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DC Motor Control

The main drive motor controller is the control unit of the drive motor, while the electric vehicle drive motor mainly has DC motor, asynchronous motor, permanent magnet synchronous motor, switch magneto-resisting motor and other types. Different types of drive motors also have different control principles. The control principles of these commonly used drive motors are described below. 1. DC motor control DC motor is divided into excitation winding type and permanent magnet DC motor. The former has excitation windings and the magnetic field can be controlled by DC excitation current, while the latter has no excitation windings and the permanent magnet's magnetic field is uncontrollable. The excitation winding DC motor can be further divided into his excitation, and recidivism, string reciprocity and re-motivation, as shown in following picture.


Type of excitation winding DC motor a ) he recited b ) and c ) string re-motivation d ) re-motivation

Although various types of DC motors have different excitation methods, their working principles are basically the same. Figure 5-35 shows the working principle diagram of the DC motor. The force between the magnetic flux generated by the stator poles and the current in the rotor armature winding generates torque. The direction of current in the armature winding is changed by the brush, so that the armature rotates to generate torque in the same direction. The torque T is proportional to the product of the magnetic flux ф and the armature current /a


2. Asynchronous motor control Asynchronous motors can be divided into winding rotors and cage types according to the rotor structure. Due to the high cost, maintenance and lack of robustness of winding rotor motors, cage-free asynchronous motors are widely used, especially in electric vehicles. figure show the schematic of the workings of the asynchronous motor. The symmetrical three-phase AC current is applied in the symmetrical winding, which causes a rotating magnetic field to be generated in the synthesized. The rotor conductor cuts the stator rotating magnetic field to sense the electromotive force, thus generating current in the closed conductor; The rotor conductor will be subjected to electromagnetic forces, which will form an electromagnetic torque.


It is difficult to control the flux and torque independently by using the conventional method to adjust the windings A, B, and C of the asynchronous motor, but the three-phase current (iA,iB, iC) of the stationary coordinate system is converted to a two-phase current (iM, iT) of the stationary coordinate system using the coordinate transformation shown in the following picture.




3. Permanent magnetic sync motor control The permanent magnet synchronous motor is a synchronous motor that generates a synchronous rotating magnetic field by permanent magnet excitation. Depending on the permanent magnet's mounting position on the rotor, the permanent magnet synchronous motor is divided into surface and built-in. Compared with the surface permanent magnet synchronous motor, the built-in permanent magnet synchronous motor has a high convexity rate, which can produce additional magnetic resistance torque components, which is very useful in the operation of constant power; And embedding permanent magnets into rotors maintains mechanical integrity at high speeds, so the electric vehicle field is dominated by built-in permanent magnet synchronous motors. Figures 5-40 show the working schematic of the permanent magnet synchronous motor. The symmetrical three-phase AC current is applied in the symmetrical winding, which causes a rotating magnetic field to be generated in the synthesized. There is no need for excitation windings on the rotor, and the main magnetic field is established by permanent magnets; The magnetic field generated by the permanent magnet interacts with the stator rotating magnetic field to produce torque. The torque expression is as follows: Similar to asynchronous motors, the permanent magnetized synchronous motor model is equivalent to a DC motor, with a coordinate transformation converting the three-phase current (iA, i B, iC) of the stationary coordinate system to a two-phase current (id, i q) under the synchronous rotation coordinate system, which corresponds to its excitation current If, and iq corresponds to the armature current Ia that generates torque. Therefore, the control of the permanent magnet synchronous motor using vector control method is similar to dc motor control, i.e. by applying current id to adjust flux, adjust current id. the size of the motor output torque can be adjusted. Of course, due to the high convexity, the change in current i can also affect the size of the motor output torque to some extent.


Compared to asynchronous motor control, permanent magnet synchronous motors require rotor position sensors (mostly rotary also presses) to accurately detect rotor positions, whereas asynchronous motors require only speed sensors. Although positionless sensor technology is available in the industrial sector, position sensors are still required for automotive applications with wide operating range, high accuracy requirements, and high performance requirements for NVH (Noise), Vibration (Vibration) acronyms (Harshness). Figure 5-41 shows a typical permanent magnet synchronous motor control block diagram. Comparing the control block diagram of the asynchronous motor, we can see that the main difference between the permanent magnet synchronous motor and the asynchronous motor is the acquisition of the position angle of the synchronous rotation coordinate system, the permanent magnet synchronous motor can be obtained directly through the position sensor detection, and the asynchronous motor needs to estimate the transition frequency before it can be calculated, and the calculation of the transition frequency is influenced by the motor parameters, so the control of the permanent magnet synchronous motor is relatively simpler. Compared with asynchronous motors, permanent magnet synchronous motors, although slightly higher in cost, but with its high efficiency, high torque and power density, constant power speed range and other advantages, has been widely used in the field of electric vehicles, has gradually become the mainstream of vehicle-driven motors.





4. Switch magnetic resistance motor control

Switched reluctance motor has the advantages of simple structure, reliable operation and low price, so it has also received attention in the field of electric vehicles. The picture shows a four-phase 8/6-pole switched reluctance motor. Only one phase of the winding is shown in the figure. Because the stator and rotor poles are of salient pole structure, the inductance L of each phase winding changes with the rotor position, as shown in the following picture.


The operation of the switched reluctance motor follows the "principle of minimum reluctance", that is, the magnetic flux is always closed along the path of the least reluctance. As shown in the following picture, when the B-phase winding is excited, in order to reduce the magnetic resistance of the magnetic circuit, the rotor rotates clockwise until the rotor pole 2 is opposite to the stator pole B. At this time, the magnetic resistance of the magnetic circuit is the smallest (inductance maximum). Then, the excitation of winding B is cut off, and excitation is applied to winding A. The reluctance torque makes rotor pole 1 and stator pole A face each other. The torque direction generally points to the closest pair of magnetic poles facing each other. Therefore, according to the feedback signal of the rotor position sensor, the phase windings are turned on in the order of B-A-D-C, so that the rotor continuously rotates in the clockwise direction.









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