Compared to the doubly fed machine, the brushless doubly fed machine (BDFM) has high reliability and low maintenance requirements. First, by taking the negative conjugation of the control motor variables in rotor reference frame, a state-space model of BDFM is derived. It is then transformed into synchronous reference frame, called synchronous reference frame state-space model (SSSM). In this way, all the variables of the SSSM are DC under the static state. Second, on the basis of the analysis of static equations, the possible output torque limits are obtained. Third, the causes of losing control are analyzed by the flux and the torque derivatives. A new control strategy called synthetic vector direct torque control (SVDTC) is proposed to solve the losing control problems of the conventional direct torque control (DTC). Finally, the correctness of the results of this paper is verified by calculation examples and simulation results, the losing control problems can be solved, and the theoretical output capacity limits can be reached using SVDTC.

In direct torque control (DTC) of the brushless doubly fed machine (BDFM) system, the inverter switching voltage vectors cannot always meet the control requirements, and the torque will lose control. For the losing control problem, this paper presents a solution of indirectly controlling torque by controlling the angle difference between the power motor (PM) stator flux and the control motor (CM) stator flux (called as the flux-angle-difference). Firstly, based on the CM static coordinate system BDFM model, the derivative equations of CM stator flux amplitude, the torque, and the flux-angle-difference are deduced. The losing control problem of BDFM’s DTC is studied by utilizing the CM stator flux amplitude and the torque derivatives. From the flux-angle-difference derivative, it is found that the phase angles of the flux-angle-difference derivative curves remain unchanged. Based on this property, by replacing the torque hysteresis comparator of conventional DTC with a flux-angle-difference hysteresis comparator, a modified control strategy called flux-angle-difference feedback control (FADFC) is proposed to solve the losing control problem. Finally, the validity and the good dynamic characteristic of the FADFC strategy are verified by simulation results.

The authors wish to make the following corrections to this paper