U.S. patent number 3,795,334 [Application Number 05/314,460] was granted by the patent office on 1974-03-05 for method for transporting loads safely by utilizing a permanent magnet type lifting magnet.
This patent grant is currently assigned to Hitachi Ltd.. Invention is credited to Kunio Harada, Shinnosuke Ishida, Susumu Itoh, Tadashi Kiyoshe, Akiyoshi Sotodate, Reizi Takeuchi, Tadashi Wachi.
United States Patent |
3,795,334 |
Ishida , et al. |
March 5, 1974 |
METHOD FOR TRANSPORTING LOADS SAFELY BY UTILIZING A PERMANENT
MAGNET TYPE LIFTING MAGNET
Abstract
A method for transporting loads safely utilizing a permanent
magnet type lifting magnet having a permanent magnet provided in
its magnetic circuit and an exciting winding for controlling the
magnetic flux developed in said magnetic circuit and adaptable to
be operated in an attraction mode in which a load is to be
attracted, a lifting mode in which the attracted load is lifted and
a transportation mode in which the load is transported, said method
comprises the steps of applying an exciting current of a
predetermined value to the exciting winding for producing an
attractive magnetic force during the attraction mode, reducing the
exciting current to a value of zero or less thereby making the
magnetic flux less than the residual magnetic flux due to its
remanence during the lifting mode and increasing the exciting
current to larger than zero during at least the initial period of
the operations in the transportation mode, thereby providing the
lifting magnet with magnetic flux larger than the residual magnetic
flux corresponding to the remanence under the condition in the
lifting mode.
Inventors: |
Ishida; Shinnosuke (Hitachi,
JA), Harada; Kunio (Hitachi, JA), Kiyoshe;
Tadashi (Hitachi, JA), Takeuchi; Reizi (Katsuta,
JA), Itoh; Susumu (Hitachi, JA), Sotodate;
Akiyoshi (Hitachi, JA), Wachi; Tadashi (Hitachi,
JA) |
Assignee: |
Hitachi Ltd. (Tokyo,
JA)
|
Family
ID: |
14294490 |
Appl.
No.: |
05/314,460 |
Filed: |
December 12, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Dec 13, 1971 [JA] |
|
|
71-101208 |
|
Current U.S.
Class: |
414/814;
294/65.5; 414/787; 414/737 |
Current CPC
Class: |
H01F
7/206 (20130101); B66C 1/08 (20130101); H01F
2007/208 (20130101) |
Current International
Class: |
B66C
1/08 (20060101); B66C 1/00 (20060101); H01F
7/20 (20060101); B66c 001/06 () |
Field of
Search: |
;214/152,1BS,1BT,1BH,1BV
;294/65.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sheridan; Robert G.
Assistant Examiner: Abraham; George F.
Attorney, Agent or Firm: Craig & Antonelli
Claims
1. A method of transporting loads by utilizing a permanent magnet
type lifting magnet having a permanent magnet provided in its
magnetic circuit and an exciting winding for controlling the
magnetic flux produced in said magnetic circuit, said method
comprising the steps of:
a. applying an exciting current of a first value to said exciting
winding during the operations in an attraction mode in which a load
to be transported is attracted to the lifting magnet,
b. reducing the exciting current to a second value of zero or less
thereby making the magnetic flux produced by the circuit less than
the residual magnetic flux due to its remanence during the
operations in a lifting mode in which the load is lifted, and
c. increasing the exciting current to a third value larger than
zero thereby making the magnetic flux produced by the circuit
larger than the residual magnetic flux corresponding to the
remanence under the condition in the lifting mode during the
operations in a transportation mode in
2. A method according to claim 1, wherein the third value of the
exciting current is held larger than zero thereby providing the
lifting magnet with magnetic flux larger than the value
corresponding to the remanence under the condition in the lifting
mode during the operations in the
3. A method according to claim 2, wherein the exciting current
during the operations in the transportation mode is not larger than
said first value.
4. A method according to claim 1, wherein said exciting current
during the operations in the transportation mode is once increased
to substantially the same value as said first value and then
reduced to and held at a
5. A method according to claim 1, wherein said third value of the
exciting current is larger than said first value.
