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Electrical Contact Bounce and the Control Dynamics of Snap-Action Switch£S

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Electrical Contact Bounce and the Control Dynamics of Snap-Action Switch£S ELECTRICAL CONTACT BOUNCE AND THE CONTROL DYNAMICS OF SNAP-ACTION SWITCH£S By JOHN WILLIAM McBRIDE ~B.Sc). A thesis submitted to the C.N.A.A for the degree of Doctor of Philosophy, and in partial f~lfilment of the requirements for that degree. Sponsoring Establishment: Plymouth Polytechnic, Plymouth, Devon. Collaborating Establishment: Arrow-Hart lEurope) Ltd, Plymouth, Devon. May 1986. ( \ , I i ·J I 0 .; -: . I DECLARATION I declare that this thesis is the result of my investigations only, and is no[ submitted in candidature for the award of any other degree. During the research programme I was nut registered for the award of any oth~r C.N.A.A or University Degree. ADVANCED STUDIES During the researc~ programme I undertooK a ~ourse of advanced studies. These included the use of the scanning electron microscope, and surface analysis techniques. I also attended several one day seminars, and presented a paper at the 31st Holm conference on e~ectrical contacts. ELECTRICAL CONTACT BOUNCE AND THE CONTROL DYNAMICS OF SNAP-ACTION SWITCHES By, J.W. McBride ABSTRACT Experimental and theoretical studies are made of a typical snap-action rocker switch, to establish the wear mechanisms in the pivoting contact. The rocker switch, used extensive~ in consumer goods, operates in the medium duty current range, (1 - 30 Amps). Highspeed photographic studies have shown that the main cause of wear is arcing, occurri~ during separation and bounce at the pivot contacts. To reduce the bounce a computer-based mathematical model of the system dynamics is developed and opcimised; this results in recommended design changes. These changes are tested under tull current endurance conditions, and show significant improvements in wear. The model of the switch dynamics relates the mathematics of motion to the bounce occuring at the pivot contact, without the influence of current. ' To show the effect of current and arcing, an automatic test system is developed for the controlled testing of electrical contacts. The system has the ability to evaluate arc energy, bounc~ times, and contact resistance. The results presented detail the influence of d.c current on contact bounce time, and identify the Lmportance of the subsequent bounce time; which is defined for a single make operation, as the total duration of the bounces occurr·.,,J after the first bounce. To compare the erosion profiles of the switch and test system, the system is operated under full load current endurance conditions, to evaluate wear. This comparison shows tnat the wear in the real switch contacts is greater, as result of the additional contact movement of slip and rolling. ACKNOWLEDGEMENTS I would like to thank all members of staff associated with this project, in the Faculty of Technology at Plymouth Polytechnic. In particular: Professor O.J. Mapps, of the Department of Electrical and Electronic engineering, my main supervisor; for his advice and encouragement during the period of research. ·or P.J White, of Electrical Engineering, (second supervisor); for many hours of useful discussion on associated topics. Mr G. Roberts, of Mechanical Engineering, {joint second supervisor); for his clear and useful observations. Mr A. Kendrick, of Mechanical Engineering, (Technician); for building the automated test apparatus to a very high standard. The members of staff in the Department of Mathematics and Computing, for their help and advice, with the computer modelling; and technicians of the fine work shop in electrical engineering for their help with equipment. I would also like to thank, Mr J. Allen (Engineering Manager), Mr B. Weeks, (Engineer), and all other staff associated with this project at Arrow-Hart (Europe) Ltd. I would like to thank Arrow-Hart (Europe) Ltd, and the S.E.R.C, for their financial support during this project. Finally, I would like to thank my family ror their support, and dedicate this work to my daughter Kirsty. Nomenclature Because this thesis spans the areas of both mechanical and electrical engineering, some of the nomenclature has had to be changed from that used normally. The following nomenclature is used unless specified otherwise. B = Data from films, the blade displacement from the horizontal. E = Arc energy. E = Strain energy stored Ln ~ontacts at impact. 