4160V Service Voltage Drop When Energizing 6000HP VFD Causes CNC Low Voltage Alarm

A 4160V service supplies a 6000 HP Siemens VFD powering a tester motor and also feeds a 4160V to 480V transformer for other factory equipment. When energizing the VFD, a significant inrush current causes a voltage drop triggering low voltage alarms on CNC equipment. The VFD is three-phase, recently rebuilt and tested by Siemens, and includes multiple cells contributing to the large inrush current, which is higher after periods of inactivity. The inrush is due to DC link capacitors discharging at startup, typically limited by a resistor bypassed after a few seconds, though it is unclear if this is internal or requires external components. Siemens has confirmed the drive is operating normally. Suggestions include enabling or increasing the soft start ramp time on the VFD. Installing a capacitor bank on the 480V side is unlikely to mitigate the voltage drop since AC capacitor banks only help with very short current spikes. The issue appears inherent to the VFD design and startup characteristics rather than the supply system.
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