Output phase loss

What does E.LF mean?

An output phase loss fault occurs if one of the three output phases (U, V, W) to the motor is lost during inverter operation (excluding specific conditions). This can cause motor damage due to imbalanced currents and will stop the inverter output. Protection is selected via Pr. 251.

• Open circuit in one of the motor output phases (U, V, or W) due to a broken cable, loose terminal connection, or faulty contactor.
• Motor winding open circuit (e.g., thermal protector tripped inside motor, damaged winding).
• Faulty output IGBT (Insulated Gate Bipolar Transistor) or its gate driver circuit within the inverter.
• Severe imbalance in motor load distribution across phases, leading to current sensing discrepancies.
• Loose connection at the inverter's output terminals (U, V, W) to the motor cable.

Click steps to track your progress.

1. 1

   Power down the inverter and use a multimeter to measure continuity of each motor cable phase (U, V, W) from inverter terminal to motor terminal.

2. 2

   Check the torque on all motor cable connections at both the inverter output terminals and the motor terminal box.

3. 3

   With motor cables disconnected from the inverter, measure the resistance between motor phases (U-V, V-W, W-U) to verify balanced winding resistance.

4. 4

   Inspect any intermediate contactors, disconnects, or fuses in the motor output circuit for signs of failure or open contacts.

5. 5

   Monitor the inverter's output voltage (U-V, V-W, V-U) during operation (if possible) to identify the missing phase.