Impulse method to calculate the frequency response of the electromagnetic forces on whirling cage rotors

An impulse method to calculate the frequency response of the electromagnetic forces acting between the rotor and stator of a cage induction motor when the rotor is in whirling motion is presented. Time-stepping finite element analysis is used to solve the magnetic field and the forces are calculated from the airgap field based on the principle of virtual work. The impulse response method is applied to the finite element analysis by moving the rotor from its central position for a short period of time. This displacement excitation disturbs the magnetic field, and this produces forces between the rotor and stator. Using spectral analysis techniques, the frequency response function is calculated using the excitation and response signals. The forces are calculated from the frequency response function. The forces calculated by impulse response method are compared with those calculated by a conventional computation. The results show very good agreement. The use of the impulse method to calculate the forces in electrical machines is also discussed.