A Short Course In The Testing Of Electrical Machinery For Non-Electrical Students (1911)

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PREFACE IN presenting these brief notes the authors feel that an explanation of their object is necessary. At Columbia University practically all of the engineering students are required to take courses in the electrical laboratories, testing both direct-current and alterna ting-current machinery. Students in Mining, Mechanical, Metallurgical, Chemical, Civil Engineering, etc ., do not have those courses in the theory of electrical machinery, which are really necessary for a proper comprehendon of the machines with which they work in the laboratory it is unreasonable to expect them to consult various text-books to prepare themselves on the theory involved in the tests, and it is with the intention of filling the needs of these men that the notes have been compiIed. Before giving specific directions regarding the test to be performed, a brief analysis of the characteristics of the machine is attempted in so far as is possible in such a limited space the reasons for the behavior of the machine are given. It is, of course, realized that a complete analysis of the different types of machines is impossible and it is questionable whether a com plete analysis would serve the purpose. It has been the inten tion of the writers to present the subject-matter in such a manner that the student not well versed in eIectricaI theory can get the most out of it in the short time allotted to the electrical courses. In some of the tests, methods are described which may not be strictly according to the standard practice if a gain in sim plicity and ease of performance is to be obtained by a sacrifice in accuracy of the test of a few tenths of a per cent, it is thought justifiabIe to use the simpler methodof testing. While the notes are being put into printed form specificdly for our use, they may possibIy be found of use in other schooIs where the conditions are similar to those at Columbia. The authors wish to express their indebtedness to Professor Geo. F. Sever, who first developed the eIectrica1 laboratory work for the non-electrical students at Columbia, and whose original scheduIe of experiments served as a guide in arranging this work also to Mr. F. L. Mason, who has rendered valuable assistance in the preparation of the book. J. H. M. F. W. a. COLUMBIA U N RSITY, . September, rgII. LIST OF D. C. EXPERIMENTS I. I FALL O F P O E N A AL L ON G A CONDUCTOR C ARRYING C URRENT. 2. M A S E O N F A T R VATURE C IRCUI iw T n SHUNT F IELD R ESISTANCES. TESTING OF ELECTRICAL rilCHINE Y DIRECT CURRENTTESTS - c c Fall of Potential along a Conductor Carrying Current. a If an electromotive force is impressed upon a circuit, as for instance a wire, a current of electricity will flow along it. The relation between the current, resistance and difference of potential between any two points on the conductor is given by Ohms law, which expresses the equality of the impressed force and the reacting force. It is found that the reacting force varies directly with the current and this fact may be expressed by the equation, E IR from which tve obtain, I where X-current flowing in amperes E difference of potential in volts, between the hvo points considered R resistance of the conductor in ohms, between the two points considered. It is a fact that all conductors offer more or less resistance to the flow of an electric currcnt and experiment shotvs that for any particular conductor, the resistancc variesdirectIy as its length and inversely as its area of cross-section...