Hi everybody

Recently started getting some readings from a large motor that I'm confused as to what the issue may be. firstly spec

Asset is a Kiln fan- Motor is a Brook Crompton 280kw 990rpm. runs on a VSD/VFD. hard to get completely consistent readings but usually take around 830rpm. readings have been low for years and within the last two months have started to creep up in mm/s ovr and g's ovr signal only on the NDE, the DE has continue to remain low at vert-1.878 mm/s horiz-1/747vmm/s and axial -1.215 mm/s. This may be an obvious sign to more experienced VA analyst but I'm new to the game and not totally sure what could be causing this increase. The dominate frequency is at 618hz with sidebands around at 13.8hz, the same as the running speed of the motor at the time it was taken. From what I was taught sidebands are never a good sign. this is why I'm slightly more concerned than normal. Also because this asset is critical to the plants production, I don't want to have to replace it until I know that there's a defect that's getting worse. I thought that an electrical fault would have sideband of either the pole pass frequency or 2x line frequency and that was it, so I've kind of ruled out the possibility of it being an electrical issue although I don't know for a certainty. the Temperature around the NDE and DE bearings are normal and haven't increased since this issue has arisen. 

All general checks have been made including the Fan blades, the coupling, loose bolts. cracks in the concrete.

Any Ideas would be greatly appreciated 

many thanks

James Peters

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Original Post

Inspect the motor fan shroud and other attachments, like coupling guard, for looseness. Check motor base bolts for tightness. Check for cooling fan damage or debris. Calculate bearing fault frequencies and harmonics and compare to accurate shaft speed and vibration frequencies. Lubricate bearings while using vibration analyzer or ultrasound meter to detect any change in audible sound or vibrations/ultrasound levels.

Walt

Ralph Stewart posted:

Is the fundamental frequency you have,(possibly 45x),which the sidebands appear to be modulating, equal to a multiple of 1x running speed? 45X? Is this order the same no matter the speed of the motor?

Hi ralph

it looks to be, ive attached a picture which shows the multiples of the 1x. 

I will have a look tomorrow as see if we can run it at a lower speed to check the orders.

cheers

James 

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Walt Strong posted:

Inspect the motor fan shroud and other attachments, like coupling guard, for looseness. Check motor base bolts for tightness. Check for cooling fan damage or debris. Calculate bearing fault frequencies and harmonics and compare to accurate shaft speed and vibration frequencies. Lubricate bearings while using vibration analyzer or ultrasound meter to detect any change in audible sound or vibrations/ultrasound levels.

Walt

Thanks for the response Walt

I'll check the motor cooling fan but the other bits I've already checked and verified. I can't find information for the bearings in the motor, so I am a bit stuck there. do you think it could be a bearing issue?. 

cheers 

James

James

Is the motor making a modulating noise or knocking/impacting noise? How are those motor feet attached to the side of the motor stator? Got more pics from different angles you can share?

Put your cursor on 1x rpm, verify the majority of the peaks are 1x harmonics or close to 1x rpm with cursor adjustments. Yes these peaks can also be side bands.

In the past I have seen this pattern indicate:

Inner race/fluting bearing fault (waveform is not showing classic bearing impacting pattern, G values are quite high, will typically show high values on problem bearing only, have the peak to peak G values in your time waveform continued to increase over time?), cooling fan loose on motor shaft (strobe light inspection should be able to rule this out).

Worn/dry coupling (no details on its design, typically would see this pattern on both ends of motor and the coupling side fan bearing not just NDE of motor)

Loose iron on the rotor (rotor stack, lamination's loose to motor shaft) would typically see this vibration pattern on both ends of the motor, not just NDE, with high 1x rpm present and knocking noise

Dave Reynolds posted:

James

Is the motor making a modulating noise or knocking/impacting noise? How are those motor feet attached to the side of the motor stator? Got more pics from different angles you can share?

Put your cursor on 1x rpm, verify the majority of the peaks are 1x harmonics or close to 1x rpm with cursor adjustments. Yes these peaks can also be side bands.

In the past I have seen this pattern indicate:

Inner race/fluting bearing fault (waveform is not showing classic bearing impacting pattern, G values are quite high, will typically show high values on problem bearing only, have the peak to peak G values in your time waveform continued to increase over time?), cooling fan loose on motor shaft (strobe light inspection should be able to rule this out).

Worn/dry coupling (no details on its design, typically would see this pattern on both ends of motor and the coupling side fan bearing not just NDE of motor)

Loose iron on the rotor (rotor stack, lamination's loose to motor shaft) would typically see this vibration pattern on both ends of the motor, not just NDE, with high 1x rpm present and knocking noise

Hiya Dave

Thanks for responding, appreciate the time you've taken to have a look.

