"MAGNETIC LAG" MOTOR.
The following description deals with another form of motor, namely,
depending on "magnetic lag" or hysteresis, its peculiarity being that in
it the attractive effects or phases while lagging behind the phases of
current which produce them, are manifested simultaneously and not
successively. The phenomenon utilized thus at an early stage by Mr.
Tesla, was not generally believed in by scientific men, and Prof. Ayrton
was probably first to advocate it or to elucidate the reason of its
supposed existence.
Fig. 54 is a side view of the motor, in elevation. Fig. 55 is a
part-sectional view at right angles to Fig. 54. Fig. 56 is an end view
in elevation and part section of a modification, and Fig. 57 is a
similar view of another modification.
In Figs. 54 and 55, A designates a base or stand, and B B the
supporting-frame of the motor. Bolted to the supporting-frame are two
magnetic cores or pole-pieces C C', of iron or soft steel. These may be
subdivided or laminated, in which case hard iron or steel plates or bars
should be used, or they should be wound with closed coils. D is a
circular disc armature, built up of sections or plates of iron and
mounted in the frame between the pole-pieces C C', curved to conform to
the circular shape thereof. This disc may be wound with a number of
closed coils E. F F are the main energizing coils, supported by the
supporting-frame, so as to include within their magnetizing influence
both the pole-pieces C C' and the armature D. The pole-pieces C C'
project out beyond the coils F F on opposite sides, as indicated in the
drawings. If an alternating current be passed through the coils F F,
rotation of the armature will be produced, and this rotation is
explained by the following apparent action, or mode of operation: An
impulse of current in the coils F F establishes two polarities in the
motor. The protruding end of pole-piece C, for instance, will be of one
sign, and the corresponding end of pole-piece C' will be of the opposite
sign. The armature also exhibits two poles at right angles to the coils
F F, like poles to those in the pole-pieces being on the same side of
the coils. While the current is flowing there is no appreciable tendency
to rotation developed; but after each current impulse ceases or begins
to fall, the magnetism in the armature and in the ends of the
pole-pieces C C' lags or continues to manifest itself, which produces a
rotation of the armature by the repellent force between the more closely
approximating points of maximum magnetic effect. This effect is
continued by the reversal of current, the polarities of field and
armature being simply reversed. One or both of the elements--the
armature or field--may be wound with closed induced coils to intensify
this effect. Although in the illustrations but one of the fields is
shown, each element of the motor really constitutes a field, wound with
the closed coils, the currents being induced mainly in those
convolutions or coils which are parallel to the coils F F.
[Illustration: FIG. 54.]
[Illustration: FIG. 55.]
A modified form of this motor is shown in Fig. 56. In this form G is one
of two standards that support the bearings for the armature-shaft. H H
are uprights or sides of a frame, preferably magnetic, the ends C C' of
which are bent in the manner indicated, to conform to the shape of the
armature D and form field-magnet poles. The construction of the armature
may be the same as in the previous figure, or it may be simply a
magnetic disc or cylinder, as shown, and a coil or coils F F are
secured in position to surround both the armature and the poles C C'.
The armature is detachable from its shaft, the latter being passed
through the armature after it has been inserted in position. The
operation of this form of motor is the same in principle as that
previously described and needs no further explanation.
[Illustration: FIG. 56.]
[Illustration: FIG. 57.]
One of the most important features in alternating current motors is,
however, that they should be adapted to and capable of running
efficiently on the alternating circuits in present use, in which almost
without exception the generators yield a very high number of
alternations. Such a motor, of the type under consideration, Mr. Tesla
has designed by a development of the principle of the motor shown in
Fig. 56, making a multipolar motor, which is illustrated in Fig. 57. In
the construction of this motor he employs an annular magnetic frame J,
with inwardly-extending ribs or projections K, the ends of which all
bend or turn in one direction and are generally shaped to conform to the
curved surface of the armature. Coils F F are wound from one part K to
the one next adjacent, the ends or loops of each coil or group of wires
being carried over toward the shaft, so as to form U-shaped groups
of convolutions at each end of the armature. The pole-pieces C C', being
substantially concentric with the armature, form ledges, along which the
coils are laid and should project to some extent beyond the the coils,
as shown. The cylindrical or drum armature D is of the same construction
as in the other motors described, and is mounted to rotate within the
annular frame J and between the U-shaped ends or bends of the
coils F. The coils F are connected in multiple or in series with a
source of alternating currents, and are so wound that with a current or
current impulse of given direction they will make the alternate
pole-pieces C of one polarity and the other pole-pieces C' of the
opposite polarity. The principle of the operation of this motor is the
same as the other above described, for, considering any two pole-pieces
C C', a current impulse passing in the coil which bridges them or is
wound over both tends to establish polarities in their ends of opposite
sign and to set up in the armature core between them a polarity of the
same sign as that of the nearest pole-piece C. Upon the fall or
cessation of the current impulse that established these polarities the
magnetism which lags behind the current phase, and which continues to
manifest itself in the polar projections C C' and the armature, produces
by repulsion a rotation of the armature. The effect is continued by each
reversal of the current. What occurs in the case of one pair of
pole-pieces occurs simultaneously in all, so that the tendency to
rotation of the armature is measured by the sum of all the forces
exerted by the pole-pieces, as above described. In this motor also the
magnetic lag or effect is intensified by winding one or both cores with
closed induced coils. The armature core is shown as thus wound. When
closed coils are used, the cores should be laminated.
It is evident that a pulsatory as well as an alternating current might
be used to drive or operate the motors above described.
It will be understood that the degree of subdivision, the mass of the
iron in the cores, their size and the number of alternations in the
current employed to run the motor, must be taken into consideration in
order to properly construct this motor. In other words, in all such
motors the proper relations between the number of alternations and the
mass, size, or quality of the iron must be preserved in order to secure
the best results.