Compared with traditional electric excitation motors, permanent magnet motors, especially rare earth permanent magnet motors, have simple structure and reliable operation. Small volume and light weight; Low loss and high efficiency; The shape and size of the motor can be flexible and diverse. Therefore, the application range is extremely wide, almost all over the fields of aerospace, national defense, industrial and agricultural production and daily life. The main features and applications of several typical permanent magnet motors are introduced below.
1. Compared with traditional generators, rare earth permanent magnet synchronous generators don’t need slip rings and brush devices, with simple structure and reduced failure rate. The rare earth permanent magnet can also increase the air gap magnetic density, increase the motor speed to the optimal value and improve the power-to-mass ratio. Rare earth permanent magnet generators are almost all used in contemporary aviation and aerospace generators. Its typical products are 150 kVA 14-pole 12 000 r/min ~ 21 000 r/min and 100 kVA 60 000 r/min rare earth cobalt permanent magnet synchronous generators manufactured by General Electric Company of America. The first rare earth permanent magnet motor developed in China is a 3 kW 20 000 r/min permanent magnet generator.
Permanent magnet generators are also used as auxiliary exciters for large turbo-generators. In 1980s, China successfully developed the world’s largest rare earth permanent magnet auxiliary exciter with capacity of 40 kVA～160 kVA, and equipped with 200 MW ~ 600 MW turbo-generators, which greatly improved the reliability of power station operation.
At present, small generators driven by internal combustion engines, permanent magnet generators for vehicles, and small permanent magnet wind generators directly driven by wind wheels are gradually being popularized.
2. High-efficiency permanent magnet synchronous motor Compared with induction motor, permanent magnet synchronous motor does not need reactive excitation current, which can significantly improve the power factor (up to 1 or even capacitive), reduce the stator current and stator resistance loss, and there is no rotor copper loss during stable operation, thus reducing the fan (small capacity motor can even remove the fan) and the corresponding wind friction loss. Compared with induction motor of the same specification, the efficiency can be increased by 2 ~ 8 percentage points. Moreover, the permanent magnet synchronous motor can keep high efficiency and power factor in the rated load range of 25% ~ 120%, which makes the energy saving effect more remarkable when running under light load. Generally, this kind of motor is equipped with a starting winding on the rotor, which has the ability to start directly at a certain frequency and voltage. At present, it is mainly used in oil fields, textile and chemical fiber industries, ceramic and glass industries, fans and pumps with long annual operation time, etc.
The NdFeB permanent magnet synchronous motor with high efficiency and high starting torque independently developed by our country can solve the problem of “big horse-drawn cart” in oilfield application. The starting torque is 50% ~ 100% larger than that of the induction motor, which can replace the induction motor with a larger base number, and the power saving rate is about 20%.
In the textile industry, the load moment of inertia is large, which requires high traction torque. Reasonable design of no-load leakage coefficient, salient pole ratio, rotor resistance, permanent magnet size and stator winding turns of permanent magnet synchronous motor can improve the traction performance of permanent magnet motor and promote its application in new textile and chemical fiber industries.
The fans and pumps used in large-scale power stations, mines, petroleum, chemical and other industries are large energy consumers, but the efficiency and power factor of the motors used at present are low. Using NdFeB permanent magnets not only improves the efficiency and power factor, saves energy, but also has a brushless structure, which improves the reliability of operation. At present, 1 120kW permanent magnet synchronous motor is the world’s most powerful asynchronous starting high-efficiency rare earth permanent magnet motor. Its efficiency is higher than 96.5% (the same specification motor efficiency is 95%), and its power factor is 0.94, which can replace the ordinary motor with 1 ~ 2 power grades larger than it.
3. AC servo permanent magnet motor and brushless DC permanent magnet motor now more and more use variable frequency power supply and AC motor to form AC speed control system instead of DC motor speed control system. In AC motors, the speed of permanent magnet synchronous motor keeps a constant relationship with the frequency of power supply during stable operation, so that it can be directly used in open-loop variable frequency speed control system. This kind of motor is usually started by the gradual increase of the frequency of the frequency converter. It is not necessary to set the starting winding on the rotor, and the brush and commutator are omitted, so the maintenance is convenient.
Self-synchronous permanent magnet motor is composed of permanent magnet synchronous motor powered by frequency converter and closed-loop control system of rotor position, which not only has excellent speed regulation performance of electrically excited DC motor, but also realizes brushless. It is mainly used in occasions with high control accuracy and reliability, such as aviation, aerospace, CNC machine tools, machining centers, robots, electric vehicles, computer peripherals, etc.
At present, the NdFeB permanent magnet synchronous motor and drive system with wide speed range and Gao Heng power speed ratio have been developed, with the speed ratio of 1: 22 500 and the limit speed of 9 000 r/min. The characteristics of high efficiency, small vibration, low noise and high torque density of permanent magnet motor are the most ideal motors in electric vehicles, machine tools and other driving devices.
With the continuous improvement of people’s living standard, the requirements for household appliances are getting higher and higher. For example, the household air conditioner is not only a big power consumer, but also the main source of noise. Its development trend is to use permanent magnet brushless DC motor with stepless speed regulation. It can automatically adjust to a suitable speed according to the change of room temperature and run for a long time, reducing noise and vibration, making people feel more comfortable, and saving 1/3 of electricity compared with the air conditioner without speed regulation. Other refrigerators, washing machines, dust collectors, fans, etc. are gradually changing to brushless DC motors.
4. Permanent magnet DC motor DC motor adopts permanent magnet excitation, which not only retains the good speed regulation characteristics and mechanical characteristics of electrically excited DC motor, but also has the characteristics of simple structure and technology, small volume, low copper consumption, high efficiency, etc. because excitation winding and excitation loss are omitted. Therefore, permanent magnet DC motors are widely used from household appliances, portable electronic devices, electric tools to precision speed and position transmission systems that require good dynamic performance. Among micro DC motors under 50W, permanent magnet motors account for 92%, while those under 10 W account for more than 99%.
At present, China’s automobile industry is developing rapidly, and the automobile industry is the largest user of permanent magnet motors, which are the key components of automobiles. In an ultra-luxury car, there are more than 70 motors with different purposes, most of which are low-voltage permanent magnet DC micromotors. When NdFeB permanent magnets and planetary gears are used in starter motors for automobiles and motorcycles, the quality of starter motors can be reduced by half.
Classification of Permanent Magnet Motors
There are many kinds of permanent magnets. According to the function of motor, it can be roughly divided into two categories: permanent magnet generator and permanent magnet motor.
Permanent magnet motors can be divided into permanent magnet DC motors and permanent magnet AC motors. The permanent magnet AC motor refers to the multi-phase synchronous motor with permanent magnet rotor, so it is often called permanent magnet synchronous motor (PMSM).
Permanent magnet DC motors can be divided into permanent magnet brushless DC motors and permanent magnet brushless DC motors (BLDCM) if they are classified according to whether there are electric switches or commutators.
Nowadays, the theory and technology of modern power electronics are developing greatly in the world. With the advent of power electronic devices, such as MOSFET, IGBT and MCT, the control devices have undergone fundamental changes. Since F. Blaceke put forward the principle of vector control of AC motor in 1971, the development of vector control technology has initiated a new era of AC servo drive control, and various high-performance microprocessors have been continuously pushed out, further accelerating the development of AC servo system instead of DC servo system. It is an inevitable trend that AC-I servo system replaces DC servo system. However, permanent magnet synchronous motor (PMSM) with sinusoidal back emf and brushless DC motor (BLIX~) with trapezoidal back emf will surely become the mainstream of developing high-performance AC servo system because of their excellent performance.