Electromagnetic forces in cage induction motors with rotor eccentricity

The paper deals with the electromagnetic forces in induction machines when the rotor is performing eccentric motion with respect to the stator. The studied eccentric motions of the rigid cage rotor are cylindrical circular whirling motion, symmetric conical whirling motion and the combination of these two basic modes of eccentric motions. The multi-slice, time stepping finite element analysis is used for solving the magnetic field, and the forces are calculated from the air gap by a method based on the principle of virtual work. The forces are measured for a test motor equipped with active magnetic bearings. The active magnetic bearings are used to generate the eccentric rotor motions and also to measure the electromagnetic forces. The measured and calculated forces show relatively good agreement. The results show that superposition is valid when determining forces. This means that the calculated forces of different motions can be combined and the result is the force of the combined motion.