Improvements in and relating to Alternating Current Dynamo Electric Generators.
Year: 1897

page_text: Date of Application, 5th Oct., 1897-Accepted, 13th Nov., 1897
COMPLETE SPECIFICATION.
Improvements in and relating to Alternating Current Dynamo
Electric Generators.
I, CHARLES SCHENCK BRADLEY, of Avon, in the County of Livingston, in the
State of New York, United States of America, Electrical Engineer, do hereby
declare the nature of this invention, and in what manner the same is to be per-
formed, to be particularly described and ascertained in and by the following
statement:
This invention relates to dynamo electric generators, and its object is to obtain
the production of a current or currents of variable frequency or rate of alternation
with a prime mover operating at practically constant speed. This invention is
specially designed for service in connection with induction motors which, when
provided with secondary windings of constant resistance, require a low frequency
and electromotive force in starting and an increased frequency and electromotive
force as the speed rises. In carrying out my invention. I develop a rotary
magnetic field by currents furnished by an exciter in one element of the generator
and drive the said exciter by the primo mover at a variable speed, thus producing
any desired speed of the magnetic poles with relation to the co-operating element
of the generator, the winding of which is connected with a distribution circuit
containing the induction motors. The connections with the exciter are 80
arranged that the direction of polar travel of the rotary field is opposite to the
bodily movement of the core in which the said rotary field is set up, thus
establishing a differential speed of polar travel with relation to the generating
winding. The circuit including the magnetising generator winding is provided
with adjusting devices for varying its capacity or inductance or both to render the
circuit resonant to the different rates of alternation which may be set up by the
exciter. The result is preferably effected by adjustable condensers controlled by
the same device which regulates the speed of the exciter, so that resonance or
approximate resonance may be maintained in the exciter circuits throughout its
working range of speed. The invention is especially applicable to plants in which
a number of motors are simultaneously varied in speed as, for example, in operat-
ing cars and trains of cars. In such cases the organisation I provide forms a
self-contained equipment.
In the accompanying diagrammatic drawings Figure 1 is a diagram showing
apparatus employed in carrying my invention into effect, the speed varying devices
of the exciter being omitted for clearness of illustration; Figure 2 is a diagram of
the speed varying devices for the exciter; and Figure 3 illustrates a modified
form of generator winding. 1, 2, and 3 represent a series of induction motors in
the primary element of which is set up a rotary magnetic field by polyphase
currents in the mains a, b, c. The motor secondaries are provided with closed
circuits which may be of constant resistances; these are shown as simple
squirrel-cage" windings. The mains may be charged with polyphase currents
of any order, as tri-phase, quarter-phase, and the like. Figure 1 shows the
system organised for tri-phase currents; Figure 3 illustrates it for quarter-phase
currents. a, b and o̟ are mains connected with three symmetrical points of the
"6

