Showing papers in "Journal of the Institution of Electrical Engineers in 1922"

The production of noise and vibration by certain squirrel-cage induction motors

Abstract: (1) A suggestion is made that the high-pitched notes emitted by some induction motors are due to a side-pull arising from an unsymmetrical field which may be produced when certain numbers of rotor slots are used. (2) A simple case is first considered and the dissymmetry shown. (3) and (4) The forces produced are indicated and their effects considered. (5) The field is analysed and found to include pairs of components, such that in each pair the numbers of poles differ by two. The interferences between such pairs of fields produce the effects observed. (6) An elementary investigation is made which gives an expression for the frequency of the note produced. (7) Illustrative examples are considered. (8) The principle is extended. (9) A more general investigation is made. (10) A rule is developed for determining what numbers of rotor slots should be avoided. (11) Some experimental results are given.

Electric motor starters

Abstract: The paper deals with the practical development of an electric motor starter from the first principles relating to current, torque, and resistance values, to the data required by the salesman. First it describes the method adopted of working in percentages for all calculations; defines what is meant by starting, accelerating and running torques; refers to the lack of data regarding starting conditions; and gives tables of stored energy in rotors, for the normal load torque during acceleration and for the stored energy of a number of typical machines. Accelerations are classified as "natural regular", "natural irregular" "forced regular" and "forced irregular" and typical diagrams with automatic contactor and handoperated starters are shown. A formula is given for estimating accelerating times and torques, and the number of notches from the point of view of breaking capacity is then considered. The rating of the resistance elements, and points arising out of continuous and intermittent rating, together with diagrams for air cooling, and radiation and diffusion curves for oil-immersed resistances, are next dealt with. Certain difficulties for the salesman in tabulating the horse-power and resistance sizes are referred to, and the necessity of higher standard temperature-rises is urged. Unbalanced rotor currents and the methods of calculating suitable resistance values are dealt with, and the accuracy which can be obtained on test is shown. Eddy-current, rheostatic stator, auto-transformer and star-delta starters are referred to, and the entirely different type of current and torque curves obtained are shown. Methods of rapidly determining the starter proportions for various duties are given. Certain points regarding the rating of auto-transformers are raised, and a test showing the effect of unequal tap voltages is given

The efficiency of overlapping joints in copper and aluminium busbar conductors

Abstract: The object of the investigation was to determine the amount of the contact resistance between the surfaces of an overlapping joint in rectangular straight busbar conductors of copper and aluminium under various conditions of surface preparation and pressure, and hence to ascertain the efficiency of such joints. An efficient joint was regarded as one in which the total resistance, which includes effects due to stream-line distortion, does not exceed that of an equal length of continuous conductor on either side of the joint. As a preliminary it was necessary to determine and allow for the effect of stream-line distortion under various conditions of overlap. Generally full-sized models were investigated, and the two surfaces with varying amounts of overlap were pressed together by means of an hydraulic ram or by bolts or special clamps. Numerical results are deduced and tables and graphs given for the contact resistance per unit of surface area under various conditions of pressure and amount of overlap, and after various modes of cleaning and preparing the surface, including the interposition of tinfoil or the soldering of the two bars together. A series of graphs are given which show the efficiency of copper and aluminium joints in terms of the above defined standard for a wide range of values of busbar sizes and for any value of specific contact resistance likely to occur under practical conditions, and hence the amount of overlap required to obtain an efficient joint. The efficiency of making joints by means of clamps and bolts was also considered and the pressure occurring at such joints estimated by comparison of the contact resistance obtained when the same joint was held together in the hydraulic press. The effect of modifying the numbers, size and position of the bolts used was also investigated.

Test-results obtained from a three-phase shunt commutator motor

Abstract: The subject-matter of this paper consists of the results of tests on a three-phase shunt commutator motor manufactured by the Swedish General Electric Company. There are three main divisions:—(a) The operation of the motor, as normally supplied by the manufacturers, with symmetrical brush-gear setting.(b) The operation of the motor with variable brush-gear setting.(c) That portion of the theory, with the vector diagrams, necessary for understanding the operation of the motor.

