SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

In the popular literature there are many opinions, much confusion, and even debates regarding the
distinction between AC and DC motors. In this report, the descriptors AC and DC refer to the
type of ideal supply current (through the machine terminals) required to operate a motor from a
supply without the use of any type of motor controller. Therefore any motor that will not turn
continuously when connected to a battery is defined as an AC motor. Among the many definitions
of AC and DC motors, the reason for the above definition is that all (non-homopolar) machines
have alternating flux linkage and induced voltage and require a current source that alternates in
polarity in order to produce useful torque. Even the commutator DC machine has this property—
it is the only the half-cycle-reversing action of the commutator that allows it to operate from a DC
source. This distinction is clear in Figure 2.3, with the exception that the universal motor (and
other brushed/“series AC” types) can operate from either source. The distinction between AC and
DC motors will be revisited at the end of the chapter.
A second categorization (based on the presence or absence of a brush-commutator system) is
shown in Figure 2.4. Both the induction and synchronous motors do not have brushes and thus do
not have the limitations and problems associated with brushes. Essentially all modern
synchronous motors are brushless, including wound-field machines since brushless exciters have
replaced the slip ring and brush systems in most machines. (An exception could be made for lowcost,
low-power applications, such as residential/consumer gasoline generator sets, but even a
portion of these use brushless exciters.) That the induction motor does not require a brushcommutator
system is one reason for the induction motor’s robustness, although it should be
noted that the induction motor is not usually called a “brushless motor.”
Figure 2.4 – Motors categorized by presence or absence of brushes.
A third categorization is one based on power supply type; this is shown in Figure 2.5. Only
continuous motors are shown (thus stepper motors are excluded). Loosely, the three types of ideal
voltage/current supplies are DC, single-phase AC, and three-phase AC. Motors using DC supplies
can be controlled by controlling the average voltage; the counterpart technique for single-phase
AC supplies is phase angle control (phase chopping). Both DC and single-phase AC control is
accomplished by “adjusting” or “limiting” the power supply without markedly changing its
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