SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

CHAPTER 1 - Introduction
Historically (c. 1900– ) the three major types of electric machines have been the brush-
commutator DC, synchronous, and induction machines. From the earliest times variable speed
operation of motors was desired. This was not possible with the synchronous machine; its
primary function has been to generate grid power. The DC motor is amenable to controlled speed
operation and a great many schemes have been devised over the years. Early methods were
characterized by low efficiency or complexity. One of the most widely adopted was the Ward
Leonard system (essentially an adjustable DC voltage source produced mechanically). The speed
of the induction motor is not as easily controlled, since it is essentially a synchronous motor with
the field replaced by a cage that allows self-starting. Simple schemes consisted of tap or polechanging
switches, yielding a few discrete speeds. Continuous control could be had with a
wound-rotor machine but at the expense of efficiency. Elaborate multi-machine speed control
systems were employed to change frequency or to recover slip-energy. Alongside the simple
mechanical means and multi-machine schemes appeared those that employed the first controlled
power devices of type and function too numerous to mention. Additional information on this rich
history can be found in [91], [90], [23, vols.1,2,4,5].
Although progress was continuous it was primarily along the same lines it had always been, until
the “second electronics revolution” that came when the commercial thyristor was introduced
around 1958. The principles of speed control in the machines obviously did not change, only the
means by which it could be achieved. With the solid-state controlled rectifier came the inverter,
which originally used originally load- or force- commutated thyristors. Variable frequency
operation was thus finally achieved by solid-state means but only at large power levels. The
possibility of pulse width modulation (PWM) came as self-turn-off thyristors and power
transistors were developed, allowing variable frequency operation at lower power levels. The
thyristor was used to build chopper drives for DC motors and variable-voltage variable-frequency
(VVVF) adjustable speed drives for induction motors (both six-step and PWM). The inverter
could have also been used to achieve variable speed operation of the synchronous machine but
this was not common.
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