is called field weakening. It is used to increase RPM above base
speed (or base frequency) which is where the controller is unable to
further increase voltage applied to the armature. In this example
field weakening is accomplished by displacing the magnets (field)
thereby decreasing the flux linking the armature (coils). Lowering
flux in the coils reduces the generated voltage. That lowers
generated voltage on the armature and demands higher RPM to balance
with the applied voltage.
With all types of field weakening
(FW), the reduced flux linking the armature will also reduce the
torque per ampere. One can increase the vehicle speed using FW, but
not the peak power from the motor. FW is commonly used in EVs, but
rarely by displacement techniques.