Field Weakening
Field weakening, also called flux weakening, is a control region in which a motor drive reduces effective flux linkage so the motor's back electromotive force remains within the voltage the inverter can supply at high speed.
What changes above base speed
In a permanent-magnet motor, the controller commonly applies negative direct-axis current that opposes part of the magnet flux linking the stator. This does not normally demagnetize or turn down the magnets themselves. Induction and electrically excited motors achieve the reduced effective field differently.
Field weakening extends the usable speed range, but some current capacity is then spent changing flux rather than producing torque. Available torque and efficiency usually decline, while current, voltage, temperature, rotor-speed, and demagnetization limits still apply.
For the complete relationship between base speed, current maps, and inverter voltage, see Motor Control and Efficiency Maps.