SISTEMAS ELÉCTRICOS DE POTENCIA

2. IET POWER AND ENERGY SERIES 56 Condition Monitoring of Rotating Electrical Machines

3. Other volumes in this series: Power circuit breaker theory and design C.H. Flurscheim (Editor) Industrial microwave heating A.C. Metaxas and R.J. Meredith Insulators for high voltages J.S.T. Looms Variable frequency AC motor drive systems D. Finney SF6 switchgear H.M. Ryan and G.R. Jones Conduction and induction heating E.J. Davies Statistical techniques for high voltage engineering W. Hauschild and W. Mosch Uninterruptible power supplies J. Platts and J.D. St Aubyn (Editors) Digital protection for power systems A.T. Johns and S.K. Salman Electricity economics and planning T.W. Berrie Vacuum switchgear A. Greenwood Electrical safety: a guide to causes and prevention of hazards J. Maxwell Adams Electricity distribution network design, 2nd edition, E. Lakervi and E.J. Holmes Artificial intelligence techniques in power systems K. Warwick, A.O. Ekwue and R. Aggarwal (Editors) Power system commissioning and maintenance practice K. Harker Engineers’ handbook of industrial microwave heating R.J. Meredith Small electric motors H. Moczala et al. AC–DC power system analysis J. Arrillaga and B.C. Smith High voltage direct current transmission, 2nd edition J. Arrillaga Flexible AC Transmission Systems (FACTS) Y-H. Song (Editor) Embedded generation N. Jenkins et al. High voltage engineering and testing, 2nd edition H.M. Ryan (Editor) Overvoltage protection of low-voltage systems, revised edition P. Hasse The lightning flash V. Cooray Control techniques drives and controls handbook W. Drury (Editor) Voltage quality in electrical power systems J. Schlabbach et al. Electrical steels for rotating machines P. Beckley The electric car: development and future of battery, hybrid and fuel-cell cars M. Westbrook Power systems electromagnetic transients simulation J. Arrillaga and N. Watson Advances in high voltage engineering M. Haddad and D. Warne Electrical operation of electrostatic precipitators K. Parker Thermal power plant simulation and control D. Flynn Economic evaluation of projects in the electricity supply industry H. Khatib Propulsion systems for hybrid vehicles J. Miller Distribution switchgear S. Stewart Protection of electricity distribution networks, 2nd edition J. Gers and E. Holmes Wood pole overhead lines B. Wareing Electric fuses, 3rd edition A. Wright and G. Newbery Wind power integration: connection and system operational aspects B. Fox et al. Short circuit currents J. Schlabbach Nuclear power J. Wood Condition assessment of high voltage insulation in power system equipment R.E. James and Q. Su Power system protection, 4 volumes Volume 1 Volume 4 Volume 7 Volume 8 Volume 10 Volume 11 Volume 13 Volume 14 Volume 15 Volume 16 Volume 18 Volume 19 Volume 21 Volume 22 Volume 24 Volume 25 Volume 26 Volume 27 Volume 29 Volume 30 Volume 31 Volume 32 Volume 33 Volume 34 Volume 35 Volume 36 Volume 37 Volume 38 Volume 39 Volume 40 Volume 41 Volume 43 Volume 44 Volume 45 Volume 46 Volume 47 Volume 48 Volume 49 Volume 50 Volume 51 Volume 52 Volume 53 Volume 905

4. Condition Monitoring of Rotating Electrical Machines Peter Tavner, Li Ran, Jim Penman and Howard Sedding The Institution of Engineering and Technology

5. Published by The Institution of Engineering and Technology, London, United Kingdom © 2008 The Institution of Engineering and Technology First published 2008 This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any form or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers at the undermentioned address: The Institution of Engineering and Technology Michael Faraday House Six Hills Way, Stevenage Herts, SG1 2AY, United Kingdom www.theiet.org While the author and the publishers believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgement when making use of them. Neither the author nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. The moral rights of the author to be identified as author of this work have been asserted by her in accordance with the Copyright, Designs and Patents Act 1988. British Library Cataloguing in Publication Data A catalogue record for this product is available from the British Library ISBN 978-0-86341-739-9 Typeset in India by Newgen Imaging Systems (P) Ltd, Chennai Printed in the UK by Athenaeum Press Ltd, Gateshead, Tyne & Wear

6. This book is dedicated to Ying Lu Considerate la vostra semenza; Fatti non foste a viver come bruti, Ma per seguir virtute e conoscenza. Consider the seed from which you were made; You were not made to live like brutes, But to pursue virtue and knowledge. Dante’s Inferno, Canto XXVI, lines 118–120, Ulysses

7. The photograph is of 3 phase, 290 kW, 6.6 kV, 60 Hz, 6-pole. 1188 rev/min squirrel cage induction motors, manufactured by ATB Laurence Scott Ltd at Norwich in the UK, driving pumps on the offshore Buzzard platform in the North Sea.

8. Contents Preface xiii Acknowledgments xvii Nomenclature xix 1 Introduction to condition monitoring 1.1 Introduction 1.2 The need for monitoring 1.3 What and when to monitor 1.4 Scope of the text 1.5 References 1 1 4 7 9 10 2 Construction, operation and failure modes of electrical machines 2.1 Introduction 2.2 Materials and temperature 2.3 Construction of electrical machines 2.3.1 General 2.3.2 Stator core and frame 2.3.3 Rotors 2.3.4 Windings 2.3.5 Enclosures 2.3.6 Connections 2.3.7 Summary 2.4 Structure of electrical machines and their types 2.5 Machine specification and failure modes 2.6 Insulation ageing mechanisms 2.6.1 General 2.6.2 Thermal ageing 2.6.3 Electrical ageing 2.6.4 Mechanical ageing 2.6.5 Environmental ageing 2.6.6 Synergism between ageing stresses 2.7 Insulation failure modes 2.7.1 General 2.7.2 Stator winding insulation 2.7.3 Stator winding faults 13 13 14 16 16 18 18 18 20 26 26 26 33 35 35 36 36 37 38 39 39 39 40 45

9. viii Condition monitoring of rotating electrical machines 2.7.4 Other failure modes 2.8.1 Stator core faults 2.8.2 Connection faults (high-voltage motors and generators) 2.8.3 Water coolant faults (all machines) 2.8.4 Bearing faults 2.8.5 Shaft voltages Conclusion References Rotor winding faults 50 54 54 2.8 54 56 56 56 59 59 2.9 2.10 3 Reliability of machines and typical failure rates 3.1 Introduction 3.2 Definition of terms 3.3 Failure sequence and effect on monitoring 3.4 Typical root causes and failure modes 3.4.1 General 3.4.2 Root causes 3.4.3 Failure modes 3.5 Reliability analysis 3.6 Machinery structure 3.7 Typical failure rates and MTBFs 3.8 Conclusion 3.9 References 61 61 61 63 65 65 65 66 66 69 71 75 76 4 Instrumentation requirements 4.1 Introduction 4.2 Temperature measurement 4.3 Vibration measurement 4.3.1 General 4.3.2 Displacement transducers 4.3.3 Velocity transducers 4.3.4 Accelerometers 4.4 Force and torque measurement 4.5 Electrical and magnetic measurement 4.6 Wear and debris measurement 4.7 Signal conditioning 4.8 Data acquisition 4.9 Conclusion 4.10 References 79 79 81 88 88 89 91 92 94 97 100 102 104 106 106 5 Signal processing requirements 5.1 Introduction 5.2 Spectral analysis 5.3 High-order spectral analysis 109 109 110 115

10. ix List of contents 5.4 5.5 Correlation analysis Signal processing for vibration 5.5.1 General 5.5.2 Cepstrum analysis 5.5.3 Time averaging and trend analysis Wavelet analysis Conclusion References 116 118 118 118 120 121 125 125 5.6 5.7 5.8 6 Temperature monitoring 6.1 Introduction 6.2 Local temperature measurement 6.3 Hot-spot measurement and thermal images 6.4 Bulk measurement 6.5 Conclusion 6.6 References 127 127 127 132 132 134 134 7 Chemical monitoring 7.1 Introduction 7.2 Insulation degradation 7.3 Factors that affect detection 7.4 Insulation degradation detection 7.4.1 Particulate detection: core monitors 7.4.2 Particulate detection: chemical analysis 7.4.3 Gas analysis off-line 7.4.4 Gas analysis on-line 7.5 Lubrication oil and bearing degradation 7.6 Oil degradation detection 7.7 Wear debris detection 7.7.1 General 7.7.2 Ferromagnetic techniques 7.7.3 Other wear debris detection techniques 7.8 Conclusion 7.9 References 137 137 137 138 142 142 146 148 149 152 153 153 153 154 155 157 157 8 Vibration monitoring 8.1 Introduction 8.2 Stator core response 8.2.1 8.2.2 8.2.3 8.3 Stator end-winding response 8.4 Rotor response 8.4.1 Transverse response 8.4.2 Torsional response 159 159 159 159 161 164 167 168 168 171 General Calculation of natural modes Stator electromagnetic force wave

