MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND FIELD.
In this form of motor, Mr. Tesla's object is to design and build
machines wherein the maxima of the magnetic effects of the armature and
field will more nearly coincide than in some of the types previously
under consideration. These types are: First, motors having two or more
energizing circuits of the same electrical character, and in the
operation of which the currents used differ primarily in phase; second,
motors with a plurality of energizing circuits of different electrical
character, in or by means of which the difference of phase is produced
artificially, and, third, motors with a plurality of energizing
circuits, the currents in one being induced from currents in another.
Considering the structural and operative conditions of any one of
them--as, for example, that first named--the armature which is mounted
to rotate in obedience to the co-operative influence or action of the
energizing circuits has coils wound upon it which are closed upon
themselves and in which currents are induced by the energizing-currents
with the object and result of energizing the armature-core; but under
any such conditions as must exist in these motors, it is obvious that a
certain time must elapse between the manifestations of an energizing
current impulse in the field coils, and the corresponding magnetic state
or phase in the armature established by the current induced thereby;
consequently a given magnetic influence or effect in the field which is
the direct result of a primary current impulse will have become more or
less weakened or lost before the corresponding effect in the armature
indirectly produced has reached its maximum. This is a condition
unfavorable to efficient working in certain cases--as, for instance,
when the progress of the resultant poles or points of maximum attraction
is very great, or when a very high number of alternations is
employed--for it is apparent that a stronger tendency to rotation will
be maintained if the maximum magnetic attractions or conditions in both
armature and field coincide, the energy developed by a motor being
measured by the product of the magnetic quantities of the armature and
field.
To secure this coincidence of maximum magnetic effects, Mr. Tesla has
devised various means, as explained below. Fig. 68 is a diagrammatic
illustration of a Tesla motor system in which the alternating currents
proceed from independent sources and differ primarily in phase.
[Illustration: FIG. 68.]
[Illustration: FIG. 69.]
A designates the field-magnet or magnetic frame of the motor; B B,
oppositely located pole-pieces adapted to receive the coils of one
energizing circuit; and C C, similar pole-pieces for the coils of the
other energizing circuit. These circuits are designated, respectively,
by D E, the conductor D'' forming a common return to the generator G.
Between these poles is mounted an armature--for example, a ring or
annular armature, wound with a series of coils F, forming a closed
circuit or circuits. The action or operation of a motor thus constructed
is now well understood. It will be observed, however, that the magnetism
of poles B, for example, established by a current impulse in the coils
thereon, precedes the magnetic effect set up in the armature by the
induced current in coils F. Consequently the mutual attraction between
the armature and field-poles is considerably reduced. The same
conditions will be found to exist if, instead of assuming the poles B or
C as acting independently, we regard the ideal resultant of both acting
together, which is the real condition. To remedy this, the motor field
is constructed with secondary poles B' C', which are situated between
the others. These pole-pieces are wound with coils D' E', the former in
derivation to the coils D, the latter to coils E. The main or primary
coils D and E are wound for a different self-induction from that of the
coils D' and E', the relations being so fixed that if the currents in D
and E differ, for example, by a quarter-phase, the currents in each
secondary coil, as D' E', will differ from those in its appropriate
primary D or E by, say, forty-five degrees, or one-eighth of a period.
Now, assuming that an impulse or alternation in circuit or branch E is
just beginning, while in the branch D it is just falling from maximum,
the conditions are those of a quarter-phase difference. The ideal
resultant of the attractive forces of the two sets of poles B C
therefore may be considered as progressing from poles B to poles C,
while the impulse in E is rising to maximum, and that in D is falling to
zero or minimum. The polarity set up in the armature, however, lags
behind the manifestations of field magnetism, and hence the maximum
points of attraction in armature and field, instead of coinciding, are
angularly displaced. This effect is counteracted by the supplemental
poles B' C'. The magnetic phases of these poles succeed those of poles B
C by the same, or nearly the same, period of time as elapses between the
effect of the poles B C and the corresponding induced effect in the
armature; hence the magnetic conditions of poles B' C' and of the
armature more nearly coincide and a better result is obtained. As poles
B' C' act in conjunction with the poles in the armature established by
poles B C, so in turn poles C B act similarly with the poles set up by
B' C', respectively. Under such conditions the retardation of the
magnetic effect of the armature and that of the secondary poles will
bring the maximum of the two more nearly into coincidence and a
correspondingly stronger torque or magnetic attraction secured.
In such a disposition as is shown in Fig. 68 it will be observed that
as the adjacent pole-pieces of either circuit are of like polarity they
will have a certain weakening effect upon one another. Mr. Tesla
therefore prefers to remove the secondary poles from the direct
influence of the others. This may be done by constructing a motor with
two independent sets of fields, and with either one or two armatures
electrically connected, or by using two armatures and one field. These
modifications are illustrated further on.
[Illustration: FIG. 70.]
[Illustration: FIG. 71.]
Fig. 69 is a diagrammatic illustration of a motor and system in which
the difference of phase is artificially produced. There are two coils D
D in one branch and two coils E E in another branch of the main circuit
from the generator G. These two circuits or branches are of different
self-induction, one, as D, being higher than the other. This is
graphically indicated by making coils D much larger than coils E. By
reason of the difference in the electrical character of the two
circuits, the phases of current in one are retarded to a greater extent
than the other. Let this difference be thirty degrees. A motor thus
constructed will rotate under the action of an alternating current; but
as happens in the case previously described the corresponding magnetic
effects of the armature and field do not coincide owing to the time that
elapses between a given magnetic effect in the armature and the
condition of the field that produces it. The secondary or supplemental
poles B' C' are therefore availed of. There being thirty degrees
difference of phase between the currents in coils D E, the magnetic
effect of poles B' C' should correspond to that produced by a current
differing from the current in coils D or E by fifteen degrees. This we
can attain by winding each supplemental pole B' C' with two coils H H'.
The coils H are included in a derived circuit having the same
self-induction as circuit D, and coils H' in a circuit having the same
self-induction as circuit E, so that if these circuits differ by thirty
degrees the magnetism of poles B' C' will correspond to that produced by
a current differing from that in either D or E by fifteen degrees. This
is true in all other cases. For example, if in Fig. 68 the coils D' E'
be replaced by the coils H H' included in the derived circuits, the
magnetism of the poles B' C' will correspond in effect or phase, if it
may be so termed, to that produced by a current differing from that in
either circuit D or E by forty-five degrees, or one-eighth of a period.
This invention as applied to a derived circuit motor is illustrated in
Figs. 70 and 71. The former is an end view of the motor with the
armature in section and a diagram of connections, and Fig. 71 a vertical
section through the field. These figures are also drawn to show one of
the dispositions of two fields that may be adopted in carrying out the
principle. The poles B B C C are in one field, the remaining poles in
the other. The former are wound with primary coils I J and secondary
coils I' J', the latter with coils K L. The primary coils I J are in
derived circuits, between which, by reason of their different
self-induction, there is a difference of phase, say, of thirty degrees.
The coils I' K are in circuit with one another, as also are coils J' L,
and there should be a difference of phase between the currents in coils
K and L and their corresponding primaries of, say, fifteen degrees. If
the poles B C are at right angles, the armature-coils should be
connected directly across, or a single armature core wound from end to
end may be used; but if the poles B C be in line there should be an
angular displacement of the armature coils, as will be well understood.
The operation will be understood from the foregoing. The maximum
magnetic condition of a pair of poles, as B' B', coincides closely with
the maximum effect in the armature, which lags behind the corresponding
condition in poles B B.