I am running a vertical motor 4160 V and 1800 RPM unloaded. I checked the mechanical integrity of all parts, bearings housings, stator and rotor. When I measure the vibrations, I get cycling vibration at 2 x the rotation between 0.07 to 0.22 inc/s. Can anyone help understand why this maybe?
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Cycling Vibration on 1800 RPM Vertical Motor 1
- Thread starter RM12
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- Thread starter
- #3
By Cycling I mean that the values increase to 0.22 gradually, 0.07,0.075,0.089,0.01.....0.22 and then the reverse happens. It isn't a beat frequency with the way it eventually increases or decreases.
Also, these values are being measured radially.
Also, these values are being measured radially.
GregLocock
Automotive
so let's see a graph of vibration vs time
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
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1
- #6
electricpete
Electrical
As Greg said, more data would help. trend over time. Spectra, TWF, photo, all of that can help in possibly unexpected ways.
I have seen Tmoose's idea used productively when evaluating beating on an unloaded motor. Typically the beat frequency is so low (due to low slip) that you'll never find it with high res spectrum, you just need to time it. BUT typically the beat is at 4x running speed for a 4-pole or 2x for a 2-pole. You wouldn't get anything electrical at 60hz close to your your 4x unless there were electronics in the power supply (and even then there would have to be additional phase asymmetries).
If no electronics, my best guess: what is the thrust bearing configuration in this motor? (part number if possible). I have seen unloaded (lower than expected downthrust) vertical motors do a lot of strange things including oscillating vibration levels and oddball frequencies. Have seen that with spherical roller thrust bearings and with angle contact bearings (these bearings depend on downthrust to develop their radial stiffness). I'm led to believe it is more likely when the thrust bearing is far underloaded. In one of our vertical motors, it is necessary to reduce endplay in order to near zero before running the motor uncoupled. Typically the abnormal vibration goes away when you add downthrust by coupling a pump or whatever to them.
A little further out into the wild guess territory, assuming you're running this motor uncoupled, is there a coupling hub mounted? If so is it interference fit, or slip fit with split rings. The slip fit coupling might possibly wander although I'd expect 1x more than 2x.
I have seen Tmoose's idea used productively when evaluating beating on an unloaded motor. Typically the beat frequency is so low (due to low slip) that you'll never find it with high res spectrum, you just need to time it. BUT typically the beat is at 4x running speed for a 4-pole or 2x for a 2-pole. You wouldn't get anything electrical at 60hz close to your your 4x unless there were electronics in the power supply (and even then there would have to be additional phase asymmetries).
If no electronics, my best guess: what is the thrust bearing configuration in this motor? (part number if possible). I have seen unloaded (lower than expected downthrust) vertical motors do a lot of strange things including oscillating vibration levels and oddball frequencies. Have seen that with spherical roller thrust bearings and with angle contact bearings (these bearings depend on downthrust to develop their radial stiffness). I'm led to believe it is more likely when the thrust bearing is far underloaded. In one of our vertical motors, it is necessary to reduce endplay in order to near zero before running the motor uncoupled. Typically the abnormal vibration goes away when you add downthrust by coupling a pump or whatever to them.
A little further out into the wild guess territory, assuming you're running this motor uncoupled, is there a coupling hub mounted? If so is it interference fit, or slip fit with split rings. The slip fit coupling might possibly wander although I'd expect 1x more than 2x.
GregLocock
Automotive
Measuring beating (heterodyning) is a bunch of laughs. Typically I ended up using envelope analysis of the time signal. It's a very good idea to synthesise two sine waves in excel or octave or whatever and study how the envelope and waveform changes as you mess around with the frequency and amplitude of the second signal.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
- Thread starter
- #9
Sorry for not responding earlier to the thread.
This thread turned out more informative than I expected.
Coming to the conclusion to the original question (OP), I had to do a bump test and low-behold the natural frequency was close to the running speed between 1659 - 1750 and it created a varying vibration frequency. I left it on a rubber base and ran like a Cadillac.
We then arranged the pump head associated with that motor and had the same results as having it on a rubber.
Had I read @Strong 's post earlier could have solved it sooner, but I was hit with the same idea right after.
This thread turned out more informative than I expected.
