Can I operate a 500kW/1500rpms motor at 35Hz with 400kW inverter?
Unless the load is a fan or centrifugal pump, the same torque will be required to drive the load at the lower speed. Your power calculation implies that the torque will be the same. The power will be reduce to about 70% of rated because power is torque multiple by speed. However current is proportional to torque, so the current will not be reduced. VFDs are inherently limited to their rated current, they can not provide more current when the voltage is lowered even though that means that less power is being converted.