Selecting the right filter capacitor to suppress electromagnetic interference

a. Curve analysis

The air conditioner is powered by the elimination method, and the outdoor unit is allowed to work alone, while the indoor unit is not working. The test curve of continuous interference voltage generated by the outdoor unit controller and compressor is shown in Figure 2.

It can be seen from Figure 2 that the peak curve and average curve of the measured continuous interference voltage do not exceed the limit line, and there is a certain margin, so the possibility of interference of outdoor unit controller and compressor can be eliminated.

When the indoor unit is not working and the outdoor unit and the liquid separator are powered on, the continuous interference voltage curve obtained from the test is shown in Figure 3.

b. Solution

Comparing the two test curves, it can be concluded that the electromagnetic interference is caused by the working of the separator controller.

The characteristics of the insulating materials in capacitors are the important restricting factors of the comprehensive performance of capacitors. The actual capacitor is not pure capacitance, it is a series network composed of equivalent inductance, capacitance and equivalent resistance. When choosing the type of capacitor, the working frequency is an important factor. The maximum frequency of a capacitor is usually limited by its inductance and lead length. At some frequencies, capacitor will produce self resonance oscillation due to inductance. The resonance frequency of the capacitor is determined by the equivalent inductance and capacitance. The larger the inductance of the capacitor is, the lower the resonance frequency is, that is, the worse the high frequency filtering effect of the capacitor is. The equivalent inductance has a great relationship with the lead length of the capacitor. The longer the lead, the larger the inductance and the lower the resonance frequency of the capacitor. Therefore, in practical application, the lead of the capacitor should be as short as possible. In addition, the dielectric parameters of the capacitor are affected by the temperature and voltage, which will change the capacitance value. When choosing capacitors, we should choose them correctly according to the voltage and temperature characteristics of different dielectric capacitors.

Commonly used capacitors are: aluminum electrolytic capacitor, tantalum electrolytic capacitor, paper dielectric capacitor, polyester film capacitor, mica and ceramic capacitor, polystyrene capacitor, through capacitor.

Because the low frequency characteristic of the polymer film capacitor is better, we choose a 0.1 μ f polymer film capacitor to weld directly on the controller of the separator. Figure 4 and table 2 are the test curves and test values of continuous interference voltage with filter capacitor added.