Space Vector Modulated – Direct Torque Controlled (DTC – SVM ...

2. Voltage Source Inverter Fed Induction Motor Drive
cycles the switching sequence is deterministic. In RPWM methods the switching
frequency or the switching sequence change randomly.
One of the proposed random modulation techniques is a method with randomly
varied lengths of coincident switching and sampling time of the modulator. This method
was named RPWM 1. The sampling and switching cycles in DEPWM with RPWM 1 is
comparable shown in Fig. 2.23. The reference voltage vectors U c , which are calculated
in one sampling time T s and realized in the next switching time T sw are shown. In drive
systems the controller mostly operates in synchronism with modulator and in RPWM 1
arises problems in the control system, when it works with variable sampling frequency.
An additional control algorithm with variable sampling frequency is difficult tin a
digital implementation.
a)
(1)
U c
(2)
U c
(3)
U c
(K)
U c
( n−1)
U c
(n)
U c
( n+1)
U c
sampling cycles 1 2 3 ... n-1 n ...
switching cycles 1 2 3 ... n-1 n ...
T
s
= T sw
b)
(1)
U c
(2)
U c
(3)
U c
(K)
U c
( n−1)
U c
(n)
U c
( n+1)
U c
sampling cycles 1 2 3 ... n-1 n
switching cycles 1 2 3 ... n-1 n ...
T
s
= T sw
...
Fig. 2.23. Sampling and switching cycles a) DEPWM, b) RPWM 1
For elimination of these disadvantages random modulation techniques were
proposed, which operate with a fixed switching and sampling frequency. These methods
randomly change switching sequence in the interval. Three of these methods are shown
in Fig. 2.24 [6].
First of them (Fig. 2.24a) is random lead-lag modulation (RLL). In this method pulse
position is either commencing at the beginning of the switching interval (leading-edge
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