Study On Permanent Magnet Synchronous Motor Control At Low-speed

Permanent magnet synchronous motor (PMSM) is a complex object with a nonlinear and strong coupling model. In 80’s of the last century the vector control strategy was proposed. The vector control strategy is an effective control strategy for PMSM so that permanent magnet synchronous motors get developed rapidly. Nowadays permanent magnet synchronous motors are widely used in various fields. Because of the advantages of small sizes, high efficiency, easy maintenance and high precision, permanent magnet synchronous motors have gradually replaced DC motors, becoming an important part of servo systems. The driver of permanent magnet synchronous motor is a frequency conversion system based on digital processing chip DSP. It can control the motor speed in closed-loop, and various control algorithms can be applied in the driver to improve the control performance of the motor. For existing permanent magnet synchronous motor driver products, the control performance at rated speed is much more concerned, while the control performance at low-speed is not satisfactory. The control of permanent magnet synchronous motor at low-speed is studied in this thesis, expecting to improve the control effect, and expand applications of the permanent magnet synchronous motor at low-speed. In addition to the controller design, permanent magnet synchronous motor speed control system contains many other modules, which may introduce errors. At low-speed, the impact of these error items cannot be ignored. In the thesis, speed algorithm is improved to improve the accuracy of feedback speed and inverter dead effect is compensated, to ensure the control voltage modulated correctly.The speed is calculated with position and time. The position information is obtained by a rotary encoder which is mounted coaxially with the rotor, and the time information is obtained from the high frequency clock pulses in DSP internal modules. Because of the quantization error, different speed algorithms have different precisions. Different speed algorithms are compared in the thesis and the impact to motor speed control by using different speed algorithm is presented by simulating experiments. Finally, the implementation of variable parameter M/T algorithm with high precision both at low-speed and high-speed is given out.Inverter dead-time effect is caused by the dead-time. It leads to the stator current zero clamping phenomenon, and increases the torque ripple of the motor. In this thesis, space vector pulse width modulation (SVPWM) procedure is under consideration to describe the error voltage vector caused by dead-time. The compensation method of dead-time effect is given. Finally the validity of the compensation method is verified through the experiments in the current loop.Finally, a low-speed control strategy is built at the motor experiment platform based on DSP28377 by using the improved speed algorithm and dead-time effect compensation. The rated speed of the motor in the experiment is 3000rpm. The low-speed control experiments were carried out at 100rpm and 50rpm conditions. The results showed that the steady speed precision is improved and the speed fluctuation is smaller compared to the original system.