//integras/b&h/Eer/Final_06-09-02/prelimsElectrical Engineer's Reference Book //integras/b&h/Eer/Final_06-09-02/prelimsImportant notice Many practical techniques described in this book involve potentially dangerous applications of electricity and engineering equipment. The authors, editors and publishers cannot take responsibility for any personal, professional or financial risk involved in carrying out these techniques, or any resulting injury, accident or loss. The techniques described in this book should only be implemented by professional and fully qualified electrical engineers using their own professional judgement and due regard to health and safety issues. //integras/b&h/Eer/Final_06-09-02/prelimsElectrical Engineer's Reference Book Sixteenthedition M. A. Laughton CEng., FIEE D. J. Warne CEng., FIEE OXFORDAMSTERDAMBOSTONNEWYORK LONDONPARISSAN DIEGOSAN FRANCISCO SINGAPORESYDNEYTOKYO //integras/b&h/Eer/Final_06-09-02/prelimsNewnes An imprint of Elsevier Science Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington, MA 01803 A division of Reed Educational and Professional Publishing Ltd A member of the Reed Elsevier plc group First published in 1945 by George Newnes Ltd Fifteenth edition 1993 Sixteenth edition 2003 Copyright # Elsevier Science, 2003. All rights reserved No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 46373 For information on all Newnes publications visit our website at www.newnespress.com Typeset in India by Integra Software Services Pvt. Ltd, Pondicherry 605 005, India. www.integra-india.com Printed and bound in Great Britain //integras/b&h/Eer/Final_06-09-02/prelimsContentsPreface Section A General Principles 1Units, Mathematics and Physical Quantities International unit system . Mathematics . Physical quantities . Physical properties . Electricity 2Electrotechnology Nomenclature . Thermal effects . Electrochemical effects . Magnetic field effects . Electric field effects . Electromagnetic field effects . Electrical discharges 3Network Analysis Introduction . Basic network analysis . Power-system network analysis Section B Materials & Processes 4Fundamental Properties of Materials Introduction . Mechanical properties . Thermal properties .Electrically conducting materials . Magnetic materials .Dielectric materials . Optical materials . The plasmastate5Conductors and Superconductors Conducting materials . Superconductors 6Semiconductors, Thick and Thin-Film Microcircuits Silicon, silicon dioxide, thick- and thin-film technology . Thick- and thin-film microcircuits 7Insulation Insulating materials . Properties and testing . Gaseous dielectrics . Liquid dielectrics . Semi-fluid and fusible materials . Varnishes, enamels, paints and lacquers . Solid dielectrics . Composite solid/liquid dielectrics . Irradiation effects . Fundamentals of dielectric theory . Polymeric insulation for high voltage outdoor applications 8Magnetic Materials Ferromagnetics . Electrical steels including silicon steels . Soft irons and relay steels . Ferrites . Nickeliron alloys . Ironcobalt alloys . Permanent magnet materials 9Electroheat and Materials Processing Introduction . Direct resistance heating . Indirect resistance heating . Electric ovens and furnaces . Induction heating . Metal melting . Dielectric heating . Ultraviolet processes . Plasma torches . Semiconductor plasma processing . Lasers 10Welding and Soldering Arc welding . Resistance welding . Fuses . Contacts . Special alloys . Solders . Rare and precious metals . Temperature-sensitive bimetals . Nuclear-reactor materials . Amorphous materials Section C Control 11Electrical Measurement Introduction . Terminology . The role of measurement traceability in product quality . National and international measurement standards . Direct-acting analogue measuring instruments . Integrating (energy) metering . Electronic instrumentation . Oscilloscopes . Potentiometers and bridges . Measuring and protection transformers . Magnetic measurements . Transducers . Data recording 12Industrial Instrumentation Introduction . Temperature . Flow . Pressure . Level transducers . Position transducers . Velocity and acceleration . Strain gauges, loadcells and weighing . Fieldbus systems . Installation notes 13Control Systems Introduction . Laplace transforms and the transferfunction . Block diagrams . Feedback . Generally desirableand acceptable behaviour . Stability . Classification ofsystem and static accuracy. Transient behaviour .Root-locus method . Frequency-response methods .State-space description . Sampled-data systems .Some necessary mathematical preliminaries . Sampler andzero-order hold . Block diagrams . Closed-loop systems .Stability . Example . Dead-beat response . Simulation .Multivariable control . Dealing with non linear elements .//integras/b&h/Eer/Final_06-09-02/prelimsDisturbances . Ratio control . Transit delays . Stability . Industrial controllers . Digital control algorithms . Auto-tuners . Practical tuning methods 14Digital Control Systems Introduction . Logic families . Combinational logic . Storage . Timers and monostables . Arithmetic circuits . Counters and shift registers . Sequencing and event driven logic . Analog interfacing . Practical considerations . Data sheet notations 15Microprocessors Introduction . Structured design of programmable logic systems . Microprogrammable systems . Programmable systems . Processor instruction sets . Program structures . Reduced instruction set computers (RISC) . Software design . Embedded systems 16Programmable Controllers Introduction . The programmable controller . Programming methods . Numerics . Distributed systems and fieldbus . Graphics . Software engineering . Safety Section D Power Electronics and Drives 17Power Semiconductor Devices Junction diodes . Bipolar power transistors and Darlingtons . Thyristors . Schottky barrier diodes . MOSFET . The insulated gate bipolar transistor (IGBT) 18Electronic Power Conversion Electronic power conversion principles . Switch-mode power supplies . D.c/a.c. conversion . A.c./d.c. conversion . A.c./a.c. conversion . Resonant techniques . Modular systems . Further reading 19Electrical Machine Drives Introduction . Fundamental control requirements for electrical machines . Drive power circuits . Drive control . Applications and drive selection . Electromagnetic compatibility 20Motors and Actuators Energy conversion . Electromagnetic devices . Industrial rotary and linear motors Section E Environment 21Lighting Light and vision . Quantities and units . Photometric concepts . Lighting design technology . Lamps . Lighting design . Design techniques . Lighting applications 22Environmental Control Introduction . Environmental comfort . Energy requirements . Heating and warm-air systems . Control . Energy conservation . Interfaces and associated data 23Electromagnetic Compatibility Introduction . Common terms . The EMC model . EMC requirements . Product design . Device selection . Printed circuit boards . Interfaces . Power supplies and power-line filters . Signal line filters . Enclosure design . Interface cable connections . Golden rules for effective design for EMC . System design . Buildings . Conformity assessment . EMC testing and measurements . Management plans 24Health and Safety The scope of electrical safety considerations . The nature of electrical injuries . Failure of electrical equipment 25Hazardous Area Technology A brief UK history . General certification requirements . Gas group and temperature class . Explosion protection concepts . ATEX certification . Global view . Useful websites Section F Power Generation 26Prime Movers Steam generating plant . Steam turbine plant . Gas turbine plant . Hydroelectric plant . Diesel-engine plant 27Alternative Energy Sources Introduction . Solar . Marine energy . Hydro . Wind . Geothermal energy. Biofuels . Direct conversion . Fuel cells . Heat pumps 28Alternating Current Generators Introduction . Airgap flux and open-circuit e.m.f. . Alternating current windings . Coils and insulation . Temperature rise . Output equation . Armature reaction . Reactances and time constants . Steady-state operation . Synchronising . Operating charts . On-load excitation . Sudden three phase short circuit . Excitation systems . Turbogenerators . Generatortransformer connection . Hydrogenerators . Salient-pole generators other than hydrogenerators . Synchronous compensators . Induction generators . Standards 29Batteries Introduction . Cells and batteries . Primary cells . Secondary cells and batteries . Battery applications . Anodising . Electrodeposition . Hydrogen and oxygen electrolysis Section G Transmission and Distribution 30Overhead Lines General . Conductors and earth wires . Conductor fittings . Electrical characteristics . Insulators . Supports . Lightning . Loadings //integras/b&h/Eer/Final_06-09-02/prelims31Cables Introduction . Cable components . General wiring cables and flexible cords . Supply distribution cables . Transmission cables . Current-carrying capacity . Jointing and accessories . Cable fault location 32HVDC Introduction . Applications of HVDC . Principles of HVDC converters . Transmission arrangements . Converter station design . Insulation co-ordination of HVDC converter stations . HVDC thyristor valves . Design of harmonic filters for HVDC converters . Reactive power considerations . Control ofHVDC . A.c. system damping controls . Interaction between a.c. and d.c. systems . Multiterminal HVDC systems . Future trends 33Power Transformers Introduction . Magnetic circuit . Windings and insulation . Connections . Three-winding transformers . Quadrature booster transformers . On-load tap changing . Cooling . Fittings . Parallel operation . Auto-transformers . Special types . Testing . Maintenance . Surge protection . Purchasing specifications 34Switchgear Circuit-switching devices . Materials . Primary-circuit-protection devices . LV switchgear . HV secondary distribution switchgear . HV primary distribution switchgear . HV transmission switchgear . Generator switchgear . Switching conditions . Switchgear testing . Diagnostic monitoring . Electromagnetic compatibility . Future developments 35Protection Overcurrent and earth leakage protection . Application of protective systems . Testing and commissioning . Overvoltage protection 36Electromagnetic Transients Introduction . Basic concepts of transient analysis . Protection of system and equipment against transient overvoltage . Power system simulators . Waveforms associated with the electromagnetic transient phenomena 37Optical Fibres in Power Systems Introduction . Optical fibre fundamentals . Optical fibre cables . British and International Standards . Optical fibre telemetry on overhead power lines . Power equipment monitoring with optical fibre sensors 38Installation Layout . Regulations and specifications . High-voltage supplies . Fault currents . Substations . Wiring systems . Lighting and small power . Floor trunking . Stand-by and emergency supplies . Special buildings . Low-voltage switchgear and protection . Transformers . Power-factor