Let A B represent the poles of an alternating current motor, of which C is the armature wound with coils D, closed upon themselves, as is now the general practice in motors of this kind. The poles A, which alternate with poles B, are wound with coils of ordinary or coarse wire E in such direction as to make them of alternate north and south polarity, as indicated in the diagram by the characters N S. Over these coils, or in other inductive relation to the same, are wound long fine-wire coils F F, and in the same direction throughout as the coils E. These coils are secondaries, in which currents of very high potential are induced. All the coils E in one series are connected, and all the secondaries F in another.
Fig. 91.
On the intermediate poles B are wound fine-wire energizing coils G, which are connected in series with one another, and also with the series of secondary coils F, the direction of winding being such that a current-impulse induced from the primary coils E imparts the same magnetism to the poles B as that produced in poles A by the primary impulse. This condition is indicated by the characters N' S'.
In the circuit formed by the two sets of coils F and G is introduced a condenser H; otherwise this circuit is closed upon itself, while the free ends of the circuit of coils E are connected to a source of alternating currents. As the condenser capacity which is needed in any particular motor of this kind is dependent upon the rate of alternation or the potential, or both, its size or cost, as before explained, may be brought within economical limits for use with the ordinary circuits if the potential of the secondary circuit in the motor be sufficiently high. By giving to the condenser proper values, any desired difference of phase between the primary and secondary energizing circuits may be obtained.
CHAPTER XXIII.
Tesla Polyphase Transformer.
Applying the polyphase principle to the construction of transformers as well to the motors already noticed, Mr. Tesla has invented some very interesting forms, which he considers free from the defects of earlier and, at present, more familiar forms. In these transformers he provides a series of inducing coils and corresponding induced coils, which are generally wound upon a core closed upon itself, usually a ring of laminated iron.