The Institution of Electrical Engineers

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© 2001: The Institution of Electrical Engineers
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British Library Cataloguing in Publication Data
Control Techniques drives and controls handbook.
(lEE power series; no. 35)
1. Electric motors 2. Electric controllers 3. Electric driving
I. Drury, W. I1. Control Techniques Drives PIc
621.4' 6
ISBN 0 85296 793 4
Typeset by Newgen Imaging Systems, India
Printed in England by Cambridge University Press, Cambridge

Variable-speed drives remain a key component of the boom in all aspects of automation and energy saving which is becoming of ever greater importance throughout the world.
The words of Harry Ward Leonard first uttered on
18 November 1896 in his paper entitled 'Volts versus o h m s - s p e e d regulation of electric motors' still hold true:
'The operation by means of electric motors of elevators, locomotives, printing presses, travelling cranes, turrets on men-of-war, pumps, ventilating fans, air compressors, horseless vehicles and many other electric motor applica- tions too numerous to mention in detail, all involve the des- irability of operating an electric motor under perfect and economical control at any desired speed from rest to full speed'.
It can, and should, be argued that electrical variable-speed drives have facilitated the automation revolution. They have, like so many enabling technologies, developed rapidly, fuel- led by their success, stretched by demands never dreamed possible a generation earlier. The development cycle of drives products is now such that product ranges have expected lifetimes of only three to five years - a problem in itself to many OEM customers whose own products have a much longer design life.
The world of variable-speed drives is an exciting and rapidly moving one. To predict the future and the pace of devel- opment is difficult. A historical perspective is helpful and, for those who need any convincing, shows how quickly things are moving:
Figure P. 1 Michael Faraday (1791-1867)
1820
1821
Oersted was the first to note that a compass needle is deflected when an electric current is applied to a wire close to the compass - the fundamental principle of an electric motor.
Faraday built two devices to produce what he called electromagnetic rotation: that is a continuous circular motion from the circular magnetic force around a wire. This was the initial stage of his pioneering work.
1824
1825
1831
Arago discovered that if a copper disc is rotated rapidly beneath a suspended magnet, then the magnet also rotates in the same direction as the disc.
Babbage and Herschel demonstrated the inversion of
Arago's experiment by rotating a magnet beneath a pivoted disc causing the disc to rotate. This was truly induced rotation and just a simple step away from the first induction motor, a step which was not taken for half a century.
Using an induction ring, Faraday made one of his greatest discoveries - electromagnetic induction: the induction of electricity i n a wire by means of the electromagnetic effect of a current in another wire.

