fieldweakening

Fieldweakening is a control strategy used in electric motors to extend the speed range beyond the machine’s base operating speed. It achieves this by reducing the effective magnetic flux in the air gap, which lowers the back-EMF (electromotive force) the machine generates at high rotational speeds. By weakening the field, the motor can run at higher speeds on the same supply voltage, albeit with reduced torque capability.

In practice, fieldweakening is implemented through motor drives that manipulate the excitation and stator currents. In wound-field machines, it involves reducing the excitation current to weaken the magnetic field. In permanent-magnet machines and brushless configurations, it is commonly realized by generating a small negative direct-axis current (id) in the dq synchronous reference frame, or by other control actions that oppose the flux from the permanent magnets. This allows higher speeds while keeping the stator voltage within the available supply limits.

Fieldweakening is widely used in applications requiring high-speed operation without increasing voltage or hardware, such as electric vehicles, aerospace actuators, and high-speed machine tools. It comes with trade-offs: the torque that can be produced at high speed is reduced because torque is proportional to the product of flux and current, and excessive weakening can increase thermal stress or reduce efficiency. Proper fieldweakening requires careful control to balance speed, torque, voltage, and heating within the machine’s design limits.