FIELD OF INVENTION
This invention relates generally to an apparatus for contact less detection of DC
load current. More particularly, this invention relates to a portable apparatus for
detecting characteristics of electrical load current and its operating duration, as
they occur in direct current (DC) circuits, for example in electric arc welding
circuit. The detected current can provide a signal indicating a current value
beyond a pre-determined value, which can be used to operate an integrating
timer.
BACKGROUND OF THE INVENTION
In welding or fabrication shops having electric arc welding facilities, wages and
incentives payable to welding operators are generally based on, among other
factors, the size and quantity of welding rods deposited on a work piece and the
operating duration related to such tasks. There are known relationships between
welding rod size and load current drawn, and quantity of rods used and
operating duration.
Similarly there are situations where the run-hour information of a DC load, like
for example in DC cranes, etc. may be required. This is needed for indicating the
time left in the warranty period of the machine as many manufacturers warranty
their machines for certain run hours.
Known devices for detecting DC load current use shunts which require power
circuit alteration for adding the shunt. Some others used Hall effect sensors
which are very costly for such small equipments when we consider installing one
such sensor in each welding machine carrying current from 0-600 Amperes.
For measuring the load and its duration, the instruments should be of portable
type and a number of such instruments are required.
Thus there exists a need for providing a more effective way of detecting DC load
current.
SUMMARY OF THE INVENTION
One object of this invention is to provide a portable instrument for detecting
characteristics of electrical load current and its operating duration without any
alteration of power circuit.
Another object of the invention is to provide a low cost solution of the problem
(as compared to shunts or Hall sensors used for DC measurement or detection).
Yet another object of the inventions is to provide a contact less detection of DC
load current and its operating duration.
The invention uses the magnetic field generated for current sensing without any
special formation of the current carrying conductor. The sensor is not physically
connected to the conductor and there is no limit as to how high a current it can
sense. Existing current measurement systems have limits of current range
where it operates but the present invention can be utilized for any range and any
value of current i.e., same sensing arrangement can be used for a wide range of
current measurement.
The current carrying conductor which is carrying the DC load current of a
machine to be monitored will have a magnetic field generated around it. The
field strength D of the magnetic field around the current carrying conductor can
be shown as
B = (u0NI)/(4hI)
where
uo = permeability of air,
N = number of conductors, which in the present case is 1;
I = current through the conductor carrying a DC current and
I = perpendicular distance from the conductor
Mo and N being constants,
B=KI/I
where:
I = current through the conductor carrying a DC current and I = perpendicular
distance from the conductor.
In a minute magnetic switch if the two magnetically and electrically conductive
pieces of material which require a certain field strength Bi to make them attract
each other and kept at a distance of li from the conductor carrying DC current,
the current li required for closing the switch can be obtained from
Bi = KIi/li
Therefore, by measuring the distance where a given minute magnetic switch
closes, the current which is flowing through the conductor carrying DC current
can be estimated.
In other words the distance I can be calibrated in current scale and the minute
magnetic switch can be put at a distance for obtaining a signal for the equivalent
amount of DC current flow corresponding to this distance.
This signal of current flow of a certain preset DC value can be used for any
recording device, indicating device, integrating device, a protective device or an
alarm.
For measuring the run-hour of a DC load, such a signal can be used in an
integrating time recording device.
For estimating, detecting and recording run-hour of DC current of welding
machines the present invention uses a distance calibrated magnetic contact
which uses the welding lead% own magnetic field due to the current flowing
through it. This does not require any alteration of the welding lead and, senses /
estimates the current just by being positioned at a defined distance. The
distance where it senses the current, is an estimation of the current through the
welding lead.
The low cost DC current estimation and detection of the present invention may
be used in protective, indicative and recording devices for DC current / load.
Thus the present invention provides an apparatus for contactless detection of DC
load current, said apparatus comprising a U-clamp for looping round a cable
carrying a DC current to be detected; at least one set of sensing elements
provided on said U-clamp for sensing the presence of a DC current in said cable;
and a magnetic switch provided in said set of sensing elements; said magnetic
switch set at a distance form said cable suitable for detecting a preset amount of
current.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with the help of the accompanying
drawings, where
Figure 1 shows the perspective view of a detecting apparatus of the
present invention which can be fixed around a DC current carrying
conductor ;
Figure 2 shows the schematic drawing of the detecting apparatus of the
present invention ;
Figure 3 shows the block diagram of the detection method ;
Figure 4 shows the principle of operation of the apparatus of the present
invention ;
Figure 5 shows the U-clamp and the flexible sleeves used in the
apparatus ;
Figure 6 shows the insulator plate for carrying the magnetic switch ;
Figure 7 shows the terminals and the minute magnetic switch ;
Figure 8 shows a sectional view of the assembled apparatus of the
present invention ;
Figure 9 shows a sectional view of the assembled apparatus positioned
around the cable ;
Figure 10 shows a schematic representation of welding time recording by
the apparatus.
