Page 334 - Power Electronics Handbook

324 D.C. link frequency changers
13.3.2 Bi-directional switching
All the waveforms cotlsidered so far have been unidirectional, that is, the
instantaneous voltage in any half cycle has been either positive or negative,
but never both. For bi-directional switching the output swings positive and
negative in a half cycle. The simplest unidirectional system was the
quasi-square waveform, shown in Figure 13.33, and it is interesting to note
that this can be obtained by combining two square waves phase shifted
from each other, as in Figure 13.41, the output being zero when the
waveforms are in anti-phase. This single-pulse system was then extended to
two-pulse waveforms, as in Figure 13.36. The two-pulse bi-directional

(C)
Flgurc 13.41 Production of a quasi-square wave by combining two-phase shifted square
waves: (a) first square wave; (b) phase-shifted second square wave; (c) quasi-square wave

Nmre 13.42 A two-pulse bi-directional wave