| Permanent magnet synchronous motors are widely used industry production and human lives.As for the high performance control methods for permanent magnet synchronous motors,the most widely used ones are vector control and direct torque control.The presented researches related to direct torque control are mainly focused on two aspects: the minimization of torque ripples and stator flux observation.However,as researches move forward,a new kind of special problem in direct torque control has been revealed gradually,called the losing control problem.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 will also change.In heavy load or high speed operation,the torque may lose control.This paper presents an intensive investigation on this issue.With the help of relevant derivative curves,the primary cause for the losing control problem is analyzed.Further study reveals that the effect of a certain voltage vector on the load angle is irrelevant with 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 direct load angle control is proposed and the original superiorities of the direct torque control are well preserved.The proposed method which has a load-invariant validity is especially suitable for permanent magnet synchronous motor drives where the load changes significantly and is easy to implement.The application limitations and both advantages and disadvantages of this new method is illustrated in detail.Comparison between the direct torque control and the proposed method illustrates that the load capacity and speed range can be obviously extended by utilizing the proposed method.This work is of great importance in providing reference for further improving the performance of direct torque control. |
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