Cause : DC Bus Overvoltage comes when the DC bus voltage exceeded the trip point. When a Yaskawa U1000 drive displays a "DC Bus Overvoltage" alarm, it indicates that the voltage level on the DC bus of the drive has exceeded the acceptable threshold. 

Voltage in the control circuit exceeded the trip point.
For 200 V class: approximately 450 V
For 400 V class: approximately 900 V

1. Surge voltage present in the drive input power.
   • Install a DC link choke or an AC reactor.
   • Voltage surge can result from a thyristor convertor and a phase advancing capacitor operating on the same drive input power system.
2. The motor is short-circuited.
3. Check the motor power cable, relay terminals and motor terminal box for short circuits.
4. Correct grounding shorts and turn the power back on.
5. Ground current has overcharged the main circuit capacitors via the drive input power.