introduction
The operating principle of a permanent magnet synchronous motor is the same as that of an electrically excited synchronous motor, but it uses magnetic flux provided by permanent magnets instead of the excitation windings of the latter, making the motor structure simpler. In recent years, improvements in the performance of permanent magnet materials and advancements in power electronics technology have driven the development of new principles and structures of permanent magnet synchronous motors, effectively promoting the development of motor product technology, variety, and functions. Some permanent magnet synchronous motors have formed a series of products, with capacities ranging from small to large, currently reaching the megawatt level, and their application range is becoming increasingly wide. Their status is becoming increasingly important, rapidly expanding from military to civilian use, and from special to general applications. They not only dominate in micro-motors but also demonstrate strong vitality in electric propulsion systems.
1. Hot Topics
Permanent magnet synchronous motors use permanent magnets for excitation, which has unparalleled advantages over electrically excited motors.
1) High efficiency: After embedding permanent magnet material on the rotor, the rotor and stator magnetic field run synchronously during normal operation. There is no induced current in the rotor winding, and there is no rotor resistance and hysteresis loss, which improves the efficiency of the motor.
2) High power factor: Permanent magnet synchronous motors have no induced current excitation in the rotor, and the stator windings exhibit resistive load. The motor's power factor is close to 1, reducing stator current and improving motor efficiency. Simultaneously, the improved power factor enhances the grid quality factor, reduces transmission line losses, and allows for lower transmission capacity, saving on grid investment.
3) High starting torque: In equipment requiring high starting torque (such as oilfield pumping motors), a smaller capacity permanent magnet motor can replace a larger capacity Y-series motor. If a 37kW permanent magnet synchronous motor replaces a 45kW-55kW Y-series motor, it effectively solves the problem of "oversized motor for a small load," saves on equipment investment costs, and improves system operating efficiency.
4) Good performance indicators: When the Y-series motor operates at 60% load, its efficiency decreases by 15%, its power factor decreases by 30%, and its performance indicators decrease by 40%; while the efficiency and power factor of the permanent magnet synchronous motor decrease very little. When the motor is only at 20% load, its performance indicators are still more than 80% of the full load.
5) Low temperature rise: There is no resistance loss in the rotor winding and almost no reactive current in the stator winding, so the motor temperature rise is low.
6) Small size, light weight, and less material consumption: The volume, weight, and materials used in a permanent magnet synchronous motor of the same capacity can be reduced by about 30%.
7) It can be made into a large gap, which is convenient for constructing new magnetic circuits.
8) The armature reaction is small and the overload resistance is strong.
2. Current Development Status
The development of permanent magnet synchronous motors is closely related to the development of permanent magnet materials. The emergence of new permanent magnet materials has greatly promoted the development of permanent magnet synchronous motors. In the 1980s, neodymium iron boron rare earth permanent magnet materials were introduced. Due to the abundance of neodymium resources, inexpensive iron replaced expensive cobalt, resulting in a relatively low price. Neodymium iron boron rare earth permanent magnet materials have excellent magnetic properties, which has greatly promoted the development of permanent magnet synchronous motors.
2.1 Development Achievements
my country attaches great importance to the research and development of neodymium iron boron permanent magnet motors and has included them in the National "863" Program. After years of research and development, fruitful results have been achieved, and 22 typical specifications of 5 types of high-performance permanent magnet synchronous motor prototypes have been developed.
1) Three typical specifications of high-efficiency, high-starting-torque permanent magnet synchronous motor prototypes successfully resolved the four mutually restrictive contradictions of high starting torque, good energy-saving effect, no demagnetization at high temperatures, and reasonable cost. Table 1 shows a performance comparison between a 37kW rare-earth permanent magnet synchronous motor and an induction motor developed in my country for oilfield pumping units. Table 2 shows a performance comparison between a newly developed 1120kW rare-earth permanent magnet synchronous motor, an induction motor, and an electrically excited synchronous motor for fan and pump operations in my country.
