Thanks Arie,
I'll set that up and give it a try. I was just looking at my Technical Asso. wall chart and it shows only 3200 lines. I'll kick it on up there. I guess I need to back down on the FMAX too, our are set way out there to 240,000 I'll try that.
R. Bell Baton Rouge, LA |
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Here is the data I took on the 2HACC point which has been trending up. I still need to go take some current analysis reading and determine the db...This scan is raw data I just took at 6400 lines & 0-200 htz
What do you think?
R. Bell Baton Rouge, LA  |
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Sorry,
I forgot to mention the the slip freq is 23.3 X 2 pole=46.8 cpm so those peaks are at excactly that distance...46.8 Let me zoom one in. R. Bell Baton Rouge, LA |
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Looks like even in the velocity I have a 2XRBPF with multiple pole pass freq peaks at 46-48 cpm. Same in the ACC...
I'm pretty positive there is some essentric rotation and perhaps cracked rotor bars as well. This motor is about 16 years old now... R. Bell Baton Rouge, LA |
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Rod,
1) Need to correct:
Above will be good for 2p>3. For 2p=2 the rule of thumb is 1000 .. 3000 hz. Your 350 hp motor may have 40 bars so RBPF is 2400 hz. Therefore be sure to select an envellop filter that captures 2400 hz 2) IMO, don't worry too much about 2*Fline in envellop spectrum. It is always there since motors with perfectly uniform airgap are few. It is not an indication for a failure mode. Regards, Arie Mol |
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After going back and looking at the Tecnical Associates wall chart, it says to 12000cpm at 3200 lines of resolution. I actually used 6400 and the FMAX was 12,000cpm... Also I had that dominating 1X peak at .58 ips which is on the wall chart as well. I checked for soft foot last week, and there was none...
The way I used to look for RBPF was using the whole FMAX out to 240,000 in ACC and 140,000 cpm in velocity. But I didn't try using that high resolution on the full scale. Our regular route data is set up for 800 lines of resolution, and some are 1,600... I took a motor current reading on the three lines coming into the breaker and the db was 47, which is in the moderate range. I'd assume you can see all the data both around 12,000cpm, and out at the RBPF further out. I'll have to try that tomorrow. I'm already home. Those spectrums I took on the 0-200htz scale this morning look identical to the wall chart. R. Bell Baton Rouge, LA |
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I agree 47 db falls somewhere around moderate. Not quite enough to make a definite call but nowhere near normal either.
Do you happen to know the number of bars and the % load during the test? With that info (along with horsepower = 350 and speed = 3600rpm), we can finetune the estimate of the severity a little bit (although it's not an exact science). |
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Not sure excactly, but it is like the other ones next to it.
The one I just got back from the shop that had the bearing problems has 38 bars...There's probably a large peak out around the 138-140,000cpm range I would bet. I'll check that tomorrow morning. I know I have one point there at the INBD bearing, the velocity, and ACC which also has a dominating 1X peak which is consistent with rotor bars and essentric rotation. The motor and blower is about 90% loaded well above the 70% needed. If there is a formula can you pass that along to me? I'm sure it's in my books someplace. Thanks
R. Bell Baton Rouge, LA |
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The attached article was at one time posted here for free download:
http://turbolab.tamu.edu/pubs/papers.html Since they only keep the last three years, it's no longer available there. Equation 2 on page 3 of 8 gives the following formula to estimate the number of broken bars: n_broken=2*R/[10^(db/20)+pp] where n_broken is the estimated number of broken bars R is the total number of rotor bars pp is number of pole pairs (i.e. for 4-pole motor pp=2) dB = dB down in curent at full load There are a number of other similar formula's around. 5081095892_t31chapter07.pdf (194 Kb, 53 downloads) |
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Thats very good! Appreciate that. I'm sending it email to my office. I'll look at it in detail tomorrow...
I was pretty please about my last case with the other 350hp blower motor. Soon as I get it back again and load it up. I'll monitor the other one, but I know it's going downhill, so it's just a matter of time. It's the first time I've got an overhauled motor back from the shop and found a bearing problem. I confirmed that with them a couple of days ago... Hooorah for the vibration program! Good night all...And thanks!
R. Bell Baton Rouge, LA |
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Heres another spectrum sample I took about 15 minutes ago on the blower motor I've been looking at.
I took the FMAX out to 360 cpm but didn't see a 2X RBPF so I zoomed in to the 1X showing a 2XLF sideband. Keeping in mind the current readings yesterday were at 47db...Kept the scale at about 175,000 cpm Any comments on this? Thanks,
R. Bell Baton Rouge, LA  |
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Looks like you probably have 35 rotor bars. Is that why you took the spectrum ? (to identify the number of bars?).
Then the formula estimates about a third of a broken bar. n_broken=2*R/[10^(db/20)+pp]=2*35/(10^(47/20)+1)~0.3 |
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Yes, I took the high resolution readings yesterday at an FMAX of 12,000 cpm, then today I jumped out to 360,000 cpm to get the overall picture. Did all this at 6400 lines of resolution.
I see 35 rotor bars myself, and the 2X line freq side bands, and also the 1X 2X at the lower freq. associated with cracking rotor bars or shorted rings, and or shorted laminations. Pretty sure thats the problem, now all I should have to do is focus on the severity. I'm trying to get some currect readings with my probe and post them here, soon as I figure out how to save them...(:>  Last reading I had it was at 47dbR. Bell Baton Rouge, LA |
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This will be my last post on this blower until after we pull it and have it back and reinstalled. Unless I get more feedback.
This isn't the best software to show this rotor bar frequency, but it's the best I can do.??? Notice the condition ratio at the bottom of the page. It swung anywhere from 32.5db up to 45db. So it looks to me these faults are in process. Most of the readings were in the 40-45db range. But it is continuing to get worse. Guess I could have increased the resolution on this as well. I'll try that next time. Regards, R. Bell Baton Rouge, LA  |
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