Encoderless direct torque controller for limited speed range applications of brushless doubly fed reluctance motors

The paper proposes a direct torque control (DTC) algorithm suitable for low-variable-frequency operation of the brushless doubly fed reluctance machine (BDFRM) and considers aspects of its practical implementation. The simulation and experimental results show that a small BDFRM prototype can successfully operate down to zero supply frequency of the inverter-fed (secondary) winding, unlike cage induction or many other alternating current machines with DTC. This BDFRM advantage is a consequence of using a flux estimation technique not relying on the secondary voltage integration and therefore avoiding the well-known problems at low secondary frequencies, being typical for the BDFRM normal operation. The developed algorithm also offers the prospect for optimizing the machine performance in a manner similar to conventional vector controllers but with control actions executed in a stationary reference frame as usual for all traditional DTC methods. The maximum torque per inverter ampere control strategy has been chosen as a case study.