11. x Condition monitoring of rotating electrical machines 8.5 Bearing response 8.5.1 8.5.2 8.5.3 Monitoring techniques 8.6.1 Overall level monitoring 8.6.2 Frequency spectrum monitoring 8.6.3 Faults detectable from the stator force wave 8.6.4 Torsional oscillation monitoring 8.6.5 Shock pulse monitoring Conclusion References 173 173 173 175 176 177 179 182 183 187 189 189 General Rolling element bearings Sleeve bearings 8.6 8.7 8.8 9 Electrical techniques: current, flux and power monitoring 9.1 Introduction 9.2 Generator and motor stator faults 9.2.1 Generator stator winding fault detection 9.2.2 Stator current monitoring for stator faults 9.2.3 Brushgear fault detection 9.2.4 Rotor-mounted search coils 9.3 Generator rotor faults 9.3.1 General 9.3.2 Earth leakage faults on-line 9.3.3 Turn-to-turn faults on-line 9.3.4 Turn-to-turn and earth leakage faults off-line 9.4 Motor rotor faults 9.4.1 General 9.4.2 Airgap search coils 9.4.3 Stator current monitoring for rotor faults 9.4.4 Rotor current monitoring 9.5 Generator and motor comprehensive methods 9.5.1 General 9.5.2 Shaft flux 9.5.3 Stator current 9.5.4 Power 9.5.5 Shaft voltage or current 9.5.6 Mechanical and electrical interaction 9.6 Effects of variable speed operation 9.7 Conclusion 9.8 References 193 193 193 193 193 194 194 194 194 195 196 204 207 207 207 207 210 212 212 213 217 217 219 221 221 224 224 10 Electrical techniques: discharge monitoring 10.1 Introduction 10.2 Background to discharge detection 10.3 Early discharge detection methods 229 229 229 231

12. xi List of contents 10.3.1 10.3.2 10.3.3 10.3.4 10.3.5 Detection problems Modern discharge detection methods Conclusion References RF coupling method Earth loop transient method Capacitive coupling method Wideband RF method Insulation remanent life 231 233 235 236 236 238 239 241 241 10.4 10.5 10.6 10.7 11 Application of artificial intelligence techniques 11.1 Introduction 11.2 Expert systems 11.3 Fuzzy logic 11.4 Artificial neural networks 11.4.1 General 11.4.2 Supervised learning 11.4.3 Unsupervised learning 11.5 Conclusion 11.6 References 245 245 246 250 253 253 254 256 260 261 12 Condition-based maintenance and asset management 12.1 Introduction 12.2 Condition-based maintenance 12.3 Life-cycle costing 12.4 Asset management 12.5 Conclusion 12.6 References 263 263 263 265 265 267 268 Appendix Failure modes and root causes in rotating electrical machines 269 Index 277

14. Preface Condition monitoring of engineering plant has increased in importance as more engineering processes become automated and the manpower needed to operate and supervise plant is reduced. However, electrical machinery has traditionally been thought of as reliable and requiring little attention, except at infrequent intervals when the plant is shut down for inspection. Indeed the traditional application of fast- acting protective relays to electrical machines has rather reduced the attention that operators pay to the equipment. Rotating electrical machines, however, are at the heart of most engineering pro- cesses and as they are designed to tighter margins there is a growing need, for reliability’s sake, to monitor their behaviour and performance on-line. This book is a guide to the techniques available. The subject of condition monitoring of elec- trical machines as a whole covers a very wide field including rotating machines and transformers. To restrict the field the authors deal with rotating machines only and with techniques that can be applied when those machines are in operation, neglecting the many off-line inspection techniques. The first edition of this book, Condition Monitoring of Electrical Machines, was written by Peter Tavner and Jim Penman and published in 1987 by Research Studies Press,withtheintentionofbringingtogetheranumberofstrandsofworkactiveatthat timefrombothindustryandacademia.Inacademiatherewasagrowingconfidencein the mathematical analysis of electrical machines, in computer modelling of complex equivalent circuits and in the application of finite-element methods to predict their magneticfields. Inindustrytherewasgrowinginterestinprovidingbettermonitoring for larger electrical machines as rising maintenance costs competed with the heavy financial impact of large machine failures. The original book was primarily aimed at larger machines involved in energy production, such as turbine generators and hydro generators, boiler feed pumps, gas compressors and reactor gas circulators. This was because at that time those were the only plant items costly enough to warrant condition-monitoring attention. It also reflected the fact that one of the authors worked in the nationalised generating utility, colouring his approach to the subject. The original book showed that, in respect of condition monitoring, electrical machines are unusual when compared with most other energy conversion rotating plant. The all-embracing nature of the electromagnetic field in the energy conversion process, whichistheraisond’êtreoftheelectricalmachine, enablesoperatorstoinfer far more about their operation from their terminal conditions than is usually the case with non-electrical rotating machinery. In this earlier work the authors were inspired

15. xiv Condition monitoring of rotating electrical machines by a much earlier book by Professor Miles Walker, The Diagnosing of Trouble in Electrical Machines, first published in 1921. Our book covered the elemental aspects of electrical machine condition monitor- ingbutexposedanumberofimportantfacetsofunderstandingthathavesubsequently lead to a great deal of further work, namely the electromagnetic behaviour of electrical machines, the dynamic behaviour of electrical machines, particularly associated with the control now available with modern power electronics, the behaviour of electrical machine insulation systems. • • • Each of these facets have now matured and are a rich source of fundamental knowledge that has been related to the behaviour of machines in their operating state, especially under fault conditions. Two examples of this further work are Professor Peter Vas’, Parameter Estimation, Condition Monitoring and Diagnosis ofElectricalMachines, publishedin1996; andGregC.Stone’s, ElectricalInsulation for Rotating Machines, Design, Evaluation, Ageing, Testing and Repair, published in 2004. The economics of industry has also changed, particularly as result of the privati- sation and deregulation of the energy industry in many countries, placing far greater emphasis on the importance of reliable operation of plant and machinery, throughout the whole life cycle, regardless of its first capital cost. Finally the availability of advanced electronics and software in powerful instru- mentation, computers and digital signal processors has simplified and extended our ability to instrument and analyse machinery, not least in the important area of visu- alising the results of complex condition-monitoring analysis. As a result, condition monitoring is now being applied to a wider range of systems, from fault-tolerant drives of a few hundred watts in the aerospace industry, to machinery of several hun- dred megawatts in major capital plant. The value of the fundamental contribution to these advances by many analysts over the last 20 years cannot be underestimated and they will play a major part in the future. In this new book, Condition Monitoring of Rotating Electrical Machines, the original authors have been joined by their colleague Dr Li Ran, an expert in power electronics and control, and Dr Howard Sedding, an expert in the monitoring of electrical insulation systems. Together we have decided to build upon the earlier book, retainingthesamelimitswesetoutatthestartofthispreface, mergingourown experience with that of the important machine analysts through the years to bring the reader a thoroughly up-to-date but practicable set of techniques that reflect the work of the last 20 years. The book is aimed at professional engineers in the energy, process engineering and manufacturing industries, research workers and students. We have placed an additional limit on the book and that is to consider the machine itself rather than its control systems. While recognising the enormous growth of the application of electronic variable speed drives in industry, we do not deal with their specific problems except in passing. We acknowledge that this is important for future growth but leave this area of investigation to a future author.

16. xv Preface The examples of faults have concentrated on conventional machines rather than the emerging brushless, reluctance, permanent magnet and unusual topology machines. This is because the industry is still dominated by these conventional machines. The ‘failure mode’ information for newer designs has not yet emerged but will be based on earlier machine experience. In this edition we have omitted case studies because the range of application of condition-monitoring techniques on elec- trical machines is now so wide and complex that it is difficult to select appropriate applications from which general conclusions can be drawn. Wehaveintroduceda‘Nomenclature’sectionandextendedthereferencestocover major recent journal papers and books that have illuminated the subject, including some of the older seminal works, which still deserve scrutiny. The authors have also taken the opportunity to correct errors in the previous book, rearrange the material presentedandaddimportantinformationaboutfailuremechanisms,reliability,instru- mentation, signal processing and the management of rotating machine assets as these factors critically affect the way in which condition monitoring needs to be applied. Finally, the diagrams and photographs representing the machines, the monitoring systems and the signal processing used have been updated where appropriate. Peter Tavner Durham University, 2006

18. Acknowledgments Peter Tavner and Li Ran acknowledge the assistance they have had from Durham University in preparing this book, particularly from Barbara Gilderoy and Denise Normanforassistingintransferringthematerialfromourpreviouseditiontothisnew one and to Chris Orton and Julie Morgan-Dodds for carefully drawing many of the diagrams. Theyalsoacknowledgethehelpofstudentsandresearchassistants, includ- ing Xiang Jianping, Michael Wilkinson, Fabio Spinato and Mark Knowles, for their contributiontothebookthroughproofreading, discussionsandtheirunderstandingof the problems of machine reliability and monitoring. Peter Tavner acknowledges the help of Dr Jim Bumby at Durham University for providing advice on the frequencies of vibration, current, flux and power associated with faults. The authors acknowledge the assistance of companies who have contributed photographs and diagrams, in particular Brush Turbogenerators (Loughborough, UK; Plzeˇ n, Czech Republic; and Ridderkirk, Netherlands), Marelli Motori S.p.A (Arzignano, Italy), Dong Feng Electrical Machinery Ltd (Deyang, China) and Laurence, Scott & Electromotors Ltd (Norwich, UK). The photograph on the front cover is of 3290 kW, 6.6 kV, 60 Hz, six-pole, 1188 rev/min induction motors, manufactured by Laurence, Scott & Electromo- tors Ltd at Norwich in the UK, driving pumps on the offshore Buzzard platform in the North Sea.