correction . Earthing . Inspection and testing Section H Power Systems 39Power System Planning The changing electricity supply industry (ESI) . Nature of an electrical power system . Types of generating plant and characteristics . Security and reliability of a power system . Revenue collection . Environmental sustainable planning 40Power System Operation and Control Introduction . Objectives and requirements . System description . Data acquisition and telemetering . Decentralised control: excitation systems and control characteristics of synchronous machines . Decentralised control: electronic turbine controllers . Decentralised control: substation automation . Decentralised control: pulse controllers for voltage control with tap-changing transformers. Centralised control . System operation . System control in liberalised electricity markets . Distribution automation and demand side management . Reliability considerations for system control 41Reactive Power Plant and FACTS Controllers Introduction . Basic concepts . Variations of voltage with load . The management of vars . The development of FACTS controllers . Shunt compensation . Series compensation . Controllers with shunt and series components . Special aspects of var compensation . Future prospects 42Electricity Economics and Trading Introduction . Summary of electricity pricing principles . Electricity markets . Market models . Reactive market 43Power Quality Introduction . Definition of power quality terms . Sources of problems . Effects of power quality problems . Measuring power quality . Amelioration of power quality problems . Power quality codes and standards Section I Sectors of Electricity Use 44Road Transport Electrical equipment of road transport vehicles . Light rail transit . Battery vehicles . Road traffic control and information systems 45Railways Railway electrification . Diesel-electric traction . Systems, EMC and standards . Railway signalling and control 46Ships Introduction . Regulations . Conditions of service . D.c. installations . A.c. installations . Earthing . Machines //integras/b&h/Eer/Final_06-09-02/prelimsand transformers . Switchgear . Cables . Emergency power . Steering gear . Refrigerated cargo spaces . Lighting . Heating . Watertight doors . Ventilating fans . Radio interference and electromagnetic compatibility . Deck auxiliaries . Remote and automatic control systems . Tankers . Steam plant . Generators . Diesel engines . Electric propulsion 47Aircraft Introduction . Engine technology . Wing technology . Integrated active controls . Flight-control systems . Systems technology . Hydraulic systems . Air-frame mounted accessory drives . Electrohydraulic flight controls . Electromechanical flight controls . Aircraft electric power . Summary of power systems . Environmental control system . Digital power/digital load management 48Mining Applications General . Power supplies . Winders . Underground transport . Coal-face layout . Power loaders . Heading machines . Flameproof and intrinsically safe equipment . Gate-end boxes . Flameproof motors . Cables, couplers, plugs and sockets . Drilling machines . Underground lighting . Monitoring and control 49Standards and Certification Introduction . Organisations preparing electrical standards . The structure and application of standards . Testing, certification and approval to standard recommendations . Sources of standards information Index //integras/b&h/Eer/Final_06-09-02/prelimsPrefaceThe Electrical Engineer's Reference Book was first published in1945:itsoriginalaims,toreflectthestateoftheartin electricalscienceandtechnology,havebeenkeptinview throughoutthesucceedingdecadesduringwhichsub-sequent editions have appeared at regular intervals. Publicationofaneweditiongivestheopportunityto respondtomanyofthechangesoccurringinthepractice ofelectricalengineering,reflectingnotonlythecurrent commercialandenvironmentalconcernsofsociety,but alsoindustrialpracticeandexperienceplusacademic insightsintofundamentals.Forthis16thedition,thirty-ninechaptersareeithernew,havebeenextensively rewritten,oraugmentedandupdatedwithnewmaterial. As in earlier editions this wide range of material is brought within the scope of a single volume. To maintain the overall lengthwithinthepossibleboundssomeoftheolder material has been deleted to make way for new text. Theorganisationofthebookhasbeenrecastinthe followingformatwiththeaimoffacilitatingquickaccess to information. GeneralPrinciples(Chapters13)coversbasicscientific backgroundmaterialrelevanttoelectricalengineering.It includeschaptersonunits,mathematicsandphysical quantities, electrotechnology and network analysis. Materials&Processes(Chapters410)describesthe fundamentalsandrangeofmaterialsencounteredin electricalengineeringintermsoftheirelectromechanical, thermoelectricandelectromagneticproperties.Included arechaptersonthefundamentalpropertiesofmaterials, conductorsandsuperconductors,semiconductors,insu-lation,magneticmaterials,electroheatandmaterialspro-cessing and welding and soldering. Control(Chapters1116)isalargelynewsectionwithsix chaptersonelectricalmeasurementandinstruments, industrialinstrumentationforprocesscontrol,classical controlsystemstheory,fundamentalsofdigitalcontrol, microprocessors and programmable controllers. PowerElectronicsandDrives(Chapters1720)reflectthe significanceofupto50%ofallelectricalpowerpassing throughsemiconductorconversion.Thesubjectsincluded ofgreatestimportancetoindustry,particularlythose relatedtotheareaofelectricalvariablespeeddrives, comprisepowersemiconductordevices,electronic powerconversion,electricalmachinedrives,motorsand actuators. Environment (Chapters 2125) is a new section of particular relevance to current concerns in this area including lighting, environmentalcontrol,electromagneticcompatibility, health and safety, and hazardous area technology. PowerGeneration(Chapters2629)seessomeration-alisation of contributions to previous editions in the largely mechanicalengineeringareaofprimemovers,butwithan expandedtreatmentoftheincreasinglyimportanttopicof alternativeenergysources,alongwithfurtherchapterson alternating current generators and batteries. TransmissionandDistribution(Chapters3038)iscon-cernedwiththemethodsandequipmentinvolvedinthe deliveryofelectricpowerfromthegeneratortothe consumer.Itdealswithoverheadlines,cables,HVDC transmission,powertransformers,switchgear,protection, andopticalfibresinpowersystemsandaspectsof installationwithanadditionalchapteronthenatureof electromagnetic transients. PowerSystems(Chapters3943)gatherstogetherthose topicsconcernedwithpresentdaypowersystemplanning andpowersystemoperationandcontrol,togetherwith aspectsofrelatedreactivepowerplantandFACTS controllers.Chaptersareincludedonelectricityeconomics and trading in the liberalised electricity supply industry now existinginmanycountries,plusananalysisofthepower supply quality necessary for modern industrialised nations. SectorsofElectricityUse(Chapters4449)isaconcluding sectioncomprisingchaptersonthespecialrequirementsof agriculture and horticulture, roads, railways, ships, aircraft, andminingwithafinalchapterprovidingapreliminary guide to Standards and Certification. Although every effort has been made to cover the scope of electricalengineering,thenatureofthesubjectandthe mannerinwhichitisevolvingmakesitinevitablethat improvementsandadditionsarepossibleanddesirable.In ordertoensurethatthereferenceinformationprovided remainsaccurateandrelevant,communicationsfrom professionalengineersareinvitedandallaregivencareful considerationintherevisionandpreparationofnew editions of the book. Theexpertcontributionsmadebyalltheauthorsinvolved and their patience through the editorial process is gratefully acknowledged. M. A. Laughton D. F. Warne 2002 //integras/b&h/Eer/Final_06-09-02/prelimsElectrical Engineer's Reference Bookonline editionAs this book goes to press an online electronic version is also in preparation. The online edition will feature.the complete text of the book.access to the latest revisions (a rolling chapter-by-chapter revision will take place between print editions).additional material not included in the print versionTo find out more, please visit the Electrical Engineer's Reference Book web page:http://www.bh.com/newness?isbn=0750646373or send an e-mail to newnes@elsevier.com//integras/b&h/Eer/Final_06-09-02/partSection A General Principles //integras/b&h/Eer/Final_06-09-02/part//integras/b&h/eer/Final_06-09-02/eerc0011 Units, Mathematics and Physical Quantities1.11/3 1.1.11/3 1.1.21/3 1.1.3Notes1/3 1.1.41/3 1.1.51/4 1.1.61/4 1.1.71/4 1.2Mathematics1/4 1.2.11/6 1.2.21/7 1.2.31/9 1.2.4Series1/9 1.2.51/9 1.2.61/10 1.2.71/10 1.2.81/10 1.2.91/13 1.2.101/13 1.31/17 1.3.1Energy1/17 1.3.21/19 1.41/26 1.5Electricity1/26 1.5.11/26 1.5.21/26 1.5.31/28 M G Say PhD, MSc, CEng, ACGI, DIC, FIEE, FRSE Formerly of Heriot-Watt University M A Laughton BASc, PhD, DSc(Eng), FREng, CEng, FIEE Formerly of Queen Mary & Westfield College, University of London (Section 1.2.10) Contents International unit system Base units Supplementary units Derived units Auxiliary units Conversion factors CGS electrostatic and electromagnetic units Trigonometric relations Exponential and hyperbolic relations Bessel functions Fourier series Derivatives and integrals Laplace transforms Binary numeration Power ratio Matrices and vectors Physical quantities Structure of matter Physical properties Charges at rest Charges in motion Charges in acceleration //integras/b&h/eer/Final_06-09-02/eerc001//integras/b&h/eer/Final_06-09-02/eerc001Thisreferencesectionprovides(a)astatementofthe International System (SI) of Units, with conversion factors; (b)basicmathematicalfunctions,seriesandtables;and (c) some physical properties of materials. 1.1International unit system TheInternationalSystemofUnits(SI)isametricsystem givingafullycoherentsetofunitsforscience,technology and engineering, involving no conversion factors. The starting point is the selection and definition of a minimum set of inde-pendent `base' units. From these, `derived' units are obtained byformingproductsorquotientsinvariouscombinations, againwithoutnumericalfactors.Forconvenience,certain combinations are given shortened names. A single SI unit of energy (joule ==kilogrammetre-squared persecond-squared) is, for example, applied to energy of any kind, whether it be kinetic, potential, electrical, thermal, chemical . . . , thus unify-ing usage throughout science and technology. The SI system has seven base units, and two supplement-ary units of angle. Combinations of these are derived for all otherunits.Eachphysicalquantityhasaquantitysymbol (e.g. m for mass, P for power) that represents it in physical equations,andaunitsymbol(e.g.kgforkilogram,Wfor watt) to indicate its SI unit of measure. 