Current-zero disconnection of induction motors

The disconnection of an induction motor from its supply, by the interruption of the stator currents as they pass through their zero values, is analysed using permissible approximations. The disconnection by current chopping, of a single motor as well as a group of motors, is also analysed. For a single machine, the rotor currents given by the two analyses are very close even for a small machine, because the difference is determined by the open-circuit, rather than the short-circuit, decrements during the quarter cycle taken for current-zero interruption. Computed results for a small and a medium-sized motor, obtained by numerical integration, are presented to validate the approximate analysis of current-zero disconnection, and to confirm that the transients on reconnection of the machine to the supply are almost independent of the way in which the disconnection takes place. Thus, quite accurate predictions of transients following current-zero disconnection are obtained if current chopping is assumed; this simplifies the computations. It is found that the discrepancies are generally least when a chopping instant ¿/6 rad after the current zero is chosen. The general conclusion is that the computationally convenient method based on current chopping is always applicable, whether or not current-zero interruption takes place.