As shown in Figure 1, a first set of sensing elements F of the apparatus
of the present invention comprises an insulated U-clamp C used as
means to loop around a conductor 2 (Figure 2) carrying DC. The U-
clamp C has threads all throughout its length and provided with
insulating sleeve around it. This U-clamp C supports the detector
apparatus around the conductor 2 carrying DC current and also other
elements of the apparatus.
The sensing clement F further comprises a minute magnetic switch E
mounted on an electrically insulated plate D. The arrangement is such
that the switch E lies perpendicular to the cable 2 which passes through
the U-clamp C. The minute magnetic switch E is placed on a slit made of
its size on the insulating plate D and also supported and terminated
electrically at terminal connectors B for giving signal connections.
Fasteners A at both sides of the plate D are provided for fixing the
insulating plate D on the U-clamp C so that its position on the arms of
the clamp C can be adjusted to suit various currents.
To hold the detector around the cable the insulating plate D should be fixed near
the cable 2 in U-clamp C using fasteners A. This set of parts will ensure the
closing of minute switch E as soon as any low value DC current passes through
the cable whose magnetic field can make the minute switch E to close.
A second set of sensing elements F provided on U-clamp C comprises similar
elements as in the first set F, like electrically insulating plate D, magnetic switch
E, fasteners A and terminals B for giving signal connections. The second set of
sensors r is used to sense a certain current by adjusting its position on the u-
clamp C by its own set of fasteners.
Thus there are two signals from this device fixed around a cable - one for
detecting the presence of a low DC current (often the lowest possible sensing
current for the device) and the other for detecting the presence of a high DC
current which can be set to any current value based on its distance from the
cable carrying DC current inside the loop of the U-clamp C.
A typical use of the detection apparatus in a welding machine arc-hour recording
system is shown m Figure 2.
A welding machine 1 is shown having positive (+) and negative (-) terminals.
The sensing elements of the detection apparatus (as discussed in detail with
reference to Figure 1) is fixed on the positive welding lead 2.
This is done by opening out all parts of the detection apparatus and then putting
the U-clamp C of detection elements F around a cable 2 carrying DC current and
then assembling the detection apparatus around the cable 2.
The contacts of the minute switch E close whenever there is an arc formed due
to striking of welding rod on an earthed conductor (may be a job to be welded).
The operating duration signal generated by switch E is carried to a recording
device 3 through cable pair 4.
The device 3 can be an integrating type time recording device which records the
hour, minute and seconds of the period during which the arcing was done.
The sensing switch E is set at a distance which is based on analytical calculation
as discussed earlier. The field strength at which the magnetic switch wilt close
can be obtained from the manufacturers catalogue.
The switch can be set at a distance which is suitable for detecting that a preset
amount of current is there which justifies an electric arc.
The same principle can be applied for detecting conditions of over current, under
current, etc.
WE CLAIM :
1. An apparatus for contactless detection of DC load current
comprising:
a U-clamp (C ) for looping round a cable (2) carrying a DC current to
be detected ;
at least onset of sensing elements (F) provided on said U-clamp for
sensing the presence of a DC current in said cable ; and
a magnetic switch (E) provided in said set of sensing elements ;
said magnetic switch set at a distance from said cable suitable for
detecting a preset amount of current.
2. The apparatus as claimed in claim 1, wherein said set of sensing
elements are provided in a suitable housing.
3. The apparatus as claimed in claim 1, wherein a second set of sensing
elements (F') can be provided for sensing a DC current by adjusting the
position of said second set of sensing elements on said U-clamp.
4. The apparatus as claimed in claim 3, wherein the signal generated by
said magnetic switch can be coupled to an indicating device, a recording
device, an integrating device, a protective device or an alarm.
5. The apparatus as claimed in claim 4, wherein said signal generated by
said magnetic switch is coupled to an integrated recording device (3) for
measuring and recording the run-hour of the DC load.
6. An apparatus for contactless detection of DC load current
substantially as herein described and illustrated in the accompanying
drawings.

The invention discloses an apparatus for contactless detection of DC load
current which comprises a U-clamp (C ) for looping round a cable (2)
carrying a DC current to be detected, at least one set of sensing
elements (F) provided on said U-clamp for sensing the presence of a DC
current in said cable, and a magnetic switch (E) provided in said
set of sensing elements, said magnetic switch set at a distance from said
cable suitable for detecting a preset amount of current.