Electrical Engineering Laboratory Notes; For Seniors in E. E. First Term Work (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1906 Excerpt: ... ELECTRICAL RELATIONS IN THREE-PHASE CIRCUITS. This exercise is intended for an experimental study of relations in polyphase circuits and also for familiarizing the students with the methods of measuring current, pressures and power in such circuits. The fundamental facts about polyphase currents are supposed to be known, but it may be well to recall here the origin of these systems and to make a short review of their applications. A simple alternating current is entirely satisfactory for lighting, but up to this time the inventors have been unable to construct a satisfactory single-phase motor for ordinary commercial work. The singlephase commutator-motor is well adapted for railway work only, and the single-phase induction motor, although used to some extent, is not a very good machine except in small sizes. But if the actions of two or more alternating currents differing in phase are combined in one motor, tne motor can be very much improved, because then at no time is the power equal to zero in all component circuits or phases at once. Such' combinations of several alternating currents mutually interlinked into one system, are called polyphase systems, and their origin was a demand for a good alternating current motor. The motors used on polyphase circuits are: induction motors, based on the familiar principle of the revolving magnetic field, and synchronous motors, which are polyphase alternators operated as motors. Of all possible combinations of alternating currents only two simplest ones came into general use: the two-phase system, consisting of two currents with a phase displacement of 900, or quarter of a period (Fig. 1), and the three-phase system consisting of three currents symmetrically displaced in phase by 1200, or one-third of a period (Fig....