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.