Current sensing apparatus

A current sensing apparatus suitable for sensing the amount of current flowing in a power feeder cable without the necessity for any ohmic connection between the meter and the circuit being measured that does not require separation of the wires in a cable and in fact utilizes both current carrying wires simultaneously includes a magnetically permeable core suitable for concentrating the magnetic fields generated by the current flowing in the pair of wires and includes a single coil disposed upon the core. The core includes a pair of poles which are placed proximate to the feeder cable and the coil yields an induced output voltage proportionate to the current flowing in the feeder cable and may be held in position on the feeder cable with the aid of a resilient member which permits ready removal of the sensing apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a current sensing apparatus, and more 
particularly, to a current sensing apparatus that does not require any 
ohmic connection between the sensing apparatus and the feeder cable 
carrying the current. 
The term "ohmic connection" as used herein refers to a direct wire 
connection, such as by a metal contact probe or clamp as employed in 
conventional measurement instruments. 
2. Description of the Relevant Art 
Many devices are available for measuring AC electrical current flowing in 
an electrically conductive wire. Generally, these instruments are of a 
split-core current transformer type in which the transformer core is 
opened and then clamped around an insulated single conductor to measure 
the current flowing in it. However, portable instruments operable without 
an ohmic connection have been limited to making current measurements on a 
single wire (line) since, if the open transformer core encompasses a 
conductor pair the magnetic field passing through the core will be 
canceled yielding either no output or an erroneous output. 
The most pertinent reference of which we are aware is U.S. Pat. No. 
3,056,922 issued to W.E. DuVall, et al on Oct. 2, 1962. The apparatus 
disclosed therein includes a coil having a magnetic core extending 
therethrough. A pair of pole pieces are connected to opposite ends of the 
core and extend, essentially in the form of an L, from the core to the 
pole tips, which are opposed to one another, and define therebetween an 
air gap. This air gap is made substantially smaller than one of the 
conductors in the cable in which the measurement is to be made. 
Preferably, the current sensor is made sufficiently small to be 
substantially shielded by one of the conductors from the magnetic field of 
the other conductors. Means are provided to attach the pick-up head to the 
cable in which the current is being measured with the air gap extending 
parallel to one of the conductors. Preferably, the sensing head is 
positioned with its air gap on a line which passes through the centers of 
the conductors in the cable under measurement. However, the apparatus 
disclosed therein was not meant to accurately measure the current flowing 
in the cable, but rather to provide an induced output voltage which would 
indicate that current had started to flow and no attempt was made to 
obtain linearity (ratio constant) between the current flowing in the 
conductor and the induced voltage in the pick-up coil. The air gap between 
the pole pieces was made much smaller than the cable to be measured and 
consequently the magnetic field passing through the core, via the pole 
pieces, included only the field generated by the current flowing in one of 
the conductors and made no use of the field formed by the current flowing 
in the other conductor. 
In order to overcome the limitations of the known art and provide a means 
for providing a directly proportional and constant ratio between induced 
coil voltage and current flowing in a cable pair the apparatus of the 
instant invention was conceived. 
Therefore, it is an object of the present invention to provide a current 
sensing apparatus that is capable of measuring current flowing in a pair 
of wires of a power feeder cable. 
It is yet another object of the present invention to provide a current 
sensing apparatus suitable for portable field use easily attachable to a 
feeder cable and removable therefrom without requiring ohmic contact. 
It is yet another object of the present invention to provide a current 
sensing apparatus that is accurate, linear, and provides a constant ratio 
of induced output voltage for current flowing in a cable pair. 
It is still yet another object of the present invention to provide a 
current sensing apparatus that utilizes the magnetic fields generated by 
current flowing in a pair of wires of a feeder cable to provide an induced 
output voltage proportioned to the current flowing. 
It is still yet another object of the present invention to provide a 
current sensing apparatus that can accurately measure current flowing in a 
pair of conductors, has an extended current range and is suitable for use 
with a plurality of feeder cables just by changing the proportionality 
(ratio) constant. 
It is yet still another object of the present invention to provide an 
inexpensive current sensing apparatus which may be utilized together with 
other instruments to provide an accurate power rating for equipment in 
use. 
It is yet further another object of the present invention to provide a 
sensing apparatus capable of sensing current flowing in a pair of wires in 
a power feeder cable and provide an output voltage proportionate to the 
amount of current flowing in the feeder cable. 
