Under Voltage Fault

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This fault occurs when the DC Bus voltage drops below a minimum threshold during operation. For 200-240V AC rated controllers, this trip happens at 210V DC bus (150V AC incoming); for 380-460V AC rated controllers, it's 390V DC bus (275V AC incoming). Persistent undervoltage can cause erratic controller behavior and motor performance issues.

• Significant AC supply voltage sag below specified threshold (e.g., 150V AC for 200-240V system) during motor operation.
• Excessive instantaneous load demand from the motor causing the DC bus to dip temporarily.
• Loose or high-resistance connection in the AC input power circuit (e.g., contactor, fuses, L1/L2/L3 terminals).
• Degraded or undersized DC bus capacitors reducing ride-through capability during voltage sags.
• Transient overcurrent condition drawing excessive power, leading to voltage drop across supply impedance.

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1. 1

   1. Monitor incoming AC line voltage (L1-L2, L2-L3, L3-L1) at the controller's input terminals using a data logger or oscilloscope during operation to identify sag events.

2. 2

   2. Inspect and tighten all AC input power connections, including main contactor, input fuses, and controller L1, L2, L3 terminals, to ensure low resistance.

3. 3

   3. Verify the motor load is within the controller's specified capabilities; consider reducing acceleration/deceleration times (e.g., P1-03, P1-04) if excessive load transients are suspected.

4. 4

   4. Measure the DC bus voltage (P+ to N-) during operation. Compare the tripping voltage to the controller's specified undervoltage trip threshold (e.g., 210V DC for 240V AC unit).

5. 5

   5. Check the health of input rectifier diodes (using diode test function) and DC bus capacitors (for bulging or leakage) if voltage dips persist despite stable AC input.