Last time I mentioned a case study.
It is a 250Hp motor, 1800 rpm. In the field it is coupled to the piston pump through a gearbox. (The gearbox ratio is not an integer).
The first slide is the current signature from the field, motor coupled to the pump through a gearbox. There are numerous sidebands created by the impacting from the pump.
The slide #2 shows the same, but the range is only 54 to 66Hz. The first sideband due to rotor bar is marked 59.75 Hz (0.168 Amp). Yes the load was very low, ~13 Amps out of 35 amps full load (the motor is 4160 Volts). Note the decibel ratio of 37.7 dB. The first sideband related to receip. impacting is at ~ 57 Hz, far away from the main peak.
The current analysis was considered inconclusive due to low load.
Later the motor was send to the shop and was load tested with DC generator. See the results on slide #3. The load was adjusted almost the same as in the field. You can notice that the sidebands related to the rotor bars are virtually identical in the field and in the shop.
Then the load was increased to full load. See slide #4 (Motor coupled to DC gen. full load 20-100Hz). There are numerous sidebands related to the rotor bars. Detailed spectrum (54 to 66Hz) is on slide #5. The actual decibel ratio was 35.2 dB, lower than usual 54 dB.
The rotor was bench tested and one bar was found broken beyond any reasonable doubt. However the motor was put back into operation, and almost 2 years later it pumps and pumps.
jank

This message has been edited. Last edited by: jank,

central_alberta_thread.ppt (916 Kb, 14 downloads) ppt file, current signatures

Thx Jan. That's another great case study.

Another example of multiple pole-pass sidebands in the current signature being associated with a broken rotor bar. And those multiple pole pass sidebands stay there both on the generator and in the field. Howard, do you have any reply?

Those other sidebands (3hz) show up in the field when driving a piston pump which has cylcical torque. I'm guessing 3hz is either pump speed or a submultiple of pump speed? Do you know pump speed and is it a 2-cycle or 4-cycle?

Also interesting the machine keeps running for years with that 38db difference. Is it stable? How is the vibration? Is it cast alumuinum rotor?

This message has been edited. Last edited by: electricpete,

Well it’s been nearly 2 months since I first posted this query regarding the current spectrum taken on a 2-pole compressor motor. I guess I didn’t realize this was such a controversial topic…

Anyway, while I still haven’t learned anything in regards to the motor I originally posted on, I do have 2 separate case histories attached. These were the first 2 cases that I ever diagnosed using motor current data.

The 1st case involved a (1000 HP / 6-pole / 4160 volt) motor operating a centerhung ID fan. In this case there were multiple pole-pass sidebands spaced around line frequency. When the motor was removed and the rotor inspected of the 56 rotor bars 5 consecutive bars were broken on both ends plus 3 adjacent bars were broken on one end.

The 2nd case involved a (1000 HP / 4-pole / 4160 volt) motor that operated an overhung grind mill. In this case there was only 1 pole-pass sideband spaced around line frequency. When this motor was removed and the rotor was inspected 23 bars of the 46 bars were either broken or cracked.

I thank all of you for your passionate contributions to this board…

Comparisons_of_2_Separate_Rotor_Issues.doc (1,853 Kb, 30 downloads)

Michel:

I have to apologize for any of my part of what is happening to your original question. I was defending a position on another string and the folks involved have decided to carry it over onto everything else when I post.

Not sure of the motives, just trying to help. That is why I stopped posting on this string.

too many egos.

Howard