SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 5.27 – Stationary regulator in synchronous frame.
It must be emphasized that this is only a representation of the compensators that are physically
operating in the stationary frame—control is not actually executed in the synchronous frame.
Figure 5.27 shows that as the frequency of operation increases, the magnitude of the crosscoupling
terms increases, causing phase lag in the output (the d-component is made larger while
the q-component is made smaller). It is already known that the compensator will experience
phase lag and a reduction of gain with increasing frequency—viewing the compensator in the
synchronous frame simply exposes the fact in a different form. Finally, the cross coupling forms
an oscillator but the oscillations are not present in steady state. However, they can be excited by
transients and small-signal resonance is possible [78, pp.343-344]; this unwanted feature is not
present in the synchronous regulator described next.
Now the same investigation can be repeated for the synchronous regulator. This was encountered
as Figure 5.12 but again the current control will be isolated from the torque control. The
simplified synchronous regulator in the synchronous frame is shown in Figure 5.28.
Figure 5.28 – Synchronous regulator in synchronous frame.
As with the stationary regulator in its native frame, the synchronous regulator in its native frame
exhibits no cross coupling. This demonstrates that it will not have the phase lag problems the
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