SUMMARY OF THE INVENTION 
A current sensing apparatus suitable for sensing the amount of current 
flowing in a power feeder cable, according to the principles of the 
present invention, comprises; a magnetically permeable core suitable for 
concentrating the magnetic fields generated by a current flowing in a pair 
of wires in the power feeder cable. The core includes at least two pole 
pieces and an air gap therebetween sufficiently large to receive the 
feeder cable wire therebetween. A single coil is disposed upon the core 
and generates an induced voltage in response to the feeder cable current 
flow when positioned proximate thereto. 
The foregoing and other objects and advantages will appear from the 
description to follow. In the description reference is made to the 
accompanying drawing which forms a part hereof and in which is shown by 
way of illustration a plurality of embodiments in which the invention may 
be practiced. These embodiments will be described in sufficient detail to 
enable those skilled in the art to practice the invention, and it is to be 
understood that other embodiments may be utilized and that structural 
changes may be made without departing from the scope of the invention. The 
following detailed description is, therefore, not be taken in a limiting 
sense, and the scope of the present invention is best defined by the 
appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the Figures, and in particular, to FIGS. 1 and 2 which 
disclose a current sensing apparatus 10 positioned on a relatively large 
feeder cable 12, shown in FIG. 1, and on a relatively small feeder cable 
14, as shown in FIG. 2. The feeder cable 12 includes two wires 16 and 18, 
or as shown in FIG. 2, may include three wires 20, 22 and 24, wire 24 
being a ground wire which may be included in some feeder cables. 
Alternatively a braided aluminum shield wire or braid may be utilized 
(FIG. 7), which would yield the same results as if it were not present, 
since the aluminum ground braid has no effect on the magnetic fields 
(magnetic lines of flux) created by current flowing in the conductor 
wires. With the device of the instant invention it makes no difference 
whether the AC current flowing is flowing in a wire pair 16 and 18 or pair 
20 and 22 since, as will be explained hereinafter, the current sensing 
apparatus 10 relies on the position of the wire pair within the cable 
wherein the magnetic fields generated by the current flow in both wires 
aid or are concentrated (summed) for operation thereof. In order to retain 
the current sensing apparatus 10 in position on the the power feeder 
cables 12 and 14 an elongated resilient or sponge like member 26 is placed 
above and beneath the cable. The current sensing apparatus is slipped 
thereover compressing the sponge like member 26 and maintaining the 
transversely extending pole pieces 28 and 28' longitudinally above and 
below the power feeder cables 12 or 14 essentially parallel to the wires 
16, 18 and 20, 24 carrying the load current that is to be measured by the 
current sensing apparatus 10. 
The pole pieces 28 and 28' are disposed transversely from the core member 
30 which, when viewed in cross-section, is generally U-shaped with pole 
pieces 28 and 28' extending transversely from the distal ends of the arms 
32 and 34 thereof, best shown in FIGS. 4, 6 and 7. The core member 30 and 
its integral pole pieces 28 and 28' may be formed of a single magnetically 
permeable metal. Preferably, in order the achieve linearity (a constant 
output voltage for a given current flowing in a cable pair) a laminated 
core, formed of two dissimilar materials 36 and 38, is utilized (see FIGS. 
4, 5 and 6). Core members 36 and 38 are formed so that one core member 36 
can be received into the opening of the other core member 38 having an 
identical configuration. Preferably, this laminated configuration utilizes 
a core member consisting of one layer of 0.014 inch M5 electrical steel 
backed with one layer of 0.050 inch conventional steel. The M5 steel was 
bonded to the backing of the conventional structural steel with an epoxy 
resin, in a conventional manner. Thus, by bonding 0.014 inch M5 electrical 
steel on to 0.050 inch thick strip structural steel a satisfactory linear 
core was fabricated. Whether a single or laminated core is utilized the 
pole pieces 28 and 28' are positioned parallel to each other and in the 
same plane as the arms 32 and 34 of the U-shaped core. 
Referring now to FIGS. 3, 4, 5 and 6 the laminated construction of the 
current sensing apparatus 10 becomes readily apparent. The U-shaped core 
30 shown in FIG. 4 includes essentially identical core members 36 and 38, 
generally U-shaped when viewed in cross-section. The central portion 40 of 
the core member 30 may be provided with a half bobbin 42 as shown in FIG. 
6 or a full bobbin 44 as shown in FIG. 7. The full bobbin 44 of course, is 
provided with a slot sufficiently wide to recline and accommodate the 
central portion 40 of the core member 30. Thereafter the coil member 46 
may be readily wound thereon. The size of wire utilized for the coil 
member 46 is not critical, since the coil carries very little current. 
