Environmental Engineering Reference
In-Depth Information
other sector), all that is necessary is to use appropriate durations of vectors
U
1
,
U
2
,
and
U
0
or
U
7
. It can also be shown that SVPWM is optimum from the standpoint of
minimization of ripple current in an
L
-
R
load. Because the load ripple current of an
electric machine 'sees' the machine leakage inductance, the ripple current is a
scaled version of the flux linkage ripple since the leakage inductance is basically
linear. Flux linkage ripple is in essence a deviation in volt-seconds appearing across
an inductor, so minimum ripple current exists only if minimum voltage ripple is
impressed. SVPWM can be said to be optimum if this deviation in the load current
vector, due to ripple current, for several switching states becomes small and if the
cycle time is as short as possible. These conditions are met if
Only the four inverter states adjacent to the reference vector are used. In rea-
lity, only three inverter states are used since it is desirable to permit only a
single switch transition per switching cycle.
●
A cycle is defined by three successive switching states only. During a given
cycle, the average voltage vector is equal to the reference vector as shown in
Figure 6.14.
●
U
1
t
U
2
U
3
T
k
T
0
/2
T
k
+1
T
0
/2
T
2
T
Figure 6.14 Switching pattern of SVPWM
To visualize this in more detail, refer again to Figure 6.13 and note that a
succession of the vectors
U
1
,or(
a
,
b
,
c
) = (1,0,0),
U
2
,or(
a
,
b
,
c
) = (1,1,0), and
U
7
,or
(
a
,
b
,
c
) = (1,1,1,), will be used to synthesize the reference vector
U
k
. In this instance
the nomenclature
U
k
is used to connote which vectors in the given sector are
adjacent to the reference. Also, only a single switch transition takes place in
moving from one state to the next at the inverter output. In general, SVPWM seeks