xiv Preface
1832
1838
1845
1870
1873
1879
1885
1886
1889
1890
The induction ring was the first electric transformer.
In a second series of experiments in the same year he discovered magneto-electric induction: the produc- tion of a steady electric current. To do this, Faraday attached two wires through a sliding contact to a copper disc, the first commutator, an approach sug- gested to him by Amp+re. By rotating the disc between the poles of a horseshoe magnet he obtained a continuous direct current. This was the first gen- erator. Faraday's scientific work laid the foundations of all subsequent electro-technology. From his experiments came devices which led directly to the modem electric motor, generator and transformer.
Pixii produced the first magneto-electric machine.
Lenz discovered that a D.C. generator could be used equally well as a motor. Jacobi used a battery-fed
D.C. motor to propel a boat on the River Neva.
Interestingly, Jacobi himself pointed out that bat- teries were inadequate for p r o p u l s i o n - a problem which is still being worked on today.
Wheatstone and Cooke patented the use of electro- magnets instead of permanent magnets for the field system of the dynamo. Over twenty years were to elapse before the principle of self excitation was to be established by Wilde, Wheatstone, Varley and the
Siemens brothers.
Gramme introduced a ring armature somewhat more advanced than that proposed by Pacinotte in 1860, which led to the multibar commutator and the modem D.C. machine.
Gramme demonstrated, at the Vienna Exhibition, the use of one machine as a generator supplying power over a distance of 1 km to drive a similar machine as a motor. This simple experiment did a great deal to establish the credibility of the D.C. motor.
Bailey developed a motor in which he replaced the rotating magnet of Babbage and Herschel by a rotating magnetic field, produced by switching of direct current at appropriately staggered intervals to four pole pieces. With its rotation induced by a rotating magnetic field it was thus the first commu- tatorless induction motor.
Ferraris produced a motor in which a rotating mag- netic field was established by passing single-phase alternating current through windings in space quad- rature. This was the first alternating-current com- mutatorless induction motor, a single-phase machine which Dobrowolsky later acknowledged as the inspiration for his polyphase machine.
Tesla developed the first polyphase induction motor.
He deliberately generated four-phase polyphase currents and supplied them to a machine which had a four-phase stator. He used several types of rotor, including one with a soft-iron salient-pole construc- tion - a reluctance m o t o r - and one with two short-circuited windings in space quadrature - the polyphase induction motor.
Dobrowolsky, working independently from Tesla, introduced the three-phase squirrel-cage induction motor.
Dobrowolsky introduced a three-phase induction motor with a polyphase slip-ring rotor into which resistors could be connected for starting and control.
The speed of these motors depends fundamentally upon pole number and supply frequency. Rotor resistance control for the slip-ring motor was intro- duced immediately, but this is equivalent to armature resistance control of a D.C. machine and is inher- ently inefficient.
By 1890 there was a well established D.C. motor, D.C. central generating stations, three-phase A.C. generation and a simple three-phase motor with enormous potential but which was inherently a single-speed machine. There was as yet no way of efficiently controlling the speed of a motor over the full range, from zero to top speed. i t
V O L T S VS. OttMS.
SPEED ]{EOULA'rION OF ]~r.ECTRIC ~fOTC, R~,
BY ~I. WARD LEO.N'ARD.
L
Figure P.2 1 l Oth meeting of the American Institute of
Electrical Engineers, New York, 18 November
1896
1896
1904
C
The work of Ward Leonard clearly marks the birth of efficient, wide-range, electrical variable-speed drives. The system he proposed was of course based upon the inherently variable-speed D.C. machine
(which had hitherto been controlled by variable armature resistors). His work was not universally accepted at the time and attracted much criticism, understandably, as it required three machines of similar rating to do the job of one. Today, however, all D.C. drives are based upon his control philoso- phy, with only the implementation changing from multimotor schemes through the era of grid-con- trolled mercury-arc rectifiers to thyristors and, more re-cently, in demanding dynamic applications, to bi- polar transistors, field-effect transistors (FETs), insulated-gate bipolar transistors (IGBTs)...
Kramer made the first significant move with respect to frequency changing in 1904 by introducing a
D.C. link between the slip rings and the A.C. supply.
This involved the use of two A.C. ~ D.C. motor sets. The D.C. link was later to become a familiar sight in many A.C. drive technologies. Subsequent advances in A.C. motor speed control were based upon purely electrical means of frequency and vol- tage conversion. Progress has followed the advances in the field of semiconductors (power and signal/ control).

