The rotor and stator form a constantly changing rotating magnetic field.  The magnetic field generated by current flow in the magnet wire coils in the stator is enhanced by the stator core.  This rotating three-phase magnetic field cuts through the rotor bars and induces a voltage which causes current flow in the rotor and the creation of a magnetic field.  The magnetic field in the rotor tries to lock in with the polarity, at any instant in time, with the stator field.  It is for this reason, the rotor and stator magnetic fields interaction is extremely sensitive to rotor position, axially, radially or a combination of both.  The rotor is constantly trying to center within the magnetic field.  Any unbalance or misalignment results in distortions in the magnetic coupling between the rotor and stator. 

Of particular concern are motors with sleeve bearings.  There is generally more axial play in motors with sleeve bearing than motors with rolling element bearings.  Prior to coupling sleeve bearing motors it is a good practice to run the motor and mark the rotor position when it is in magnetic center.  Then couple the motor keeping the rotor in that magnetic center position.   Rolling element bearing rotors can also be out of magnetic center, but – it not as common an occurrence.

Current signature is an excellent method for identifying axial movement of the rotor.  The current distortion caused by the axial movement causes a noticeable distortion at the fifth harmonic of the fundamental for 60 HZ that would be 300 HZ.  The distortion causes a split peak at the fifth harmonic.

Confirmation of the axial movement is also a simple task.  When the motor is de-energized, place a mark on the rotor shaft near the bearing housing.   Start the motor.  When the motor is running observe the mark that was placed on the shaft by monitoring with a strobe tachometer.  The mark will be moving in and out if axial movement is occurring.  If axial movement is present, the motor should be shutdown, uncoupled, magnetic center identified, followed by re-coupling and alignment based upon the correct location of magnetic center.  A very slight movement, just a few millimeters is all that is necessary to cause this indication.