Showing papers in "Journal of the Institution of Electrical Engineers in 1921"

Some developments in multi-speed cascade induction motors

Abstract: The paper is divided into four Sections and an Appendix. (1) Section I reviews previous work and shows that earlier conceptions were capable of very wide generalization. By this means many new combinations are brought to light, both in principle and in construction distinct from any hitherto described. (2) Section II is a discussion of the theory of the cascade motor from a new point of view, which avoids the difficulties that have hitherto prevented the subject becoming known in its full generality. (3) Section III and the Appendix describe some new rotor windings which remove some existing limitations of the cascade type. Hitherto single-speed machines running faster than 500 r.p.m. (on 50 periods) could not be built, but the present paper describes a machine capable of running at 750 r.p.m. With three speeds in the ratios 1 500/750/500 there are often either too few speeds or too great an interval between them. Apparatus is described giving as many more speeds as desired, with smaller intervals between them, particularly a machine giving speeds in the ratios 750/500/375/300 and having the characteristics of known cascade machines at every speed. (4) Section IV describes stator windings corresponding to the above rotor windings.

The distribution of electricity

Abstract: The principal conditions of economic distribution are determined by conflicting technical and financial considerations. Certain approximations are shown to be possible which assist in deciding the principal technical conditions. The necessity for several working pressures is shown. A general expression is obtained for capital expenditure on mains and substations. The effect of load factor on losses is considered and a formula, established for general use. The cost of supply is analysed and appropriate expressions obtained. The approximations reached enable Kelvin's law to be applied to decide economical current densities and pressure-drop. A consideration of expenditure on a power distribution network indicates a method of arriving at the economical number and size of substations. The special considerations accompanying the use of high-pressure cables and of overhead lines are enumerated, and the method of arriving at the economical working pressures for various parts of the system is shown.

The whitened cube as a precision integrating photometer

Abstract: The paper indicates how a whitened cube can be made to give accurate results when used in the determination of the average candle-powers of lamps having different polar distributions. As a preliminary, the relative effects of different portions of the interior of the cube in producing illumination on the window are determined, and the results used to calculate the illuminations produced by different types of light distribution. The ratios of these illuminations to that furnished by a uniform point source of unit intensity give apparent average candle-powers in the cube. It is then shown by experiment that candle-powers obtained by the usual methods of integrating photometry agree with these calculated values. Thus it is possible to determine by how much these apparent candle-powers differ from true candlepowers deduced by point-to-point methods, and hence to obtain true candle-powers from experimentally determined apparent values. In the special case when a comparison is made between lamps having identical polar distributions, it is shown that the usual method gives accurate results.

Temperature limits of large alternators

Abstract: The paper discusses existing practice of specifying temperature-rise of electrical machinery by thermometers, and points out that the B.E.S.A. Rules are more satisfactory but are not generally adopted, probably on account of the higher limits.The relations between surface temperatures and internal or hot-spot temperatures are discussed, together with the influence of the load on the temperatures of different parts of a machine.Location of hot spots in the axial direction is considered, as well as the heat flow through the insulation.The influence of the thickness of insulation on the relation between internal and surface temperatures is dealt with, and a diagram showing the radial temperature distribution is given. The importance of considering the voltage of a machine in connection with temperature-rise on account of the greater temperature gradient through the insulation is pointed out.Consideration is given to the question of operating large turbo plants on ultimate temperature rather than on temperature-rise, on account of the increased load obtainable at low air temperatures.A plea is made to operating engineers to call for the fitting of embedded temperature detectors, although temperature guarantees based on this method may not be obtainable. A series of tests on mica-insulated bars is described, including heating runs on bars insulated with mica wraps containing varying percentages of mica, and it is shown that the percentage of mica has a direct bearing on the breakdown voltage which the bars will stand after heating.The results of bending tests on bars before and after heating are shown. The conclusion arrived at is that mica insulation, as used in modern turbo-alternators, retains its mechanical and electrical properties at temperatures up to 200° C. It is suggested that the limit of the ultimate temperature should be raised to 160° C. instead of the usual value of 125°C.

