Showing papers in "Journal of the Institution of Electrical Engineers in 1932"

Studies in radio transmission

Abstract: With the advent of short-wave telegraphy, a new technique of echo and facsimile measurement has been developed, facilitating signal-intensity measurements in the wavelength range between 14 and 50 metres. Part (1) of this paper deals with the method adopted of making facsimile measurements, and describes the results obtained, whilst in Part (2) the author discusses transmission problems in the light of these data and suggests the theoretical methods to be employed in solving such problems. Part (3) discusses the problem of scattering, while in Part (4) the author deals with theory.

The inductance of linear conductors of rectangular section

Abstract: A short approximate formula for estimating the inductance of parallel linear conductors of rectangular section is developed. This formula is shown, theoretically and experimentally, to be reliable to within 2 per cent, provided that the length of the conductors is great compared with their section and spacing. An additional formula is put forward for estimating the end effect in those cases where the conductors cannot be assumed to be infinitely long.As the spacing between the lead and return conductors increases, the approximate formula approaches asymptotically to the theoretical formula from which it was developed. If this spacing exceeds half the perimeter of either conductor the difference between the approximate formula and the theoretical formula is less than ½ per cent.

The technique of the high-speed cathode-ray oscillograph

Abstract: After an introduction in which the need for high-speed oscillography and the nature of the problems are explained, an equipment built for recording the “uncontrolled” surges of a 1 000 000-volt impulse generator is described. Attention is concentrated on the problems of discharge tubes, beam traps, sweep circuits, and screening. Examples of the oscillograms obtained are given.

The penetration of alternating magnetic flux in wire ropes

Abstract: When a wire rope is arranged in a laminated magnet yoke and magnetized by alternating current in accordance with the arrangement of magnetic circuit which has been described in a previous paper, †it is of importance to know how the magnetic flux becomes distributed across the section of the rope. In order to obtain some experimental data on this question, a sample of wire rope was constructed with a built-in search coil surrouniding each layer. By an examination of the oscillograms obtained from these search coils the flux density has been obtained for each layer of the rope for two values of the a.c. supply frequency. By means of ballistic tests the flux density has also been obtained for each layer when the wire rope is magnetized by direct current.

A practical investigation into the design of brushes and brush holders

Abstract: A systematic investigation with the aid of an oscillograph into the behaviour of brushes running on a short-circuited commutator is described. Each brush was divided into two or more parts insulated from each other so that the distribution of current across the face of the brush could be examined. The difficult problem of reversible running received special consideration. The commutator contained a slightly high bar and two flats in order to determine the relative merits of different brush and brush-holder arrangements on an imperfect commutator. A distinction is drawn between the effect of "sudden" irregularities as exhibited by high bars, and of a "gradual" irregulalrity such as eccentricity of the commutator. The advantages of using a pressure finger of high inertia and the importance of low inertia in the brush itself are demonstrated. Finally, suggestions are made concerning the lines along which further investigation should proceed.

Measurement of the mutual impedance of circuits with earth return

Abstract: The object of this paper is to place on record the results of tests carried out at three different sites in England at which measurements were made of the mutual impedance between two earthed circuits. Results are given for different separations from 5 to 800 metres and for frequencies form 5 to 00 metres and for frequencies from 200 to 3 000 cycles per sec. The results are shown to be in good agreement with the theory on this subject,and the resistivity of the earth was found to be 250 ohms/cm3 at one site and 6 000 ohms/cm2 at the others.

