Undervoltage

What does USF mean?

This fault indicates that the incoming line supply voltage to the drive has dropped below its safe operating threshold, or there is an internal issue with the drive's precharge resistor. This can be caused by insufficient line supply, transient voltage dips, or a damaged internal component. An undervoltage condition will cause the drive to trip to protect its internal circuitry and ensure stable operation.

• Incoming line voltage at drive input (L1, L2, L3) dropping below the drive's minimum operating threshold (e.g., <340VAC for a 480V class drive).
• Tripped upstream circuit breaker or blown fuses in the incoming power supply path to the drive.
• Internal failure of the drive's pre-charge resistor or its bypass contactor, preventing proper DC bus charging.
• Significant degradation or failure of internal DC bus capacitors, leading to an inability to maintain stable DC bus voltage.
• Intermittent connection at the main power source or within the drive's input power circuit.

Click steps to track your progress.

1. 1

   1. Measure the incoming line voltage at the drive input terminals (L1, L2, L3) using a true RMS multimeter. Verify it remains above the drive's minimum specified operating voltage.

2. 2

   2. Inspect and test all upstream circuit breakers and fuses in the incoming power supply path. Reset tripped breakers or replace blown fuses.

3. 3

   3. Monitor the DC bus voltage at the +DC and -DC terminals during startup and operation (should stabilize at nominal DC bus voltage, e.g., 675VDC for 480V AC input).

4. 4

   4. If accessible, inspect the internal pre-charge resistor and its associated contactor for signs of damage or operational failure (e.g., contact sticking).

5. 5

   5. Contact the utility provider to investigate persistent low line voltage or brownout conditions impacting the facility's power supply.