Saturday, July 2, 2016

Electric Overdrive

Variable Field Brushless Motor

This 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.

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