I have a 300 HP Louis Allis AC motor that has Pole Pass frequency sidebands surrounding run speed harmonics, an indicator or rotor problem. We repaired this motor back in October, and no indication of a rotor bar problem was noted. We have the motor in the shop now, and still have no signs of a rotor problem. We attempted to run the motor from our test panel, and it sounded awful. We thought for sure the rotor was bad, and the problem would stick out like a sore thumb. We ran a core-loss test, a surge test, a hipot test, and did an open stator test on the stator an all tests passed with flying colors. The rotor was also tested, and no problems were found. The back bearing housing was found to have excessive clearances, but not that bad. The only sign we have of a problem is the noise, and when we started the motor acroos the line we were drawing 200 Amps on two legs, and 80 Amps on the third leg. Does anyone have any information that may help me get to the root of this problem??
80 amps across one leg.... hmmmmmm. I would try and find the source of this problem first. Wouldn't a coil to coil resistance test flag a coil short? If you dont have a supply current problem, it might be in the stator coils.
As far as a rotor bar problem, do you have pole pass frequency sidbands at 1X around 1X ? If so, you could induce a voltage to the rotor (while out of the motor stator)and inspect the rotor for hot spots with thermography.
Good luck !!
Posts: 13 | Location: Baton Rouge La | Registered: 08 March 2005
What was the % load current at that time as far as i know the load current should be >75%. what is the rated current of the motor. if you do a motor current analysis (mca) test it should confirm what you see in the vibration spectrum.most data collectors can do mca testing i assume your data collector/software is CSI.
Posts: 3 | Location: TRINIDAD | Registered: 09 March 2005
The only sign we have of a problem is the noise, and when we started the motor acroos the line we were drawing 200 Amps on two legs, and 80 Amps on the third leg. Does anyone have any information that may help me get to the root of this problem??
Was this reading starting current or steady running current? (if running it is most likely voltage unbalance, if starting please not it can be tricky to measure starting current).
If running was it loaded or unloaded? What is the voltage.
Several years ago I was collecting data 300 HP AC motor turning an overhung fan. I began seeing Pole Pass Freq. Sidebands around running speed harmonics. As time went on the sidebands grew and the ocsillating vibration grew. The customer sent the motor out during a shutdown and the motor shop said the ODE bearing housing was oversized, but the rotor passed all tests. When the motor came back, the vibration had disappeared. The following summer the problem started again and the Motor Shop again said the same thing. Now the customer has installed a new motor. I still believe there was a rotor issue, but I can't prove it. It's just a gut feeling.
Also, many years ago, I saw the same thing on a 150 HP motor, but the problem there was a cracked shaft.
Presence of pole pass SBs in a vibration spectrum is not a sufficient symptom of a broken bar. A better indicator is pole pass SBs around LF in the current spectrum.
Unbalanced current seems to be unrelated to the rotor bar problem and probably is the one causing audible noise. It will likely produce some peaks in the upper region of the vibration spectrum.
Posts: 980 | Location: Texas | Registered: 22 February 2005
Pole pass sidebands around multiples of runspeed is more often an indicator of end-ring issues. A weak or loose end ring can cause a motor thrum and may eventually lead to burnt rotor bars. This can be a slow long-term process.
Can you post the full-spectrum where the zoom at 2LF came from?
Posts: 63 | Location: Lightning Capitol of the US | Registered: 23 February 2005
Try this, Mark the power leads coming in Black, Red, Yellow and the motor leads the same, Black to Black, Red to Red, and Yellow to Yellow.
Take amp readings and record the amps to the color of the power leads and motor leads. Then switch power leads like your changing rotation. Do what I call a double switch CW to CCW and back to CW each time moving the leads to another motor lead.
If you have a power problems the amps will stay the same on the power leads. And if you have motor problems the amps will stay the same on the motor leads.
Also you said the motor sounds bad, does the noise go away when the power is shut off? Or does it sound bad until it comes to a stop?
GregG
Posts: 18 | Location: Oregon | Registered: 13 April 2004
Art Crawford in his Vibration book, vol. 2, page 81 makes a statement that an out-of-round rotor with good bars will also produce 1x with pole pass sidebands. There is no any explanation given but it is reasonable to think that he is correct. (Pete, you may possibly also agree with the above).
Again, a current signature analysis will be the ultimate verification.
