Axial force on induction rotor

[Brad1979]

On an AC induction motor, if the rotor and stator aren't aligned (that is, the rotor is shifted axially with respect to the stator), when the motor is turned on, will there be an axial force on the rotor? In other words, will the stator and rotor try to align themselves once power is applied?

 

[jraef]

Given the small air gap the point is likely moot, but yes, it would "try" until the vibration destroys it.

"Will work for (the memory of) salami"

 

[Brad1979]

jraef,

Can you explain your comment a little more? What does the size of the air gap have to do with the relative axial positions of the stator and rotor? Were you thinking I was asking about radial alignment?

 

[electricpete]

There is an axial magnetic centering force, but it is very weak. Typically mentioned as only a few hundred pounds even for large motors. If air ducts in rotor and stator, their relative positions can affect the axial magnetic force also.

Some more discusion
thread237-264609

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(2B)+(2B)' ?

 

[waross]

I have seen fractional HP motors where the rotor was deliberately shifted and the axial force was used to put bearing force on a friction clutch.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

The answer is yes, the stator and rotor will try to align themselves. In most cases, this is only a problem with sleeve (babbitt) bearings or roller bearings.

In the case of the babbitt bearing, there is usually up to 1/2" of endplay allowed and, if during alignment, the shaft is not located on magnetic center then damage will result. If the shaft is pulled to the inner or outer extreme of the endplay, the shaft journal will ride against the inner or outer thrust face and, because there is no 'mechanism' for delivering oil to the thrust face, bearing failure will result.

In the case of the roller bearing, it is more difficult to 'accomplish' this type of misalignment because there is very little endplay in the bearing. However, if you are 'forceful' in the alignment you can end up with axial thrust load on the bearing. In this case, the ends of the rollers will ride against both the inner and outer bearing races on alternate sides and this will result in quick bearing failure.

Ball bearings are more forgiving. I imagine that in the extreme case that it would be possible to cause a bearing failure due to axial misalignment but this, being the extreme case, is not common.