Fig. 2,411.—Westinghouse air cooled choke coil particularly suitable for outdoor use. The method of mounting is such that insulation for any desired voltage is readily obtained with the same type of porcelain, and mounting in any position is possible. The coil is a helix of aluminum rod, about 15 inches in diameter and containing about 30 turns. Bracing clamps are provided to give mechanical strength to the helix, and the rod used is of sufficient diameter to carry 200 amperes. The coil is supported on two insulating columns made up of porcelain insulators, which, except for the end pieces, are interchangeable. The number of insulators used in the columns depends on the voltage of the circuit in which the coil is to be used. The apparatus can be mounted in any position convenient for the wiring, on floor, wall, or ceiling. It is intended principally for the protection of transformers. Where greater reactance than is afforded by a single coil is desired on the higher voltage circuits, it is recommended that two or more coils be connected in series, one coil being used for each 22,000 line voltage. This coil should not be used for generators. The insulating columns are supported on substantial cast iron blocks on wooden bases.
Ques. How are choke coils cooled?
Ans. By air, or by oil.
Ques. For what service are oil cooled choke coils used?
Ans. On circuits of pressures above 25,000 volts, choke coils immersed in oil, as are transformer coils, have advantages in that the coil is amply insulated not only from the ground but against side flash, and that copper of comparatively small section may be used without undue heating.
Fig. 2,412.—Westinghouse air cooled choke coil, for voltages of from 2,200 to 110,000. In construction, the coils are made of aluminum rod wound into a helix of about 15 inches in diameter and having 20 turns. The helix is supported on two insulators. For mechanical reasons it is necessary to have the aluminum rod of sufficient size to secure rigidity, consequently every coil has a capacity of 200 amperes and may be used on any circuit up to that capacity. The coils are insulated according to the standard practice for disconnecting switches, the insulators being mounted on wooden pins supported by a wooden base. This apparatus can be mounted in any position. The wiring of a station or sub-station is facilitated because the protection may be placed so as to simply form part of the wiring. The coils are symmetrical so that it is immaterial which end is connected to the line or to the apparatus.
"Static" Interrupters.—A static interrupter is a combination of a choke coil and a condenser, the two being mounted together and placed in a tank and oil insulated.
It is used on high pressure circuits and its function is to so delay the erroneously called "static" wave in its entry into the transformer coil, that a considerable portion of the latter will become charged before the terminal will have reached full pressure.