Induction motor scalar control
If the motor nameplate says 400 V, 50 Hz, wye connection, that is the line to line voltage that must be supplied for the wye connection 8 V/Hz is required. If you re-connect to the delta connection, you can supply 4.62 V/Hz. That would be 231 V for 50 Hz or 277 V for 60 Hz. If you do not provide the rated V/Hz the motor can not produce the rated torque without overheating. Rated V/Hz is required to produce the magnetizing flux for which the motor is designed.