Note on induction-machine performance with a variable-frequency supply

Previous work on the variable-frequency performance of induction machines is reviewed, and, for completeness, references to other aspects of transient performance are included. It is shown that results based on the analysis of constant-current performance cannot be expected to correlate well with actual performance under forced-voltage conditions. Attention is drawn to the simplicity of the direct solution of the complete set of 2-axis equations for variable-frequency variable-voltage conditions, and appropriate methods of solution by digital and analogue computer are outlined. These methods are used to study briefly the factors influencing machine stability and improved techniques of achieving rapid speed changing. In connection with stability, using frequency-response and step-response tests, it is shown that parameters of special interest are operating frequency, rotor resistance and inertia. In particular, it is shown that there is an operating frequency for which damping is a minimum. In connection with speed changing, it is shown that continuous (rather than step) variations in frequency lead to quicker speed changes with lower current requirements. In the case of speed reversal, the method eliminates the disadvantageous feature of plugging, namely that rotor kinetic energy is dissipated as heat in the winding.