Principle, characteristics and application of permanent magnet synchronous motor

The motor is very important for industry and agriculture. This paper will introduce the definition, classification and classification of motor drive, and introduce the principle, characteristics and application of permanent magnet synchronous motor in detail.

Motor definition

The so-called motor, as its name suggests, is a kind of power component that converts electrical energy and mechanical energy into each other. When electrical energy is converted into mechanical energy, the electric motor exhibits the operating characteristics of the electric motor; when electrical energy is converted into mechanical energy, the electric motor exhibits the operating characteristics of the electric generator. The motor is mainly composed of rotor, stator winding, speed sensor, housing, cooling and other components.

Motor classification

Divided by structure and working principle: DC motor, asynchronous motor, synchronous motor.

Divided by the type of working power: can be divided into DC motor and AC motor.

AC motors can also be divided into single-phase motors and three-phase motors.

DC motors can be divided according to structure and working principle: brushless DC motor and brushed DC motor.

Brushed DC motors can be divided into: permanent magnet DC motors and electromagnetic DC motors.

Electromagnetic DC motor division: series-excited DC motor, shunt DC motor, separately excited DC motor and compound excitation DC motor.

Permanent magnet DC motor division: rare earth permanent magnet DC motor, ferrite permanent magnet DC motor and AlNiCo permanent magnet DC motor.

Divided by structure and working principle: can be divided into DC motor, asynchronous motor, synchronous motor.

Synchronous motors can be divided into: permanent magnet synchronous motors, reluctance synchronous motors and hysteresis synchronous motors.

Asynchronous motors can be divided into: induction motors and AC commutator motors.

Induction motors can be divided into three-phase asynchronous motors, single-phase asynchronous motors and shaded-pole asynchronous motors.

The AC commutator motor can be divided into: single-phase series-excited motor, AC-DC motor and repulsive motor.

According to the starting and running modes: capacitor-starting single-phase asynchronous motor, capacitor-operated single-phase asynchronous motor, capacitor-starting single-phase asynchronous motor and split-phase single-phase asynchronous motor.

Divided by purpose: drive motor and control motor.

Permanent magnet synchronous motor

The so-called permanent magnet refers to the addition of permanent magnets when manufacturing the rotor of the motor, so that the performance of the motor is further improved. The so-called synchronization means that the rotational speed of the rotor is always consistent with the current frequency of the stator winding. Therefore, by controlling the input current frequency of the stator winding of the motor, the vehicle speed of the electric vehicle will eventually be controlled. How to adjust the current frequency is the problem to be solved in the electronic control part.

Characteristics of permanent magnet synchronous motor

The permanent magnet motor has a higher power/mass ratio, smaller volume and lighter weight. It has larger output torque than other types of motors, and the motor's limit speed and braking performance are also excellent. Therefore, the permanent magnet synchronous motor has become The electric motor with the most applications in electric vehicles today. However, when the permanent magnet material is subjected to vibration, high temperature and overload current, its magnetic permeability may be degraded or demagnetization may occur, which may reduce the performance of the permanent magnet motor. In addition, the rare earth permanent magnet synchronous motor uses rare earth materials, and the manufacturing cost is not stable.

Permanent magnet synchronous motor and asynchronous motor

In addition to permanent magnet synchronous motors, asynchronous motors have also received extensive attention due to the use of Tesla. Compared to synchronous motors, the rotational speed of the rotor of the motor is always less than the rotational speed of the rotating magnetic field (generated by the stator winding current). Therefore, the rotor appears to be "inconsistent" with the current frequency of the stator winding, which is why it is called an asynchronous motor.

Compared with permanent magnet synchronous motors, asynchronous motors have the advantages of low cost and simple process; of course, the disadvantage is that their power density and torque density are lower than those of permanent magnet synchronous motors. Why does Tesla ModelS use asynchronous motors instead of permanent magnet synchronous motors? In addition to the main reason for controlling costs, it is also important that larger ModelS bodies have enough space to place relatively large asynchronous motors. factor.

How does a permanent magnet synchronous motor generate power?

In an AC asynchronous motor, the rotor magnetic field is formed in two steps: the first step is that the stator rotating magnetic field first induces current in the rotor winding; the second step is to induce current to generate the rotor magnetic field. Under the action of Lenz's law, the rotor follows the rotating magnetic field of the stator, but it "can never catch up", so it is called an asynchronous motor. If the current in the rotor winding is not induced by the rotating magnetic field of the stator, but is generated by itself, the rotor magnetic field is independent of the rotating magnetic field of the stator, and the magnetic pole direction is fixed. Then, according to the principle of homosexual repulsive and opposite-phase attracting, the stator The rotating magnetic field pulls the rotor to rotate and rotates the rotor field and the rotor in synchronism with the stator's rotating magnetic field. This is how the synchronous motor works.

