Short Circuiting Device for Stopping Dental Motors.
Year: 1894

page_text: Date of Application, 24th Nov., 1891-Accepted, 5th Jan., 1895
Short Circuiting Device for Stopping Dental Motors.
I, FRANCIS NAPIER DENISON, of 26 Denison Square, in the City of Toronto,
in the County of York, in the Province of Ontario, Canada, Electrical Expert, 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 :-
My invention relates to a short circuiting device for the instantaneous stoppage
of an electric motor more particularly adapted for running a dental engine and
referring particularly to a former Patent of mine for improvements in dental motor
outfits granted on the 7th day of June, 1894, under Number 11,100 in which a
peculiar construction of foot switch is used.
The object of the invention is to provide means for short-circuiting the armature
of the motor no matter at what rate of speed the motor armature shaft may be
rotating, so as to thereby stop the shaft instantaneously without the necessity
of employing any means to decrease the speed of rotation of the armature and
it consists essentially of two contact plates preferably arc-shaped in form
insulated from the base plate of the switch and each connected to the armature of
the motor but separated from each other and a spring contact plate connected to
the bottom of the toe-plug of the pedal of the foot switch, which plug is of
insulating material, this contact plate being arranged when the heel of the pedal
of the foot switch is depressed to bridge the contact plates and thus short circuit
the armature no matter into what position the toe may be swung for the purpose
of speeding the shaft of the motor, the foot switch being otherwise constructed as
described in my former Patent above referred to with the exception that there is no
provision made for operating the brake therein provided which brake by my present
device is entirely dispensed with.
Fig. 1, represents a view partially in perspective and partially diagrammatic
showing the foot switch, wiring, motor, hand switch, shunt field and connection, the
foot switch being shown in an enlarged perspective view, portion of the plate or
frame of the switch being broken away.
Fig. 2, is a plan view of the rheostat inverted.
•
In the drawings like letters of reference indicate corresponding parts in each
figure.
For the purpose of clearly illustrating my invention I shall first describe the
major portion of the foot switch in detail as in my former Patent and in so doing
will point out the difference in construction necessary to accomplish the present
object I have in view.
U, is the base plate of the foot switch, which is made hollow as shown, and,
V, is the pedal which is stiffly pivoted, swivelled and arranged as described in my
former Patent.
U, U, are hollow stop pins attached to or forming part of the base plate, U.
W, is a bar of insulating material secured to the bottom of the base plate, U.
A great deal of the bar, W, is broken away so as to exhibit details of
construction.
X, is a bar of insulating material pivoted at the centre by the screw pin, ., to
the bar, W.
X. X², are plungers, which are connected to pins in the ends of the pivot
bar, X, by the links, a¹, and a³, extending into the slots, a³, and, .z³, in the

page_text: bars. The plungers, X¹, and, X³, are preferably provided with convex pushing
caps, x, x
V, is a downwardly extending plug of insulating material secured to the
forward end of the pedal. The plug, V, is surrounded at the top by a ring, V³,
of insulating material, which is designed to limit the downward movement of
the plug.
v, is a boss formed underneath the heel portion of the pedal, V. The boss v³,
is provided with a rubber cushion, V10, which is designed to limit the downward
movement of the heel portion of the pedal.
Y, Y, are contact plates preferably arc-shaped, which are secured to the
bottom of the base plate, U, and, are insulated from it and each other by the
plate Y¹.
V³, is a spring contact plate secured on the reduced lower end of the
plug, V, by a nut, v, of insulating material. V, is a spring contact plate
rigidly secured at the bottom of the insulating plug, V4, by the screw, v³. It will
be seen that the plates, V³, and, V, are insulated from one another.
The spring plate, V, is normally held in contact with the plates, Y, Y, when
the motor is at rest and the plate, V6, is held out of contact with one of the
plates, Z, of the flat rheostat, Z, which is situated beneath the base plate, U,
except when the foot switch is pressed at the toe to throw in the motor. The
plates, Z', are separated from each other and are insulated from the rheostat by
the plate, Z*.
Z³, is a binding screw on one end plate of the rheostat.
w, w³, are contact blocks secured at one end of the bar, W, and, w³, w³, w, are
bars flush with the inner side of the bar.
X³, X, are contact plates secured to the front side of the pivoted bar, X. The
plates, X, X, have at one end the spring ends, X, X, which are continuously
held in contact with the contact blocks, w, w, secured to the bar, W, and at the
other spring ends, X, X³, which are designed to be respectively brought in
contact with the bars, to³, w³, or, w³, w, as shown in Fig. 3, this depending upon
which of the plungers X', or, X', is pressed down.
The rheostat above mentioned in this instance as indicated in Fig. 2, has the
greatest amount of resistance at the right hand side, and the resistance
decreases from right to left, the left hand binding post having no resistance
at all.
The binding screw, Z, is connected by the wire, ', to the contact bar, w³. The
bar, to, is connected to the bar, w, by the wire, :'.
W, W, W, W³, are binding posts of the motor and switch board circuits
which are secured in the bottom of the bar, W.
2, 21, are wires connecting the contact plate, Y, with the binding post, W*,
and, W.
3, is a wire connecting the wire, ³, and contact plate, w, with the binding
post, W
4, is a wire connecting the contact block, w, with the binding pcst, W.
5, is a wire connecting the contact block, wi, with the binding post, W.
6, is a wire connected at one end to the screw, vs, and consequently to the contact
plate, V, and extending up through the insulating pin, V, underneath the pedal
through the opening, v, down through the hollow pin, V', as indicated by dotted
lines forward to the binding post, W.
7, is the positive wire through which, when the switch, 8, is thrown into the
position shown in dotted lines, the current passes ordinarily when the foot switch is
not thrown in through the wire, 9, shunt field, 10, wire, 11, and back by the
negative wire, 12.
When the forward end of the pedal of the switch is pressed down the circuit is
completed and passes through the positive wire, 7, into the binding post W, thence by
the wire, 6, to and through the insulating plug, 4, contact plate, V, one of the plates,,
of the rheostat, through the rheostat out on the binding post, Z3, at the left hand

