Noise and vibration occuring at 120, 126 and 162 motor harmonics

[Screw1]

I'm coping with a new compressor drive train showing noise and vibrations peaks at 120, 126th and 162th motor order. I haven't found a plausible explanation for this orders. What can be the cause of such high motor orders? They don't coincidence with any of the typical frequencies one can expect when dealing with gearbox drive arrangements (gear mesh frequency, bull gear frequency, pinion frequency, ...)

 

[MachineryWatch]

The most common high frequency vibration on induction motors is Rotor Bar Pass Frequency (RBPF) and harmonics of RBPF. The fundamental is simply the number of rotor bars X RPM. There are usually somewhere between 30-70 rotor bars on a motor. RBPF is usually surrounded by sidebands spaced away from the center frequency by 2X Line Frequency. I suspect you are seeing some combination of the RBPF harmonics and the sidebands.

Skip Hartman

http://www.machinerywatch.com

 

[vanstoja]

If you have a centrifugal compressor, impeller blade passing (IBP) frequency harmonics and amplitude modulated 1X sidebands could be involved. If you have 40 impeller blades then the 3rd and 4th IBP harmonics would be possible sources, though the 126X peak would have to be a somewhat implausible +6X sideband of 3 IBP (120X). The 162X peak is a more plausible +2X sideband to 4 IBP. Sometimes complex modulation effects can suppress exact IBP harmonics and instead amplify certain of the sideband peaks. While you refer to "motor" order, I presume you don't have an electric motor in your compressor drivetrain and that your "order" is compressor shaft speed.

 

[electricpete]

All good comments.

It would probably help if you can provide any of the following info which is available:

Is vib higher on motor or compressor?
What is motor speed?
What is compressor speed?
Is there a vfd?
Describe gearbox configuration including # of teeth
Describe compressor
If we're talking a machine 1200 rpm or lower it's possible this might be ringing from bearing impacting. Are you sure these frequencies clearly syncronous (exact multiples of running speed? Are these sleeve bearings or anti-friction and if anti-friction do you have manufacturer and part number?

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[Screw1]

Thanks for all comments and suggestions.
Some extra data:
- it's a screw compressor with 4 lobes on male and 6 lobes on female
- vibration peaks remain at same frequencies, regardless of gear sets (different transmission ratios have been tried)
- motor speed is around 1500 rpm (4 pole AC motor)

Rotor Bar Pass Frequency? Is this something similar as Slot Frequency?

 

[vanstoja]

Rotor bar pass frequency is the same as rotor slot passing frequency and results from the non-magnetic, air-filled slot over the rotor bars interrupting the motor's magnetic flux field. The 40 blade compressor impeller I cited above could equally as well be 40 or so rotor bars in your drivemotor. Since rotor slot passing(RSP) hamonics sidebands are usually multiples of twice line frequency or 2 times 50 Hz=100Hz in your case rather than shaft rotational sidebands, you need to determine the number of rotor bars in the motor and check if your peak frequencies of 3000, 3150 and 4050Hz are RSP harmonics or 2 times line frequency(2*L) sidebands of RSP harmonics. Since your 4 pole motor might typically have 36 stator slots and the number of rotor bars is generally within 25% of stator slot number, something like 42 rotor bars would be a possibility which would make the 3150Hz peak the 3rd RSP harmonic. However, the 3000 Hz peak would then be an implausible 3RSP-3L and 4050 Hz would be an implausible 4RSP-3L source. RSP harmonics as high as 3 and 4 are entirely plausible vibration sources in 3-phase motors.