Natural Commutation of Current-Source Thyristor Inverters by Cage-Rotor Induction Machines

A new type of brushless cage-rotor induction machine has recently been developed [1]-[3] which is capable of providing enough counter electromotive force (EMF) when interfaced with a current-source inverter (CSI) to naturally commutate the thyristors in a high-voltage dc-link adjustable-frequency converter. The basis of this system is that by stator electromagnetic design, the induction motor may have the terminal characteristics of a dc-field synchronous machine yet still retain the torque and constructional features of a conventional induction machine. Four-quadrant operation has been demonstrated on 35- and 150-hp cage motors, and most important, leading power factor operation of the entire unit at full load is compatible with high power natural commutation of thyristor inverters. The single rotating machine has two distinct and different stator pole-pitches which must be excited from a common CSI. The analysis determines the magnitude of the CEMF available from the asynchronous condenser winding and the consequent operation of the machine in both natural commutation and in a dc-link current pulsing mode for startup.