Does the shielded compressor work? Does the motherboard's filter circuit not work?

●The speaker does not work properly?

1. Background

In a store in a certain area, when the air conditioner was turned on, the audio system in the store produced interference, causing the sound to be unclear. There were no interfering devices around, and there was no problem when the audio system was turned on alone. However, interference occurred after the air conditioner compressor was started. The outlet tried to move the audio equipment and adjust the circuit breaker, but the problem could not be solved.

2. Are shielded compressors effective?

After-sales staff used tin foil to shield the compressor, similar to the effect of a Faraday cage, but it didn't work. This is because audio signals are usually coupled through wire conduction. Verification: Another set of power lines were introduced from other stores for air conditioners, and the interference of the audio disappeared. The power supply was reconnected to the store, and the fault reappeared. For this kind of conduction coupling, filtering, absorption and other means can be used. Generally, the mainboard has an EMI filter circuit.

3. Does the motherboard's filter circuit not work?

Generally, the mainboard of the air conditioner outdoor unit has a bidirectional filter circuit, as shown in the figure below. Is this circuit not working? After further checking the electric control cabinet in the store, it was found that the electric control cabinet was not effectively grounded. The fault was eliminated after re-grounding the electric control cabinet. In the filter circuit, common-mode interference must form a low-resistance loop with the PE ground wire to release the interference energy. If there is no ground wire, the filter circuit cannot start the filtering function, and the compressor, IPM, and switch circuit interference filtering. All PE (protective ground) not only has safety protection, but also plays an important role in interference and anti-interference.