Preface xv
8
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B
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Vou Ch. K r i l m e r .
Figure P.3 Elektrotechnische Zeitschrift, volume 31, 30
July 1908
1911 Schrage introduced a system based upon an induc- tion motor with a commutator on the rotor. This machine proved to be very popular, requiring no auxiliary machines, and was very reliable. It found large markets particularly in the textile industry and some other niche applications, and is still sold today although in rapidly reducing numbers.
1923 The introduction of the ignitron made controlled rectification possible. The thyratron and grid-con- trolled mercury rectifiers made life easier in 1928.
This made possible the direct control of voltage applied to the armature of a D.C. machine so as to apply the philosophy of Ward Leonard control without additional machines.
1930 The ideas of inversion (D.C. to variable frequency/ voltage A.C. which is the basis for the present day inverter) had been established, the use of forced commutation by means of switched capacitors was introduced.
1931 Direct A.C. to A.C. conversion by means of cyclo- converters was introduced for the railway service.
1932 Nyquist stability criterion developed.
1938 Bode stability criterion developed.
1950 The introduction of silicon into power switches replacing the bulky and relatively inefficient mer- cury-arc rectifiers (MAR). By 1960 thyristors
(SCRs) had become available and the key enabling technology for drives had arrived. D.C. drives and cycloconverters quickly embraced the new silicon technology, at first using techniques with origins in the MAR forerunners. The faster switching perfor- mance of the new silicon, however, opened many new doors notably in the field of forced commu- tation-the way was clear for commercial variable- frequency inverters.
1957 Back to back reversing D.C. drive introduced.
1960s Power semiconductor voltage and current ratings grow and performance characteristics improve.
Inverters became commercially viable, notably in industries such as textiles where a single (bulk) inverter was used to feed large numbers of induction motors (or reluctance motors, despite their low power factor, where synchronisation was required).
1963 Gain-bandwidth relationships of power converters investigated.
1970 The 1970s saw a new and very significant revolution hit the variable-speed drives market - packaging. Up until this time the static variable-speed drive design process had essentially concentrated on perfor- mance/functionality. Both A.C. and D.C. drives of even low rating were broadly speaking custom built/ hand crafted. This approach resulted in bulky, high- cost drives the very uniqueness of which often compromised reliability and meant service support was difficult. The drives industry was not fulfilling its potential.
1970s A.C. motor drives had made great advances in terms of performance but still lacked the dynamic performance to really challenge the D.C. drive in demanding pro- cess applications. Since the early 1970s considerable interest was being generated in the field-oriented control of A.C. machines. This technique pioneered by Blaschke and further developed by Leonhard opened up the opportunity for A.C. drives not only to match the performance of a D.C. drive but to improve upon it. The processing requirements were such that in its early days commercial exploitation was restric- ted to large drives such as mill motor drives, boiler- feed pump drives. Siemens was very much in the fore- front of commercialising field orientation and was also rationalising the numerous alternative drive topo- logies which had proliferated and, although stimu- lating to the academic, were confusing to drive users:
• D.C. drives
• single converter
• double converter
-
- circulating current free circulating current
• A.C. drives
• voltage (phase) control
• voltage-fed inverters
•
- quasi square V/f
- quasi square V/f with D.C. link chopper
- pulse-width modulated (PWM)
-
- current-source inverters induction motor synchronous machine
• static Kramer drive
• cycloconverter
1972
1973
Siemens launched the SIMOPAC integrated motor with ratings up to 70 kW. This was a D.C. motor with integrated converter including line reactors!
A new approach to drives in terms of packaging.
Utilising 19-inch rack principles, a cubicle mounting standard well used in the process industry, compact, high-specification ranges of D.C. drives in modular form became available off the shelf. Companies such as AEG, Thorn Automation, Mawdsley's and
Control Techniques pioneered this work. A new era of drive design had started.

xvi Preface
Figure P.4 D.C. drive module (Control Techniques)
1979
1983
1985
Further advances in packaging design were made possible by the introduction of isolated thyristor packages.
In 1983 plastic mouldings made their first significant impact in drives. Bipolar transistor technology also arrived, which eliminated bulky auxiliary commu- tation circuits.
Takahashi and Noguchi published a paper on direct torque control (DTC) in the IEEE. (This date is included not because of its technical significance rather as a point of interest as DTC has received much attention recently.)
Figure P.5 Plastic mouldings introduced into drives
(Control Techniques)
Figure P.6 Digital D.C. drive with microprocessor and ASIC
(Control Techniques)
1986
1988
1989
1990
1992
Great advances were being made at this time in the field of microprocessors making possible cost- effective digital drives at low powers. Further drives were introduced containing application-specific integrated circuits (ASIC), which up to that time had only been used in exceptionally large volume/ domestic applications. Further, new plastic materials were introduced which gave structural strength, weight, size, assembly and cost advantage.
IGBT technology was introduced to the drives market. IGBTs heralded the era of relatively quiet variable-speed drives (and introduced a few pro- blems, some of which have led to substantial aca- demic activity and a very few of which have required more pragmatic treatment).
The first implementation of the field-orientation or flux-vector drive was introduced to the high-volume, lower power market. It found immediate application in machine tool spindle drives and has grown rapidly in application (and rating) since. It should be said that the name vector has been prostituted by some in the drives industry with voltage vector and other such names/techniques, causing confusion and frus- tration to customers.
The trend to smaller drive products which were also simpler to design was given a significant boost by
Mitsubishi which introduced intelligent power modules, integrating into the semiconductor package necessary gate drive and protection functions.
A new packaging trend e m e r g e d - the book- form shape which had previously been applied to servo drives was now being applied to the broader

