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Introduction on permanent magnet motor

The development history of permanent magnet motor

The development of permanent magnet motors is closely related to the development of permanent magnet materials.China is the earliest country in the world to discover the magnetic specialty of permanent magnet materials and apply it to practice. More than 2,000 years ago, China used the magnetic specialty of permanent magnet materials to make a compass. The role has become one of the four major inventions of ancient China.

The world’s first motor in the world in the 1920s was permanent magnet motor to generate a magnetic field by permanent magnet. However, the permanent magnet material used at that time was natural magnet ore (Fe3O4). The magnetic energy density was very low. The motor made with it was large and was soon replaced by the excitation motor.

With the needs of the rapid development of various motors and the invention of current magnetic chargers, people have conducted in -depth research on the mechanism, composition and manufacturing technology of permanent magnet materials, and have been discovered by carbon steel and tungsten steel (maximum magnetic energy accumulated about 2.7 kj/m3 ) Various permanent magnet materials such as cobalt steel (maximum magnetic energy accumulated about 7.2 kj/m3). Especially in the 1930s, aluminum -nickel cobalt permanent magnets (maximum magnetic energy accumulation of 85 kJ/m3) and the permanent magnet of the ferrite permanent magnet (maximum magnetic energy accumulation can reach 40 kj/m3) in the 1950s, and the magnetic performance has greatly improved, and various micro and small motors use permanent magnets. The power of the permanent magnet motor is as small as milliled to several dozen watts. It has been widely used in  industrial and agricultural production and daily life, and the output has increased sharply. Correspondingly, this period has also made breakthrough progress in the design theory, calculation methods, capacity charging and manufacturing technology of the permanent magnet motor, and forms a set of analysis and research methods represented by the permanent magnetic map interpretation method. However, the stubborn force of aluminum nickel cobalt permanent magnet (36 ~ 160 ka/m), the remaining magnetic density of the permanent magnet of the iron oxygen is not high (0.2 ~ 0.44 t), which limits their application range in the motor. Until the 1960s and 1980s, the rare earth cobalt permanent magnet and the iron boron permanent magnet (both collectively referred to as rare earth permanent magnets) came out one after another. The excellent magnetic properties are particularly suitable for manufacturing motors, so that the development of permanent magnet motors has entered a new historical period.

The characteristics and application of permanent magnet motor

Compared with the traditional electrical magnetic motor, permanent magnet motor, especially the rare earth permanent magnet motor, has simple structure, reliable operation, small volume, light quality, small loss, high efficiency, the shape and size of the motor can be flexible and diverse. Therefore, the scope of application is extremely wide, and it is almost all over the fields of industrial and agricultural production and daily life. Here are the main features of several typical permanent magnet motors and their main application occasions.

1. Rare earth permanent magnet synchronous generator does not require a collection ring and brush device compared to the traditional generator. The structure is simple and reduced the failure rate. By rare earth permanent magnet, it can also increase the air gap magnetic tight, and increase the  speed to the optimal value, and increase the power quality ratio. Almost all of contemporary aviation and aerospace generators use rare earth permanent magnet generators.The permanent magnet generator was also used as an associate incentive machine for large steam wheel generators. At present, the internal combustion engine driving small generator, the permanent magnet generator, and the vanguard direct -driven small permanent magnet air generator for independent power supply are gradually being promoted.

2. Compared with the inductive motor, for efficient permanent magnet synchronous motor, there is no need to have no power incentive current, which can significantly improve the power factor (can reach 1, even capability), reduce the loss of stator current and stator resistance, and the there is no rotor copper consumption during stable operation, and it can reduce the fan (small -capacity motor can even remove the fan) and the corresponding wind loss. The efficiency can increase by 2 to 8 percentage points than the same specification sensor. Moreover, the permanent magnet synchronous motor can maintain higher efficiency and power factor within the range of 25% to 120% of the rated load, making the energy daily effect more significant during the runtime of light load. This type of motor generally sets up 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 the fields of oil fields, textile chemical fiber industries, ceramic glass industries and fan pumps with long annual operating time.

