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Beating / modulating noise and vibration

Beating / modulating noise and vibration

Beating / modulating noise and vibration

(OP)
I have a drive train comprising of a motor (4 Pole 3 phase induction motor, 50Hz), resilient coupling and a compressor (screw compressor- 4 male lobes / 5 female lobe combination). These compressors make a strong modulating / beating noise, which gives  a perceptional feel that something is wrong with the machine. I tried measuring noise as well as vibration data on the machine. Both the data indicate strong sidebands of 1.6Hz, which we hear as beating frequency. I have attached the measured vibration data for reference.

Since this machine is a direct drive machine (no gears), the rotational inertia on the driven side (compressor) is around 30% lesser compared to the regular machines. This is the only difference I could see it from the design point of view. Or is the absence of gears in this machine is by itself does not have the flywheel kind of effect, which I am not sure.

The sidebands are there in most of the response frequencies. The  response frequencies are all multiples of the male lobe frequencies, as expected in a typical positive displacement compressor.

I also did a motor current signature analysis (MCSA) measurements and observed that the data is fine, with only a 30dB of 250Hz (5th harmonic) component in addition to the line frequency of 50Hz. I suspected whether I could get some side bands related to pole passing frequency, but I didn't get it.

Kindly looking for your valuable suggestions & views in this problem.

Regards & Thanks in Advance
Jeyaselvan  

RE: Beating / modulating noise and vibration

Is this machine running by itself or are there other compressors of the same design running close by?

http://www.machinerywatch.com

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