The mean error of an electricity meter

Abstract: Questions concerning the accuracy of electricity meters in actual service have reference to the error of registration over a period, rather than the speed error at any particular value of the power in the circuit. There will be a certain load at which the speed error is the same as the error of registration over the period in question, and the purpose of the author is to investigate a method of ascertaining this load. From a consideration of the sources of error in electricity meters it appears that, in meters in which a constant frictional torque is the only disturbing factor, the error of registration over a period is the speed error at the average load over that period. In meters having in addition a disturbing torque proportional to the square of the speed, the error over a period will be slightly slower than the error at the average speed. A discussion of the effect of varying power factor leads to the result that the error of registration due to this cause is equal to that determined by a speed test at an average power factor deduced from the readings of an energy meter and a meter integrating the wattless component of the power in the circuit.

Electric driving in the paper mill, on heat-economy lines

Abstract: The various uses for which steam is required in paper manufacture are first discussed.Then follow particulars of the various classes of machinery to be driven, and comments on the selection of suitable electrical gear. The heating requirements for heat economy, and the various types of power plant by which this can be attained are discussed in detail, together with recommendations on how each type of plant will be used for the different types of paper mills.The utilization of existing prime movers on “heateconomy” lines is specially shown.The factors governing the choice of electric supply are then dealt with.Finally, the driving of the variable-speed end of the paper machine proper is discussed in detail. The requirements for a successful drive are first enumerated, then various means of attaining that drive, not only with a single motor, but alternatively with several motors, both electrically and mechanically interlocked, are investigated.A Table at the conclusion of the paper gives particulars of machines which have been converted for electrical working in the United Kingdom, and the total number of machines in the country.

The transmission of electric waves along wires. A vector method involving only elementary mathematics

Abstract: The first part of the paper embodies a determination, using elementary mathematics only, of the fundamental equations of a long transmission line. The interpretation ofthe equations is discussed. Subsequently, the discussion is extended to the growth of the wave system.

A precision form of the Kelvin electrostatic voltmeter

Abstract: The paper describes a form of electrostatic voltmeter for precision measurements The instrument follows the general arrangement of the Kelvin type The chief modifications are : (1) Very considerable reduction in weight, permitting (2) the use of a bifilar suspension to overcome the effect of elastic fatigue: (3) a modification of the shape of the needle to produce a more satisfactory ?scale law? for both small and large deflections: and (4) the provision of various mechanical details to facilitate construction and adjustment

Properties and characteristics of insulating materials

Abstract: The subject of insulating materials has been considered in this paper under five main headings, of which four are dealt with in this abstract. The fifth section was chiefly concerned with the properties of well-known forms of moulded insulation, which were considered in more detail in an earlier paper by the present author. Section 1 here deals with the manufacture and properties of porcelain, and describes the various processes in common use, and their application to transmission and terminal insulators. Section 2 deals with mica insulation, the physical properties of mica, its use in its natural form as an insulator, and the various fabricated insulators in which it plays an important part. Section 3 treats of fibrous insulation, including papers and cloths and various insulators made with these materials as a base, such as "Micarta", etc. Section 4 deals with liquid insulation, including transformer and switch oils and varnishes employed in impregnation and dipping processes in connection with coils, etc., used in electrical apparatus.

Abnormal pressure-rise in transformers and its remedy

Abstract: In a high-tension transformer, abnormal potential gradients likely to cause a breakdown appear usually at the foot of the bushing and at the end coils. The author describes briefly a theory which is applicable to both these cases and from which he concludes that these dangers may be due to the incorrect arrangement of electrostatic capacities. Following this theory, he shows that, by properly grading the capacities, it is possible to avoid to a great extent the dangers in both the bushing and the end coils. The correct gradation ot the capacities is shown to be as follows:—(1) The shunt capacity (Cs) must be graded so as to increase with the distance from the earthed end of the coil, or from the top of the bushing.(2) The capacity (Cg) to earth, or to the high-tension conductor, should be graded so as to decrease with the distance.(3) The ratio Cs/Cg should be as great as possible.(4) It is preferable to use a resistance in series with the capacity Cs.Experiments show that where a transformer is exposed to excessive stress a potential gradient of about 20 times the normal value may occur at the end coil, and this can be reduced to one-third this value by the device proposed. The author's method also prevents breakdowns due to other causes, such as the decrease in the natural frequency of the coil.