Abstract: To solve the problem of high amplitude ripples of stator flux linkage and torque in the traditional threelevel direct torque control(DTC) algorithm for brushless doublefed machine(BDFM)， an improved threelevel direct torque algorithm for brushless doublyfed machines based on fixed synthesizing vectors was presented. The selectivity of voltage vector was added in order to deduce the torque ripple and the flux linkage ripple. A simulation model of threelevel DTC algorithm for BDFM was established in Matlab/Simulink software environment and the effectiveness of the proposed control algorithm was testified in this model. The simulation results show that the torque ripples and the flux linkage ripples were effectively reduced， the locus of the stator flux linkage was better and the rotor speed was more stable by this improved control method. In addition， the advantage of fast torque response in the DTC algorithm was maintained. The dynamic performance and robustness was improved in this DTC system.

Key words: brushless doublyfed machine, direct torque control(DTC), threelevel inverter, fixed synthesizing vectors, torque ripples, flux linkage ripples