Heat losses due to load current in direct-current armature windings

Abstract: Designers of alternating-current machinery have long been able to make rapid estimates, by means of published curves, of the heating effect due to current displacement in slot-wound conductors. The wave-shape of the current in the windings of direct-current armatures is complex, since it is rich in harmonics. The estimation of the heating effect due to current displacement in direct-current armatures has hitherto necessitated laborious calculation, for each case on its own merits. The busy designer rarely has the time to carry out such a long process, and it is probable that in most cases guesswork is resorted to. In this paper an attempt is made to provide curves, by the use of which the heating of direct-current armatures may be estimated rapidly. It is believed that results are obtainable with fair accuracy, and with almost the same rapidity as is associated with the estimation of the loss in an alternating-current machine. It is not possible to construct one curve for use in all cases; a ?standard? curve is given, which refers to a single-turn full-pitch winding, having 15 slots per pole and 45 commutator segments per pole. Two additional curves give coefficients which enable other full-pitch single-turn windings to be dealt with. The heating ratios so obtained do not differ from those obtained from individual calculations by more than 2 per cent in any practical case. Curves are also given for windings which differ from the standard winding in having (a) two turns per coil, (b) three turns per coil, (c) chording. The question of the critical height of conductor, and the question of the use of unequal heights for the two layers in a slot, are alsotouched upon. It is, perhaps, not generally realized that current displacement is present at all times in a direct-current armature winding; it is not confined to the period of commutation. A set of current-distribution curves has been calculated for one case, to show this. The author hopes to deal with the problem of the rotary convertor, and the problem of unusual winding arrangements designed to reduce heating losses, in a subsequent paper.

Modern communication systems

Abstract: In the first part of the paper the principles underlying modern communication engineering are discussed. The second part of the paper deals with some characteristic examples of the multiple utilization of transmission systems, such as 2-band systems and simultaneous telephony and telegraphy using short waves.

The design of a distribution system in a rural area

Abstract: The present stage of rural development is reviewed and the general considerations affecting design of equipment are discussed. It was realized that an examination of the design of a concrete scheme would unduly limit the scope of the paper, and consequently an abstract scheme embodying average conditions found within Great Britain was employed. This was termed a ?Model Scheme? and covered 400 square miles. Average conditions, including those of average density of population (150), average load and average revenue, were sought and determined. Such information having been acquired, the general and then the detailed design are discussed, involving the construction of overhead lines and substation equipment best suited to the purpose, having regard to the limits imposed by financial considerations. Estimates of capital expenditure and revenue over a period of years are given for the ?Model Scheme?, supplemented by estimates of similarly devised schemes based on densities of 100 and of 75 people per square mile, such densities being representative of the countryside. Based on such statistics and data as are now available, and on a satisfactory tariff, the paper shows that electrification of sparsely populated rural areas is a practical proposition from a technical and financial aspect.

The resonance method of wave-form analysis

Abstract: The paper describes briefly the principles of the resonance method of wave-form analysis. A short account is included of the other methods of analysis that are available, and a historical resume of the resonance method is give. The desirability of basing the analysis on a measurement of the voltage across the inductance coil in the resonance circuit is demonstrated from theoretical and practical considerations. The most suitable form of analyser is described and the advantages of the thermionic voltmeter for this prupose are indicated. A selection of the results obtained are included to to illustrate the applications of the method.

