INSULATION AND DESIGN OF ELECTRICAL WINDINGS BY A. P. M. FLEMING,
M. LE. E. AND R. JOHNSON, A. M. I. E. E. WITH DIAGRAMS LONGMANS,
GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK, BOMBAY, AND
CALCUTTA PREFACE IT is generally recognised that insulation
constitutes the most vulnerable part of electrical machinery, and
manufacturers and users alike are confronted with the problem of
how to ensure the maintenance of electrical service while dependent
on materials known to be of an unreliable character. The extremely
unmechanical nature and general unsuitability of the commercial
insulating materials for withstanding the high temperatures and
stresses occurring in service, has discouraged any wide-spread
scientific investigations of directly practical application. As a
result, therefore, insulation problems have in the past been solved
largely by process of trial and error. The necessity for greater
attention to these problems has been forced upon engineers by the
advent of high voltages and larger and more costly units. Modern
scientific research has thrown much light on the electrical
behaviour of dielectrics, and much scattered data has been
published dealing with the properties of insulating materials. This
information, however, has not been available heretofore in a
co-related form whereby it can be used as a fundamental basis for
the practical insulation of electrical apparatus. In this treatise
the authors have endeavoured to set forth the underlying principles
and methods whereby the design of insulation can be carried out
with precision, and have embodied the results of many years of
practical experience in connection with insulating problems.
CONTENTS I. PHYSICAL CHARACTERISTICSOF DIELECTRICS 1 II.
ELECTROSTATIC CONDITIONS IN PRACTICAL WORKING .... 37 III.
INSULATING MATERIALS 51 IV. DESIGN OF INSULATION AND WINDINGS 116
V. INSULATION TESTS . 176 VI. THE DRYING AND HANDLING OF ELECTRICAL
WINDINGS . . 198 VII. INSULATION FAILURES 214 INDEX 219 CHAPTER I
PHYSICAL CHARACTERISTICS OF DIELECTRICS Introduction. In
considering the physical characteristics of insu lating materials
it is important to appreciate that all matter to some extent
possesses the property of electrical conductivity, and that in this
respect the difference between conductors and insulators is not so
much one of kind as of degree. The exhaustive investigations of Sir
J. J. Thompson and others show that the conductivity of gases is
explained by the presence and motion of infinitely small
electrically charged particles termed ions, which may be considered
to consist of atoms or groups of atoms, or of very much smaller
negatively charged particles known as electrons. Largely as a
result of these investigations the electron theory has been
developed, which has served to throw much light on the electrical
behaviour of matter. In applying this theory to the study of
electrical conductivity it is assumed that every atom has
associated with it one or more detachable electrons whose aggregate
negative charge is balanced by an equivalent positive charge on the
atom, and that an ion possessing a definite positive or negative
charge is formed by the addition of an electron to, or its removal
from, one of these neutral atoms or combinations of atoms. If the
cohesion between atoms and their electrons is overcome, the ions
thus formed are free to move under the influence of an electric
force, and when inmotion in a definite direction constitute what is
ordinarily termed an electric current, the magnitude of which is
determined by the number of the ions and the velocity with which
they move. The theory thus presents a mental picture of the
conducting 2 INSULATION AND DESIGN OF ELECTRICAL WINDINGS process
which, in so far as it is at present understood, is the same for
all matter, whether in a solid, liquid, or gaseous form...
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