2) High-efficiency, high-traction synchronous neodymium iron boron permanent magnet synchronous motors for chemical fiber machinery (6 specifications). Compared with existing motors, the developed motors have improved power factor, efficiency, and maximum torque multiple to varying degrees. The out-of-step torque is 3.59 times that of the original, and the traction torque is increased by 3 times.
3) The machine tool spindle uses a 7.5kW high constant power speed ratio neodymium iron boron permanent magnet synchronous motor and drive system. The developed permanent magnet synchronous motor speed regulation system has a speed regulation range of 0.4r/min to 9000r/min (the speed regulation range of domestic spindle induction motors of the same specifications is only 8r/min to 8000r/min), and the constant power speed regulation ratio reaches 1:6.
4) Permanent Magnet Synchronous Motor and Drive System for Electric Vehicles. A 7.5kW permanent magnet synchronous motor system for a light-duty electric bus was developed. The motor weighs 45kg, the magnet weight is 0.92kg, the rated speed is 3000r/min, and the maximum speed is 5500r/min. The prototype system has an overall rated efficiency of 89.1%, a 1-hour continuous torque density of 0.74 N·m/kg (air-cooled), a 15-minute continuous torque density of 1.123 N·m/kg (compared to the Japanese AISIMAW prototype, which has a 1-hour continuous torque density of 0.78 N·m/kg) (oil-cooled), and a 15-minute continuous torque density of 1.178 N·m/kg.
5) High-starting-capacity NdFeB permanent magnet starter motor (4 prototype specifications). The developed motor replaces part of the original permanent magnet poles with inexpensive soft iron auxiliary poles, saving approximately 30% of NdFeB permanent magnet material.
2.2 Existing Problems
While achieving the above results in the development of high-performance permanent magnet synchronous motors, some problems have also been encountered, which require further in-depth research and exploration.
1) Irreversible demagnetization problem. If designed or used improperly, permanent magnet synchronous motors may experience irreversible demagnetization, or loss of magnetization, under excessively high (neodymium iron boron permanent magnet) or excessively low (ferrite permanent magnet) temperatures, due to the armature reaction caused by the impact current, or under severe mechanical vibration. This can lead to a decrease in motor performance or even render the motor unusable.
Therefore, it is necessary to research and develop methods and devices suitable for use in motor manufacturing plants to check the thermal stability of permanent magnet materials, and to analyze the demagnetization resistance of various structural types, so as to take corresponding measures during design and manufacturing to ensure that permanent magnet synchronous motors do not lose magnetism.
2) Cost Issues. Ferrite permanent magnet synchronous motors are widely used because their simple structure and manufacturing process, reduced weight, and overall cost are generally lower than those of electrically excited motors. However, rare-earth permanent magnets are currently relatively expensive, making rare-earth permanent magnet motors generally more expensive than electrically excited motors. This higher cost needs to be compensated for by their higher performance and lower operating costs. Therefore, the design process requires comparing performance and price based on specific application requirements, as well as innovating and optimizing the structural process to reduce costs.
3) Control Issues. Permanent magnet synchronous motors (PMSMs) can maintain their magnetic field without external energy, but this makes external adjustment and control of the magnetic field extremely difficult. However, with the development of power electronic devices and control technologies such as MOSFETs and IGBTs, most PMSMs can be used in applications without magnetic field control, only armature control. The design needs to combine three new technologies—permanent magnet materials, power electronic devices, and microcomputer control—to enable PMSMs to operate under new conditions. Furthermore, PMSM AC servo systems using PMSMs as actuators face challenges. Because PMSMs themselves are nonlinear, strongly coupled, and time-varying systems, and the servo object also exhibits significant uncertainty and nonlinearity, coupled with the system's susceptibility to varying degrees of interference, adopting advanced control strategies and advanced control system implementation methods (such as DSP-based control) to improve the overall intelligence and digitalization of the system should be a major breakthrough in the development of high-performance PMSM servo systems.
3. Development Trends
Permanent magnet synchronous motors (PMSMs) have been increasingly widely used in industrial production and daily life due to their high efficiency, high power density, simple structure, and significant energy-saving effects. In particular, the successful development of high-heat-resistant, high-magnetic-performance neodymium iron boron permanent magnets and the further development and improvement of power electronic components in recent years have ushered in a new era for the research and development of rare-earth PMSMs both domestically and internationally. This will lead to a qualitative leap in both theoretical research and application, and is currently developing towards ultra-high speed, high torque, high power, miniaturization, and high functionality.
