Power apparatus and electric machines
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- Electrical and electronic engineering [28]
- Power systems and applications [28]
- Power apparatus and electric machines [28]
- Switchgear [16]
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- 1941 [28]
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- J.W. Gibson [4]
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The paper is divided into three main sections. The first of these deals with the method of manufacture and formation of the discs, and with the forward and reverse voltage/current and voltage/resistance characteristics and the effects of temperature variations, ageing and capacitance. This section includes a brief account of a theory of the operation of the rectifier.The second section describes the design of actual rectifiers for resistive or inductive loads and also a new method of design for battery-charging rectifiers, the mathematical theory of which is given in an appendix. A new method of construction is described which enables the output of a rectifier to be increased.In the final section, a description is given of some of the applications of the rectifier, including the use of oil-cooling for large outputs.
The paper deals in detail only with the two types of metal rectifier which have found general application in industry, namely the copper-oxide and selenium rectifiers.The history of these is given briefly, with particular reference to the main developments which have brought them to their present state of efficiency. This section leads up to modern methods of manufacture.The fundamental direct-current electrical characteristics, upon which the performance depends, and the manner in which they are influenced by various factors, are described.The simple basic theory which governs all types of rectifier is discussed with reference to metal rectifiers in relation to their chemical and physical forms.Finally, the method by which the correct working conditions are deduced from the characteristics is described.
The paper is divided into three main sections. The first of these deals with the method of manufacture and formation of the discs, and with the forward and reverse voltage/current and voltage/resistance characteristics and the effects of temperature variations, ageing and capacitance. This section includes a brief account of a theory of the operation of the rectifier.The second section describes the design of actual rectifiers for resistive or inductive loads and also a new method of design for battery-charging rectifiers, the mathematical theory of which is given in an appendix. A new method of construction is described which enables the output of a rectifier to be increased.In the final section, a description is given of some of the applications of the rectifier, including the use of oil-cooling for large outputs.
In this paper the author advances the view that leakage protection problems must be considered upon a more practical basis, necessitating some departure from the present practice of almost purely academic treatment of the subject. Piecemeal considerations of separate sections of the whole earthing problem, earth electrodes, fuses, earth-continuity conductors, testing, etc., result in obscuration of the question, as efficient leakage protection entails the perfect functioning of each and every section of the protective system, at the same time and without reasonable chance of failure. The view is also expressed that in the search for immunity from shock, the equally important matter of protection against fire risk has been overlooked.The point has been reached where the increasing expense and decreasing security of earthing has led to some consideration of alternatives, notably multiple neutral earthing, with the connection of the consumer's casings to the neutral line, and the use of earth-leakage circuit-breakers. The rural area now represents the field of greatest difficulty combined with greatest urgency in the matter of protective questions, but the author discusses protective problems as a whole. The view is advanced that the only logical way to limit leakage voltages is by means of devices responsive to predetermined voltage limits, such as are now employed in the largest rural areas in Europe.The problems are examined in as simple a manner as possible, without recourse to complicated mathematical formulae or very involved calculations, for two reasons : first, because the divergence between theory and practice is more marked in earthing problems than in any other aspect of engineering practice, and the problems are not therefore amenable to calculation or paper consideration, and second, because these problems are normally encountered by mains and installation engineers in the course of their normal duties, and if the industry is to benefit from the practical experiences of these men over the past 20 years then any discussion of the questions involved must be placed upon the most practical basis possible.
The measurement of pole-face losses in laminated and solid materials has been made by a novel method employing a homopolar machine in which the ordinary core losses do not occur. The “retardation method” has been employed to measure the losses, using a stroboscope to measure the speeds and a tuning-fork-controlled siphon recorder to measure the time-intervals. Using this method, the hysteresis loss can be separated from the eddy-current loss with considerable accuracy since the observations can be continued to very low speeds. The investigation covers a large range of variables, and empirical formulae have been developed for the hysteresis and eddy-current losses separately. It has been found that the hysteresis loss is independent of the thickness of the laminations, and is the same in both solid and laminated poles, while the eddy-current loss is proportional to the first power of the thickness. The results show that the slot width and air gap should be treated as separate variables. The exponent of B (the average magnetic induction in the air gap) has been found to vary considerably with the air gap and the slot pitch; the loss formulae therefore contain a variable exponent for B. Since the pole-face loss is, apart from a very small friction loss, the only loss involved, the accuracy of measurement should be of a high order. The results are based on a large number of observations, but only a few of the curves obtained are included in the paper.
