Fig. 2,779.—Sectional elevation of portable outdoor transformer type sub-station. The high voltage switching and protective apparatus is mounted, out of the way, on the roof of the car, but is operated from the switchboard with a standard remote control handle. The transformer is carried directly over the truck at the uncovered end of the car and the low-tension leads from it run in conduit beneath the floor and up into the cab, (which contains the converter and switchboard) to the converter. The positive lead runs through a conduit and ends in a terminal on the roof. The energy thus makes a complete circuit of the car leaving at a point close to that at which it entered. The low pressure alternating current as well as the direct current positive leads are carried below the car floor in iron conduit supported from the channel frame. The field wires are carried through this conduit to the rheostat. Wiring for the lights is arranged to supply two, 5 light clusters. One is fed with the 600 volt direct current and the other with 420 volt alternating current. All lighting conductors are carried in metal moulding carried between the flanges of the channel iron ribs. High wiring is carried entirely on the roof of the car where it is entirely out of the way and where the operator cannot come in contact with it. The switchboard should be of the utmost simplicity. Usually the negative and equalizer switches, and the field break-up switch are mounted on the frame of the converter. The double throw switch for starting and running the converter can be mounted under the floor of the car and operated by handle at the switchboard. The rheostat can be mounted back of the switchboard on brackets bolted to the car super-structure. The switchboard need only carry the positive knife switch and circuit breaker, and the alternating current ammeter, voltmeter and power factor meter. Sometimes a watthour meter is added. The positive lead is brought out through a conduit on the roof of the car and is arranged for bolting to the positive feeder. The negative and equalizer terminals are located at the cab end of the car and are arranged so that connection can be easily made from them to the ground and, if necessary, to an equalizer circuit. There is usually a sliding door at each end of the cab and two windows on each side. Above the doors, transoms, extending the width of the cab, are arranged to drop so that a current of air will circulate through the cab under the roof, carrying out the heated air. There are also several ventilating holes beneath the converter in the floor of the car. These provisions insure a constant circulation of air through the car which carries away all heated air.
Ques. State the effect of strengthening converter field.
Ans. A leading current is set up which gives a rise of voltage in the reactance coil.
Hence when a heavy current passes through the series coil of a compound wound converter and tends to produce a leading current, the reactance coil will balance it, and improve the power factor of the whole line.
Fig. 2,780.—Westinghouse 300 kw. converter in portable sub-station.
Portable Sub-Stations.—A portable sub-station constitutes a spare equipment for practically any number of permanent sub-stations and renders unnecessary the installation of spare equipment in each.
It can be used to increase the capacity of a permanent sub-station when the load is unusually heavy, or to provide service while a permanent sub-station is being overhauled or rebuilt.