20. Nomenclature Symbol A A A(t) Explanation effective cross-sectional area of a coil, m2 availability, A = MTBF/(MTBF + MTTR) availability function of a population of components as a function of time scaling factor of time in a mother wavelet transform scale parameter in a power law expression strain coefficients for the strain energy of the stator core cross-sectional area of a tooth, m2 scale parameter in a Weibull function resistance temperature coefficient, degree/ohm skew angle of a stator, degree bispectrum radial flux density in an airgap, Tesla stator or rotor side instantaneous radial flux density, Tesla time-shifting parameter in a mother wavelet transform shape parameter in a Weibull function half-angle subtended by a shorted turn, degrees volumetric concentration of a degradation product in a machine Carter factor to account for airgap slotting inverse wavelet transform cepstrum function damping constant of a support system, N/m/s damping factor for rotor vibrations rolling element diameter, m Young’s modulus of a material stored energy in an electrical system, Joules specific unbalance e = mr/M, m instantaneous induced EMF, V strain in a material MTBF of a component, θ = 1/λ hours space position in the stator field, degrees space position in the rotor field, degrees parameter from shock pulse measurement of a rolling element bearing a α αnand bn αt α αr αs B(f1,f2) B b1or b2 b β β C C C(y,v) C(t) c D d E Ee e e(t) ? θ θ1 θ2 F

21. xx Condition Monitoring of Rotating Electrical Machines Fm(θ,t) forcing function on a rotor or stator expressed in circumferential angle θ and time t, N failure mode probability density, a Weibull function forward or backward Fourier transform first critical or natural frequency of a rotor system, Hz stator or rotor side instantaneous magnetomotive force (MMF), N1I1 or N2I2, ampere-turns electrical supply side frequency = 1/T, Hz PWM switching frequency, Hz higher mthnatural frequencies of the stator core, Hz mechanical vibration frequency on the stator side, Hz the nthcomponent of an unbalanced forcing function mechanical rotational frequency = N/60, Hz strain gauge factor degree of residual unbalance as denoted by the quantity G = eω generalised power spectral function of frequency, fk, the kthharmonic complex conjugate of G(fk) stiffness function of an mthnatural frequencies of the stator core generalised periodic function of time, tn acceleration due to gravity, m/s2 airgap length, mm natural frequency function an nthsolution of the balance equation heat transfer coefficient from an insulation surface, W/m2K tooth depth, m stator or rotor side rms current, A stator or rotor side instantaneous current, A polar moment of inertia of the core cylinder, joules2 integer constant, indicates the stator MMF space harmonics, 1, 3, 5, 7… heat transfer coefficient through an insulating material, W/mK stiffness constant of a support system, N/m reflection coefficient in the recurrent surge oscillography (RSO) test integer number of commutator segments in a DC machine Hall effect constant of an electronic material integer constant, indicates eccentricity order number, which is zero for static eccentricity and a low integer value for dynamic eccentricity integer constant, indicates the circumferential modes in a vibrating stator core integer constant, indicates the lengthwise modes in a vibrating stator core stator winding factor for the nthharmonic active length of a core, m F(t) F or F−1 f0 f1or f2(t) fse fsw fm fsm fn frm G G G(fk) G∗(fk) G(m) G(tn) g g gn(z) h ht I1or I2 i1or i2(t) J k k k kr kc k/nq ke kc kl kwn L

22. xxi Nomenclature inductance of a coil, H integer number of stator time harmonics or rotor winding fault harmonics magnetic permeance instantaneous failure rate or hazard function of a component or machine, failures/component/year failure rate of a component or machine varying with time, failures/component/year mass of a rotating system, kg mass of a support system, kg integer constant equivalent unbalance mass on a shaft, kg permeability in a magnetic field integer number of stator or rotor side turns of a coil speed of a machine rotor, rev/min integer number of rotor slots integer number of stator slots number of charge carriers per unit volume in a semiconductor integer constant, 1, 2, 3, 4… integer number of rolling elements in a rolling element bearing stator side power, watt instantaneous stator side power, watt integer number of pole pairs heat flow, watt/m2 maximum partial discharge recorded in partial discharge tests using a calibrated coupler, mv electronic charge, coulomb integer phase number change in resistance resistance, ohms shock pulse meter reading reliability or survivor function of a population of components as a function of time, failures/machine/year auto-correlation function on a time function f (t) with a delay of g t cross-correlation function between time functions f (t) andh(t) with a delay of gt resistance of a device made of the metal at 0 ˚C, ohm resistance, ohm effective radius of an equivalent unbalanced mass, m mean radius of a core, m radius of airgap, m constant related to the stiffness of a winding, insulation and tooth components L ? ? λ λ(t) M Ms m m µ N1or N2 N Nr Ns n n nb P1 p1(t) p Q Qm q q ?R R R R(t) Rff(t) Rfh(t) R0 RT r rmean rairgap S

23. xxii Condition Monitoring of Rotating Electrical Machines slip of an induction machine, between 0 and 1 torque, Nm temperature, ˚C period of a wave, sec volumetric vibration kinetic energy, joules/m3 trispectrum radial thickness of a stator core annulus, m density of a material, kg/m3 electrical conductivity of a region, ohm.m radial and tangential Maxwell stress in the airgap, N/m2 time duration of an overheating incident, s residence time of an overheating product in a machine, or leakage factor, s time delay in a correlation function, s lateral displacement of a machine rotor, µm radial and peripheral displacements in a strained stator core, µm rms voltage, volts machine volume, m3 volumetric strain potential energy, joules/m3 velocity of the rotor, relative to the travelling flux wave produced by the stator, m/s volumetric rate of production of a detectable substance, m3/s background rate of production of the substance, m3/s Poisson’s ratio of a material flux, Webers contact angle with races of a rolling element bearing, degree electrical phase angle of a stator MMF wave Fs, degree angular frequency of an electrical supply, rad/s first critical or natural angular frequency of a rotor system, rad/s electrical supply side angular frequency, rad/s mechanical angular vibration frequency on the stator side, rad/s mechanical rotational angular frequency = 2pN/60, rad/s angular velocity of an eccentricity, rad/s mother wavelet function of time work function for strain energy in stator core wavelet transform weight per unit length per unit circumferential angle of a stator core cylinder, N/m weights of a core yoke, teeth, insulation and windings, respectively, kg second harmonic magnetising reactance, ohm second harmonic leakage reactance, ohm longitudinal distance from the centre of a machine, m surge impedance of a winding, ohm s T T T T? T(f1,f2,f3) τ0 ρ σ σrand σq τw τr τ u urand uθ V V V? v ˙ ν ˙ νb ν φ φ φ ψ ψ0 ψse ψsm ψrm ψecc ψ(t) W W(a,b) w wy,wt,wiandww Xm2 X12 z Z0

24. Chapter 1 Introduction tocondition monitoring 1.1 Introduction Rotating electrical machines permeateall areas of modernlifeat boththedomestic andindustriallevel.Theaveragemodernhomeinthedevelopedworldcontains20–30 electric motorsintherange0–1kW forclocks, toys, domestic appliances, aircondi- tioningorheatingsystems.Moderncarsuseelectricmotorsforwindows, windscreen wipers, starting andnow evenfor propulsioninhybridvehicles. A modernS-series Mercedes-Benz car is reported to incorporate more than 120 separate electrical machines. Themajorityofsmallerapplicationsofelectricalmachinesdonotrequiremonitor- ing, thecomponentsaresufficiently reliablethatthey canoutlivethelifeoftheparent product. However, modernsociety depends, directly orindirectly, uponmachinesof greaterrating andcomplexity inordertosupportanincreasedstandardof living. Theelectricity weusesofreely isgeneratedinpowerplantsby machineswhose ratingcanexceed1000MW andwhichhaveevolvedtoastateofgreatsophistication. These power plants are supported by fossil fuel and nuclear energy industries that involve the transport of raw materials using pumps, compressors and conveyors insophisticatedengineering processes incorporating rotating electrical machines of powers ranging from 100 kW to 100 MW. These have been joined by a growing renewableenergy industry using many of theseandnewtechniquestoextractenergy fromrenewablesourcesoftenincombinationwithtraditional sources. Thesteelusedincarswillhavebeenrolledusinglargeelectricalmachinesandatan earlierstagethefurnaceswill havebeenchargedusingmoreelectrical machines.Our waterandwastesystemsarealsodrivenby electrical machines, asaretheprocesses that produce the raw materials for the agricultural, chemical and pharmaceutical industries. Without all these our society, as it exists at the moment, would cease to function. Theoverall pictureisthatelectrical machinescomeinmany sizesandfulfil their function either independently or as part of a highly complex process in which all elements must function smoothly so that production can be maintained. It is the usageof electrical machinery inthelatterrolethathasrisendramatically towardsthe endof thetwentiethcentury, andthereisnoreasontosuspect that thistrendwill do anything other thanaccelerateinthetwenty-first century. However, historically the function of an individual electrical machine was seen as separable fromthe rest of theelectromechanical system. Itmustberememberedthatthepower-to-weightratio

