Cause : DC under voltage comes when the intermediate circuit voltage (DC link) drops below the under voltage limit, the frequency converter checks if a 24 V DC back-up supply is connected.

• If no 24 V DC back- up supply is connected, the frequency converter trips after a fixed time delay.
• The time delay varies with unit size.
• Check that the supply voltage matches the frequency converter voltage.
• Perform an input voltage test.