What is a permanent magnet synchronous motor?
Permanent magnet synchronous motor (PMSM) is a kind of synchronous motor with permanent magnet instead of winding. Permanent magnet synchronous motor can be divided into radial, axial or horizontal according to the flux mode. Depending on the layout of its components, all kinds of PMSM have different performance in efficiency, volume, weight and working speed.
The operation principle of permanent magnet synchronous motor is the same as that of electric excitation synchronous motor, but it uses the flux provided by permanent magnet to replace the excitation winding of the latter, which makes the structure of the motor simpler. PMSM is a synchronous motor with synchronous rotating magnetic field excited by permanent magnet. Permanent magnet generates rotating magnetic field as rotor, and three-phase stator winding induces three-phase symmetrical current through armature reaction under the action of rotating magnetic field. At this time, the kinetic energy of the rotor is converted into electrical energy, and PMSM is used as a generator; in addition, when three-phase symmetrical current is applied to the stator side, the three-phase stator current generates a rotating magnetic field in space due to the space position difference of the three-phase stator. The rotor rotating magnetic field is moved by electromagnetic force, and the electric energy is converted into kinetic energy, and PMSM is used as a motor.
Characteristics of permanent magnet motor
PMSM adopts permanent magnet excitation, which has incomparable advantages of electric excitation motor.
- (1) High efficiency: after the permanent magnet material is embedded in the rotor, the rotor and stator magnetic field operate synchronously in normal operation, the rotor winding has no induced current, there is no rotor resistance and hysteresis loss, which improves the motor efficiency.
- (2) High power factor: there is no induced current excitation in PMSM rotor, and the stator winding presents resistive load. The power factor of the motor is close to 1, which reduces the stator current and improves the efficiency of the motor. At the same time, the improvement of power factor can improve the quality factor of power grid, reduce the loss of transmission and transformation lines, and reduce the transmission and transformation capacitance, thus saving the investment of power grid.
- (3) Large starting torque: in the equipment that needs large starting torque (such as oil field pumping motor), the smaller capacity permanent magnet motor can replace the larger capacity Y series motor. If 37KW permanent magnet synchronous motor replaces 45KW ~ 55kW Y series motor, the phenomenon of “big horse pulling small car” can be solved, the equipment investment cost can be saved and the operation efficiency of the system can be improved.
- (4) Good force and energy index: when y series motor works under 60% load, the efficiency decreases by 15%, the power factor decreases by 30%, and the force energy index decreases by 40%; while the efficiency and power factor of PMSM decrease little, when the motor has only 20% load, its power index is still more than 80% of 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 temperature rise of the motor is low.
- (6) Small size, light weight, less consumables: the volume, weight and materials used for PMSM with the same capacity can be reduced by about 30%.
- (7) It is easy to form a new magnetic circuit.
- (8) Armature reaction is small, anti overload ability is strong.
Problems of permanent magnet motor
In the process of developing high performance permanent magnet synchronous motor, there are some problems
- (1) Irreversible demagnetization. If it is not designed or used properly, the permanent magnet synchronous motor may produce irreversible demagnetization or loss of excitation when the temperature is too high (NdFeB permanent magnet) or too low (ferrite permanent magnet) under the action of armature reaction generated by impact current or severe mechanical vibration, so that the performance of the motor will decline or even can not be used. Therefore, it is necessary to research and develop the methods and devices for checking the thermal stability of permanent magnet materials suitable for motor manufacturers, and analyze the anti demagnetization ability of various structural types, so as to adopt corresponding measures to ensure the permanent magnet synchronous motor does not lose excitation when designing and manufacturing.
- (2) Cost. Ferrite permanent magnet synchronous motor (PMSM) is widely used because of its simple structure, low weight and low total cost. Because the price of rare earth permanent magnet is still relatively expensive, the cost of rare earth permanent magnet motor is generally higher than that of electric excitation motor, which needs to be compensated by its high performance and operating cost saving. In the design, it is necessary not only to compare the performance and price according to the specific application situation and requirements, but also to innovate the structure technology and optimize the design to reduce the cost.
