saving the cost of, space occupied by, and losses in, the latter. Motors are also operated directly at this voltage in order to drive arc-lighting dynamos or other apparatus. The generators GG are connected in parallel to the bus bars, B, B, B, through switches and oil circuit breakers (Fig. 172) or fuses CC. Means are also provided for bringing each machine into synchronism with those already running, before it is connected to the main bus bars, the usual indicating lamp or other device being employed for the purpose. The field magnets of all the generators are supplied in parallel from a set of direct current exciters, EE, in the ordinary manner. It is well to have a storage battery in parallel with the exciters in order to increase the reliability of the field excitation. The strengths of the fields and therefore E.M.F's of the generators are independently regulated by means of a rheostat, R, placed in each field circuit, which may be operated electrically. From the main bus bars, feeders FF lead out through oil circuit breakers or fuses, D, and run to the sub-station or stations from which the energy is to be distributed. The drop on the feeders is either 5 or 10 per cent, giving 6300 or 6000 volts at the substation. These feeders are usually underground cables of the form described in the chapter on Underground Conductors. After the feeders enter the sub-station, they again pass through fuses or oil circuit breakers, KK, and then to the high tension bus bars, HHH, from which the primary circuits of the static transformers, S, are supplied through oil circuit breakers, LL. In these transformers the energy is stepped down from 6300 or 6000 to about 165 volts, and is then fed into rotary converters, R, which change it into direct current at about 270 volts. The ratio of conversion from single- or two-phase to direct current is 1: √2 1:1.41, since the latter corresponds to the maximum value of the former (p. 114). The three-phase E.M.F. is √32 times that of a single-phase E.M.F. obtained from the same rotary converter (p. 145). Hence the ratio of conversion from three-phase to direct current is √3 ÷ 2: √2 = .613 : 1 1:1.63 with a sine wave of E.M.F., and is not capable of much change, as stated on page 97. On account of the latter fact, the regulation of the direct current voltage is effected by means of induction regulators I of the form shown in Fig. 167 inserted in = These the secondary circuits of the stepdown transformers S. regulators are capable of raising or lowering by 30 volts, or about 10 per cent, the pressure produced by the rotary converters, which is 270 volts, hence the range of regulation is from 240 to 300 volts. Each rotary, R, of 1000 k.w. capacity is provided with its own regulator rated at about 130 k.w. The static transformers, PS, of 350 k.w. capacity each, are connected in groups of three with no cross connections between the groups on the secondary side; i.e., there are no low-tension alternating current bus bars. Each group of three transformers feeds one rotary converter, to which the secondaries of the group are directly wired, there being no means of switching any rotary from one to another group of transformers. This arrangement is adopted to avoid the use of switches in the low-tension, heavy current alternating circuits, as well as to avoid the transference of stray direct currents from one rotary converter to another through the alternating current connections, which transference is likely to take place when two or more rotaries are electrically connected to the same low-tension alternating current bus bars. In most cases the rotary converters are operated as six-phase machines. The purpose of this arrangement is to reduce the copper losses in the rotary armatures, and consequently raise the capacity of the rotaries with the same temperature rise. As rotary converters have no field distortion their capacity is determined solely by their heating limit and their commutating ability, so that any means of reducing the heating correspondingly increases the number of kilowatts which a given machine will convert, provided there is sufficient field strength to reverse the currents in the armature coils as their commutator segments pass under the brushes. It has been shown* that a machine which will deliver 100 kilowatts without overheating when driven. mechanically as a generator, will deliver 131 kilowatts with the same temperature rise when run as a three-phase rotary converter, and will deliver 194 kilowatts when run as a six-phase rotary converter, other conditions remaining the same. This is allowing for the internal losses of the converter, and assuming that the impressed E.M.F. is in phase with the counter E.M.F. of the machine. If wattless currents are not carefully balanced out, or *Steinmetz, The Electrical World, Dec. 17, 1898. are used for purposes of regulation, the output of the machine with either number of phases falls off somewhat, but the six phases still show about the same advantage over the three phases. Assuming a wattless component amounting to 30 per cent of the total alternating current input of the machine, the same 100-k.w. generator would deliver 122 kilowatts as a three-phase rotary and 167 as a six-phase rotary. In addition to the reduction of the heating, the six-phase arrangement distributes the heating much more uniformly around the armature of the rotary than do three phases, with which the heating is rather badly concentrated in a few coils. While the term "six-phase" conveys an idea of considerable complexity, it requires but very little modification of the usual three-phase arrangement. The generators and high-tension transmission lines are three-phase, the six phases being derived from the step-down transformers, which are of the usual single-phase type, and three in number, but have each two electrically independent secondaries. These six secondaries of the step-down transformers are connected in two separate A arrangements, one secondary of each transformer being reversed with respect to the other, thus producing currents differing 180° in phase. The connections of the three transformers are shown in Fig. 171, the primaries P being fed from the high-tension three-phase circuit arranged in A, the potential being either 6300 or 6000 volts, depending upon whether a drop of 5 or 10 per cent occurs on the feeders. Special taps are led from the primary coil to enable either voltage to be used. Each transformer has two secondaries which are connected in double A fashion, as represented, to give six-phase currents at about 165 volts which are carried through the induction regulator I. From the latter the six-phase energy passes to the rotary converter, R in which it is changed to direct current energy at about 270 volts. This energy is supplied to the outer bus bars M and T, from which it is distributed through feeders N to the ordinary three-wire network of conductors similar to that described on page 103. The drop on the feeders, mains and housewiring reduces the potential to about 230 volts, so that the lamps receive about 115 volts on each side of the system. Storage batteries are commonly employed in connection with these systems, being placed in the sub-stations, and charged from the direct current side of the rotary connectors as represented at A in Fig. 171. The batteries serve to subdivide the potential for the three-wire system, and enable a more uniform load to be maintained on the generating plant, feeders, transformers, converters, etc., or allow them to be shut down temporarily. It affords also a means of starting up the rotary converters from their direct current sides. The connections of a somewhat different distributing plant are shown in Fig. 173, which represents the arthe North rangement at Avenue Sub-station of the Chicago Edison Company. In this case two three-phase rotary converters of 100 k.w. each are used, and the primary pressure is 4500 volts; otherwise the installation is similar to that described in connection with Fig. 171. Substantially the same system of three-phase transmission and direct current distribution is employed for electric railways, the only im- contact portant differences being the facts that the direct current is produced 550 or 600 volts and the distribution is by twowire instead of by three-wire circuits. In spite of the step-down transformation and the con Fig. 172. Oil Circuit Breakers. |