ELECTRIC MOTORS
AND DRIVES
Fundamentals, Types, and Applications
Fourth Edition
AUSTIN HUGHES AND BILL DRURY
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PREFACE
This fourth edition is again intended primarily for nonspecialist users or students of
electric motors and drives. From the outset the aim has been to bridge the gap
between specialist textbooks (which are pitched at a level which is too academic for
the average user) and the more prosaic handbooks which are full of detailed
information but provide little opportunity for the development of any real insight.
We intend to continue what has been a successful formula by providing the reader
with an understanding of how each motor and drive system works, in the belief that
it is only by knowing what should happen (and why) that informed judgements and
sound com parisons can be made.
The fact that the boo k now has joint authors resulted directly from the
publisher’s successful reviewing process, which canvassed expert opinions about
a prospective fou rth edition. It identified several new topics needed to bring the
work up to date, but these areas were not ones that the original author (AH) was
equipped to address, having long since retired. Fortunately, one of the reviewers
(WD) turned out to be a willing co-author: he is not only an industrialist (and
author) with vast experience in the field, but, at least as importantly, shares the
philosophy that guided the first three versions. We enjoy collabo rating and hope
and believe that our synergy will prove of benefit to our readers.
Given that the book is aimed at readers from a range of disciplines, sections of
the book are of necessity devoted to introductory material. The first two chapters
therefore provide a gentle introduction to electromagnetic energy conversion and
power electronics. Many of the basic ideas introduced here crop up frequently
throughout the book (and indeed are deliberately repeated to emphasize their
importance), so unless the reader is already well versed in the fundamentals it would
be wise to absorb the first two chapters before tackling the later material. At various
points later in the book we include more tutorial material, e.g. in Chapter 7 where
we prepare the ground for unraveling the mysteries of field-oriented control. A
grasp of basic closed-loop principles is also required in order to understand the
operation of the various drives, so further introductory material is included in
Appendix 1.
The book covers all of the most important types of motor and drive, including
conventional and brushless d.c., induction motor, synchronous motors of all types,
switched reluctance, and stepping motors (but not highly customized or applica-
tion-specific systems, e.g. digital hard disk drives). The induction motor and
induction motor drives are given most weight, reflecting their dominant market
position in terms of numbers. Conventional d.c. machines are deliberately intro-
duced early on, despite their declining importance: this is partly because under-
standing is relatively easy, but primarily because the fundamental principles that
ix
j
emerge carry forward to other motors. Similarly, d.c. drives are tackled first, because
experience shows that readers who manage to gra sp the principles of the d.c. drive
will find this knowhow invaluable in dealing with other more challenging types.
The third edition has been completely revised and updated. Major additions
include an extensive (but largely non-mathematical) treatment of both field-
oriented and direct torque control in both induction and synchronous motor drives;
a new chapter on permanent magnet brushless machines; new material dealing with
self-excited machines, including wind-power generation; and increased emphasis
throughout on the inherent ability of electrical machines to act either as a motor or
a generator.
Younger readers may be unaware of the radical changes that have taken place
over the past 50 years, so a couple of paragraphs are appropriate to put the current
scene into perspective. For more than a century, many different types of motor were
developed, and each became closely associated with a particular application.
Traction, for example, was seen as the exclusive preserve of the series d.c. motor,
whereas the shunt d.c. motor, though outwardly indistinguishable, was seen as
being quite unsuited to traction applications. The cage induction motor was (and
still is) the most numerous type but was judged as being suited only to applications
which called for constant speed. The reason for the plethora of motor types was that
there was no easy way of varying the supply voltage and/or frequency to obtain
speed control, and desi gners were therefore forced to seek ways of providing for
control of speed within the motor itself. All sorts of ingenious arrangements and
interconnections of motor windings were invented, but even the best motors had
a limited operating range, and they all required bulky electromechanical control
gear.
All this changed from the early 1960s, when power electronics began to make
an impact. The first major breakthrough came with the thyristor, which provided
a relatively cheap, compact, and easily controlled variablespeed drive using the d.c.
motor. In the 1970s the second major breakthro ugh resulted from the development
of power electronic inverters, providing a 3phase variable-frequency supply for the
cage induction motor and thereby enabling its speed to be controlled. These major
developments resulted in the demise of many of the special motors, leaving the
majority of applications in the hands of comparatively few types . The switch from
analogue to digital control also represented significant progress, but it was the
availability of cheap digital processors that sparked the most recent leap forward.
Real time modeling and simulation are now incorporated as standard into induction
and synchronous motor drives, thereby allowing them to achieve levels of dynamic
performance that had long been considered impossible.
The informal style of the book reflects our belief that the difficulty of coming to
grips with new ideas should not be disguised. The level at which to pitch the
material was based on feedback from previous editions which supported our view
that a mainly descriptive approach with physical explanations would be most
x Preface
appropriate, with mathematics kept to a minimum to assist digestion. The most
important concepts (such as the inherent e.m.f. feedback in motors, the need for
a switching strategy in converters, and the importance of stored energy) are
deliberately reiterated to reinforce understanding, but should not prove too tire-
some for readers who have already ‘got the message’. We have deliberately not
included any computed magnetic field plots, nor any results from the excellent
motor simulation packages that are now available because experience suggests that
simplified diagrams are actually better as learning vehic les.
Finally, we welcome feedback, either via the publisher, or using the e-mail
addresses below.
Austin Hughes (a.hughes@leeds.ac.uk)
Bill Drury (w.drury@btinternet.com)
14 October 2012
Preface xi
