Page 231 - Power Electronics Handbook

Chapter 10

Direct 8.c. frequency converters

10.1 Introduction

An a.c. output of a given frequency and voltage can be produced either
from a d.c. source, or directly from another a.c. source of a different
frequency and voltage. Frequency converters can therefore be classified
broadly into two types, d.c. to ax., called d.c. line frequency converters or
inverters, and a.c. to a.c., known as direct a.c. frequency converters or
cycloconverters.
Inverters and cycloconverters were introduced in Chapter 6, and in that
chapter a simplified operating mode, i.e. single-phase resistive load, was
used in order to illustrate the circuit principles. In this chapter
cycloconverters will be analysed in greater detail, in particular it will be
shown that they form an extension to controlled rectifier circuits, as
described in Chapter 9, and can be treated similarly. It is important to keep
this similarity in mind since, although controlled rectification forms one of
the most frequently used power electronic circuits, the cycloconverter is
often regarded as something mysterious and complicated. This reputation
has no doubt been gained due to the apparently rather formidable
arrangements which cycloconverter systems require, but it will be seen that
all such systems can be broken down into basic controlled rectifier circuits,
which are interconnected.
Although inverters are described in a later chapter, it is important to
recall the main differences between it and the cycloconverter, as
introduced in Chapter 6. These are as follows:

(i) An inverter uses fewer power components than a cycloconverter and
is usually simpler in construction, even when the commutation
components are considered. This often results in a cheaper system,
although in very large current drives the power devices, which are
usually thyristors, are much smaller than for a comparable inverter
circuit, since they now have a lower duty cycle, so offsetting the cost
advantage. In low-power circuits the voltage rating principally
determines the cost of power components, so that this no longer
applies. It should also be remembered that inverter thyristors need to
be specially selected for fast turn-off times and high dv/dt, which
results in more expensive components. The thyristor-firing circuits
22 I