ANOTHER TYPE OF TESLA INDUCTION MOTOR.
It will have been gathered by all who are interested in the advance of
the electrical arts, and who follow carefully, step by step, the work of
pioneers, that Mr. Tesla has been foremost to utilize inductive effects
in permanently closed circuits, in the operation of alternating motors.
In this chapter one simple type of such a motor is described and
illustrated, which will serve as an exemplification of the principle.
Let it be assumed that an ordinary alternating current generator is
connected up in a circuit of practically no self-induction, such, for
example, as a circuit containing incandescent lamps only. On the
operation of the machine, alternating currents will be developed in the
circuit, and the phases of these currents will theoretically coincide
with the phases of the impressed electromotive force. Such currents may
be regarded and designated as the "unretarded currents."
It will be understood, of course, that in practice there is always more
or less self-induction in the circuit, which modifies to a corresponding
extent these conditions; but for convenience this may be disregarded in
the consideration of the principle of operation, since the same laws
apply. Assume next that a path of currents be formed across any two
points of the above circuit, consisting, for example, of the primary of
an induction device. The phases of the currents passing through the
primary, owing to the self-induction of the same, will not coincide with
the phases of the impressed electromotive force, but will lag behind,
such lag being directly proportional to the self-induction and inversely
proportional to the resistance of the said coil. The insertion of this
coil will also cause a lagging or retardation of the currents traversing
and delivered by the generator behind the impressed electromotive force,
such lag being the mean or resultant of the lag of the current through
the primary alone and of the "unretarded current" in the entire working
circuit. Next consider the conditions imposed by the association in
inductive relation with the primary coil, of a secondary coil. The
current generated in the secondary coil will react upon the primary
current, modifying the retardation of the same, according to the amount
of self-induction and resistance in the secondary circuit. If the
secondary circuit has but little self-induction--as, for instance, when
it contains incandescent lamps only--it will increase the actual
difference of phase between its own and the primary current, first, by
diminishing the lag between the primary current and the impressed
electromotive force, and, second, by its own lag or retardation behind
the impressed electromotive force. On the other hand, if the secondary
circuit have a high self-induction, its lag behind the current in the
primary is directly increased, while it will be still further increased
if the primary have a very low self-induction. The better results are
obtained when the primary has a low self-induction.
[Illustration: FIG. 75.]
[Illustration: FIG. 76.]
Fig. 75 is a diagram of a Tesla motor embodying this principle. Fig. 76
is a similar diagram of a modification of the same. In Fig. 75 let A
designate the field-magnet of a motor which, as in all these motors, is
built up of sections or plates. B C are polar projections upon which the
coils are wound. Upon one pair of these poles, as C, are wound primary
coils D, which are directly connected to the circuit of an alternating
current generator G. On the same poles are also wound secondary coils F,
either side by side or over or under the primary coils, and these are
connected with other coils E, which surround the poles B B. The
currents in both primary and secondary coils in such a motor will be
retarded or will lag behind the impressed electromotive force; but to
secure a proper difference in phase between the primary and secondary
currents themselves, Mr. Tesla increases the resistance of the circuit
of the secondary and reduces as much as practicable its self-induction.
This is done by using for the secondary circuit, particularly in the
coils E, wire of comparatively small diameter and having but few turns
around the cores; or by using some conductor of higher specific
resistance, such as German silver; or by introducing at some point in
the secondary circuit an artificial resistance R. Thus the
self-induction of the secondary is kept down and its resistance
increased, with the result of decreasing the lag between the impressed
electro-motive force and the current in the primary coils and increasing
the difference of phase between the primary and secondary currents.
In the disposition shown in Fig. 76, the lag in the secondary is
increased by increasing the self-induction of that circuit, while the
increasing tendency of the primary to lag is counteracted by inserting
therein a dead resistance. The primary coils D in this case have a low
self-induction and high resistance, while the coils E F, included in the
secondary circuit, have a high self-induction and low resistance. This
may be done by the proper winding of the coils; or in the circuit
including the secondary coils E F, we may introduce a self-induction
coil S, while in the primary circuit from the generator G and including
coils D, there may be inserted a dead resistance R. By this means the
difference of phase between the primary and secondary is increased. It
is evident that both means of increasing the difference of
phase--namely, by the special winding as well as by the supplemental or
external inductive and dead resistance--may be employed conjointly.
In the operation of this motor the current impulses in the primary coils
induce currents in the secondary coils, and by the conjoint action of
the two the points of greatest magnetic attraction are shifted or
rotated.
In practice it is found desirable to wind the armature with closed coils
in which currents are induced by the action thereon of the primaries.