Page 232 - Power Electronics Handbook

222 Direct a.c. frequency converters
for inverters are simpler than for cycloconverters and this results in a
further reduction in system cost.
(ii) An inverter can work from a d.c. or an a.c. source, which has first
been rectified and smoothed by relatively cheap components. A
cycloconverter, on the other hand, can only work from an a.c. input,
so if a d.c. supply is available only it is necessary to first convert it to
a.c. before it can feed the cycloconverter. Such a d.c. to a.c. inverter
can add appreciably to the original cycloconverter system cost, even if
rotating converters are used. Therefore an inverter is much more
versatile than a cycloconverter and is the system most frequently
used, although, as will be seen later, a cycloconverter is used for
special applications, usually when the supply is a.c.
(iii) Both inverters and cycloconverters can regenerate power from the
load to the supply, but whereas an inverter requires a fairly complex
control system to do this, regeneration occurs as part of the natural
process in a cycloconverter. Regeneration is frequently required in
high-power motor-drive systems, if frequent stops and starts are
necessary, and in this application a cycloconverter has the obvious
edge over an inverter.
(iv) Both inverters and cycloconverters (three-phase) can provide a
stepless output frequency variation. For an inverter this can vary
from any value below to any above the base frequency, since the
input is converted to a d.c. line. For cycloconverters, on the other
hand, the output frequency is usually limited to about one third that
of the input and at any rate to below that of the supply, unless forced
commutation is used, which is not desirable. The load current
waveforms from inverters can be made to resemble those of
cycloconverters by using pulse-width voltage-control techniques, as
will be seen in Chapter 13.
(v) The inverter has an intermediate d.c. store so that when operating
from an a.c. supply the power factor imposed on this source is high,
irrespective of that of the load. A cycloconverter does not have any
equivalent storage capability so that the load power factor is reflected
directly to the supply. By the very nature of the phase control
involved in a cycloconverter, the power factor is always lagging,
therefore a 60" leading or 60" lagging load angle would both produce
an approximately 60" lagging supply power factor angle.
As a generality, an inverter is used where a wide frequency variation is
required and a cycloconverter is preferred when most of the output
requirements are at low frequency.

10.2 Cycloconverter principles

Cycloconverters can operate in one of two modes, envelope or
phase control. In both systems the circuit arrangement is identical, it is the
operating mode which determines the type of converter. In Chapter 6 the
phase-controlled system was introduced and this is the commonest method
in use, being described in this section to illustrate the fundamental