SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 5.21 – Flux weakening in the rotor reference frame.
For this reason the q- and d- components of the current SV are sometimes called the torqueproducing
and flux-producing components, respectively, and this is drawn in the literature as
Figure 5.22, where 1
C and 2
C are constants.40
Figure 5.22 – "Torque" and "flux" control.
For a salient machine the concept in Figure 5.22 would need to be modified in consideration of
Figure 5.18-b. Continuing again with the assumption of a nonsalient machine, the bEMF is the
derivative of the d-axis flux. Thus, advancing the current SV (by setting a negative reference
value for flux in Figure 5.22) would achieve flux weakening that would allow operation above
base speed. This could be implemented as shown in Figure 5.23, were “S.C.” is a speed
compensator and “F.C.” is a flux controller. (For simplicity, both the flux loop is open, but using
an estimator it could be made closed-loop.) As before, if a salient machine is used the flux
weakening scheme of Figure 5.23 would likely need to be coordinated with the torque controller
of Figure 5.18-b. [113] discusses concurrent flux weakening and torque control.
40 In induction motor control (to achieve the most torque per current) the value of flux is held constant and
in order to achieve this either the FOC control is used or the “volts per Hz” control (mentioned in Chapter
4) is used.
227