page_text: side of the rheostat, and by the wire, z¹, to the contact bar, w³, thence by the
contact plate, X³, to the contact block, w, thence by the wire, 4, to the binding
post, W thence by the wire, 13, to the motor, through the motor and by the
wire, 14, back to the binding post, W, from the binding post, W³, by the wire, 5,
to the contact block, w¹, thence by the contact plate, X, to the contact bar, w*,
thence by the wire, 3, to the binding post W₁, thence by the wire, 12, back to
through and to the switch board, J. The course above described is that shown
when the plunger, X', is pressed down and the bar, X, in the position shown.
It will of course, be understood, that the direction of rotation of armature shaft
will be reversed as in my former patent and that the nearer the pedal is swung to
the left hand side of the figure so as to bring the contact plate, V6, upon the plates
nearer the left hand side, the less the resistance will be (Fig. 2) and consequently
the greater the speed of the motor.
It will now be seen that in order to produce a short circuit through the
armature of the motor and stop the rotation of the armature shaft that the
heel of the pedal, V, may be pressed down so as to bring the contact plate, V6,
upwardly out of contact with the plates, Z', thereby throwing the current
entirely out of the foot switch. The spring contact plate, V³, however, is
brought against the contact plates, Y, Y, so as to bridge them and an induced
current will be produced in the armature short circuit through the wires, 14,
binding post, W³, wire, 2¹, plate, Y, contact plate, V³, plate, Y, wire, 2, binding
post, W, and wire, 13.
It will thus be seen that the motor will be really acting as a dynamo for the
time being and that an instant stoppage of the armature shatt will be effected.
By means of the short circuiting device above described the instant stoppage of
armature shaft can be effected in the same time at the highest speed as well as at
the slowest speed. In my device it will also be seen that owing to the slow speed
of armature when running at its maximum the short circuiting of armature can be
effected with the fully magnetized field as the induced current in the armature has
not time to rise to dangerous proportions on account of the armature coming to
rest instantaneously.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is :-
1. The combination with a motor and two contact plates connected to the
positive and negative wires of the main circuit respectively and a contact plate
to join these contact plates, so as to throw the current into the poles of the
armature and a shunt field constantly in circuit, of two contact plates insulated
from each other and connected by separate wires to the poles of the armature and
a spring contact plate designed to be brought into engagement so as to bridge the
two latter plates and form a sliding contact between them immediately the contact
plate joining the plates completing the main circuit is removed, so as to thereby
establish a short circuit through the armature and convert the motor into a dynamo
as and for the purpose specified.
2. The combination with a motor and two contact plates connected to the positive
and negative wires of the main circuit respectively through a series of resistance
coils provided with contact plates, and a sliding spring contact plate designed to be
brought over one or the other of these plates, of a spring contact plate insulated
from the spring contact plate movable over the resistance coil plates and designed
to bridge the two contact plates connected to the poles of the armature immediately
upon the contact plate of the resistance coil being removed from or the other of
the resistance coil plates, so as to thereby establish a short circuit through the
armature and convert the motor into a dynamo as and for the purpose specified.
3. The combination with a motor and shunt field and circuit completed through
the foot switch as specified, and the foot switch pedal pivoted and swivelled as
specified and having a plug of insulating material extending downwardly from the

page_text: toe portion through which runs the main circuit wire to the spring contact plate
designed to be movable upon the plates of the resistance coil, so as to vary the
amount of current through the main circuit and consequently the rate of speed of
the motor, of two arc-shaped contact plates insulated from each other and each
connected to the poles of the armature and a spring contact plate secured near the
bottom of the toe plug of the pedal insulated from the spring contact plate movable
upon the plates of the resistance coil, and so arranged that when the heel of the
pedal is pressed down such contact spring will be brought into sliding contact with
the arc-shaped plates so as to bridge them as and for the purpose specified.
Dated this 24th day of November 1894.
G. F. REDFERN & Co.,
4, South Street, Finsbury, London, Agents for the Applicant.