SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 3.9 – Developed view showing stator MMF waves of Figure 3.7 and Figure 3.8.
Prior to the development of controlled drives, synchronous machines were line-connected and
operated at line frequency and induction motors were driven by variable frequency drives. In the
synchronous machine, torque production requires that the rotor and stator fields maintain a fixed
relationship in space but it was sufficient to think about the constant rotation of the rotating field
and let the torque angle vary slowly as determined by the load and the controller. 13 In the
induction motor, the steady-state equivalent circuit was all that was considered and the drive
varied frequency and voltage to control speed but had poor transient performance. In contrast to
both, a modern synchronous machine drive (or vector induction motor drive) seeks to control the
instantaneous position of the rotating MMF of the stator with respect to the position of the rotor
field. Thus instead of visualizing the constant synchronous rotation, it is essential to develop a
mental connection between the instantaneous position of current (the reference slider of Figure
3.6) and the position of the peak of the stator MMF wave. The instantaneous angular position of
the MMF is always the same as that of the current. This means that if the current is stationary the
stator MMF will also be stationary, yet retains its cosinusoidal distribution in space. This concept
13 The load angle (and thus the power factor) can be controlled by adjusting field current and this is an
important aspect in synchronous machine control. However, in large machines the angle is varied by a
compensator to ensure stability. It is a “slow-changing” steady state control problem where (aside from the
controller dynamics required to ensure stability) it is the time-average value that must be controlled.
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