Improvements in Controlling Devices for Electric Motors.
Year: 1899

page_text: Date of Application, 22nd Nov., 1899-Accepted, 6th Jan., 1900
COMPLETE SPECIFICATION.
Improvements in Controlling Devices for Electric Motors.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by William B.
Potter, residing at Schenectady, in the County of Schenectady, State of New
York, United States of America, Electrical Engineer,) and in what manner the
same is to be performed, to be particularly described and ascertained in and by
the following statement:-
Our invention relates to controlling devices for electric motors; and has for
its object to provide a current-limiting device for apparatus of this kind, by
which the manipulation of the controller to increase current flow will be prevented
at such times as the motors are already taking as much current as is proper
for them to carry. The arrangement is such that although the current cannot
be turned on to the motors faster than is desired, yet the limiting device has no
effect on the backward movement of the handle.
To carry out the purposes of our invention, we employ in one of the motor
leads a current-measuring coil, preferably the coil of an electro-magnet, and an
armature within the influence of the coil, the motion of the latter being utilized
to check the rotation of the controller; its action may be direct or indirect.
The passage of current through it may be constant, as when it is included directly
in circuit with the motor, or it may be so arranged (and we prefer to have it so),
that only when the current rises to a predetermined maximum will it pass in
its entirety through the coil of the electro-magnet. In this way we get a distinct
and positive response in the automatic device.
The preferred means of carrying out this part of the invention is to place a
circuit-breaker in shunt to the coil, so, arranged that only when the current
rises to such a degree as to open the circuit, does the entire current pass through
the coil; the resistance of the circuits being adjusted to effect this.
This invention also comprises means for adjusting the effect of the coil of the
current limiting device to the current flow in the motors so that, where these
are operated by the series-parallel system now in common use, the current limiting
device will act at the proper point, whether the motors be in series or in parallel;
it being well known that the total flow of current in the circuit between trolley
and ground is approximately twice as great in one case as in the other. This part
of the invention we utilize in several different ways, either by dividing the coil
into number of parts corresponding to the number of motors, and so arranging
the connections that these parts will be series paralleled as the motors are; or
we may employ a single coil in series with one of the motors. While this will
only regulate according to the current flow in the particular motor selected,
under many conditions of service, where motors are so nearly alike as is the
modern practice, this will be found sufficient. The need for a device of the kind
arises from the fact that not only is it a strain upon the motor insulation to turn
on the current too rapidly, before the counter-electro-motive force of the motors
has risen to the intended degree, but the same act produces disagreeable jerking

page_text: and rocking of the cars, which is entirely obviated where the resistance is cut
out only as the speed of the car rises, in the way for which the apparatus is
designed.
The accompanying drawings show an embodiment of the invention, Fig. 1 being
a plan, partly in section and with the cover removed, of a controlling device
equipped with the invention, Fig. 2 being a rear elevation of parts of the device;
Figs. 3 and 5 are diagrams of circuits, and Fig. 4 is a diagram of the circuits of
a controlling device having the invention applied thereto, and with cut-out
switches of a form adapted to its exercise.
In Fig. 1, A is the back of the controller, B is its case, C is the reversing
switch, in which c is one of the brushes; K is the controlling switch cylinder the
contacts of which may effect any desired change of connections for regulating
the motors, although in practice the series parallel form of controller is preferred.
It is provided with a disc D having notches d³ in which, as the controller is
turned, the roller d of the pawl DI may drop. The pawl is attracted by the
spring D2, one end of which is secured to the pin d' projecting downward from the
cap plate of the controller. This is a well known step by step device, used in all sorts
of controllers. To the controller cylinder the disc E is fixed, (seen best in Fig. 2)
having notches e of such shape that the controller may be rotated in one direction,
whether the pawl corresponding to the notches is in them or not. With these
notches the pawl O engages when in its upper position in Fig. 2. The pawl is
retracted by the upper spring H, the tension which is adjusted by the adjusting
screw I, so that the current limit is changed as desired. The pawl is attached to
armature O¹ of an electro-magnet, of which F is the coil, F¹ the core, and F2 one
of the pole pieces. A stop f prevents the armature 0¹ from increasing its air
gap too much. Fig. 2 shows the same parts, similarly lettered except that
the cap L of the controller is shown in place. K' is the shaft of the controller, C
is the reversing switch, and S is a stop for the controller handle. The disc E
in this figure is shown passing through the back of the controller in the slot A¹,
but the whole device may if desired be arranged within the case without departing
from this invention.
In Fig. 3 we show the arrangement outlined in the statement of invention, by
which the current is caused to pass through the coil F only when the current
exceeds the pre-determined amount at which the stop should act.
While in practice a part of the current always passes in the coil, with the
connections illustrated, it is not sufficient to actuate the armature, and the main
volume of current passes through the circuit-breaker CB. The series coil M
of the circuit-breaker is set by the spring n to trip its armature at just that point
at which it is desired to actuate the armature C. As the current increases in the
circuit it reaches this point, the circuit-breaker opens, and the entire current
is then suddenly passed through the coil F, bringing down the armature O
quickly and positively.
In the practical operation of a device of the class described. it is of great
importance to preserve the effect of the coil proportionate to the current in the
motor or motors. It is well known, for instance, that in the series parallel com-
bination, the current in the circuit between the trolley and ground is, in the paral-
lel position, double that flowing when the motors are in series. It is manifest
that the current limiting device would ordinarily continue to limit the current
How in the parallel position below that requisite for the proper operation of the
motors; or if connected so as to be effective in the parallel position would allow too
much current to flow in series. In this invention we so arrange the device that
this difficulty is obviated. Several different methods may be adopted, two only
of which we will describe; but we of course intend to cover by the claims any
arrangement of the device which will permit its effort to be automatically varied
in accordance with a desired change of current flow in the motors.
The first arrangement for effecting the described purpose is that illustrated in
Fig. 3. Here the coil F is shown in series with one of the motors, so that it is

