Improvements in Alternating Current Electric Motors.
Year: 1894

page_text: Date of Application, 27th Feb., 1894
Complete Specification Left, 21st Aug., 1894-Accepted, 22nd Sept., 1894
PROVISIONAL SPECIFICATION.
Improvements in Alternating Current Electric Motors.
A communication by CHARLES S. BRADLEY, of Avon, in the County of
Livingston and State of New York, United States of America, Electrical
Engineer.
I, WILLIAM PHILLIPS THOMPSON F.C.S. M.I.M.E. of The Agency for Foreign
Patent Solicitors 6, Lord Street, Liverpool and 6, Bank Street, Manchester, both
in the County of Lancaster and 323 High Holborn in the County of Middlesex
Civil Engineer, do hereby declare the nature of this invention to be as follows:-
This invention relates to that type of alternating current motor in which a rotary
magnetic field is developed in one element and acts inductively upon a secondary
circuit enclosing the other element. The element carrying the secondary circuit
will move almost in synchronism with the rotary field when such a motor is
operating under no load. At such times there being almost no slip between the
rotary field and the secondary circuit there is no induction developed in the latter
and the motor has little torque. When the motor is loaded the secondary circuit
lags behind the rotary field creating a certain amount of so-called "slip" between
the secondary circuit and the poles of the rotary field. The induction in the
secondary circuit is greater as the percentage of this slip increases or as the
motor is more heavily loaded. When starting under load the induction upon
the secondary circuit of such a motor is so great as to cause the poles developed by
the said secondary circuit to overpower the poles of the rotary field, and the
motor operates with small torque. As the motor rises in speed, the percentage of
slip becomes less and the induction is reduced to an amount where the poles
developed thereby can co-operate more effectively with the poles of the rotary
field, and the strongest torque is developed at a speed where the motor approaches
synchronism. It is the object of the invention to enable such a motor to be started
under load with strong torque and to vary the speed and torque as may be desired
during the operation of the motor. The invention is carried out by providing the
motor with the rotary field operating upon a common secondary element and providing
in the secondary circuit which polarizes such element two paths for the current, one
of which will be selected when the poles of the rotary fields occupy the same
inductive relation to the secondary circuit, and the other of which will be selected
when the poles occupy a different inductive position with reference to the
secondary circuit.
Two fixed laminated ring field maguet poles are provided side by side and wound
with polarizing coils adapted for connection with a polyphase alternating current
circuit so that when the polyphase currents flow a rotary or shifting magnetic
field will be developed in the field-magnets. The coils may in some cases be
wound upon internally projecting teeth so as to develop the rotary or shifting
magnetic field,. and they may be grouped so as to produce a bipolar or multipolar
system. In inductive relation to the two field-magnet cores placed a rotary
secondary element which may be provided with two laminated cores or may have a
single core extending throughout the length of the armature. Upon this element
is placed a low resistance secondary circuit preferably composed of a series of
longitudinal copper bars set in radial grooves formed in the pile of lamina
constituting the core and united at the ends by transverse conducting plates.
forming a completely closed metallic circuit, commonly known as the "squirrel
cage
type of winding. When the poles of the rotary field occupy the same
"

page_text: inductive position with relation to the secondary element the two electro-motive-
forces set up in the copper bars by the two field-magnets will be in the same
direction, and induced current will flow from one conducting head at the end of the
armature to the other through the bars on one side and back by way of the bars on
the other side of the armature. When the motor is starting and the slip is very
great this induced current is so strong as to develop secondary poles which
overpower the primary poles. It is customary to overcome this difficulty in
starting an induction motor into operation by the use of external resistance
connected with the circuits through proper conducting rings and brushes. In order
to dispose of the use of such external resistance and its accompanying structure, au
auxiliary path is provided in the secondary circuit by connecting the conducting
bars by conductors of comparatively high resistance, the points of connection
between the several bars being made at a point between the two field-magnets.
