Control method of permanent magnet synchronous motor

We have introduced the design of permanent magnet synchronous motors before. It can be seen that there are three design optimization methods, and everyone must have mastered them. So what are the control methods of permanent magnet synchronous motors? Please read the detailed introduction below!

With the continuous development of permanent magnet synchronous motors, their control skills have also attracted much attention. At present, the typical control skills of permanent magnet synchronous motors are similar to those of asynchronous motors, including constant voltage frequency ratio control, vector control, and direct torque control.

1. Constant voltage frequency ratio control

Constant voltage frequency ratio control (also known as constant flux control method) is to control the motor speed by changing the frequency of the power supply while maintaining the flux track of the motor to change as required, that is, when the motor speed changes (power supply voltage When the frequency changes), the motor supply voltage is adjusted accordingly according to certain rules. Below the fundamental frequency, in order to keep the air gap flux unchanged, the stator terminal voltage and the stator power supply frequency are always controlled harmoniously, and the ratio between the two is a constant. It is called constant voltage frequency ratio control. At low frequencies, the stator voltage drop should be compensated by increasing the motor supply voltage. When the frequency increases, the motor supply voltage needs to be kept at an additional voltage value to make the magnetic flux consistent with the frequency. Inversely proportional reduction is used to complete the control of field weakening.

The control method has a simple structure and low implementation cost. However, this control method cannot achieve instantaneous control of torque to ensure the dynamic response of the motor, and at low frequencies, the motor torque output is often insufficient. Therefore, motors using this control method are generally used in situations that do not require high dynamic functions, such as the textile industry, air compressors, high-power centrifugal fans, water pumps, cement rotary kilns, etc.

2. Vector control

Vector control can be divided into vector control based on the rotor magnetic field, vector control based on the air gap magnetic field and vector control based on the stator magnetic field according to the different positioning magnetic fields. Among them, because the latter two positioning methods cannot completely decouple the alternating and direct-axis currents of the motor, vector control based on the rotor magnetic field is currently widely used.

Vector control theoretically solves the nonlinear and strong accidental problems in motor control, completes the high-function control of AC motors, and makes AC servo drive gradually replace DC servo drive and engine drive, becoming the mainstream drive system. Nowadays, international research on vector control has become very mature, and the products developed have been widely used in various industrial fields.

3. Direct torque control

Direct torque control is a method of controlling the torque of a three-phase motor with a frequency converter. The method is to calculate the estimated values ​​of motor flux and torque based on the measured motor voltage and current. After controlling the torque, the motor speed can also be controlled. Direct torque control is a patent of the European ABB company. .

In direct torque control, the stator magnetic field is obtained by integrating the stator voltage. The torque is estimated based on the inner product of the estimated stator flux vector and the measured current vector. The magnetic flux and torque will be compared with the reference value. If the error between the magnetic flux or torque and the reference value exceeds the allowable value, the power crystal in the frequency converter will switch so that the error in the magnetic flux or torque can be reduced quickly. Therefore, direct torque control can also be regarded as a hysteresis or relay control.

The controller in the permanent magnet synchronous motor control system generally uses PID control. PID control has the characteristics of simple structure, easy adjustment, and considerable reliability. Moreover, the control algorithm does not rely on the mathematical model of the controlled object. However, the permanent magnet synchronous motor is a strongly coupled nonlinear system. If it only relies on simple PID control, it will be difficult for the motor to complete high-function operation. For this reason, in order to improve the static and dynamic characteristics of the motor and ensure its efficient and stable operation, many experts and scholars at home and abroad combine control algorithms such as sliding film control, adaptive control, fuzzy control and even neural network control with traditional vector control and direct torque control. The control is combined into motor control to obtain motor operation targets.