Dear Analysts,

We are facing an issue on our cement plant. It's raw mill main drive motor of 4300kw. On no load condition, vibration is normal but as the feed come, vibration reaches to 4-6 mm/sec (fluctuating between 4 and 6) and whole vibration is on 85Hz (which is natural frequency of end shield bearing housing as told by manufacturer). According to manufacturer, this frequency is being excited by some forcing frequency which is unknown.

Second thing which is observed is vibration amplitude is greatly influenced by line frequency. Vibration exceed 4mm/s as line frequency exceeds 50 hz, and it reduces to 2mm/s as line frequency drops below 50 hz i.e 49.9 hz.

Thirdly, at no load, dominant frequency is 2Lf (0.3mm/s) whereas at load conditions 2Lf is there (same magnitude i.e. 0.3mm/s) but dominant frequency is 85Hz as discussed above.

Fourth observation is, 2Lf and 85hz disappears as the motor is switched OFF.

Spectrums are attached for reference. Please guide.

PS: An year ago, insulation resistance of this motor was dropped below allowed value and then it was sent to vendor where is rotor was cleaned and painted/polished.

Attachments

Original Post
John from PA posted:

Hmmm, load dependent.  When is the last time you did a good look at the coupling?  Alignment, lubrication, wear, etc.

Thanks for reply. Coupling pin bushes are replaced with new, bearings are also replaced along with bearing housing. Alignment verified and found no issue of misalignment.

Yes BECAR, this is recent vibration change and it all started after activity mentioned in PS of initial post. Before it was running smoothly with vibration no more than 3. We want to dig for the cause of this vibration and this strange behavior. Specially its relation with Lf. Since it's 6 pole motor, if we consider maximum fluctuation in Lf, it brings 10rpm speed change at maximum. Vibration magnitude changes drastically with this difference.

One other thing is, at no load vibration magnitude is not influenced by Lf. This change in magnitude with change in Lf is observed only at load.

Dear Becar, motor is coupled with a single stage gearbox (GMF~640hz) which then drives the sun of a planetary gearbox box. Details of planetary gearbox are unknown and we've requested the supplier to provide the details. Sun of planetary gearbox runs at 40rpm. If dun has even 50 teeth then it's GMF will be around 33 Hz which is far less than 85 Hz.

We've collected vibration on gearbox body and it's noticed that two peaks of around 84.5 Hz and 86.9Hz with almost same magnitude i.e. ~1 mm/s.

But before motor maintenance that I mentioned in the starting post, there was no such issue. 2Lf is also visible in the vibration which tilts is toward motor problem.

Dear Becar, motor rated rpm is 995. We noticed from the display of control room where values of Lf and vibration values from installed vibration sensors are coming that fluctuation of Lf (which is almost between 49.8 and 50.2 Hz) causes  change of vibration from 2mm/s to 4mm/s or even more. When Lf is 50.2Hz, vibration will be 4 or 5 mm/s and when Lf drops below 50hz, vibration values decreases to around 2mm/s. Maximum fluctuation in Lf is about 0.5Hz.

Speed(RPM) = 120 f(Hz) / P(Poles)
So Fluctuation in speed will be 120*0.5/6=10RPM

So we can say that maximum fluctuation in RPM due to fluctuation in Lf is 10rpm causes change in vibration from 2mm/s to 4mm/s.

Abid.Khan posted:

Dear Becar, motor rated rpm is 995. We noticed from the display of control room where values of Lf and vibration values from installed vibration sensors are coming that fluctuation of Lf (which is almost between 49.8 and 50.2 Hz) causes  change of vibration from 2mm/s to 4mm/s or even more. When Lf is 50.2Hz, vibration will be 4 or 5 mm/s and when Lf drops below 50hz, vibration values decreases to around 2mm/s. Maximum fluctuation in Lf is about 0.5Hz.

Speed(RPM) = 120 f(Hz) / P(Poles)
So Fluctuation in speed will be 120*0.5/6=10RPM

So we can say that maximum fluctuation in RPM due to fluctuation in Lf is 10rpm causes change in vibration from 2mm/s to 4mm/s.

Thus phenomenon is quite strange for us and it only in load condition. This fluctuation is not observed in no load condition.

I wouldn't say your situation is strange when you wouldn't say it was not present before. Speed and vibration fluctuations you are witnessing are nothing special at machines with changing load. And raw cement mills are definitelly in that group. When you are 100% sure it is a new phenomena, I would bet it is related to some motor's drop of power after overhaul and could be related to electric motor issues. I would also check if anything changed on the electric network this machine is connected to. Enough kV for the motor? 

We'll check if anything changed on the electric network this machine is connected to.

We also acquire current and flux data. This data you can see that data in attached file (file that is attached to starting post).

Can there be any source of 85Hz in motor? Or if the source of 85 Hz is planetary gearbox, can a frequency from that far location excites motor outboard housing natural frequency? Motor and gearbox are coupled by flexible coupling and installed on separate foundations. And what further test do suggest for motor rotor condition?

Or if the source of 85 Hz is planetary gearbox, can a frequency from that far location excites motor outboard housing natural frequency?

Sure, it can. But there also comes the same question, why it appeared after motor overhaul.

What type of bearings are there? I suppose fluid type?

Motor and gearbox are coupled by flexible coupling and installed on separate foundations.

I don't like the fact about the separate foundation. It also could be causing some problems.

 

I must say I suspect two other options rather then bearing frequency:

- something is exciting your motor from the side of the gearbox / planetary gearbox, 50 teeth for sun gear sounds a low number, you also have to check the planetary teeth rotation speed

- or there is some electric fault weakness on the motor but I don't understand how is it possible that the line frequency is fluctuating?? OK, rotation frequency, but line frequency should be constant at 50 Hz 

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