25. 2 Conditionmonitoring ofrotating electrical machines of electrical machines has beenmuchlower thansteam, diesel andgas engines and consequently theirreliability hasbeenmuchhigher. It is against this background that the basic principles of protective relaying evolved. Protectionisdesignedtointerceptfaultsasthey occurandtoinitiateaction that ensures that the minimumof further damage occurs before failure. In its basic formthefunctionof theprotectiverelay isoutlinedinFigure1.1. Flag Initiate executive action Current of voltage signal Figure1.1 Thebasic functionofanelectrical protectiverelay Thesignal providedby thetransducerwill beintheformof acurrentorvoltage andwill beinterpretedby therelay asanacceptableorunacceptablelevel, according toapre-setvaluedeterminedbytherelaydesignerorthemaintenancestaff.Ifthepre- setvalueisexceededthentherelay will initiatefurtherelectromechanical actionthat will oftenresultindisconnectionoftheelectrical machine, anditwill flagthefactthat afault, orevenfailure, hasbeenidentified. Thisisasimplistic view of theprotective relay, whichwasconfiguredusing electromechanical devicessuchasrelaystocarry out their function, as the name implies. However, nowadays most protective relays usedigital processorstodeploy awiderangeof functions, andareprogrammableto allow moresophisticatedcriteriaforinitiating interruptprocedurestobeapplied; for example, to block therestart of a motor until it has cooled to anacceptabledegree. Figure1.2showsatypical modernprogrammablerelay forfulfillingsuchafunction. From what has been said earlier it is apparent that protective relaying can be regarded as a formof monitoring, and indeed it is widely used with great success. Moderndigital relayshavealsostartedtofulfil amonitoring functionsincethey can recordthevoltagesandcurrentstheymeasureforaperiodbeforeandafteranyfault.In factmanyfailureinvestigationsonelectrical machines, involvingrootcauseanalysis, start with the download and analysis of the digital protective relay data, which can usually bedisplayedclearly inanExcel spreadsheet. Virtually all electrical machine protectionsystemsembody someformof protectiondevice, andontypical machines they areusedinsomeorall of thefollowing schemes earthfaultprotection, overcurrentprotection, differential currentprotection, under- andovervoltageprotection, • • • •

26. Introductiontoconditionmonitoring 3 Figure1.2 Atypicalmoderndigitalmotorrelay.[Source: GE PowerSystems,USA] negativephasesequenceprotection, fieldfailureprotection, reversepowerprotection, overspeedprotection, excessivevibrationprotection, thermal overloadprotection. • • • • • • Thislistisrepresentativeratherthanexhaustive. It is important to stress the fact that protection is basically designed to act only once a fault has occurred and it will normally initiate some executive action. In the words of Electricity Training Association’s Power Systems Protection [1], ‘the functionof protectiveequipment is not thepreventiveoneits namewouldimply, in thatittakesactiononly afterafaulthasoccurred; itistheambulanceatthefootof the cliff ratherthanthefenceatthetop’. Conditionmonitoringneedstoestablishitself as the‘fenceattheclifftop’.Theexecutiveactionmayverywell bethedisconnectionof thepieceof machinery fromthesupply. Suchactionisacceptableif theitemof plant isreadily dissociatedfromtheprocessitisinvolvedwith, orif itexistssubstantially inisolation. If, however, thepieceof plantisvital totheoperationof aprocessthen an unscheduled shutdown of the complete process may occur. The losses involved may thenbesignificantly greaterthanthoseresulting simply fromthelossof output

27. 4 Conditionmonitoring ofrotating electrical machines during a scheduled shutdown. It must also bebornein mind that thecapital cost of anindividual machineis moreoftenthannot small comparedwiththecapital costs involved in a plant shutdown. Maintenance is most effective when it is planned to servicemanyitemsinthecourseofasingleoutage.Insummary,conditionmonitoring of anelectrical machineis not necessarily aimedsolely at themachineitself, but at thewiderhealthof theprocessof whichitispart. 1.2 Theneed for monitoring Thenotion of thescheduled shutdown or outageintroduces us logically to thecase tobemadeonbehalf of monitoring. By conditionmonitoring wemeanthecontinu- ous evaluation of the health of plant and equipment throughout its serviceable life. Condition monitoring and protection areclosely related functions. Theapproach to the implementation of each is, however, quite different. Also the advantages that accruedueto monitoring areentirely different to thoseto beexpectedfromprotec- tion. This is principally because monitoring should be designed to pre-empt faults, whereas protection is essentially retroactive. Condition monitoring can, in many cases, be extended to provide primary protection, but its real function must always betoattempttorecognisethedevelopmentof faultsatanearly stage. Suchadvanced warning is desirable since it allows maintenance staff greater freedom to schedule outages in the most convenient manner, resulting in lower down time and lower capitalisedlosses. We have said that advanced warnings of malfunction, as provided by monitor- ing, are desirable. Are they? We must justify this because the implementation of a monitoring systemcaninvolvetheoperatorinconsiderableexpense. Thereareother questionstobeansweredtoo, forexample: Once one has chosen to embark upon a programme of monitoring what form shouldittake? Should the monitoring be intermittent, regular at fixed time intervals, or continuous? If oneemploysafixedtimeinterval maintenanceprogrammethenisitnecessary tomonitoratall? Monitoringcangeneratelargequantitiesofdata; howcanthisinformationbebest usedtominimisefutureexpenditure? Finally, andperhapsmostimportantly, howmuchneedstobespentonmonitoring inordertomakeittruly effective? • • • • • Thesequestionsdonothavesimpleanswersbutwecangetsomeindicationsby considering the magnitude of the maintenance and replacement burden that indus- try is continually facing, and the implications for the costs of various maintenance strategies. Wecouldconsiderthreedifferentcoursesof action breakdownmaintenance, fixed-timeinterval orplannedmaintenance, condition-basedmaintenance. • • •

28. Introductiontoconditionmonitoring 5 Table1.1 Expenditure on plant per employee of selected industriesadaptedfromNealeReport, 1979[2] Industry Annual investment/employee inplantandmachinery, £ NorthSeaoil andgas Oil refining Electricity supply Chemical industry Ironandsteel Watersupply Textilemanufacture Instrumentationmanufacture Electrical engineering manufacture 160000 14000 8000 2400 1800 800 600 400 400 Method (1) demands no more than a ‘run it until it breaks then replace it’ strategy, whilemethod(2) mayormaynotincludeadegreeofmonitoringtoaidintheplanning of machinery outages. Thefinal scenariomethod(3) requiresadefinitecommitment tomonitoring. ThescaleofinvestmentcanbeseenfromfiguresprovidedbytheNealeReport[2], publishedin1979. Thisinformationis30yearsoldandcomesfromaperiodbeforea long periodof privatisationbutisstill invaluable. Table1.1showstheannual invest- mentperemployeeinplantandmachinery.Wehavemodifiedthesevaluesinorderto reflect morerealistically today’scostsandhaveselectedthoseindustriesthat would haveahighproportionof expenditureinelectrical machinery andancillary plant. Thesamereportshowsthattheaverageannual expenditureonmaintenancewas 80 percent of theamount annually investedinplant andmachinery. Thefigures for someselectedindustriesandindustrialgroupingsareshowninTable1.2,whichshows the annual maintenance expenditure as a percentage of the annual plant investment expenditure. This is a high figure in real terms and anything that helps to reduce it mustbewelcome. TheHewlett-PackardJournal hasquotedthestaggering figureof $200 billion as the annual maintenance bill for US business, and a growth rate of 12 per cent. Now only afractionof this sumwill bespent onmaintaining electrical machinery, butevenif itamountstoafractionof onepercentof thetotal itisstill an enormousamountof money. Therearegreatincentivestomaintainplantmoreefficiently, particularly whenit is estimatedthat approximately 70 per cent of themaintenancework carriedout by companiesthatusenoplanning atall may beclassifiedasemergency work andmust bedoneat premiumcosts. It is apparent that careful thought shouldbegivento the most appropriate formof maintenance planning. Breakdown maintenance can only be effective when there is a substantial amount of redundant capacity or spares are available, and a single breakdown does not cause the failure of a complete system.

