Undervoltage or No Voltage

What does USF mean?

The input line voltage supplied to the soft starter has dropped below the permissible operating limits or is entirely absent. This condition prevents proper operation and can lead to control issues or motor stoppage.

• Loss of one or more phases of the incoming three-phase supply voltage to the soft starter's L1, L2, L3 terminals.
• Supply voltage sag below the minimum operating threshold (e.g., during heavy load transients in the electrical system).
• Blown fuse or tripped circuit breaker in the upstream power distribution system feeding the soft starter.
• Loose or corroded electrical connection at the soft starter's input power terminals or upstream disconnect.
• Incorrect undervoltage trip level (USF parameter) configured too high for the application's actual voltage stability.

Click steps to track your progress.

1. 1

   1. Use a true-RMS voltmeter to measure the phase-to-phase voltages (L1-L2, L2-L3, L3-L1) at the soft starter's input terminals.

2. 2

   2. Check all upstream circuit breakers and fuses (including control circuit fuses if applicable) for tripped status or continuity.

3. 3

   3. Inspect all power connections at the soft starter's L1, L2, L3 terminals and the main incoming disconnect for tightness and corrosion.

4. 4

   4. Monitor the incoming supply voltage over a period using a power quality analyzer to detect intermittent sags or drops.

5. 5

   5. Review the soft starter's undervoltage trip threshold (e.g., USF parameter) to ensure it is set appropriately for the nominal supply voltage.