DEMAGNETIZING CIRCUIT FOR DC MAGNETS
FIELD OF INVENTION
The present invention relates to demagnetizing of the DC magnets. More
particularly, the present invention relates to modification in the currently use DC
magnet circuits with the use of the fixed resistances, especially that in the EOT
cranes.
BACKGROUND ART
When a lifting magnet is used mounting on a crane or creeper, it's working
capacity in carry processes, such as a steel rod, is good. To the electromagnet
used with a lifting magnet, a direct current is supplied from the battery source of
a crane, or a power plant for exclusive use, and an electromagnet is excited and
demagnetized by the switching operation of an electric power switch at it. In
order to acquire a powerful suction force and adsorption power, the back
electromotive force of high tension generates this electromagnet in the self-
inductance effect of a coil at the moment of Kai of an electric power switch by
constituting from a mass coil and a mass iron core, and energizing a high
current. This back electromotive force usually reaches by about 5 to 6 times the
service voltage, and brings an adverse effect to spark generating in an electric
power switch part, and damage-by-fire / consumption pan of contact at power-
system apparatus.
In the prior art a Chinese utility Patent application CN201122480U discloses a
demagnetizing device of electromagnet. The demagnetization device comprises
a working iron core, an electromagnetic coil and an electromagnetic coil working
control circuit, wherein the electromagnetic coil is also electrically connected
with a current attenuation oscillation circuit which is connected with the
electromagnetic coil working control circuit in parallel, and a control relay used
for controlling the electromagnet through the operating current or the damped
oscillation current is electrically connected in the electromagnetic coil working
control circuit; after the direct current electromagnet of the electromagnetic coil
is powered off, a charging capacitor is charged by the alternating current power

supply through the current attenuation oscillation circuit and the electromagnetic
coil, the charged current gradually becomes smaller along with the continuously
increase of the voltage at both ends of the charging capacitor, the current
passing through the electromagnetic coil is alternatively changed, therefore, the
purpose of ensuring an electromagnetic cylinder to be quickly demagnetized is
achieved; the problem of remnant magnetism of the improved electromagnetic
coil is solved, to ensure a bunker scale to be automatically reset after long term
operation, therefore, the labour production efficiency is enhanced.
Currently all dc magnet circuits invariably use a demagnetizing powered circuitry
for demagnetizing the magnets. A typical circuit used is shown in figure 1. In the
circuit of Fig.1 disclosed by this application, the right reverse connection circuit
of parallel connection and cross connection which has electric power switch
contact to a main power supply cable run as a changeover switch part,
respectively is provided. When 'LIFT' contact was given from the master
controller, lift contactor marked 'L' picked up. During this time only resistance
R1' (5 ohms) came in series with the magnet marked 'M1' and 'M2' (its value
being 5 ohms). An equal resistance in series with the magnet 'MV and 'M2'
dropped the line voltage (460V dc) to the required 230V dc. Thus the magnet got
magnetized to lift the load (at 230V dc in this typical case).
When 'DROP' contact was made in the master controller (to drop the load), lift
contactor 'L' dropped and the drop contactor 'D' picked up. This action applied a
voltage in the reverse polarity across the magnet 'M1' and 'M2'. During dropping
action the resistances in line with the magnet were 'R1\ 'R2', 'R5' and 'R4'. This
action dropped the load. Then master controller was brought to the neutral
position and made ready to perform the next lifting action.
Note that the 800 ohms (r3) resistance shown in the figure was used only for the
control circuitry of the contactor coils of 'L' and 'D'. This economy resistance was
for reducing the voltage level of the 460V dc coils of 'L' and 'D'.
At the time of right connection or reverse connection, this circuit holds the switch
of a separate short circuit to close, and returns the switch of this short circuit to
open after the fixed time of power supply cut off. While preventing spark
generating accompanying the excessive back electromotive force generated on

an electromagnet at the time of power supply cut off, short-time reverse
excitation of the electromagnet is carried out by operation of a changeover
switch part after power supply cut off, the remnant magnetization of the
adsorbate is demagnetized, and the adsorbate is made to secede from this
electromagnet by this operation.
Drawbacks of the Known Art:
The drawbacks of this system are
1. The circuit is complicated
2. The cost of the panel is higher due to use of dropping contacts/ contactor.
Overall 4 power contacts are in use
3. Application of reverse polarity voltage during drop action reduces the life of
the magnet coil.
SUMMARY OF THE INVENTION
The present invention is proposed in order to eliminate inconvenient and
malfunction in the aforementioned demagnetising circuit, and an object of the
present invention is to provide the circuit which carries out full demagnetization
of the remnant magnetization of the adsorbate rationally about a lifting magnet
etc. If a hydraulic operation valve/ switch is promptly returned to off state, when
an electromagnet maintains an adsorbed state, even if it detaches from an
operating pedal etc., the circuitry, disclosed in the present invention, configured
to continue the carry process safely, while shortening secession time further.
Another purpose of the present invention is to provide a simple, compact, and
inexpensive control circuit.
Yet another purpose of the present invention is to provide the control circuit
which can avoid the fortuitous accident which control machinery is not worn out
and also contact opens by vibration etc.
Therefore the primary objective of the invention was to eliminate the dropping
contacts from the circuit and achieve the desired function by use of the fixed

