Fig. 88. Plan of Power House.
Railway generators or dynamos for direct current are usually built with compound-wound fields, so that, as the load increases, they will automatically raise the voltage at their terminals to compensate for the drop in the feeders and to maintain a constant potential at the cars. Thus, if the line loss on a system is 10 per cent, or 50 volts at full load, the generators will be provided with shunt fields of sufficient strength to give 500 volts at no load, and with series field coils which will add to the field strength enough to give 550 volts at full load. The amount of “compounding”—which is the term applied to this method of increasing voltage—may be any amount within reasonable limits. The pressure maintained at different companies’ electric-railway power houses varies, but is usually between 500 and 600 volts.
Alternating-Current Generators. Alternating-current generators used for generating alternating current to be distributed at high tension, are generally constructed to give a three-phase current at 25 cycles per second. The voltage of these alternating-current generators is sometimes the voltage at which the power is to be transmitted, if the distances are not too great. A number of stations have alternating-current generators giving 6,600 volts at their terminals, which is a voltage well adapted to high-tension distribution within the limits of a large city. However, generators giving 11,000 volts at their terminals are now becoming common. For higher voltages than this, it is considered necessary to use step-up transformers, in order to raise the voltage to the proper pressure for transmission over long distances. In such cases there is no object in having a high generator voltage. At such stations the voltage of the generators adopted may be anything desired, and it varies according to the ideas of the constructing engineer. Voltages of 400, 1,000, and 2,300 are among those in most common use.
Double-Current Generators. Double-current generators are sometimes used, which generators will give direct current at a commutator at one end of the armature for use on a 500-volt direct-current distribution system supplying the trolley direct. The other end of the armature has collector rings from which the three-phase alternating current is obtained, which can be taken to step-up transformers and raised to a sufficient pressure, for high-tension transmission to substations at distant parts of the road. The same generator can therefore be used on both the direct-current and the high-tension alternating-current distribution.
General Plan of Power Stations. The general plan of an electric-railway power station is usually such that the building can be extended and more boilers, engines and generators added without disturbing the symmetrical design of the station. Thus, the boilers and engines are placed as in [Fig. 88], in parallel rows, although almost invariably in different rooms separated by a fire wall. By adding to the row of engines and to the row of boilers, the station capacity can be increased. Other arrangements are sometimes required by circumstances; but this is the most common arrangement and gives the greatest capacity with the minimum amount of steam piping. Large stations are sometimes constructed with a boiler room of several floors and with boilers on each floor, in order to save ground space and bring the boilers near to the large engine units so that there will not be an excessive amount of steam piping.
Fig. 89a. G. E. Circuit Breaker.
Switchboards. Direct-current stations have switchboards, which may be considered under two general classes—generator boards and feeder boards. Each board consists of panels.
Generator D. C. Panels. The generator panel usually contains an automatic circuit breaker which will open the main circuit to the generator in case of an overload due to a short circuit. These circuit breakers consist of a coil in the main circuit, which acts upon a solenoid. When the current in the coil exceeds a certain amount, the solenoid is drawn in, and a trigger is tripped which allows the circuit breaker to fly open under the pressure of a spring. In the General Electric circuit breaker, the main contact is made by heavy copper jaws, but the last breaking of the contact is made between points which are under the influence of a magnetic field. This magnetic field blows out the heavy arc that would otherwise be established. On the I-T-E, the Westinghouse and most other types of circuit breaker, the breaking of the contact takes place between carbon points, which are not so readily destroyed by an arc as are copper contacts, and which are more cheaply renewed. The main contact through the circuit breaker, in either type, is made between copper jaws of sufficient cross-section for carrying the current without heating. These jaws open before the current is finally broken by the smaller contacts which take the final arc.