Note: Descriptions are shown in the official language in which they were submitted.
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~ CURRENT MEASUREMENT SHUNT
; The invention described herein relates to electric
instruments and particularly to a current measuring shunt.
Shunts are used in connection with electrical
instruments for measuring the flow of electrical current in
circuits, particularly circuits carrying heavy current. The
shunt carries most of the current to be measured through a
resistance calibrated in relation to the instrument with
which it is used so that there is a predetermined
relationship between the current through the shunt and the
vbltage drop across the shunt. The electrical measuring
instrument is usually a sensitive milli~olt meter connected
across the shunt to measure the voltage drop, the instrument
itself carrying only a small fraction of the total curren~
being measured, most of which bypasses the instrument and
flows through the shunt. In this case, it is difficult to
maintain its resistance constant because of the heat
generated in the measuring device.
Most shunts currently in use comprise two massive
solid terminal blocks usually formed of copper or brass
castings or extrusions. Extending between the terminal
blocks are a plurality of spaced strips of conducting
material, the ends of which are soldered or welded in slots
machined in the opposing faces of the terminal blocks. These
shunts are costly to build because skilled labour is required
to machine the slots in the terminal blocks accurately and
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because scrap losses are high due to the large amount of
metal in any terminal blocks that must be scrapped because of
machining errors. Furthermore, assembly is difficult because
of the weight of the massive terminal blocks which makes them
difficult to handle.
Also it is necessary to maintain the temperature of
the shunt constant and the device therefore is cooled by air,
but since variation in current flow produces different levels
of heating, a uniform flow of air thereover cannot possibly
maintain a constant temperature. Sînce the resistance of the
copper and pieces varies about 0.4% per degree centigrade and
since the resistance of the welded joints and copper is an
appreciable percentage of the total resistance, the accuracy
of the device is limited to about 1%.
Moreover, known current measuring shunts are not
compensated, that is, the device i8 not protectecl against the
influence of stray magnetic fields. In those cases, the
current is estimated by measuring the voltage drop across at
least a portion of the shunt, and it will be apparent that
the voltage developed will not be a true indication of the
current because of the effect that the induced voltage will
have on the reading. Such stray magnetic fields also distort
the current distribution in the shunt so that it may be
greater in one part than in another. Regardless of where the
sensing or voltage measuring elements are positioned in the
shunt, they will not be capable of sensing an average flow of
current under all conditions of operation
Other known current measuring devices determine the
current values by measuring the flux density surrounding a
current carrying conductor. They consist of a plurality of
L-shaped magnetic yoke sections assembled to form a square
surrounding the bus bar but spaced from each other at their
ends to form air gaps therebetween. The sensing elements
located within the gaps respond to the flux flow thereacross
and thereby provide a reading of the current flowing in the
bus bar assembly. The primary disadvantage of this kind of
construction is the iron of the yoke must be operated at a
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flu~ density less than that which will cause saturation of
the iron. In the event a large number of ampere turns are
involved, such as 500,000, in order to prevent the iron from
saturation, it is necessary to provide Large air gaps which
result in inaccuracy in the reading taken.
The above-cited disadvantages indicate the need for
an improved current measuring shunt capable of both
eliminating the problems connected with known devices and
extending the range to measure current of high magnitude.
The primary object of my invention, therefore, is
to provide a relatively small device capable of measuring
current values up to about 1,000 amperes of frequencies, up
to 40-50 kilohertz and within an accuracy range of about 1%.
Another object of my invention is to provide
current measuring shunt wlth substantial immunity to
electromagnetic effects from adjacent stray field generators.
Yet, another object of my invention is to provide a
shunt which is simple in construction, for~ed o~ relatively
lightweight component parts, and hence is less costly and
easier to manufacture.
SUMMARY OF THE INVENTION
The present invention amply satisfies all of the
foregoing requirements of current measuring shunts
simultaneously and does it, moreover, with a minimum of
expense and complexity.
Broadly, the invention permits the measurement of
current without the influence of stray magnetic fields by
forming two bus bars made from conductive material and formed
by standard etching method on the opposite surfaces of
printed circuit ~p.c.) board. Bus bars are compris~d of a
plurality of conductive portions and portions located on one
surface of printed board are connected with conductive
portions on the opposite surface of the board through holes
in the p.c. board. The particular way of connecting the
conductive portions divides the current into two parallel
branches such that the current flows in a transposed
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configuration. Bus bars are provided with a pair o~
conductive bars that are produced on both surfaces of printed
board to provide a voltage signal proportional to the current
passing through said bus bars.
While the specification concludes with claims
particularly pointing out and distinctly claiming the subject
matter which we regard as our invention, it is believed the
invention will be better understood from the following
description taken in connection with the accompanying
drawings in which:
Figure 1 shows a current measuring shunt.
Referring to the drawing, there is illustrated in
Figure 1 a typical installation of shunt comprising two bus
bars 1 and 2 that are divided in a plurality of conductive
portions. Bus bars 1 and 2 made from a conductive material
such as copper and are formed by standard etchi.ng method on
printed circuit board 3. The conductlve portions of bar 1
belonging to one of the surfaces of printed board 3 are
connected with conductive portions of bus bar 2 belonging to
the opposite surface of printed circuit board 3 through
holes, made in printed board 3.
Current measurement bars 4 and 5 are accordingly
connected by fastening suitable leads to the ends of bus bars
1 and 2 and also are produced on the surfaces of the printed
circuit board 3.
The current is divided into two parallel branches
both of which provide for current flow in a transposed
configuration (shown by arrows on Fig. 1) so as to eliminate
the effect of stray magnetic fields.
The connections of conductive portions through
holes in printed circuit board is performed either by
conducting pins soldered on both sides of the printed circuit
board or by plating the inside walls of the holes by standard
plating techni~ues.
Numerous and varied other arrangements may be
utilized by those skilled in the art without departlng from
the spirit and scope of the invention.
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