I've just gone to take some more readings with it running at 29.75hz on the inverter, I'll post up the spectrum in a bit.

I've seen the acceleration vibration increase and then fluctuate from being very low to high G's even though the speed taken was consistent.

There is a noise that sounds like impacting but only when you put your ear to the motor nde very position. It comes in waves where it is noisy and then dies down.

Unfortunately no bearing details on this motor so I don't have a definite way of ruling it out. 

I've attached some more pictures of motor feet and nameplate etc, just for you're interest.

Many thanks

James

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fburgos posted:

Many running speed harmonics, is this region 40x-50x speed dependant (motor problem) or  fixed at 600-700Hz (resonance mounting)

I've attached some more readings for you to have a look at because i'm not sure, the amplitude has decrease with the lower Rpm but we still have many peaks in the 600-700hz area. 

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Where are you mounting the accelerometer at Motor NDE-Vertical and how is it mounted? Is the location and mounting very consistent between measurements?

The vibration data does not indicate a severe bearing fault, since vibrations are significantly lower at a lower shaft speed and also low in the horizontal direction. AC motor with VFD is prone to EDM (fluting), so perhaps a mild case is developing. Clearly a structural natural frequency is being excited in the vertical direction. There are several candidates for natural frequency including motor case/frame, support skid/frame, terminal box (cover or entire box), wire terminations inside terminal box, cooling fan shroud, and coupling guard. Measurements with accelerometer or microphone (within 1" of surface) could help locate the resonant component and "hot spot".

At some point, the motor will need new bearings. It would be worthwhile to find out what bearings are needed, and also calculate the fault frequencies for future diagnostics.

Walt

Walt Strong posted:

Where are you mounting the accelerometer at Motor NDE-Vertical and how is it mounted? Is the location and mounting very consistent between measurements?

The vibration data does not indicate a severe bearing fault, since vibrations are significantly lower at a lower shaft speed and also low in the horizontal direction. AC motor with VFD is prone to EDM (fluting), so perhaps a mild case is developing. Clearly a structural natural frequency is being excited in the vertical direction. There are several candidates for natural frequency including motor case/frame, support skid/frame, terminal box (cover or entire box), wire terminations inside terminal box, cooling fan shroud, and coupling guard. Measurements with accelerometer or microphone (within 1" of surface) could help locate the resonant component and "hot spot".

At some point, the motor will need new bearings. It would be worthwhile to find out what bearings are needed, and also calculate the fault frequencies for future diagnostics.

Walt

Hi Walt 

So to take the nde vert I've put it as close to the cooling fan cover without touching it, the cover is slightly longer than the typical type which means the measurements aren't being taken directly above where the nde bearings is. I've just a yellow marker to locate the point at which I take it to make it repeatable. The accelerometer has a strong magnet which is what I use for most of my readings, it's not the best but it is the good enough for this type. 

Glad to hear it's not a serious issue like a bearing default or electrical issue. Thanks for all your help Walt , appreciate it 

Many thanks

James

James,

I was wondering if you have checked to see if the "possible 45x" pattern from your 1st data posted moved when your speed was reduce?

Have you confirm the data you are seeing is definitely not Slotpass with 1x sidebands as opposed to a bearing defect?

Have you tried greasing the motor bearings lightly while the problem data is showing in the spectrum when you take the data and then retake the data after greasing, for a change in the signal?

Just wondering.

Ralph

Hi James, 

keep up following the behavior of the deterioration of bearing, there are peaks in high frequency spectrum, I recommend to take acceleration spectrum, to see more details what happen in high frequiencies. 

this issue looks like Dinamic Excentricity when the rotor may be out of round, or the bearing may be loose enough to allow the rotor to travel in eccentric path.

because  RBF (Rotor Bar Frequency ) with side band ( 1x rpm) is associated to Dinamic Excentricity.

please keep this equipment under close monitoring, and update us ..

Regards. 

ramonruiz posted:

Hi James, 

keep up following the behavior of the deterioration of bearing, there are peaks in high frequency spectrum, I recommend to take acceleration spectrum, to see more details what happen in high frequiencies. 

this issue looks like Dinamic Excentricity when the rotor may be out of round, or the bearing may be loose enough to allow the rotor to travel in eccentric path.

because  RBF (Rotor Bar Frequency ) with side band ( 1x rpm) is associated to Dinamic Excentricity.

please keep this equipment under close monitoring, and update us ..

Regards. 

thanks for your reply Ramonruiz

ill keep monitoring it for now and just have to wait and see if there any other symptoms that start emerging. My accelerometer already takes acceleration data too so i have that too look at as well.

oh okay ill bearing that in mind. thanks very much

James 

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