SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

then there would be periods during which only one phase would be connected to the bus (which
results in zero potential since the neutral of the load is isolated), so that is the minimum. Perhaps
there are schemes with gating durations between the two but these are not common. Therefore, in
the 120° inverter at least one transistor per leg must be controlled (full-on or PWM) over 120° of
the electrical half-cycle; during the remaining 60° both transistors must be OFF. In the 180°
inverter both transistors in each leg must be controlled (full-on or PWM) at all times. The gating
periods for 120° and 180° inverters are shown in Figure 4.8. The shaded blocks show the periods
during which either transistor in one phase leg is controlled. In the 120° inverter it is seen that
there are two 60° segments per electrical cycle (360°) where both transistors in a leg are OFF.
This is true for each phase and the OFF periods are staggered between the phases, thus there will
always be exactly two phase legs controlled at any given time. In the 180° inverter it is clear that
the transistors in each leg are controlled at all times, thus all three legs are controlled at all times.
Due to finite transistor turn-off times a small delay (called dead-time or a blanking pulse) must be
separate the transistor switching in one leg, as discussed later.
Figure 4.8 – Gating schemes for 120° and 180° inverters.
Inverter Control Methods
The 120° and 180° inverters can be controlled in many different ways. As one reference puts it,
“…the number and combination of permutations seem to be endless” [128]. In another, “…the
enormous amount of material published makes it challenging for a user to identify basic
modulation principles and apply them to particular implementations. Much reported research
has presented “new” or “improved” PWM techniques, which are often only a straightforward
variation of a previous approach. But it can at times be quite difficult to see how they are
related” [70, p.95]. These quotations were referring to PWM methods in general, but the problem
is compounded in motor control because of the commutation/phasing and PWM are often
inseparable from one another. Only the PWM methods commonly used in BPMS motor control
will be discussed here. These are presented not only to give some perspective on motor control
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