Fig. 791.—Parallel-series system of distribution. It consists of groups of series connected receptive devices, the groups being arranged in the circuit in parallel.
Ques. When is a parallel-series system used?
Ans. When it is desired to operate a number of lamps or motors on a line where voltage is several times that required to operate a single lamp or motor.
The parallel-series system is employed chiefly in the lighting circuit on electric traction lines; here, usually five 110 volt lamps are connected to the source of supply which has a pressure of 550 volts.
Center of Distribution.—It is important to determine the point at which the feeders should be attached to the mains in order to minimize the amount of copper required. The method employed is similar to that used in determining the best location of a power plant as regards amount of copper required. The center of distribution may be called the electrical center of gravity of the system, and is found by separately obtaining the center of gravity of straight sections and then determining the total resultant and point of application of this resultant of the straight sections.
Feeders (feeding cables or conductors) are run from the source of supply to the distributing centers, and, as these feeders are in many cases of considerable length, a substantial loss of pressure generally occurs in them. The pressure at the source of supply, however, is so regulated as to compensate for the drop in the feeders, and the pressure at the distributing centers is thus kept constant; or the same result is obtained by the use of regulating devices in the feeders. The essential condition in most systems is that the pressure at the distributing centers shall be kept practically constant, irrespective of the load.
Edison Three Wire System.—In electric lighting systems used up to about 1897, it was not considered practicable to use incandescent lamps requiring a pressure exceeding 120 volts. This limited the operating voltage of parallel systems, and necessitated the use of conductors of large size and weight, especially where the current had to be transmitted a considerable distance.
The effect of this limiting voltage is more apparent when it is clearly understood that the size of wire required to carry a current depends upon the amperes and not upon the volts.
A wire capable of carrying a current of 10 amperes at 20 volts, can carry 10 amperes at 20,000 volts or any other voltage. Therefore, since the amount of electric energy or power transmitted through a conductor is equal to the amperes multiplied by the volts, it is clear that by increasing the voltage, the power transmitting capacity of a current can be almost indefinitely increased without increasing the size of the conducting wire. This is the reason why considerations of economy dictate the use of the highest voltages possible in long distance transmissions. The voltage of the current is determined, however, by the requirements of the apparatus to be operated.
Incandescent lamps usually require a pressure of 110 volts, and the current required by a 16 candle power lamp at that voltage is about ½ ampere. Therefore if the lamp be designed for a pressure of 220 volts, the current will be reduced to ¼ ampere, and the same size of wire could be used to feed twice as many lamps.