China independently developed high -efficiency permanent magnet synchronization motor can solve the “big horse to drive small car” problem in the application of oilfield. The starting torque is 50% to 100% larger than the induction motor. The inductive motor is about 20%.

The load rotation in the textile chemical fiber industry is large and requires high torque. Reasonably design the empty load leakage coefficient, convex ratio, rotor resistance, permanent magnet size and stator winding turns of the permanent magnet synchronous motor, which can improve the entry performance of the permanent magnet motor and promote it to the new type of textile and chemical fiber industry.

Hundreds of kilowatts, Milk, Petroleum, Chemical and other industries are used by hundreds of kW and Muwa -grade fans and pump motors. Improve efficiency and power factor, save energy, and improve the reliability of operation.

3. AC servo permanent magnet motor and brushless DC permanent magnet motor now increasingly use the inverter power supply and AC electric motor set to replace the DC motor speed regulation system. In the AC motor, the speed of the permanent magnet synchronous motor maintains a constant relationship with the power frequency when running stable, so that it can be used directly for the frequency conversion speed regulating system that can be used to open the loop. This type of motor usually starts from the gradual increase of the frequency of the inverter. It can be set up on the rotor without setting a starting winding, and the brush and the directioner are saved to maintain convenient maintenance.

The permanent magnet synchronous motor of the inverter and the rotor position closed -loop control system constitutes a self -synchronous permanent magnet motor. It not only has the excellent speed regulating performance of the electromotive -mobilizing DC motor, but also realizes brushless. It is mainly used for high -control accuracy and high reliability occasions, such as aviation, aerospace, CNC machine tools, processing centers, robots, electric vehicles, computer peripheral equipment, etc.

The NdFEB permanent magnet synchronous motor and drive system with wide speed range and high constant power speed ratio have been developed. The speed ratio is as high as 1:22,500 and the limit speed is as high as 9 000 r/min. Permanent magnet synchronous motor is the most ideal motor in electric vehicle, machine tool and other driving devices because of its high efficiency, low vibration, low noise and high torque density.

With the continuous improvement of people’s living standards, the requirements for household appliances are getting higher and higher. For example, household air conditioners are not only power consumption, but also the main source of noise, and its development trend is to use permanent magnet brushless DC motors that can be stepless speed regulation. It can automatically adjust to the appropriate speed for a long time according to the change of room temperature, reduce noise and vibration, make people feel more comfortable, but also save 1/3 than the air conditioner without speed regulation. Others such as refrigerators, washing machines, dust collectors, fans, etc., are also gradually switching to brushless DC motors.

4. Permanent magnet DC motor after the use of permanent magnet excitation, not only retains the good speed regulation characteristics and mechanical characteristics of the electric excitation DC motor, but also has a simple structure, small volume, less copper, high efficiency because of the elimination of excitation winding and excitation loss. Therefore, from household appliances, portable electronic equipment, power tools to precision speed and position transmission systems that require good dynamic performance, permanent magnet DC motors are widely used. In micro DC motors below 500 W, permanent magnet motors account for 92%, while permanent magnet motors below 10 W account for more than 99%.

At present, China’s automobile industry is developing rapidly, the automobile industry is the largest user of permanent magnet motor, motor is a key part of the car, a super luxury car, a variety of different uses of the motor up to more than 70 sets, most of which are low-voltage permanent magnet DC micromotors. Starting motor for cars and motorcycles, using NdFeb permanent magnet and using decelerating planetary gear, can reduce the mass of the starting motor by half.

Classification of permanent magnet motors

There are many kinds of permanent magnet motors. According to the motor function can be roughly divided into permanent magnet generator and permanent magnet motor two categories.

