Improvements in Variable Speed Electric Motors.
Year: 1893

page_text: Date of Application, 10th Jan., 1893-Accepted, 18th Feb., 1893
COMPLETE SPECIFICATION.
Improvements in Variable Speed Electric Motors.
I, CHARLES ARTHUR ALLISON, of 52 Chancery Lane, London, in the County
of Middlesex, Patent Agent, do hereby declare the nature of this invention (which
has been communicated to me from abroad by Carl Hering, a citizen of the United
States, residing at Philadelphia, in the County of Philadelphia and State of
Pennsylvania, Electrical Engineer,) and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:―
The object of the present invention is to provide a means for running an electric.
motor efficiently at greatly varying speeds, with increase of torque as the speed
diminishes, or in other words, to obtain the equivalent of a double cone pulley for
varying the speeds by purely electrical means and without great loss in the efficiency
of the motor.
Such a motor is especially applicable to electric railroads, elevators, cranes, etc.,
on which the motors are started frequently with more than their normal load and
ought therefore to have even a greater torque at the starting speed. This is
obtained by means of my invention.
The principle of the invention is as follows:-Suppose two similar electric
motors have their revolving armature shafts coupled together, and suppose the
field of the first one be stationary, whilst that of the second one be capable of
revolving around the shaft. If the currents in the fields and armatures be regulated
so that the speeds of the two armatures in their own field are absolutely the same,
then there will be no tendency for the movable field to revolve; but if by regulating
the field or armature currents or both, of either or of both of the motors, the speed
of the armature in its field is made to differ in the two motors, the revolving field
will rotate at a speed equal to the difference between the speeds of the two motors,
and in a direction depending on whether the second motor is running faster or
slower than the first. Furthermore, the torque of the revolving field will be very
great at small speeds and will diminish as the speed increases.
If, for instance, the speed of the first motor be made 1000 revolutions per minute,
and that of the second be made 999, then the movable field will revolve once a
minute; or if the speed of the second motor be made 1001, its field will revolve
once a minute in the opposite direction. By reversing the currents so as to tend to
reverse the direction of rotation of one of the armatures, the field will revolve with
a speed equal to the sum of the two speeds.
By simply manipulating the currents slightly, variations of speed can be obtained
in either direction between zero and the sum of the two motor speeds. The two
armatures in this case simply revolve in their bearings, while the driving pulley,
(or the car wheel of an electric car) of the combined motors is connected to the
revolving field. The armatures may revolve continuously while the driving pulley.
is at rest.
It is evident that the same principle may be carried out by coupling the first
armature to the second field and letting the second armature drive the driving
pulley, or to couple the fields and let them both revolve, while one armature is
fixed and the other drives, in which latter case only one field might be used in
common; but the latter does not admit of such good regulation.
It is evident, therefore, that the terms field and armature are in general inter-
changeable in this description of my invention.
It is evident also that instead of coupling the armatures at their shafts, they may
be coupled by means of belts or other gearing, which may have a ratio of one to
one, or any other; in the latter case there will be a compounding difference in the
relations of the speeds.

page_text: In using the words "stationary part" I mean that that part does not revolve,
but such part can of course be placed on a moving car or elevator.
In the accompanying drawings
1:
Fig. 1, shows in side elevation a construction that might be employed in carrying
out the invention.
A, indicates the stationary motor or motor with a stationary field and B, a motor
whose field and armature can both revolve. In the present instance this armature
is shown coupled or connected mechanically in any desired way with the armature
of A.
P, is the driving pulley or wheel to which the power of the motor is imparted,
said pulley or wheel being properly connected with that one of the parts of B, that
is not mechanically coupled or connected with A. The pulley P, is merely typical
of any part or device driven by the motor.
It is evident that the motors may be shunt, series or compound wound or one of
them of one kind and the other of another kind and that they may be variously
connected with each other as, for instance, in series, in multiple arc or in various
cross connections between the two armatures and the two fields.
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 :-
A duplex electric motor consisting of two electric motors both having the usual
parts, namely, an armature and a field, one of which parts is fixed, two of which
revolve and are mechanically coupled, and the fourth of which is capable of
revolving and is connected to the work.
Dated this 10th day of January 1893.
ALLISON BROS.,
Agents for the Applicant.