These notes on Dynamo Design are not intended to supersede the more complete handbooks on the special branch of electrical engineering of which this is only a part. In the forthcoming new (seventh) edition of the Author's Dynamo-Electric Machinery many other examples of design will be found. The present short work, intended primarily for the Author's own students, is purposely confined to continuous-current generators. As it will be used by engineers chiefly in Great Britain, in her Colonies, and in the United States, the calculations and data have been expressed in inch measures. But the Author has adopted throughout the decimal subdivision of the inch; small lengths being given in mils, and small areas of cross-section in square mils, or, sometimes also, in circular mils, to suit American practice.

In the section on Armature Winding Schemes special attention is given to series-parallel windings, and to the doctrine of the "equivalent ring."

Dynamo Design is an art not to be acquired without practice and experience; and like other branches of engineering design it reposes upon certain fundamental scientific principles. These can be laid down definitely, and taught with precision. But in the application of them in design to meet the varied needs of an ever expanding industry there is wide scope for choice and for individual preference. Time and experience have indeed taught the general lines along which dynamo design must proceed. But no one yet ever designed a successful dynamo by mere rules. A grasp of principles, electrical and mechanical; a knowledge of machinery and its construction; an acquaintance with the successful forms that exist; and a perception of the reasons why they are successful  these and many other things are requisite in the designer who is to produce machines that will hold their own in the competition of to-day

In his treatise on Dynamo-Electric Machines the author has treated the subject broadly, and with some reference not only to the historical evolution of the various types of machine, but also to the abstract theory which must be acquired if a thorough grasp of the subject is to be attained. But there are many engineers who have followed some course of instruction in the theoretical part of the sciences of magnetism and electricity, who yet have no knowledge of the way in which that theory is applied in dynamo design. The immediate end and aim, therefore, of the present book is to give to such a working insight into the procedure of dynamo design as carried out in recent years for the construction of continuous current dynamos of modern type. Considerations of space, and the desire not to enter too far upon the topics treated of in the author's other works, Dynamo-Electric Machinery, Polyphase Electric Currents, The Electromagnet and Elementary Lessons in Electricity and Magnetism, have determined him to confine the present publication strictly to the design of Continuous Current Generators, and of these to treat only of the principal kind, leaving aside small machines, and special types such as arc-lighting machines. These are treated of in his larger work on Dynamo-Electric Machinery, To that work those readers are referred who, for want of previous general acquaintance with the subject, find the considerations laid down in the following chapters to assume points that are not familiar to them. The Author assumes, indeed, that his reader has some acquaintance with such matters as elementary magnetism and the magnetic properties of iron, permeability and hysteresis. He also assumes a general knowledge of electric conduction and insulation, and of the elements of electrical measurement.

The present work does hot go into the theories of armature winding, nor into the practical modes of carrying it out in the shop. For these also he refers the reader to his larger treatise.

After all, however fundamental the necessity of scientific principles, sound theories, and rules derived from the experience and practice of others, dynamo design remains an art. It needs the eye to see, as well as the mind to understand.

All dynamo design is based upon a knowledge of the magnetic properties of iron and steel. During the past twenty years thousands of brands of various qualities have been subjected to test as to their magnetic properties by scientific authorities, and there exists an extensive literature on the subject. The principal thing to know is the appropriate density of the magnetic flux, and the amount of excitation required to produce it, in any given specimen. In this book the letter N is used to denote the magnetic flux, that is to say, the total number of magnetic lines, carried by any iron core. If the area of section of this core is denoted by the letter A, the density of the magnetic flux will be equal to N -f- A. When the sectional area is given in square inches the letter used to denote the flux-density (i.e. the number of magnetic lines per square inch) is B. In cases where the area is given in square centimetres, the letter used for the flux-density will be B. The magnetizing forces required to excite any required flux-density in the magnetic circuit of a dynamo are obtained by causing an electric current to circulate around the iron core. It is found that the magnetizing force thus produced is proportional both to the amount of current (i.e. the number of amperes) so flowing, and to the number of times it circulates around the core (i.e. the number of turns in the magnetizing coil). In other words, the magnetizing force is proportional to the number of ampere-turns. For brevity we sometimes describe the total number of ampere-turns of circulation of current around a core as "the excitation." It goes without saying that the higher the flux-density required, and the greater the length of the iron through which the magnetic flux is to be driven, the greater is the amount of excitation needed. To drive a magnetic flux through air requires a much greater amount of excitation than is required for an equal flux-density through an equal length of iron. The coefficients used in calculating air-gaps are mentioned on p. 28.

In treatises on theoretical magnetism it- is usual to describe magnetizing forces in terms of a theoretical unit (derived from the Centimetre-gramme-second system), which is such that if applied to an air-core one centimetre in length it would produce a flux-density of one line per square centimetre.


DOWNLOAD FREE ELECTRICAL ENGINEERING BOOKS:
Design of Dynamos