Introduction

The field weakening function (Maximum Field Weakening current) applies a percentage of available motor current (Rated motor current) on the field flux (d-axis), instead of the torque component (q-axis) to increase the motor speed above what your voltage would typically allow at the cost of reduced torque output. Note that for a brushless DC motor, torque is approximately proportional to the motor phase current.

Caution

• Excessive field weakening can cause a motor to auto-rotate when not commanding motor power.
• Excessive field weakening can reduce the quality of back EMF, reducing sensorless driveability.
• Operating above rated motor speed increases the likelihood of overvoltage faults and the possibility of damage to the controller. This is because when you are no longer commanding torque from the motor, the current is no longer applied to the flux field and the normal no-load speed limit (Rated motor speed) reapplies. Voltage spikes can be estimated by the following: actual Motor rpm * Rated system voltage / Rated motor speed.

Implementation

Users by default (user access level 0) can configure field weakening from 0 to 25%. [implemented in v6+ software]

For example, with Rated motor current equal to 100A, and Maximum Field Weakening current set to 25%. You will have 75A available for producing torque and 25A for reducing the flux field strength to achieve a higher speed.

Some motors respond better to field weakening than others. We suggest incrementing the Maximum Field Weakening current slowly, as a small can change can sometimes result in large speed changes. Some motors, however, require large changes for even marginal increases in speed, and likely drastically reducing motor efficiency.

Configuration parameters