Extended high-speed operation via electronic winding-change method for AC motors

In many applications, it is often needed to extend the speed range of an electric motor. In permanent-magnet ac motors (PMAC), the maximum speed is limited by the available voltage for a given counter electromotive force value. The extension of the speed range can be achieved to some extent by using the field-weakening principle, resulting in constant-power characteristics, but is limited by machine parameters and inverter rating. In an induction-motor-based spindle drive for machine-tool applications, the wye/delta switchover method remains popular for extending the constant-power range without sacrificing the torque capability at higher speeds. Conventionally, the wye/delta method employs mechanical contactors that have limited life and are associated with dead time in the range of tens to hundreds of milliseconds due to mechanical constraints. Power-electronic versions of the contactors have been proposed, but they are rather complicated and involve many switching devices. This paper describes a novel and simple switching technique involving only two three-phase diode bridges and two insulated gate bipolar transistor (IGBT) switches to extend the speed range of ac machines (induction as well as permanent-magnet type). The proposed method requires a center tap to be placed in each phase of the motor with access to the ends and the center tap, making the motor a nine-lead machine. Test results carried out on a nonsalient-type PMAC motor and an induction motor are given to support the idea.