SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Overview of Voltage Source Inverters
There are many different inverter types reported in the literature, such as matrix, multi-level, soft-
switched, and various resonant topologies. Only hard-switched, two-level, three-phase inverters
are discussed here because they are the only common topologies in brushless permanent magnet
motor control. The current source inverter (CSI) is only used in very large power ratings [75]
thus the voltage source (or stiff) inverter (VSI) is the dominant topology, even though it is
ultimately motor current that we desire to control. In a three-phase VSI there are three output
terminals, each connected to a leg (also known as a half-bridge) that is connected across the DC
link or bus. The bus could be supplied by a DC power supply or it could be created using a
rectifier and a large bulk capacitor as shown in Figure 4.1. At no time can both transistors in one
leg be ON otherwise they will short-circuit the bus (a shoot-through fault). Since each terminal
can be connected to only two potentials (with respect to the bus midpoint) this is called a two-
level inverter.
Figure 4.1 – VSI topology with controlled-rectifier front end.
In the VSI topology the output voltage waveform and frequency are controlled only by the
transistor switching scheme (which will be called commutation regardless of whether a motor is
considered). However, the magnitude of the voltage can be controlled using the transistors or by
using the controlled rectifier at the input. (This is true even if the inverter is fed from a DC
supply—the controlled rectifier is simply replaced with a chopper.)
In order to get the largest possible amplitude, the output waveforms are synthesized about the
midpoint of the DC bus, which is labeled M as shown in Figure 4.2. Many sources in the
literature specify output voltages in terms of V BUS because that it is the actual bus voltage. But
the output waveforms are AC, thus it makes more sense to the author to specify them in terms of
V DC as shown (the output is at maximum when it reaches V DC , as opposed to V BUS /2).
A
161