stator slot pass frequency vibration 3

[electricpete]

In my experience vibration at rotor bar pass frequency and 2*LF sidebands are relatively common for induction motors. But I have not seen stator slot pass frequency, And there seems to be a lot of discussion of it in some other forums.

Has anyone ever seen stator slot pass frequency vibration?
Any literature or explanation for it?

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[acmotorengineer]

Oh yeah - particularly with form wound, 2 pole stators where the coils are dropped in the slot to permit air to flow down the core.

A 700HP 2P Horiz Chiller compressor motor was brought in with a vibration spectrum diagnosed as rotor problems - Rotor Bar Pass Frequency with 2X line frequency side bands.

Long story short it was vibrating slot teeth -- normal to the design. As the rotor bar passes the tooth causing a deflection of the tooth -- i.e. rotor bar pass frequency.

I suspect that a lot of normal stator loose coil and lamination problems are diagnosed as rotor bar problems.

 

[aolalde]

Hi Electripete.

We should analyze first why rotor bars side frequency bands appear. It is not the rotor slot vibration as far as I know, but due to fractured rotor bar conductors and because uneven current distribution in the rotor circuit will add magnetic attraction and repulsion forces. The working electric frequency of an induction rotor is very low as compared to that in the stator frequency.

The magnetic forces in the stator have the fundamental line frequency and their harmonics. When high and low frequencies interact the high frequency is modulated by the low frequency. I think that combination generates side band frequencies.

In conclusion we must combine at least two weaves far apart in frequency to modulate the carrier (high frequency) and as a consequence generate side band frequencies. That condition is created when the stator magnetic forces interact with the rotor magnetic forces. In my opinion any defect on the stator winding will result in the line frequency or its harmonics. The special case of loose stator conductors will result in 2X LF.

 

[aolalde]

Hi Electripete.

Making a second thought you should have a “beat” when the stator winding has electric problems.
The beat will result from combining two very close frequencies. The synchronous or line frequency and the mechanical rotor speed (for instance 1800 rpm and 1750 rpm).

 

[electricpete]

Thanks for all the comments. You guys have touched on a lot of very interesting topics for me. If I have time I'd like to come back and talk some more about RBPF pattern.

But for purposes of this thread, I am really interested in Stator Slot pass frequency. Has anyone seen it and what significance would you attach to it.

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[electricpete]

I guess I will share some of my own opinion.

I have been lucky enough to accumulate 5 think binders of articles on motor electromagnetic vibration. Mostly with the generous help of an occasional participant of this forum (I'll let him remain anonymous unless he wants to chime in).

In all of that I see many very detailed explanation for RBPF +/-2*LF types of pattern based on analysis of radial magnetic force (btw I have condensed it into my own simplified explanation... maybe I will start a new thread on that subject).

I see no mention at all of stator slot pass frequency. And my own model does not predict taht when analysing radial magnetic force.

So my thinking up until now is that stator slot pass frequency is not likely to show up on a motor.

But I have seen a few threads on maintenanceforums.com and vibration institute mentioning stator slot pass frequency seen in motors. One case in particular stator slot pass frequency with 1x sidebands. (Could of course also be a series of 1x harmonics that happened to peak at a resonant frequency SSPF but I don't subscribe to coincidences like that.) In that case the number of rotor and stator slots violated one of the rules published in EA Magazine which was supposedly related to torque considerations. My recollection of the details is a little fuzzy. If I get a chance I will go back and get more details. But I was wondering if maybe somehow this pattern might be related to torque oscillations (not related to radial magnetic pull) or somehow related to zig-zag flux pattern.

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[vanstoja]

Stator slot passing (SSP) frequency harmonics and 1X sidebands have been found in water-cooled canned motor pumps (CMP)where stator cans may have surface depressions due to setback of the stator slot wedges with respect to the stator punching bore. Wedgetop surfaces are generally deliberately recessed below the stator bore to avoid hangup and possible "wrinkling" of the thin (17 to 30+ mil thick)stator can during its axial insertion into the stator bore at pump assembly. In pressurized water flow systems, stator can surface depressions over stator slots may increase with operating time due to compacting of the winding insulation in the slots. SSP peaks in vibration spectra are due to fluid perturbations at each stator winding slot as Taylor vortex can annulus motor-cooling flow is swept helically over the surface of the stator can. Because the pumprotor is mounted in fluid film radial bearings, variable rotor eccentricity due to rotor whirling precession can change the rotor to stator clearance gap during each revolotion causing amplitude modulation of SSP peaks, ie. 1X sidebands. I have searched for but have not been able to uncover any plausible hypothesis for electromagnetically-induced stator slot passing vibration so I believe that hydrodynamic or aerodynamic sources associated with overslot depressions are the best explanation for SSP peaks in a spectrum. Non-canned air-cooled motors might have overslot depressions between wedges and punching bore but I would expect any air-cooled motor SSP sources to be much weaker than with water-cooled motors due to the lesser density of air and the much lower pressure perturbations.

 

[mc5w]

Also, I believe that normal magnetostriction would cause a stator tooth to expand and contract. Plus, normal magnetic forces between rotor and stator would cause each stator tooth to bend one way and then the other way as the rotor poles pass over.

A loose lamination can create a h#!! of a lot of noise - had this experience with an old mercury vapor barn light. I have also had an instance of a well aligned shaft coupling for a 1/2 HP motor and lubrication pump that was rather noisy. Throw in a bad bearing or a worn gearbox and you can have all kinds of fun.

Plus, radial vane motor fans, when they are turning fast enough ( 3,450 RPM ) or large enough are rather obnoxious. Just remember that leaf blowers use radial vane fans and some communities have banned or restricted operation of these things because they are just as obnoxious as a jet plane idling at X feet away.