Sufficient turns should be utilized to provide an output induced voltage 
ratio of between 1 and 2.5 millivolts per ampere flowing in the power 
feeder cable on which the measurements are to be taken. Thus, with the 
instant apparatus a high impedance measuring device may be utilized so 
that the output, leads 48 and 50 of coil member 46 may be coupled to a 
digital voltmeter or any other type of apparatus which may be calibrated 
in amperes so that the ratio of millivolts to amperes will remain constant 
over the desired current range. 
Also, as is well known by those knowledgeable in the art, by coupling a 
fixed AC voltage (equal to that appearing across the current pair) to the 
device utilized to read the output induced voltage a power reading may be 
obtained by providing conventional means for multiplying the line voltage 
times the current flowing in the feeder cable, taking into consideration 
the power factor thereof, in a conventional manner. 
Referring now to FIG. 7 which shows a cross-sectional view of one 
embodiment of the instant invention affixed to a feeder cable 12 having 
wires 16 and 18 disposed therein and carrying the current which must be 
measured by the current sensing apparatus 10 includes an insulating medium 
or material 52 which surrounds the conductive wires 16 and 18 or may 
include wires 22 and 24 of a smaller size insulated wire. Although the 
thickness of the insulated cable or wires may vary the resilient or sponge 
like member 26 is suitable to fill the gap between the pole pieces 28 and 
28' thus positioning the current sensing apparatus 10 and maintaining it 
on the power feeder cable until the reading is obtained. 
It is to be noted that because of a difference in thickness of the 
insulating material or the distance of the pole pieces 28 and 28' from the 
conducting wires 22 and 24 as compared to wires 16 and 18 a different 
ratio (current flowing compared to the induced voltage provided at the 
output wires 48 and 50 of coil member 46) will be obtained. However, for 
the same type of cable located in a different position, the same ratio 
will be obtained and it will be linear over the entire current range of 
measurement. Some types of power feeder cables include an aluminum braid 
54 surrounding the conductive wire. This braid does not interfere with the 
measurements taken by the current sensing apparatus 10, since the braid 
does not interfere with the path of the magnetic fields or lines of flux. 
In FIGS. 8, 9 and 9A, there is shown an alternative embodiment of the 
current sensing apparatus 60 which includes a pair of L-shaped core 
members 62 and 64 that are provided with transverse pole pieces 66 and 68, 
respectively. One portion 70 of core member 62 has a bobbin 74 disposed 
thereon, adapted to receive a coil 76 provided with output leads 78 and 80 
suitable for coupling to a voltmeter, not shown, in order to read the 
induced voltage appearing when the current sensing apparatus 60 is applied 
to a current carrying power feeder cable. The portions 70 and 72 of cores 
62 and 64 are slidably clamped with a ring member 82 to enable core 64 to 
be moved in the direction of arrows 84 for accommodating various sizes of 
cable which may be placed between the pole pieces 66 and 68 and also 
provides a constant current to voltage ratio. In the embodiment disclosed 
in FIG. 9A, portion 70 of core member 62 is cut to be flush with the edge 
of bobbin 74 and core member 64 is inverted and inserted together with the 
portion 72 of core member 64 into the opening 86 provided in the bobbin 
74, permitting slidable movement between core members 62 and 64, thus 
permitting pole pieces 66 and 68 to be moved to accommodate various cable 
feeder sizes and in addition provides a constant induced current to 
voltage ratio. Here again the transversely disposed pole pieces 66 and 68 
are placed upon the feeder cable longitudinally with the conductive wires 
(parallel therewith), with the feeder cable being disposed therebetween. 
Refer now to FIGS. 10 and 11 which disclose yet another alternative 
embodiment of the instant invention. FIG. 10 is a top plan view of a 
current sensing apparatus 90 which includes a core member 92, generally 
E-shaped when viewed in cross-section as shown in FIG. 11. The central 
portion 94 of core member 92 may be an integral portion of the E-shaped 
lamination or, alternatively, may be comprised a centrally disposed 
conically shaped portion 96, shown in dotted lines on FIGS. 10 and 11, 
having sloped surfaces 98 and 100 provided on the outwardly extending 
distal edges in order to reduce the resistivity of the flux path when the 
current sensing apparatus 90 is positioned close to the power feeder cable 
92, which includes conductive wires 104, 106 and ground wire 108 and the 
conventional insulating material 110 covering the conductive wires. The 
central portion 96 may be affixed to the flat portion 112 of core member 
92 in a conventional manner. A coil 114 is wound around the central 
portion 94 or 96, if a conventional manner, providing the required number 
of turns as indicated earlier, and having two output wires 115 and 117 
suitable for affixment to the indicating volt meter to be used to read the 
induced voltage generated in the coil by current flowing in the wires 104 
and 106. 
