Figs. 2,195 to 2,197.—Diagram showing electric railway system. Three phase current is generated at the main station where it passes to step up transformers to increase the pressure a suitable amount for economical transmission. At various points along the railway line are sub-stations, where the three phase current is reduced in pressure to 500 or 600 volts by step down transformers, and converted into direct current by rotary converters. The relatively low pressure direct current is then conveyed by "feeders" to the rails, this resulting in a considerable saving in copper.
Mixed Current Systems.—It is often desirable to transmit electrical energy in the form of alternating current, and distribute it as direct current or vice versa.
Such systems may be classed as mixed current systems. The usual conversion is from alternating current to direct current because of the saving in copper secured by the use of alternating current in transmission, especially in the case of long distance lines. Such conversion involves the use of a rotary converter, motor generator set, or rectifier, according to the conditions of service.
Fig. 2,198.—Example of converter sub-station, showing the Brooklyn Edison Co. Madison sub-station. The transformers are seen on the left, the converter shown at the right is a Westinghouse synchronous booster rotary converter, consisting of a standard rotary converter in combination with a revolving armature alternator mounted on the same shaft with the converter and having the same number of poles. The function of the machine is to convert and regulate the pressure. By varying the field excitation of the alternator, the A. C. voltage impressed on the rotary converter proper can be increased or decreased as desired. Thus, the D. C. voltage delivered by the converter is varied accordingly. This type of converter is well adapted for any application for which a relatively wide variation, either automatic or non-automatic, in direct current voltage is necessary. Also especially for serving incandescent lighting systems where considerable voltage variation is required for the compensation of drop in long feeders, for operation in parallel with storage batteries and for electrolytic work where extreme variations in voltage are required by changes in the resistance of the electrolytic cells.
The suburban trolley forms a good example of a mixed system, in which alternating current is generated at the central station and transmitted to sub-stations, where it is transformed to low pressure, and converted into direct current for use on the line. Fig. 2,195 shows the interior of a sub-station of this kind.