AC Motor

This is a attribute rarely quantified of AC motors for multiple reasons. The primary reason is that most people have a great difficulty in understanding the parameters of magnetic flux density. Since the field is a non-tangible three dimensional construct that nominally decreases in magnitude with the inverse squared of the distance from the source (in free space) the possibly interfering magnetic source must be close to in contact. Additionally the orientation of the fields in the motor and the outside interference matters. As if this wasn't enough to make this too complicated, there is then the question of alternate field paths from the possible passive paramagnetic and diamagnetic materials around the motor and other source.

I take back my opening statement. The primary reason that this is rarely quantified attribute is that it almost never becomes a problem. The other reason is the very few times that it does become a problem, the solution is usually a simple strategically placed magnetic shield.

The rotor is a fraction of a mm from the stator that carry hugh current which induce magnetic flux in rotor. In any real situation, the rotor will be further away from other magnetic source external to the motor by many order. MAgnetic field strength decreases according to inverse square law, so you can expect the two magnetic field strength, one due to internal induction, the other from far away extrenal source will be so hughly different, that it is unlikely the external magnetic source will have any appreciable effect.