An experimental and analytical investigation of earthed receiving aerials

Abstract: The paper is concerned with the behaviour of earthed open- aerial receiving systems. The analysis of such systems is based on the classical transmission-line equations, as modified by Moullin to apply to distributed excitation.Part 1 gives the theoretical formulae so obtained for the case of a plain aerial with uniformly distributed constants, and the detail of the variation with frequency of the resistance and reactance components of the effective impedance of such an aerial.It is shown that successive resonances (i.e. conditions of zero reactance) are given by an equation of the formλ=(4h/n)(1+Ah2)where n is any integer and A is a small factor depending on n and on the attenuation constant of the aerial considered as a transmission line. The corresponding values of the resistance are shown to be Rh/2 (approximately) when n is odd, and a very high value when n is even, R being the total effective resistance per unit length of the aerial.The actual variation of the impedance of a plain uniform aerial as a function of frequency was determined experimentally, and a detailed analysis of the results in relation to the theory is given.The observed variations of resistance and reactance are shown to be in substantial agreement with those deduced from the analysis, but it appears that the actual length of the aerial (h) must be replaced by an effective length h+ e, e being about 5 per cent of h. This is in agreement with a more rigid analysis by Abraham, and with experimental results obtained by Wilmotte. The observed aerial resistance was found to be mainly due to parasitic eddy-current and dielectric losses. It was reduced to less than one-third of its value by using a single-wire “earth screen.”The validity of the fundamental assumptions, to a useful degree of approximation, having been established by measurement, Part 2 of the paper is devoted to obtaining additional information by purely analytical methods. The analysis is mainly based on the case of a plain aerial divided into three parts in each of which a uniformly distributed e.m.f. is assumed to be induced, the intensities being different in the three parts.By an analytical manipulation of this three-part case a line integral formula is found for the effective e.m.f. induced in a uniform aerial by a non-uniform field (Section 7). The variation of the effective e.m.f. induced by a uniform field is considered as a function of frequency and aerial height, and it is shown that no advantage is gained by making h>λ/4 unless certain parts of the aerial are compressed (Section 8).The relative effectiveness of different parts of a receiving aerial is considered in Section (9), and it is found that in an L aerial short compared with the wavelength the addition of a down-lead from the open end nearly to the ground may actually increase the total effective e.m.f. in spite of the reversed e.m.f. induced in the added length.An earthed aerial of this shape with the vertical members of length λ/4 and the horizontal member of length λ/2 is shown to have a figure-of-eight polar diagram with a law of the form cos(½12πcosΘ (Section 10).The effective e.m.f. formulae of the three-element case are applied to the Franklin suppressed half-wave construction and afford means of allowing for the effect of aerial resistance in this system (Section 11).In Section (12) the effective e.m.f. of the present formulation is related to the original conception of “effective height.” It is suggested that the idea of “effective height” is one which has to a great extent outlived its usefulness.The analysis of a receiving aerial considered as a collector of energy from the incident wave is given in Section (13). The figure of merit from this point of view is |e6|2/Re, which is determined as a function of h and λ. It reaches a maximum when h is very approximately λ/4, and thereafter oscillates with rapidly diminishing amplitude. Considered as a collector of energy the suppressed half-wave construction will not apparently give an increase in useful power proportional to the increase in height, owing to the corresponding increase in Re.Current and potential distributions in receiving and transmitting aerials are discussed in Section (14). It is shown that they depend on the distribution of the exciting field, and that the current distribution in a receiving aerial may be totally different from that in a transmitting aerial of the same height. Current nodes in a receiving aerial may be any distance apart from 0 to λ, and are not necessarily separated by half a wavelength as in a transmitting aerial with a point source of excitation

The attenuation of short wireless waves at the surface of the Earth

Abstract: An apparatus is described for measuring relative field intensities of wireless waves of the order of 20 metres, and the results of a series of measurements taken with it on wavelengths of approximately 25 metres for distances of from 200 ft. to 60 miles from the transmitter are shown as "Intensity/Distance" curves. For distances greater than 2 miles the decrease of intensity is found to be approximately proportional to the inverse square of the distance as predicted by the theory of Sommerfeld. For shorter distances, however, the curves are much straighter than predicted by theory. This is attributed to penetration of the waves to layers of higher conductivity below the surface. This is supported by the increase in the variations in attenuation with change of surface observed on a wavelength of 18 metres and by measurements on the "tilt" of the electric vector of the wave-front. In all cases, marked changes in intensity and attenuation were found to occur with apparently slight changes of surface and slope of the ground.

Errors in power-factor measurement due to terminal losses on short lengths of cable

Abstract: It is frequently necessary for economic reasons to carry out tests on short cable samples. Measurements of a.c. leakance and power factor on short lengths of power or submarine cables may be subject to considerable error unless due allowance is made for end effects, which are of minor importance on long lengths. The application of the simple guard wire, which is adequate for d.c. testing, is not sufficient to secure accurate results. The nature of the errors which arise is discussed, a theory of the cause of the errors is developed, and recommendations are made as to the form of guard which will enable accurate results to be obtained.

Accuracy of measurements made with hot-filament cathode-ray tubes of the gas-focused type

Abstract: The rapid development of the hot-filament type of cathode-ray tube is outlined, andvarious sources of error which are encountered in its use as a measuring instrument are discussed, including errors inherent to the tube, especially those due to the effect of gas conduction, then errors of manipulation and those due to external influences are reviewed. Recording is next dealt with, including photographic methods, and various methods of calibrating are considered. The chief part of the paper, however, deals with what is known as the ?threshold effect?, a quantitative examination of which is given, together with graphs showing its relation to the various variables, such as frequency and the effect of resistance in series with the deflecting plates. Suggestions are put forward as to the cause of the threshold effect, and the paper concludes with recommendations for accurate working. In an Appendix particulars relating to the latest pattern of low-voltage tube are given, and its sensitivity is discussed.

