Page 294 - Power Electronics Handbook

Chapter 13
DOC. link frequency changers

13.1 Introduction

The basic principles of a d.c. link frequency changer, or inverter, were
introduced in Chapter 6 and it was seen there that an inverter resembles a
chopper in requiring forced commutation if power thyristors are used as
the switching semiconductors. The inverter defers in the important aspect
that its output is a.c. and not d.c., as was the case for a chopper.
This chapter considers inverters with a view to classifying the multitude
of circuits that exist. A classification system based on the commutation
method was used in Chapter 12, and a similar technique can be used for
inverters. There are, however, several other considerations which did not
apply for choppers. First, inverters fall into two major groups:

(i) Push-pull inverters, where the load must be centre tapped, or a
separate centre-tapped transformer used to supply the load.
Push-pull inverters find frequent application for lower-power
inverter circuits, where transistors are used as the switching
semiconductors, so avoiding the need for commutation circuitry and
sometimes, as will be seen in the next section, for any separate base
drive circuitry as well.
(ii) Bridge inverters, in which a centre-tapped load is not essential.
Second, most inverter circuits, whether bridge or push-pull and
irrespective of the commutation system used, can be operated in several
voltage-control modes. Apart from varying the value of the fundamental
a.c. voltage, this also gives a measure of control over harmonics in the
waveform. Although choppers were stated as having two control modes,
variable frequency and fixed frequency variable mark space, both operate
on the principle of giving unidirectional output voltage pulses, the
mark-to-space ratio of which is controlled. They cannot be grouped
according to a voltage-control method, as is done with inverters.
Several factors were selected in Chapter 11 to enable a comparison to be
made between the various commutation systems used, especially in
relation to chopper circuits. Seven points may be considered when looking
at a goodness factor for inverters:
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