Improvements in Systems of Electric Distribution, particularly applicable to Electric Railway Systems.
Year: 1899

page_text: Date of Application, 8th Dec., 1899-Accepted, 20th Jan., 1900
Improvements in Systems of Electric Distribution, particularly
applicable to Electric Railway Systems.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of our
said invention (which has been communicated to us from abroad by The General
Electric Company, of Schenectady, in the County of Schenectady, State of New
York, United States of America, Electrical Engineers,) 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 systems of electrical distribution of the kind in which
one side of the circuit is earthed, and is particularly applicable to electric railway
systems; its object being to obviate the damage due to the electrolytic effects of
the earth currents returning to the generator station by other paths than the rails
or return conductor.
These currents result from different potentials occurring at different parts of
the ground return circuit created by the return currents in the conductors. Ob-
viously, currents will flow through the earth from points of high potential to
points of low potential, and any metallic bodies, such as pipes or armoured cables
lying in the paths of these earth currents are more or less injured by the
electrolytic effects of these currents.
We prevent these currents by bringing all parts of the return circuits to the
same potential, so that the return currents are confined to the return circuit con-
ductors. We accomplish this by running feeders to suitable points of the ground
or rail circuit, and corresponding feeders to the trolley wire or its equivalent.
It is evident that by generating a current of the proper amount in any one
of the rail feeders, the potential at the point where it is joined to the rail, may be
made equal to that at any other part of the rail circuit, as for example, at the
generating station.
Apparatus for more or less automatically producing currents approximately
accomplishing this result by the use of feeders, with suitable generators in the
feeders, have been described heretofore, but we believe the following described
apparatus to be novel and an improvement on these.
For each pair of feeders is supplied a combined motor and generator, with their
armatures rigidly coupled, the two machines making one set.
The armature of the generator G in Fig. 1, is connected in series with the rail
feeder, and the armature of the motor M is connected in series with the trolley
wire feeder.
The shunt field windings may be connected between any two conductors having
approximately a constant difference of potential, being across the main busbars
in Fig. 1.
The connections are so arranged that when M revolves because of the current
in the feeder L, the electro motive-force generated by G is in such a direction as
to assist the natural flow of current in the feeder E.

page_text: As the armature of the motor M is in series with the feeder L, it follows that
there will be a torque produced proportional to this current.
The speed of the motor and with it the generator, will increase until this torque
is balanced by the reaction of the generator. This condition of balance will occur
when the current generated is approximately proportional to the current in the
feeder L, that is, assuming the two fields to be constant in relative strength, the
current generated in the feeder G will be proportional to the current in the
feeder L.
If by adjusting the relative strengths of the fields by rheostats, the current of the
generator G will be made equal to that of the feeder L, then there will be at most
only a slight difference of potential between the two ends of the feeder E, under
which circumstances there can be no appreciable earth currents created by
this system of distribution.
As certain differences of potential are allowable between different portions of
the earthed circuit, such a close adjustment would not be necessary.
When the adjustment of the field strengths has once been made for any par-
ticular feeder or sets of feeders, the apparatus will, in virtue of the constant pro-
portionality of the motor and generator currents, keep the potentials at the further
end of the rail feeder at the desired value, although these currents may vary
over such values as commonly occur in traction work.
At the very low currents the proportionality does not so closely hold. but as
the differences of potential caused by such small currents would not cause damage,
the need of close regulation in the apparatus is not urgent.
Matters attaining to the proportioning of the feeder resistances and motor gene-
rators are now well known in the electrical arts.
It will probably be found advisable to over-compound the station generator
sufficiently to compensate for the drop in the voltage in the motor armature. The
amount of this drop will depend on the amount of energy it has been decided shall
be expended in the rail circuits and feeders.
Having now particularly described and ascertained the nature of our said inven
tion (as communicated to us from abroad) and in what manner the same is to
be performed, we declare that what we claim is:-
1. In a system of distribution having earthed conductors, the use of a motor-
generator set having the armatures of the motor and generator members con-
nected in series with the line and earth conductor feeders respectively, substantially
as and for the purpose herein before described.
Dat thi 001
1000
THE BRITISH THOMSON HOUSTON CO., LTD.
ALFRED CLEMENTS,
Assistant Secretary.
W. MARTINDALE,
}
T. A. THOMPSON,
Directors.