The purpose of the line filter is to suppress electromagnetic noise. The effects of noise can be divided into the following two types:
Emissions: To reduce the noise generated by the equipment and affect the power supply or other equipment below the allowable value of regulations (such as FCC part 15), such as noise generated by switching power supplies.
Immunity: To reduce the noise entering the device to such an extent that it does not cause abnormal operation of the device, such as an instrument used in a broadcasting station.
Power filter installation1. There is no electromagnetic coupling path in the power filter.
1 power input line is too long;
2 The input and output lines of the power filter are too close.
Both of these are incorrect installations. The essence of the problem is that there is a significant electromagnetic coupling path between the input wire of the filter and its output wire. In this way, the EMI signal present at one end of the filter escapes the suppression of the filter and is directly coupled to the other end of the filter without the attenuation of the filter. Therefore, the filter input and output must be effectively separated first.
In addition, as the above two types of power supply filters are installed inside the shielding of the device, the EMI signals on the internal circuits and components of the device are directly coupled to the outside of the device due to the EMI signal generated by the radiation on the (power) terminal of the filter. Going, the device shielding loses the suppression of EMI radiation generated by internal components and circuits. Of course, if there is an EMI signal on the filter (power supply), it will also be coupled to the components and circuits inside the device due to radiation, thereby damaging the suppression of the EMI signal by the filter and the shield. So it won't work.
2, can not bundle the cable
In general, when installing a power filter in an electronic device or system, be careful not to bundle the wires between the filter (power) end and the (load) end when strapping the device cable, as this is undoubtedly exacerbated. The electromagnetic coupling between the input and output of the filter severely impairs the ability of the filter and device shield to suppress EMI signals.
3, try to avoid using long grounding wire
It is advisable to connect the output of the power filter to the inverter or the motor with a wiring length of not more than 30 cm.
Because an excessively long ground line means a large increase in grounding inductance and resistance, it can severely damage the filter's common mode rejection. A better method is to secure the shield of the filter to the housing at the power inlet of the unit with metal screws and star spring washers.
4, the power filter input line, output line must be pulled apart
The input and output lines of the power filter must be pulled apart. Avoid paralleling to avoid reducing the filter performance.
5, the power filter housing and the chassis must be in good contact
The inverter-specific filter metal case and the chassis must ensure good surface contact and connect the ground wire.
6, the connection line of the power filter should use twisted pair
The input and output connection lines of the power filter are preferably shielded twisted pairs, which can effectively eliminate some high frequency interference signals.
Power filter parametersThe power filter is a passive device composed of a filter inductor and a capacitor. It allows the power frequency (50Hz, 60Hz, 400Hz) to pass, and has a large attenuation of the electromagnetic interference signal. The power supply filter is bidirectionally reversible, which can prevent electromagnetic interference on the power grid from entering the device through the power supply, and can prevent the electromagnetic interference generated by the device itself from polluting the power grid, and is an effective configuration for suppressing conducted interference.
Insertion loss
Insertion loss is an important technical indicator of the filter. In the test system where the output impedance of the signal source and the impedance of the receiver are both 50Ω, the output voltage of the signal source is U1 when the filter is not connected. When the filter is connected, the voltage of the signal source is measured at the output of the filter. U2, the insertion loss of the filter is: Il = 20log (U1/U2). Since the interference mode has two modes: common mode interference and differential mode interference, the insertion loss is further divided into common mode insertion loss and differential mode insertion loss, which respectively characterize the suppression of common mode noise and differential mode noise.
The insertion loss is measured in accordance with GB7343-87 (same as CISPR No.17), and the test schematic is shown in Fig. 3.
Fig.3. Insertion loss test schematic
Leakage current
Leakage current (meaning only for AC line filters): Current at either end of the filter housing to the AC line at rated AC voltage. Its value mainly depends on the leakage current of the common mode capacitor Cy. Depending on the environment in which the filter is used, various safety standards have different provisions for leakage current, typically 0.5 to 5.0 mA. The test circuit principle of leakage current is shown in Fig. 4.
Fig4. Schematic diagram of leakage current test circuit
Pressure test method
According to the national standard: the voltage starts from zero and slowly rises to the test voltage at a speed of no more than 150V/s for 1 minute, then slowly drops to zero. The application method includes between the filter lead terminals and the lead terminal and the filter metal case.
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