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:

\[\frac {-~ f [\%]} {Field~ Weakening~ Ending~ Speed - Field~ Weakening~ Starting~ Speed}\]\[unit~ given~ in~ :~ \bigg[ \frac {\%~ rated~ stator~ magnetic~ field} {rpm}\bigg]\]
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Fig. 2: Percentage of field weakening in respect to motor speed

Note

While the field weakening feature is enabled, it is assumed that the Drive is in its normal conditions with no fault.