Heavy Electrical Engineering (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. 1908 Excerpt: ...the dielectric. In this way the required thickness of dielectric is further decreased. By a combination of the above two principles, we may reduce See O'Garman, Jour0. I. K. E., vol. xxx. p. 60S. the necessary thickness of material considerably below that which the curves in Fig. 95 would indicate to be necessary. Lastly, Jona and others have ascertained experimentally that for cables with very small cores at high voltages, the insulation required is, when correctly applied, very appreciably less than that calculated, there being some other phenomena as yet unexplained. For the purposes of the following investigations, the radial thickness of the insulation will be taken from the curves in Fig. 96.0 A minimum radius of curvature of 3 mm will be taken for voltages over 5000, and a thin sheath of lead will, as proposed by O'Gorman and Jona, be placed round the stranded core so that it shall conform to this curvature. At 40 000 volts the diameter of core for low values of the kilowatts transmitted comes out less than this lowest value shown, and in these cases an aluminium or a tubular conductor of the minimum outside diameter might be used. For high voltages it might often be advisable to use an aluminium conductor, since the radial depth of the required insulation decreases rapidly with increasing radius of curvature of the core. Jona has shown that if a stranded core is not covered with a lead sheath, the stress is liable to be increased to 1,2--1,4 times the amount obtaining for a smooth core. To allow for the room taken by this lead sheath, which, however, may be very thin, and also for the fact that the core is composed of stranded wire, the real copper section is increased by one third, and this figure is called the apparent section. Since the cores are...