3.1 Ultra-high speed motor
Permanent magnet synchronous motors (PMSMs) do not require excitation windings, have a relatively simple structure, no heat source in the magnetic field, do not require cooling devices, and utilize materials with high coercivity, allowing for larger air gap lengths and thus enabling significant speed increases. Motors with speeds of (2-3) x 10 r/min have already been manufactured, such as the 150kW, 23000r/min radial air gap rotor rare-earth permanent magnet generator for aviation developed by General Electric in the United States, and the 7.2kW, 27000r/min external rotor motor for electric vehicles. Motors with speeds of hundreds of thousands of revolutions per minute are currently under development.
3.2 High-torque, high-power motor
The successful development of heat-resistant, high-magnetic-performance neodymium iron boron permanent magnet materials will enable their important applications in high-power permanent magnet synchronous motors. The demand for high-power motors is growing in transportation and industrial sectors such as electric vehicles, hybrid (internal combustion engine and electric motor combined) vehicles, trains, elevators, machine tools, and robots.
Ship propulsion motors require low speed and high torque. In 1986, Siemens of Germany developed a 1095kW, 230r/min six-phase permanent magnet synchronous motor for ship propulsion. Compared to previously used DC motors, its size was reduced by approximately 60%, and losses by approximately 20%. Additionally, a 1760kW permanent magnet synchronous propulsion motor was tested on the U.S. 212 submarine, with its length and effective volume reduced by 40% compared to traditional DC propulsion motors. ABB of Switzerland has built over 300 electrically propelled ships with a maximum installed capacity of 2 x 19MW. Its 400kW to 3MW permanent magnet synchronous motors are used in the "Com-paetA~ipod" podded electric propulsion system. In 1987, the French company Gérard Industries developed a 400kW, 500r/min permanent magnet motor prototype, which also reduced its size by 40% compared to DC motors. In 1996, a 12-phase, 1800kW, 180r/min permanent magnet propulsion motor and control device were completed and all ship trials were completed. In the same year, Britain exhibited a design model of the light stealth frigate "Hailar". The ship is equipped with two 21MW permanent magnet synchronous motors that directly drive the propellers during cruising or stealth operations.
3.3 Miniaturization
Because neodymium iron boron permanent magnets have a very high maximum energy product, and especially because they can be made into ultra-thin permanent magnets, it has become possible to manufacture ultra-miniature and low-inertia motors that were previously difficult to produce. Currently, ultra-miniature motors with diameters of only a few millimeters have been developed for use as drive sources in medical micro-machines, robotic arms for eye surgery, and robots for pipeline inspection. The world's smallest permanent magnet motor, with an outer diameter of 0.8 mm and a length of 1.2 mm, has already been manufactured.
3.4 High Functionality
Traditional motors are difficult to use in special environments such as high temperature, high vacuum, or confined spaces. Rare-earth permanent magnet motors, however, can withstand high temperatures (referring to samarium cobalt or high heat-resistant neodymium iron boron magnets) and are small in size, perfectly meeting these special requirements. Motors operating in special environments, such as robotic arms in aerospace equipment, inspection robots in nuclear energy equipment, and semiconductor manufacturing devices, require high-temperature and high-vacuum motors. A 150W, 3000r/min three-phase four-pole permanent magnet motor has been developed, operating at 200°C–300°C and a vacuum of 133.3 x 10 Pa. It has a diameter of 105mm and a length of 145mm and uses Sm2Co permanent magnets with excellent high-temperature characteristics.
4. Conclusion
In the 21st century, with the rapid development of science and technology, the emergence of new high technologies, and the increasing awareness of energy conservation and environmental protection, the future of permanent magnet synchronous motors is bright. In particular, high-performance rare-earth permanent magnet synchronous motors and their servo systems will become increasingly diversified in structure as their technology develops rapidly and matures, and will gain a wider range of development space and wider applications.
