Unbalanced magnetic pull in cage induction motors with asymmetry in rotor structures

Rotor-cage faults, eccentric rotors or unsuitable numbers of rotor slots cause unbalanced magnetic pull in induction motors. The calculation of the forces is based on time-stepping, finite-element analysis of the magnetic field. The field is assumed to be two-dimensional, and the forces are calculated from the air-gap field using a method based on the principle of virtual work. The simulations done on a 30 kW two-pole motor and a 15 kW four-pole motor show that each of the three types of asymmetry can produce forces larger than the weights of the rotors.