page_text: governed entirely by the current flow in that motor. In practice this will be an
efficacious way of effecting the adjustment spoken of, for it is manifest that
practically the current flow in the two motors will be identical.
'Another way of accomplishing the same results is shown in Fig. 5. Here the
coil F is divided into two parts and one of the parts is put into circuit with
each of the two motors. In the first of the diagrams in the figure, the two motors
are shown in series; in this arrangement the two coils are also in series. In the
second, where the motors are in parallel, the coils are also shown in parallel, but
each in series with its own motor, and of course the effect of each corresponds to
the current flow in the particular motor with which it is connected. Where a
number of motors are employed, this is the best arrangement; if the flow of
current in the respective motors should be different, the effect of the different coils
would still preserve a general relation to the average current requirements of all
the motors.
A still further difficulty arises with the ordinary series-parallel controller,
which we aim to obviate:-with the coil connected to one of the motors, as just
indicated with reference to Fig. 3, if the cut-out switches be operated in the
ordinary way some arrangement must be made by which the coil will still be
left in circuit with whichever motor may happen to be active. Otherwise, if the
coil, for instance were in series with motor number one, and that motor should be
cut out, there would be no current in the coil and the device would be inoperative.
We have therefore arranged the connections of the ordinary cut-out switches in
the controller so that, whichever motor is cut out, the coil F will still be left in
series with the remaining motor or motors. To render this more clear we show in
Fig. 4 a diagram of a series parallel controlling device of well known form with
the current limiting device of our invention applied thereto, and with proper con-
nections for the cut-out switches. The controlling cylinder is shown developed in
plane, as is also the reversing switch RS. The fixed contacts are numbered on
the controller from 1 to 9, and on the reversing switch from 10 to 17. The cut-
out switches are lettered CO, GO². These switches have back contacts P1, P2.
1" is simply a bridging contact, while P2 connects one blade of the cut-out switch
directly to ground, the other being open circuited.
The series position of the motors, with the resistance R cut-out, may be traced
as follows: From the trolley T to the contact 1, then by the contact plates of the
controller cylinder and the cross-connection to contact 6, through the coil F,
across one blade of the switch CO¹, contact 17 on the reversing switch, contact 16,
through the armature of the motor M', back to the reversing switch, through the
field of motor M', across the other blade of the switch 00, to contact 8 on the
controller, contact 7, across switch CO2, to the reversing switch at contact 13,
contact 12, and so through the armature of the motor M², back to the reversing
switch, through the field of M² to ground.
When contact 8 bridges the contacts 8 and 8b, a shunt is thrown around the
second motor; the circuit of the latter is then opened, and is re-established when
contact 7 touches contact 7b, so that the two motors are connected in parallel.
This is a well known type of controller.
Assume now. that the switch CO¹ is thrown so as to connect its blades by the
back contact ; this will cut out motor M. The circuit is then from the trolley
to contact 6, through the coil F, the first blade of the switch CO2, by back
contact to the other blade of CO2, then back to the controller at contact 8,
contact 7, through CO to the reversing switch, at contact 13, then to contact 12,
through the armature of motor M² back to the reversing switch, through the field
of motor M, to ground; the coil F being thus still maintained in circuit. With
the switch CO2 thrown the motor M2 is cut out, the circuit being from contact 6ª
through the coil F, the first blade of switch CO¹ in its. illustrated position,
contact 17 on the reversing switch, contact 16, through the armature of motor M³,
back to the reversing switch, through the field of motor M', across the second
blade of switch CO', to contact 8 on the controller, contact 7, to the first blade of

