Rotor Alignment 5

[Rwinder]

I thought I would ask for your help on the subject of rotor alignment in an induction motor. If alignment is off, can this cause preloading or excessive axial loading on the bearings?

thank you in advance.

 

[electricpete]

If there is eccentricity of the air gap, this can lead to loading of the bearings, particularly for 2-pole motor.

 

[edison123]

Rwinder,

Assuming it is a horizontal motor and there is an axial misalignment of rotor within the stator, there will be more axial thrust on the locating bearing (ball bearing, spherical roller etc.). Also, the stator current will be higher with more electromagnetic noise from the motor.

 

[jbartos]

Suggestion to edison123 (Electrical) Nov 15, 2003
Rwinder,
Assuming it is a horizontal motor and there is an axial misalignment of rotor within the stator, there will be more axial thrust on the locating bearing (ball bearing, spherical roller etc.).
///It depends on the coupling. If fixed then the axial thrust will be biggest. Compare with MagnaDrive coupling solutions. Visit

http://www.magnadrive.com

etc. for more info\\ Also, the stator current will be higher with more electromagnetic noise from the motor.
///It depends. Yes, somewhat; however, the rotating dynamics can make it indiscernible for all practical purposes.\\

 

[Rwinder]

It is an horizontal 3 phase induction motor with the alignment off by 1/2" from rotor core to motor frame core.
I have never seen one off this much before. Thank you for your helpful tips.

 

[edison123]

Rwinder,

May be if you insert the rotor other way about (rotate rotor 180 deg), the cores will align.

 

[edison123]

correction '(rotate rotor 180 deg horizontally)'.

 

[electricpete]

On first response I didn't realize you were talking about axial alignment. I agree with edison/kumar's comments about that.

I am not sure what is meant by flipping the rotor... there is only one shaft end on the rotor and stator, right?

I have seen in sleeve bearing machines there is either an adjustment of the stator core moving axially on the stator frame (bearings/rotor stay in same position... stator moves) or an adjustement of the bearings within their housing.

Curious to know if the motor had been in service before and whether it had problems.

If placed in service in sleeve bearing machine, rotor position is normally determined by the load/coupling. But with rotor off magnetic center you can still get a lot of hunting particularly with flex coupling. Assuming the end float is properly controlled by the driven machine and limited end float coupling, you shouldn't under any circumstances end up running on the shaft shoulder.

For rolling bearing machine it would certainly create a preload on the bearings .

 

[edison123]

pete,

The stator-rotor core alignment is fixed by the anit-friction bearing centres. If the stator/rotor core is equidistant from both the bearing centres, then rotor can be inserted either way. However, in most motors, stator/rotor core is not equidistant from the bearing centre. In such a case, it is possible to insert the rotor the wrong way (in a repair shop or even in OEM shop) so that there is a axial misalignment of cores. To correct it, you just flip the rotor 180 deg horizontally (so that your DE becomes ODE). I have done it many times due to original careless motor assembly by someone.

 

[electricpete]

The stator casing has only one end with opening for the shaft... wouldn't that only allow the rotor to be installed only one way?

Unless perhaps when the top half of stator casing can be flipped end-for-end to swap which end of the stator casing is open?

 

[ScottyUK]

I think I see Edison's point:

If you have motor in pieces for overhaul, so you have the stator body, the rotor, and the DE and NDE end frames, you could put the frames onto the wrong ends of the stator and re-assemble it back-to-front. Kinda careless to do it though.

 

[Mendit]

Flipping the rotor from one end to the other usually ends up with the terminal box on the wrong side (F1 and /or F2)

Rotor thrust load when unloaded is vitually zero, this is demonstrated by the fact that a sleeve bearing motor on test (uncoupled) can be moved off center with the force of pencil in the center hole. However as the laod is applied and more lines of flux are cut, the thrust forces increase considerably. The amount of force is determined by the motor rpm also, low RPM motors have smaller air gap than 2 pole motors and also greater thrust under off set conditions.

 

[THooper]

Check out thread237-9327 and thread237-23043

 

[edison123]

Mendit,

In an induction motor, the flux from no-load to full load virtually remains constant for a constant apllied voltage. So, the thrust load (in anti-friction bearings) due to stator/rotor axial magnetic centre misalignment remains the same from no-load to full load.

In an sleeve bearing machine, if there is sufficient gaps between shaft shoulders and the bearing end faces, then the rotor will pull itself to the correct magnetic centre immediately on application of voltage and will remain in that position from no-load to full load.

 

[Mendit]

Let me clarify, in a no load condition running almost at synchronous speed it is easy to hold the rotor off center.
This force is almost zero, as demonstrated by the pencil test. However if we hold the rotor off center and load up the motor, the eddycurrent effects from the displaced rotor increase and the force to move the rotor back to center become greater.

 

[jbartos]

Suggestion: 1/2" axial misalignment appears to be big even for very large and inaccurate applications. It may pay off to make the physical/mechanical corrections.