An equation for the global speed stability bounds in the variable frequency induction motor drive

Results of algebraic formulation for the global frequency stability bounds in the adjustable-speed induction motor drive are presented. The analysis is based on identifying system regenerative bounds with those of the self-excited generation mode. Under the condition, the speed limits will be determined by motor inherent capacity for own saturation while the associated steady-state oscillations within the global voltage boundaries will be determined by external systems exciting parameters. For the inverter-fed motor, the dc source exciting parameters, inverter dc-link filter, machine pole-pairs and inertia of rotating masses will, therefore, have no influence on the resulting speed limits. The approach has evolved in an equilibrium state of motor saturation that allowed introduction of an equivalent active conductance function for a stabilizing slip parameter that will set the stability limits in terms of motor short-circuit constants and saturated state. The equation brings into focus the inherent resistance-induced speed instabilities.