Preface xvii iii~?' ..... i~i~ ~ ~?~iii!i~!~i
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F i g u r e P.7 I G B T p o w e r s t a g e in a n A.C. d r i v e ( C o n t r o l
T e c h n i q u e s )
F i g u r e P.9 B o o k f o r m s h a p e o f d r i v e ( C o n t r o l T e c h n i q u e s )
• • i~,~iiii~,~,+~i>~,~ ~
F i g u r e P.8 V e c t o r d r i v e ( C o n t r o l T e c h n i q u e s )
1993
1996
1998
2000 industrial A.C. drives market. The trend continues today but there is not a consensus that this is the most suitable shape for all market segments.
Another innovation in packaging - at the low-power end of the spectrum when a DIN rail mounting
0.4 kW inverter package, similar to that used widely in equipment such as contactors and control relays, was launched. The first drive with a built-in supply side filter fully compliant with, the then impending,
EU regulations on conducted EMC was introduced.
The first truly universal drive was launched which met the diverse requirements of a general purpose open-loop vector drive, a closed-loop flux-vector drive, a servo drive and a sinusoidal supply converter with the selection purely by parameter selection.
The integrated D.C. motor launched in 1972 was not a great commercial success - much has been learned since those days. In 1998 integrated A.C. motor drives were introduced onto the market. These pro- ducts are, for the most part, open-loop inverter- driven induction motors and were initially targeted on replacing mechanical variable-speed drives.
A radical servo drive was introduced with the posi- tion and speed loop embedded in the encoder hous- ing on the motor itself. This brought with it the advantage of processing the position information close to the source thereby avoiding problems of noise etc., and allowed dramatic improvements in control resolution, stiffness of the drive and reduced the number of wires between the drive and the motor.

xviii Preface
Figure P. 12 Integrated A.C. motor (courtesy Leroy Somer)
Figure P. 10 DIN rail mounting drive with built-in EMC filter (Control Techniques)
Figure P. 11 Universal A.C. drive modules (Control Tech- niques)
A review of the time lines presented above illustrates that development within the drives industry continues at an ever increasing pace. Fundamental changes in the product, from a customer perspective, are still emerging, accessing ever more applications driven by automation and quality.
This book covers the present state of development, or rather commercial exploitation, of industrial A.C. and D.C. vari- able-speed drives and associated systems. It is intended primarily for the use of professional engineers who specify or design systems which incorporate variable-speed drives.
The theory of both the driven motor and the drive is explained in practical terms, with reference to fundamental theory being made only where necessary. Information on how to apply drive systems is included, as are examples of what is available within commercially offered drives and indications of what can be achieved using them. Emphasis is placed on industrial drives in the range 0.37 kW to 1 MW.
The practical emphasis of the book has led to two unfor- tunate but I fear unavoidable consequences. First, some of the theory behind the technology contained in the book has had to be omitted or abridged in the interests of simplicity and volume. Second, in such a practical book it has proved difficult to avoid reference to proprietary equipment. In such circumstances a tendency towards referencing the products of Control Techniques is inevitable. It should be clear to readers that these products are described for illu- mination and explanation of the technology. The lEE, publisher of this book, does not endorse these products or their use in any way.
This edition of the Control Techniques Drives and Con- trols Handbook has been created with contributions from engineers both within Control Techniques itself as well as sister companies within the family that is Emerson. I would in particular like to thank Dr Pete Barrass, Ray Brister,

Dr Mike Cade, Vikas Desai, Dr Colin Hargis, Jim Lynch,
John Orrells, Bleddyn Powell, Alex Rothwell, Michael
Turner and Peter Worland.
Preface xix