Coming to the conclusion to the original question (OP), I had to do a bump test and low-behold the natural frequency was close to the running speed between 1659 - 1750 and it created a varying vibration frequency. I left it on a rubber base and ran like a Cadillac.
We then arranged the pump head associated with that motor and had the same results as having it on a rubber.
Had I read @Strong 's post earlier could have solved it sooner, but I was hit with the same idea right after.
electricpete
Electrical
Yes, it is a standard NEMA/EASA requirement for shop testing that the mounted natural frequency should be checked to ensure sufficient separation from running speed enough to meet the requirements of either a resilient mount or a rigid mount.
But I have to admit I've never heard of resonance causing oscillation of the magnitude of the vibration (although I can see a theoretical mechanism if machine speed varies over time or stiffness varies over time)
And I don't understand what you're suggesting
It doesn't make sense to me that bump test natural frequency near running speed would explain cycling vibration at twice running speed. I suspect there must be more to the story somewhere.
But I have to admit I've never heard of resonance causing oscillation of the magnitude of the vibration (although I can see a theoretical mechanism if machine speed varies over time or stiffness varies over time)
And I don't understand what you're suggesting
It doesn't make sense to me that bump test natural frequency near running speed would explain cycling vibration at twice running speed. I suspect there must be more to the story somewhere.
- Thread starter
- #11
I have to admit to it also that I am yet to understand completely why it would make the vibration vary between a range.
For now, my theory is that the motor since it's not a synchronous motor, the motor rpm oscillates as well which oscillates the vibration with its natural frequency.
It's a farfetched idea because the interval between varying vibration was quite high. For averaging of 5 seconds, the values seemed to have changed after at least 30 seconds plus the varying of motor speed unloaded is said in other words having no slip or 1799-1800 in this case.
Also, although I placed the motor on the rubber base or the pump head, I still noticed some cycling (0.015-0.03) but not as bad without them. Based on this it would shatter the premise of natural frequency affecting the cycling or oscillating vibration.
I will try to collect the existing data, waveforms etc. for us to visualize and analyze here.
For now, my theory is that the motor since it's not a synchronous motor, the motor rpm oscillates as well which oscillates the vibration with its natural frequency.
It's a farfetched idea because the interval between varying vibration was quite high. For averaging of 5 seconds, the values seemed to have changed after at least 30 seconds plus the varying of motor speed unloaded is said in other words having no slip or 1799-1800 in this case.
Also, although I placed the motor on the rubber base or the pump head, I still noticed some cycling (0.015-0.03) but not as bad without them. Based on this it would shatter the premise of natural frequency affecting the cycling or oscillating vibration.
I will try to collect the existing data, waveforms etc. for us to visualize and analyze here.
- Thread starter
- #13
This time it wasn't cycling 2 times when on rubber or pump head.
It was 1 time and 2 times cycling without the pump head and it did not matter where I measured it radially around the motor.
The initial and final test were both done at the shop.
Initially we had the motor run in the shop without the pump head and directly mounted on its flange. This is where we had the varying vibration consistent and repetitive.
After the bump test was conducted, we decided to place it on rubber, and it worked. Finally, we asked the pump head to be sent to us and it worked just the same as on the rubber with low vibrations.
I do understand the structure of the pump head stool can make it highly directional, and we tested the natural frequency to find different frequencies in the direction where there is no cutout.
I am waiting until the motor is installed back in the field and loaded to see difference in the vibration. The bearing configuration is two thrusts down and the endplay set at 1 thousandth.
It was 1 time and 2 times cycling without the pump head and it did not matter where I measured it radially around the motor.
The initial and final test were both done at the shop.
Initially we had the motor run in the shop without the pump head and directly mounted on its flange. This is where we had the varying vibration consistent and repetitive.
After the bump test was conducted, we decided to place it on rubber, and it worked. Finally, we asked the pump head to be sent to us and it worked just the same as on the rubber with low vibrations.
I do understand the structure of the pump head stool can make it highly directional, and we tested the natural frequency to find different frequencies in the direction where there is no cutout.
I am waiting until the motor is installed back in the field and loaded to see difference in the vibration. The bearing configuration is two thrusts down and the endplay set at 1 thousandth.
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