A Digital Model for Three-Phase Induction Machines

A digital model for a three-phase induction machine is developed, which is particularly adapted for studying its dynamic performance when fed from an inverter. Conventionally, the induction motor is analyzed in terms of its d-q variables, while the operation of the inverter generally needs continuous monitoring of the state of its phase quantities. Thus in a study of the composite inverter-induction-machine system, one is faced with the problem of matching the two sets of variables. The proposed model overcomes this problem by describing the machine behavior directly in terms of the stator phase variables, at the same time retaining a computational simplicity comparable to that of the d-q equations. Furthermore, it is shown that the machine equations, expressed in terms of the stator terminal variables, can conveniently handle the various terminal conditions, like open phases, that may arise when the machine is fed from an inverter. Detailed equations in a form suitable for digital solution are also presented for possible terminal constraints of this nature, with or without the machine neutral connected to supply.