magnetic center of motor

[Noor]

we are using ansaldo motor of following specification:
6.3 KV,493 rpm,129 amp,1MW power.
the motor is using sleeve bearing with max. temperature of 75 C.The shaft of motor changes its position when we strart it i.e it comes out approx. 12mm when we start it (in discouple form).We call this phenomenon as magnetic center of motor.My question is 1)what is meant by magnetic center of motor2)why position of shaft varies when we on/off the motor3)how it is calculated because siemens and ABB motors of same capacity have shaft position indicator to reflect the position of shaft after startup.

 

[RichT]

I believe that the rotor simply centers when the stator windings are energized. It should center in the same place each time the power is applied. I'm not sure there is a way to calculate this position. The old fashioned way is to simply mark the shaft with the motor running. Then with the power off the rotor (shaft) can be moved to the mark in order to get the coupling alignment right.