1.1.1Baseunits Definitionsofthesevenbaseunitshavebeenlaiddownin thefollowingterms.Thequantitysymbolisgiveninitalic, theunitsymbol(withitsstandardabbreviation)inroman type.Asmeasurementsbecomemoreprecise,changesare occasionally made in the definitions. Length:l,metre(m)Themetrewasdefinedin1983as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second. Mass:m,kilogram(kg)Themassoftheinternational prototype(ablockofplatinumpreservedatthe International Bureau of Weights and Measures, Se vres). Time: t, second (s)The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. Electric current: i, ampere (A)The current which, main-tained in two straight parallel conductors of infinite length, of negligible circular cross-section and 1 m apart in vacuum, pro-duces a force equal to 2 =107 newton per metre of length. Thermodynamic temperature: T, kelvin (K)The fraction 1/273.16ofthethermodynamic(absolute)temperatureof the triple point of water. Luminous intensity: I, candela (cd)The luminous intensity in the perpendicular direction of a surface of 1/600 000 m2 of a blackbodyatthetemperature of freezingplatinumundera pressure of 101 325 newton per square metre. Amount of substance: Q, mole (mol)The amount of sub-stance of a system which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. The elementary entity must be specified and may be an atom, a molecule, an ion, an electron . . . , or a specified group of such entities. 1.1.2Supplementaryunits Planeangle:c,u.. . . ,radian(rad)Theplaneangle between two radii of a circle which cut off on the circumfer-ence of the circle an arc of length equal to the radius. Solid angle: , steradian (sr)The solid angle which, having its vertex at the centre of a sphere, cuts off an area of the surface of the sphere equal to a square having sides equal to the radius. International unit system1/3 1.1.3Notes TemperatureAtzeroK,bodiespossessnothermal energy.Specifiedpoints(273.16and373.16 K)define theCelsius(centigrade)scale(0and100C).Intermsof intervals, 1C ==1 K.Intermsoflevels,ascaleCelsius temperature 0.corresponds to (0.273.16) K. ForceTheSIunitisthenewton(N).Aforceof1 N endows a mass of 1 kg with an acceleration of 1 m/s2. WeightTheweightofamassdependsongravitational effect.Thestandardweightofamassof1 kgatthesurface of the earth is 9.807 N. 1.1.4Derivedunits All physical quantities have units derived from the base and supplementarySIunits,andsomeofthemhavebeengiven names for convenience in use. A tabulation of those of inter-est in electrical technology is appended to the list in Table 1.1. Table1.1SIbase,supplementaryandderivedunits QuantityUnitDerivationUnit namesymbol Lengthmetre Masskilogram Timesecond Electriccurrentampere Thermodynamic temperaturekelvin Luminous intensitycandela Amountofmole substance Planeangleradian Solidanglesteradian Forcenewton Pressure,stresspascal Energyjoule Powerwatt Electriccharge, fluxcoulomb Magneticfluxweber Electricpotentialvolt Magneticflux densitytesla Resistanceohm Inductancehenry Capacitancefarad Conductancesiemens Frequencyhertz Luminousfluxlumen Illuminancelux Radiation activitybecquerel Absorbeddosegray Massdensitykilogramper cubicmetre Dynamic viscositypascal-second Concentrationmolepercubic m kg s A K cd mol rad sr kg m/s2 N N/m2 Pa Nm,WsJ J/sW AsC VsWb J/CV s Wb/m2 T V/A Wb/A,Vs/AH C/V,As/VF A/VS 1 Hz cdsrlm lm/m2 lx s 1 Bq J/kgGy kg/m3 Pa s mol/ 3metrem Linearvelocitymetrepersecondm/s Linearmetrepersecond- m/s2 accelerationsquared Angularvelocityradianpersecondrad/s cont'd //integras/b&h/eer/Final_06-09-02/eerc0011/4Units, mathematics and physical quantities Table1.1(continued ) QuantityUnitDerivationUnit namesymbol Angularradianpersecond-accelerationsquaredrad/s2 TorquenewtonmetreN m Electricfield strengthvoltpermetreV/m Magneticfield strengthamperepermetreA/m Currentdensityamperepersquare metreA/m2 Resistivityohmmetre m ConductivitysiemenspermetreS/m PermeabilityhenrypermetreH/m PermittivityfaradpermetreF/m Thermal capacityjouleperkelvinJ/K Specificheatjouleperkilogram capacitykelvinJ/(kg K) Thermalwattpermetre conductivitykelvinW/(m K) Luminancecandelaper squaremetrecd/m2 DecimalmultiplesandsubmultiplesofSIunitsareindi-cated by prefix letters as listed in Table 1.2. Thus, kA is the unitsymbolforkiloampere,andmFthatformicrofarad. Thereisapreferenceintechnologyforstepsof103. Prefixesforthekilogramareexpressedintermsofthe gram: thus, 1000 kg =1 Mg, not 1 kkg. Table1.2Decimalprefixes 1.1.5Auxiliaryunits Somequantitiesarestillusedinspecialfields(suchas vacuum physics, irradiation, etc.) having non-SI units. Some of these are given in Table 1.3 with their SI equivalents. 1.1.6Conversionfactors Imperialandothernon-SIunitsstillinusearelistedin Table 1.4, expressed in the most convenient multiples or sub-multiples of the basic SI unit [] under classified headings. 1.1.7 CGSelectrostatic and electromagnetic units Althoughobsolescent,electrostaticandelectromagnetic units(e.s.u.,e.m.u.)appearinolderworksofreference. Neithersystemis`rationalised',norarethetwomutually compatible.Ine.s.u.theelectricspaceconstantis.0 =1,in e.m.u. the magnetic space constant is j0 =1; but the SI units takeaccountofthefactthat1/H(.0j0)isthevelocityof electromagneticwavepropagationinfreespace.Table1.5 listsSIunitswiththeequivalentnumbernofe.s.u.and e.m.u. Where these lack names, they are expressed as SI unit nameswiththeprefix`st'(`electrostatic')fore.s.u.and`ab' (`absolute')fore.m.u.Thus,1 Vcorrespondsto102/3stV and to 108 abV, so that 1 stV =300 V and 1 abV =108V. 1.2Mathematics MathematicalsymbolismissetoutinTable1.6.Thissub-sectiongivestrigonometricandhyperbolicrelations,series (includingFourierseriesforanumberofcommonwave forms),binaryenumerationandalistofcommonderiva-tives and integrals. 1018 exaE 1015 petaP 1012 teraT 109 gigaG 106 megaM 103 kilok 102 hectoh 101 decada 101 decid 103 millim 106 micro j.109 nanon 1012 picop 1015 femtof 1018 attoa 102 centic Table1.3Auxiliaryunits QuantitySymbolSIQuantitySymbolSI AngleMass degree()/180radtonnet1000kg minute(/) second(/ /)Nucleonics,Radiation becquerelBq1.0s 1 AreagrayGy1.0J/kg area100m 2 curieCi3.7 1010 Bq hectareha0.01km2 radrd0.01Gy barnbarn1028 m 2 roentgenR2.6 104 C/kg EnergyPressure ergerg0.1mJbarb100kPa caloriecal4.186JtorrTorr133.3Pa electron-volteV0.160aJTime gauss-oerstedGaOe7.96mJ/m3 minutemin60s Forcehourh3600s dynedyn10mNdayd86 400s Length A ngstromA 0.1mm Volume litre1orL1.0dm3 //integras/b&h/eer/Final_06-09-02/eerc001Mathematics1/5 Table1.4Conversionfactors Length[m]Density[kg/m,kg/m3] 1mil25.40 mm1 lb/in17.86 kg/m 1in25.40 mm1 lb/ft1.488 kg/m 1ft 1yd 1fathom 1mile 0.3048 m 0.9144 m 1.829 m 1.6093 km 1 lb/yd 1 lb/in3 1 lb/ft3 1 ton/yd3 0.496 kg/m 27.68 Mg/m3 16.02 kg/m3 1329 kg/m3 1nauticalmile1.852 km Area[m2] 1circularmil 1in2 1ft2 1yd2 1acre 1mile2 Volume[m3] 1 in3 1 ft3 1 yd3 1 UKgal 506.7 mm 2 645.2 mm 2 0.0929 m 2 0.8361 m 2 4047 m 2 2.590 km2 16.39 cm 3 0.0283 m 3 0.7646 m 3 4.546 dm3 Flowrate[kg/s,m 3/s] 1 lb/h 1 ton/h 1 lb/s 1 ft3/h 1 ft3/s 1 gal/h 1 gal/min 1 gal/s Force[N],Pressure[Pa] 1 dyn 1 kgf 1 ozf 0.1260 g/s 0.2822 kg/s 0.4536 kg/s 7.866 cm 3/s 0.0283 m 3/s 1.263 cm 3/s 75.77 cm 3/s 4.546 dm 3/s 10.0 mN 9.807 N 0.278 N 1 lbf4.445 N Velocity[m/s,rad/s] Acceleration [m/s2,rad/s2] 1ft/min 1in/s 1ft/s 1mile/h 1knot 1deg/s 5.080 mm/s 25.40 mm/s 0.3048 m/s 0.4470 m/s 0.5144 m/s 17.45 mrad/s 1 tonf 1 dyn/cm2 1 lbf/ft2 1 lbf/in2 1 tonf/ft2 1 tonf/in2 1 kgf/m2 1 kgf/cm2 9.964 kN 0.10 Pa 47.88 Pa 6.895 kPa 107.2 kPa 15.44 MPa 9.807 Pa 98.07 kPa 1rev/min0.1047 rad/s1 mmHg133.3 Pa 1rev/s 1ft/s2 1mile/hpers 6.283 rad/s 0.3048 m/s2 0.4470 m/s2 1 inHg 1 inH2O 1 ftH2O 3.386 kPa 149.1 Pa 2.989 kPa Mass[kg]Torque[N m] 1oz28.35 g1 ozfin7.062 nN m 1lb0.454 kg1 lbfin0.113 N m 1slug14.59 kg1 lbfft1.356 N m 1cwt50.80 kg1 tonfft3.307 kN m 1UKton1016 kg1 kgfm9.806 N m Energy[J],Power[W] 1ftlbf 1mkgf 1Btu 1therm 1hph 1kWh 1.356 J 9.807 J 1055 J 105.5 kJ 2.685 MJ 3.60 MJ Inertia[kg m 2] Momentum[kg m/s,kg m 2/s] 1 ozin2 1 lbin2 1 lbft2 1 slugft2 1 tonft2 0.018 g m 2 0.293 g m 2 0.0421 kg m 2 1.355 kg m 2 94.30 kg m 2 1Btu/h 1ftlbf/s 0.293 W 1.356 W 1 lbft/s 1 lbft2/s 0.138 kg m/s 0.042 kg m 2/s 1mkgf/s9.807 W 1hp 745.9 WViscosity[Pa s,m 2/s] Thermalquantities[W,J,kg,K] 1 W/in2 1 Btu/(ft2 h) 1 Btu/(ft3 h) 1 Btu/(fth F) 1 ftlbf/lb 1.550 kW/m2 3.155 W/m2 10.35 W/m3 1.731 W/(m K) 2.989 J/kg 1 poise 1 kgfs/m2 1 lbfs/ft2 1 lbfh/ft2 1stokes 1 in 2/s 1 ft2/s 9.807 Pa s 9.807 Pa s 47.88 Pa s 172.4 kPa s 1.0 cm 2/s 6.452 cm 2/s 929.0 cm 2/s 1 Btu/lb 1 Btu/ft3 1 ftlbf/(lb F) 1 Btu/(lb F) 1 Btu/(ft3F) 2326 J/kg 37.26 KJ/m3 5.380 J/(kg K) 4.187 kJ/(kg K) 67.07 kJ/m 3 K Illumination[cd,lm] 1 lm/ft2 1 cd/ft2 1 cd/in2 10.76 lm/m2 10.76 cd/m2 1550 cd/m2 //integras/b&h/eer/Final_06-09-02/eerc0011/6Units, mathematics and physical quantities Table1.5RelationbetweenSI,e.s.ande.m.units Quantity Length Mass Time Force Torque Energy Power Charge,electricflux density Potential,e.m.f. Electricfieldstrength Current density Magneticflux density Mag.fd.strength M.M.F. Resistivity Conductivity Permeability(abs) Permittivity(abs) Resistance Conductance Inductance Capacitance Reluctance Permeance SIunit m kg s N N m J W C C/m2 V V/m A A/m2 Wb T A/m A m S/m H/m F/m S H F A/Wb Wb/A 102 103 1 105 107 107 107 3 109 3 105 102/3 104/3 3 109 3 105 102/3 106/3 12107 12109 109/9 9 109 1013/36.36109 1011/9 9 1011 1012/9 9 1011 361011 1011/36.e.s.u. Equivalentnumbernof e.m.u. cm102 cm g 103 g s1s dyn105 dyn dyn cm107 dyn cm erg107 erg erg/s107 erg/s stC101 abC stC/cm2 105 abC/cm2 stV108 abV stV/cm106 abV/cm stA101 abA stA/cm2 105 abA/cm2 stWb108 Mx stWb/cm2 104 Gs stA/cm4103 Oe stA4101 Gb st cm1011 ab cm stS/cm1011 abS/cm 107/4. 41011 st109 ab stS109 abS stH109 cm cm9 1011 abF 4108 Gb/Mx 109/4. Mx/Gb Gb =gilbert;Gs =gauss;Mx =maxwell;Oe =oersted. 1.2.1Trigonometricrelations Thetrigonometricfunctions(sine,cosine,tangent,cosecant, secant, cotangent) of an angle 0are based on the circle, given byx 2 y 2 =h2.Lettworadiiofthecircleencloseanangle0.andformthesectorareaSc =(h2)(0/2)shownshadedin Figure1.1(left):then0.canbedefinedas2Sc/h2. Theright-angled triangle with sides h (hypotenuse), a (adjacent side) and p (opposite side) give ratios defining the trigonometric functions sin 0 = phcosec0 = 1 sin 0 = hp cos 0 = ahsec0 = 1 cos 0 = ha tan 0 = pacotan0 = 1 tan 0 = ap In any triangle (Figure 1.1, right) with angles, A, B and C at thecornersopposite,respectively,tosidesa,bandc,then A B C = rad (180) and the following relations hold: a = b cos C c cos B b = c cos A a cos C c = a cos B b cos A a sin A = b sin B = c sin C a= b2 c 2 2bc cos A (a b)(a b) = (sin A sin B)(sin A sin B)=Other useful relationships are: sin(x y) = sin xcos y cos xsin ycos(x y) = cos xcos y sin xsin ytan(x y) = (tan xtan y)(1 tan xtan y)=2sin2 x =1 (1 cos 2x) cos x = 1 (1 cos 2x)2 22sin2 x cosx = 1sin3 x = 1 (3 sin x sin 3x)43cosx =1 (3 cos x cos 3x)4 cossin sin x sin y = 21 (x y)=1 (x y)2sin 2 cos cossin cos x cos y = 21 (x y)=1 (x y)2sin 2 cos tan x tan y = sin(x y) cos xcos y sin2 x sin2 y = sin(x y)sin(x y)=2 cosx cos2 y = sin(x y)sin(x y)=2 cosx sin2 y = cos(x y)cos(x y)=d(sin x)dx = cos xsin xdx = cos x k d(cos x)dx = sin xcos xdx = sin x k d(tan x)dx = sec2 x tan xdx = ln [ cos x[ k Values of sin 0, cos 0 and tan 0 for 0=< 0 < 90=(or 0 < 0 . = 1 RUN_CMD AUTO_MODE MAN_CMDMAN_MODE& AUTO_MODE Function Block (FBD) language RUN_CMD : = AUTO_CMD & AUTO_MODEOR (MAN_CMD & MAN_MODE & NOT AUTO_MODE)Structured Text (ST) Language Figure 16.53The five programming methods defined in IEC 1131-3 Figure 16.54Data Movement: (a) Allen Bradley PLC-5; (b) Siemens S5; (c) GEM-80 I/O) into the rung, and the -- instruction puts the valuefromtherungtothespecifiedaddress.InFigure 16.54(c)the(binary)valuefrom16bitinputwordA12is placed into 16 bit storage word G24. LD AUTO_CMD AND AUTO_MODE OR (MAN_CMD AND MAN_MODE ANDN AUTO_MODE ) STRUN_CMD Instruction List (IL) Language ReadyLmp-1 Start_PB FlllSDV1 Full & P1Full & P2 DI sch 2SDV3DI sch 1SDV21 done2 done Close 1SDV4Close 2SDV 5 1 Closed 2 Closed Walt Lmp2 Sequential Function Chart (SFC) Language BCD/binaryconversionisavailablewith--and --instructions,the directionof the con-version being obvious. IntheABBMaster,thepointsbetweenwhichdataisto be transferred are simply linked on the logic diagram. 