Research On The Flux-weakening Control Of Direct Torque Control For Permanent Magnet Synchronous Motor

Permanent magnet synchronous motor has been extensively applied in various fields due to its high efficiency, high power factor and simple structure, especially on urban rail transit and electric vehicles. Currently, there are two major directions on studies of control strategies of permanent magnet synchronous motor: vector control and direct torque control. The main advantages of direct torque control include strong robustness, simple control thoughts and rapid running noise etc.. Consequently, there are more and more studies and applications of direct torque control in recent years. The studies on direct torque control mainly include the following aspects: flux observer, low torque ripple technology, position sensorless technology and flux-weakening control which the paper focuses on. Compared to the electric excitation synchronous motor, weak magnetic running of permanent magnet synchronous motor is more difficult. Improving weak magnetic running performance of permanent magnet synchronous motor is of great significance for improving the motor performance. The research contents of the paper include:(1) Analyzed the characteristics of several conventional flux observers of direct torque control. Performed principle and performance researches for the improved amplitude limited compensate integrator. Through simulation comparison, it has been verified that compared with the improved amplitude limited compensate integrator and traditional pure integrator,drift flux caused by DC error signal can be effectively inhibited to improve flux observation precision.(2) Proposed a maximum torque per ampere(MTPA) control method of robustness self-tuning. The method doesn’t rely on electric machine parameters and has good robustness.It is applicable for digital control. It studied how to apply MTPA strategy in direct torque control. It set salient pole and non-salient pole type permanent magnet synchronous motor as research object respectively and derived MTPA control conditions of the two respectively.Compared MTPA control and constant magnetic flux control through simulation. The simulation results have proved that MTPA control can effectively reduce the reactive loss of motor at light load, and improve operating efficiency of the motor.(3) Illuminated demagnetization characteristics of permanent magnet, principle and implementation methods of the flux-weakening control of direct torque control for permanent magnet synchronous motor. Analyzed the stability of flux-weakening control of direct torque control and obtained the result that torque-angle exceeding the limits is the root cause of weak magnetic motor out of control. Analyzed the impact on weak magnetic running ability from load rate. Proposed a method of flux-weakening control of direct torque control with torque-angle limiting control for permanent magnet synchronous motor. The simulationresults have proved that torque-angle limiting control can effectively prevent step out and out of control caused by volatility of torque angle, and improved weak magnetic depth. Max.rotating speed of the motor can reach to 5.5 times of the base speed.(4) Designed the DSP software system of permanent magnet synchronous motor direct torque control, including flow chart design of digital flux observer, main program and A/D interrupt subprograms.