METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.
In that class of motors in which two or more sets of energizing magnets
are employed, and in which by artificial means a certain interval of
time is made to elapse between the respective maximum or minimum periods
or phases of their magnetic attraction or effect, the interval or
difference in phase between the two sets of magnets is limited in
extent. It is desirable, however, for the economical working of such
motors that the strength or attraction of one set of magnets should be
maximum, at the time when that of the other set is minimum, and
conversely; but these conditions have not heretofore been realized
except in cases where the two currents have been obtained from
independent sources in the same or different machines. Mr. Tesla has
therefore devised a motor embodying conditions that approach more nearly
the theoretical requirements of perfect working, or in other words, he
produces artificially a difference of magnetic phase by means of a
current from a single primary source sufficient in extent to meet the
requirements of practical and economical working. He employs a motor
with two sets of energizing or field magnets, each wound with coils
connected with a source of alternating or rapidly-varying currents, but
forming two separate paths or circuits. The magnets of one set are
protected to a certain extent from the energizing action of the current
by means of a magnetic shield or screen interposed between the magnet
and its energizing coil. This shield is properly adapted to the
conditions of particular cases, so as to shield or protect the main core
from magnetization until it has become itself saturated and no longer
capable of containing all the lines of force produced by the current. It
will be seen that by this means the energizing action begins in the
protected set of magnets a certain arbitrarily-determined period of time
later than in the other, and that by this means alone or in conjunction
with other means or devices heretofore employed a practical difference
of magnetic phase may readily be secured.
Fig. 58 is a view of a motor, partly in section, with a diagram
illustrating the invention. Fig. 59 is a similar view of a modification
of the same.
[Illustration: FIG. 58.]
[Illustration: FIG. 59.]
In Fig. 58, which exhibits the simplest form of the invention, A A is
the field-magnet of a motor, having, say, eight poles or
inwardly-projecting cores B and C. The cores B form one set of magnets
and are energized by coils D. The cores C, forming the other set are
energized by coils E, and the coils are connected, preferably, in series
with one another, in two derived or branched circuits, F G,
respectively, from a suitable source of current. Each coil E is
surrounded by a magnetic shield H, which is preferably composed of an
annealed, insulated, or oxidized iron wire wrapped or wound on the coils
in the manner indicated so as to form a closed magnetic circuit around
the coils and between the same and the magnetic cores C. Between the
pole pieces or cores B C is mounted the armature K, which, as is usual
in this type of machines, is wound with coils L closed upon themselves.
The operation resulting from this disposition is as follows: If a
current impulse be directed through the two circuits of the motor, it
will quickly energize the cores B, but not so the cores C, for the
reason that in passing through the coils E there is encountered the
influence of the closed magnetic circuits formed by the shields H. The
first effect is to retard effectively the current impulse in circuit G,
while at the same time the proportion of current which does pass does
not magnetize the cores C, which are shielded or screened by the
shields H. As the increasing electromotive force then urges more current
through the coils E, the iron wire H becomes magnetically saturated and
incapable of carrying all the lines of force, and hence ceases to
protect the cores C, which becomes magnetized, developing their maximum
effect after an interval of time subsequent to the similar manifestation
of strength in the other set of magnets, the extent of which is
arbitrarily determined by the thickness of the shield H, and other
well-understood conditions.
From the above it will be seen that the apparatus or device acts in two
ways. First, by retarding the current, and, second, by retarding the
magnetization of one set of the cores, from which its effectiveness will
readily appear.
Many modifications of the principle of this invention are possible. One
useful and efficient application of the invention is shown in Fig. 59.
In this figure a motor is shown similar in all respects to that above
described, except that the iron wire H, which is wrapped around the
coils E, is in this case connected in series with the coils D. The
iron-wire coils H, are connected and wound, so as to have little or no
self-induction, and being added to the resistance of the circuit F, the
action of the current in that circuit will be accelerated, while in the
other circuit G it will be retarded. The shield H may be made in many
forms, as will be understood, and used in different ways, as appears
from the foregoing description.
As a modification of his type of motor with "shielded" fields, Mr. Tesla
has constructed a motor with a field-magnet having two sets of poles or
inwardly-projecting cores and placed side by side, so as practically to
form two fields of force and alternately disposed--that is to say, with
the poles of one set or field opposite the spaces between the other. He
then connects the free ends of one set of poles by means of laminated
iron bands or bridge-pieces of considerably smaller cross-section than
the cores themselves, whereby the cores will all form parts of complete
magnetic circuits. When the coils on each set of magnets are connected
in multiple circuits or branches from a source of alternating currents,
electromotive forces are set up in or impressed upon each circuit
simultaneously; but the coils on the magnetically bridged or shunted
cores will have, by reason of the closed magnetic circuits, a high
self-induction, which retards the current, permitting at the beginning
of each impulse but little current to pass. On the other hand, no such
opposition being encountered in the other set of coils, the current
passes freely through them, magnetizing the poles on which they are
wound. As soon, however, as the laminated bridges become saturated and
incapable of carrying all the lines of force which the rising
electromotive force, and consequently increased current, produce, free
poles are developed at the ends of the cores, which, acting in
conjunction with the others, produce rotation of the armature.
The construction in detail by which this invention is illustrated is
shown in the accompanying drawings.
[Illustration: FIG. 60.]
[Illustration: FIG. 61.]
Fig. 60 is a view in side elevation of a motor embodying the principle.
Fig. 61 is a vertical cross-section of the motor. A is the frame of the
motor, which should be built up of sheets of iron punched out to the
desired shape and bolted together with insulation between the sheets.
When complete, the frame makes a field-magnet with inwardly projecting
pole-pieces B and C. To adapt them to the requirements of this
particular case these pole-pieces are out of line with one another,
those marked B surrounding one end of the armature and the others, as C,
the opposite end, and they are disposed alternately--that is to say, the
pole-pieces of one set occur in line with the spaces between those of
the other sets.
The armature D is of cylindrical form, and is also laminated in the
usual way and is wound longitudinally with coils closed upon themselves.
The pole-pieces C are connected or shunted by bridge-pieces E. These may
be made independently and attached to the pole-pieces, or they may be
parts of the forms or blanks stamped or punched out of sheet-iron. Their
size or mass is determined by various conditions, such as the strength
of the current to be employed, the mass or size of the cores to which
they are applied, and other familiar conditions.
Coils F surround the pole-pieces B, and other coils G are wound on the
pole-pieces C. These coils are connected in series in two circuits,
which are branches of a circuit from a generator of alternating
currents, and they may be so wound, or the respective circuits in which
they are included may be so arranged, that the circuit of coils G will
have, independently of the particular construction described, a higher
self-induction than the other circuit or branch.
The function of the shunts or bridges E is that they shall form with the
cores C a closed magnetic circuit for a current up to a predetermined
strength, so that when saturated by such current and unable to carry
more lines of force than such a current produces they will to no further
appreciable extent interfere with the development, by a stronger
current, of free magnetic poles at the ends of the cores C.
In such a motor the current is so retarded in the coils G, and the
manifestation of the free magnetism in the poles C is so delayed beyond
the period of maximum magnetic effect in poles B, that a strong torque
is produced and the motor operates with approximately the power
developed in a motor of this kind energized by independently generated
currents differing by a full quarter phase.