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

2. Voltage Source Inverter Fed Induction Motor Drive
The main disadvantage of the RPWM 1 method (Fig. 2.23b) is variable switching
frequency. For elimination of this disadvantage RPWM 2 [119] was proposed, which
operates with fixed sampling frequency and variable switching frequency. The principle
of this method is shown in Fig. 2.25.
T s
(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
switching cycles
∆t
1 2 3 ... n-1 n ...
1 2 3 ... n-1 n
T sw
...
Fig. 2.25. Sampling and switching cycles in RPWM 2 technique
In this method the start of each switching cycles is delayed with respect to that of the
coincident sampling cycle by a random varied time interval
∆ t . It is given as:
∆ t = rT s
(2.39)
where r denotes a random number between 0 and 1. Time interval
the sake of minimum switching time of inverter.
∆ t is limited for
Fig. 2.26. The output line to line voltage harmonics content a) RPWM 1, b) RPWM 2
Corresponding spectra for the RPWM 1 and RPWM 2 techniques are shown in Fig.
2.26a and 2.26b respectively. It can be seen that the harmonic clusters typical for the
determination modulation (compared to Fig. 2.17) are practically eliminated by the
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