This cross-connection may be made by means of wires having more or less
resistance as required. Means are also provided by which the poles of one rotary
field may be made to lag behind the poles of the other field. This may be
accomplished by mounting a contact arm provided with three or more insulated
conducting brushes in operative relation to a series of contact spots connecting at
proper points with the winding of one field-magnet; for example, in a triphase
system there would be three such arms electrically connected with the terminals of
the triphase circuit and insulated from one another, and from suitable points of the
winding of one field-magnet will be led connections to contact studs, so that when
the three brushes referred to are shifted over the contact studs, the point of entry
of the triphase currents into the magnetizing coils of the field-magnet will be varied
so as to give the poles a definite amount of lead or lag over the poles of the other
field-magnet which may be directly connected with the triphase circuit, so that by
giving the contact arms a partial rotation, by means of a suitable handle, the poles
of one element may be shifted with respect to the poles of the other element,
and since these poles both act on the armature it will be evident that one secondary
current will lag a definite amount behind the other secondary current, and the two
induced currents thus set up will more or less oppose one another, thus shunting
some of the current through the path afforded by the connections between the
several bars at points between the two field-magnets. In this way the path afforded
for the current through this high resistance path or shunt although continuously
closed will not draw any current unless one of the induced currents lags behind
the other, since when the two induced currents are coincident in phase they will
both unite and flow across the low resistance path afforded by the copper bars,
but when they differ the phases oppose one another to a greater or less degree
and force current over the high resistance path. Thus we have at hand a means
of controlling the speed and torque of the motor without the introduction of
external resistance. In starting the motor the connections upon one field-magnet
will be shifted so as to bring its poles to such a degree of displacement with
reference to the poles of the other field-magnet that the two induced currents will act
in opposition. Every conducting bar of the secondary circuit then has two electro-
motive-forces in opposite directions, and the only path for the current is by way
of the high resistance shunt connecting the successive bars, so that a weak current
flows through the secondary circuit and the secondary poles cannot overpower the
poles of the rotary field, and the motor will therefore start with a strong torque.
When it rises in speed and the amount of slip becomes less, the connections with
one field-magnet may be shifted so as to bring its poles into alignment with the
poles of the other field-magnet, thus reducing the degree of opposition of the
electro-motive-forces and permitting such induced currents as are developed to
exert a greater magnetizing strength upon the secondary element, and finally the
poles may be given such an adjustment that the two induced currents have their
two phases co-ordinate in time and direction, and this current flowing over the low
resistance system develops a maximum magnetizing effect. The high resistance
shunt formed by the connections between the several bars may be made by means

page_text: of loops of wire or thin conducting rings. In cases where independent secondary
elements are provided to co-operate with the two rotary fields, these elements may
be connected at three or more points by wires and suitable resistance wires be
connected across these several connecting wires, so that when the two circuits act in
conjunction current may flow over one to the other through the connecting wires,
and when they act in opposition the current will be forced to traverse the cross-
connections of higher resistance. In some cases instead of shifting the poles of one
element with reference to those of the other by a switch as described, the whole
body of one field-magnet may move circumferentially with relation to that of
the other.
Dated this bath day of Wahnuary 1804
COMPLETE SPECIFICATION.
Improvements in Alternating Current Electric Motors.
A communication by CHARLES S. BRADLEY, of Avon, in the County of
Livingston and State of New York, United States of America, Electrical
Engineer.
I, WILLIAM PHILLIPS THOMPSON, F.C.S., M.I.M.E., of The Agency for
Foreign Patent Solicitors, 6, Lord Street, Liverpool, and 6, Bank Street,
Manchester, both in the County of Lancaster, and 323, High Holborn, in the
County of Middlesex, Civil Engineer, do hereby declare the nature of this
invention and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :-
This invention relates to that type of alternating current motors in which a
rotary magnetic field is developed in one element and acts inductively upon a
secondary circuit enclosing the other element. The element carrying the secondary
circuit will move almost in synchronism with the rotary field when such a motor is
operating under no load. At such times there being almost no slip between the
rotary field and the secondary circuit there is no induction developed in the latter
and the motor has little torque. When the motor is loaded the secondary circuit
lags behind the rotary field creating a certain amount of so-called "slip" between
the secondary circuit and the poles of the rotary field. The induction in the
secondary circuit is greater as the percentage of this slip increases or as the motor
is more heavily loaded.
the
When starting under load the induction upon
secondary circuit of such a motor is so great as to cause the poles developed by the
said secondary circuit to overpower the poles of the rotary field, and the motor
operates with small torque. As the motor rises in speed the percentage of slip
becomes less and the induction is reduced to an amount where the poles developed
thereby can co-operate more effectively with the poles of the rotary field; and the
strongest torque is developed at a speed where the motor approaches synchronism.
It is the object of this invention to enable such a motor to be started under load
with strong torque and to vary the speed and torque as may be desired during the
operation of the motor. This invention is carried out by providing the motor with
two rotary fields operating upon a common secondary element and providing in the
secondary circuit which polarizes such element two paths for the current, one of
which will be selected when the poles of the rotary fields occupy the same
inductive relation to the secondary circuit, and the other of which will be selected
when the poles occupy a different inductive position with reference to the secondary
circuit. The several features of novelty of this invention will be more particularly
hereinafter described and will be definitely indicated in the claims appended to this
Specification.