Description
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention is relating to a method for transporting loads
safely by utilizing a permanent magnet type lifting magnet.
2. DESCRIPTION OF THE PRIOR ART
A permanent magnet type lifting magnet, which utilizes a permanent
magnet provided in its magnetic circuit and an exciting winding for
controlling the intensity of magnetic field through the magnetic
circuit to provide the lifting magnet with a necessary attractive
force or reduce the force, is employed to transport loads of
magnetizable material such as steel and iron in such a way that an
exciting current is first supplied to the exciting winding for a
few seconds thereby providing the lifting magnet with a magnetic
force sufficient to attract a load to be transported, the
attractive force of the lifting magnet is held during the
transportion of the subject by its high residual magnetic flux and
high coercive force after removal of the exciting current and then
a negative exciting current, i.e. an exciting current flowing in a
reverse direction to the original exciting current, is applied to
the exciting winding thereby erasing the residual magnetic flux to
release the load from the lifting magnet. This type of lifting
magnet is advantageous in that the exciting current is applied to
the exciting winding only for a short time when the load is
attracted to or released from the lifting magnet so that the
electric power consumption is very low and that the load is held by
the lifting magnet even at failure of the power supply.
This type of a permanent magnet type lifting magnet is usually
operated in such a way that: First, in order to attract a load, an
exciting current of a predetermined value is applied to the
exciting winding for a short time of 2 to 3 seconds, as shown in
FIG. 1B, thereby to change the magnetic force or the magnetic flux
density to point a on a hysteresis curve, as shown in FIG. 1A in
which the ordinate and abscissa represent the magnetic flux density
and the intensity of excitation, respectively, corresponding to the
intensity H.sub.1 of the excitation. Then the excitation is removed
and thereby the magnet flux density of the magnet changes to point
b corresponding to its remanence B.sub.1. Under this condition the
lifting magnet is capable of lifting and transporting the load by
its residual magnetic force due to the remanence B.sub.1. Finally
the exciting winding is supplied with a negative exciting current
for providing a negative excitation H.sub.2 thereby to change the
magnetic flux density to point c to release the load.
The permanent magnet type lifting magnet may be operated in a
different manner such that, as shown in FIGS. 2A and 2B first an
exciting current of a predetermined value is applied to the
exciting winding for providing a magnetic force, or a magnetic flux
density corresponding to point a in FIG. 2A for attracting a load
and then the exciting current is reduced to a smaller value, as
shown in FIG. 2B. The load is lifted and transported, as holding
the small exciting current, by a magnetic force due to the magnetic
flux density B.sub.2 corresponding to point d in FIG. 2A. The
exciting current is finally changed to a negative value for
releasing the load.
However, in the case of transporting loads in the manner as
described referring to FIGS. 1A and 1B, if the weight of the load
is very near to or slightly exceeds the allowable maximum weight
for the lifting magnet or if the lifting magnet is subject to
abnormal vibrations or sudden change of its moving speed during
transportation of even a light load, it is not assured that there
is no dangerous fall of the load from the magnet.
On the other hand, in the case of transporting loads in the manner
as described referring to FIGS. 2A and 2B, the load is held by the
sum of the magnetic flux due to an applied small exciting current
and that due to its remanence during the lift and transportation of
the load. Therefore, failure of the power supply may cause the
lifting magnet to fail to hold the load especially when the load is
too heavy to be held by only the residual magnetic flux due to its
remanence.
Thus the conventional methods have problems of dangerous fall of
loads during the transportation.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for
transporting loads reliably and safely utilizing a permanent magnet
type lifting magnet.
Another object of the present invention is to provide a method for
transporting loads free from such dangerous fall of loads utilizing
a permanent magnet type lifting magnet even at failure of the power
supply.
The present invention is based on such principal concept that the
attractive force of the lifting magnet, which is proportional to
the square of its magnetic flux density, is always held larger
during the operations in a transportation mode in which loads are
transported than that during the operations in a lifting mode in
which the loads to be transported are lifted and the atractive
force produced by the lifting magnet is checked as to whether it is
sufficiently strong to hold the loads securely during the
transportation of loads by changing its attractive force to a
predetermined value prior to the transportation.
