Page 295 - Power Electronics Handbook

Inverter circuits 285

(i) Is it essential that the load be centre tapped, with the two halves
magnetically coupled? Such a push-pull inverter severely limits the
type of applications for which it may be used.
(ii) What is the harmonic content of the output waveform? Inverters with
high harmonic content, which need to provide a sine wave output,
require large and bulky filters. Even if the output is not sinusoidal, a
low harmonic content is often advantageous. For instance, in an
induction motor it is the fundamental component of the waveform
which produces the useful torque, whereas the harmonics result in
losses.
(iii) What is the complexity of the inverter and control electronics
required to produce the output voltage waveform?
(iv) Can the inverter operate at high frequencies with relatively high
efficiency?
(v) What are the maximum and minimum values of the fundamental
r.m.s. output voltage?
(vi) Is the commutation voltage increased in proportion to the load being
commutated? This is desirable to allow optimisation of commutation
components.
(vii) Is the current rating of the main thyristor increased by the
commutation capacitor reset pulse?
Two other factors were considered when dealing with choppers, namely:
(i) In the advent of a commutation failure will commutation be
re-attempted and will it be successful?
(ii) Does a low-impedance fault current path exist across the supply?
For an inverter a commutation failure almost always results in a
low-impedance path across the source, which is protected by fast-acting
fuses. Therefore commutation of the thyristors cannot be re-attempted.
The above factors are considered again in the following sections with
reference to typical inverter systems.
This chapter follows a format similar to Chapter 12, which described
choppers. The various inverter circuits are first introduced, both those
using transistors, which do not require commutation, and those with forced
commutated thyristors. This is then followed by the techniques used to
control the output voltage from an inverter. The design of inverter circuits,
both with and without commutation, is then described. The chapter
concludes with a description of the current-fed inverter and the control
electronics used in inverter circuits.

13.2 Inverter circuits

This section first introduces the various forms of inverter configurations,
followed by a description of transistor inverters, popularly used for low- to
medium-power applications. For high-power applications thyristor circuits
are required and the basic commutation techniques used for these are
described. The section concludes with a description of the modifications
made to the basic commutated circuits for enhancement of certain
performance factors.