Showing papers in "Journal of the Institution of Electrical Engineers in 1923"

A mathematical development of the theory of the magnetomotive force of windings

Abstract: The mathematical theory, of the magnetomotive force developed by armature windings is developed and presented in a new manner, and in such a form that it is at once directly applicable not only to, the more, common cases but also to abnormal cases, such as unbalanced windings, and to pole-changing windings, etc. An attempt has been made to present the subject matter in simple mathematical language and to give the various series concerned in as simple a form as possible. In general, the conclusions arrived at iiu the various sections of the paper have been succinctly stated. The method of treatment adopted is to start from the Fourier series representing the M.M.F. distribution due to a single coil, and to build up from this series the corresponding series for any complete winding. The effect of the value of the coil-span is discussed, it being shown that, in general, a single coil gives rise to an M.M.F. distribution corresponding to the joint effect of all numbers of poles from 2, 4, 6, etc., upwards. The effect of the spread of the coil-side is briefly discussed, and data relating to the various winding factors concerned are given in a simple graphical form applicable to harmonics of all orders, whether odd or even. The resulting M.M.F. due to a number of coils carrying the same current is then deduced from the effect of a single coil, heteropolar arrangements also being discussed. In the sections dealing with alternating-current windings, a similar method is employed. It is shown that all types of polyphase windings fall simply into one category, the treatment being perfectly general and directly applicable to all types of windings, whether normal polyphase, full-pitch or chorded, or a winding intended for pole-changing. The effects of higher harmonics in the current wave are discussed, and data relating to the harmonics present in the M.M.F. developed by polyphase windings are given.

The rating of cables for intermittent or fluctuating loads

Abstract: The extent to which the rating of a cable is affected by an intermittent or fluctuating load as compared with continuous running conditions is investigated theoretically, and formulae are given by means of which the effect of any type of loading can be calculated.It is shown by experimental determinations on various sizes and types of cables that the thermal time-constant can be calculated either from a heating or cooling curve of a particular cable, or from the specific heat and mass of the various components of the cable.Tables are given showing the rating for the cables for which load tables are given in the I.E.E. Wiring Rules, on the same basis as for motors, i.e. ½-hour and 1-hour ratings.

The electric battery vehicle

Abstract: In the original the paper was intended to review the whole of the position of the electric battery vehicle as it stands to-day. In the present abstract the descriptive sections dealing with the battery, chassis and charging equipment have been omitted, attention being confined to the more important sections.The paper opens with a reference to the early failure of the electric battery vehicle and to the misconceptions which exist as to its proper duty.The battery is not dealt with in detail, but a table showing the characteristics of the two chief types is included, while the motor is dealt with by a brief reference to the main factors controlling its design.The question of transmission is discussed at length, as is also the subject of control. Under the heading of transmission the special circumstances affecting differential action are noted.As information regarding the methods of charging is not readily accessible, it was decided to devote a section to this subject which is probably of greater interest to the user than that of the battery itself.Space does not permit of any considerations of the financial aspect of the question. For information on this point reference should be made to the papers by Mitchell and Watson * and L. Brookman.† in which the matter is discussed at length.

The design of inductances for high-frequency circuits

Abstract: This paper consists of an investigation of the proportions and the arrangement of the winding of high-frequency inductances in order to obtain the minimum ratio of R/L. Both stranded and solid wire coils are dealt with, and the unavoidable limitations arising from the space factor are considered. Results and tables are given which enable simple comparisons to be made between stranded and solid wire coils having either single- or multiple-layer windings.

The use of single-core sheathed cables for alternating currents

Abstract: It is frequently a matter of convenience to use single-core lead-covered cables for transmitting power by means of alternating current, and in such cases the magnitude of the loss arising from the eddy currents induced in the lead sheath becomes a matter of importance. Recently some power-supply authorities have refused to connect to their lines any distributing system depending upon the use of such cables.The eddy losses in the sheaths of these cables may be divided into two groups, called herein “sheath eddies,” and “sheath circuit eddies.” The latter occur only when one or more cables have their sheaths connected at more than one place.The losses are analysed for a single cable, a pair of single-phase cables, and a set of three-phase cables.It is shown that the sheath eddies are in all cases negligible, and that the sheath circuit eddies may be kept within reasonable limits by proper regulations regarding the spacing of the cables.It is suggested that the I.E.E. Wiring Rules should contain such regulations, and that so long as these are adhered to there is no justification for refusing to connect single-core lead-covered cables to any ordinary a.c. system. The necessary data are given for the framing of the regulations.Similar questions arise regarding armoured cables. The results in this case are awaiting the conclusion of the experiments now being carried on at the University of Birmingham.