page_text: In the accompanying drawings which illustrate this invention,
Figure 1 is a part sectional view of a motor embodying the improvements;
Fig. 2 is a diagram showing a mode of shifting the poles of one magnetic field
so as to give said poles a lead or lag over the poles of the other magnetic field;
Fig. 3 is a diagram showing a modified form of a secondary element ;
Fig. 4 is a diagram illustrating one mode of connecting the secondary circuit so
as to provide the selective paths referred to above;
Fig. 5 is another diagram showing another mode of making the connections; and
Figs. 6 and 7 are sectional views of a modification in which the relative positions
of the poles in the two magnetic fields may be changed by bodily shifting one of
the elements in which one magnetic field is developed.
Referring first to Fig. 1, A and B represent two stationary elements fixed upon
a suitable base and formed of superposed laminæ of soft iron. The polarizing coils
are wound upon internally projecting teeth, indicated at C, so as to develop in the
ring core a rotary magnetic field when connections with the windings are made
with a polyphase alternating current circuit in a manner well understood in the
art. In inductive relation to the two cores is placed a rotary element D which
may be provided as shown in the drawings with two independent laminated cores E
and F, or may have a single core extending throughout both cores of A. and B.
Upon this element is placed a low resistance secondary circuit, preferably composed
of a series of longitudinal copper bars G set in radial grooves formed in the pile
of laminæ constituting the core and united at the ends by conducting plates H, I,
forming a completely closed metallic circuit. When the poles of the rotary field
developed in the elements A and B occupy the same inductive position with relation
to the secondary element the two electro-motive-forces acting in any of the copper
bars will be in the same direction, and the induced current will flow from one
conducting head H through the bars to the other conducting head I and back by
way of the bars on the other side of the element. When the motor is starting and
the slip is very great this induced current is so strong as to induce poles in the
secondary element which overpower the poles of the rotary field. It is customary
in this type of motor to introduce into the secondary circuit an external resistance
connected with the circuit by means of brushes and contact rings, and this resistance
serves to tone the effect of the current when the motor starting, reducing the
polarization of the armature to a degree at which it will co-operate to better
advantage with the poles of the rotary field. In order to obviate the necessity of
employing such an external resistance and to dispense with the contact rings and
brushes, an auxiliary path is provided in the secondary circuit by connecting the bars
by conductors of comparatively high resistance at a point between the rotary fields.
This is indicated in Fig. 1 by the looped lines K connecting successive bars of the
secondary circuit. Means are also provided by which the poles of one rotary field
may be shifted so as to give them a lag or lead over the poles of the other field.
For example, in Figs. 1 and 2 upon a boss attached to one of the standards is
mounted a sleeve carrying three contact arms adapted to lead a triphase current to
three equidistant points of the winding of the element A. By giving these contact
arms a partial rotation by means of the handle L the poles of the element A may
be shifted with respect to the poles of the element B so that the poles of the two
rotary fields may be made to co-operate in developing an electro-motive-force in
the several secondary conductors or may be made to oppose one another.
acting in conjunction the current induced in the secondary circuit will pass directly
across the bars from one head H to the other, but when they oppose one another,
more or less current, depending on the degree of opposition, will be shunted
through the high resistance path formed by the loop K connecting the several
bars. This path although continuously closed takes no current when the electro-
motive-forces induced in any bar by the two rotary fields are coincident in phase,
as a path of low resistance is open at such times, but when the phases differ the
two electro-motive-forces oppose each other more or less and unite in sending
current over the high resistance path. Thus we have at hand a means of
When
✓

page_text: controlling the speed and torque of the motor without the introduction of external
resistance. In starting the motor the handle L is shifted so as to bring the poles
of one rotary field in opposition to those of the other In any conducting bar of
the secondary circuit there is then in action two electro-motive-forces opposed in
direction and the only path for the current is through the high resistance circuit K
which by reason of its high resistance weakens the strength of the current and
prevents the secondary poles from overpowering the poles of the rotary field.
The motor therefore starts with strong torque. As the motor rises in speed and the
amount of slip and therefore the amount of induction decreases between the poles.
of the field and the secondary element, the handle L is shifted so as to bring the
poles of A into a closer or absolute conjunctive relation to the poles of B, thus
reducing the degree of opposition of the electro-motive-forces, when the current
will flow across the bars of the secondary circuit from one head to the other and
the secondary current will maintain the pole strength of the secondary element.
These provisions result in effect in weakening the strong induced current at
starting by forcing it to traverse a path of high resistance and in offering to it a
path of low resistance when the motor increases its speed and when there is
consequently a less percentage of slip and a weaker induced current, thus permitting
the establishment of a strong torque at any speed, or permitting the speed of the
motor to be varied when operating under load.
