How to control permanent magnet synchronous motor?

1. Working principle of permanent magnet synchronous motor

Permanent magnet synchronous motors achieve power transmission through the magnetic force between the magnetic field generated by the stator and the magnetic field generated by the rotor. The teeth on the stator generate a rotating magnetic field through three-phase alternating current, while the rotor generates a constant magnetic field through permanent magnets. When the frequency of the stator's rotating magnetic field is consistent with the frequency of the rotor's magnetic field, the magnetic forces cancel each other out, allowing the rotor to operate synchronously. This synchronous operation feature makes permanent magnet synchronous motors widely used in the industrial field.

2. How to control permanent magnet synchronous motor?

1. Vector control:

Vector control is one of the most widely used permanent magnet synchronous motor control strategies. It is based on the mathematical model of the motor and space vector modulation technology, and achieves precise control of the motor by controlling the rotor magnetic field and stator current of the motor. Vector control can achieve high dynamic performance and high efficiency, and is suitable for applications under various load conditions.

The FOC algorithm flow can be described as follows:

(1) First, the motor phase current is sampled to obtain the current components Ia, Ib, and Ic in the three-phase stationary coordinate system;

(2) Then, Clark transformation is performed on Ia, Ib, and Ic to obtain the current components Iα and Iβ in the two-phase stationary coordinate system;

(3) Then, Iα and Iβ are subjected to Park transformation to obtain the current components Id and Iq in the synchronous selection coordinate system;

(4) Calculate the difference between Id and Iq and the given Id and Iq respectively, and use the difference as the input of the two current loop PI controllers (please note that when dual closed-loop id=0 vector control is used, the value of Id is generally selected as 0, and the input of Iq is the output of the speed loop PI controller, and the input of the speed loop PI controller is the difference between the actual speed n measured by the sensor and the given reference speed n*);

(5) Perform inverse Park transformation on the output voltages Ud and Uq of the current loop PI controller to obtain Uα and Uβ;

(6) Then, Iα and Iβ are input into the space pulse width vector modulation technology module for modulation and output PWM wave signal, thereby controlling the on and off of the six switching tubes in the three-phase inverter, inverting the DC voltage into a voltage signal that is infinitely close to a sine wave, thereby driving the permanent magnet synchronous motor to rotate and realizing closed-loop control.

Figure 1.1 Permanent magnet synchronous motor vector control torque control block diagram

2. Direct Torque Control (DTC):

Direct torque control uses Bang-Bang control (hysteresis control) to generate PWM signals and optimally control the switching state of the inverter, thereby obtaining high dynamic performance of the torque. Its basic operation is to transmit the error between the flux torque set value and the actual value of the flux torque to the hysteresis comparator, and obtain the appropriate motor space vector through the offline operation switch table, thereby realizing the speed control of the motor.

Here’s how it works:

Different from vector control technology, DTC uses Bang-Bang control (hysteresis control) to generate PWM signals and optimally control the switching state of the inverter, thereby obtaining high dynamic performance of torque. DTC has its own characteristics, which largely solves some problems existing in vector control, such as complex calculation characteristics, susceptibility to changes in motor parameters, and actual performance is difficult to achieve theoretical analysis results.

DTC abandons the decoupling idea in traditional vector control, and instead replaces the rotor flux orientation with the stator flux orientation, cancels the rotating coordinate transformation, and weakens the system's dependence on motor parameters. It detects the motor stator voltage and current in real time, calculates the torque and flux amplitudes, and compares them with the given values ​​of torque and flux respectively. The obtained difference is used to control the amplitude of the stator flux and the angle of the vector relative to the flux. The torque and flux regulator directly outputs the required spatial voltage vector, thereby achieving the purpose of direct control of flux and torque.

(1) Build a basic direct torque control framework

Figure 2.1 Permanent magnet synchronous motor direct control torque control block diagram

Figure 2.2 Schematic diagram of permanent magnet synchronous motor direct control torque control system

(2) Find the required module in Simulink

(3) Use Simulink to build a direct torque model

(4) Set the corresponding parameters

(5) Run the simulation and observe the corresponding data

speed:

Torque:

Current:

The control strategies of permanent magnet synchronous motors include vector control, direct torque control, etc. Each control strategy has its own characteristics and scope of application. Choosing a suitable control strategy can achieve precise control of permanent magnet synchronous motors and improve the performance and efficiency of the motors.