Permanent magnets, and the relation of their properties to the constitution of magnet steels

Abstract: The first portion of the paper deals with the properties required in a permanent magnet. It is shown that in all cases, provided the magnet is properly proportioned to its work, the true criterion is the (BH)max. value. The relationship existing between the values of saturation density, remanent density, coercive force and (BH)max. is then considered, and a modified form of Kennelly's or Lamont's law is developed, which enables this relationship to be expressed both analytically and graphically. It is shown that in actual practice magnets of homogeneous composition obey this law very closely indeed.The paper next deals with the case of magnets which are microscopically not of uniform composition, and which contain two principal constituents. The relation between the magnetic properties of the magnet and the grouping of the constituents is considered, in addition to its relation to their magnetic properties. The constituents may fall under three headings :(a) non-magnetic, (b) magnetic and possessed of high coercive force (magnetically hard), and (c) magnetically soft, i.e. magnetic but of low coercive force. Evidence shows that the highest values of (BH)max. are obtained when the magnet, consists uniformly of a material of class (b), and that with the right steels and appropriate heat treatment other constituents are absent. The constituent in question is, however, only a transition product between the other two classes (a) and (c), and its presence in the pure state is therefore not always easy of attainment.Data taken chiefly from modern cobalt steels are given in support of the theories put forward, and some suggestions are made for further research and development work in this connection.

Steam turbine blading

Abstract: The paper is a review of the present position of the subject of steam turbine blading. Section (1) describes the causes and effects of corrosion and erosion. The most important materials in use are enumerated and compared. Section (2) deals with the form of the blade passage in impulse and reaction machines, and with the energy losses occurring in the blading. Some meLhods of tapering the blades are given. Section (3) details the stresses in the blades and describes the types of blade fastenings, lacing and shrouding. Section (4) gives the methods used for obtaining a large exhaust area. Section (5) deals with vibration and its causes, and shows a method of correcting for the effect of centrifugal force. The lines on which the problem is being attacked are described. Section (6) is devoted, to the methods of manufacture by-milling and by drop forging. A bibliography is appended.

The effect of local conditions on radio direction-finding installations

Abstract: This paper is an account of some of the experiments carried out during the past year by the Sub-Committee of the Radio Research Board on Wireless Direction Finding. The majority of the work was done either at the Radio Research Station, Slough, or the National Physical Laboratory, Teddington. The paper summarizes the work of previous investigators on the same lines, and indicates why further reasearch was necessary. It then describes experiments which provide quantitative data as regards the effect of metal work, coils, aerials, overhead wires and trees on a direction-finding set in their vicinity, and gives definite figures which show the extent in certain cases of errors produced by mountains and buildings.

Wireless direction-finding in steel ships

Abstract: The problems of direction-finding which are peculiar to ships are discussed. A method of regarding the action of a ship on wireless waves is given, as well as means of correcting for the deviations so produced. The precautions to be observed in the calibration of a ship direction-finder are indicated. A laboratory method of investigating the action of metal structures is described, and the use of multiple-aerial systems is considered.

Electric arc welding apparatus and equipment

Abstract: The intention of this paper is to deal with the physical features of the iron arc as used for welding ferrous metals, so far as those features are known, indicating some of the lacunae in present knowledge; and to describe the general types of apparatus and equipment which have been developed, illustrated by some examples of each type, and some of recent design which promise well. The two branches of the subject are directly connected, because the apparatus must conform in its performance to the conditions of the welding arc and to the requirements of the welder. Operating or workshop methods are touched on only so far as they illustrate or determine, or are determined by, the physics of the welding arc and the practical requirements of welding. Working methods and applications of arc welding have been described in several recent contributions to other Institutions and Associations, in technical journals, and in books, reports, etc. A short list of the more important of these within the author's knowledge is set out in Appendix II at the end of the paper.

