Mains undervoltage

What does LU mean?

This fault indicates that the incoming mains voltage has dropped below the permissible operating threshold for the drive. By default, this condition is configured as a Warning, meaning it alerts the operator and is entered into the history buffer, but it does not immediately inhibit pulses. Continued operation under undervoltage can lead to reduced motor performance or potential damage if not resolved promptly.

• Utility grid voltage sag or brownout conditions affecting the facility's power supply.
• Overloaded electrical circuit within the plant, causing excessive voltage drop across cables and transformers.
• Incorrectly set undervoltage trip threshold parameter (e.g., P0-12) in the drive, causing premature warning/trip.
• Loose or corroded connections on the incoming mains supply lines to the drive's input terminals (L1, L2, L3).
• Faulty isolation transformer or line reactor connected upstream of the drive, causing excessive voltage drop under load.

Click steps to track your progress.

1. 1

   Measure the actual incoming mains voltage (line-to-line) at the drive's input terminals (L1, L2, L3) using a true RMS multimeter; compare to the drive's rated voltage (e.g., 400V).

2. 2

   Check the drive's undervoltage trip threshold parameter (e.g., P0-12) and adjust if it is set too high (e.g., typically 340V for a 400V system).

3. 3

   Inspect all power connections from the main distribution panel to the drive's input terminals for looseness or corrosion; torque to manufacturer's specification.

4. 4

   Monitor the mains voltage over time using a power quality analyzer to detect intermittent sags or prolonged low voltage conditions.

5. 5

   Verify the sizing and condition of any upstream transformers or line reactors; check for excessive voltage drop across them under load.