Field Weakening
Field weakening is a motor control technique used to increase the maximum speed of a motor beyond its normal maximum speed.
Under normal operation, a motor controller can only increase motor speed until it reaches approximately 100% duty cycle. At this point, the controller is already applying nearly the full battery voltage to the motor and cannot increase the motor voltage any further.
Field Weakening works by injecting additional current into the motor in a way that reduces the motor’s effective magnetic field. This reduces the back-EMF generated by the motor, allowing it to spin faster than would normally be possible at the available battery voltage.
On VESC controllers, Field Weakening is configured using the Field Weakening Current Max setting.
Field Weakening can be used to:
- Increase top speed
- Extend the speed range of a motor
- Reduce the need for a higher voltage battery
- Reduce the need for a higher KV motor
⚠️ Field Weakening does not increase torque. In fact, as field weakening increases, the available motor torque generally decreases.
⚠️ Field Weakening is not free speed. Increasing field weakening current increases motor losses and heat generation within both the motor and controller.
⚠️ Excessive field weakening can result in:
- Increased motor temperatures
- Increased controller temperatures
- Reduced efficiency
- Reduced runtime
- Reduced motor torque at high speed
- Motor instability
- Loss of speed regulation
In extreme cases, excessive field weakening can cause the motor to become unstable enough that the controller can no longer accurately regulate motor speed. This may result in the motor continuing to accelerate beyond the commanded speed, sometimes referred to as a runaway condition.
Generally, motors with higher inductance are better suited to field weakening than motors with very low inductance. The effectiveness and stability of field weakening can vary significantly between different motor designs, so results should not be assumed based solely on motor size or power rating.
For these reasons, field weakening should generally be used only when additional speed is required beyond what can be achieved through battery voltage and motor selection alone. Field weakening is an advanced motor control technique and should only be configured by experienced users. Any increases should be made gradually and tested after each adjustment to ensure the motor and controller remain stable and operate within safe temperature limits.
Example
A motor may naturally reach a maximum speed of 5,000 RPM at 100% duty cycle when powered from a particular battery. By applying field weakening, the controller may be able to increase the motor speed to 6,000 RPM or higher, depending on the motor, controller, and field weakening settings.
In simple terms: field weakening is a technique that allows a motor to spin faster than its normal maximum speed by reducing the effect of the motor’s magnetic field.