Robots Atlas>ROBOTS ATLAS
Robotics

Field weakening

ActivePublished: 21 October 2026Updated: 21 October 2026Published
Key innovation
Lets an AC motor run above its base speed, in the constant-power region, by actively weakening the magnetic flux (negative d-axis current) once the inverter voltage reaches its limit.
Category
Robotics
Abstraction level
Pattern
Operation level
Robot control
Use cases
Electric-vehicle (EV) traction drivesServo drives and CNC machine-tool spindlesHumanoid-robot actuators and wide-speed-range drivesVariable-speed compressors and pumpsPower tools and household-appliance drives

How it works

1) The drive runs FOC: stator current is transformed into the rotating dq frame (Park transformation), where the d-axis controls flux and the q-axis controls torque. 2) Below base speed it keeps id ≈ 0 (for surface-PM PMSM) or the MTPA point, maximizing torque per amp. 3) When the required voltage hits the inverter limit (voltage constraint), the controller injects a negative d-axis current (id < 0). 4) The negative id produces flux opposing the magnet flux, reducing the net flux linkage and lowering the back-EMF. 5) This keeps the voltage within limits so speed can rise above base speed — in the constant-power region. 6) Meanwhile the current limit (current circle) forces the q-axis component down, so available torque decreases as speed increases.

Problem solved

The inverter voltage is limited, while the back-EMF grows with speed and flux. Once base speed is reached, the back-EMF matches the available voltage and the speed cannot increase further. Field weakening addresses this by lowering the flux, and therefore the back-EMF, so the drive can reach higher speeds without exceeding the voltage limit.