What are the matters that need to be paid attention to when wiring the power filter?

1. Precautions for installing the power filter

     (1) There should be no electromagnetic coupling path in the power filter

         ① The power input line is too long;

         ② The input line and output line of the power filter are too close.

       Both of these are incorrect installation methods. The essence of the problem is that there is an obvious electromagnetic coupling path between the input wire of the filter and its output wire. In this way, the EMI signal existing at one end of the filter will escape the suppression of the filter and directly couple to the other end of the filter without attenuation by the filter. Therefore, the filter input and output must be effectively separated first.   

       In addition, if the power filter is installed inside the device shield in the above two methods, the EMI signals on the internal circuits and components of the device will be directly coupled to the outside of the device due to the radiation on the (power) end lead of the filter to generate EMI signals, so that the device shield 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 destroying the filter and shielding's suppression of EMI signals. So it has no effect.

  (2) Do not bundle the cables together

       Generally speaking, when installing a power filter in an electronic device or system, it should be noted that when bundling the equipment cables, the wires at the filter (power supply) end and the (load) end must not be bundled together, because this will undoubtedly aggravate the electromagnetic coupling between the filter input and output ends, and seriously damage the filter and equipment shielding's ability to suppress EMI signals.

  (3) Try to avoid using long grounding wires

       The length of the wiring connecting the output end of the power filter to the inverter or motor should not exceed 30 cm.

       Because an overly long grounding wire means a significant increase in grounding inductance and resistance, it will seriously damage the filter's common-mode suppression ability. A better method is to use metal screws and star-shaped spring washers to firmly fix the filter's shielding to the housing at the device's power inlet.

 (4) The input and output lines of the power filter must be kept apart. The input and output lines of the power filter must be kept apart and must not be parallel to avoid reducing the filter efficiency.

 (5) The power filter housing must be in good contact with the chassis housing. The metal shell of the inverter dedicated filter must be in good contact with the chassis housing and the ground wire must be connected.

 (6) The connection line of the power filter should preferably use twisted pair. The input and output connection lines of the power filter should preferably use shielded twisted pair, which can effectively eliminate some high-frequency interference signals.

2. Basic structure of the power filter.

       The power filter is generally designed as a passive filter composed of only resistors, capacitors and inductors, without active components such as transistors. The right picture is an example of a power filter. The power supply is connected to the top of the power filter. There is a common-mode inductor at the power supply end, that is, the two lines of the power supply are wound on the iron core in the same direction. If there is a common-mode signal on the power line, the magnetic field generated by it in the common-mode inductor will be added, so there is a larger impedance, while the magnetic field generated by the differential-mode signal in the common-mode inductor will cancel each other, so it can flow through the common-mode inductor. The current flowing through the power supply is mainly differential mode, but noise may also appear in the form of differential mode. To suppress differential mode noise, differential mode inductors or individual inductors for each phase are required.

       Special safety decoupling capacitors are used on power supply filters, which are divided into two categories: X capacitors and Y capacitors:

       X capacitors: suppress differential mode interference (interference between power lines).

       Y capacitors: suppress common mode interference (interference between each group of power lines to the ground).

       Since the increase of Y capacitance will increase the leakage current of the appliance, and the leakage current of the appliance has a specified range, the Y capacitor cannot be too large and is generally smaller than the X capacitor.

        X capacitors and Y capacitors are safety capacitors, that is, they will not cause electric shock or affect personal safety after failure. Both have self-healing function, which will restore the local short-circuited part to its original insulation state.