Cause : Undervoltage comes when DC link bus voltage has dropped below the undervoltage level. the input power supply to the Fuji FRENIC-VG drive is below the required voltage level. This can prevent the drive from operating correctly and protect it from damage.

"lu" fault in Fuji FRENIC-VG drive indicate "Undervoltage" condition. This means the drive has detected an abnormal delay in starting the motor. This fault typically occurs when the drive is commanded to start, but there is a significant delay before the motor begins to operate, or the motor does not start at all.

Troubleshooting Steps:

1. Inspect Load and Motor:

   • Check the motor and the connected load to ensure there are no mechanical issues, such as binding, that could prevent the motor from starting.
   • Ensure that the load is not too heavy for the motor to handle during startup.

2. Check Power Supply:

   • Verify that the drive and motor are receiving the correct voltage and that the power supply is stable.
   • Check for any signs of low voltage or power fluctuations that could affect startup.

3. Inspect Motor and Wiring:

   • Inspect the motor for any signs of faults, such as damaged windings or insulation issues.
   • Check the wiring between the drive and the motor for loose connections, open circuits, or damaged cables.

4. Review Drive Parameters:

   • Review the drive’s start-up parameters, such as acceleration time, torque limits, and current limits, to ensure they are set appropriately for the application.
   • If necessary, adjust these parameters to reduce the likelihood of a start delay.

5. Check External Control Signals:

   • If the drive is controlled by an external system (e.g., PLC), check the control signals to ensure that the start command is being sent without delay.
   • Ensure that there are no communication issues between the external controller and the drive.

6. Inspect Mechanical Brake (if applicable):

   • If the motor has a mechanical brake, ensure that it is releasing properly when the start command is given.
   • Check the brake's wiring and control signals to verify correct operation.

7. Monitor for Overload Conditions:

   • Ensure that the motor is not being overloaded at startup. If the load is too heavy, consider reducing the load or using a soft start feature.