. Electric traction and transmission engineering . Alternating-current Motors. — The advantages in-cident to the use of the alternating-current motor arethe lower first cost of the low-tension distribution system, TYPES AND PERFORMANCE CURVES OF MOTORS. 29 the substitution of the simple and efficient transformersubstation for the converter substation, and the reductionof the cost of operation. It is not advisable to employhigh trolley potentials in cities or densely populated sub-urban districts, but for trunk Hne operation, requiringan infrequent service, economical operation dictates hightro
. Electric traction and transmission engineering . Alternating-current Motors. — The advantages in-cident to the use of the alternating-current motor arethe lower first cost of the low-tension distribution system, TYPES AND PERFORMANCE CURVES OF MOTORS. 29 the substitution of the simple and efficient transformersubstation for the converter substation, and the reductionof the cost of operation. It is not advisable to employhigh trolley potentials in cities or densely populated sub-urban districts, but for trunk Hne operation, requiringan infrequent service, economical operation dictates hightrolley potentials; in many cases transformation to a lowermotor voltage is effected by transformers on the cars orlocomotives. In alternating-current traction, controller sys-tems may be utilized which do not entail the large energylosses incident to starting direct-current motors. Three-phase generation is more economical than single-phase generation of E,, The current from the formersystem may be converted into a two-phase current by means. TROLLEYS Fig. 12. of a Scott transformer, each phase of which supplies single-phase current to the motors on one side of the 12 shows the scheme of connections. A single-phaseload drawn from one phase of a large polyphase systemunbalances the system for use otherwise. Phase convertersor modifiers are used to restore this balance. There are several types of alternating-current single-phase railway motors at present in operation, but of thesethe compensated series motor is the only one used in thiscountry. 30 TRACTION AND TRANSMISSION. Series Motors, — Consider a direct-current armaturemounted within a single-phase alternating magnetic field,as in Fig. 13. When the armature is stationary an electro-motive force will be induced in the armature turns, dueto the alternating flux which passes between the fieldpoles. The greatest ^s will be induced in the turnsperpendicular to the field axis, since these turns link with
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