Permanent Magnet Synchronous Servo Motor (PMSM) Drive Principles

With the rapid development of modern motor technology, modern power electronics technology, microelectronics technology, permanent magnet materials technology, AC variable speed technology, and control technology, permanent magnet AC servo technology has made great strides. The performance of permanent magnet AC servo systems is continually improving, and prices are becoming reasonable, making permanent magnet AC servo systems a trend in modern electric servo drive systems, especially in high-precision and high-performance servo drive fields. Permanent magnet AC servo systems have the following advantages:

1. The motor has no brushes and commutators, making it reliable, easy to maintain, and easy to service.
2. The stator winding dissipates heat quickly.
3. The rotor inertia is small, making it easy to improve the system's responsiveness.
4. Suitable for high-speed and large torque working conditions.
5. Smaller and lighter than motors of the same power, widely used in machine tools, mechanical equipment, transport structures, printing equipment, assembly robots, processing machinery, high-speed winding machines, textile machinery, and other fields, meeting the development needs of the transmission field.

The permanent magnet AC servo system's drive has evolved from analog to hybrid to fully digital. Fully digital servo drives not only overcome the drawbacks of analog servos, such as large dispersion, low reliability, and zero drift, but also fully utilize digital control's advantages in precision and flexible control methods, making servo drives more reliable and structurally simpler. Currently, high-performance servo systems mostly use permanent magnet AC servo systems, including permanent magnet synchronous AC servo motors and fully digital AC permanent magnet synchronous servo drives. The servo drive has two parts: drive hardware and control algorithms. Control algorithms are one of the key technologies that determine the performance of AC servo systems and are the main part of foreign AC servo technology blockades and the core of technical monopolies.

Basic Structure of AC Permanent Magnet Servo System

The main components of an AC permanent magnet synchronous servo drive include a servo control unit, a power drive unit, a communication interface unit, a servo motor, and corresponding feedback detection components. The servo control unit includes a position controller, speed controller, torque and current controller, etc. Our AC permanent magnet synchronous drive integrates advanced control technology and strategies to make it highly suitable for high-precision and high-performance servo drive fields, showcasing its powerful intelligence and flexibility, unparalleled by traditional drive systems.

Currently, the mainstream servo drives all use digital signal processors (DSPs) as the control core, which has the advantages of being able to implement relatively complex control algorithms, digitization, networking, and intelligence. Power components generally use intelligent power modules (IPMs) as the core design of the drive circuit, which integrates the drive circuit and has overvoltage, overcurrent, overheating, and undervoltage fault detection and protection circuits. A soft-start circuit is also added to the main loop to reduce the impact on the drive during the startup process.

Servo drives can generally be divided into two relatively independent functional modules: the power board (drive board) and the control board. The power board (drive board) is the high-power part, including two units: one is the power drive unit IPM used to drive the motor, and the other is the switching power supply unit that provides digital and analog power for the entire system.

The control board is the weak power part and the control core of the motor. It is also the carrier of the servo drive's technical core control algorithm. The control board outputs PWM signals through corresponding algorithms, which are used as drive signals for the drive circuit to control the power conversion of the inverter, thereby achieving the purpose of controlling the three-phase permanent magnet synchronous AC servo motor.

Power Drive Unit

The power drive unit first uses a three-phase full-bridge rectifier circuit to rectify the input three-phase power or city power to obtain the corresponding DC power. Then, the rectified three-phase power or city power is used to drive the three-phase permanent magnet synchronous AC servo motor through a three-phase sine PWM voltage type inverter. The entire process of the power drive unit can be simply described as an AC-DC-AC process. The rectifier unit (AC-DC) mainly uses a three-phase full-bridge uncontrolled rectifier circuit topology.

The inverter part (DC-AC) uses intelligent power modules (IPMs) that integrate drive circuits, protection circuits, and power switches. The main topology structure uses a three-phase inverter circuit principle and pulse width modulation (PWM) technology to change the frequency of the inverter output waveform by changing the on-off time of the power transistor. By changing the half-cycle duty cycle of the transistor, the size of the inverter output voltage's half-wave value is changed to achieve the goal of regulating power.

Control Unit

The control unit is the core of the entire AC servo system and realizes the position control, speed control, torque, and current control of the system. The digital signal processor (DSP) used not only has fast data processing capabilities but also integrates rich specialized integrated circuits for motor control, such as A/D converters, PWM generators, timer circuits, asynchronous communication circuits, CAN bus receivers, and high-speed programmable static RAM and large-capacity program storage units. The servo drive uses the magnetic field orientation control principle (FOC) and coordinate transformation to realize vector control (VC), and combines the sine wave pulse width modulation (SPWM) control mode to control the motor. The vector control of the permanent magnet synchronous motor generally controls the stator current or voltage through the detection or estimation of the position and magnitude of the rotor magnetic field, so that the motor torque is related only to the magnetic field and current, similar to the control method of the DC motor, and high control performance can be obtained. For permanent magnet synchronous motors, the rotor magnetic field position is the same as the mechanical position of the rotor, so the rotor magnetic field position can be known by detecting the actual position of the rotor, making the vector control of the permanent magnet synchronous motor simpler than that of the induction motor.

Servo Drive Control of Permanent Magnet Synchronous Servo Motor (PMSM)

When the servo drive controls the permanent magnet synchronous servo motor (PMSM), it can work in current (torque), speed, and position control modes, respectively. Since the permanent magnet synchronous servo motor (PMSM) uses permanent magnets for excitation, its magnetic field can be considered constant, and the motor speed is the synchronous speed, i.e., the slip is zero. These conditions significantly reduce the complexity of the mathematical model of the AC servo drive when driving the AC permanent magnet servo motor.

In conclusion, the Permanent Magnet Synchronous Servo Motor (PMSM) is a crucial component in modern servo drive systems due to its high precision, reliability, and responsiveness. The fully digital AC permanent magnet synchronous servo drive has overcome the limitations of analog servos and utilized advanced control algorithms and strategies to achieve superior performance and flexibility. The power drive unit and control unit work together to drive and control the PMSM, with a simple and efficient AC-DC-AC process. The control algorithms, including magnetic field orientation control and vector control, enable high control performance and simplify the control process for permanent magnet synchronous motors. The widespread use of PMSMs in various industries demonstrates their importance in modern servo drive systems.