- (3) Control issues. Permanent magnet synchronous motor can maintain its magnetic field without external energy, but it is very difficult to adjust and control its magnetic field from the outside. However, with the development of MOSFET, IGBT and other power electronic devices and control technology, most of the permanent magnet synchronous motor in the application, can not control the magnetic field but only the armature control. It is necessary to combine the three new technologies of permanent magnet material, power electronic devices and microcomputer control to make the permanent magnet synchronous motor operate in a brand-new working condition. In addition, permanent magnet synchronous motor (PMSM) is a kind of nonlinear, strong coupling and time-varying system. At the same time, its servo object also has strong uncertainty and nonlinearity. In addition, the system is easy to be disturbed in varying degrees. Therefore, advanced control strategy and advanced control system are adopted in the system The current mode (such as DSP control) can improve the intelligent and digital level of the system as a whole, which should be a major breakthrough in the development of high-performance permanent magnet synchronous motor servo system.
Development trend of permanent magnet motor
Permanent magnet synchronous motor (PMSM) has been widely used in industrial production and daily life because of its high efficiency, high specific power, simple structure and remarkable energy saving effect. Especially in recent years, with the successful development of NdFeB permanent magnet with high heat resistance and high magnetic performance and the further development and improvement of power electronic components, the research and development of rare earth permanent magnet synchronous motor has entered a new period at home and abroad. It will produce a qualitative leap in the field of theoretical research and application. At present, it is moving towards ultra-high speed, high torque, high power, miniaturization and high functionality to develop.
Ultra high speed motor
Permanent magnet synchronous motor does not need excitation winding, its structure is relatively simple, there is no heat source in the magnetic field, no cooling device is needed, the coercivity of the material is high, and the length of air gap can be taken as a large value, which makes it possible to greatly increase the speed. At present, (2-3) × 104R / min motors have been made, such as 150KW, 23000r / min radial air gap rotor structure aviation rare earth permanent magnet generator developed by General Electric Company of the United States, and the outer rotor type is used for 7.2kw, 27000r / min motor for electric vehicles.
High torque and high power motor
The successful development of heat-resistant and high magnetic properties NdFeB permanent magnet material will make it an important application in high-power permanent magnet synchronous motor. In transportation and industry, such as electric vehicles, hybrid electric vehicles, trains, elevators, machine tools, robots, etc., the demand for high-power motors is growing. Ship propulsion motor requires low speed and high torque. A 1095kw, 230r / min six phase permanent magnet synchronous motor was developed by Siemens in 1986. Compared with the DC motor used in the past, its volume and loss can be reduced by 60% and 20% respectively. In addition, the length and effective volume of 1760kw PMSM installed in u-212 submarine are reduced by 40% compared with the traditional DC propulsion motor. ABB has built more than 300 electric propulsion ships with a maximum installed capacity of 2 × 19mw. Its 400KW to 3MW permanent magnet synchronous motors are used in the “com pact Azipod” pod electric propulsion system. Compared with the DC motor, the volume of the 400 kW, 500 R / min permanent magnet motor prototype developed by French rimont Industrial Company in 1987 also reduced by 40%. In 1996, 12 phase, 1800kW, 180r / min permanent magnet propulsion motor and control device have been developed and all ship tests have been completed. In the same year, Britain exhibited the design model of the light stealth frigate HNA. The ship is equipped with two 21mw permanent magnet synchronous motors to drive the propeller directly during cruising or stealth.
Because the maximum energy product of NdFeB permanent magnet is very high, especially it can be made into ultra-thin permanent magnet, the ultramicro and low inertia motor which was difficult to make in the past can be realized. At present, subminiature motors with diameters less than a few millimeters have been developed as driving sources for medical micro machines, robot arms for eye surgery or robots for pipeline inspection. The smallest permanent magnet motor with an outer diameter of 0.8mm and a length of 1.2mm has been made.
It is difficult to use traditional motor in high temperature, high vacuum or narrow space. However, rare earth permanent magnet motor can withstand high temperature (refer to samarium cobalt or high heat resistance NdFeB magnet), and its volume is small, which can meet these special requirements. High temperature motors and high vacuum motors are needed in special environments such as manipulator, inspection robot of atomic energy equipment and semiconductor manufacturing device. A three-phase four pole permanent magnet motor with a diameter of 105mm and a length of 145mm has been developed, which operates at a high temperature of 200 ℃ ~ 300 ℃ and a vacuum of 133.3 × 10 – 6pa. The Sm2Co17 permanent magnet with good high temperature characteristics is used.