0 Introduction:
With the continuous improvement of electrification and automation, motors are widely used in various fields. Although the characteristics of motors themselves have been greatly improved, the electromagnetic compatibility (EMC) problem between motors and drive systems is becoming increasingly serious. Taking automobiles as an example, a car is equipped with a large number of electronic devices, and the EMC issue is also very complex. The strongest source of interference is the ignition system, which generates strong radiated interference in the surrounding space during operation, significantly affecting the normal operation of other electronic devices in the vehicle. In addition, the windshield wiper motor is also a major source of interference.
Solving the electromagnetic compatibility issues between various electronic modules and systems in automobiles, improving the reliability and safety of automobiles, and ensuring that certain operations on a normally driving automobile do not interfere with the normal operation of surrounding equipment has become a very important and urgent research topic.
1. Test method for electromagnetic interference characteristics of wiper motor
1.1 Components of a typical testing device
Typical test equipment consists of a power supply, a measurement receiver, a grounding plate, and an artificial power network (Line Impedance Stabilization Network LISN).
(1) Measurement Receiver. Since the object of measurement is a weak, continuous signal, and also a pulse signal with relatively large amplitude, the measurement receiver itself must have low noise, high sensitivity, a large dynamic range of the detector, low input impedance, and strong overload capacity of the pre-amplifier circuit. The detector should have multiple detection functions to meet different needs (peak, quasi-peak, average, and RMS detection). The receiver's accuracy across the entire measurement frequency band must be within ±2dB.
(2) Power supply. The 12V power supply system should be 13.5±0.5V, and the 24V system should be 27±1.0V. The power supply must be properly filtered so that the radio frequency noise in the power supply is more than 6dB lower than the limit specified in the test plan.
(3) Artificial Power Network (LISN). Also known as a line impedance stabilization network, an artificial power network provides a stable impedance between the reference ground and terminals of the device under test (DUT) or between terminals within the radio frequency range. It isolates unwanted signals and noise from the power supply from the DUT, coupling only the DUT's interference voltage to the measurement receiver input. It is primarily used to measure the continuous interference voltage conducted along the power line to the conductors of the device under test (Equipment Under Test). The artificial power network should have an inductor nominally rated at 5μH, and its measurement port impedance should be [insert value here].
(4) Grounding plate. When measuring conducted emissions, a grounding plate can improve accuracy. It should be made of copper plate or galvanized steel plate with a thickness of at least 0.5 mm.
1.2 Test Methods
According to the requirements of the national standard GB18655-2002 "Limits and methods of measurement for radio interference characteristics used to protect vehicle-mounted receivers", an artificial power supply network with a 5μH port impedance was used, and the EMCScan electromagnetic interference measurement system independently developed by the laboratory was used to complete the conducted interference test of the automotive wiper motor.
When measuring voltage on all power lines, the reference point should be the casing of the device under test (DUT), or as close as possible to the ground plane. The test cable should be placed 50mm above the grounding plate. Typically, if the DUT is grounded at the far end of the return line, the voltage of each conductor (including the return and incoming lines) should be referenced to the grounding plate voltage. This test involves a near-end grounding return line. When measuring voltage on a DUT with near-end grounding (power return line less than 200mm), the test equipment and the DUT should be arranged and connected as shown in Figure 1.
Figure 1. Typical setup and connection diagram for conducted emission experiments (device under test with the power return line grounded near the end).
1. Power supply; 2. Artificial power network; 3. EUT; 4. Test harness; 5. Grounding plate; 6. Insulation layer (50mm thick);
2. Testing and Results Analysis
2.1 Pre-test analysis
Wiper motors are DC motors, and the electromagnetic interference generated by the brushes of DC motors includes both common-mode and non-mode interference. The main methods for suppressing electromagnetic interference from DC motors include adding capacitors, inductors, and grounding. For common-mode interference, capacitors should be connected between each lead of the wiper motor and ground; however, for non-mode interference, capacitors should be connected across the power supply leads. Generally, interference generated by carbon brushes is usually non-mode, occurring when the carbon brushes disconnect from the commutator contacts.