16.4.4Data comparison NumericalvaluesoftenneedtobecomparedinPLCpro-grams;typicalexamplesareabatchcountersayingthe requirednumberofitemshavebeendelivered,oralarm circuits indicating, say, a temperature has gone above some safety level. Thesecomparisonsareperformedbyelementswhich have the generalised form of Figure 16.55, with two numer-ical inputs corresponding to the values to be compared, and a binary (on/off) output which is true if the specified condi-tion is met. Many comparisons are possible; most PLCs provide AGreater Than B(A>B) AGreater Than orEqualtoB(A>B) AEquals B(A @B) ALess ThanorEqual toB(A500 kWEarlycagemotorswith`A'and`E'classinsulationrequirecare overwindingtemperatureunderinvertercontrol.Powersaving onlargedrivesimportant. Openloopinductionmotordrivespredominate. 19.5.1.10Paperandtissue DrivedutyRatingrangeComments PapermachineandpumpsUpto500 kWEnvironmentdifficultwithwater,steamandpaperpulppresent. Pipeventmotorscommon.Oftennonstandarda.c.andd.c.motors. Usuallycloselyco-ordinateddrivesinapaperline. Closedloopinductionmotordrivesandd.c.drives. Windersandreelers5to100 kWConstantkWrangeoverbuild-uprange.Fourquadrantoperationwith regenerativebraking.IP23motorenclosurewithfilteriscommon. Closedloopinductionmotordrivesandd.c.drives. 19.5.1.11Printing DrivedutyRatingrangeComments Printingpress Folders,unwind& rewindstands Upto200 kW Upto100 kW Somespecialco-axialmotordesignsforseriesconnectiononline. Fieldweakeningforwidespeedrange.4-quadrantwithslowramp accelerationandinch/crawlcontrolplusemergencystop.Pipevent whereinkfumesmaybeahazard. Closedloopinductionmotordrivesandd.c.drives. Oftenintegratedinprintinglinedrivewithpressdriveandunwindstand driveunder`master'control.Otherwiseasabove. Closedloopinductionmotordrivesandd.c.drives. 19.5.1.12Packaging DrivedutyRatingrangeComments Boxing,stamping, folding,wrapping Upto75 kWMostly4-quadrantwithslowrampaccelerationwithinch/crawlandE/stop. Oftenintegratedlinecontrol. PMServodrivesarewidelyusedinprecisionpackagingmachines. Closedloopinductionmotordrivesandsomed.c.drivesarealsoused. 19.5.1.13Engineeringindustries DrivedutyRatingrangeComments Testrigsof manytypes Upto >&15 MWTestrigdrivesrequirecarefulengineering.Oftenhighspeedwithfast response,accuratespeedandtorquemeasurement,usually4-quadrant withfieldweakeningcontrol.Enginetestrigsrequirespecialknowledge ofthrottlecontroldrive/absorbchangeoverandpowermeasurement. Drivecontrol/monitoringparticularlyimportant. Closedloopinductionmotordrivesandd.c.drives. PMservodrivesarealsousedforprecisionapplications. //integras/b&h/Eer/Final_06-09-02/eerc01919/32Electrical machine drives 19.5.1.14Wireandcable DrivedutyRatingrange Comments Bunchersandstranders10to150 kW Generallymultipledriveswithcageorbow,capstanplustakeupdrives underintegratedcontrol.Constanttorqueexcepttake-upwith4-quadrant acceleration/decelerationwithinch/crawl/E-stopcontrols.Motorsrequire filterprotectionagainstmetaldustentry. Closedloopinductionmotordrivesandd.c.drives. Capstan5to100 kW Asabove. Take-upand5to50 kWAsabovebutconstantkWoverbuild-upratio. unwindstands Extruders5to150 kWSeeextrudersunderplasticsindustrybutcontroloftenbeintegratedin cablelinedrives. Armourers10to150 kWAsBuncher/Stranderdriveabove. Caterpillars1.5to30 kWConstanttorquedutyandlowkWratinginviewoflowhauloffcablespeeds. Oftenintegratedincablelinedrives.Motorprotectiongenerallynoproblem. Closedloopinductionmotordrivesandd.c.drives. 19.5.1.15Hydraulics DrivedutyRatingrangeComments PumpandmotorUpto250 kWHydraulicfluidisacontaminationrisk.Pipeventoftenused. testrigs Generallyconstanttorquetomedium/highspeedswith4-quadrantdrive. Speedtorqueandpowermeasurementoftenrequiredwithfulldrive monitoringonendurancerigs. Closedloopinductionmotordrivesandd.c.drives. 19.5.1.16Electricmotorsandalternators Driveduty RatingrangeComments A.c.andd.c.motors/ Upto>15 MWAllrotatingelectricalmachinemanufacturers generators/alternatorshaveelaboratetestbedrigs,supplyingtheir testbedrigsownrotatingmachinesandobtainingcontrol systemsfromthedrivesindustrytotheirownrequirements.Closedloopinductionmotordrivesandd.c.drives.19.5.1.17Textiles DrivedutyRatingrangeComments Ringframemachines, cardingmachines,looms Upto150 kWDifficultenvironmentinwhichIP55enclosurehasbecomeastandard; ringframeSchrageandd.c.thyristordrive.Usepipeventilation. Alldrivesconstanttorque4-quadrantforspeedmodulation(ringframe) orbestspeedholdingaccuracywithslowrampacceleration/deceleration oncardingdrives.Speciala.c.cageloommotorsarehightorque,highslip designs.Todaya.c.inverterdrivespredominatesincetheircharacteristics areparticularlysuitable. 19.5.1.18Foods,biscuitandconfection DrivedutyRatingrangeComments Extruder5to400 kWHoseproofmotorsforplantcleaning.Continuousproduction requiringhighlevelsofreliability,controlandmonitoring. Otherwiseasplasticsindustryextruders. Mixer5to150 kWAsaboveandseechemicalindustrymixerdrive. Conveyors0.5to120 kWAsaboveandseematerialhandlingindustryconveyordrive. //integras/b&h/Eer/Final_06-09-02/eerc019Bibliography and further reading19/33 19.6Electromagnetic compatibility Electromagnetic compatibility of electrical drive system is a complex but vitally important subject. By their very nature byrapidlyswitchingvoltagesand/orcurrents,drivescan formveryeffectivenoisegenerators.Thedesignofthe drivemustthereforebeverycarefullyconsideredfrom the early stages firstly to ensure that the drives own controls arenotinterferedwithandsecondlythatthedrivecan beappliedinasystemwithoutadverseeffectsonother equipment. Variousnationalandinternationalstandardsexistfor EMCfewhavebeenwrittenwithpowerelectronicdrives in mind. Drive manufacturers are now making great efforts todesigndrivescompliantwiththeexistingandfuture standardstheyshouldbeconsultedforadviceonappro-priate standards and compliance thereto. Bibliography and further reading Thedocumentslistedherehavebeenselectedtoprovide thereaderwithusefulsourcesofinformationandfurther readingrelatingtoElectricalVariableSpeeddrivesand their application. 1 MOLTGEN,G.,`ConverterEngineering',JohnWiley, ISBN0-471-90561-5(1984)(Areferencefor fundamen-tal power converter operations and relationships) 2 CHALMERS,B.J.,`ElectricMotorHandbook', Butterworths,ISBN0-408-00707-9(1988)(Apractical reference book covering many aspects of characteristics, specification, design, selection, commissioning and main-tenance) 3 Vas,P.,SensorlessVectorandDirectTorqueControl, OxfordUniversityPress,ISBN0-198-56465-1(1998) (Generalbackground to the theoryofvector control ofmotors) //integras/b&h/Eer/Final_06-09-02/eerc019//integras/b&h/eer/Final_06-09-02/eerc02020Motors and Actuators20.120/3 20.220/3 20.2.1Electromagnets20/3 20.2.220/3 20.2.3Actuators20/4 20.2.420/5 20.2.520/6 20.2.6Separators20/7 20.2.7Clutches20/8 20.2.8Couplings20/8 20.2.9Brakes20/9 20.2.1020/9 20.2.11Vibrators20/10 20.2.1220/10 20.2.1320/12 20.2.1420/13 20.320/15 20.3.120/15 20.3.220/17 20.3.320/24 20.3.420/29 20.3.520/31 20.3.620/31 20.3.720/32 20.3.820/35 20.3.9Testing20/35 20.3.1020/40 M G Say PhD, MSc, CEng, FRSE, FIERE, AGCI, DIC Formerly of Heriot-Watt University J F Eastham Contents Energy conversion Electromagnetic devices Tractive electromagnets Lifting magnets Crack detectors Magnetic chucks Relays and contactors Miniature circuit-breakers Particle accelerators Industrial rotary and linear motors Prototype machines D.c. motors Three-phase induction motors Three-phase commutator motors Synchronous motors Reluctance motors Single-phase motors Motor ratings and dimensions Linear motors //integras/b&h/eer/Final_06-09-02/eerc020//integras/b&h/eer/Final_06-09-02/eerc020Electromagnetic devices20/3 20.1Energy conversion Electromagneticmachinesconvertelectricalintomechan-ical energy in devices with a limited stroke (actuator, brake, relay etc.) or continuous angular rotation (motor), or linear motion (linear motor). Mechanicalenergyinvolvesaforcefm actingoveradis-tance x or a torque Mm acting over an angular displacement 0. Electricalenergyinvolvesthedisplacementofachargeq (a current i for a time t) through a potential difference (p.d.) v. The energies W and corresponding powers P =dW/dt are Mechanical : Wm = fmx Pm = fm (dxdt) = fmu(linear) Wm = Mm0. Pm = Mm(d0dt) = Mm.r (rotary) Electrical : We = vqPe = v(dqdt) = vi whereu =dx/dtisthetranslationalspeedand.r =d0/dtis therotationalspeed.Inanelectromagneticmachinethe basic physical conversion mechanism between the two forms ofenergyisthemagneticfield,acharacteristicpropertyof electric current. The elements of electromagnetic/mechanical conversion are set out in Sections 2.4.1 to 2.4.3. 20.2Electromagnetic devices 20.2.1Electromagnets Electromagnets for stroke-limited devices (e.g. actuators) are such that estimation of the flux distribution in the air gap (the workingregion)isdifficult.Thetotalmagnetomotiveforce (m.m.f.) produced by a current i in an N-turn coil is F =Ni. 20.2.1.1Coil windings Most coils for magnetic-circuit excitation are wound by one of the following (usually automated) methods: (1) on a for-mer with end-cheeks; (2) on a bobbin that forms an integral partofthecoilandcomprisesamouldedorfabricated constructionofasuitableinsultant;or(3)byawinding machinethatfeedsinsulatedwireintoaself-supporting form, with an epoxy-resin binder. Thecoil design is basedon the provisionof the required m.m.f.foraspecifiedvoltage(orcurrent),withanaccept-able coil temperature rise on a specified duty cycle. D.c.excitationFordirectcurrent(d.c.),the currentiata coilterminalvoltagevisdeterminedbythecoilresistance R =V/i =jLmtN/a,wheretheNturnshaveameanturn lengthLmt andthe conductor,ofresistivity,j,has a cross-sectional area a. Then a = jLmt(Ni)V= jLmtFV for a total m.m.f. F. The current cannot be determined until thecoolingconditionsareestablished.Lettheconductor currentdensitybeJ,sothati =Ja;thenN =F/Ja.The totalconductingcross-sectionofthecoilisNaandthe gross cross-sectionalarea ofthe wound coilis Na/k, where k is the space factor. ThepowertakenbythecoilisP =Vi, and theconsequent temperatureriseoncontinuousoperationis0m =P/cS.Herec is a cooling coefficient representing the power dissipation per unit of the coil surface area S per degree Celsius rise of surface temperature above ambient. The value of 0m for continuously ratedcoilsisusuallyspecified.Onintermittentorshort-time rating the rise is a function of the thermal capacity of the coil. A.c.excitationForalternatingcurrent(a.c.)thecurrenti atvoltageVisdeterminedbythecoilimpedance Z =R j.Latangularfrequency..Ana.c.coiltherefore tends to have fewer turns than one for d.c. Further, the coil inductance L varies widely, depending on the saturation of theferromagneticpartsandinparticularonthelengthof theairgap.Awidegapincreasesthemagneticreluctance and reduces L, but as the gap length reduces (e.g. by move-ment of the working parts) the inductance rises. If .L 4 R, asisusual,theroot-mean-square(r.m.s.)valueofthe m.m.f.approximatestoF =VN/.LwithLestimatedfor the range of air gap lengths. Inpractice,performanceisbasedondataobtainedon test.Aparticularfeatureisthedouble-frequencyfluctu-ationof themechanicalforce, whichproducesa character-istic`chatter'intheclosedpositionofthedevice;thismay have to be mitigated by means of a shading ring. 20.2.1.2Coil design SpacefactorAsimplecoilwoundfromacircular-section wire of diameter d, and insulated to a diameter di, will pack downinamannerthatisaffectedbythemethodofwind-ing,onelayerpartlyoccupyingthetroughsinthelayer beneathit;thespacefactormaythenapproximateto k =0.85(d/di)2. Conductors of small diameter bed less effect-ively, and the space factor is reduced. CoolingcoefficientAtypicalvalueofthecoolingcoeffi-cient c is 0.075 W/m2 per C above ambient. However, cool-ing conditions vary widely with the efficacy of ventilation. 