Field Weakening

In many applications, it is required to extend the velocity range of an electric motor. In-wheel Automotive applications, gearless wind turbines and fast robotics are good examples of such applications. As shown in Fig. 1, “Field Weakening” is a feature that increases the speed range of a PM motor by weakening the magnetic field of the rotor linearly over the speed range.

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Fig. 1: Rated and extended speed range of a BLDC

As shown in Fig. 2, the user should set 3 parameters for this feature:

  1. Field Weakening Starting Speed
  2. Field Weakening Ending Speed
  3. Percentage of Field Weakening (f)

Once the motor speed hits “Field Weakening Starting Speed”, the field weakening procedure starts to linearly reduce the magnetic field of rotor until the motor speed hits “Field Weakening Ending Speed”. The user can also determine how many percentage of rotor magnetic field should be finally reduced. As a result, the slope of rotor field reduction would be:

\[\frac {-~ f [\%]} {Field~ Weakening~ Ending~ Speed - Field~ Weakening~ Starting~ Speed}\]\[unit~ given~ in~ :~ \bigg[ \frac {\%~ rated~ stator~ magnetic~ field} {rpm}\bigg]\]
../../../../../../_images/percentage_field_weakening1.jpeg

Fig. 2: Percentage of field weakening in respect to motor speed

Note

The field weakening feature can not be enabled when the drive is in fault state.

Attention

Use this feature at your own risk, it can seriously damage your hardware (including catching fire) when the values are set wrongly! Even though not all risks can be eliminate, these measures can reduce them:

  • This is an expert feature! Please make sure you have sufficient knowledge about BLDC motors.
  • Change all values gradually and with caution, check results often.
  • Never use field weakening to achieve velocities that correspond to more than 80 Volts of peak back EMF voltage. Otherwise the drive may start burning!
  • Avoid switching off the drive while in motion - especially at high velocities! Otherwise the drive may start burning!
  • Avoid using the torque control mode when field weakening is activated. It is better to use velocity control or position control. Otherwise the system can become unstable, the drive may start burning or mechanical components may brake.

Parameters needed for Field Weakening can be found in Subindices 4-8 of the object Torque Controller.