With the popularization and use of electric vehicles, the electromagnetic compatibility problem of electric vehicle motor controller directly affects the safety and reliability of electric vehicles, and becomes an urgent problem to be solved, which attracts much attention of the industry. Based on the requirements of EMC radiation emission standard and the analysis and verification of electromagnetic radiation emission test results of electric vehicle motor controller, this paper puts forward specific measures to suppress electromagnetic interference of motor controller, which are proved to be effective by experiments. In recent years, under the pressure of dual crisis of energy and environment, many countries have started the research and development of electric vehicles. In the limited interior space, the use of high-frequency switches and the integration of a large number of electronic components have brought many electromagnetic compatibility problems to electric vehicles.
In this study, a typical domestic electric vehicle motor controller was tested. The power of motor controller is 12 kw, and the control method is vector control. The motor controller mainly includes power electronic system and control electronic system, whose control object is permanent magnet synchronous motor. The power electronic system includes the main circuit of the PWM inverter and its driving module, and the control electronic system mainly includes the control system of the PWM inverter and the motor control module. There are power supply cables and rotary speed measuring cables between motor control system and motor. According to “GB/T 18387-2008 electric vehicle electromagnetic field emission intensity limit and measurement method, broadband, 9kHz-30MHz” radiation emission standard, magnetic field radiation emission test (ring antenna) is carried out on selected motor controller. The test results show that the magnetic field radiated by the system in high frequency range is very large, exceeding the limit, and does not meet the standard. Requirement. The motor control system mainly includes power electronic system and control electronic system, whose control object is permanent magnet motor. The power electronic system includes the main circuit of the PWM inverter and its driving module, and the control electronic system mainly includes the control system of the PWM inverter and the motor control module. There are power supply cables and rotary speed measuring cables between motor control system and motor. Combining with the test results, this paper analyses the causes of the problems based on the differential mode radiation and common mode radiation of motor control system, radiation emission of inverters and radiation emission of shell shielding. In the formula above, s denotes the distance between parallel two lines, and R denotes the radius length of the line. [1] In power electronic systems of motor controllers, the switching frequency is usually about tens of kHz, which will produce high du/dt and di/dt. They interact with stray inductance of busbar on motor controllers and will inevitably produce serious electromagnetic disturbance. These disturbance signals propagate through common mode and differential mode circuits and radiate outward through cables. Voltage source inverters use space vector pulse width modulation (SVPWM) technology. Because the three-phase output voltage of the motor is not absolutely symmetrical, it is difficult for the neutral point voltage of the motor to be zero, so the common-mode voltage is generated. At the same time, the PWM pulse has a high du/dt, so there is a large common-mode voltage and a high du/dt between the inverter and the motor, which forms the main interference source.
The poor shielding of the shell of the motor controller can also cause electromagnetic interference.
At this time, because of the poor shielding of the shell, the electromagnetic signal of the control circuit inside the shell can be transmitted through the form of “field”, and the various electromagnetic interference noise outside the shell can also enter the control circuit to influence the control circuit. There are many reasons for electromagnetic interference of motor controller, the most important of which is cable, because cable can be used as an antenna to transmit electromagnetic signals around it. At the same time, due to the limited space available for installation of equipment on electric vehicles, it is impossible to keep the installed cables at safe distances.
This paper divides the cables used in the motor controller system into two categories, as shown in Table 1. Other signal lines adopt cable with shielding layer, the shielding layer of cable adopts single-ended grounding mode, and is close to the chassis wiring. The space vector pulse width modulation (SVPWM) method is used in the voltage source inverters used in the motor controller system. Because the three-phase output voltage of the motor is not absolutely symmetrical, it is difficult for the neutral point of the motor to be zero, so the common-mode voltage is generated. At the same time, the switching on and off of PWM pulse control switch will produce high du/dt, so there is a large common-mode voltage and high du/dt between the inverter and the motor, which forms the main interference source. Therefore, in electric vehicles, the motor controller and the motor should be as close as possible, so that the connecting cable can be as short as possible to reduce the area of common-mode current loop. At the same time, the connecting cable should be close to the chassis with a shielding layer, whose shielding layer overlaps with the motor controller and motor 360 degrees respectively, so as to make the cable shielding layer and motor control. Together with the motor, the device forms a complete shield to reduce common-mode radiation [2]. Electromagnetic radiation propagates along space in the form of “field”, so cutting off the propagation of radiation disturbance through the shielding effect of the shell is an effective method to suppress electromagnetic radiation. The shielding effect of the shell depends on the conductivity continuity of the shell. The better the conductivity continuity of the shell, the more the electromagnetic field coupled to the shielded circuit attenuates through reflection loss and absorption loss. The total shielding effectiveness of the shell is equal to the sum of the reflection loss and absorption loss of the electromagnetic field passing through the shell. From the point of view of power electronics technology, the motor controller system of electric vehicle is the combination of inverter and speed motor controller. This kind of equipment can produce a lot of radio frequency noise. The shell of the motor controller in this test is made of aluminium. The electromagnetic shielding of the shell should first pay attention to the opening of each cable and the gap of the shell. Therefore, metal connectors are used between the shell and cable of the controller, and the cables are shielded cables, and the 360 degree overlap between the cable and the connector is ensured to suppress the radiation emission of electromagnetic signals; the screw spacing of the shell should be short, at the same time, solid conductive rubber strips are added as sealing gaskets; and the gap of the aluminum shell is bonded with conductive cloth. In order to reduce the electromagnetic interference caused by the gap [3].
In addition, the power module and control circuit of the motor controller are separated by aluminium plate. At the same time, the aluminium shell is grounded reliably, which further strengthens the shielding ability of the motor controller. It not only ensures that the electromagnetic energy around the inverter does not enter the control circuit to affect the function of the equipment, thermostatic element but also makes the electromagnetic energy generated in the control circuit or power module. It does not spread out. In this paper, the EMI problem of motor controller is analyzed based on the results of magnetic field radiation emission test. After repeated verification, an effective suppression method is proposed to meet the reliability and safety of motor controller of pure electric vehicle.