The function of a low-pass filter circuit is to pass low frequencies and block high frequencies. It significantly attenuates high-frequency electromagnetic interference signals, suppressing interference and weakening the output of conducted interference. Figure 2 shows a commonly used low-pass filter circuit. Capacitors C1, C2, C4, and C5, and the common-mode inductor Lc are typically used to suppress common-mode interference; while C3 and C6 are generally used to suppress heterogeneous-mode interference. Typical values for capacitors C1, C2, C4, and C5 are between 1000 and 4700 pF; typical values for C3 and C6 are between 0.1 and 47 μF; and typical values for the common-mode inductor Lc range from 0.01 to 3 mH. As analyzed above, the main component of electromagnetic interference generated by the wiper motor is conducted interference, and the conducted interference generated by the motor is all differential-mode interference. To suppress electromagnetic interference from the wiper motor, only some parts of the low-pass filter circuit in Figure 2 are needed to achieve the purpose of suppressing electromagnetic interference. Therefore, this test uses an L-shaped low-pass filter (i.e., composed of one inductor and one capacitor).
Figure 2 shows a low-pass filter commonly used at the motor power input.
For an L-type low-pass filter, its resonant frequency is:
Figure 3. Connection diagram of experimental equipment
2.2 Test Result Analysis
Based on analysis, this paper proposes a method to suppress electromagnetic interference using an L-shaped low-pass filter (one inductor and one capacitor). During the experiment, a capacitor + inductor configuration was used for testing; the capacitor value was determined first, followed by the inductor value.
First, the test was conducted without a filter, and the test results are shown in Figure 4.
Figure 4 shows the test results without a filter.
As shown in Figure 4, the test without a filter revealed that the conducted interference of the wiper motor is mainly within 1MHz, with the interference being relatively large in the 150kHz~400kHz frequency range.
Next, select the inductor and determine the optimal value of the capacitor. Based on the optimal capacitor value, select the optimal test inductor value by changing the size of the inductor.
Conducted interference was tested by connecting a filter capacitor in parallel to the input terminal of the wiper motor power supply. The selected capacitors were 3.3μF, 10μF, and 39μF, respectively, as shown in Figures 5, 6, and 7.
Figure 7 shows the test results for a capacitance of 39μF.
The test results show that the interference suppression effect is best when the capacitor is 10μF, reaching about 10dB.
Finally, a 10μF capacitor was selected. Without changing the capacitance, only the inductance was changed. The inductances were successively selected as 82μH, 220μH, and 302μH. The results are shown in Figures 8, 9, and 10, respectively.
Figure 8 shows the test results for a capacitor of 10μF and an inductor of 82μH.
Figure 10 shows the test results for a capacitor of 10μF and an inductor of 302μH.
The tests show that adding an inductor significantly improves the suppression effect. The best suppression effect against electromagnetic interference is achieved when the capacitor is 10μF and the inductor is 220μH.
3. Experimental Conclusions
Based on the above experimental results and analysis, the inductance value of the L-type filter was determined to be 220uH, and the capacitance value to be 10pF. It can effectively suppress conducted interference in the 150kHz~500kHz frequency range, reducing it by at least 20dB, and up to 30dB at some frequencies. Although the interference is not minimal in the 2MHz~20MHz frequency range, it does not affect its electromagnetic compatibility.
References:
[1]AndersenP.AnoverviewofautomotiveEMCstandards[C].IEEEInternationalSymposiumon
ElectromagneticCompatibility,2006,3(8):13-189
[2] Meng Jin, Ma Weiming, Liu Dezhi, et al. Time-domain model and simulation study of conducted electromagnetic interference in AC generator rectifier system [J].
Proceedings of the Chinese Society for Electrical Engineering, 2002, 22(6):75-801
[3] Liang Zhenguang, Tang Renyuan. Electromagnetic compatibility issues of electric motors [J]. Small and Medium-sized Electric Machines, 2004, 31(2): 63-66
[4] Yu Jihui, Zheng Yali, Zou Zhixing. Simulation study on crosstalk and radiation of in-vehicle wiring [J]. Journal of System Simulation, 2008.
20(17): 4738-4748
[5] Ouyang Juan. EM Characteristics Analysis and Simulation Study of Car Windshield Wiper Drive System, Master's Thesis, Southwest Jiaotong University, 2009: 40-44
