Page 399 - Power Electronics Handbook

388 Power semiconductor circuit applications

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There are two devices which have been predominantly used as switches
in electronic commutator motors, namely thyristors and transistors. Where
the power to be handled is large, thyristors are the obvious choice. The
thyristor is not capable of amplification-mode operation, but at the
high-power levels the low efficiency of such a system would be
unacceptable. Another characteristic of a thyristor is that it is unidirection-
al in operation and for bi-directional switching two devices can be
connected in a parallel back-to-back arrangement or a bi-directional device
such as a triac can be used.
Transistors are much more versatile since they can be operated in
amplification or switching mode, Figure 14.40 showing several arrange-
ments that may be used. Figure 14.40(a) is intended for unidirectional
operation, whereas Figures 14.40(b) to 14.40(d) give bi-directional capa-
bility. Since only one sensor is used in (c) and (d) these are generally the
preferred arrangement.
The motor winding shown in Figure 14.39 is inefficient due to poor
winding utilisation. Figure 14.41 shows an alternative arrangement which
overcomes this disadvantage and also illustrates another form of closed
winding. The table shows that the switches have purposely been operated
so as to produce a stepped output voltage waveform, which reduces the
motor voltage waveform harmonic content and gives greater efficiency.

14.3.3.4 Advantages of an electronic cornmumor
As mentioned earlier, a d.c. motor has very desirable characteristics for a
wide range of applications, its major disadvantage being its mechanical
commutator, which leads to the following limitations:
(i) The commutator and brushes are prone to sparking with a
consequent production of radiated interference. This can be a serious
hazard when operated in certain environments, such as in airborne
equipment.