Dynamic instability in permanent-magnet synchronous/stepping motors

An analytical study of dynamic instability in hybrid permanent-magnet stepping motors is described, the principal features of which can be applied, more generally, to a wide range of variable-frequency, permanent-magnet synchronous machines. The analysis enables the characteristics of electromagnetic damping torque coefficient against stepping frequency to be obtained for any specified practical value of rotor perturbation amplitude. From these characteristics the overall stability of the motor/drive combination can be deduced. Predicted and experimental results are presented for a 200 steps/rev, bifilar-wound, hybrid-type motor.