An improved direct torque control of brushless DC motors using twelve voltage space vectors

In direct torque control (DTC) of brushless dc (BLDC) motors the same voltage vector is applied during the whole sample period, and maybe some sequential sample intervals, which results high ripple in the stator flux linkage and torque. One proposed solution to minimize these weak points is achieved by increasing the number of candidate voltage vectors that can be selected in each sector. This is realized by multilevel inverters. In low voltage applications, use of multi switches in each phase causes cost increase. In addition, some of the voltage vectors may not be used depending on the selected technique. In this paper, use of two and three phase conduction modes at the same time is proposed. Based on the proposed scheme, there are twelve voltage vectors and so twelve sectors in the voltage vector plane. Each sector corresponds to one vector. In addition to torque ripple reduction, especially at commutation instants, due to employing the full conduction mode at some intervals, more capacity of the brushless dc motor will be used. This paper contains the principles of two phase and full conduction modes, combination of these two modes and builds up the proposed scheme, respectively. The validity of the proposed combined switching mode is verified through the simulation results.