page_text: generating winding of a generator 4. The magnetising winding 5 of this generator
is driven by a prime mover, acting for example on a pulley 6, see Figure 2. 7 is
an exciter which is geared or otherwise connected with the shaft which carries
the winding 5 by a variable speed device, shown as two cone pulleys 8 and 8"
with an intervening coupling or belt 9 and a shifter 10. The windings of the
generator and exciter are shown diagrammatically, similar reference letters being
used to show corresponding parts. The field magnet of the exciter may be charged
by a direct current or in any suitable way. Its armature is tapped at a plurality
of symmetrical points leading through ring contacts 11, 11", and 11b, upon which
bear brushes leading to the magnetising winding of the generator through corre- 1
sponding brushes and ring contacts 12, 12", and 12. Between the exciter
armature and the generator are devices 13, 13, and 13 for varying the capacity
inductance product of the circuits. By a proper adjustment the circuits may
be rendered resonant to any rate of alternation and when so adjusted the opposing
effect of inductance is eliminated and electromotive force and resistance are the
only factors affecting the strength of current, and a large current will flow in the
circuits permitting an exciter of small size to be employed to set up the rotary
field of the generator. Resonance may be established by adjusting either the
capacity or the inductance of the circuits, so that the product of the capacity (in
25,300
microfarads) and the inductance (in henries) will equal
R representing
K²
the number of alternations per second in the magnetising winding. I prefer to
use a condenser or condensers as the adjusting element, and one such is shown
for each branch of the tri-phase circuit at 13, 13" and 13b, being arranged so that
more or less surface may be cut in by the same operation of the handle 14 which
operates the shifter and speed regulator. The parts are so arranged that the
capacity will be least when the motors are being started, since the rate supplied
to the motor in starting should be slow, which requires the speed of the rotary
field to be closed to that of the revolving core; that is to say, the rotary field
and therefore the exciter should have a maximum speed and maximum rate,
which will be resonant to or in tune with the circuit when the latter has the
least capacity. As the motors rise in speed the capacity should be increased by
a movement of the controlling handle, and the rate of the exciter is slowed down
by the accompanying movement of the shifter. The brushes 12, 12" and 12 are
connected with the exciter leads in such order as to develop a rotary magnetic
field on the magnetising winding of the generator in an opposite direction to the
motion imparted to the core by the shaft; thus the actual speed of travel of the
rotary field poles relatively to the generating winding on the core 4 is differential,
and may be of any value from zero to the maximum difference of speed. Thus it
will be seen that in starting up the handle may be thrown to the left giving a high
speed to the exciter armature and a low speed of cutting of the generating winding
by the lines of force of the rotary field in element 5, by reason of the close
approximation in speed of the rotary magnetic poles and the core 5 and in opposite
directions. Thus the winding in core 4 which is the generating winding is cut
but slowly and a low rate of alternation is supplied to the consumption cir-
cuit a, b, c. The motors then start with their primaries excited by currents of
low rate and low electromotive force. The handle is then gradually shifted to
the right, producing an increased rate and electromotive force in the circuits a, b, c,
and driving the induction motors at increased speed.
It will of course be understood that it is relative, motion of the two elements
of the generator which controls the generation of current, and that either may a
be the rotating element, and that the arrangement of the 'two elements as to
outside and inside part is also reversible. It is not necessary that the exciter
should deliver polyphase currents to the magnetising circuit, as single phase
currents will set up a rotary magnetic field under the reaction of the co-operating
element of the generator, especially when induction motors are connected with the
distribution circuit. Such an arrangement is shown in Figure 3 of the drawing
..

page_text: in which also the generating circuits are shown as quarter phase. In Figure 1
the condensers are shown in series relation to the coils of the magnetising winding;
they may also be in multiple are relation as shown in Figure 3.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is:
1. An alternating current electric generator provided with means for varying
the rate of alternation, and adjusting devices to vary the product of capacity and
inductance to produce resonance at the several rates, arranged substantially as
herein before described and as illustrated by the accompanying drawings.
2. An alternating current generator provided with a rotary field winding on its
magnetising element, an alternating current exciter for furnishing the magnetis-
ing current, and means for varying the rate of alternation of the exciter, sub-
stantially as herein before described and as illustrated by the accompanying
drawings.
3. An alternating curre.. generator provided with a rotary field winding on its
magnetising element, an exciter of variable rate of alternation and adjusting
devices for varying the product of capacity and inductance to produce electrical
resonance, substantially as hereinbefore described and as illustrated by the accom-
panying drawings.
4. The hereinbefore described system comprising one or more induction motors,
a generator having its generating winding connected therewith and a rotary field
winding on its magnetising element, and means for varying the speed of the
rotary field, substantially as set forth and as illustrated by the accompanying
drawings.
5. The herebefore described system comprising one or more induction motors,
an induction generator having its secondary connected with the said motor and a
rotary winding on its primary, means for varying the speed of the rotary field,
and adjusting devices for varying the capacity inductance product of the rotary
field circuits to vary the speed of the motor, substantially as hereinbefore
described and as illustrated by the accompanying drawings
Dated this 4th dox of October 1807
W. E. HEYS & SON,
Agents for the Applicant.