Rotor laminations alignment with Stator laminations.

[AidyG123]

Hi everyone,

First post on here. I am a maintenance engineer working primarily with Motors and Generators. This question has bothered me for a while now, and I've finally decided to sign up to a forum to gain some further knowledge.

I am interested to know what the consequences are for a motor if the rotor laminations and not aligned with stator laminations. By this I mean axial misalignment.

Many thanks!

Aidan G

 

[edison123]

Rotor would try to pull into axial magnetic center thus creating an axial load on the bearings. In anti-friction bearings, the locating bearing will get overloaded and hence overheated if the axial misalignment is severe.

Muthu

http://www.edison.co.in

 

[ScottyUK]

Don't forget that in many generators the rotor is a steel forging, not laminated. The comments above re. magnetic centre still apply.

 

[waross]

AidyG123; are you talking about overall alignment or the alignment of individual laminations?
The rotor and the stator each have a magnetic center and the magnetic centers should align.
As for the alignment of individual laminations, interesting question.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BrianE22]

I would think you would lose some magnetic efficiency also. Less torque for the same current for a motor.

 

[electricpete]

overall alignment or the alignment of individual laminations?

I understand why you asked, considering the particular wording of the op question. I'm going to assume overall alignment, as in rotor iron centered on stator iron. Considering that the interlaminar insulation is much thinner than the lamination itself AND that the airgap depth is several times the thickness of a single laminations, I'd say individual lamination alignment is irrelevant even if you could achieve it.

Less torque for the same current for a motor

Yup. higher no-load current, lower power factor at any load.

As far as overloading bearings, I wouldn't think it's typically a problem since the centering forces tend to be relatively low compared to other forces that would be present. But you never know, maybe edison has seen something I haven't (it wouldn't be the first time).

Being off of magnetic center is often cited as a cause of axial "hunting". I tend to think this hunting concern from magnetic centering is overblown. For one thing there is of course no low frequency component (few hz) to magnetic centering force. Magnetic force is one of several forces that may be acting axially (others include air forces and forces from the coupling). In a sleeve bearing machine, the coupling and magnetic centering force are effectively parallel springs holding the rotor axially in position. However the "magnetic spring" is nonlinear such that if the coupling holds the rotor off-center, then the spring constant of the magnetic spring is lower and thus the motor rotor is more susceptible to movement in response to dynamic air forces or from small dynamic movements of driven machine. Also changes in vent duct alignment (in addition to end alignment) might cause decrease in the magnetic spring constant, depending on the configuration. These reductions in effective spring constant of the magnetic centering force are the only logical way I can figure anyone identifying lack of magnetic centering as a contributor to hunting.



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(2B)+(2B)' ?

 

[AidyG123]

Hi everyone,

I really appreciate all your replies.

Sorry i was not clear on my question.

I did indeed mean the general alignment of the stator and the rotor.

All the replies have been interesting reads, so thank you.

 

[Tmoose]

I think there is a bigger danger of not setting up a sleeve bearing motor on mag center with an LEF ( limited end float) coupling driving a fan or pump with it's own fixed thrust bearing. When thermal expansion of the fan or pump shaft (which can be a half inch or more on a long, hot, device) pushes the motor rotor axially, the spinning rotor might make contact with the bumpers or whatever physically limits the motor rotor axial travel. THAT would make a mess in no time.
I believe that is the "fixed" bearing is almost always the inboard bearing, so axial thermal growth does not affect the other component very much.

Perhaps similar to what the honorable EPete said, my few experiences are that even a 500 HP sleeve bearing motor running solo can quite easily be pushed axially. I suppose under load the restoring force might be greater.

I'd still want to be sure to confirm mag center before setting or aligning motor driven equipment. Some big motors have nicely labeled movable mag center indicators so the OEM can set them rather than relying on surface on the end bell, and that means the indicator pointer could be bumped in handling. Also some OEMs scribe lines on the rotor shaft for the center, and the maximum limits. If a millright mistook the " all the way in" scribe line for the mag center line, the motor would be on the verge of running "on the bumper " the moment it went back on line, and any thermal growth would make contact.

 

[dArsonval]

All of the contributions so far have merit.

Referring to the original question of what are the consequences for a motor rotor not being
aligned with the stator when it's powered... can additionally be described as:

Over time, the motor's Electrical Center will destroy the motor's Mechanical Center if they both
do not coincide or align with each other.

Obviously one beats up on the other and the weakest link is going to go first.

A motor coupled to a pump pulling axially on the pump destroys an impeller for example.

Or, the electrical center works on a bearing fit eventually wearing it out of tolerance.
The destroyed portion may be the shoulder on a shaft journal, or the bearing itself taking the bulk
of the "preload" eventually failing.

An example of where the winding goes before the mechanical aspect of the motor
is where a motor's rotor shaft fit is not correct for whatever reason. Maybe it left the factory
that way. Or a new shaft was made for a motor and the "fit" was not quite right.

What can happen in that situation is, while cold...the rotor moves or slides on the shaft in turn
seizing in a position other than electrical center after heating up. The failure cascades from there.

John