POLYPHASE CURRENTS.

17

These motors may be operated in three different ways : 1. Bythe alternate currents of the source only. 2. By a combined ac-tion of these and of induced currents. 3. By the joint action ofalternate and continuous currents.

The simplest form of a synchronous motor is obtained by wind-ing a laminated ring provided with pole projections with fourcoils, and connecting the same in the manner before indicated.An iron disc having a segment cut away on each side may be used

Fin 10.

as an armature. Such a motor is shown in Fig. 9. The discbeing arranged to rotate freely within the ring in close proximityto the projections, it is evident that as the poles are shifted itwill, owing to its tendency to place itself in such a position as toembrace the greatest number of the lines of force, closely followthe movement of the poles, and its motion will be synchronouswith that of the armature of the generator; that is, in the peculiardisposition shown in Fig. 9, in which the armature produces byone revolution two current impulses in each of the circuits. Itis evident that if, by one revolution of the armature, a greaternumber of impulses is produced, the speed of the motor will becorrespondingly increased. Considering that the attraction ex-erted upon the disc is greatest when the same is in close proximityto the poles, it follows that such a motor will maintain exactlythe same speed at all loads within the limits of its capacity.

To facilitate the starting, the disc may be provided with a coilclosed upon itself. The advantage secured by such a coil is evi-dent. On the start the currents set up in the coil strongly ener-