29. 6 Conditionmonitoring ofrotating electrical machines Table1.2 Annual maintenance expenditure as a percentage of annual capital investment in plant, for selected industries.AdaptedfromtheNealeReport, 1979[2] Industry Maintenanceexpenditure/plant expenditure, % Printing Instrumentationmanufacture Mechanical engineering Textilemanufacture Watersupply Gassupply Electricity supply Electrical machinery manufacture Chemical manufacture Marineengineering Ironandsteel manufacture Coal production 160 150 100 82 80 80 80 80 78 50 42 26 The question to be answered in such circumstances is why is there a significant redundancy? Andshoulditbeallowedtocontinue? Many sectors of industry, and particularly theelectricity, water and gas utilities andtherailways, haveadoptedmaintenanceplanningbasedonreplacementandover- haul atfixedtimeperiods, sothatoutagework canbescheduled, anddiversionsand loadscanbeplanned. Suchscheduling isusually plannedonthebasisof plantmon- itoring, whichis typically doneonadiscontinuous basis. Therearemany estimates of the savings that accrue by adopting such an approach and an average reduction figureof 60percentof thetotal maintenanceburdenmay beconsideredreasonable. This is good news, but it must be treated cautiously because such a maintenance policy makesheavy demandsuponscarce, skilledmanpower. Itisalsoestimatedthat only 10 per cent of components replacedduring fixed-interval maintenanceoutages actually needtobereplacedatthattime. Theobviousimplicationisthat90 percent of whatisreplacedneednotbe. Suchconsiderations, andtherealisationthatmodernelectrical machinesandthe processes they operate in are growing in complexity, leads one to the conclusion that continuous condition monitoring of certain critical items of plant can lead to significantbenefits. Thesebenefitsaccruein greaterplantefficiency, reducedcapitalisedlossesduetobreakdown, reducedreplacementcosts. • • • The plant operator can also be updated with information on the performance of his machinery. This will help him to improve the day-to-day operational availability

30. Introductiontoconditionmonitoring 7 and efficiency of the plant. Condition monitoring should give information relevant to both the operational and maintenance functions. There is an added bonus in that bettermaintenancegivesbettersafety. In the longer term, condition monitoring also allows the operator to build up a database that can be used for trend analysis, so that further improvements can be made in the scheduling of maintenance. Such information should also be used advantageously by plant manufacturers and designers in order to further improve productreliability. Thisstepeffectively closestheloop. In view of this, how much needs to be spent on monitoring? This depends on the value of the process in which the machine works, and estimates vary, but they are never less than 1 per cent of the capital value of the plant being monitored. A moretypical (andprobably morerealistic) figurewouldbe5percentforthegeneral runof industrial processes, whilespecial requirementsforhighvalueprocesses, such as those found in the offshore oil and gas industry, may push a realistic figure to greaterthan10percent. 1.3 What and when tomonitor Nowthatwehaveexaminedsomeof theadvantagestobegainedfromacommitment toconditionmonitoringwecanbrieflyaddressthequestions,whatshouldwemonitor, andwhen? Thequestionof whattomonitorhastwoimplications. Whatmachines? Whatparameters? • • Thefirst part is moreeasily answered. In view of thecapital costs involved in pro- viding monitoring, whetherittakestheformof apermanentinstallationwithitsown local intelligence, or a handheld device used periodically by a skilled operator, it is unlikely that electrical machines with ratings of less than 20 kW would benefit. Thereare, of course, exceptionstothiswhereasmallermachinehasavital function intheperformanceof alargersystem. Itwill pay dividendstocarefully considerthe implications of losing theoutput of anindividual pieceof machinery inthecontext of acompletesystem. Larger electrical drives, which support generating, process or production plant if a high margin of spare capacity exists, will benefit from monitoring, although perhaps not continuous monitoring. One could include induced and forced-draught boiler fan drives, boiler water feed pump drives, and cooling water pump drives in power stations in this category. It must be borne in mind, however, that suc- cessful monitoring can allow a big reduction in the requirement for on-site spare capacity. Machines that have a high penalty in lost output costs need to be monitored continually. Large generators naturally fall into this category since lost output can exceed £600000 per day for a large machine in a high-efficiency power station. A similar approach would apply to large propulsion motors and large process drive motors.

31. 8 Conditionmonitoring ofrotating electrical machines Theconclusionisthattherearemachinestowhichmonitoring isreadily applica- ble, but there are other circumstances where careful assessment is needed before deciding. One must always be mindful of the scale of the maintenance burden, however, and not be driven to false economies on the basis that ‘nothing has gone wrong so far’. On the other hand one must bear in mind the complexities of the monitoringsystemitself anditsownmaintenanceburden. Nothingcanbeworsethan investing incomplex monitoring equipment, whichbecauseof poordesignormain- tenancegivesrisetolargenumbersof falsealarmsandleadstotheequipmentbeing ignored. The parameters to be monitored are essentially those that will provide the operator andmaintainer withsufficient details tomakeinformeddecisions onoper- ation and maintenance scheduling, but which ensure security of plant operation. Automatic on-line monitoring has only recently begun to make an impact in the area of electrical machines. Traditionally quantities, such as line currents and volt- ages, coolant temperatures, and bearing vibration levels, have been measured and will continue to be used. Other quantities, involving the sensing of pyrolysed products in cooling gases and oils, have recently been introduced, as have tech- niques for measuring contamination levels in bearing lubricants. Other specialist methods, involving the accurate measurement of rotational speed, or the sens- ing of leakage fluxes, are being developed in order to monitor a variety of fault conditions. As the ready availability of sophisticated electronic and microprocessor-based systems is increasingly translated into monitoring hardware, the more variables it is possible to consider, and the more comprehensive the monitoring can be. This trendwill befurtheracceleratedasthecostsof computingpowerfall still further, and thecomplexity of microprocessors increases. Suchdevelopments areessential both because of the complexity of the plant being monitored and the complexity of the monitoring signalsthemselves. The question of when to monitor is more easily answered. One should monitor whenitiscost-effectivetodoso, orwhenthereareover-riding safety considerations to be observed. The assessment of cost-effectiveness can be a relatively complex matter, butingeneral termsmonitoringisworthwhilewhenthenetannual savingsare increasedby itsuse. Thenetannual savingisthedifferencebetweenthegrossannual saving andtheannual costs. Thecostsof monitoring includetheinitial investigation, purchase, and installation charges, the staff training costs, and the costs associated withthedataacquisition. Thisexpenditurecanbewrittenoff overthelifetimeof the monitoring systemandset against thesavings accrued. Wehavealready considered these savings in some detail earlier in this chapter, and it is sufficient to say that it is not uncommon for the capital costs of a wisely chosen monitoring system to be retrievedinthefirstyearof itsoperational life. Finally it is tempting to think that, with such a degree of monitoring power becoming available, theprotectiveandmonitoring functionscouldbemerged. With thedevelopmentof morepowerful digital protectionandimprovedsupervisory con- trol anddataacquisition(SCADA) systemsthisishappening butcaremustbetaken andoperational experiencemustbeestablishedbeforethesefunctionsmerge.

32. Introductiontoconditionmonitoring 9 1.4 Scopeof thetext Sometimeagotheauthorsrecognisedtheneedtodraw togetherintoasinglesource an account of the techniques available to anyone wishing to involve themselves in themonitoring of electrical machines. The list of books on the subject of electrical machine condition monitoring is short, the first historic reference of seminal interest being Walker [3], followed by thepresent authors’ first edition[4] andthenby Vas [5]. Themost up-to-datebook, by Stoneetal., isaimedatwinding andinsulationproblems[6]. Thejournal literatureonconditionmonitoring of electrical machinesisgrowing rapidly. In fact, one author has said that it has picked up at a fervent pace and anotherhascalleditanexplosion, althoughthegrowthisnotnecessarily indirections most useful to industry. There are a number of general survey papers of condition- monitoringtechniquesformachinesof whichthemostrelevantareFinley andBurke [7], SinghandAl Kazzaz [8], HanandSong [9] andNandi etal. [10]. Raohasgivenanoverviewof conditionmonitoringinhishandbook [11], includ- ing achapteronelectrical machines, whileBarron[12] gives asuccinct mechanical engineer’sview of conditionmonitoring whichisuseful asanoverview. Itisimpor- tantinelectrical machinesmonitoringforabridgetobedevelopedbetweenelectrical andmechanical engineers. Condition monitoring is an area of technology that is extremely wide-ranging, requiring knowledgeof theconstructionandperformanceof themachinestobemonitored, theway they fail inservice, theanalysisof electrical, magnetic, vibrationandchemical signals, thedesignof microprocessor-basedinstrumentation, theprocessing of thesesignalsandtheirpresentationinacomprehensibleway. • • • • • Inabook of this lengthit is not possibleto enter into adetailedstudy of eacharea. Theartof conditionmonitoringisminimalist, toextractthecorrectinformationfrom themachinethatenablesus, withaminimumof analysis, togiveacleardetectionof anincipientfailuremode. Wehaveinsteadsetourselvestheobjectiveof covering thecompletemonitoring field as it relates to electrical machinery, in a manner that will be useful to anyone wishing tobecomefamiliarwiththesubject forthefirst time, andwill assist people actively engagedinconditionmonitoringtogainaperspectiveof newdevelopments. To restrict thefield theauthors deal withrotating machines only and withtech- niques that can be applied when those machines are in operation, neglecting the many off-lineinspectiontechniques. Wehaveaddedtwo further restrictions. While recognisingtheenormousgrowthinrecentyearsof theapplicationof electronicvari- able speed drives to industry, this edition does not deal with the specific problems of condition-monitoring electrical machines driven at variable speed. In imposing this limit we acknowledge that variable speed drives will be an important area for future growth in condition monitoring, but this technology will be founded firmly

