The radial forces acting on a rotor of a four-pole cage induction motor are studied in the case when the rotor has an odd number of slots. The calculation is based on time-stepping, finite element analysis of the magnetic field. The field is assumed to be two-dimensional, and the field equation and the circuit equations for the windings are solved together. The forces are computed from the air-gap field by a method based on Maxwell's stress. A typical unbalanced magnetic pull obtained in the simulations of a 15 kW motor was a force vector rotating at a high frequency but having a relatively constant magnitude. The frequency of rotation depends on the number of rotor slots. For the constructions studied it was typically about one kilohertz. In the worst cases, the magnitude of the force was over 30 times the weight of the rotor. The largest forces were obtained for those numbers of rotor slots that are close to the number of stator slots.
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