I would like to know if I can made extra tests to determine what is the problem with theses 2 AC induction motors. I join the vibration spectrums for the 2 motors. Theses spectrums are the highest in amplitude. I have a high main frequency with 120 Hz sideband. I suspect rotor/stator bar problem, but i just want to confirm with other test.

There is any electrical test also that can be made ?

thanks

spectres_moteurs_électriques.doc (54 Kb, 49 downloads) vibration spectrum

Two things come to my mind. Skewed airgap - hardly much to do since levels not high. Or a real bar problem in rotor. Not likely, but if you like to test, you can disconnect the motor from supply, connect some 5 to 10 VAC from any variac, rotary variable transformer to one phase, add a standard old pointer voltmeter to the leads and then slowly turn the rotor by hand. If pointer is steady, rotor is fine. If pointer varies bound to your rotor, the losses vary and could indicate a nonsymmetrical rotor. One reason can be a broken bar or other similar mechanical faults that can affect the magnetic flux. If you would have the motors here, I would not rise an issue on the oil pump motor. I would maybe check the air compressors base for being in good condition. Could be easier to judge this if you could add the other points HVA and a simple picture.

here is the complete vibration spectrums for the 2 machines.

pictures not avaliable yet

a+

Probleme_électrique.doc (336 Kb, 21 downloads)

Pat,

Is this a sudden onset or a chronic problem? Unless there has been a drastic change, like Arne said it probably is not a big problem.

If it is a sudden onset problem I would recommend that you collect a velocity spectrum, at 12000 cpm fmax, 3200 lines and 3 averages, in the vertical and horizontal directions. Look for sidebands spaced at slip x the number of poles (in your case 47X4=188 cpm) around running speed harmonics. That might mean broken rotor bars or some other rotor problem. It will also give you a good look for the same sidebands around 2 x line freq.

The amplitude shown on M1 at 98,860 is 56 x running speed which is slot pass I would guess, but it is about 5 x higher vertically than horizontally. Highly directional vibration is a trait of an eccentric stator, although it is usually associated with the amplitude at 2 x line frequency.

The thing that I can figure out is the peals at 9000 and 18000 cpm. That is about 5.14 and 10.28 orders which doesn't seen normal for bearings and I can't think of anything else in that range. Do you have the bearing frequencies?

Good Luck,