Showing papers in "Journal of the Institution of Electrical Engineers in 1962"

The permanent-magnet synchronous motor

Abstract: A synchronous motor, constructed by modifying an induction motor by fitting a permanent magnet inside the squirrel-cage rotor, is very useful for drives where synchronous operation is required or where the drop in speed of an induction motor is too great. The paper presents a theory of operation of this type of motor and suggests an empirical design method. The analysis is based on the 2-axis theory of the salient-pole synchronous motor and shows how the properties of the permanent magnet can be taken into account. The principal difficulty in designing the motor arises from the demagnetizing effect on the magnet of the heavy currents which flow when the motor is started up and synchronized. The analysis consists of two parts. First, the performance characteristics are related to the direct-axis and quadrature-axis characteristics, which show separately the variation of flux and magnetomotive force on each axis. A method is then developed for calculating the axis characteristics from the B/H curve of the permanent magnet and the dimensions of the machine. In this way the performance can be predetermined. The method is verified by tests on an existing machine. Finally, two other designs, which could be expected to give improved performance, have been worked out.

A reference distortion system using modulated noise

Abstract: A reference distortion device has been developed, initially for use in the subjective assessment of pulse-code-modulated telephony links. Noise is amplitude-modulated by the speech signal, and then added to the signal in variable proportion, to produce controllable distortion comparable with the effect of quantizing noise.

Arc interruption and thermal reignition

Abstract: For a short period following are interruption, reignition can be produced by a continuous energy process. The paper develops the theory of measurements of are resistance, power loss and reignition voltage during this thermal reignition period of recovery. Experiments are described in which these measurements are made during the first 100μs of the free recovery of ares in air burning freely between 4mm diameter carbon electrodes. Direct-current pulses of 150 ms duration and of 10, 20 and 40A are used.The results give variations in resistance, power loss and reignition voltage which in the main can be explained by thermal considerations, although the first few microseconds of recovery are difficult to explain in a like manner. A clear distinction is drawn between power-loss voltage and reignition voltage.Assuming a linear relationship between the internal energy and the electrical conductivity of a thermal gas, a linearized theory of interruption is developed which appears to have advantages over previous interruption theories and shows reasonable agreement with the experimental results.

Diversity reception and automatic phase correction

Abstract: The interference between the direct ray and a ground-reflected ray gives rise at the receiver of a communication link to a sinusoidal field pattern in the vertical plane consisting of nodes and maxima. The position and pattern wavelength of this field depend on the receiver and transmitter heights and spacing and on propagation conditions via an effective curvature parameter, C. This parameter varies in time, and is the cause of most fading at a single antenna. Its upper limit, which may be as high as 2.5 for a small percentage of the time, determines minimum antenna heights for line-of-sight transmission under extreme conditions. C is normally about ¾. Its range of variation determines the optimum spacing of a pair of diversity antennae, and suitable design formulae are given. Experiments using a pair of mirrors, a varying transmitter frequency, and photographs of oscillograph traces indicate an extreme lower value of C over water of −1.5. A moving-film display shows that conditions can vary rapidly from minute to minute, although at other times the display is steady for hours at a time.An automatic phasing junction has been designed for insertion in the feed from two diversity antennae, the drive for the phasing element being taken from the receiver. A combined signal nowhere smaller than the greater of the received signals is obtained from the combining unit, and when the diversity spacing is chosen with regard to the extreme values of curvature obtained on the path, an excellent overall response is ensured. Some preliminary figures are given for the performance of an improved combining network and phase-control apparatus operating in conjunction with a height-diversity microwave system over a 69-mile overwater path.

A new linear oscillating motor

Abstract: The paper describes the performance of a simple motor which produces directly a linear oscillatory motion. The amplitude range is of the order of 5–50 cm, and at an amplitude of 25 cm, frequencies of 10–12 c/s have been achieved. The moving member is a conducting metal ring sliding over a laminated bar of iron. The coils are at both ends at a distance apart greater than the oscillatory amplitude. The motor is operated from a single-phase 50 c/s supply.An analysis of the induction mechanism is attempted and several practical machines are described.

Asynchronous performance characteristics of turbo-generators

Abstract: The paper presents a theoretical analysis of the asynchronous operation of turbo-generators, and equations are developed for predicting performance characteristics for two important cases in which the field circuit is either open-circuited or short-circuited through a resistor. The basic problem is that of calculating the effects of slip-frequency eddy currents induced in the surface of the highly saturated rotor forging, and the results of an approximate non-linear analysis are used for this purpose.Results of asynchronous tests conducted on a 60MW generator at Marchwood power station in March, 1960, are presented, and the accuracy and validity of the theoretical treatment are examined by comparison with these and earlier test results. The agreement obtained is considered to be sufficiently encouraging to warrant the use of the method of calculation as a basis for assessing the permissible asynchronous loading of turbo-generators.