The effect of shearing stress on the span calculations of a suspended cable

Abstract: The paper presents the complete equation of a suspended cable, taking into account its density, elasticity and rigidity, and determines the extent to which the effect of shearing stress should be considered for practical span calculations. The criterion of the importance of flexural rigidity can be written in the form e=100 × 2γ=100Ed 2 D 3 /ρ(S/2) 6 per cent where γ is the percentage error caused by neglecting flexural rigidity in calculating the span from the usual formulae, and γ=w 2 0 EI 0 /T 3 0 . Here d is the diameter of the wire, ρ the volume density, ω 0 the linear density, I 0 the moment of inertia of the section, all in the unstretched condition, E Young's modulus, T0 the lowest tension, D the deflection and S the span. The ordinary catenary equation is sufficiently accurate for most practical purposes, but when λ <2γ and with the lower tension, where λ=T 0 /EQ and Q is the cross-sectional area, the effect of flexural rigidity is more important than that of elasticity.

The compound characteristic in regenerative braking with direct-current traction

Abstract: This paper deals with a particular form of regenerative braking for d.c. trains, effected by the application of a differential compound field excitation to the traction motors Speed/torque and speed/current curves are derived mathematically and experimentally, and are compared. A peculiar property of the speed/torque characteristic is a pronounced maximum value, to ignore which might be very serious.

The drive of power station auxiliaries

Abstract: The paper deals only with the supply, of power for the auxiliaries of a modern power station, no attempt being made to discuss the relative merits of apparatus such as evaporators, ejectors, etc.The following aspects of the problem are dealt with :—(1) Consideration of the different types of auxiliaries used in a station.(2) Consideration of the different types of supplies available.(3) Suitability of different auxiliaries and supplies for various conditions of station working.Seven alternative schemes for the supply to the auxiliaries of a proposed new station are described and tabulated, and the advantages and disadvantages of each are considered in turn.Certain of these are recommended by the authors, this recommendation being based on the principle that no expense should be spared to secure a simple and reliable system of station auxiliaries.Some observations are made on features to be taken care of when auxiliary supplies are being laid down.

Induction motors used as synchronous machines

Abstract: The object of the paper is to determine the overload capacity of an induction motor when operated synchronously, and the pulling-in capacity at synchronism. Incidentally, in view of suggestions made in this connection in periodicals, experimental data have been obtained for (1) two methods of using a star-connected rotor for excitation, and (2) the resulting heating of rotor windings under the two methods of excitation. Further readings have been taken so as to enable curves of efficiency, power factor and stator current to be drawn. The conclusions are that the advantages of synchronous operation of induction motors are most pronounced when the method is utilized at times of light load and up to about 70 per cent of full load. At heavier loads there is a logs of efficiency and only a slight improvement in power factor. Owing to the necessity of adjusting the excitation to the load, synchronous operation is unsuitable in the case of fluctuating loads.

Directional wireless telegraphy in aircraft

Abstract: The paper deals with the operation of directional wireless telegraphy in aircraft where the directional aerials are installed in the aircraft. The main difficulty in this work is to deal with the errors which arise. Curves are given showing the errors obtained, and formulae are developed showing the factors on which the errors depend. Methods of predicting and correcting the errors are given. Curves of variation in bearings of transmitting stations are given and discussed. A possible cause of these variations is given.

Domestic load building. A few suggestions upon propaganda work

Abstract: The object of this paper is to show in a simple manner some of the channels open to electric supply undertakings for securing the domestic load. As conditions differ in almost every district, the subject must be approached from a different angle in each case. In certain districts it might be fatal to adopt a too progressive attitude, and one must therefore lead up to the point by suggestion; in others it may be found desirable to combine the development scheme, such as an educative campaign by actual demonstration, with Press and other advertisements. These methods are briefly discussed and reference is made to the value of the load to the undertaking, and the various proportions it can take. A skeleton scheme of one year's campaign is put forward, and, in view of the decision of certain authorities to embark upon this development, it is hoped that the remarks may be of interest. The fundamental essentials on the question to tariffs, load characteristics, hire of appliances, etc., are discussed, and a chart showing one week's demand of a housing estate, where cookers, heaters, etc., are installed, is given. The advantage of an electrically equipped household to the consumer and its appeal to women, also the assistance given by the latter in achieving the object, are referred to.

A bridge method for determining dielectric losses at wireless frequencies

Abstract: The method described below was devised with a view to the investigation of the losses which occur in soil.The principle employed was that of a modified form of Wheatstone bridge, and a description is given of the arrangement of the apparatus used.It was found possible satisfactorily to measure at various frequencies the capacity and equivalent resistance of the condensers employed, and the latter was found to be unexpectedly large.Some suggestions are added that may be of service to those who contemplate similar work.

