High-frequency instabilities in variable-reluctance stepping motors

An analytical study of the dynamic instability of a variable-reluctance stepping motor is described. This type of instability occurs at speeds where the machine is operating essentially as a synchronous device, and is characterised by the presence of a low-frequency velocity oscillation superimposed on the synchronous speed. The frequency of the mechanical oscillation and the applied stepping frequency are not related rationally. The analytical method allows the determination of all the important features of the disturbance; in particular, the amplitude and frequency for a given stepping rate. The sources of positive and negative electromagnetic damping are identified and discussed and the instability is shown to be due to what is commonly termed `parametric excitation¿. Predicted and experimental results for a commercial multistator stepping motor are presented.