POLYPHASE CURRENTS.

33

tlie collecting-1 trashes a' V o' d', connected by wires g g h h, re-spectively, with the terminals of coils c and d.

Assume, for sake of illustration, that the coils d i> are to re-ceive the maximum and coils c c at the same instant the mini-mum current, so that the polar line may be midway between thecoils d d. The rings a b would therefore be connected to thecontinuous armature-coil at its neutral points with respect to thefield, or the point corresponding with that of the ordinary com-mutator brushes, and between which exists the greatest differ-ence of potential; while rings e d would be connected to twopoints in the coil, between which exists no difference of potential.The best results will be obtained by making these connections atpoints equidistant from one another, as shown. These connec-tions are easiest made by using wires l between the rings and theloops or wires j, connecting the coil i to the segments of thecommutator k. When the converters are made in this manner,it is evident that the phases of the currents in the sections of thegenerator coil will be reproduced in the converter coils. Forexample, after turning through an arc of ninety degrees the con-ductors l l, which before conveyed the maximum current, willreceive the minimum current by reason of the change in theposition of their coils, and it is evident that for the same reasonthe current in these coils has gradually fallen from the maximumto the minimum in passing through the arc of ninety degrees.In this special plan of connections, the rotation of the magneticpoles of the converter will be synchronous with that of thearmature coils of the generator, and the result will be the same,whether the energizing circuits are derivations from a continuousarmature coil or from independent coils, as in Mr. Tesla’sother devices.

In Fig. 25, the brushes m m are shown in dotted lines in theirproper normal position. In practice these brushes may be re-moved from the commutator and the field of the generatorexcited by an external source of current; or the brushes may beallowed to remain on the commutator and to take off a convertedcurrent to excite the field, or to be used for other purposes.

In a certain well-known class of machines known as the “opencircuit,” the armature contains a number of coils the terminals ofwhich connect to commutator segments, the coils being connectedacross the armature in pairs. This type of machine is repre-sented in Fig. 20. In this machine each pair of coils goes