Coil ignition systems

Abstract: The paper first deals with the general principles of, and the conditions to be fulfilled by, coil ignition apparatus for internal combustion engines. It then describes a simplified treatment of the induction coil, regarded as a closely-coupled system with damping due to secondary loading, an assumption which is found to give results agreeing fairly well with practice. From this theory is developed a basis of coil design, enabling optimum values to be chosen so as to meet given conditions and indicating the limits which are met under extreme conditions of operation, generally at high engine speeds and low battery voltages. Methods of extending the range of operation, when necessary, are then dealt with, and special designs are described which have been developed with a view to meeting extreme conditions of operation. A further section deals briefly with the constructional features of modern coil systems, and with the principal points embodied in the various details.

Bushing insulators for outdoor transformers

Abstract: The paper has four sections which discuss:?(1)The electrical characteristics which outdoor bushings should possess for satisfactory operation; (2) the testing of bushings; (3) the factors which control the design of bushings; and (4) types of bushings. The paper deals particularly with bushings for outdoor transformers, but other bushings are briefly considered, e.g. those for oil circuit-breakers and also wall and roof bushings and cable terminations. (1) Electrical Characteristics. Desirable characteristics are discussed, the bushing being considered firstly as part of a transformer and therefore subject to all the transformer tests, and secondly as part of a transmission line and therefore subject to over-potentials which originate in the line. Bushing as part of transformer.?The transformer with its bushings is assumed to be tested in accordance with B.S.S. No. 171-1927. Values are thus obtained for the 1-minute test voltages, particularly for 3-phase transformers with neutral earthed or insulated. Methods of bushing failure by flash-over and by puncture are considered, and figures are given for the various flash-over voltages, viz. air-end flash-over under dry and wet conditions and also oil-end flash-over. Bushing as part of transmission line.?The transmission-line insulators are assumed to conform to the test requirements of B.S.S. No. 137-1930. The magnitudes of over-potentials which originate in the line and impinge on the transformer are considered. A law is given for the relative strengths of transformer insulation as measured by 1-minute test values and as measured under impulse conditions. The law is used to obtain flash-over characteristics for ?weak links? in the line insulation. Recommended bushing flash-over characteristics are given. The two sets of flash-over figures now obtained are considered together and final recommendations are made for bushing characteristics. For comparison the characteristics are given for bushings to Central Electricity Board requirements and also for bushings in accordance with the new British Standard Specification B.S.S. No. 223-1931. Indoor transformer bushings ; bushings for oil circuitbreakers and cable terminations ; wall bushings ; roof bushings.?The differences between these and outdoor transformer bushings are considered. Suggested characteristics are given. Lightning arrestors.?These are briefly considered. (2) Testing of Bushings. Dry flash-over voltage.?Effects of barometric pressure, air temperature and humidity. Wet flash-over voltage.?The effects of intensity of rainfall and resistivity of water are considered. Duration of test.?Figures are given of the times taken to carry out various tests. Temperature-rise of metal parts.?The use of thermocouples to measure temperature-rise is described, and results of typical tests are given. Temperature-cycle test.?This test is described. (3)Design of Bushings. Features of electrical design are first considered. Temperature ; duration of test ; shape of electrodes.?These factors are considered as affecting the electric strength of insulation. Permissible voltage gradients.?These are given for porcelain, varnish-paper insulation, air, oil, solid compound, and semifluid compound. Laws of corona and puncture.?These laws are developed for bushings having two or three different dielectrics. Flash-over voltages.?Curves are given connecting axial lengths over bushing surface with flash-over voltages for dry, wet and under oil conditions. Heating of bushings.?Brief consideration is given to the heating of the centre conductor, allowing for skin effect, and to the heating of the earth flange due to hysteresis and eddy-current losses. Features of mechanical design and construction are also considered. Clamps.?For dealing with large currents. Joints.?The making of oil-tight and weather-tight joints. (4) Types of Bushings. The following types are described and discussed:?All-porcelain air-core; compound-filled; oil-filled; condenser; all-porcelain multi-tubular; all-porcelain multi-tubular embodying condenser feature. The incorporation of a current transformer as part of the bushing is described, and capacitance coupling is briefly considered. In conclusion the following Appendices are given:- I. Development of law connecting dry and wet flash-over voltages. II. Corona formation in air-core porcelain bushings. III. Radial electrostatic stresses in bushings having three different dielectrics. IV. Statistics of heavy rainfall.

