Year: 1889

page_text: A communication from LEMUEL W. SERRELL and HARVEY L. LUFKIN, both of the
City, County, and State of New York, United States of America.
I, ALFRED JULIUS BOULT of 323 High Holborn in the County of Middlesex,
M.I.M.E. 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 electric motors and has for its object to provide an electric
motor which is adapted to be used in constant current circuits, and which will be self-
regulating and run at practically a constant speed under all variations of load or work
to be performed, and which shall be saving of potential under all stages of the
developement of its powers and practically uniform in efficiency; and the invention
consists in the general arrangement of circuits and devices substantially as hereinafter
described, whereby these ends are attained.
In the accompanying drawings are illustrated the principles of the invention
diagrammatically, and it may be carried out in practice by the use of any well known
devices which are adapted to be used as indicated, and which will readily suggest
themselves to those skilled in the art.
Fig. 1 shows the variable resistance interposed in the differential circuit midway
between the terminals thereof.
Fig. 2 shows the variable resistance interposed in the differential circuit between the
coils and armature; and Fig. 3 shows an arrangement for cutting out the coils of the
differential circuit.
Heretofore the usual way of controlling the power and speed of electric motors used
in series or constant current circuits has been to connect in multiple are with them the
shunt or variable resistance circuit around the motor, or to throw into and out of

page_text: circuit more or less of the field magnet coils. It has also been proposed in compound
wound arc motors to make use of a demagnetizing field circuit of comparatively high
resistance, forming a shunt across the terminals of the motor including a variable
resistance controlled by a regulating magnet arranged in or governed by the armature
or field circuit of the motor.
It is obvious that when electric motors are used in series circuit with other motors
or translating devices, it is desirable that when running free the motor should offer the
least possible resistance to the passage of the current and as the load increases it is
desirable to increase the power of the motor and to diminish ineffective or dead
resistance to the greatest extent, and to cause the motor to operate with a practically
uniform efficiency from slight load to full load.
Referring to the drawing / represents the main line and t, t¹ the terminals of the
motor; a represents the armature and b, b¹ the brushes thereof; f is the main field
magnet or exciting circuit, and d is the differential field magnet or demagnetizing
circuit; r,, etc. are a series of resistances in or loops of the differential coil;
s represents a suitable switch for cutting in or out more or less of the coils and g is
a governor of any suitable construction being shown as a ball governor which controls
the switch.
The differential circuit is of such character as to be of very low resistance relatively to
the resistance of the main energizing circuit, and even when all the coils of the variablo
resistance are included in the circuit the resistance is as low as possible and in this way the
internal resistance of the motor may be made very low especially when working under
a light load and under these conditions, when the motor is running free, the
resistance of the motor is at a minimum, as the currents have two paths across the
field open to it, and gradually, as the load is superimposed, one of these paths, that is
through the differential coil, becomes more and more closed to the passage of the
current, until when the motor running under its maximum load practically all of the
current flows through the main exciting coil and armature.
This will be better understood when it is considered that theoretically the number of
volts generated in the armature depends upon the number of lines of force cut per
unit of time by the Ampere turns of wire in the armature, and the Amperes in the
armature are assumed to be constant, while the number of turns or convolutions of
wire upon the armature and its speed of rotation are also constant and the regulation
is, therefore, dependent upon the variations in the magnetic strength of the field, and
this is done by automatically varying the current flowing through the differential or
demagnetizing circuit by means of the governor and switching devices for including
more or less of the resistances or loops in the differential circuit. In this way the
regulation is effected by automatically varying the resistance in the differential circuit
so that the resultant magnetic moment of the field shall be proportional to the load
upon or the work done by the armature.
It has been found that this arrangement is exceedingly simple, delicate and effective,
permitting all the advantages of a series wound motor and still obtaining practically
perfect regulation.
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 correspondents, I declare that what I claim is―
1. The combination with the main exciting coil of an electric motor of a differential
coil in parallel with the main exciting coil both coils being in series with the armature
and automatic devices so adjusted electrically and mechanically as to compel more or
less of the current to flow through the differential coil in accordance with the work
done substantially as described.
2. The combination with the main exciting coil of an electric motor of a differential
coil in parallel with the main exciting coil which differential coil is connected in
series circuit with variable resistance devices and both coils are connected in series
with the armature the action of the variable resistance being controlled or regulated
by a governor in accordance with the work done substantially as described.

page_text: 3. The combination with a series wound electric motor having a differential coil in
parallel with the main field exciting coil and both coils in series with the armature of
a variable resistance in the differential circuit a governor controlling the variable
resistance so that as the speed of the motor unduly increases less and less resistance
shall be offered to the passage of the current through the differential coil and as the
speed of the motor unduly diminishes more and more resistance shall be offered to the
passage of the current through the differential coil substantially as described.
Dated this 2nd day of February 1889.