33. 10 Conditionmonitoring ofrotating electrical machines uponthebehaviourof machinesatconstantspeed, whichthisbook will address. We havealsoconcentratedonconventional machinesratherthantheemergingbrushless, reluctance, permanent magnet and unusual topology machines. This is because the industry isstill dominatedby theseconventional machinesandthefailuremodesfor emerging designs are not yet clear, but will be based upon conventional machine experience. The text is divisible into four sections. The first section, Chapters 2 and 3, is essentially ‘adescriptionof thepatient, thethingsthatcangowrong withhim, anda general guidetothediagnosis’. Chapter 2 gives abroadguidetoelectrical machine constructionwithdescriptionsof thematerialsandspecificationlimitsuponthem. It also details examples of faults that can occur and through a number of tables starts toclassify theprincipal failuremodes androot causes. Chapter 3 thendescribes the general principlesof reliability theory anditsapplicability toelectrical machines. In particularithighlightstheimportanceoffailuremodesinpredictingfailureprobability andintroducestheideaof conditionmonitoring addressing therootcausesof failure modesthathaveaslow failuresequence. Thesecondsection, Chapters4and5, givesadetailedaccountof specific instru- mentationandsignalprocessingtechniques.Wetreatinstrumentationatthefunctional level andassumeacertainbasic knowledgeof thetechniquesof spectral analysisof signals. Thethirdsection, Chapters6–10, givesadetailedaccountof specific monitoring techniques,startingwiththermalandthenchemicaldegradationmethods,progressing ontomechanical andfinally electrical methods, considering firstterminal conditions andfinally dischargemonitoring of electrical machineinsulationsystems. Thereare areasof overlapbetweeneachof themonitoringmethods. Asfaraspossiblewehave subdivided thetechniques within each chapter into thetypes and parts of machines on which they are used. In these sections we describe current practice and discuss someof thenew developmentsnow being introduced. Thefourthand final section, Chapters 11 and 12, considers first theapplication of artificial intelligencetotheconditionmonitoring of machinesandthentheuseof conditionmonitoring onthemaintenanceplanning andassetmanagementof plant. We have tried to be mindful of the fact that, when describing developments in arelatively new subject area, acomprehensivebibliography is of theutmost impor- tance. We have in general quoted the major recent journal papers and books that haveilluminatedthesubject. Wehaveonly quotedconferencepaperswherethey are essential toidentifyaparticularlyrelevantmodernpoint.Wehavealsoincludedsome of theolderseminal works, whichstill deservescrutiny. Itisinevitablethattherewill beomissions but hopefully it will providetheinterestedreader withauseful source of additional material. 1.5 References 1. ElectricalTrainingAssociation.PowerSystemProtection,Vol.1: Principlesand Components. Stevenage: PeterPeregrinus; 1981.

34. Introductiontoconditionmonitoring 11 2. Neale N. and Associates. A Guide to the Condition Monitoring of Machinery. London: HerMajesty’sStationary Office; 1979. WalkerM. TheDiagnosing ofTroubleinElectrical Machines. London: Library Press; 1924. Tavner P.J. and Penman J. Condition Monitoring of Electrical Machines. Letchworth: ResearchStudiesPressandJohnWiley & Sons; 1987. VasP.ParameterEstimation, ConditionMonitoringandDiagnosisofElectrical Machines. Oxford: ClarendonPress; 1996. Stone G.C., Boulter E.A., Culbert I. and Dhirani H. Electrical Insulation for RotatingMachines, Design, Evaluation, Aging, Testing, andRepair.NewYork: Wiley–IEEE Press; 2004. Finley W.R. and Burke R.R. Troubleshooting motor problems. IEEE Transac- tionsonIndustry Applications1994, 30(5): 1383–97. SinghG.K.andAl KazzazS.A.S.Inductionmachinedriveconditionmonitoring anddiagnosticresearch–asurvey.ElectricPowerSystemsResearch2003,64(2): 145–58. HanY.andSongY.H.Conditionmonitoringtechniquesforelectrical equipment – aliteraturesurvey. IEEE TransactionsonPower Delivery 200318(1): 4–13. Nandi S., Toliyat H.A. and Li X. Conditionmonitoring and fault diagnosis of electrical motors – a review. IEEE Transactions on Energy Conversion 2005, 20(4): 719–29. RaoB.K.N. HandbookofConditionMonitoring. Oxford: Elsevier; 1996. BarronR. Engineering ConditionMonitoring: Practice, MethodsandApplica- tions. Harlow: Longman; 1996. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

36. Chapter 2 Construction, operation and failuremodesof electrical machines 2.1 Introduction This chapter could also be subtitled ‘the way rotating electrical machines fail in service’. Rotating electrical machines convert electrical to mechanical energy, or viceversa, andthey achievethis by magnetically coupling electrical circuits across anairgapthatpermitsrotationalfreedomofoneofthesecircuits.Mechanicalenergyis transmittedintooroutof themachineviaadrivetrainthatismechanically connected tooneof theelectric circuits. Anexampleof oneof thelargestelectromagnetic energy conversionunitsinthe world, at 1111 megavolt-amperes (MVA), is showninFigure2.1. Theconstruction Figure2.1 View of a 1111 MVA, 24 kV, 50 Hz steam turbine-driven, hydrogen- cooled, two-poleturbinegenerator installedina nuclear power station intheCzechRepublic. Thegenerator exciter is ontheleft, theturbine generator isinthecentreof thepictureandthelowpressureturbineto theright. [Source: BrushTurbogenerators]

37. 14 Conditionmonitoring ofrotating electrical machines of electrical machinesissimilar, whetherlargeorsmall, asshownlaterinthechapter andtheiroperational weaknessesaredominatedby thesameprinciples. Thepurpose of this chapter is to explain their constructional principles and the main causes of failure. Thechapterisillustratedwithalargenumberof photographstodemonstrate tothereaderthesalientfeaturesof electrical machines. 2.2 Materialsand temperature The magnetic and electric circuits essential to machines require materials of high permeability andlow resistivity, respectively, andthesearegenerally metals. Metals withgoodmagneticandelectrical propertiesdonotnecessarily havehighmechanical strength. Indeedtheatomicstructureof agoodconductorissuchthatitwill naturally have a low yield strength and high ductility. Yet the magnetic and electric circuits of the machine must bear the mechanical loads imposed upon themby the transfer of energy across the airgap. Furthermore, the magnetic and electrical circuits must be separated by insulating materials, such as films, fibres and resins, which have evenweakermechanical properties. Table2.1 setsouttheelastic moduli andtensile strengthof materialsusedinelectrical machinesandhighlightstherelativeweakness of electrical steel, conductor and insulating materials. Right from the outset then, thereisaconflictbetweentheelectrical andmechanical requirementsof thevarious partsof anelectrical machine, whichthedesignermustattempttoresolve. Table2.1 Mechanical propertiesofmaterialsusedinelectrical machines Material Elastic modulus, GPa Tensilestrength, MPa Hightensilesteel Structural steel Electrical steel Copper Aluminium Epoxy-mica-glasscomposite Mouldedorganic/inorganic resin Phenol-formaldehyderesins 210 210 220 120 70 60 1800 290–830 450 210 310 275 48 35 5 3 However, there is a further complication. The transfer of energy inevitably involves the dissipation of heat, by ohmic losses in the electric circuit and by eddy currentandhysteresislossesinthemagnetic circuit. Theperformanceof theinsulat- ingmaterialsthatkeepthesecircuitsapartishighly dependentupontemperature, and deteriorates rapidly at higher temperatures. Materials that can sustain these higher temperaturesbecomeprogressively moreexpensiveandtheirmechanical anddielec- tric propertiesareoftenworsethanlowertemperaturematerials. Table2.2 classifies

