In [Fig. 89]a is seen a General Electric circuit breaker with the magnetic blow-out coils at the top, the solenoid at the left, and the handle for resetting the circuit breaker at the bottom. The small handle for tripping the circuit breaker, when it is desired to open the circuit by hand, is shown just under the solenoid.
An I-T-E circuit breaker is shown in [Fig. 89]b. This is of the type previously mentioned, in which the break occurs between carbon contacts and there is no magnetic blow-out.
Fig. 89b. I-T-E Circuit Breaker.
In addition to the circuit breaker there is usually an ammeter, to indicate the current passing from the generator; and a rheostat handle, geared to a rheostat back of the board, for cutting in and out more or less resistance in the shunt field coils of the generator so as to reduce or raise the voltage. There is a small switch for opening and closing the circuit through the shunt field coils.
The main leads from the generator pass through two single-pole quick-break knife switches. The most recent practice is to have the switches on the switchboard in only the positive and negative leads from the generator, leaving connection to the equalizer to be made by a switch located on or near the generator. However, all three leads may be taken to the switchboard, and a three-pole knife switch may be used instead of the positive and negative switches spoken of.
In [Fig. 90] is given a simple diagram of the general relative connection of generators and feeders in a direct-current railway power station. It is seen that the generators are connected in parallel across the positive and negative bus bar. There is a third bus bar—called an “equalizing bus”—which connects in parallel the series coils of all the generator fields. The object of this equalizer is to prevent the weakening of the series field of any one generator, so as to allow it to take current and to act as a motor instead of as a generator.
Starting Up a Generator. Suppose that a new generator is to be started up and connected to the bus bars in addition to others already in operation. The engine of that generator is first brought up to speed. The switch controlling the shunt field circuit is then closed, causing current to flow through the shunt fields; and the generator begins to “build up,” its voltage gradually rising until it approximates that upon the bus bars. Before the generator is thrown in parallel with the others by connecting it with the bus bars, it is important that its voltage be nearly the same as that of the bus bars. Otherwise, when connected to the bus bars, it might take more than its share of the load; while, on the other hand, if its voltage were too low, it might act as a motor, taking current from the bus bars. The voltage of the bus bars in a railway station is constantly fluctuating, owing to the varying load and to the fact that generators are often compounded, as before mentioned, in order to compensate for the line loss.
Fig. 90. Connection of Generators and Feeders.