Magnetostriction in grain-oriented silicon-iron

Abstract: An apparatus is described with which magnetostrictive changes of length in strip samples can be measured with accuracy under direct and alternating conditions at various flux densities. The factors controlling the magnetostrictive pattern in grain-oriented silicon-iron have been studied. It has been found that considerable reduction in magnetostriction can be obtained by heat treatment at a suitable temperature, the time at temperature and the rate of cooling both having an effect on the ultimate value of the magnetostriction. With material heat-treated under optimum conditions, cooling under an applied load and magnetic annealing give no subsequent reduction. Tensile stress applied along the direction of flux produces a marked decrease in magnetostriction, as does, to a smaller extent, compressive stress applied at right angles to the sheet. It is found that a compressive stress of only 3001bf/in2 along the direction of flux is very deleterious, so that in transformer construction the use of wavy or undulating sheets, which on clamping would have such induced stress components, should be avoided. Bending and handling have little effect on the material, provided that it is restored to its original form and that the applied stresses have caused no permanent deformation. The importance of these measurements to the transformer engineer is discussed.

Pole-change motors using phase-mixing techniques

Abstract: The paper outlines the development of a new type of pole-change motor giving three or five speeds. The development is an extension of the principles of `phase-mixing?, which were explained in an earlier paper which described a continuously-variable-speed brushless induction motor. In the earlier machine a fraction of the periphery could not be used owing to the continuously variable pole-pitch requirement, and the stator windings in the active arc were fed from phase-shifting regulators. The present paper shows how acceptance of a finite number of specific speeds may be exploited to enable the whole of the periphery of the machine to be used, and, at the same time, to replace the phase-shifting regulators by switches. The complexity of the switch arrangement depends on the number of speeds required. The paper concludes by describing 2-speed machines based on the phase-mixing principle in which the number of terminals on the machine is reduced to six. Theoretical investigation of the harmonic effects is considered, and this material is supported by experimental results obtained from several machines. High values of copper utilization and low harmonic content are shown to be possible with the phase-mixing technique.

The 400 kV grid system for England and Wales

Abstract: The paper describes, in Part 1, a study of probable long-term requirements of the main transmission system in England and Wales. The possibility of continuing to extend at 275 kV is compared with the alternative of introducing a higher voltage, either 400 or 500 kV, and the reasons for deciding to adopt 400 kV as the next step in transmission voltage are given. The research work associated with the development of overhead lines suitable for operating at 400 kV in England and Wales is described in Part 2 of the paper, while Part 3 gives a summary of the main features of the design of the 400 kV substation equipment, overhead lines and cables which will be used to implement the proposals.

An oscillating synchronous linear machine

Abstract: The paper describes the development of a new type of a.c. generator in which the moving member travels between a d.c. pole structure in a straight line with reciprocating motion. Tha induced e.m.f. appears in the moving member, which consists of a single loop of conducting material which embraces and moves along the core. The core carries stationary coils which experience an induced alternating e.m.f. by transformer action from the moving loop. These coils are not associated with the air-gap between the d.c. poles and are more easily cooled than the windings of a rotary machine. The loop, which is confined to the air-gap, can be run at high temperature, since it carries no insulation. The machine can be used as a generator to convert mechanical power supplied directly from a piston. It can also be used as a synchronous motor for such purposes as the driving of compressors without cranks; when used in this manner the machine is self-starting. Essentially a single-phase machine, the equivalent of multi-polar rotary machines can be constructed which will generate e.m.f.'s at 50c/swith mechanical oscillations at frequencies lower than 3 000 per minute. The construction of the machine is simple, since the core is made up entirely from rectangular stampings, while the windings consist of four transformer-type coils and a conducting loop. An experimental machine is described, development of which consisted largely of testing devices to minimize the internal impedance Test results are given. The paper includes a proposed design for a 33 kVA generator.

A diversity combiner giving total power transfer

Abstract: A method is described of combining two or more carrier signals, which may be from a space-diversity aerial system, in such a way that the output power is always the sum of the individual signal powers. This gives the optimum signal/noise ratio under the commonly assumed Rayleigh distributed fading conditions. The control signals are taken from the i.f. amplifier of the main receiver; no extra detecting facilities are therefore required. However, a standby receiver can be incorporated, and the combiner can easily be adapted to act as a change-over switch, if required.

A new method of ionospheric forecasting

Abstract: The older method of ionospheric predictions based on solar and ionospheric indices is described and the difficulties are illustrated. A new method which has been in use for three years and which makes no use of solar indices is described. The method also provides world charts of measured values of fF2 which form part of a permanent volume covering the whole range of ionospheric variation.

Progress in aircraft aerials

Abstract: Aircraft aerials must satisfy severe environmental and aerodynamic conditions, besides meeting particular electrical requirements. These conditions are discussed in the paper, and solutions shown to some of the problems. In particular, aerials for h.f. and v.h.f. communication and v.h.f. navigation systems are described. Problems associated with aerial systems for automatic landing equipments are discussed, and a number of solutions are proposed. At this stage, virtually the beginning of multiple systems, it is improbable that the solutions proposed will be final.The basic problems of v.h.f. aerial siting have not been discussed fully, but references are given to other papers on this aspect of the subject. No attempt has been made to describe aerials for l.f. navigation aids, or centimetric aerial systems whose requirements can generally be considered apart from the aircraft.