High-speed wireless telegraphy

Abstract: This paper consists in the main of a record of experimental progress directed towards the mechanicalization of wireless telegraphy in as simple and portable a form as possible.One of the great difficulties in a modern war on a large scale was shown to be that of providing at short notice the trained personnel necessary for the manipulation of scientific and highly technical apparatus. Simplicity in handling such apparatus is therefore a matter of great importance.Experimental work has now arrived at a point at which it has been shown by practical experience that apparatus can be produced which, in the hands of operators of average ability, enables wireless telegraphy to be carried on by mechanical operations both in transmission and in reception, either by itself or in continuation of a line system; and this at all speeds up to the maximum of Wheatstone apparatus.

Electro-farming, or the applications of electricity to agriculture

Abstract: The term "electro-farming" is defined, and the importance of agriculture as the world's largest industry is emphasized. Based on experimental work conducted on a commercial scale, the author considers that the highest efficiency in farming can be reached only by electrical methods. The applications of electricity on the farm are many and are shown in an exhaustive table under the following classification:-(1) Uses in the farm buildings, (2) on the farm land, (3) crop treatment, (4) in the farm house. The farmer is really a manufacturer, but his present methods are based on abundant cheap labour and unlimited time. Before the engineer can apply modern manufacturing principles he must understand the technicalities of farming. Data are given to show that the farm load is an attractive one from the central station standpoint, and a simple distribution system is outlined. Restricted maximum demand is advocated as a basis for power rates, as it encourages the extended use of electricity. Simple and robust design is essential in all farm machinery. The use of electric motors is discussed and tables are given of motor sizes required for various duties and of the work done per unit consumed. Inexpensive and simple arrangements of portable motors are described. In the Appendix, which follows the classification stated above, examples are given of the profitable use of electric light, e. g. in avoiding milk loss, increasing winter egg production, etc. A suitable wiring system is described. Other farm-building uses of electricity considered in some detail are milk sterilization, and the incubation and rearing of poultry. Reference is made to the weather reports issued by wireless from the Meteorological Office. Irrigation is considered, and the present position of electric ploughing is discussed at length. Single-rope haulage is the most suitable method at present available and is adapted for use in small fields. In the case of

Phantom telephone circuits, and combined telegraph and telephone circuits, worked at audio frequencies

Abstract: The superposing of additional circuits on telegraph and telephone conductors, so as to obtain two or more channels of independent communication from the same physical circuit, now occupies a very important place in the design of such circuits. The loading of telephone circuits, combined with the use of thermionic amplifiers in them, now renders it possible to provide efficient long-distance telephonic communication?including the provision of phantom circuits?on small-gauge conductors in underground cables carrying a large number of circuits. The provision of these cables involves the gradual replacement of overhead open telephone circuits in a large measure, and constitutes a revolution in modern circuit provision. The object of this paper is to review the present position of the art as applied to superposed circuits worked at audio frequencies. The theory of simultaneous telegraph and telephone working on the same circuit is first dealt with, and the limitations of this method of working are stated. The comparative efficiency of side and phantom circuits is then dealt with, and the principal problems arising from the loading and relaying by amplifiers of phantom circuits are outlined. The solution of the problems is indicated. Finally an appendix is given, dealing with the derivation of formulae for the calculation of electric wave filters, for application to the circuits dealt with.

Protective apparatus for turbo-generators

Abstract: The paper deals briefly with the causes of breakdowns in turbo-alternators, particularly in the stator winding, and discusses means for limiting damage in case breakdowns do occur. Some of the systems of alternator protection which are at present in use are dealt with briefly and their advantages and disadvantages pointed out, particularly as regards protection for faults between turns of the same phase winding. A new system of protection is proposed which has the advantage of protection against faults between turns. The system has been tried out in the factory, but the author recommends that it be tested under actual operating conditions. An investigation has been made of the phenomena occurring when the field circuit is opened, and it is shown that it is permissible to open the main field circuit without any discharge resistance. Different methods at present in use of opening the main or exciter field are discussed, and it is shown that they unnecessarily prolong the discharge of the field and the destructive effect of the fault currents. It is therefore recommended that a simple circuit breaker without discharge resistance and operated directly by the fault-current relays be provided in the main field circuit.

An investigation of transmitting aerial resistances

Abstract: This paper, as its title implies, deals with the subject of the resistance of transmitting aerials.It describes an attempt which has been made to reduce to a minimum the energy losses in the neighbourhood of: such aerials, which are known to be very inefficient.It is shown that favourable results are obtained by interposing a screen of wires between the aerial and earth, and using this screen as a counterpoise earth instead of using the usual buried earth.The losses in such a system are analysed and it is shown that a large fraction of the earth losses are eliminated whensuch a screen is used.Formulae are devised for predetermining the resistance of screened aerials, in terms of the conductivity of the earth and the surface dielectric loss.Such aerials have the drawback that the wires of the screen itself may form an oscillating system, and under certain conditions oscillations are set up in this system which waste energy. Methods of dealing with and eliminating these losses are discussed.Finally, the method of screening is compared with Alexanderson's multiple-earth system, and it is concluded that a combination of the two methods should give the ibest results.

