TRANSFORMER. This transformer is particularly well adapted to all installations where the use of oil is not permis sible, or where, on account of the size of the units, self-cooling is impracticable and the cost of water required by water-cooled transformers is prohibitive. Their high efficiency at all loads, low fire risk, economy of floor space and other desirable features have resulted in a large and increasing demand for this apparatus for those installations in the foundry and arsenal in which large amounts of power must be con tinuously handled.
Electric transmission lines for moderate dis tances are usually o. rated at the following potentials: 2,200, 6,4 1 1 11,000, 16,500, 22,000 and 33,000 volts. The standard lines of air blast transformers are designed for these voltages and on account of the drop in potential on the transmission lines, taps are brought out from the high tension winding at such points that the transformers can be operated on poten tials approximately 3%, 63 and 10 per cent lower than their normal voltage. This arrange
ment of taps on the high tension winding per mits the use of duplicate transformers at either end of transmission lines and at intermediate points, which is of great advantage in many cases, particularly on large systems.
Air-blast transformers are cooled by a forced blast of air delivered by a blower. One blower usually supplies all of the transformers in any particular station; but it is strongly ' recom mended that a duplicate blower equipment be added. This arrangement permits one unit to be held in reserve. The transformers are gen erally placed above an air-chamber in which a pressure is maintained slightly above that of the surrounding and the blower may deliver air directly into this chamber, or, if it is more convenient, the blower can be located at a dis tance from the transformers, feeding into a conduit which leads to the air-chamber.