A 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 BDFRM and considers aspects of its practical implementation. The simulation and experimental results generated for a small prototype will show that the machine can successfully operate down to zero supply frequency of the inverter-fed (secondary) winding, unlike cage induction or many other traditional AC machines with DTC. This undoubt advantage of the BDFRM 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, typical for the BDFRM normal operation. The developed algorithm also offers the prospect of the machine performance being optimised in a manner similar to conventional vector controllers but with control actions executed in a stationary reference frame as usual for all the DTC methods. The maximum torque per inverter ampere (MTPIA) control strategy has been selected as a case study.