38. Construction, operationandfailuremodesofelectrical machines 15 Table2.2 Temperaturecapabilitiesofinsulating materials Class Material Temperaturerating to giveanacceptablelife underprescribed industrial conditions,◦C O orY obsolete Oleo-resinousnatural fibre materials, cotton, silk, paper, wood withoutimpregnation. Natural fibrematerials, cotton, silk, paperandwoodimpregnated, coatedorimmersedindielectric liquid, suchasoil. Synthetic-resinimpregnatedor enamelledwirenotcontaining fibrousmaterialssuchascotton, silk orpaperbutincluding phenolics, alkydsandleatheroid. Combinationsof mica, glassand paperwithnatural organic bonding, impregnating orcoating substances including shellac, bitumenand polyesterresins. Combinationsof mica, glass, film andpaperwithsynthetic inorganic bonding, impregnating orcoating substancesincluding epoxy and polyesterresins. Combinationsmica, paper, glassor asbestoswithsynthetic bonding, impregnating orcoating substances including epoxy, polymideand siliconeresins. Combinationsof asbestos, mica, glass, porcelain, quartz orother silicateswithorwithoutahigh temperaturesynthetic bonding, impregnating orcoating substance including silicone. Thesecan includehigh-temperaturearamid calendaredpaperslikeNomex. 90 105 A 120 E 130 B F 155 180 H 220 C thecommoninsulatingmaterialsusedinelectrical machinesandshowstherelatively low temperaturesatwhichthey arepermittedtooperate. Uncertaintiesaboutthetemperatureswithinamachinemeanthatthedesigneris forced to restrict the maximummeasurable operating temperature to an even lower

39. 16 Conditionmonitoring ofrotating electrical machines value than that given in Table 2.2, taken fromthe IEC standard [1], for the appro- priateinsulation, inorder toprovideasafety factor during operation. It is clear that the heat dissipated within a machine must be removed effectively if design limits are to be met. For example, in the 1111 MVA turbine generator shown in Figure 2.1 with losses of the order of 12 MW, if cooling stopped the average temperature of thegenerator body wouldexceedany of themaximumpermittedinsulationtem- peratures within12 seconds. Theproblemis exacerbatedbecausethelosses arenot evenly distributed and in practice at some locations the rise in temperature will be evenfaster thanthis. So cooling andits distributionbecomeavital part of machine design. The health of an electrical machine, its failure modes and root causes, are ultimately related to the materials of which it is made, the mechanical and elec- trical stresses those materials are subjected to and the temperatures they attain in service. InChapter1weexplainedhowelectrical machinesareprotectedby relays, which sense serious disruptions of the current flowing in the windings and operate to trip or disconnect the machine. However, when fault currents are flowing the machine has already failed as an electrical device. Electrical or mechanical failure modes arealways precededby deteriorationof oneof themechanical, electrical, magnetic, insulation or cooling components of themachine. This is thecaseregardless of the type of electrical machine. If this deterioration takes a significant period of time andcanbedetectedby measurement, thenthat root causedetectionwill beameans of monitoring the machine before a failure mode develops. The heart of condition monitoring is toderivemethods tomeasure, as directly as possible, parameters that indicaterootcausedeteriorationactivityandprovidesufficientwarningofimpending failureinorderthatthemachinemay betakenoff forrepairormay betrippedbefore seriousdamageoccurs. A degree of protection could be achieved by making the protective relays especially sensitive and providing an alarm indication before tripping occurs. Experience has shown that this is a precarious mode of condition monitoring leading to false alarms and a lack of confidence in the monitoring process. The following sections show how the construction, specification, operation and types of fault can lead to the identification of generic failure mode root causes in the machine. 2.3 Construction of electrical machines 2.3.1 General Thebasicconstructional featuresoftheelectrical machineareshowninFigure2.2(a). Therotor, whichusually has arelatively highinertia, is normally supportedontwo bearings, whichmaybemountedonseparatepedestalsorincorporatedintotheenclo- sureof themachine, asshowninFigure2.2(a). Somelarger, slower-speedmachines incorporate a single non-drive end bearing and rely on the prime mover or driven plant and its bearings for the remaining support. Rolling element bearings are used

40. Construction, operationandfailuremodesofelectrical machines 17 (a) (b) Figure2.2 (a) Medium-sizedsynchronous generator. Sectionthrougha 125 MVA, 15 kV, 60 Hz, two-pole, air-cooled, brushless excitation turbine gen- erator showing the fabricated main frame of the machine, stator core, winding,rotorandontherightthemainexciterofthemachine.(b)Large synchronousgenerator.Constructionofthestatorcoreofa500MW,two- pole, hydrogen-cooled turbine generator showing the fabricated inner frameofthemachineandthesegmentedlaminationsbeinginsertedinto thatframe. [Source: BrushTurbogenerators] onsmaller-sizemachineswhereshaftperipheral velocitiesarelow, andsleevebear- ings with hydrodynamic oil films are used for larger machines. Vertically mounted machines will incorporate a thrust bearing usually at the low end of the enclosure. This may be a relatively modest angular contact ball bearing for a small, vertically

41. 18 Conditionmonitoring ofrotating electrical machines mountedpumpmotorbutcouldbealargehydrodynamic oil filmthrustpadMichell- type bearing for a hydro-type generator where the rotor may weigh 100 tonnes or more(seeFigure2.6(b)). 2.3.2 Stator coreandframe The stators of all AC machines are constructed from lightly insulated laminations of electrical steel. As Table 2.1 shows, electrical steels are strong but the silicon, incorporated into the alloy, to raise the resistance and impart magnetic properties, weakens thematerial comparedtostructural steel, making it brittle. Furthermore, if thelaminatedstructureistohavethecohesionnecessary totransmittheloadtorque, andhavelow levels of vibrationwhencarrying themagnetic flux, it must befirmly clampedbetweencastorfabricatedend-platesthataresecuredtoacylindrical frame intowhichthecoreiskeyed.Thecoreisconstructedwithintheframeandcompressed beforetheclampingplatesareapplied.Theframestructureanditsclampingareclearly visibleinFigures 2.2(b), 2.3(a) and2.5(b). Onlarger machines theclamping plates aretightenedby largebolts(seeFigure2.3(a)), butonsmallermachinesinterlocking keys or evenwelds areusedtosecuretheclamping plates, or thecoreitself may be weldedorcleated. InaDC machinethelaminatedstatorfieldpolesareboltedtoarolled-steel yoke thathasmuchgreaterinherentstrengththanalaminatedcore(Figure2.7(a)). 2.3.3 Rotors Thedesignof therotorwill dependontheparticulartypeof machine. AC induction andDC motorshavelaminatedrotorswherethelaminationsareclampedtogetherand shrunk ontothesteel shaft(Figure2.7(a)). Turbine-typegeneratorshavelarge, solid, forged-steel rotorsthatarelongandthin(Figure2.3(b)), whilehydro-typegenerators havelarge, short, fatrotorswithlaminatedpoleshoesboltedontoafabricatedspider (Figure2.6(b)). Whereair or gas cooling is necessary anaxial or radial fanmay be fittedateitherorbothendsof therotorshaft. However, smallermachinesrely solely onaircirculationasaresultof thewindageof therotoritself, whichisusually slotted toaccepttherotorwindings(Figure2.9). The rotor windings of generators are constructed of hard-drawn copper and are insulated with rigid epoxy or formaldehyde resin and impregnated into a woven material. Onsquirrel cageinductionmotorsthewinding may consistof lightly insu- latedcopperbarsdrivenintotheslotsinthelaminatedrotororof aluminiumbarscast directly intotherotor. Therotorwindingsof aDC machineorwoundrotorinduction motor arerather similar to aconventional AC stator winding that is describedlater. Typical inductionmotorandgeneratorrotorsareshowninFigures2.3and2.10. 2.3.4 Windings Thestatorwindingsof all high-voltageAC machinescompriseconductorbarsmade up of hard-drawn, higher strength copper subconductors that may be connected in

42. Construction, operationandfailuremodesofelectrical machines 19 (a) (b) Figure2.3 (a) Large synchronous generator. Stators for 2500 MW, two-pole, hydrogen-cooled turbine generators. The stator nearest the camera is wound. The stator furthest from the camera is awaiting winding. (b) Large synchronous generator. Rotor for a 500 MW, two-pole, hydrogen-cooled turbine generator showing rotor forging and rotor windingbeforethefittingofendbells.[Source: BrushTurbogenerators]

