Field Weakening for PMSMs

Field-weakening [FW] (aka flux-weakening) is a control technique for increasing the speed of an electric motor when the motor stator winding voltage reaches the inverter output voltage limit. FW increases the motor speed by reducing the motor flux. FW is used in applications to achieve higher motor speed when lower torque is needed.

PMSM FW control injects a negative d-axis stator current [Id] to reduce the motor flux, which thereby reduces the effective torque constant (current-to-torque conversion), which in turns means the motor operates at lower efficiency.

Most vendors support FW control for PMSMs. However, this often requires a specific set of configuration parameters in the drive. Contact the vendor for the details.

In SERVOsoft, one motor record in the database can store both NoFW and FW characteristic curves. Sometimes vendors provide data for one or the other, and sometimes for both NoFW and FW curves. In SERVOsoft, the user specifies which curve to use by specifying the Inverter Control Mode as shown, which is found on the Main and the Motor Performance forms.

If Control mode = Auto and the selected motor has both NoFW and FW curves available, then the FW curve is used. Otherwise, the available curve is used.


In SERVOsoft, there are a few areas that highlight the use of FW, including System Check and the Motor Performance form.

System Check
Motor Performance Form

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