The second system, used for series distribution on alternating-current circuits consists of a constant-potential transformer, stepping down the line voltage to that required for the total number of lamps on the system, allowing 83 volts for each lamp, and in series with the lamps is a reactive coil, the reactance of which is automatically regu lated, as the load is increased or decreased, in order to keep the current in the line con stant. Fig. 47 shows such a regulator and Fig. 48 shows this regu lator connected in circuit. The inductance is varied by the move ment of the coil so as to include more or less iron in the magnetic circuit. Since the inductance in series with the lamps is high on light loads, the power factor is greatly reduced as in the constant-current transformer; and the circuits should, preferably, be run fully loaded.
60 to 65 lamps on a circuit is the usual maximum limit.
While used primarily for arc-light circuits, the same systems, designed for lower currents, are very readily applied to series incan descent systems.
The introduction of certain flaming or luminous arcs requiring direct current for their operation has led to the use of the mercury arc rectifier in connection with series circuits on alternating-current systems. A constant-current transformer is used to regulate for the proper constant current in its second ary winding, and this secondary current is rectified by means of the mercury arc rectifier for the lamp circuit. In the recent outfits the rectifier tubes are immersed in oil for cooling. While this rectifier was first introduced for the operation of luminous arc lamps, there is no reason why it should not be used with any series lamp requiring direct current, provided the system is designed for the current taken by such lamps. With this system any commer cial frequency may be used. Sets are constructed for 25-, 50-, and 75-light circuits. They have a combined effi ciency, transformer and rectifier tube, of 85% to 90%, and operate at a power factor of from 65% to 70%. Fig. 49 gives a diagram of the circuit and rectifier connections used with a single tube outfit.
leading to the bus bars at the station, or to the secondaries of con stant-potential transformers. Fig. 50 shows a diagram of ten lamps connected in parallel. The current delivered by the machine de pends directly on the number of lamps connected in service, the vol tage of the system being kept constant.
Inasmuch as the flow of current in a conductor is always accom panied by a fall of potential equal to the product of the current flow ing into the resist ance of the con ductor, the lamps at the end of the system shown will not have as high a voltage impressed upon them as those nearer the ma chine. This drop in potential is the most seri ous obstacle that we have to over come in multiple systems, and various schemes have been adopted to aid in this regulation. The systems may be classified as: 1. Cylindrical conductors, parallel feeding.
2. Conical 3. Cylindrical anti-parallel feeding.
4. Conical " " " In the cylindrical conductor, parallel-feeding system, the con ductors, A, B, C, D, Fig. 50, are of the same size throughout and are fed at the same end by the generator. The voltage is a minimum at the lamps E and a maximum at the lamps F; the value of the voltage at any lamp being readily calculated.
By a conical or tapering conductor is meant a conductor whose diameter is so proportioned throughout its length that the current, divided by the cross-section, or the current density, is a constant quantity. Such a conductor is approximated in practice by using smaller sizes of wire as the current in the lines becomes less.
In an anti-parallel system, the current is fed to the lamps from opposite ends of the system, as shown in Fig. 51.
Multiple-Wire Systems. In order to take advantage of a higher voltage for distribution of power to the lighting circuits, three- and five-wire systems have been introduced, the three-wire system being used to a very large extent. In this system, three conductors are used, the voltage from each outside conductor to the middle neutral conductor being the same as for a simple parallel system. Fig. 52 gives a diagram of this. By this system the amount of copper required for a giv en number of lamps is from five-sixteenths to three eighths of the amount required for a two-wire dis tribution, depending on the size of the neutral con ductor. The saving of copper together with the disadvantages of the system is more fully treated in the paper on "Power Transmission."