Here is another chapter about magnetic centre.
Firstly, when running an asynchronous 2p=20 pole motor with sleeve bearings at no load and coupled by means of limited end float coupling and axially aligned at magnetic centre scribe, a strong humming noise and vibration can be observed. Vibration on pedestal bearings is low however vibration on stator frame is high in radial (hor/vert) direction. Dominant frequency is 2*Fline and to a lesser extend 2*2*Fline.
The remarkable thing is that these vibration frequency components are low-frequent amplitude modulated, this is audible as the humming noise. In the vibration spectrum the 2*Fline comes with sidebands equal to the low-frequent amplitude modulation frequency. Here comes the mystery:
From 90 % to 105 % of nominal voltage (and at nominal frequency 50 hz, rotor spinning 5.00 hz) the modulation frequency varies from 2.3 hz to 2.9 hz. Modulation amplitude reaches a maximum at approx. 95 % of nominal voltage where modulation frequency is (or happens to be?) the same as half rotor speed.
Secondly, when running uncoupled with rotor free to move in axial direction the rotor is severely drifting back and forth indicating magnetic centre is not constant during one revolution of the rotor.
Question: what is the source of this strong voltage dependant 2.3 hz … 2.9 hz fluctuation of 2 * Fline?
I suspect some kind of axial / radial asymmetry in the magnetic circuit.
Any thoughts appreciated!
Regards,
Arie Mol

Maybe there is some motor shaft axial movement occuring even with the rotor coupled. Can you see it? What kind of coupling?
Is the rotor level?

Less likely - some kind of lateral subsynchronous instability like a whirl.

Pete,
Motor is coupled with an electrical machine through a cardan coupling (motor under test at a test field). This electrical machine is not energised, only functions as an axial rotor fixation. A to Q: no axial displacement occuring.
Motor is level as rotor does not drift to end play once de-energised.
Airgap uniform within +/- 10 %, rotor not skew in stator bore.
Subsynch oil film instability, I would expect such to occur at 10 times higher rotational speed and 10 times lower rotor weight. No prox probes installed.
So many options ruled out already, that is my challenge. And my reason for posting.
Thanks for reply.
Arie Mol

I did not say oil whirl, I said subsynchronous instability or whirl which is a broader term. Machines operating above critical speed and particularly above twice critical speed are more susceptiblel to subsync instabilities. The fact that your rotor is heavy may help for oil whirl but probablay pushes critical speed way down which is a direction that pushes you closer to many instabilities (of course low speed pushes the other direction away from instabilities). Hysteretic whirl is one that comes to mind for motors since loose motor rotor cores can contribute... see Den Hartog or Shock and Vib handbook etc. Although again I never said likely... Do you have coastdown data to evaluate critical speeds?

Does the pattern continue when uncoupled?

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

Some axial asymmetry is always to be expected, and in your case, I believe it is the source of a small axial force, that is not constant during one revolution.
The large axial movement when uncoupled may be caused by rotor iron being longer than the stator iron. The neutral position in that case is not a single point but a range. Within that range, the axial force is close to zero. The manufacturers scribe mark will put the rotor to the middle of that range.
The axial oscillation frequency depends on voltage. The higher voltage the stiffer is the “electrical spring”, hence higher frequency.
The 100 Hz is to be found on all multi-pole motors. It would be interesting to know how many slots you have. It can be as low as 120, it means 2 slots per pole per phase. Even when it is q=3 (total of 180 slots, the approximation of the sine-wave is very crude. The result is double line frequency. Note also that the magnetizing current (no load) is quite high.
jank

I agree with the comments about uncoupled conditon. I have seen axial oscillation while uncoupled and I never considered it a problem.

I don't understand the connection between assymetry and vibration frequency down around 1/2x. If it is rotor assymetry - then we expect oscillation either at pole pass frequency or 1x. Why do these symptoms suggest assymetry?

By axial asymmetry, I mean for example this: The stator iron may not be ideally straight. Imagine a 2-pole motor. The laminations on one side are in a perfect plane. But on the other side, they are not. Let the stack be wider on the top for example. When the flux flows from bottom to the top, there will be some axial force towards the non-ideal side. When the flux flows in the horizontal direction, the force will be different.
Those variations may cause axial movement in the motors with friction bearings. The movement of course cannot follow 120Hz. But the asymmetrical rotor and asymmetrical rotor may give rise to forces slow enough to cause a movement. The problem is that the electrical force that keeps returning the rotor to neutral position drops to zero when the neutral position is actually reached. Hence, any disturbance (such as air flow) can disturb the ideal position.
In an ideal motor, the energy in the airgap is constant. If the stator and the rotor iron are the same length, any axial movement will change the volume of the airgap, in other words it will change the energy in the airgap. The change of energy will appear as an axial force returning the rotor to the neutral position. The oscillations depend on the mass of the rotor and the flux density in the airgap. Increasing the voltage will increase the force and in the end the frequency of the axial oscillations. It seems to me very natural that this movement will affect vibration at 120Hz (or 100 Hz) as well.
jank