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Motor Vibration at 1/2 times line frequency
6

Motor Vibration at 1/2 times line frequency

Motor Vibration at 1/2 times line frequency

(OP)
I have a 2-pole 665hp 60hz horizontal motor with sleeve bearings driving a rotary compressor. The motor has has increasing amplitude at exactly 1800cpm... currently at 0.27ips.
Here are the measurements in inches per second:
POS 1A 0.07
POS 1H 0.09
POS 1V 0.28
POS 2H 0.08
POS 2V 0.21
POS 3H 0.11
POS 3V 0.08
POS 4A 0.11
POS 4H 0.09
POS 4V 0.08

Here is the spectrum which shows high peak at 1800, lower peak at 1x (3584), and even lower peak at 7200: http://209.15.74.79/pub/mab_chiller_24.jpg

Here is log spectrum which also shows some slight harmonics of 7200: http://209.15.74.79/pub/mab_chiller_24_log_scale.jpg

Here is the historical magnitude of the 1800cpm spike at the 1V position (in ips):
2/6/01 - 0.01
6/15/01 - 0.12
8/13/01 - 0.13
11/26/01 - 0.20
1/23/02 - 0.27

Spike energy measurements are very low. I don't have time waveform yet.

Note the the 1800 vibration which is driving the high overall amplitudes is very directional (much higher in vertical than horizontal). This would normally suggest that resonance is a factor, although I suspect the excitation is also changing for some reasons.

I'm not sure if this is a hermetically sealed motor (I will check).

Has anyone every seen high 1/2 * Line frequency? What is the significance? Any suggestions?


RE: Motor Vibration at 1/2 times line frequency

2
First, I would balance the 1X down as much as possible because the 1/2 X will follow downward. Then, assuming that you do not have tilt pad type bearings,  there may be inadequate oil film or oil whip. Solving that could be a problem because you have to balance (opps! Perhaps a poor play on words) juggle between reducing oil whip by means that will also cause an increase in bearing temperature. First I would try the next heavier grade of oil. Then try reducing the bearing clearance which of course you can only do by re-babbiting the bearings

RE: Motor Vibration at 1/2 times line frequency

(OP)
Motorman - thanks for the info. In this case the vibration is slightly higher than 1/2 times running at 1800/3585=50.2% of running speed.  

Have you ever seen it (oil whirl) at precisely 1/2 line frequency.  

It seems like the even fraction of line frequency cannot just be a coincidence. If it is oil whirl then maybe the magnetic force is somehow participating in determining the trajectory of the rotor?

Is there any other simple measurements/troubleshooting you might suggest to confirm the problem before I try the new oil or balance? (balance would be a big effort... doesn't seem quite justified when 1x is so low...I'd be inclined to try the new oil first).

I have seen low-magnitude 1800 cpm vibration before on many 2-pole motors including ball-bearing types (very low magnitude only visible on log scale).  Is that some unrelated phenomenon?

RE: Motor Vibration at 1/2 times line frequency

(OP)
Is high vertical (compared to horizontal) typical for oil whirl?

RE: Motor Vibration at 1/2 times line frequency

2
Hi electricpete, One of the most common problems I have seen with a high vertical in/sec reading is something is "loose" possibly the outer bearing shell to the top housing has worn and should be a metal to metal fit and no more than 1/2 thousandths clearance fit. Or the babbitt itself has started to wear out of tolerance, very sensitive with 2-pole motors...

Shut motor down and put a dial indicator as close to where the shaft exits out of the motor. Start up motor and watch indicator as motor is starting, if it jumps more than .003 something is very loose...

Kind Regards,
motorhead

RE: Motor Vibration at 1/2 times line frequency

(OP)
I read that oil whirl can be influenced by forces on the rotor near 1/2 times running speed.
I can conjure up an (apparently) logical explanation why the magnetic field force will sustain a highly-directional oil whirl orbit at 1800cpm (I can't explain how it starts, but once started it will continue).
I have a written explanation below and graphical explanation here -> http://reliability-magazine.com/pub/mab_chiller_whirl.jpg

ASSUMPTIONS:
* Shaft is rotating at 3585 rpm (slightly less than 60hz)
* Field is rotating at 3600 rpm (60hz)
* ASSUME that shaft is orbiting within bearing at 1800 rpm (30hz).
* Assume that at t=0 North pole of field is at bottom (6:00) and “high spot” of orbit is at bottom (6:00).

ANALYSIS:
* Attractive magnetic force is highest when the high spot aligns with a pole.
* Maximum downward force occurs at t=0.
* Maximum upward force occurs at t=(1/60) sec when high spot is at top (12:00) and south pole is at top (12:00).
* Maximum downward force occurs again at t=(2/60) sec when high spot is again at bottom and N pole is again at bottom.
* Maximum force never occurs in horizontal direction because high spot and pole never reach there at the same time.

CONCLUSIONS:
* The vertical force is periodic with a period of 2/60 (1800cpm), and will sustain the shaft orbit at 1800cpm.
* The horizontal force is smaller.
* Vibration will be directional.

There are a few problems with my analysis:
#1 - The flux has some tangential component and the force will also have some tangential component.
#2 - This model predicts force in-phase with motion. That may be characteristic of a system well below resonance (dominated by spring constant, mass negligible), but I would expect that high magnitudes would occur under resonant type conditions where force leads motion by 90 degrees).
#3 - I have not said anything about oil forces. I don't know much about them other than that they will sustain vibration at approx 1/2 speed when excited at approx half speed. Any more comments about how oil forces would fit into this picture?

I suspect that better modeling of item #1 will help to resolve item #2 (any comments?). At any rate, an exact model of magnetic forces may not required... I think it is enough to show that a 2-pole field rotating at 3600cpm (7200 poles passing given point per minute) CAN give a directional force at 1800cpm if the shaft is already whirling at 1800cpm.

Lots of speculation on my part isn't really productive, but it's kind of interesting. I'd like to hear any comments on that.  We will start trying some of your suggestions shortly.

Does anyone have any links to info on oil whirl?

RE: Motor Vibration at 1/2 times line frequency

electricpete:- try bently nevada at www.bently.com.
they are really helpfull.
kind regds.

RE: Motor Vibration at 1/2 times line frequency

electricpete

try www.agilent.com

I have from Hewlett Packard (now Agilent):
Application Note 243-1 "Effective Machinery Measurements using Dynamic Signal Analyzers".  My copy dates from 1994 though I believe it can be downloaded from the above site.  If not, they will mail it to you.  I have used it many times regarding motor vibration and can recommend it (it contains a description of oil whirl).

Re the oil whirl/whip, i understand this sets up a vibration at typ. 0.43 to 0.48 x rotational speed.  If true, then you would not get a synchronous relationship with any electromag forces, or at least it would come and go.  

Re the looseness, this often comes with a lot of sidebands (as the vibration isn't sinusoidal), which aren't there on your spectrum.  But I can't think of any other likely cause, other than external to the motor.

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