page_text: the switch CO2, and by the back contact P2 direct to ground. It will be observed
that the coil F is thus in circuit whichever motor may happen to be cut out.
The operation of the invention is as follows: with the connections shown in
Fig. 3, as the controller is rotated from its off position, the current flows through
the resistance R to the first motor M, then through the coil N of the circuit-
breaker and by the shunt around the coil F, thence through the motor M² to
ground at G. Owing to the low resistance of the shunt, only a small current
passes through the coil F and therefore its armature is not attracted sufficiently
to overcome the spring H. (See Fig. 2). If the controller be so rotated that the
resistance is cut-out too rapidly, the current rises in coil N so that the armature
of the circuit-breaker is drawn down, opening the shunt and sending the entire
current through the coil F, which then attracts armature 01 and the pawl 0
attached to it. However the coil F is connected, when its armature is attracted,
the pawl enters the next one of the notches e as the disc E rotates arresting the
forward movement of the controller; but this has no effect on the backward move-
ment of the handle, the bevel of the notches e being such that the pawl O is forced
out by them by the rotation backward of the disc E. When the current has so
fallen that the spring of the circuit-breaker overcomes the attraction of the
coil N, the contacts are closed and the circuit is established as before.
From the description of the operation as outlined in Fig. 3, the operation of the
device will be apparent, when connected according to any of the modifications
shown, as in Fig. 4 or Fig. 5, or if connected without a circuit breaker.
By our invention a great gain in the smoothness of operation, in tram-cars par-
ticularly, is obtained, since in these the moderate weights and the great power
of the motors make the cars liable to sudden lunges if the power is turned on too
quickly. Some economy is also obtained by the use of the device, although this
is not an important item. We are the first, so far as we know, to proportion the
action of the current-limiting device to the requirements of the motors in such a
way as to adapt this proportioning to the different motor connections; and also
the first to effect this by series paralleling the coils of an electro-magnet in accor-
dance with the relative connection of the different motors, or to arrange cut-out
switches which do not interfere with the current limiting device.
Having now particularly described and ascertained the nature of our said in-
vention (as communicated to us from abroad) and in what manner the same is
to be performed, we declare that what we claim is:-
1. A current limiting device comprising an electro-magnet in circuit with the
apparatus to be protected, its coil carrying only a small part of the current during
normal working a current regulator, a stop for the regulator controlled by the
electro-magnet, and means for throwing the entire current through the magnet
coil when the current rises to a predeterminate maximum.
2. In a current limiting device, a magnet controlling a current regulator, a
shunt around the magnet-coil, and means for opening the shunt when the current
in the divided circuit rises to a given value.
3. In combination, motors and a controller, a ratchet upon the controller, and
a pawl controlled by an electro-magnet in the main circuit; with a shunt circuit
around the electro-magnet controlled by a circuit breaker the coil of which is
in the main circuit.
4. A current limiting device, a motor or motors, and means for proportioning
the effect of the current limiting device to the current flow in the motor circuit.
5. A plurality of electric motors, a series parallel controller, a current limiting
device, and means for proportioning the effect of the current-limiting device to the
current flow in the motors irrespective of the motor-combinations effected by the
controller.
6. A plurality of motors, cut-out switches, and a current limiting device; in
combination with connections arranged to leave the current limiting device always
in series with at least one active motor.

page_text: 7. A plurality of electric motors, a current limiting device having coils in series
with each motor, a series-parallel controller, and connections such that as the
motors are thrown into series or parallel, the coils upon the current limiting
device are connected in circuit in a similar relation.
8. The combination of a number of electric motors with a motor-controller, a
cut-out switch for cach motor, and a current limiting device consisting of an
electro-magnet and a stop for the controller; the cut-outs being so connected that
as any one of them is thrown, it shifts the coil of the electro-magnet to the
circuit of the other motor.
1
1000
THE BRITISH THOMSON-HOUSTON CO. LTD.
ALFRED CLEMENTS,
Assistant Secretary.
W. MARTINDALE,
A. R. MONKS,
}
Directors.