In Figs. 4 and 5 are shown two modes of establishing the high and low resistance
circuits in the secondary element. In Fig. + the loops K, K connecting successive
conducting bars at a point between the two rotary fields provide a path of
comparatively high resistance. In Fig. 5 the several bars of the circuit which
are acted upon by opposite poles of the rotary field are connected by means of
thin rings of a suitable resisting material indicated by the curved lines M, M..
When one rotary field is shifted with reference to the other and there is a difference
of phase in the electro-motive-forces developed at the two sides of the secondary
circuit more or less current will be diverted over the path M, M, adding resistance
to the circuit and weakening the magnetizing effect of the secondary current.
Other modes of accomplishing the same result might be adopted, it being only
necessary that a path of high resistance shall be offered to the secondary current.
when the motor is starting.
In Fig. 3 is diagrammatically indicated a method of accomplishing the results.
contemplated by my invention where independent secondary elements are acted
upon by the two rotary fields. In this case, as before, means are provided for
shifting the poles of one field with relation to the other, as indicated by the three
spring contacts of the line circuit with element A, and the two secondary circuits.
are connected at a number of points by conductors 0, 01, 0³, and these several
conductors are cross connected by resistances P, P', P. When the electro-motive-
forces induced in the two secondary circuits operate in conjunction, current flows
from one secondary circuit to the other and back by way of the conductors O, O¹, O²,
but when they act in opposition the current is forced to traverse the paths.
indicated by P, P1, P2, which, by reason of the high resistance offered, reduces the
magnetizing effect of the current and accomplishes the desired result. It will of
course be understood that the means of throwing the several parts of the secondary
circuit into conjunction or opposition does not necessarily involve adjustment of
the poles of the two rotary fields. For example, in Fig. 3, the secondary
elements D, D', might be mace stationary and the inducing element permitted to.
rotate.. In this case the two induced currents might be thrown into conjunction.
or opposition by shifting the points of connection of the wires O, O', O* of one
element with reference to those of the other. It will be understood that instead
of shifting the poles of one element with reference to those of the other by means
of the switch described, any means of changing the relation of the two fields to
the armature might be employed. For example, in Figs. 6 and 7 is illustrated a
system in which the element A may be shifted bodily with reference to the element B,
so that the electro-motive-force due to the poles of element A. will oppose to a

page_text: b
greater or less degree the electro-motive-force due to the element B. This result
may be accomplished in any suitable manner, the system shown comprising an
arc-shaped support in which element A is mounted so as to be capable of shifting
circumferentially and locked in any one of a number of positions of adjustment
by a set screw a, entering a recess in one of the side plates of the element, and
adapted to be inserted in one of a number of openings provided in the arc-shaped
support for the element. A two pole motor has been herein described and it is
desired to have it understood however that the invention is equally applicable to
multipolar motors. In the latter class of machines the poles shift progressively
around the ring core so as to produce in reality a rotary multipolar field.
It is
deemed unnecessary to further describe such multipolar motors as their construction
is well understood by those familiar with so-called "
rotary current motors.
Having now particularly described and ascertained the nature of the said
invention, and in what manner the same is to be performed, as communicated to
me by my foreign correspondent I declare that what I claim is:
"
1. An alternating current motor having one element wound to develop
progressively shifting magnetic poles, and the other element provided with a closed
secondary circuit inductively related to the shifting poles, means for developing
opposing electro-motive-forces in the secondary circuit and a common path of
relatively bigh resistance forming a multiple arc circuit for currents due to the
opposed electro-motive-forces.
2. An alternating current motor having two primary elements and a secondary
element inductively related to both, and means for shifting the poles of one primary
element with reference to those of the other to develop opposing electro-motive-
forces in the secondary element, said secondary element being provided with a
common path of high resistance for the current due to such opposing electro-
motive-forces.
3. An alternating current motor having two ring cores wound to develop two
rotary magnetic fields, a secondary element having a closed circuit in inductive
relation to both cores, and means for shifting the poles of one field with relation to
those of the other, to develop opposing electro-motive-forces in the secondary
circuit, the secondary circuit being provided with a path of high resistance
forming a multiple arc circuit for currents due to the opposed electro-motive-forces
for the purpose described.
4. Au alternating current motor having one element wound to develop
progressively shifting poles and the other element provided with a closed secondary
circuit of low resistance inductively related to the shifting poles, and means for
developing when desired opposing electro-motive-forces in the secondary circuit,
said circuit being provided with a high resistance shunt forming a multiple arc
circuit for currents due to the opposed electro-motive-forces.
Dated this 20th day of August 1894.