Telephone line work in the United States

Abstract: The object of this paper is to place before members of the Institution observations made during a trip to the United States upon some of the practices adopted in line construction work. The treatment is mainly descriptive and, in general, no attempt is made to estimate the advantages or disadvantages of American telephone methods in comparison with those adopted in other countries.

Directional transmission of electromagnetic waves for navigational purposes

Abstract: The paper criticizes the differences between directional transmission and reception as used in navigation and describes in particular a number of directional transmission methods which have been developed by the Royal Air Force. These methods include several in which the determination of a bearing depends on the timing of the moment of equality of signals as heard in the receiver, and also include a new class of directional transmitters in which a characteristic of the radiation, such as a particular wave-length, is alloted to each direction in azimuth.

The thermionic triode as rectifier

Abstract: The performances of the triode rectifier arrangements used in wireless telegraphy, with signals of various strengths, and with or without a superposed local heterodyne oscillation, are investigated theoretically and experimentally. The theoretical results throughout are illustrated numerically by reference to a certain well-known pattern of triode, and in the more fundamental cases the theoretical deductions are compared with precise experimental measurements made at low frequency. The agreement is found to be very close. Experimental curves for this pattern of triode are given also in cases where it has not seemed practicable to evolve theoretical formulae.

Induction-type synchronous motors

Abstract: The induction-type synchronous motor is a polyphase motor and has a cylindrical rotor. It is started as an induction motor of the slip-ring type and synchronizes itself when direct current is switched into the rotor.The characteristics and construction of this type of motor are compared with those of other polyphase motors.The self-synchronizing feature of this type of machine is discussed in detail so as to present a full physical conception of the phenomena which occur, and the subject is illustrated by means of oscillograms.A mathematical expression is deduced for the limiting conditions beyond which synchronizing is uncertain.The considerations which determine the selection of the excitation voltage are discussed, as well as the method of control and other practical points affecting the installation of these machines.

Some notes, on the design of liquid rheostats

Abstract: The paper has for its object the more definite determination of the constants of the liquid rheostat, and the more exact prediction of its performance. First, the phenomena occurring in such a rheostat under a wide variety of circumstances are considered, a distinction being drawn between the behaviour of direct and of alternating currents in the apparatus. Tests are described which bear out and amplify the conclusions, many of which are characterized by the employment, with impunity, of current densities many times larger than those generally considered to fall within the limits of safety. Representative designs of rheostats, for both small and large horse-powers, are described and criticized. A mathematical method is developed for conveniently arriving at the capacity of a given liquid starter, from thermal considerations, which form the deciding factor in this case. A more elaborate calculation is also given, capable of more general application. The effecting of artificial cooling for liquid controllers is dealt with, descriptions of the pipe-cooled and the adjustable weir types being included. The functions of a liquid slip-regulator are analysed and the usual pattern is reviewed, the conclusion being reached that it could be simplified with considerable advantage. Finally, suggestions are made with regard to specifications for governing the design and construction of this apparatus.

The effect of heat on the electric strength of some commercial insulating materials

Abstract: Of the three principal constituents of electrical machinery the greatest improvements in the future are to be looked for in the insulating materials. A knowledge of the effect of heat upon the electrical and mechanical characteristics of insulating materials is of the greatest practical importance. An investigation has been carried out to ascertain the variation in electric strength between temperatures of 30° C. and 100° C. of many of the solid insulating materials at present employed in the manufacture of electrical machinery and apparatus. These tests show that in the case of some of the materials used at present the breakdown voltage at 100° C. may be only one-half that at 30° C. Except in the case of certain mica products the electric strength of all the materials was found to decrease appreciably with increase in temperature. Consequently, all insulating materials should be judged and used in accordance with the breakdown voltage at 100° C. and not on the results of tests carried out at air temperature. Further research work would appear desirable: (a) to investigate other materials on the lines of these tests, and also the effect of heat on the mechanical properties; and (b) to develop new insulating materials with better electrical and mechanical properties at temperatures likely to be attained in modern electrical machinery.