Overvoltage or voltage shortage / Synchronous signal error

What does U9 mean?

This fault is detected when DC bus voltage decreases to 210V, momentarily drops to 200V, rises to 400V, or if input current drops to 0.4A (when frequency >= 30Hz or compressor current >= 1A) during compressor operation. It signals significant power supply instability or a fault within the main circuit components like the noise filter or ACT module. Unresolved, this can lead to severe damage to the compressor and other sensitive electronic components due to improper voltage conditions.

• Unstable or fluctuating AC line voltage supply to the outdoor unit (e.g., brownouts, sags, swells from grid issues).
• Failure of the DC bus capacitors to properly smooth or store charge, leading to rapid voltage fluctuations.
• Malfunctioning rectifier bridge converting AC input to DC bus voltage, causing voltage drops or poor regulation.
• Intermittent loss of one or more AC input phases, resulting in severely unbalanced or low DC bus voltage.
• Faulty DC bus voltage sensing circuit providing incorrect readings to the main control board.

Click steps to track your progress.

1. 1

   Using a true RMS multimeter, monitor the AC input voltage (e.g., L1-L2, L2-L3, L3-L1) at the main contactor terminals for stability.

2. 2

   Measure the DC bus voltage across the main DC link capacitors (e.g., Test Point TP1/TP2) and observe for rapid fluctuations outside 280V-380V range.

3. 3

   Inspect the rectifier bridge diodes for damage or discoloration and test for proper forward voltage drop (approx. 0.5-0.7V).

4. 4

   Check for secure connections and continuity of all input power wiring from the disconnect switch to the main control board's input terminals.

5. 5

   Verify the input current sensor (e.g., on phase L1) is providing a consistent signal above 0.4A when the compressor is operating.