Posts: 980 | Location: Texas | Registered: 22 February 2005
Thank you all very much for responding. We have reached the conclusion that the rotor is indeed bad. It is a poured rotor, and a core loss test will only test the surface. We believe the problem is deep within the rotor. In response to Greg's post; the noise does stop immediately when power is removed. Thusly confirming an electrical problem of sorts. We are going to check the rotor for out-of- roundness. I have attached a full low freq spectrum and a high freq spectrum from the drive end bearing. Any more comments or suggestions would be appreciated.
I agree with David that dynamic eccentricity is a possibility as well. Were the excessive clearances you mentioned resolved? Is it sleeve or rolling bearing?
Have you ever gotten to the bottom of the stator current unbalance. Has it occurred in more than 1 location? Only during starting or also during running? What is the motor voltage?
Can you please go through your speed and the calc to determine pole pass frequency. From your original data I see 7045 = 2*RPM which would lead me to calculate speed = 7045/2 = 3522 rpm. But you have labeled speed = 3562 rpm. Or maybe that 3562 just a fixed number from the database and not the actual speed?
We're not exactly sure what caused the clearances to go in the bearing, but we believe it is a result of the rotor surging. Not real sure yet. They are rolling element bearings (FAG 6216's). The current unblance was only witnessed once because we did not want to run the motor again after the initial run. We weren't sure if would be safe to do so. We did check the rotor for eccentricities, and none could be seen. Within a thou all the way around. The rated RPM on the motor is 3555, and the RPM in the spectrum is correct at 3562. Which gives me a slip frequncy of 38 cpm * 2 poles = 76, Pole Pass Frequency. The motor voltage is 460, amps is 330. Let me know what you think. By the way we are going to cut the rotor apart, and see what we can find. I will post pics as soon as we get into it.
This message has been edited. Last edited by: SteveG,
Posts: 20 | Location: OH | Registered: 03 March 2005
I can see looking at the horizontal scale that the spacing between peaks is in the neighborhood of 78cpm. Here is what confused me that I still don't understand. The peak with the cursor is 7045 and the peak to the right is 7200, right. That gives me a spacing 7200-7045 ~ 150 ~ 2x78. I must be reading something wrong.
If it really was a rotor problem (instead of maybe test voltage problem) causing stator current unbalance, the only way I can make senseof it is that all 3 phase currents should pulse at pole-pass frequency as the faulty position in the rotor moved accross the stator field (the same phenomenon which causes pole pass frequency sidebands around line frequency in the current spectrum). At no-load pole-pass frequency might be so low that the pulsing wasn't evident and looked like steady current. But, that tremendous change in current magnitude is just hard to believe would come from a rotor problem.
It will be interesting to see those pictures.
This message has been edited. Last edited by: electricpete,
I attached a better look at the spectrum. The pole pass sidebands are surrounding run speed harmonics, not 2x AC Line. If you look close you can see 1/2x pole pass frequency as well. Shouldn't that indicate some sort of looseness? All of my vibration data was taken off of the motor while it was still in the field and operating under normal conditions. I hope this helps...let me know. Thanks.
Steve, it is my prediction that you won't find anything wrong in the rotor. I believe you actually have a stator problem only. (Why wouldn't you check stator circuit electrically?).
The reason I am saying this is because of the presence of a high peak at 41x with high amplitude sidebands (looks like they are spaced at 7200cpm) and mechanical vibration phase reversal in the TWF. I have seen cases of a 3 phase motor running single phase and it had very similar vibration signature. Did not have time to figure it out as to why.
At the time of unbalanced currents in the stator the rotor current was ABNORMALLY high and has OVERSTATED vibration due to not perfectly equal resistance in the squirrel cage bars, thus causing sidebands around 1x.
Posts: 980 | Location: Texas | Registered: 22 February 2005
David_G You maybe right, as an old electric motor man. What happens when a 3 phase motor running on a single phase you are using the sator as a big growler checking out the rotor. Of course the volts and amps are too high. And a spectrum like what you are seeing could happen.
GregG
Posts: 18 | Location: Oregon | Registered: 13 April 2004
David, We did do an open stator test on the stator as well as a core loss test, and they both passed. The peak at 41x RPM is rotor bar pass, and the sidebands are of 2x line freq and 1x run speed or 1x line freq. I have also attached the spectrum dated 10:10:38 in G-s. Thanks for your input, it is greatly appreciated.
