POLYPHASE CURRENTS.

43

other ends are both connected to a ring, a. Collecting-brushesV c' bear on the rings 1> c, respectively, and conductors l l con-vey the currents therefrom through the coils e and e of the gen-erator. 1/ is a common return-wire to brush a'. Two indepen-dent circuits are thus formed, one including coils c of the exciterand e e of the generator, the other coils c' of the exciter and e'k' of the generator. It results from this that the operation ofthe exciter produces a progressive movement of the magneticpoles of the annular field-core of the generator, the shifting orrotary movement of the poles being synchronous with the rota-tion of the exciter armature. Considering the operative con-ditions of a system thus established, it will be found that whenthe exciter is driven so as to energize the field of the generator,the armature of the latter, if left free to turn, would rotate at aspeed practically the same as that of the exciter. If under suchconditions the coils f f' of the generator armature be closedupon themselves or short-circuited, no currents, at least theoreti-cally, will be generated in these armature coils. In practicethe presence of slight currents is observed, the existence of whichis attributable to more or less pronounced fluctuations in the in-tensity of the magnetic poles of the generator ring. So, if thearmature-coils f f' be closed through the motor, the latter willnot be turned as long as the movement of the generator armatureis synchronous with that of the exciter or of the magnetic polesof its field. If, on the contrary, the speed of the generator arm-ature be in any way checked, so that the shifting or rotation ofthe poles of the field becomes relatively .more rapid, currents willbe induced in the armature coils. This obviously follows fromthe passing of the lines of force across the armature conductors.The greater the speed of rotation of the magnetic poles relativelyto that of the armature the more rapidly the currents developedin the coils of the latter will follow one another, and the morerapidly the motor will revolve in response thereto, and this con-tinues until the armature generator is stopped entirely, as by abrake, when the motor, if properly constructed, runs at the speedwith which the magnetic poles of the generator rotate.

The effective strength of the currents developed in the arma-ture coils of the generator is dependent upon the strength of thecurrents energizing the generator and upon the number of rota-tions per unit of time of the magnetic poles of the generator;lienee the speed of the motor armature will depend in all cases