Field Weakening

Overview

As motor velocity increases so does the induced back EMF voltage. If large enough, this induced back EMF voltage can limit the inverter's ability to produce current. When this happens, it can be observed that the current feedback cannot follow the current command, and velocity cannot go above a certain limit (called base speed). This base speed depends on the motor parameters as well as the available dc bus voltage.

To go above the base speed, the field weakening loop is activated using the AXIS#.MOTOR.FIELDWEAKENING keyword. Once activated, the field weakening control loop monitors the magnitude of the motor-voltage vector. If this magnitude gets close to the available bus voltage, it increases the d-axis current to reduce the back emf voltage of the motor and, as a result, limits the magnitude of the motor-voltage vector. Therefore, it can go above the base speed of the motor while operating at Maximum Torque Torque is the tendency of a force to rotate an object about an axis. Just as a force is a push or a pull, a torque can be thought of as a twist per Volt (MTPV) condition.

The field weakening control loop is configurable using the following keywords:

• AXIS#.FIELDWEAKENING.CURRFILTER
• AXIS#.FIELDWEAKENING.LOOPBW
• AXIS#.FIELDWEAKENING.VBUSMARGIN
• AXIS#.FIELDWEAKENING.VOLTFILTER

For more information on configuring the field weakening loop, see the keywords above.

Usage and Safety Considerations

Kollmorgen recommends that the field weakening control loop is used in velocity or position modes but not in the torque Torque is the tendency of a force to rotate an object about an axis. Just as a force is a push or a pull, a torque can be thought of as a twist mode.

The field weakening algorithm works with the foldback algorithm to ensure the magnitude of the current vector applied to the motor is always within the safe limits for both motor and the drive. For more information on foldback algorithm see Foldback.

The applied q-axis and d-axis currents are also limited by:

• AXIS#.MOTOR.IPEAK
• AXIS#.MOTOR.IDMAX
• AXIS#.IPEAK
• AXIS#.IL.LIMITP
• AXIS#.IL.LIMITN

Set AXIS#.MOTOR.IDMAX appropriately to prevent demagnetization of the permanent magnets inside the motor when operating in the field weakening mode.

The field weakening algorithm can support both surface permanent magnet and interior permanent magnet motors:

AXIS#.MOTOR.TYPE = 0 or 6

• 
  
• When operating in the field weakening mode, the drive is artificially lowering the induced back emf voltage of the motor by applying whatever d-axis current that is necessary. If a fault happens while operating above base speed and the power stage disables, the d-axis current-flow stops and a back emf voltage that is higher than the bus voltage is created.
  
  To ensure operational safety of the drive in such condition, the velocity fault threshold AXIS#.VL.VFTHRESH is always limited by the over-voltage fault-threshold of the drive. AXIS#.VL.VFTHRESH is also limited by AXIS#.MOTOR.VMAX , which is the maximum mechanical velocity limit for the motor, as well as AXIS#.VL.THRESH, which is the user-defined velocity threshold.
  
  Exercise care when using field weakening because the induced back emf voltage of the motor is no longer limited by the available bus voltage of the drive.