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 EMF value. Extension of the speed range can be achieved to some extent by using the Held weakening principle resulting in constant power characteristics, but is limited by machine parameters and inverter rating. Salient pole PMAC machines (inset or buried magnet types) are better suited for field weakening compared to surface mount PMAC machines. In induction motor-based spindle drives for machine tool applications, the wye/delta switchover method was adopted and still 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 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. The start, finish, and the center tap should be accessible thereby making the motor a nine-lead machine. Adopting the proposed method, results in an AC machine with dual-torque characteristics. When used in conjunction with field weakening principle, the highspeed range can be extended in PMAC machines. The circuit used to extend the speed range is described and supported by test results carried out on a non-salient type PMAC motor and an induction motor.