Some problems in high-speed alternators, and their solution

Abstract: It is the author's purpose to present concisely some of the more interesting problems encountered, and the methods adopted to solve them. The problems, although encountered in high-speed alternators, will be found to be of interest to engineers engaged in other branches of the electrical engineering industry. The following subjects are considered:- (1) The history and failures of turbo-alternators. (2) Mechanical construction of rotors and caps supporting the coil ends. (3) Stator windings and insulation. (4) Stator slots. (5) Alternator ventilation, water cooling and the necessity for clean air supply. (6) Exciter instability. (7) Eddy currents in rotors. (8) Oscillograph tests, in explanation of exciter instability, showing the necessity for precautions when breaking the main field by means of a quick-break switch without any discharge resistance.

The calculation of air-space flux

Abstract: With the increase of alternating-current apparatus and the spread of wireless telegraphy, the calculation of airspace flux, whether electromagnetic or electrostatic, becomes daily of greater importance. At present the old formulae of Forbes are used, supplemented by certain cases developed by Maxwell, Thomson, Carter and others. This paper extends the number of these cases, and develops therefrom close approximations in forms of immediate use to the designer. The results thus obtained are compared with the older formulae, by means of examples. The following is a list of the chief cases considered :? I. The flux between equal equipotential surfaces in parallel planes. II. The flux between equal parallel equipotential surfaces in the same plane. III. Variation in Case I produced by surfaces like those in Case II. IV. The effect of the outer surfaces of two equal equipotential surfaces in parallel planes. V. The flux issuing from a taper pole-shoe. VI. The flux distribution in a stepped recess. VII. Calculation of capacities by means of the same theorems.

The improvement of power factor

Abstract: The economical limit of power factor improvement in relation to capital outlay. It is shown that if C is the capital cost of generating and transmission plant per kilowatt, and c is the cost of phase-improving plant per wattless kilovolt-ampere; then, writing α = c/C, it is economical (from the capital outlay point of view) to go on improving the power factor until sin o = α, where o is the angle of lag. The saving thereby effected is equal to 100[l - cos (o o - o)] per cent, where cos o 0 is the original power factor before improvement. A practical example is worked out to show the operation of this rule. The synchronous motor and static condenser serving exclusively for injecting leading kVA are considered. The rotary converter and the synchronous induction motor serving the double purpose of supplying power and injecting leading kVA are dealt with and their province is described. Various methods of starting the synchronous induction motor are illustrated. The paper then goes on to treat of phase advancers of the rotating type connected to the slip-rings of induction motors. The effect of such advancers upon the performance of the induction motor is illustrated by several examples, and the change in the shape of the circle diagram is worked out. The author then proceeds to give an expression for the apparent capacity of his "vibrator" in terms of the constants of the machine, and shows how to arrive at the size of vibrator required to suit any given conditions. The last section of the paper deals with meters and tariffs. Various methods of indicating kW maximum demand and wattless kVA are described and particulars are given of the tariffs employed by various power companies. Observations are made as to the fairness of these to consumers who improve their power factor.

The leakage flux between parallel pole-cores of circular cross-section

Abstract: Some of the formulae used by designers for the purpose of calculating the leakage flux passing between circular pole-cores in salient-pole machines are far from satisfactory, since they cause the flux to be considerably under-estimated. The object of this paper is to draw attention to formulae and curves from which the permeance of the leakage paths can be found with high accuracy, the results being derived by simple mathematical methods. A critical comparison of the old with the new formula? is given, illustrated by a numerical example to show the use of the new results in practice.

Experiments on modes of resonant vibrations of telephone receiver diaphragms

Abstract: (1) The well-known phenomenon of the "howling telephone" is extended by the insertion between the microphone and the receiver of a valve amplifier. (2) The extended phenomenon is used to measure the velocity of sound. (3) An investigation is made as to which member of the complicated mechanical-electrical system determines the pitch of the "howl." (4) Sand figures are obtained of some of the various modes of vibration of a receiver diaphragm. (5) The frequencies of the various modes are compared with those predicted by theory. (6) Conclusions; that the characteristic note is determined by the telephone receiver, and that the receiver diaphragm overtones are substantially in accordance with the plate theory.

Wind-driven generators

Abstract: The paper describes the various sources of power for the generation of electricity in small quantities. Former objections to wind-power sets are stated, and the paper proceeds to show how these have been overcome. Wind-driven generating sets in general are described, particular attention being given to a novel form. The actual results of a plant are given for the period from July to December.