In order to more clearly understand the operation of this embodiment as 
well as the earlier embodiments let us consider that current is flowing 
into the paper in wire 104 as indicated by the "+" mark 116 flowing in the 
opposite direction as indicated by the "dot" 118 provided in wire 106. The 
aluminum ground wire 108 has little if any effect on the results obtained 
or on our present discussion. By using the "right hand rule" it can be 
seen that with current flowing into the paper the flux lines or magnetic 
field appears in the direction of arrow 120 in wire 104 and with current 
flowing out of the paper at 118 it can be seen that the flux lines are 
generated in the direction of arrow 122. (Note: the "right hand rule" 
states that if the thumb is pointed in the direction of current flow the 
fingers will depict the direction of the flux lines around a single 
conductor). 
It can seen then that the flux lines generated by the current flowing in 
wire 104 and the current flowing in wire 106 will aid each other or be 
more concentrated only in the area directly between the two conductive 
wires and thus enters the sloped distal edges 98 and 100 of E-core 92 when 
brought in direct contact with the insulating material 110 of the power 
feeder cable 102. Thus, the central portion 94, 96 of E-core 92 causes 
concentration of the aforementioned flux lines providing a return path to 
the outer edges of the E-core back around to the source. Whether the field 
or flux lines are generated by AC current flowing in wires 104 and 106 or 
DC current flowing therein, the flux lines are summed, aided or 
concentrated in the central portion of the E-core. Thus, if AC current is 
flowing in a wire pair it will induce a voltage in coil 114 providing an 
output voltage across wires 115 and 117. In a similar manner, the other 
embodiments provide for the concentration of the lines of flux generated 
in the conductive wires by positioning the pole pieces 28 and 28' 
longitudinally (same direction as the conductive wires) along the power 
feeder cable thus, concentrating the lines of flux appearing at any 
instant in time in both wires simultaneously in the core, which then 
provides the induced voltage proportional to the current flowing in the 
pair of wires of the feeder cable. 
As explained earlier the ratio of AC current flow in the power feeder cable 
to output voltage induced in the coil may be determined for a particular 
cable width or size and once determined for the configuration and 
thickness of the coil and core will remain constant for that particular 
device. In order to maintain the necessary linearity for a large current 
range it is necessary to provide a current sensing device that is capable 
of affixment to various sizes of power feeder cables. Once this ratio is 
obtained it will be a constant which may be utilized by any computing 
system as is conventionally known. 
In order to enable the operator of the current sensing apparatus to choose 
the proper constant and/or spacing between the core pole pieces the device 
or gauge 124 (FIG. 12) may be utilized. The gauge discloses two area 
ranges for the thickness measurement as indicated by the apertures 126 and 
128 which are selected for typical thicknesses of power feeder cables, 
NM10, NM12; NM8; SE6; and NM6 and cables SE4/0, SE2/0 and SE4, 
respectively. The width of these cables may be determined from apertures 
130 and 132 set forth for the same cables. Thus, once the cable size is 
determined it may readily be determined which spacing is required for the 
pole pieces on the current sensing apparatus to be utilized. If a special 
type of cable not mentioned herein is to be utilized it will be necessary 
to determine the ratio constant to be utilized by merely permitting a 
fixed known current to flow in the power feeder cable and determine the 
actual voltage induced at the voltage coil terminals. Once the proper 
constant is determined the ratio will remain fixed over the complete 
current range of the cable. Obviously the output millivolts for accurate 
reading is a function of the sensitivity of the voltmeter providing the 
reading and/or the type of instrument to be used, whether it be a power 
reading device or merely a voltage device calibrated to indicate the 
amount of current flowing in the power feeder cable pair. 
It is well known by those knowledgeable in the art that a Hall effect 
generator may be inserted in the core in the area where the field or flux 
lines are concentrated and thereby eliminate or replace the coil therewith 
and obtain an output voltage proportional to the current flowing in the 
cable conductors in a manner similar to that taught in the present 
invention. 
Hereinbefore has been disclosed an inexpensive readily attachable and 
removable current sensing apparatus that provides accurate readings of 
current flowing in a feeder cable in terms of induced millivolts in a 
sensing cable and provides a constant which is linear over the range or 
current carrying capacity of the cable. It will be understood that various 
changes in the details, materials, arrangement of parts and operating 
conditions which have been herein described and illustrated in order to 
explain the nature of the invention may be made by those skilled in the 
art within the principles and scope of the instant invention.