The electrification of the suburban and certain main-line sections of the Great Indian Peninsula Railway

Abstract: The paper gives a general account of the suburban and main-line electrification scheme on the Great Indian Peninsula Railway. The first portion sets out the circumstances under which the scheme was initiated, the principal reasons for the decision to electrify the surburban and the main-line services, the actual and anticipated traffic, and the general programme of carrying out the work.The second portion deals with the local conditions which determined the design of a good deal of the plant and apparatus. This is followed by brief references to special points in the installation and design of the multiple-unit trains, electric locomotives, track equipment, bonding, substations, transmission system, power station, locomotive shops and car sheds, and system control. Such information as is available is given as to the capital cost of the complete scheme.The last two sections contain a number of statistics of operation and notes of operating experience, and the paper concludes with an Appendix in which are given for reference the names of contractors and principal sub-contractors, and the technical particulars of the plant supplied and installed.

Some recent advances in the design of relays for the protection of alternating-current systems

Abstract: The paper postulates that the success of any protective system depends upon the reliability and suitability of the relays employed. The requirements of an ideal protective relay are formulated, and the constructional and operatingcharacteristics of some novel designs of protective relays are compared therewith. For convenience of treatment, the relays?all of the induction type?are divided into four groups in the order of their development. It is submitted that the improved differential or balance type relay (Group A) and the novel time-selective relay (Group U)described in the paper, approach very closely to the ideal aimed at. Some novel working principles are described, but only one form of construction, capableof universal application, is followed throughout the paper, the aim being economical production and uniformity of performance.

The spreading of electromagnetic waves from a Hertzian dipole

Abstract: The previous experimental work on the spreading of a wave from a Hertzian dipole is discussed and is shown to be incomplete. An experimental investigation of the variation of the electric and magnetic fields with distance, in a wave from a source which approximates to a Hertzian dipole, is described. A long wavelength (1000 m) is used, and the results agree with theory up to distances of 0.01λ. The field re-radiated from a receiving aerial is investigated, using long waves, and is found to be in accordance with the ordinary theory. Finally it is shown, experimentally, that the field due to an aerial with its lower end connected to earth is, at a nearby point on the earth's surface, equal to twice the field which the aerial alone would give in free space. The theoretical significance of this fact is discussed.

A method for determining the effect of the Earth on the radiation from aerial systems

Abstract: In an earlier paper, the author described a graphical method for determining the magnitude and phase of the electric field in the neighbourhood of an aerial carrying a current of known distribution. The major portion of that paper neglected the effect of the earth, although an indication was given of a method for taking account of the image of the aerial in the earth. The first part of the present paper elaborates this explanation and shows how the results given in the earlier paper may be modified in order to take account of the electrical constants of the earth's surface. A formula is deduced for the field at any point near the earth's surface due to a given doublet, and it is shown how this formula may be extended in order to determine the field at various distances from an aerial carrying a current of known distribution. The second part of the paper shows how the field at any distant point may be found directly for any number of equal aerials and reflectors from a knowledge of the field at the same point due to one aerial and reflector alone. The current in the reflector wire need not be equal in magnitude or in phase to that in the aerial, and the components of the array may be either vertical or horizontal.

Symmetrical components, and their application to the phase convertor

Abstract: The object of the paper is to explain in common-sense terms the method of calculation developed in 1918 by Mr. C. Le Geyt Fortescue and generally known as the method of symmetrical components. This method shortens and simplifies the solution of problems of unbalanced polyphase systems. It has made practicable the calculating board by which such problems may be solved automatically. In order to demonstrate its wide usefulness, the method of symmetrical components is here used in an explanation of the action of the phase convertor.