43. 20 Conditionmonitoring ofrotating electrical machines (a) (b) (c) Wedge Wedge Wedge or top stick Conductors Conductors Ground insulation or slot cell Coil separator Main wall insulation Main wall insulation Insulated magnet wire Figure2.4 Sectional viewof theslotsectionthreestator windings, nottothesame scale. (a) A 400 V roundwirewinding for a 1 kW motor. (b) An11 kV, 1.5MWmotorwinding.(c) A23kV conductorbarforthestatorwinding ofa largeturbinegenerator series or parallel. Individual subconductors arecoveredwithapaper or glass-based tapeandtheassembledbarisovertapedwithasimilarmaterial impregnatedonolder designswithbitumenbutnowadayswithepoxy resins(seeFigure2.4). Intheportion oftheconductorbarembeddedinthestatorslottheinsulationsystemiscompactedby beingheatedandpressedoritmay beimpregnatedundervacuumandpressure.Inthe end-winding portion, where one coil is connected to another, the insulation system is not compacted and may be slightly altered, containing less impregnant, so that it is more flexible and therefore better able to withstand the large electromagnetic forces that part of the winding experiences. An important part of the construction is the manner of the bracing of these end windings. They are usually pulled back onto rigid insulated brackets made of impregnated laminate or steel using nylon or Terylenelacing cord. Onthelargestmachines(Figure2.5(a)) bracing ringsof glass- reinforced plastic are used with insulating bolts. The exact nature of the bracing depends upon the machine rating and the relative length of the end winding, as determined by the number of pole pairs. The yoke (or stator core) is fitted into a frame and enclosure. On smaller machines and those of standard design, the stator coreissecureddirectlyintoasimplifieddesignofamachinemainframe(Figure2.10), but onlarger machines thecorehas its owninner frame, whichis separatefromthe outer frame so that the clamped core can be removed fromthe enclosure for repair (Figure2.5(b)). 2.3.5 Enclosures Themachineenclosurecan takea widevariety of forms, depending on themanner in which the machine is cooled, and the protection it needs from the environment inwhichit will work. Whereapressurisedgas systemof cooling is usedtheenclo- surewill beathick-walledpressurevessel butforsimpleair-coolingwithanopen-air circuittheenclosurewillconsistofthin-walledducting.Typicalenclosuresareshown

44. Construction, operationandfailuremodesofelectrical machines 21 (a) (b) Figure2.5 (a) Large synchronous generator. End region of a 600 MW, two-pole, hydrogen-cooled turbine generator with water-cooled stator windings showingtheendwindingbracingstructureandthehosescarryingwater to the winding. (b) Large synchronous generator. Stator of a 600 MW, two-pole,hydrogen-cooledturbinegeneratorshowingstatorcore,frame andwindingsbeinginsertedintoitsstatorpressurehousingpreparatory tofactory testing. [Source: Dongfang Electrical Machinery, China]

45. 22 Conditionmonitoring ofrotating electrical machines (a) (b) Figure2.6 (a) Large synchronous generators. Generator Hall in the Grand CouleeHydroelectric Damshowing anumber ofhydrogenerator units, 120 MVA, 88-pole, 60 Hz, 81.8 rev/min. [Source: GrandCouleeDam, USA] (b) Large synchronous generator. Installation of the stator of a 75MVA,44-pole,50Hz,136.4rev/minhydrogeneratorinIcelandshow- ingstatorcore,frameandwindingsbeingloweredoverthe44-polerotor supportedonitsthrustbearing. [Source: BrushTurbogenerators]

46. Construction, operationandfailuremodesofelectrical machines 23 (a) (b) Figure2.7 (a) Small DC motor. Section through a 500 W, wound-field DC motor showing stator frameandbearing housings, armatureandcommutator on theleft. [Source: SEM Motors, UK] (b) LargeAC induction motor. 7 MW, 11 kV, 50 Hz, four-pole, 1486 rev/mindesignedto drivea high- speedflashgascompressorforoffshoreoil andgas. Notethedriveshaft onleftandthewater-cooledheatexchangerontopofmachine.[Source: Laurence, Scott& ElectromotorsLtd.] inFigures2.1, 2.2(a), 2.5(b), 2.7, 2.8 and2.9. Thereisanincreasing demandnowa- days to reduce the noise level from electrical machines and apart from affecting the basic design of the stator and rotor cores, this will require specially designed noise-proof enclosures.

47. 24 Conditionmonitoring ofrotating electrical machines (a) (b) Figure2.8 (a) Large AC induction motor. View of a 20 MW advanced induc- tionmotor for shippropulsionshowing lowspeeddriveshaft andheat exchangers on the machine flanks. This is a large multi-phase, multi- pole, variablespeedmotorfedbyacurrent-fedinverter.Source: Alstom, France.(b)SmallAC motor.Viewofacombined4kWmotorandinverter used in a small Nissan full-electric-vehicle, the Hyper-Mini. [Source: Hitachi, Japan]

48. Construction, operationandfailuremodesofelectrical machines 25 (a) (b) Figure2.9 (a) Medium synchronous generator. 400 V line, 40 kVA, four-pole generator for diesel genset, pilot exciter on the right. (b) Small AC motor. Induction motor, 400 V line, 40 kW. [Source: Marelli Motori S.p.A.]

49. 26 Conditionmonitoring ofrotating electrical machines 2.3.6 Connections Electrical connections are made to the windings via copper busbars or cables that leave the machine enclosure through bushings into a terminal box. The main three phasebusbarsof the1111MVA generatorarevisiblerising fromthestatorframein thecentreof Figure2.1. Thebusbarsmay belightly insulatedtoprotectthemagainst theenvironment. Thebushingsusually consistof thebusbarembeddedintoanepoxy resincasting, althoughwoundpaper bushings may beusedonolder machines. The electrical connections arewell bracedto withstandthelargeelectromagnetic forces thataredevelopedwhenfaultcurrentsflow.Theterminal enclosureallowstheproper termination of the supply cables or busbars, and must be specially designed to suit the environment in which the machine works. For example, special enclosures are required for motors that operate in inflammable areas and these incorporate baffles andseals toensurethat any flashover intheenclosuredoes not ignitegas or vapour outsidetheterminal box. Many machinesincorporatebrushgearforconnectiontotherotorwindingseither through steel or copper sliprings or through a copper commutator (Figure 2.7(a). Thecommutator is avery carefully designedcomponent inwhichcopper segments interlock with the rotor so that they can withstand the bursting forces acting upon them. Also, each segment must be well insulated fromits neighbours, and mica is normally usedforthispurpose. Slipringsareusually shrunk ontoaninsulatingsleeve mountedonaboss ontherotor shaft, andelectrical connections to thesliprings are insulatedandcarefully bracedtowithstandthecentrifugal forcesuponthem.Brushes arespringloadedandmountedinbrassbrushboxesaroundtheperiphery of therings orcommutator. Heatexchangersforthecoolingsystemof themachinearemountedontheenclo- sure or may be a part of it, as shown in Figure 2.7(b). They may be as simple as a finnedcasing tothemachinetopromoteconvectiveheattransfertothesurrounding air or they may beamorecomplex water-cooledsystemthroughwhichthecooling gasorairisducted. 2.3.7 Summary Thesedescriptionsshowthevery widerangeofmaterialsthatareusedinanelectrical machineandTable2.3givesasummaryofthesebasedonthestructureofthemachine. Inparticularitshouldbenotedhow importantinsulating materialsare, bothinterms of volumeandcostintheoverall structureof anelectrical machine. Inthefollowing sectionthisstructureof themachinewill bediscussedinmoredetail. 2.4 Structureof electrical machinesand their types Theprevious sections providedabrief descriptionof themajor constructional com- ponents of an electrical machine and the materials of which they are made. The differencebetweenthestructureforassembly andforreliability will bedescribedin Chapter3.

50. Construction, operationandfailuremodesofelectrical machines 27 Table2.3 Materialsusedintheconstructionofa typical electrical machine Subassembly Component Materials Fabricatedstructural steel Steel, copperorbrasstube Copperoraluminiumbusbarorcable Castepoxy resin Steel babbitt, hightensilesteel rolling elementsorsoftbearing alloy on bearing shells Enclosure Enclosure Heatexchangerelectrical connections Bushingsbearings Statorbody Frame Core Coreclamp Structural steel Electrical steel laminationsorrolled steel yoke Structural steel ornon-magnetic, low-conductivity alloy Statorwinding ConductorsinsulationEnd winding support Harddrawncopperorcopperwire Mica-paperorglassorfilm impregnatedwithresin Glassfibrestructural materialsand impregnatedinsulationfelts, ropesand boards Rotorwinding Conductors Insulation Endwinding support Harddrawncopperorcopperwire Mica-paperorglassorfilm impregnatedwithresin Impregnatedglassfibrerope Structural steel orforging Electrical steel laminationsorsteel forging integral withshaft Structural steel ornon-magnetic, low conductivity alloy steel, brassorcopper Carbonorcopperbrushesinbrass brushholders Rotorbody Shaft Core Coreclampsliprings Brushgear In this section the detailed structure for assembly of the electrical machine is discussedandtheeffectof differenttypesof machineuponit. Thisstructureisexem- plified by Figure 2.10. Note the similarity between this 4 kW induction motor and the125MVA synchronousgeneratorshowninFigure2.2(a). ThisstructureisalsopresentedintabularforminTable2.3andthiswill formthe basisforconsidering thefaultsinmachineslaterinthechapter. Notethatgenerators require an exciter to provide the field current for their rotor. They generally have their exciters mounted on the shaft of the main machine and a large generator can haveapilotexciterandmainexciter. Themainexciterisclearly visibleontheleftin