20.2.1.3Operating conditions Whether d.c. or a.c. excited, the current in an operating coil is affected by that movement of the working parts that closes oropenstheairgap.Letaquiescentspring-loadedrelay (Figure 20.1(a)) in the open position be connected to a direct sourcevoltageV.Thecoilcurrentbeginstoriseexponen-tially,butthearmaturedoesnotmoveuntilthemagnetic forceexceeds thespring restraint. Thereafter, theshortening gapincreasesthecoilinductance,settingupacounterelec-tromotive force (e.m.f.) and checking the current rise and the attractingforce.Finally,thearmaturereachestheclosed positionattheend-stop,dissipatingkineticenergyinnoise, bounce and mechanical deformation. The sequence of events, in terms of the gap length x, armature speed u, coil current i and time t, is shownin Figure 20.1(b). Ifthecoilisenergisedfromana.c.sourcetherearetwo further effects: the closing time depends on the instant in the cycle at which the voltage is applied and (more importantly) the operating force fluctuates. Ferromagnetic parts must be laminatedtopreventexcessivecorelossandthecounter-effects of eddy currents. Suitable sheet steel for the purpose has a core loss of less than 5 W/kg. The force fluctuation can be reduced (but not eliminated) by a shading ring (Figure 20.2) embedded in one of the pole facesflankingthegap.Currentsinducedintheringdelay partofthepoleflux.Thusthecombinationofshadedand unshaded flux gives a resultant that still fluctuates but does not at any instant fall to zero. 20.2.2Tractive electromagnets Two forms of tractive electromagnet are shown in Figure 20.3. Type (a) usually has cylindrical poles, sometimes with shoul-dered ends to retain the coils, a rectangular yoke to which the //integras/b&h/eer/Final_06-09-02/eerc02020/4Motors and actuatorsFigure 20.1Operation of a d.c. relay Figure 20.2Shading coil or ring Figure 20.3Tractive electromagnets poles are screwed or bolted, and a rectangular armature. Two excitingcoilsareused;theyareconnectedtogiveopposite polaritiesattherespectivepoleends.Type(b)has asinglecoil mountedon acylindricalcoretowhichtherectangular pole-pieces are attached. In both cases the total air gap length is the sum of the gaps at the respective poles; in some designs, however, the armature is hinged to the pole-piece at one end. In this case the free end forms the major gap. In (a) let each polar surface have an area of 250 mm2 and berequiredtoexertatotalforceof1.0 Nonthearmature when both gaps are 3.0 mm long. Then with d.c. excitation, f ==400 000B2 A giving B ==70 mT, for which H ==57 000 A-t/m. For a total gap of 6 mm the gap m.m.f. required is 340 A-t. Adding10%fortheironcircuitand25%forleakage,the totalexcitationrequiredisabout450 A-t,fromwhichthe coil design follows. Witha.c.excitationitisnecessarytoestimatetheinduct-ance in the open and closed positions, and to adjust the num-berofturnsforagivenoperatingvoltagesothatadequate forceisavailable.Thechangeofmagneticfluxbetweenthe twoextremearmaturepositionsisverymuchlessthanwith d.c. operation, so that for the same (average) force in the open position, that in the closed position is only a little greater. 20.2.3Actuators 20.2.3.1D.c. actuators Threetypical arrangementsfor d.c.actuators are shownin Figure 20.4. Form (a) is convenient for small devices as the frame can be bent from strip; it is common for overcurrent andundervoltagerelays.Form(b)mayhaveacastframe, and provides parallel flux paths through the iron. In (c) the cylindrical iron circuit presents a low reluctance, the circuit being completed by a lid attached by studs or screwed into the cylindrical body. Theironend-stopshouldprojectwellintothecoilto improvefluxconcentration.Itmaybeintegralwiththe frameorscrewedintoit(inwhichcaseitcanbeusedto locateandsecuretheoperatingcoil).Theplungerpasses throughtheframeatthethroat,thereluctanceofwhich can be reduced by minimisingthe annular gap and extend-ing the effective axial length, as shown at (b) and (c). Atypicalironcladactuatorinpartsectionisshownin Figure20.5.Withthedimensionsa ==220 mm,d ==65 mm, x ==63 mmandy ==150 mm,thecoilmaydevelopabout 15 kA-ttogiveapullof400 Nacrossa25 mmgapinthe open position. The brass pin forms a stop, and cushions the plunger by expelling air through the vent. With a flat-ended plunger the stroke is equal in length to themagneticairgap.Maximumwork(force =displace-ment) occurs with a short stroke. By using a coned plunger (see Figure 20.5), maximum work is obtained with a longer stroke.Iftheconeangleis60=,thecomparablestroke istwicethatforaflat-endedplungerforaboutthesame magneticpull.Itispossibletoobtainawidevarietyof characteristicsbymodifyingtheshapesofthestopand plunger ends. Figure 20.4D.c. actuators //integras/b&h/eer/Final_06-09-02/eerc020Electromagnetic devices20/5 Figure 20.5Iron clad `pot' actuator 20.2.3.2A.c. actuators Acommonarrangementforasingle-phasedeviceisthat shown in Figure 20.6. The E-type laminations are clamped. In the plunger, rivets should lie in a line in the flux direction tominimiseeddycurrents.Tokeepdownthe`holding' current the plunger and stop ends should be flat. Becauseofthemanyvariablesconcerned,thedesignis complicated.Anempiricalruleistoallow1.5 mm2 of plungercross-sectionforevery1 Nofforce;thiscorre-spondstoapeakfluxdensityof0.8 Tinthelaminations. Thesizeofthecoil(andthereforethemaindimensions) maybetakenashavingalength2.53timesthestroke andadepthequaltothestroke.ThenumberofturnsN isestimatedfrom N==V4.4fBmA whereBmAisthepeakfluxandfisthefrequency.Final adjustment of N is made on test; it is reduced if the force is too low. 20.2.3.3Polyphase actuators Athree-phaseactuatorhasthreelimbs.Becauseofthe phasing, the net force on the laminated bar armature assem-bly is never zero, and shading is not necessary. Thetypicalunit(Figure20.7)isforoperatingabrake. It has three limb-coils E connected in star. The armature A isshowninthelifted(energised)condition.Theplunger Figure 20.6Single-phase actuator Figure 20.7Three-phase actuator rod,fittedwithapistoninthedashpotD,cushionsthe endofthestroke.Avalveinthepistonallowsunretarded drop-out for quick brake application. 20.2.4Lifting magnets Lifting magnets are of use in the handling of iron and steel, as they dispense withhooksandslings.Themaximumload ofamagnetvarieswiththematerialtobelifted.Amagnet capableoflifting1 tofscrapmayraisea20 tloadinthe form of athick solidpiece with a flat upper surface. As the excitation is limited by temperature rise of the coil, the lifting capacityisalsodependentonthedutycycle.Forthe comparativelyarduousconditionsnormally rulingin indus-trial use, a robust and weatherproof construction is essential. 20.2.4.1Circular magnets Theessentialfeaturesofacircularmagnetareshownin Figure 20.8. As the magnetic properties of the material lifted and the air gaps between the magnet poles and the material are both arbitrary and subject to wide variation, the design ofaliftingmagnetisgenerallybasedonthermalconsider-ations. A given carcass and winding are assigned an empir-ically derived power rating such that the temperature rise of the coil is not excessive. The designer's aim is then to secure the maximum effective ampere-turn excitation and working flux density by adjustments of iron and conductor materials andheatdissipation.Allowancesindesignmustbemade forthedevelopmentofadequatepullunderconditions oflowlinevoltage(e.g.80%orlessofnominal),andhigh conductor resistivity when hot. The majority of lifting magnets, except those of small size, haveawindingofflatstrip,whichismoreadaptablethan wiresofcircularsectiontotheattainmentofagoodspace factorwiththelargeconductorareasgenerallynecessary. Aluminium is sometimes employed in preference to copper for the advantage of weight economy: the weight of a magnet is //integras/b&h/eer/Final_06-09-02/eerc02020/6Motors and actuatorsFigure 20.8Circular lifting magnet importantasitrepresentsauselessadditionalloadtobe movedeverytimeitscraneisoperated.Thewindingin Figure20.8isshowndiagrammatically:itcomprisesa number of flat spirals with heat-resistant insulation. Thegeneraldimensionsaresuchthat thediameterofthe innerpole-faceis aboutone-thirdoftheoveralldiameterd. The load lifted is proportional to d 2 and the power rating (in kilowatts) is of the order of 4d2 (with d in metres). As regards theload,onlyinexceptionalcasesdoescloseanduniform contactoccurbetweenmagnetandloadsurfaces.Theten-dencyforfluxconcentrationoversmallcontactregions develops local saturation and increases the effective gap. 20.2.4.2Rectangular magnets Materials of regular shape, such as sheets, bars, pipes, etc., are well suited to lifting by rectangular magnets. The general Table20.1Approximateliftingcapacities Circularmagnets:loadlifted(t) construction is similar to that of the circular type, the body beingformedofabox-shapedsteelcastingwithacentral projection to give the inner polar surface. Theapproximateliftingcapacitiesofcircularandrect-angular magnets are given in Table 20.1. 20.2.4.3Control Simpleon/offswitchingisnotpracticablebecauseofthe highlevelofstoredmagneticenergy.Thegeneralcontrol features needed are: (1) discharge resistors connected across thewindingjustpriortodisconnectiontoreducecontact arcing and limit inductive e.m.f; (2) auxiliary resistors intro-ducedintothecoilcircuitafterapredeterminedtimeto limitcoiltemperaturerise;and(3)reversalofpolarityata lowcurrentleveltoovercomeremanenceandsorelease small pieces such as turnings or scrap. 