. Electric traction and transmission engineering . eouter conductors, it is assumed that the cross sections of 202 TRACTION AND TRANSMISSION. the conductors will be so chosen that the loss per foot isthe same, PIz-, ii^ ^^^h conductor. The maximum voltagebetween any two conductors is assumed the same in allcases because its value determines the capacity and cost ofeach insulator. The center wire of the three-wire quarter-phase system, however, does not need to be so well insu-lated as the outside wires, and to this extent the abovecomparison is unfair to this system. Considering, however,that
. Electric traction and transmission engineering . eouter conductors, it is assumed that the cross sections of 202 TRACTION AND TRANSMISSION. the conductors will be so chosen that the loss per foot isthe same, PIz-, ii^ ^^^h conductor. The maximum voltagebetween any two conductors is assumed the same in allcases because its value determines the capacity and cost ofeach insulator. The center wire of the three-wire quarter-phase system, however, does not need to be so well insu-lated as the outside wires, and to this extent the abovecomparison is unfair to this system. Considering, however,that the conductor expense considerably exceeds the in-sulator expense in most cases, this system does not need tobe considered in comparison with the three-phase system,which, as shown in the table, is superior to all systems usingalternating currents. RELATIVE WEIGHTS OF CONDUCTORS. System. Two wires: Direct current. .. Single-phase Three wires: Three-phase Quarter-phase:Right-hand wire Center wire Left-hand Four wires: Quarter-phase. . .. TRANSMISSION LINES. 203 69. Frequency. — The Standardization Rules of theA. I. E. E. give 25 and 60 as standard frequencies. Fortransmission hnes supplying converting substations one orthe other should be used. The weights and costs of 60-cycle 50 40 \ 30 \ ?o ^ 0 Uy ^ ~~-—-i 5 CYl JLES to 1000 2000 CAPACITY IN 88. 3000 transformers are less than those for 25 cycles and the oper-ating efficiencies of the former are greater than those ofthe latter. The differences are not very great, as will beseen from the curves in Figs. 88 and 89, which refer .99 98 o ^.97 .96 .95 6( ► CYC LES ^ ^^^ ^ — 2S > CYC LES y y ^ / 1000 2000 CAPACITY IN KILOWATTS. Fig. 89. 3000 33,000-volt, plain steel, air-blast transformers. Inductionmotors for higher frequencies are also cheaper, but operateat lower power factors. At the lower frequency it is lessdifficult to operate generators and other synchronous appa- 204 TRACTION AND TRA
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