Study on the losing control problem of direct torque control in permanent magnet synchronous motor drive

In direct torque controlled permanent magnet synchronous motor drives, when the rotor speed or the load torque changes, the effect of a certain voltage vector on the torque also changes. In heavy-load or high-speed operating conditions, the torque may lose control. This paper presents an intensive investigation into this issue. With the help of relevant derivative curves, the primary cause for the losing control problem is analysed. Further study reveals that the effect of a certain voltage vector on the load angle is no relevant for the load conditions. Based on this property, by replacing the torque hysteresis comparator with a load angle hysteresis comparator, a modified direct torque control called the load-angle-feedback direct torque control is proposed and the original superiorities of the direct torque control are well preserved. Comparison between the direct torque control and the proposed method shows that the load capacity and speed operation range may be obviously extended by utilizing the proposed method.