Field weakening is a technique employed in electric motors, particularly in permanent magnet synchronous motors (PMSM) and brushless DC (BLDC) motors. It aims to extend the operational RPM range of the motor, enabling higher speeds by intentionally reducing the strength of the magnetic field. Here's a streamlined explanation of how field weakening works:

1. Normal Operation:
   • In standard operation, electric motors have an optimal magnetic field strength that ensures efficiency within a specified speed range.
2. Limitation at High Speeds:
   • As the motor speed increases, the back electromotive force (EMF) also rises, imposing a natural limit on speed and torque.
3. Field Weakening Technique:
   • To overcome this limitation, field weakening involves intentionally reducing the current supplied to the motor's field windings, thereby weakening the magnetic field.
4. Increased Speed Capability:
   • With the weakened magnetic field, the back EMF is reduced, allowing the motor to operate at higher speeds without encountering limitations imposed by the back EMF at normal operating conditions.
5. Trade-offs:
   • While field weakening extends the speed range, it comes with trade-offs. Weakening the magnetic field can reduce overall efficiency and torque generation, making it a trade-off between higher speeds and efficiency.

In essence, field weakening is a strategy to push the operational limits of electric motors, enabling them to achieve higher speeds beyond their normal range, albeit with some efficiency compromises.

TLDR; Field weakening is a technique used in electric motors, like in electric vehicles, to reach higher speeds. It involves intentionally weakening the magnetic field by reducing the current to the motor's field windings. This allows the motor to operate beyond its normal speed range but comes with trade-offs, impacting overall efficiency and torque generation. AKA More speed, less torque.