Permanent magnet motor can be divided into permanent magnet DC motor and permanent magnet AC motor. Permanent magnet AC motor refers to a polyphase synchronous motor with a permanent magnet rotor, so it is often called a permanent magnet synchronous motor (PMSM).

Permanent magnet DC motor if there is no electric drama and commutator to divide. It can be divided into permanent magnet brushless direct current motor and permanent magnet brushless direct current motor (BLDC).

In today’s world, the theory and technology of modern power electronics are developing greatly. Power electronic devices, such as MOSFET, IGBT, MCT, etc., continue to come out, and the control device has undergone fundamental changes. Since F. BlascEke proposed the principle of AC motor vector control in 1971, the development of vector control technology has created a new era of AC servo drive control, and the continuous introduction of various high-performance microprocessors has further accelerated the development of AC servo systems to replace DC servo systems. Ac servo system replacing DC servo system has become an inevitable trend. However, permanent magnet synchronous motor (PMSM) with sine wave back potential and brushless direct current motor (BLDC) with trapezoidal wave back potential will become the mainstream of developing high-performance AC servo system because of their excellent performance

Permanent magnet motor related precautions

1. Magnetic circuit structure and design calculation

In order to give full play to the magnetic properties of various permanent magnet materials, especially the excellent magnetic properties of rare earth permanent magnets, and create cost-effective permanent magnet motors, it is not possible to simply apply the structure and design calculation methods of traditional permanent magnet motors or electric excitation motors, and new design concepts must be established to re-analyze and improve the magnetic circuit structure. With the rapid development of computer hardware and software technology, as well as the continuous improvement of modern design methods such as electromagnetic field numerical calculation, optimization design and simulation technology, through the joint efforts of the academic and engineering circles, breakthrough progress has been made in the design theory, calculation method, structure process and control technology of permanent magnet motor. A complete set of analysis and research methods and computer-aided analysis and design software, which are combined with numerical calculation of electromagnetic field and analytical solution of equivalent magnetic circuit, have been formed, and are being constantly improved.

2. Control issues

After the permanent magnet motor is made, it can maintain its magnetic field without external energy, but it also makes it extremely difficult to adjust and control its magnetic field from the outside. The permanent magnet generator is difficult to adjust its output voltage and power factor from the outside, and the permanent magnet DC motor can no longer adjust its speed by changing the excitation. These limit the application range of permanent magnet motors. However, with the rapid development of power electronic devices and control technology such as MOSFET and IGBT, most permanent magnet motors can be used without magnetic field control and only armature control. It is necessary to combine the three new technologies of rare earth permanent magnet materials, power electronic devices and microcomputer control in the design, so that the permanent magnet motor can operate under new conditions.

3. Irreversible demagnetization problem

If the design or use is improper, the permanent magnet motor is too high (NdFeb permanent magnet) or too low (ferrite permanent magnet) temperature, under the action of the armature reaction generated by the impact current, or in the violent mechanical vibration may produce irreversible demagnetization, or loss of magnetism, so that the motor performance is reduced, or even can not be used. Therefore, it is necessary to research and develop methods and devices suitable for motor manufacturers to check the thermal stability of permanent magnet materials, and to analyze the anti-demagnetization ability of various structural forms, so as to adopt corresponding measures to ensure that the permanent magnet motor does not lose magnetism when designing and manufacturing.

4. Cost

Ferrite permanent magnet motor, especially micro permanent magnet DC motor, has been widely used because of its simple structure and process, reduced mass, and lower total cost than electric excitation motor. Since the current 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 savings in operating costs. In some cases, such as the voice coil motor of the computer disk drive, the performance of the Ndfeb permanent magnet is improved, the volume mass is significantly reduced, and the total cost is reduced. In the design, it is necessary to decide whether to choose according to the performance and price comparison according to the specific use occasions and requirements, and to carry out the innovation and design optimization of the structure process to reduce the cost.

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