Improvements in Alternating Current Electric Motors.
Year: 1894

page_text: Date of Application, 27th Feb., 1854
Complete Specification Left, 21st Aug., 1894-Accepted, 22nd Sept., 1891
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, Man-
chester, 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 alternating current electric motors and is particularly
applicable to that type of motor in which the magnetizing current in one of the
elements is a secondary current developed by the inductive action of a rotary
magnetic field and flows through a continuously closed circuit.. The object of the
invention is to enable the motor to start with a strong torque. The peculiarity of
the class of motors referred to is that by reason of the excessive slip between the
secondary circuit and the rotary magnetic field when the motor is starting, the
induction is so great as to cause the poles developed by the induced circuit to
overpower the poles of the rotary field, and it is customary to interpose in the
induced circuit when the motor is starting a suitable resistance which can be cut
out when the motor gets up to speed. Such resistance being external to the
circuit with which it is to co-operate involves the necessity of contact rings upon
the shaft of the rotating element and co-operating brushes. It is one object of
this invention to dispense with an external resistance. The invention is carried
out by providing two secondary circuits mounted on the same shaft, one of high
resistance and the other of low resistance, and placing in co-operative relation
thereto an inducing circuit adapted to develop a rotary magnetic field, said
inducing circuit being adjustable so as to be brought into active relation to either
of the secondary circuits. In this way the high resistance induced circuit may be
brought into action when the motor is starting, and by reason of its high
resistance lowers the magnetizing effect of the induced current and permits the
field magnet and armature poles to co-operate to the best advantage and develop a
maximum torque.
When the motor attains its speed and the percentage of slip
between the induced circuit and the poles of the rotary field is reduced and the
induction consequently more moderate the inducing circuit is shifted so
co-operate with a low resistance secondary circuit, thus developing a stronger
current at that speed and preserving the torque of the motor.
as to
It is well understood that if a polyphase alternating current be led at a number
of points into a closed circuit enclosing a ring core that a rotary magnetic field is
set up in the core and that if within this core there is placed a secondary circuit,
the conductors of which are adapted to cut the lines of force of the rotary magnetic
field, a strong secondary current may be induced in this circuit.
A ring core formed of thin lamina of sheet iron is provided in which a rotary
magnetic field is developed by suitable polarizing coils connected with a polyphase
alternating current circuit. The polyphase circuit may be connected at different
circumferential points with the coils around the ring core, and when polyphase
currents are led through such coils a rotary magnetic field is set up, the number of
rotating or shifting poles depending on the particular manner in which the coils
are wound. Within the core in which this rotary or shifting field is developed is
supported a shaft upon which are mounted two secondary circuits. These circuits

page_text: comprise a series of longitudinal copper bars arranged axially along the periphery
of a laminated iron cylinder, and the bars are united at the ends by good
conducting heads. This organization forms what may be considered as the
armature of the motor. When a rotary or shifting field is set up in the outer
core, forming the stationary portion of the system in this case, the lines of force
cutting the copper bars arranged in the armature induce strong secondary
currents. The induction is so strong when the motor is starting, by reason of the
great amount of slip, that the armature poles overpower the poles of the rotary
field, and a weak torque is the result. In order to obviate this difficulty two
armatures are provided mounted on the same shaft, and the field-magnet is
arranged in sliding bearings provided with a screw or other means of shifting it
into active relation to either armature, or in some cases in active relation to parts
of both. One of the armatures is provided with a secondary circuit of low
resistance and the other one with one of comparatively high resistance.
In
starting the motor, the field-magnet will be shifted by means of the screw or other-
wise so as to enclose the high resistance secondary element and the strength of
current developed by the excessive slip in starting is lowered so as to have a
moderate magnetizing effect upon the armature core, permitting it to co-operate
to the best advantage with the rotating field. After the motor has attained a
sufficient speed, the field is shifted longitudinally so as to enclose the other
armature, which having a lower resistance permits currents of stronger ampère
strength to flow through the circuit with the given amount of slip and will thus
maintain a high torque when the motor is running at speed. Any suitable
provision may be made in the two armature circuits to make one of relative high
resistance to the other; for example, one armature circuit may be formed of
copper bars and the other of German silver bars, or auxiliary resistance may be
inserted in one of the circuits. By this organization the speed may be varied
under a constant load by shifting the field-magnet so as to partially enclose both
armatures. In this way by a proper graduation the speed or torque may be varied
to suit the work in hand. In some cases instead of a screw adjustment a simple
lever to shift the field by a quick movement may be provided, the field-magnet
being arranged in suitable sliding bearings.
Dated this 26th day of February 1894.
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.
1, 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 inven-
tion 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 alternating current electric motors and is particularly
applicable to that type of motor in which the magnetizing current in one of the
elements is a secondary current developed by the inductive action of a rotary
magnetic field and flows through a continuously closed circuit. The object of the
invention is to enable the motor to start with a strong torque. The peculiarity of
the class of motors referred to is that by reason of the excessive slip between the
secondary circuit and the rotary magnetic field when the motor is starting, the