Metal-clad switchgear, automatic protection, and remote control, with particular reference to developments during the last seven years

Abstract: This paper outlines a picture of the progress during the last seven years in the field it deals with, and also sets out the problems of to-day and the lines of possible future practice. The period under review has seen a change of national policy in Great Britain, which practically puts an end to isolated plants for public service, and enhances the scope for economy by the running of the largest plants with interconnections between them and all other plants and loads. New conditions therefore arise, demanding a reconsideration of the problems dealt with by the author in their earlier form in 1925. The construction of the various types of metal-clad switch-gear is diagrammatically represented by illustrations brought upto date and concluding with a forecast of the full application of the principle of complete enclosure and immersion of all conductors and insulators for all voltages as the means of safeguarding life and property, and securing continuity of supply. Certain basic constructional features are dealt with more fully. These are, in general, typical of changes, some brought about by research and practical experience, and others evolved in the quest by designers for something ?different? in a period of intensified competition. An attempt is made to classify the prevalent forms of arcing contacts and busbar enclosures, and to state the effects of such details, particularly of the latter, on the general lay-out of the whole switching equipment. The use of metal-clad switchgear has of itself effected such economy in buildings that outdoor switchgear has not been widely used in Great Britain; but reasons are given in favour of outdoor construction. A review is given of the present position of rating, performance, and selection of circuit breakers, in order to lead up to a better understanding of the interpretation of these terms, and in view of the need for an international agreement on standard conditions for assessing ratings. Results of tests in a British 1500000-kVA testing station are recorded as examples. The need for economy is accepted as a fundamental object for the designer, but it should be effected by the reduction of components rather than by lowering the quality of details. Some ways and means of effecting economy along these lines are therefore proposed. The automatic isolation of faults has a particular bearing on the protection of property and supply; in the latter its performance in procuring stability is considered to be of paramount importance, and in the former its speed of operation is a criterion of equal merit. In the light of these principles the known protective systems are reviewed, and some new developments are mentioned. This study is in the main a history of the quest of the ideal, which, for the sake of economy alone, excludes the pilot cables of the earlier systems, and thus an opening occurs for the use of telephone pairs. Some original research is recorded for the avoidance of interference with the other uses of such telephone conductors arising from accidents on main lines. Further, the resistance of heavy arcs has been experimentally ascertained, with a view to making clear the necessity of using reactance instead of impedance as the true means of measuring distance for the accurate loaction and discrimination of faults in the distance types of proctection. The evolution of automatic and remote-control systems is traced from the earlier power-station control-room to the present-day design of regional control from a central room, and the forecast for the future favours the increasing application of mechanical principles, which in the first place tends to avoid errors, and in the second place takes over the responsibility of action in an emergency.

Recent developments in cathode-ray oscillographs

Abstract: The paper deals briefly with some of the more important developments in design and use of cathode-ray oscillographs. Methods of increasing the photographic or recording sensitivity are first considered, particular attention being given to (a) focusing the cathode-ray stream, (b) increasing the sensitivity of the photographic film, (c) the use of phosphorescent materials as a means of increasing photographic sensitivity and of facilitating “external” photography, and (d) increasing the exciting voltage and consequently the kinetic energy andpenetrating power of the cathode rays. With regard to (d), particular a ttention is paid to Lenard's observations on the penetrating power of high-velocity cathode rays and the application of his results to the design of high-voltage oscillographs fitted with a “window” for direct external photography.Referring to the technique of recording by cathode rays, consideration is given to the various methods of generating a “time” axis and to methods of voltage subdivision. The paper concludes with a section dealing with the recording of isolated electrical impulses.

Some notes on oil circuit-breaker design and performance

Abstract: The principal cause of inconsistency in the short-circuit performance of oil circuit-breakers is considered to be the erratic movement of the oil. Quick-break circuit-breakers are advantageous for the higher voltages, and still further improved performance and consistency is obtained by the use of suitably-designed contact shrouds. Reference is made to published tests on some of the recent designs. The principal features of design affecting arc rupture are briefly reviewed and are followed by notes on the causes and effects of the magnitude and the rate of rise of recovery voltage.The importance of short-circuit testing is emphasized, and a plea is made for facilities for testing on site. A description of short-circuit testing arrangements is given, including a detailed description of the oscillograph equipment; also general notes on the calculation of 3-phase kVA. Reports are included of recent short-circuit tests carried out on oil circuit-breakers fitted with a new type of shrouded contact.