54

INVENTIONS OF NIKOLA TESLA.

tor will run witli only one of the circuits b or c connected withthe main or generator circuit, or the two circuits may be con-nected in series. This latter plan is preferable when a currenthaving a high number of alternations per unit of time is em-ployed to drive the motor. In such case the starting of themotor is more difficult, and the dead and inductive resistancesmust take up a considerable proportion of the electromotiveforce of the circuits. Generally the conditions are so adjustedthat the electromotive force used in each of the motor circuits isthat which is required to operate the motor when its circuits arein series. The plan followed in this case is illustrated in Fig.39. In this instance the motor has twelve poles and the arma-ture has polar projections d wound with closed coils f. Theswitch used is of substantially the same construction as thatshown in the previous figure. There are, however, live contacts,designated as 5, 6, 7, 8, and 9. The motor-circuits b c, which in-clude alternate field-coils, are connected to the terminals in thefollowing order: One end of circuit c is connected to contact 9and to contact 5 through a dead resistance, i. One terminal ofcircuit b is connected to contact 7 and to contact 6 through aself-induction coil, j. The opposite terminals of both circuits areconnected to contact 8.

One of the levers, as f, of the switch is made with an exten-sion, f, or otherwise, so as to cover both contacts 5 and 6 whenshifted into the position to start the motor. It will be observedthat when in this position and with lever f' on contact 8 the cur-rent divides between the two circuits b c, which from their dif-ference in electrical character produce a progression of the polesthat starts the motor hi rotation. When the motor has attainedthe proper speed, the switch is shifted so that the levers coverthe contacts 7 and 9, thereby connecting circuits b and c in se-ries. It is found that by this disposition the motor is maintainedin rotation in synchronism with the generator. This principleof operation, which consists in converting by a change of con-nections or otherwise a double-circuit motor, or one operating bya progressive shifting of the poles, into an ordinary synchroniz-ing motor may be carried out in many other ways. For instance,instead of using the switch shown in the previous figures, wemay use a temporary ground circuit between the generator andmotor, in order to start the motor, in substantially the mannerindicated in Fig. 40. Let g in this figure represent an ordinary