Improvements in Shielding Devices for Electric Meters.
Year: 1897

page_text: Date of Application, 28th Sept., 1897-Accepted, 30th Oct., 1897
E
COMPLETE SPECIFICATION.
Improvements in Shielding Devices for Electric Meters.
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 them from abroad by Elihu Thomson,
of Swampscott, in the County of Essex, State of Massachusetts, United States of
America) and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement:-
The present invention relates to electric meters that are provided with magnetic
damping devices. Our correspondent has found that when a short circuit occurs
on a line in which an electric meter of any of the well-known types having
damping magnets, is included in circuit the sudden instantaneous current of high
value, in passing through the load coils of the meter, will set up a magnetic field
powerful enough to permanently weaken or change the magnetism of the damping
magnets, rendering the registration of the meter inaccurate to a certain degree.
The object of the present invention is to protect these magnets from magnetic
influences due to passing through the meter extremely sudden and high currents,
such as are occasioned by a momentary short circuit, attended by the blowing of
fuses or opening of circuit breakers on the line to which the meter is attached.
In the accompanying drawing, attached to and made a part of this specification,
Fig. 1 is a partial front elevation of a meter embodying the invention; Fig. 2
is a modification of the shielding device shown in Fig. 1; Figs. 3 and 4 are
modifications of the shielding device applied to the damping magnets, and Figs. 5
and 6 are further modifications.
For the purpose of illustration, the invention is shown as applied to the well-
known Thomson recording watt meter, wherein A represents the armature mounted
on shaft X and adapted to revolve within the influence of the field or load coils C.
The armature is wound with fine wire coils which are connected to a commutator,
and is connected in shunt across the supply mains of the system; the load coils
are connected in series with one or more of the mains, and the current therein
varies with the load on the consumption circuit.
To damp the rotation of the armature, a damping disc D is mounted on
the shaft X and revolves within the field of permanent magnets M and M¹.
At the upper end of the armature shaft is a worm meshing with a worm wheel R
which actuates any suitable registering mechanism.
If a heavy current, largely in excess of the normal working current, flows, either
momentarily or for a prolonged interval, through such a motor, a very intense
magnetic field is created around the series field coils. This magnetic field extends
to a point where it affects the magnetism of the magnets M and M', either causing
a loss or change in their magnetism. This will introduce inaccuracies into the
registration of the meter.

page_text: To overcome this objection each of the series or load coils C is provided with a
shield S, which consists of a conductor of good conductivity, made either of wire
or plate, and arranged so as to form a circuit parallel to the conductors of the
series coils C. In this case any sudden or instantaneous influx of current of large
amount, or a current greatly in excess of the capacity of the meter, induces a similar
large opposing current in the conductors or shields S, which prevent a sudden
increase in magnetic effect, and thus prevents injury to the magnets M and M¹.
In Fig. 2 is shown at S¹ a modification of the shield so arranged that it encloses
the coils C on two sides; at So the shield is shown as enclosing the coil on three
sides, and at $3 on all sides of the coil. At S the shield is divided so as to make
a casing surrounding the upper and lower, or inner and outer sides of the coil.
At S the shield is divided and is placed between sections or turns of the series coil.
In Fig. 3 the shields S and S¹, consisting of closed bands of wire or sheet metal
of good conductivity surround the poles of the magnets M and M', and also a
portion of the damping disc D. The series coils. C may, or may not in this case
be provided with a shield.
In Fig. 4 the shields are disposed in a slightly different manner, in that they
do not surround the damping disc D, but form closed coils around the magnets M
and M¹.
In Fig. 5 the shield S is carried as close to the magnet as possible, and com-
prises a heavy copper casting which completely surrounds the magnet with the
exception of the ends and the space between the inner surfaces of the poles.'
In Fig. 6 the magnet M is surrounded by a U-shaped sheet of heavy copper S,
which is somewhat wider than the magnet in order to protect the sides from
stray magnetic lines, and, to protect the ends of the poles, the shield is made a
trifle longer than the magnet.
These various forms may be used either singly or together in carrying out the
invention.
The invention has been above described in connection with a meter which has
an armature with a winding thereon but it is not limited to this; for the invention
will apply equally well to other types of motor mechanism.
Having now particularly described and ascertained the nature of the said
invention (as communicated to us from abroad) and in what manner the same
is to be performed, we declare that what we claim is:-
1. The combination of an electric meter comprising, as part of its motor
mechanism, a field coil or coils in circuit, with the load, and a damping magnet
with a shield of copper or like highly conducting metal forming a closed circuit
or circuits within which currents are induced upon any sudden change in the
magnetic field due to the field coils, said shields being so arranged as to shield
the damping magnets from the effects of such changes in the magnetic field of
the meter, as set forth.
2. The combination in an electric meter of an armature, field or load coils, a
magnetic damping mechanism, and a copper or like shield forming the seat of
induced currents surrounding the field coil or coils, as set forth.
3. The combination of an electric watt meter having an armature wound with
shunt coils, a series field coil or coils, permanent magnets within the field of which
revolves a disc of copper or like metal forming a magnetic damper, and a sheath
of copper or like non-magnetic but highly conducting metal enclosing more or
less completely the field coil or coils of the meter, as set forth.
4. In an electric meter, the combination of a field magnet, an armature, a
magnetic damping mechanism and means for producing a current in opposition
to that flowing in the field-magnet for preventing a sudden increase in magnetic
effect upon the passage of currents of abnormal magnitude through the field circuit.
5. In a recording watt meter, the combination of a moving armature, stationary
field coils, a shield of good conducting material surrounding each of the field coils,

page_text: a disc carried by the armature shaft, and magnets for damping the rotation of
the armature.
Dated the 28th day of September 1897.
J. C. CHAPMAN,
70, Chancery Lane, London, W.C., Chartered Patent Agent.