resistances. This made the magnet panels cheaper to build, achieved trouble-
free operation and offered a very simple design.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 shows the circuit for demagnetizing of the DC magnets using drop contact
as prior art;
Fig. 2 shows the circuit for demagnetizing of the DC magnets using fixed
resistances in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
It is to be understood that the present disclosure is an exemplification of the
principles of the invention and does not limit the invention to the illustrated
embodiments.
The demagnetizing circuit for DC magnetises shown in fig. 2, wherein when
'LIFT' contact is provided, the contactors 'L1' and 'L2' are picked up. The
dropping resistance 'DR' (6ohms) and discharge resistance 'Dl' then gets
enabled in the circuit. The 'DR' resistance is configured for reducing the voltage
(voltage drop) across the magnet (marked between 'M1 and' M2') from 460V to
210 V. Thus the applied voltage across the magnet could be changed by
adjusting the tapping in the resistance 'DR'. In the circuit the Dl acts as a dummy
resistance. The current through the said dummy resistance (D1) was (210V/
24ohms) 8.75 amperes while that through the magnet is 35 amps (210V/6
ohms). This said action is used to lift the load through the magnet.
When master controller was brought to zero position, the potential across the
'L1' and 'L2' contactors are dropped and the residual magnetism of the magnet
was discharged in the discharge resistance (Dummy resistance) 'Dl', thus
dropping the load.
As per the aforesaid discussion, the disclosed demagnetization circuit did not
use the any dropping contactors, instead, the circuit has been modified by using
the discharge resistance (demagnetisation) or dummy resistance
(magnetisation). The same function was achieved by using the fixed discharge

resistance 'Dl' across the magnet 'M1VM2. The delay in dropping the load was
insignificant when compared to the currently used circuitry.
Best mode of working the invention:
This invention has been tested in circular magnets
Magnet ohmic value/ KW: 5.6 ohms, 9.5KW





5. LIFT Master controller lift contact :
6. DROP master controller drop cont :
7. M1.M2 Magnet& connection point :
Although the foregoing description of the present invention has been shown and
described with reference to particular embodiments and applications thereof, it
has been presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the particular embodiments
and applications disclosed. It will be apparent to those having ordinary skill in the
art that a number of changes, modifications, variations, or alterations to the
invention as described herein may be made, none of which depart from the spirit
or scope of the present invention. The particular embodiments and applications
were chosen and described to provide the best illustration of the principles of the
invention and its practical application to thereby enable one of ordinary skill in
the art to utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. All such changes,
modifications, variations, and alterations should therefore be seen as being
within the scope of the present invention as determined by the appended claims
when interpreted in accordance with the breadth to which they are fairly, legally,
and equitably entitled.

CLAIMS:
1. A demagnetizing circuit of the DC magnets comprising of :-
a pair of contactors ('L1' and 'L2');
a dropping resistance 'DR' connected in series with the magnet;
a discharge resistance 'Dl' connected in parallel with the magnet; wherein in the
event of charging and under condition that the lift switch is connected, the said
contactors are picked up and the dropping resistance 'DR' reduces the voltage
across the magnet (between'MI and' M2') from 460V to 210 V and the current is
35 amps (210V/6 ohms).
2. A demagnetizing circuit of the DC magnets as claimed in claim 1, wherein the
applied voltage across the magnet could be changed by adjusting the tapping in
the resistance 'DR'.
3. A demagnetizing circuit of the DC magnets comprising of :-
a pair of contactors ('L1' and 'L2');
a dropping resistance 'DR' connected in series with the magnet;
a discharge resistance 'Dl' connected in parallel with the magnet; wherein in the
event of discharging and under condition that the lift switch is disconnected, the
potential across the said contactors are dropped and the residual magnetism of
the magnet was discharged in the discharge resistance 'Dl', thus dropping the
load.
4. A demagnetizing circuit of the DC magnets as claimed in any of the preceding
claims, wherein the circuit do not use the dropping contactors, instead the same
function was achieved by using the fixed discharge resistance 'Dl' across the
magnet which act as a dummy resistance while charging.
5. A demagnetizing circuit of the DC magnets as claimed in any of the preceding
claims, wherein the application of reverse polarity voltage across the magnets
for the purpose of discharging is eliminated

6. A demagnetizing circuit of the DC magnets as claimed in any of the preceding
claims, wherein the value of the discharge resistance 'Dl' is preferably four times
the ohmic value of the magnet ohmic value.

ABSTRACT

The present invention relates to demagnetizing of the DC magnets. More particularly, the present invention relates to modification in the currently use DC magnet circuits with the use of the fixed resistances, especially that in the EOT
cranes.