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Introduction of Permanent Magnet Synchronous Motor for New Energy Pure Electric Vehicles
The permanent magnet synchronous motor has the characteristics of high efficiency, high control accuracy, high torque density, good torque stability and low vibration and noise. Through the reasonable design of the permanent magnet magnetic circuit structure, it can obtain high magnetic field weakening performance. It has high application value and is highly valued by the electric vehicle industry at home and abroad. It is one of the most competitive electric vehicle drive motor systems.
First, the structure of permanent magnet synchronous motor
Permanent magnet synchronous motors are divided into permanent magnet synchronous motors with sine wave drive current and permanent magnet synchronous motors with square wave drive current. Introduced here is mainly a three-phase sine wave driven permanent magnet synchronous motor.
The schematic diagram of the structure of the permanent magnet synchronous motor is shown in Figure 1-81. Like the traditional motor, it is mainly composed of two parts: the stator and the rotor.
The stator is basically the same as that of an ordinary motor, consisting of an armature iron core and an armature winding. The armature core is generally made of 0.5mm silicon steel punched sheets. For motors with high efficiency indicators or high frequencies, in order to reduce iron consumption, 0.35mm low-loss cold-rolled non-oriented silicon steel sheets can be considered. Armature windings generally use distributed and short-distance windings; for motors with a large number of poles, fractional slot windings are generally used; when the electromotive force waveform needs to be further improved, sinusoidal windings or other special windings can also be considered.
The rotor is mainly composed of permanent magnets, rotor core and rotating shaft. Among them, the permanent magnets are mainly made of ferrite permanent magnets and NdFeB permanent magnets; the rotor core can be made of solid steel, or made of steel or silicon steel sheets after punching and lamination according to different magnetic pole structures.
Compared with ordinary motors, permanent magnet synchronous motors must also be equipped with rotor permanent magnet position detectors to detect the position of magnetic poles, and use this to control armature current to achieve the purpose of driving and controlling permanent magnet synchronous motors.
According to the different positions of the permanent magnets on the rotor, the magnetic pole structure of the permanent magnet synchronous motor can be divided into two types: surface type and built-in type.
(1) Surface-type rotor magnetic circuit structure In the surface-type rotor magnetic circuit structure, the permanent magnets are usually tile-shaped and located on the outer surface of the rotor core, and the direction in which the permanent magnets provide magnetic flux is radial. Surface structure is divided into two types: protruding and embedded, as shown in Figure 1-82. For motors using rare earth permanent magnet materials, since the relative recoil permeability of the permanent magnet material is close to 1, the surface protruding rotor belongs to the hidden pole rotor structure in terms of electromagnetic performance; while the two adjacent permanent magnets of the embedded rotor are There is a ferromagnetic material with high magnetic permeability between the magnetic poles, so it belongs to the salient pole rotor structure in terms of electromagnetic performance.
The surface protruding rotor structure has the advantages of simple structure, low manufacturing cost and small moment of inertia, and has been widely used in rectangular wave permanent magnet synchronous motors and sine wave permanent magnet synchronous motors with limited constant power operating range. In addition, the permanent magnet poles in the surface protruding rotor structure are easy to achieve optimal design, making it a magnetic pole shape that can make the air gap flux density waveform of the motor approach a sine wave, which can significantly improve the performance of the motor and even the entire transmission system.
The surface-embedded rotor structure can make full use of the reluctance torque generated by the asymmetry of the rotor magnetic circuit, improve the power density of the motor, and improve the dynamic performance compared with the convex type. The permanent magnet synchronous motor is used, but the leakage coefficient and manufacturing cost are larger than those of the protruding type.
(2) Built-in rotor magnetic circuit structure The permanent magnet of the built-in structure is located inside the rotor, and there is a pole piece made of ferromagnetic material between the outer surface of the permanent magnet and the inner circle of the stator core. The pole piece can be placed in a cast aluminum cage or The copper bar cage plays the role of damping or starting, and has good dynamic and steady-state performance. It is widely used in permanent magnet synchronous motors that require asynchronous starting ability or high dynamic performance. The permanent magnet in the built-in rotor is protected by the pole piece, and the reluctance torque generated by the asymmetry of the rotor magnetic circuit structure also helps to improve the overload capacity or power density of the motor, and is easy to expand the speed of the weak magnetic field.
The magnetic steel of the permanent magnet synchronous motor with the radial rotor structure is placed in the asymmetric position of the magnetic flux axis or the magnetic steel of radial and tangential magnetization is used at the same time to generate high magnetic flux density. The advantages of this structure are that the magnetic leakage coefficient is small, no magnetic isolation measures are required on the rotating shaft, the pole arc coefficient is easy to control, the mechanical strength of the rotor is high, and the rotor is not easily deformed after the permanent magnet is installed.
The rotor of the tangential rotor structure has a larger inertia, a larger magnetic leakage coefficient, and the manufacturing process and cost are increased compared with the radial type. The advantage is that the magnetic flux under one pole pitch is provided by two adjacent magnetic poles in parallel, which can obtain a larger magnetic flux per pole. Especially when the motor has a large number of poles and the radial structure cannot provide enough magnetic flux per pole, the advantages of this structure are more prominent. In addition, the reluctance torque of the permanent magnet synchronous motor with this structure can account for 40% of the total electromagnetic torque, which is very beneficial to improve the power density of the motor and expand the constant power operating range.
The hybrid structure concentrates the advantages of radial and tangential, but the structure and manufacturing process are relatively complex, and the manufacturing cost is relatively high.
Second, the working principle of permanent magnet synchronous motor
The working principle of the permanent magnet synchronous motor is shown in Figure 1-84. In the figure, n is the motor speed, n 0 is the synchronous speed, T is the torque, and θ is the power angle. The rotor of the motor is a permanent magnet, and the N and S poles are alternately arranged in the circumferential direction. The stator can be regarded as a magnetic field rotating at a speed n 0 . When the motor is running, the stator has a rotating magnetomotive force, and the rotor rotates synchronously with the rotating magnetic field of the stator like a magnetic needle rotating in a rotating magnetic field.
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