page_text: induction is so great as to cause the poles developed by the induced circuit to over-
power the poles of the rotary field, and it is customary to interpose in the induced
circuit when the motor is starting a suitable resistance which can be cut out when
the motor gets up to speed. Such resistance being external to the circuit with
which it is to co-operate involves the necessity of contact rings upon the shaft of
the rotating element and co-operating brushes. It is one object of this invention to
dispense with an external resistance. The invention is carried out by providing
two secondary circuits mounted on the same shaft, one of high resistance and the
other of low resistance, and placing in co-operative relation thereto an inducing
circuit adapted to develop a rotary magnetic field, said inducing circuit being
adjustable so as to be brought into active relation to either of the secondary
circuits. In this way the high resistance induced circuit may be brought into
action when the motor is starting, and by reason of its high resistance lowers
the magnetizing effect of the induced current and permits the field-magnet and
armature poles to co-operate to the best advantage and develop a maximum
torque. When the motor attains its speed and the percentage of slip between
the induced circuit and the poles of the rotary field is reduced and the induction
cousequently more moderate the inducing circuit is shifted so as to co-operate
with a low resistance secondary circuit, thus developing a stronger current at
that speed and preserving the torque of the motor.
In the accompanying drawings which illustrate the invention,
Figure 1 is an end elevation of a motor embodying the improvements;
Fig. 2 is a side elevation of the same with part broken away, part of the windings
being omitted for clearness of illustration;
Fig. 3 is a plan view of a modification, and Fig. 4 is an end elevation of the
same.
It is well understood that if a polyphase alternating current be led at a number
of points into a closed circuit enclosing a ring core that a rotary magnetic field is
set up in the core and that if within this core there is placed a secondary circuit,
the conductors of which are adapted to cut the lines of force of the rotary magnetic
field, a strong secondary current may be induced in this circuit. In Fig. 1
A represents a ring core formed of thin lamina of sheet iron in which a rotary
magnetic field is developed by suitable polarizing coils connected with a polyphase
alternating current circuit. The supporting standards of the field core are provided
with undercut tongues B, C, sliding in grooves in the base of the motor, and
co-operate with a screw D controlled by a hand wheel E and co-operating pinion.
Within the core A is supported a shaft upon which are mounted two secondary
circuits. These circuits comprise a series of radial copper bars F inserted in
grooves formed in a laminated ring G constituting what is generally denominated
as the armature of the motor. The bars F are in good electric connection with
copper
disks or rings H at the end of the armature. When a rotary or alternating
field is set up in the outer core the lines of force cutting the conductors F induce
strong secondary currents which polarize the armature G. The induction is so
strong when the motor is starting, by reason of the great amount of slip and the
consequent rapid cutting of lines of force, that the armature poles overpower the
poles of the rotary field and the torque is weak. In order to obviate this difficulty
two armatures are provided, one of which is indicated in full and the other in dotted
lines, either of which may be enclosed by the core of the rotary field. One of
these armatures is provided with an inducing circuit of low resistance, and the
other one of comparatively high resistance, as indicated by the dotted and full
lines. In starting the motor the field is adjusted so as to enclose the high resistance
secondary element and the strength of the current developed by the excessive slip
in starting is lowered so as to have a moderate magnetizing effect upon the armature
core, permitting it to co-operate to the best advantage with the poles of the
rotating field and develop a maximum torque. After the motor has attained a
sufficient speed the field is shifted so as to enclose the other armature, which, by
reason of the lower resistance of its circuit, permits a current of the same ampère

page_text: strength to be developed in the secondary circuit by a less amount of slip, and thus
preserves a high torque in the motor at the increased speed. Simple straight bars
are indicated in the high resistance circuit. The size or number of these bars may
be varied to any extent desired to bring the resistance of the secondary circuit to
the best working condition, or, any suitable provision be made to render the
resistance of one of the secondary circuits greater than that of the other. This
construction permits not only of an attainment of a high torque on starting but
permits the speed of the motor to be maintained irrespective of the load, or permits
the speed to be varied if desired when the load is constant. Such a result can be
accomplished by shifting the field so as to partially enclose both armatures, and by
a proper graduation the speed or torque may be varied to suit the work in hand.
In Figs. 3 and 4 is shown a simpler device for shifting the field with respect to the
secondary circuit convenient for use ou small motors. In this case a simple lever I
pivoted to the base of the motor is provided with a forked or slotted arm K
co-operating with a pin secured beneath the sliding field. By operating the lever
the field may be quickly shifted from a position of active relation to one armature
to a position of active relation to the other. The improvements herein described
are applicable to all motors of the induction type, wherein an alternating field
developed by the primary element reacts inductively upon a secondary element
and develops rotation.
Having now particularly described and ascertained the nature of the said inven-
tion, 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 a primary element, two secondary
elements, one of high and the other of low resistance, and means for shifting the
elements relatively so that the primary may co-operate with either secondary.
2. An alternating current motor having a primary element and two closed
secondary circuits on the other element, one of high and the other of low resistance,
and means for shifting the elements relatively so that the primary may co-operate
with either or both secondary circuits.
Dated this 20th day of August 1894.
WM. P. THOMPSON & Co.,
Agents.