SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Space Vector Modulation (SVM)
The overview of VSIs discussed three common methods of controlling an inverter (six-step
squarewave, six-step PWM, and PWM). The first two had an algorithm-like nature. In both cases
the inverter could be in only one of six valid commutation states and operation of the inverter
consisted of cycling through the states in order; the second case simply added PWM to control the
magnitude. The third method (PWM) was somewhat different because it was concerned with
producing arbitrary output voltages (usually sinusoidal) without any commutation states. SVM is
somewhat of a combination of the first two methods’ algorithm-like nature and the third method’s
ability to generate an arbitrary output voltage. SVM is simply a method of controlling the inverter
as a whole. It seems to have been presented first by van der Broeck, et al. [133], though [131] is
often cited as well.
States and Neutral Voltage
SVM is a 180° control method therefore it is best compared to the other 180° methods and the
120° methods are not discussed. In the 180° inverter each leg must have one transistor on at any
given time (again ignoring the dead-time between leg transistor switching). Thus the pole voltage
for each phase will be either high (H) at VDC
or low (L) at VDC
. Since there are three legs and
each can have only two states, the inverter will be in exactly one of
given time. The states can be numbered and are given by Table 4.1.
Table 4.1 – States and corresponding pole voltages of 180° inverter.
inverter leg state pole voltage
state SA SB SC vA vB vC S0 L L L –VDC –VDC –VDC S1 H L L +VDC –VDC –VDC S2 H H L +VDC +VDC –VDC S3 L H L –VDC +VDC –VDC S4 L H H –VDC +VDC +VDC S5 L L H –VDC –VDC +VDC S6 H L H +VDC –VDC +VDC S7 H H H +VDC +VDC +VDC 3
(2 8) discrete states at any
Of the eight states there are two states (S0 and S7) that result in zero voltage across any two
phase terminals; these two states are called null states and the other six are the active states. In
any active state one (two) phases will be high and two (one) will be low, therefore the neutral
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