MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.
As has been pointed out elsewhere, the lag or retardation 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
differences 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 have led Mr. Tesla to the invention of a motor having the
following general characteristics: The coils which are included in that
energizing circuit which is to have the higher self-induction are made
of coarse wire, or a conductor of relatively low resistance, and with
the greatest possible length or number of turns. In the other set of
coils a comparatively few turns of finer wire are used, or a wire of
higher resistance. Furthermore, in order to approximate the magnetic
quantities of the poles excited by these coils, Mr. Tesla employs in the
self-induction circuit cores much longer than those in the other or
resistance circuit.
Fig. 65 is a part sectional view of the motor at right angles to the
shaft. Fig. 66 is a diagram of the field circuits.
In Fig. 66, 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. There
are, therefore, used a long length or a large number of turns of coarse
wire in forming the coils of this circuit. For the circuit B, a smaller
conductor is employed, or a conductor of a higher resistance than
copper, such as German silver or iron, and the coils are wound with
fewer turns. In applying these coils to a motor, Mr. Tesla builds up a
field-magnet of plates C, of iron and steel, secured together in the
usual manner by bolts D. Each plate is formed with four (more or less)
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 Mr. Tesla
prefers to wind 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.
[Illustration: FIG. 65.]
[Illustration: FIG. 66.]
An important result secured by the construction shown here is that these
coils which are designed to have the higher self-induction are almost
completely surrounded by iron, and that the retardation is thus very
materially increased.