Stability of reluctance machines

An experimental and theoretical study of rotor oscillations in reluctance machines, with particular reference to variable-frequency systems, is described. Measured performance characteristics, both steady-state and transient, are used to indicate the basic features of the oscillation and to confirm the validity of theoretically derived results. Machine sine-wave and invertor quasisquare voltage supplies are considered. Theoretical results are presented which bring out all the significant influences on the stability performance of reluctance machines. These lead to a much improved understanding of observed behaviour of actual machines, and indicate those directions in which improved performance may be achieved. The direct-axis-rotor-resistance/quadrature-axis-rotor-resistance ratio is shown to be particularly important in this respect. The theoretical results are obtained from the characteristic equation of the machine using the method of D decomposition, previously developed in a powerful form for the treatment of multiparameter systems, and permitting the derivation of the natural frequency and damping factor. The frequency of rotor oscillation increases progressively with supply frequency, but, in most practical circumstances, lies somewhere in the range 0.03¿0.15 p.u., the supply frequency then being in the range 0.1¿0.4 p.u. Brief consideration of the relationship between the reluctance-machine instability and that occurring in induction machines is included.