Foreword
China's proven rare earth reserves account for 36.52% of the global total, making it the world's largest rare earth resource country. China has long been a major exporter of rare earths, and now, with 36.4% of the world's reserves, it supplies over 90% of the global rare earth market, making a significant contribution to its supply and stability. my country is truly the world's largest rare earth resource country, with proven reserves of approximately 65.88 million tons. my country's rare earth resources are not only abundant in quantity but also possess advantages such as a complete range of mineral types and rare earth elements, and a rational distribution of rare earth grades and deposits, laying a solid foundation for the development of my country's rare earth industry. China's rare earth resources have highly favorable mineralization conditions, a complete range of deposit types, and a wide yet relatively concentrated distribution. The geographical distribution of China's rare earth deposits is characterized by both wide coverage and relative concentration. To date, geologists have discovered thousands of deposits, mineral occurrences, and mineralized areas in more than two-thirds of the provinces (regions) of China.
As an electrical energy conversion device, the electric motor is widely used in our production and daily life and is receiving increasing attention. With the continuous development of manufacturing technology and the deepening research into the working principles of electric motors, many new types of motors have emerged. At the same time, the continuous improvement and perfection of permanent magnet materials have enabled permanent magnet motors to develop towards higher power, higher performance, and miniaturization. Furthermore, the rapid development of power electronics technology, modern control theory, high-performance microprocessors, and electrical and mechanical signal detection technologies has led to a leap forward in the performance of electric drive systems. AC permanent magnet synchronous motors constructed from rare-earth materials possess excellent low-speed performance. Combined with high-precision power electronic devices, they can achieve precise position servo control and can utilize field weakening high-speed control, widening the speed range and improving performance and efficiency. This gives AC permanent magnet synchronous motors a large application space in high-performance position servo systems.
Although AC speed control systems are widely used in China, the vast majority of devices, especially high-performance AC servo devices, rely on imports. Based on extensive research into the mechanisms and industrial design characteristics of various advanced AC servo systems both domestically and internationally, and combined with the advantages of rare-earth AC permanent magnet motors, a domestically produced device has been developed that can completely replace imported AC speed control devices on the market. It can be widely used in the feed systems of CNC machine tools and general machine tools, and is also suitable for other applications requiring AC speed control.
1. Structure of AC Permanent Magnet Synchronous Motor
Permanent magnet synchronous motors (PMSMs) come in many varieties. Based on the waveform of the induced electromotive force (EMF) in the stator windings, they can be classified into sinusoidal permanent magnet synchronous motors (PMSMs) and trapezoidal permanent magnet synchronous motors (BLDCs). The stator of a sinusoidal PMSM consists of three-phase windings and an iron core. The armature windings are often Y-connected and use short-pitch distributed windings. The air gap field is designed as a sinusoidal wave to generate a sinusoidal back EMF. The rotor uses permanent magnets instead of electrical excitation. Depending on the installation position of the permanent magnets on the rotor, sinusoidal PMSMs can be divided into three categories: projectile-mounted, embedded, and internally embedded. The motor studied in this paper is a projectile-mounted sinusoidal PMSM, as shown in Figure 1. The stator windings are generally multi-phase, and the rotor is composed of permanent magnets arranged in a certain number of pairs. In this system, the rotor has two pairs of magnetic poles, so the motor speed is n = 60f/p, where P is the number of pole pairs and f is the current frequency.
Figure 1. Structure diagram of a convex-mounted sinusoidal permanent magnet synchronous motor
Currently, there are two main control methods for three-phase synchronous motors: one is manually controlled (also known as open-loop frequency control); the other is automatically controlled (also known as closed-loop frequency control). Manually controlled motors primarily adjust rotor speed by independently controlling the frequency of the external power supply without needing to know the rotor's position information, often employing a constant voltage-to-frequency ratio open-loop control scheme. Automatically controlled permanent magnet synchronous motors also adjust rotor speed by changing the frequency of the external power supply. However, unlike manually controlled motors, the change in the external power frequency is related to the rotor's position information; the higher the rotor speed, the higher the stator energizing frequency. The rotor speed is adjusted by changing the frequency of the applied voltage (or current) to the stator windings. Since self-controlled synchronous motors do not suffer from the loss of synchronism and oscillation problems of externally controlled synchronous motors, and permanent magnet synchronous motors use permanent magnets as rotors, eliminating the need for brushes and commutators, they reduce rotor size and mass, improve system response speed and speed range, and possess the performance of DC motors. Therefore, the self-controlled AC permanent magnet synchronous motor used in this paper will naturally generate a synchronous rotating stator magnetic field when a three-phase symmetrical power supply is applied to the three-phase symmetrical windings. The rotor speed of the synchronous motor is strictly synchronized with the external power supply frequency and is independent of the load size.
2. Working principle of AC permanent magnet synchronous motor
This system employs a self-controlled AC-DC-AC voltage-type motor control method, consisting of a rectifier bridge, a three-phase inverter circuit, a control circuit, a three-phase AC permanent magnet motor, and position sensors. Its structural principle is shown in Figure 2. In Figure 2, the 50Hz mains power, after rectification, supplies power to the three-phase windings of the motor via the three-phase inverter. The rotating magnetic field synthesized by the three-phase symmetrical currents interacts with the magnetic field generated by the rotor's permanent magnets, producing torque and driving the rotor to rotate synchronously. The position sensor reads the rotor magnet position, converting it into an electrical signal to control the inverter's power devices, adjusting the current frequency and phase to maintain a stable positional relationship between the stator and rotor magnetomotive forces, thus generating constant torque. The magnitude of the current in the stator windings is determined by the load. The frequency and phase of the three-phase currents in the stator windings change with the rotor position, causing the three-phase currents to synthesize a rotating magnetic field synchronized with the rotor. The switching of the inverter circuit, composed of power electronic devices, achieves commutation of the three-phase currents, replacing the mechanical commutator.
Figure 2. Schematic diagram of the self-controlled motor structure.
Sinusoidal permanent magnet synchronous motors are self-controlled motors, except that the stator back electromotive force and current waveforms are sinusoidal and remain in phase. They can achieve the same torque characteristics as AC motors and realize constant torque speed regulation. This position servo system uses a sinusoidal permanent magnet synchronous motor to achieve position servo functionality.
III. Establishment of Mathematical Model for AC Permanent Magnet Synchronous Servo Motor
The stator of a sinusoidal PMSM is essentially the same as that of a conventional electrically excited three-phase synchronous motor, and its back electromotive force is also sinusoidal. Therefore, its mathematical model is also the same as that of an electrically excited three-phase synchronous motor. The instantaneous current flowing through the three-phase windings of the stator is shown in Figure 3. The balanced currents flowing through the three-phase stator windings are ia, ib, and ic, which are spatially 120 degrees apart. The instantaneous current expression is as follows:
3. Conclusion
The introduction of vector control theory for AC motors represents a significant leap forward in motor control theory, making AC motor control as simple as DC motor control and enabling the achievement of superior dynamic properties. The mathematical model established in this paper aligns with the structural characteristics of rare-earth AC permanent magnet synchronous servo motors. Practical application has demonstrated that AC servo systems constructed using rare-earth AC permanent magnet synchronous servo motors exhibit advantages such as simple structure, low cost, and high performance.
