POLYPHASE CURRENTS.

09

cured in position to surround both the armature and the polesc o'. The armature is detachable from its shaft, the latter beingpassed through the armature after it lias been inserted in posi-tion. The operation of this form of motor is the same in prin-ciple as that previously described and needs no further explana-tion.

One of the most important features in alternating currentmotors is, however, that they should be adapted to and capableof running efficiently on the alternating circuits in present use,in which almost without exception the generators yield a veryhigh number of alternations. Such a motor, of the type underconsideration, Mr. Tesla has designed by a development of theprinciple of the motor shown in Fig. 56, making a multipolarmotor, which is illustrated in Fig. 57. In the construction of

this motor he employs an annular magnetic frame .r, with in-wardly-extending ribs or projections k, the ends of which allbend or turn in one direction and are generally shaped to con-form to the curved surface of the armature. Coils f f are woundfrom one part k to the one next adjacent, the ends or loops ofeach coil or group of wires being carried over toward the shaft,so as to form U-shaped groups of convolutions at each end of thearmature. The pole-pieces C C', being substantially concentricwitli the armature, form ledges, along which the coils are laidand should project to some extent beyond the the coils, as shown.The cylindrical or drum armature d is of the same constructionas in the other motors described, and is mounted to rotate withinthe annular frame j and between the [J-shaped ends or bends of