20.2.5Crack detectors Electromagnetic crack detection depends on the fact that, in magneticmaterial,themagneticsusceptibilityofafaultis markedlyinferiortothatofthesurroundingmaterial.The success of the whole technique of magneticcrack detection depends largely on the care taken to ensure correct strength and direction of magnetisation.The following methods are used: (1) Needle method. The surface to be tested is explored with a smallmagneticneedle.Thisneedlecarriesapointer whichmovesoverascale,witharightandleftmotion astheneedleturnsonitspivottoalignwiththefield distortionpassingbeneathitinthedirectionofthe arrow.Thusthefaultisdetected.Thesensitivityis increased by using a mirror and light beam. (2) Powder method. The part to be tested, previously cleaned, islaidacrossthearmsofthemachine,andthecircuit-Material Magnetdiameter(m) 1.61.41.21.00.6 Skull-crackerball18151073 Slabs27231693 Pig-iron1.31.00.60.30.1 Brokenscrap0.80.50.40.30.1 Cast-ironborings1.00.70.50.30.1 Steelturnings0.50.30.20.10.005 Rectangularmagnets:platearealifted(m2) PlatestackMagnetdimensions(m) PlatethicknessLongestplateMaximumNo. (mm)(m)ofplatesin stack0.6 =0.41.0 =0.41.4 =0.42.0 =0.4 0.41.5800.92.33.54.6 12.8201.84.36.58.7 34.2102.45.58.311 66.752.86.59.713 129.533.27.511.315 2513.523.27.511.315 //integras/b&h/eer/Final_06-09-02/eerc020Electromagnetic devices20/7 Figure 20.9Magnetic pulley separator closingpush-buttonswitchdepressedandreleased quickly.Thearticleisthenremovedandsprinkledwith specialpowder,theexcessofwhichisblownawayor shakenoff;itwillthenbefoundthatthedefectsare clearly indicated by the magnetic patterns. (3) Fluidmethod.Thisresemblesthepowdermethodbut employs a fluid (e.g. paraffin) containing finely divided magnetic material in suspension. Each of these techniques can be applied to the detection of cracks or other flaws in parts which have been magnetised. Therearetwomethodsofattainingthismagnetisationin normal commercial use. In the first method the part to be tested is placed between the poles of an electromagnet, in which case the direction of the field is from pole to pole. The second method utilises the fact that a concentric magnetic field forms round an electric current. Aheavy low-voltage current is passed through the partitself,orthroughacurrent-baradjacenttoitor threaded through it. As only those cracks or flaws will be shown up which cut across the magnetic field, it will readily be understood that the first method is most suited to the detection of transverse cracks,thesecondtothedetectionoflongitudinalones. However,apparatusdesignedfortestingbymeansofthe secondmethodmaybeadaptedtothedetectionoftrans-versecracksbyencirclingthepartwithseveralturnsof cable through which the heavy current is passed. 20.2.6 Separators Thebulkhandlingofmaterial,particularlywherethepro-cess involves crushing or grinding, may require the use of a magneticseparatorforremovingunwantedortrampiron andsteel,orforquicklyseparatingferrousfromnon-ferrousscrapmetals.Successfuloperationdependsonuni-formityofthefeedthickness,andoftenaninstallation must include a suitable conveyor/feeder. 20.2.6.1Types Magnetic pulleyThis form of separator (Figure 20.9) com-prisesanumberofcircularcoresandpoles,themagnetic axisbeingthatoftheshaft.Coilsencirclethecores,with d.c.(orrectifieda.c.)excitation,andsetupamagnetic field pattern. Iron attracted to the pulley surface is removed by aid of the conveyor belt, the material being drawn away fromthemagneticfieldregion.Whenthebeltspeedor width, or the thickness of the feed, is unsuitable for a single pulley,twomaybeused,oneateachendofashortauxi-liary belt that receives its feed from the main conveyor. DrumThishasanadvantageoverthepulleytypein respect of its more effective separation. A drum can operate in conjunction with a belt conveyor if placed below the head pulleyandasuitableguide.Feedisreadilyarrangeddown a chute or directly on to the feeder tray, if one is provided. Acommontypeoffeederhasthetrayoscillatedbyan eccentricmotion,orvibratedinastraight-linemotion,at about 15 Hz. SuspensionAstructureresemblingaliftingmagnetis suspended over a conveyor belt. It operates successfully on feedscontainingawkwardshapesoftrampironatabelt speed up to 2.5 m/s. The magnet will not automatically dis-charge its load, but the large gap can contain a considerable load. The power rating is large. DiscMostmachinesutiliserotatingdiscswithasharpor serrated periphery, set above the conveyor belt and over the magnet. Separation of iron depends on the change of polar-ity of a given region of the disc as it rotates, so that ferrous particles can be released. Induction rollA powerful magnet (Figure 20.10) is providedwithareturnpathplate.Rollerssetbetweenthemare Figure 20.10Induction roll separator //integras/b&h/eer/Final_06-09-02/eerc02020/8Motors and actuatorsmagnetised by induction. Material is fed into thetop. Non-magneticpiecesfallthroughundergravity,whileferrous materialadherestotherollerandiscarriedroundand detached. Up to eight rollers in tandem may be used. WetherillThe Wetherill separator has a single magnet unit mounted either side of a conveyor belt on which the mater-ialtobetreatedispassedbeneaththeuppermagnetpole (Figure20.11).Anotherbeltisarrangedovertheupper pole of each magnet to take off the extracted ferrous mater-ial. The success of the separator depends on the shape of the magnetpoles:thelowerisflatandtheupperisarranged with a ridge to concentrate the field. As the material passes under the magnets, each ferrous particle jumps towards the upper pole and is intercepted by the take-off belt, which in turn carries it to the side where it is discharged in a continu-ousoperation.Inpractice,severalmagnetsareused;the number of products that can be separated in a single oper-ationisdeterminedbythenumberoftake-offbelts,of which there are two per magnet unit. 20.2.6.2Ore separation For dealing with material in large lumps the magnetic field musthaveadeeppenetration.Thisinvolveswideningout thepoles.Thefluxdensityisinevitablyweakened.Thus, whilefeeddepthsof250 mmareusualwithadrumof1 m diameterforthe removaloftrampiron,the depthmustbe cutto,say,75 mmwhenfeeblymagneticmaterialisoper-ated on. Animportantbranchofseparationdealswiththesub-division and concentration of ores, the constituents of which have permeabilities very much lower than that of iron. Data onthispointaregiveninTable20.2.Theprocessmaybe performed in several ways. A single product can be removed fromthebulk;severalconstituentsmayberemoved,each separately,inasingleoperation;ortheseparationmaybe carried out by a wet process. Thegeneraldesignforfeeblymagneticmaterialsdiffers fromthatfortheremovaloftrampiron,essentiallyinthe necessary flux density. A material with a permeability of 1% ofthatofironmayrequireagapdensityexceeding1.6 T, and the field must be divergent. For this purpose the lower pole over which the materialpassesis made flat;the upper pole,whetherfixedormoving,isprovidedwithacon-centratingV-edgesothatparticlestraveltoitoutofthe general bulk of the material treated. 20.2.7Clutches Theconventionalclutchconsistsessentiallyoftwomem-bers:thefieldmember,whichcarriestheexcitingwinding, andthearmaturemember,consistingvirtuallyofasteel ring which becomes attracted to the field member when the windingisenergised.Theengagingsurfacesofthesemem-bershaveafrictionliningfortakinguptheloadwhenthe clutchengages,andmeansareprovidedforspringdisen-gagement of the armature when the winding is de-energised. As the clutch rotates in operation, it is necessary to employ slip-rings and brushes for the current supply. Aspecialtypeofclutchwithadoublefrictionliningis showninsectioninFigure20.12.Inthiscasethefieldand armaturemembersrotatetogetheronthesameshaft.The other shaft carries on a spring plate the lining carrier member. Thetwofrictionsurfacesonthismemberengagebetween thearmatureandfieldmemberswhenthefieldcoilis energised. General particulars for representative sizes of this type of clutch are given in Table 20.3. 20.2.8Couplings Eddy-currentcouplingsresembleinductionmotorsinthat they develop torque by `slip', and the throughput efficiency fallswithdecreaseofspeed.Inselectingacouplingthe criticalfactorsarethespeedrangeandtheloadtorque variation therein. TheessentialfeaturesareshowninFigure20.13.The outermember(thelossdrum)ismountedontheshaft extensionofthedrivemotor,andtheinnermember(the pole system) on the driven shaft. Operation depends on the inductionofcurrentinthelossdrumbye.m.f.sresulting fromthespeeddifferencebetweenthedrivinganddriven shafts. The two types illustrated are: (1) Interdigitate. This is common for drives transferring up toabout100 kW.Thelossdrumisofplainferromag-neticmaterialoflowresistivity,normallywithforced cooling. The `claw'-shaped pole structure gives a multi-polarfieldbymeansofasingleexcitingcoil.Thereis substantial interpolar leakage flux. (2) Inductor. The toothed rotor produces an alternating flux density pattern in the loss drum by the modulation of the airgappermeance.Anannularexcitingcoil,fixedor rotary, causes the two air gaps to have opposite polarity, Figure 20.11Wetherill separator Figure 20.12Electromagnetic clutch with double friction lining //integras/b&h/eer/Final_06-09-02/eerc020Electromagnetic devices20/9 Table20.2Relativeattractionforce(iron ==100)ofvariousmaterialsApatite Argentite Biotite Bornite Cerium Chromium Corundum 0.2 0.3 3.2 0.2 15.4 3.1 0.8 Dolomite Fluorite Franklinite Garnet Haematite Ilmenite Limonite 0.2 0.1 35.4 0.4 1.3 24.7 0.8 Lithium Magnesium Magnetite Manganese Molybdenite Palladium Pyrrhoite 0.5 0.8 40.2 8.9 0.3 5.2 6.7 Quartz Rutile Siderite Strontium Titanium Tungsten Zircon 0.4 0.4 1.8 3.4 1.2 0.3 1.0 Table20.3ClutcheswithdoublefrictionliningsOverallMax.powerMax.torqueMax.speedKineticenergyMassCurrentat diameterper100(kN-m)(rev/min)at100 rev/min(kg)240 V (m)rev/min(kJ)(A) (kW) 0.633312000.53000.9 0.89599001.85201.4 1.02002070059602.1 1.2360366001113002.6 1.5670674803122003.2 1.812501254006734004.2 2.1160016025012547004.7 thefluxbetweenthemcompletingitspaththroughthe loss drum. 20.2.9Brakes Thethreebasicformsofbrakeare:(i)solenoid-operated, (ii)tractive,and(iii)athruster(electrohydraulic).Ineach caseabrake-bandor(morecommonly)abrake-shoeis pressedagainstthebrake-drum,eitherbyweightsorby springs operating through a lever. The use of springs is pre-ferable,especiallywithlargebrakes,asthecushioningof theshockduetoafallingweightintroducesadditional problems of design as well as limiting the positions in which thebrakemaybemounted.Thebrakeisreleasedbythe operating force acting against the force due to the resetting spring.Thebrakeisheldintheoffpositionforaslongas the controlling circuit is energised. Thepressureusedonthefrictionsurfacesandthecoeffi-cientoffrictionareofthesameorderasforclutches.The pressureemployedshouldbesuchastogiveareasonable rate of wear, and the figure chosen will determine the width ofshoerequiredforagivenoperatingforceandwheeldia-meter.Ingeneralpracticetherearetwobrake-shoes,each embracing about one-quarter of the wheel circumference. 