. Revised and enlarged ed. of the science of railways . y placed outside of themotor and connected with it by suitable this case, connection is made to the windingsof the armature by brushes resting on rings con-nected to the winding. The mechanical sim-plicity of the induction motor in its absence of commutator andusually of all openor moving contactsis particularly de-sirable in railwaywork. The induc-tion motor has,however, met butlittle progress inthe practical rail-way field. The rea-son for this is foundin its electricalcharacteristics. Itrequires for its suc-cessful operationtw
. Revised and enlarged ed. of the science of railways . y placed outside of themotor and connected with it by suitable this case, connection is made to the windingsof the armature by brushes resting on rings con-nected to the winding. The mechanical sim-plicity of the induction motor in its absence of commutator andusually of all openor moving contactsis particularly de-sirable in railwaywork. The induc-tion motor has,however, met butlittle progress inthe practical rail-way field. The rea-son for this is foundin its electricalcharacteristics. Itrequires for its suc-cessful operationtwo-phase or three-phase currents. There aremethods of operating an induction motor bysingle-phase currents, but in general they addconsiderably to the quantity of apparatus, reducethe output of the motor, lower the efficiency andgreatly impair the performance at low currents of more than one-phase requiremore than two conductors, so there must be atleast two conductors besides the rail return forsupplying the required currents to a car. This. Alternating Current Induction MotorComplete. ELECTRICITY A^ A MOTIVE POWER. 445 requirement has been especially disagreeable instreet car work in cities and cannot be conven-ientl}^ met on suburban lines where the cars areto run through the city streets. In cross-countryand heavy work these objections do not hold. Adouble trolley line can be adopted with success,except, possibly, at very high speed. If the cur-rent is conducted by additional rails, it is, ofcourse, necessary to add two rails. This increasesthe difficulties at crossings and in repairs to thetrack. The theoretical and practical evolution of thealternating current motor has been slow anddifficult in comparison with direct current appar-atus. Much labor has been expended upon asingle-phase motor Avith no practical outcome asfar as railway service is concerned. The induc-tion motor is essentially a constant speed tendency is to run at a certain definite spee
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