Direct Torque Control with Space Vector Modulation (DTC-SVM) of ...

Voltage source PWM inverter for PMSM supply
2 3
t2
= Msinα
ref
(3.16a)
π
3 1
t1 = M cosαref
− t2
(3.16b)
π
2
After t 1
and t 2
calculation, the remaining normalized time is reserved for zero vectors
U
0
, U7
with condition t 1
+ t 2
≤ 1.Therefore, the normalized total time for zero vectors
becomes:
t = t + t = 1 − ( t + t )
(3.17)
07 0 7 1 2
The equations (3.15a-b) for time interval of active vectors and equation (3.17) for total
time interval of zero vectors are identical for all variants of space vector modulation
(SVM) techniques.
The absence of neutral wire in star connected load provides a degree of freedom in
selecting the partitioning (zero sequence signals -ZSS) time of the two zero vectors. It is
equivalent to the freedom of injected signals in to phase signals. Therefore, it gives
different equations of t 0
and t 7
for each PWM method, but normalized duration time of
must fulfill condition in Eq. 3.17. As a consequence is only in different placement of
zero vectors U
0
, U
7
. Therefore, we can introduce the portioning factor of zero vectors,
which is defined as:
t t
k = =
t + t t
7 7
0 7 07
(3.18)
Please note that, the zero sequence signals does not change the inverter output line-toline
voltage.
From knowledge of the neutral voltage U
N 0
(see Fig. 3.4) and information what kind of
zero sequence signal (ZSS) will be injected in each phase of motor it is possible to
calculate normalized duration time of zero vectors t 0
and t 7
. In bellow Table 3.1 are
summarized different three-phase modulation techniques and remarks. Also graphical
interpretation are shown in Fig. 3.19.
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