20.2.9.1Solenoid brake Thebrakeisheld`off 'byasolenoid/plungerdeviceacting throughleverageagainstspringloading,thelatterbeing adjustabletosuitthebrake-torquerequirements.Ifthe brakeisenergisedonlyforshortperiods,withintervening periodsofrest(withthebrakeon),itisusuallypossibleto fit a coil giving more ampere-turns than are obtainable with acontinuousratingandthustouseagreaterresetting spring pressure, giving increased braking torque. 20.2.9.2Tractive brake TheexampleinFigure20.14embodiesatractiveelectro-magnet operating on inner and outer disc armatures A when the magnetising coil B is energised. The mechanical features are the adjusting wedge C, the brake-shoes D, the adjusting nutsE forthe outershoe-leverF,thetorquespringGand its adjuster H, the tie-rod J, the terminals K, and the shoe-clamping screws L. Armatures AA rest in slots in the base and tend to remain against the slot abutments as a result of spring pressure and magnetic force. The powerful mainspring forces the armatures AA apart, causing the inner to apply pressure totheright-handshoeandtheoutertotheleft-handshoe through the tie-rod J. When coil B is energised, the armatures AA mutually attract, so releasing the brake. 20.2.9.3Thruster brake Thethrusterbrakeemploysahydraulicthrustercylinder, with apiston acting underthefluidpressure producedby a smallmotor-drivenpumpunit.Thepowerconsumptionis relatively low, but there is a short time-lag in brake response. 20.2.10Magnetic chucks In cases where awkwardlyshaped ferrous-metalparts have tobe machinedinany quantity,theelectromagneticchuck formsavaluableauxiliarytovariouskindsofmachine tools. The chuck contains a number of distributed windings which when energised from a d.c. source produce a concen-trated and uniform field at the surface of the chuck, which is ground flat so as to form a suitable base-plate for accurate machining operations. The magnetic pull on ferrous mater-ials in contact with the chuck surface is sufficient to prevent movement under all normal machining stresses. When the current is switched off, the residual magnetism isinsomecasessufficienttopreventeasyremovalofthe part.Theusualformofcontrolswitchaccordinglyhasa demagnetising position. The principle can be applied to rotating chucks, in which caseslip-ringsarenecessarytoconveyexcitingcurrentto the windings. In some cases permanent-magnet chucks can be employed. Hold and release of the workpiece are effected by an operating //integras/b&h/eer/Final_06-09-02/eerc02020/10Motors and actuatorsFigure 20.13Eddy-current couplings lever which, in the off position, closes the flux paths of the magnets through high-permeability bridges and reduces the fluxthroughthework.Witheitherelectro-orpermanent-magnet forms, the workpiece may have to be demagnetised after machining. 20.2.11Vibrators Avibratorgeneratordevelopsavibro-motiveforceof adjustablemagnitudeandfrequencyforthenoise,fatigue and vibration testing of small structures and for the assess-ment of mechanical resonance. Figure 20.14Tractive electromagnetic brake 20.2.11.1Electrodynamic vibrator Figure20.15(a)showstheessentialfeaturesofanelectro-dynamic vibrator, which are those of a powerful loudspeaker mechanism in which a circular coil, carrying an alternating current and lying in a constant radial magnetic field, devel-opsvibratoryforceanddisplacementofcorresponding frequency. A construction of the form shown can be adapted todeveloptorsionalvibrationbypivotingthearmature centrally. 20.2.11.2Magnetostrictive vibrator Themagnetostrictioneffectcanbeemployedbyplacing thea.c.excitingcoilaroundastackofmagnetostrictive material(Figure20.15(b)).Mechanicalamplificationofthe very small displacement is provided by a truncated drive rod. Vibrators of this kind are generally fixed-frequency devices, but they are suitable for relatively high frequencies only. Single-frequency low-power vibrators can be constructed withpiezo-electricdrive.Aslargecrystalsarenotreadily available,thesevibratorsareusableonlyintheultrasonic frequency range. 20.2.12Relays and contactors Relaysandcontactors,a.c.ord.c.excited,arewidely employedforlow- andhigh-powerswitching.Thebasic features are shown in Figure 20.16. 20.2.12.1Contactors Theterm`contactor'appliestopower-controldevices.For d.c.operationthecontactorismadesingle-ordouble-pole asrequired.Whenthecoilisenergised,amagneticfieldis established across the air gap and the armature is attracted to the pole to close the contacts. The moving contact has a flexibleconductorattachedtoitinordertoavoidpassing currentthroughthehinge.Thedestructiveeffectsofd.c. arcsaresuchastomakenecessaryanarcshieldandmag-neticblow-outarrangement.Theblow-outwindingcarries the main current and its connection is so arranged that the arcisexpelledfromthecontactregionwhenthecontacts separate. //integras/b&h/eer/Final_06-09-02/eerc020Figure 20.15Vibrators Figure 20.16Elements of a contactor Fora.c.servicethecontactornormallyhastwoorthree poles. The magnetic circuit is laminated and the pole-face has a shading coil to reduce `chatter'. Blow-out coils may not be providedbecausetheprincipleoperateslesseffectivelyon a.c.;reliancemaybe placedon extinction atacurrentzero. A typical a.c. contactor is illustrated in Figure 20.17. Electromagnetic devices20/11 RatingsThesehavebeenstandardised.Theseverityof operatingconditionsvariesconsiderablyaccordingtothe classofservice.Althoughthecleaningactiononthecon-tactsduetofrequentoperationisdesirableinremoving cumulativehigh-resistancefilmswhichtendtoincrease heating,thisclassofservicecausesgreatercontactwear and erosion for a given loading than would occur with less frequentoperation.Conversely,veryinfrequentoperation which involves the contacts carrying current for long periods is not onerous from the viewpoint of wear and erosion but isconducivetotheformationofhigh-resistancesurface films unless a suitably low temperatureis maintained so as to limit the formation of the films. The permissible tempera-ture rise for different types of contact is given in Table 20.4. Operationmustbesatisfactorywiththeshuntwindingsat final rated temperature and with reduced operating voltage (80% of normal for d.c., 85% for a.c.). Table20.4TemperaturelimitsforcontactsTypeofcontactTemperaturerise (C) Solidcopperinair Standardrating65 Uninterruptedrating45 Solidcopperinoil45 Laminatedcopperinairorinoil40 Solidsilverorsilverfacedinair80 Carbon100 20.2.12.2Relays The electromagnetic relay operates one or more sets of con-tactsbytheattractionofamovablearmaturetowardsa magnetisedcore.TherepresentativetypesshowninFigure 20.18are:(a)the`telephone'typewithpivotedarmature; (b)the`commercial'versionof(a);(c)amercuryswitch with hinged armature; and (d) a spring-suspended armature. An important feature is the operating time. High-speed operation may be obtained by one or more of the following methods: (i) lamination of the magnetic circuit to minimise eddy-current delay; (ii) reduction of the mass of moving parts; (iii) use of a large coil power; or (iv) reduction of coil inductance. Low-speedoperation,sometimesneededtointroducea time-lag,isobtainedby:(i)useofalag(orslugging)coil comprisinganadditionalandseparateshort-circuitedloop orwinding;(ii)useofaseriesinductororshuntcapacitor; or (iii) addition of an external time-delay relay. DesignfeaturesContactsetsmaybenormallyopenor normallyclosed, and both types may be fittedon the same relaymechanism.Thearrangementisdeterminedbythe operating sequence required: i.e. make, break, change-over, make-before-break,break-before-make.Thecontactsize and materialmust bechoseninaccordancewiththe rating and electrical characteristics of the circuits controlled. Ideally,thecontactsshouldoperatecleanlyandwith nobounce.Theyshouldbeofadequatesizeandofthe mostsuitablematerial.Inextremelylow-voltagecircuits the contact resistance is usually an important consideration and special precautions may also have to be taken to ensure reliable operation under conditions of vibration or shock. Similarly,incasesofhigh-currentswitchingitmaybe necessary to ensure wide separation of the contacts or even to //integras/b&h/eer/Final_06-09-02/eerc02020/12Motors and actuatorsFigure 20.18Electromagnetic relays arrange for several gaps to operate in series. In some cases it may be necessary to use arc-suppressing circuits. Thenumberandtypeofthecontactsandspringsdeter-mines the switching operation to be performed by the relay; this factor also determines the work to be done by the mag-neticcircuit.Itfollows,therefore, thatthe choiceofa suit-able coil and iron circuit design is determined by the contact arrangement of any particular relay. Various configurations ofmagneticcircuitsandmaterialsareusedintherelays underreview,dependingupontheirparticularapplication. Forexample,inthehigh-sensitivityrelays,wheretheair gaphastobekepttoaminimum,itisnecessarytouse materialshavingaverylowresidualmagnetismandhigh permeability. The power required to operate the relay is determined by thespring-setarrangementandthemagneticcircuit.The methodofconstructionisimportant,sinceitlargelydeter-minesthesafeoperatingtemperatureofthewindingand this, in turn, governs the coil power and the maximum pull available at the armature. By increasing the area of the flux pathwhilemaintainingtheampere-turnsandcoilpower constant, the total air gap flux, and therefore the armature pull, can be increased and the increased coil area will permit cooleroperationofthecoil.Thismay,however,leadtoa relay that is physically larger than can be tolerated. In prac-tice,therefore,itismorereasonabletobuildarelayofa Figure 20.17Triple-pole a.c. contactor given size and to use other means to amplify the controlling power. The continuous power input to a given relay coil is limited onlybythemaximumtemperaturethatthecoilinsulation canwithstandwithoutbreakdown.Thistemperatureis governed by the environmental conditions as well as by the coil construction and the quality of the insulating material. Many of the functions performed by the electromagnetic relay have been taken over by solid-state switching. 20.2.13Miniature circuit-breakers Theminiaturecircuit-breaker(m.c.b.)is,forthecontrolof small motors and domestic subcircuits, considered primarily as an alternative to the fused switch. The appropriate British StandardisBS3871,whichlaysdownspecifictechnical requirements.Theusualformofthem.c.b.embodiestotal enclosure in a moulded insulating material. As the operating mechanism must be fitted with an automatic release independ-ent of the closing mechanism, the m.c.b. is such that the user cannotaltertheovercurrentsettingnorclosethebreaker underfaultconditions.Atthesametimethem.c.b.must tolerateharmlesstransientoverloadswhileclearingshort circuits. For most practical conditions, a change-over from time-delay switching to `instantaneous' tripping at currents exceeding 610 times full-load rating is suitable. 