According to the different ways of generating the self-generated magnetic field of the rotor, the synchronous motor can be divided into two types:

First, the rotor winding is connected to an external direct current (excitation current), and then the excitation current generates a rotor magnetic field, thereby rotating the rotor and the stator magnetic field synchronously. Such a synchronous motor that generates a rotor magnetic field from an exciting current is called an exciting synchronous motor.

The second is to simply embed a permanent magnet on the rotor to directly generate a magnetic field, eliminating the need for excitation current or induced current. Such a synchronous motor that generates a rotor magnetic field from a permanent magnet is called a permanent magnet synchronous motor.

Application of permanent magnet synchronous motor

Permanent magnet synchronous motors are widely used in industry and agriculture. Over the past decade, due to the emergence and use of new technologies, new processes and new devices, the excitation mode of permanent magnet synchronous motors has been continuously developed and improved. In the aspect of automatic adjustment of the excitation device, many new types of adjustment devices have been continuously developed and promoted. At present, many countries are developing and testing digital automatic adjustment excitation devices composed of microcomputers and corresponding external devices, which will enable adaptive optimal adjustment.

Permanent magnet synchronous motors require a large number of production machines in industrial and agricultural production to continuously operate in a single direction at roughly constant speeds, such as fans, pumps, compressors, and general machine tools. The permanent magnet synchronous motor has low cost, simple and reliable structure and convenient maintenance, and is very suitable for the driving of such a machine.

Of course, with the advantages of permanent magnet synchronous motors, the majority of new energy vehicle manufacturers prefer permanent magnet synchronous motors, which are also widely used in elevators.

Motor driven classification

According to the requirements of different industrial and agricultural production machinery, motor drive is divided into three types: fixed speed drive, speed control drive and precision control drive.

1. There are a large number of production machines in fixed-speed driving industrial and agricultural production that require continuous operation in a single direction at roughly constant speed, such as fans, pumps, compressors, and general machine tools. In the past, most of these machines were driven by three-phase or single-phase asynchronous motors. Asynchronous motors are low in cost, simple in structure and easy to maintain, and are very suitable for driving such machines. However, the asynchronous motor has low efficiency, low power factor and large loss, and the use of such motors is large, so a large amount of electric energy is wasted in use. Secondly, fans and pumps that are used in large quantities in industrial and agricultural areas often need to adjust their flow rates, usually by adjusting the dampers and valves, which wastes a lot of electrical energy. Since the 1970s, people used inverters to adjust the speed of asynchronous motors in fans and pumps to adjust their flow rate, and achieved considerable energy-saving effects. However, the cost of the inverter limits its use, and the low efficiency of the asynchronous motor itself still exists.

2. The speed control drive has quite a lot of working machines, and its running speed needs to be arbitrarily set and adjusted, but the speed control accuracy requirements are not very high. Such drive systems have a large number of applications in packaging machinery, food machinery, printing machinery, material handling machinery, textile machinery and transportation vehicles. The most used in this kind of speed regulation application field is the DC motor speed control system. After the development of power electronics technology and control technology in the 1970s, the variable frequency speed regulation of asynchronous motor quickly penetrated into the application field of the original DC speed control system. . This is because on the one hand, the performance price of the asynchronous motor variable frequency speed control system is comparable to that of the DC speed control system. On the other hand, the asynchronous motor has a simple manufacturing process, high efficiency, and less copper for the same power motor than the DC motor. The advantages of convenient maintenance and so on. Therefore, the asynchronous motor frequency conversion speed regulation has quickly replaced the DC speed regulation system in many occasions.

3, precision control drive

1 High-precision servo control system Servo motor plays an important role in the operation control of industrial automation. The application performance requirements of servo motor are also different. In practical applications, servo motors have various control methods, such as torque control/current control, speed control, and position control. The servo motor system has also experienced DC servo system, AC servo system, stepper motor drive system, and until recently, the most attractive permanent magnet motor AC servo system. Most of the imported automation equipment, automatic processing equipment and robots imported in recent years have adopted the AC servo system of permanent magnet synchronous motor.

2 Permanent magnet synchronous motor in information technology Today's information technology is highly developed, and various computer peripherals and office automation equipment are also highly developed. The key components of the micro-motors are in high demand, and the precision and performance requirements are also higher. . The requirements for such micromotors are miniaturization, thinning, high speed, long life, high reliability, low noise and low vibration, and the accuracy requirements are particularly high. For example, the spindle drive motor for a hard disk drive is a permanent magnet brushless DC motor, which drives the disk to rotate at a high speed of nearly 10,000 rpm, and the magnetic head performing data reading and writing on the disk is suspended at a distance of 0.1 to 0.3 μm from the surface of the disk. The accuracy requirements can be imagined. Most of the driving motors used in various information equipment such as printers, soft hard disk drives, optical disk drives, facsimile machines, copying machines, etc. are permanent magnet brushless DC motors. Limited by the technical level, such micro-motors are currently not manufactured in China, and some products are assembled in China.