5 F = The spring force in the rocker switch, and Contact force. F = Reaction forces used in the Static distribution. ,,2, 3 G = Switch dimension. H Data from films, and radial position of the plunger e.g. I= current, IJ= Supply current, Im= minimum arc current, I 4 = arc current. If= Plunger inertia, Ib= IB = blade inertia, I~= Rocker inertia. K = The spring stiffness, and the bounce reduction time. KEi = The angular kinetic enegy of the blade at main contact impact. L Lorenz Constant, and Inductance. M = The Blade mass, M = The contact mass, M = The blade mass, without contacts. ~ N = The number of switching cycles, and plunger reaction on actuator. R = The circuit resistance. R~c) = constriction resistance, RlfJ the film resistance. RttJ The total contact resistance. S Data from films, T Bounce time, T~= electrical bounce, T ~ mechanical bounce. U The contact volt drop, U5= softening, U~= melting voltage. V Volts, V~= the minimum arc voltage, V = the supply voltage. 5 W Youngs Modulus, X Main contact gap data, for the left hand contact. Y = Main contact gap data, for the right hand contact. Z = Pivot contact gap data, and displacement during bounce. a = radius of contact spot, a·= radius of ·a· spot. b = blade thickness, c = acceleration, e = The coefficient of restistution, and max compression of the spring. f = transient force, g Rocker dimension, h = Rocker dimension, i = Transient current, l Rocker dimension, m = the plunger mass, n = the number of bounces in a given make operation. p = Rocker dimension, q = Temperature, r = the radius of the plunger, and radius of spherical contact. s = Rocker dimension, t = time variable, u = contact velocity, u.= The velocity of the contact at impact. L u = The combined contact velocity in impact. 0 v = Transient voltage, x = The extension of the spring in the rocker switch, y A general variable, z Rocker dimension, GREEK SYMBOLS USED C{ The actuator angle in che rocker switch, p = The Blade angle, "[ = .The· reco.verab:i:e ~gai(J •energy at i~paci:, .~ := The therma~ co11d!Jcti:Vity, ? = .The coeffiCient :of ftfct:i:on f ·= The resis(fvlity, 'f''=> A substitution :term defined in· appendix 5. Table of Contents DECLARATION ABSTRACT ACKNOWLEDGEMENTS NOMENCLATURE TABLE OF CONTENTS CHAPTER 1 •..•.••••••••••••.•.•.••...••••••••••.•••••••••••••.•.•••••.• 1 1.1 Recent Developements in Consumer Switches •.••.••.•.••••••..•••.• 4 1.2 Future Trends in the Switch Market •••••.••.••••••..•••••.•.••.•. 5 1.3 The Hand Operated Rocker Switch ................................. / REFERENCES TO CHAPTER 1 ••••.•.••••••.••..•••••••.••..•.•••.••••••.. 11 CHAPTER 2 •..••••..•.••.•••.•.•.•••.•••••••••••••••.•••••.•••••..••••• 12 2.1 Introduction •.••••.•.•••••....•.•••••••.•••••.•••••••••••.••••• 12 2. 2 Static Contact Theory •••••.••••••••.••••••....••••••••••••••••• 13 2.2.1 Electrical Conduction Through the Area of Contact •.•...•. 15 2.2.2 Contact Resistance ....•.••••..••••••..•.•••••••••.•.••••. 16 2.2.3 The Influence of Force and Temperature on Contact ••.•..•. 17 2.2.4 The Measurement of Contact Resistance ••...•...•..•....... 21 2.3 Electrical Contact Phenomena during tne Break Operation ••.•.••. 23 2.3.1 The Electric Arc and Arc Initiation •..••••..••.•••..••••• 24 2.3.2 The Full Arc, Current and Voltage Characteristics ••••.•• 26 2.3.2.1 The Minimum Arc Current and Voltage ..•.......•..• 26 2.3.2.2 The Arc characteristics ....•.........•........... 28 2.3.2.3 The Arc Energy •.•......•......•.........••.•...•• 29 2. 3. 3 Circuit Conditions ...••..............•..•...•.......•.•... 30 2.4 £lectrical Contact Phenomena at Switch Closure ..........•....•• 33 2.4.1 Arc Ignition Prior to Impact •••..••••.••.•••••••••••••..• 34 2. 4. 2 Mechanical Impact and Bounce •••.•••••.•••••••..•••••..•.• 34 2.4.3 Impact and Bounce Applied to Electrical Contacts .•••.•••• 36· 2.4.4 Summary of Electrical Contact Bounce ••••••••••.•••••••••• 50 2.4.5 Arc Energy during Make and Bounce time •••••••.•••••••...• 51 2.4.6 The Mechanical Suppression of Contact Bounce •••••••.•.••• 52 2. 5 Contact Erosion and Contact Failure ....•••.••.•.••••.••••.•.••• 53 2. 5.1 Wear in Non-arcing Contacts ••••••••••••.•••••••••.•.•..•• 54 2.5.2 Corrosion Mechanisms as applied to Electrica1 •••••••••••• 55 2.5.3 Mass Transfer in Switching Contacts ••••••••••.••.•••••.• 56 2.5.4 The Measurement of Contact Erosion •.•.•••••••..••..•••••. 56 2.5.4.1 Erosion as a function of Mass Change •..•••••••.•• 57 2. 5. 5 The Evaluation of Contact Failure ••.••••.••.•.•...•••.•.. 59 2.5.6 The Monitoring of Contact Resistance .••••.••.•••.••••... 60 2.6 Switch Dyna mics and Contact Erosion .••••.••••••.•.•••.•••••••• 60 2.7 The Nature of this Investigation .•.•.••••••••••••.•.•.••••••..• 62 REFERENCES TO CHAPTER 2 .•........•.•.•...••••••••••••••.•••••..•••. 63 CHA.PTER 3 .•••••••••••••• , ••• , ••••• , ••••••••••••••••••••••••••••••••••· 7 2 3.1 High Speed Photography •..•••.•..•••••••••••.••..•.••...•.•..... 72 3.1.1 The Methods used in the Study of Rocker Switch •..••...•.• 75 3.1.2 The Method for the study of the Test Apparatus ..••.•••... 78 3.2 The Monitoring of Switching Transients •...•••••..•••...•.•••..• 79 3.2.1 The DL1080 and GP-IB Interface .••..••.••.•.•..•..•...•.•. SO 3.2.2 The Circuit for Monitoring Switch Transient ...•...•••...
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