20.2.13.1Tripping mechanisms Methods of achieving the required operating characteristics can be classified as (i) thermomagnetic, (ii) assisted thermal and(iii)magnetohydraulic.Inthethermomagneticmethod the time-delay is provided by a bimetal element, and the fast tripbyaseparatemagneticallyoperatedmechanismbased on a trip coil. In the assisted thermal method the bimetal is itselfsubjectedtomagneticforce.Themagnetohydraulic mechanismincorporatesasealeddashpotwithafluidand aspringrestraint,thedashpotplungerbeingofironand subjecttothemagneticpullofthetripcoil.Theessential features are illustrated in Figure 20.19. ThermomagneticThebimetalelementshowninFigure 20.19(a)maycarrythelinecurrentor,forlowcurrentrat-ings,beindependentlyheated.Itsflexureoperatesthetrip latchthroughacrank.Onovercurrentthemagneticforce actsdirectlyonthelatchbar,withorwithouttheaidof the bimetal deflection. //integras/b&h/eer/Final_06-09-02/eerc020Figure 20.19M.c.b. trip mechanisms AssistedthermalThe time-delay characteristic is provided byabimetalelement,andinstantaneoustrippingbymag-neticdeflectionofthebimetal.Theoperationisshownin Figure20.19(b).Abarofmagneticmaterialisplacedclose to the bimetal element, and the magnetic field set up by the current develops a pull on the bimetal such as to increase its deflectionandreleasethetriplatch.Themagneticeffectis proportionaltothesquareofthecurrentandsobecomes significantonovercurrent.However,asthepositionofthe bimetal element on the occurrence of a short circuit is arbi-trary,thereisnowell-definedchange-overpointatwhich instantaneous tripping occurs. The method is cheap and simple, but is difficult to design forlow-current(e.g.5 A)breakersbecausetheoperation tends to be sluggish, particularly at fault-current levels that are less than 500 A. MagnetohydraulicThis method, shown in Figure 20.19(c), combines in one composite magnetic system a spring-loaded dashpot with magnetic slug in a silicone fluid, and a normal magnetic trip. When the line current flows, the magnetic field produced by the trip coil moves the slug against the spring towards the fixed pole-piece, so reducing the reluctance of the flux path and increasing themagnetic pull on the trip lever. Ifitreachestheendofthedashpot,thepullissufficientto operatethisleverandtripthecircuit-breaker.Onsudden overcurrentexceeding610timesfull-loadvalue,thereis sufficient pull at the fixed pole-piece to attract the armature ofthetripleverregardlessofthepositionoftheslug inthe dashpot. Thecharacteristic is more definite andsatisfactory Electromagnetic devices20/13 forlow-currentratingsthanthatoftheassistedthermal mechanism. 20.2.13.2Operating features Thermal operation by bimetal elements implies that the effect-ive current rating is a function of the ambient temperature. Itisthepractice,ifcompleteambientcompensationisnot fitted, to rate m.c.b.s in such a way as to allow for the type of enclosure. With magnetohydraulic devices the tripping is independentoftheambienttemperatureoveraspecified range, the small variations due to change of viscosity of the dampingfluidbeingminimisedbyuseofafluidwitha nearly flat viscositytemperature characteristic. Thecombinationofthermalandmagneticfunctionsis noteasilycontrolledforlowcurrentratings,andfor m.c.b.swithsuchratingsthetolerancesonoperationmust be wider than they are for larger currents. Normally,m.c.b.saresuitableonlyfora.c.circuits.As withalla.c.switchgear,theproblemsofbreakingefficacy are associated not only with the actual short-circuit current but also with its asymmetry and power factor. Asm.c.b.scanbelinkedtogivetwo- andthree-pole versions,soarrangedthat afault ononepole willproduce completecircuitisolation,theriskofsingle-phasingin motorcontroliseffectivelyeliminated.Inotherdirections, however,m.c.b.scannotnecessarilyreplacefuses:theydo notpossessthehighshort-circuitbreakingcapacityofthe modernh.r.c.fuse,nordotheyhaveitsinherentfault-energylimitation.If,therefore,conditionsaresuchthat back-upprotectionhastobeprovidedform.c.b.s,the `take-over' zone should be of the order of 1.01.3 kA. 20.2.14Particle accelerators Modernacceleratorsproducehigh-energybeamsofelec-trons,ions,X-rays,neutronsormesonsfornuclear research,X-ray therapy,electronirradiation and industrial radiography. If a particle of charge e is accelerated between electrodes of p.d. V it acquires a kinetic energy eV electron-volts(1 MeV ==1.6 =1013 J).Acceleratorsareclassifiedas direct,inwhichthefullacceleratingvoltageisapplied betweenthetwoelectrodes;indirect,inwhichtheparticles travelincircularorbitsandcyclicallytraversearegionof electric or magnetic field, gaining energy in each revolution; andlinear,inwhichtheparticlestravelalongastraight path, arriving in correct phase at gaps in the structure having high-frequencyexcitation,ormoveinstepwithatravelling electromagnetic wave. 20.2.14.1Direct accelerators TheCockcroftWaltonmultipliercircuithastwobanksof series capacitors, alternately connected by rectifiers acting as change-over switches according to the output polarity of the energisingtransformer.Theupperlimitofenergy,about 2 MeV, is set by insulation. A typical target current is 100 mA. TheVandeGraaffelectrostaticgeneratoriscapableof generating a direct potential of up to about 8 MV of either polarity. It has an endless insulating belt on to which charge is sprayedfrom`spray-set'needle-pointsatabout50 kV.The charge is carried upwards to the interior of the high-voltage (h.v.) electrode, a metal sphere, to which it is transferred by means of a second spray set. H.v. insulation difficulties are overcome by operating the equipment in a tank filled with a high-pressure gas, e.g. nitrogenfreon mixture at 1500 kN/m2. Intwo-stageVandeGraaffgeneratorsforhigherenergies, negative hydrogen ions are accelerated from earth potential to //integras/b&h/eer/Final_06-09-02/eerc02020/14Motors and actuators6 MeV; they are then fired into a thin beryllium foil `stripper' whichremovestheelectronsfromtheoutershellsofthe atomandleavestheremanentionsmovingonwithlittle changeofenergybutwithapositivecharge.Thesecond stage brings these ions back to earth potential and the total energygainis12 MeV.Tobringionsontoasmalltarget the accelerating and deflecting fields must be accurately con-trolled,andscatteringlimitedbyevacuatingtheaccelerator tubes to very low pressure. The energies are sufficient for the study of nuclear reactions with the heaviest elements. 20.2.14.2Indirect (orbital) accelerators Indirect(orbital)acceleratorsmayhaveorbitsofapproxi-matelyconstantradiuswithachangingmagneticfield (betatrons and synchrotrons) or orbits consisting of a series ofarcsofcirclesofdiscreteandincreasingradiiinacon-stant magnetic field (cyclotrons and microtrons). BetatronThe betatron is unique in that the magnetic field not only directs particles into circular orbits but also accel-eratesthem.Themagnethasanalternatingfieldofwhich only one quarter-period is used. Electrons are accelerated in anevacuatedtoroidalchamberbetweenthepolesofthe magnet.Theyareinjectedatanenergycorrespondingtoa lowmagneticfield,whichbendsthemincircularorbits roundthetoroid.Across-sectionofthepolesandvacuum chamberisshowninFigure20.20.Asthemagneticflux through an electron orbit increases during the cycle of alter-nation,theelectronexperiencesatangentialforce,andits gaininenergyperrevolutionisthevoltagethatwouldbe induced in a loop of wire in the orbit. As the electron gains energy,themagneticguidefieldintensityattheorbit increasesatasuitablerate.Tokeeptheelectrononacon-stant radius from injection to peak energy requires the rate of change of intensity at the orbit to be half that of the mean fluxperunitareawithin the orbit.Atpeak energy(or ear-lier)theelectronsarecausedtomoveawayfromtheir equilibriumorbitandtostrikeatargetinsidethevacuum chamber,producingX-raysorcorrespondingenergy.The output consists of short pulses of radiation whose repetition rateisthefrequencyofthemagnetexcitation.Energy limitationsaresetbythesizeandcostofthemagnetand theradiationlosswhenahigh-energyelectronhascircular motion. SynchrotronThe synchrotron uses an annular magnetic guide fieldwhichincreasesastheparticlesgainenergy,asinthe betatron,sothattheymaintainaconstantorbitradius. Electronsareinitiallyacceleratedbytheactionofcentral `betatron bars' which saturate when the main magnetic field corresponds to an energy of 23 MeV when electrons travel at a velocity only 12% less than the velocity of light. Further gain of energy is produced by radio-frequency (r.f.) power at the frequency of orbital rotation (or a multiple of it) that is fed to resonators inside the vacuum chamber. The particles becomebunchedintheirorbitssothattheypassacross the accelerating gap in the resonator at the correct phase of the r.f. field. The limitation on electron acceleration is now mainly set by radiation losses due to circular motion. Protons are injectedat about 500 keV,which producesa velocityofonly3%ofthatoflight.Furtheracceleration changesthefrequencyoforbitalrotation.Foraproton synchrotronthemagneticguidefieldstrengthandther.f. powerfrequencyhavetobevariedaccuratelyoverlarge ranges. CyclotronThisearlyformofacceleratorconsistsofa vacuum chamberbetween the poles of a fixed-fieldmagnet containingtwohollowD-shapedelectrodeswhichload the endofaquarter-waveresonantlinesothatavoltageof frequency1020 MHzappearsacrosstheacceleratinggap betweenthe`D's.Positiveionsorprotonsareintroduced atthecentreaxisofthemagnetandareacceleratedtwice per rotation as they spiral out from the centre. The relation between particle mass m,charge e, magneticfluxdensity B andfrequencyfisf ==Be/2m.Energylimitationissetby therelativisticincreaseofmass,whichlimitsthespeedof high-energy particles so that their phase retards with respect to the r.f. field. SynchrocyclotronIn this device the energy limitation of the cyclotroncanberemovedbymodulatingtheoscillatorfre-quency to a lower value as a bunch of particles gains energy. MicrotronInthemicrotron,orelectroncyclotron,elec-tronsareacceleratedinavacuumchamberbetweenthe polesofafixed-fieldmagnet.Theorbitsconsistofaseries of discrete circular arcs which have a common tangent at a resonantcavityinwhichtheelectronsgaintheirsuccessive increasesofenergyfromanr.f.electricfield.Thehighest energyachievedwithsuchamachineis6 MeV,andmean currents are less than 1 mA. 20.2.14.3Linear accelerators Indirect accelerators of protons have so far used a resonant cavity in which drift-tube electrodes are introduced that dis-tort the fields and enable particles to be shielded from field reversals.Particlesareacceleratedbetweengapsandmove betweencentresofsuccessivegapsinonecompleteperiod of oscillation(Figure 20.21).Oscillators operating atabout 200 MHzandapulsepowerof12 MWareusedtoexcite thecavityforsomehundredsofmicroseconds.Injectionis by a CockcroftWalton or Van de Graaff device. Animportantdeviceforelectronaccelerationisthe travelling-waveaccelerator,usingmegawattpulsesofr.f. power at 3000 MHz. The power is propagated along a cylin-drical waveguideloaded with a seriesof irises.Atravelling wave is set up with an axial electric-field component, and cor-rect dimensioning of the iris hole radius a and the waveguide Figure 20.20Cross-section of a 20-MeV betatronFigure 20.21Field resonant cavity for a proton accelerator //integras/b&h/eer/Final_06-09-02/eerc020Industrial rotary and linear motors20/15 radiusb(Figure20.22)enablesthepropagationvelocity and the field-intensity/power-flow relation to be varied. An electron injected along the axis with an energy of the order of 45