Tesla's Alternating Current Motor

Since Mr. Nikola Tesla’s first work in alternating current motors was published, he has been busily engaged in working out other types, one of which is described below. In Mr. Tesla’s motors, it will be remembered, two or more energising circuits are employed, through which alternating currents differing in phase are passed, with the result of producing a progressive shifting or rotation of the poles or points of maximum effect. Various types of alternating motors have been constructed on this principle, the present being one in which the difference of phase is artificially produced, as, for instance, in cases where the motor-circuits are of different resistance and self-induction, so that the same current divided between them will be retarded in one to a greater extent than in the other, and the requisite phase difference secured in this way.

The lag or rotation of the phases of an alternating current is directly proportional to the self-induction and inversely proportional to the resistance of the circuit through which the current flows. Hence, in order to secure the proper difference of phase between the two motor-circuits, it is desirable to make the self-induction in one much higher and the resistance much lower than the self-induction and resistance, respectively, in the other. At the same time the magnetic quantities of the two poles or sets of poles which the two circuits produce should be approximately equal. These requirements, which Mr. Tesla has found to exist in motors of this kind, have led him to the invention of a motor having the following general characteristics: The coils which are included in that energising circuit which is to have the higher self-induction are made of coarse wire, or a conductor of relatively low resistance and the greatest possible length or number of turns are used. In the other set of coils a comparatively few turns of finer wire, or a wire of higher resistance, is used.

Furthermore, in order to increase the difference of phase in the two circuits, Mr. Tesla provides for the self-induction coils a considerably greater space in order to place a greater amount of copper in these coils, and in addition he envelops these coils with iron so as to increase by these means the self-induction to the greatest possible value, and he calculates the magnetic qualities so that they are practically equal in both sets of magnets.

These features of construction are shown in the engraving Fig. 2, and the principle is illustrated by the diagram Fig. 1.

In Fig. 1, let A represent the coils in one motor-circuit, and B those in the other. The circuit A is to have the higher self-induction. Mr. Tesla therefore uses a long length or a large number of turns of coarse wire in forming the coils of this circuit. For the circuit B he employs a smaller conductor, or a conductor of a higher resistance than copper, such as German silver or iron, and winds the coils with fewer turns. In applying these coils to a motor, a field-magnet is built up of plates C, of iron or steel. Each plate is formed with four long cores, E, around which is a space to receive the coil and an equal number of short projections, F, to receive the coils of the resistance-circuit. The plates are generally annular in shape, having an open space in the centre for receiving the armature G, which is preferably wound with closed coils.

An alternating current divided between the two circuits is retarded as to its phases in the circuit A to a much greater extent than in the circuit B. By reason of the relative sizes and disposition of the cores and coils, the magnetic effect of the poles E and F upon the armature closely approximate.

An important result secured by the construction shown in the motor is, that those coils which are designed to have the higher self-induction are, as above stated, almost completely surrounded by iron, by which the retardation is considerably increased. JAN. 3, 1890.