According to the present invention, for realizing the above
concept, first a predetermined value of exciting current is applied
to the exciting current during the operations in an attraction mode
in which a load to be transported is attracted thereto and then
thereby providing necessary magnetic flux for attraction of the
load and then the exciting current is reduced to zero or less,
thereby reducing the magnetic flux of the magnet to less than the
residual magnetic flux due to its remanance and then the operation
is made for lifting the load. The exciting current is again
increased to a value adaptable to provide magnetic flux larger than
the residual magnetic flux and thereby the safety is assured during
the transportation of loads.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B, 2A and 2B are graphs showing variations of the
magnetic flux density and the exciting current during the operation
of a permanent magnet type lifting magnet according to typical
conventional methods.
FIGS. 3A, 3B, 4A, 4B, 5A and 5B are similar graphs for illustrating
the operation according to the present invention.
FIG. 6 is a schematic diagram showing a control circuit for use
with the method according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Explanation will be made of the operations of a permanent magnet
type lifting magnet according to the present invention. As shown in
FIG. 3B, first, during the operations in an attraction mode in
which a load to be transported is attracted, an exciting current of
a predetermined value is applied to the exciting winding of the
lifting magnet. After the load has been attracted by the magnet the
exciting current is reduced to zero and then the operation is
changed into a lifting mode to lift the load from the floor or
ground. Thereafter, during the operations in a transportation mode
in which the load is transported, the exciting current is once
increased to substantially the same value as that applied during
the operations in the attraction mode and then reduced to a
smaller, but larger than zero, value. Finally the exciting current
is changed to a negative value for releasing the load. During the
above operations, the magnetic flux density of the lifting magnet,
the square of which is proportional to its magnetic force, changes,
as shown in FIG. 3A, according to variation of the exciting
current. That is, in the attraction mode, the exciting current
provides excitation H.sub.1, whereby the magnetic flux density is
changed to B.sub.4 at point a lying within the saturation area of
the hysteresis curve of the magnet and the load is attracted to the
lifting magnet by its magnetic force. In the lifting mode the
exciting current is reduced to zero and thereby the magnetic flux
density is changed to a smaller value B.sub.1 at point b
corresponding to its remanence. With this magnetic flux density,
the operation is made to lift the load. If the magnetic force is in
sufficient to hold the load during the transportation, the lifting
magnet is unable to lift the load. Thus it is detectable by
operations in the lifting mode whether the magnetic force due to
the residual magnetic flux is sufficiently strong to hold the load
during the transportation. Thereafter, the exciting current is
increased to exhibit excitation H.sub.1 so that the magnetic flux
density changes through point e, where its value is B.sub.3, to
point a where its value is substantially equal to B.sub.4 as
developed during the initial attraction and subsequently, the
excitation is reduced to H.sub.3 so that the magnetic flux density
changes to point d where its value is B.sub.2. The magnetic flux
density is held B.sub.2 during the operations in the transportation
mode. The values B.sub.2, B.sub.3 and B.sub.4 of the magnetic flux
density, which are held during the transportation, are larger than
B.sub.1 which is held during the lift mode operations. Thus, the
magnetic flux density is increased during the transportation as if
the air gap exciting in the magnetic circuit during the lift of
loads is eliminated thereby being capable of assuring safety
transportation. Finally the magnetic flux density is reduced by a
negative excitation H.sub.2 for releasing the loads.
In the case of operations as shown in FIGS. 4A and 4B, the lifting
matnet is subject to negative excitation during the lift mode
operations as shown in FIG. 4B, and thereafter to positive
excitation in the same manner as discribed referring to FIGS. 3A
and 3B during the transportation mode operations. However, in this
case, the magnetic flux density changes, due to the negative
excitation H.sub.4, to B.sub.5 at point f which is smaller than
B.sub.1 corresponding to the remanence of the magnet and the
difference between the magnetic flux density during the lift of
loads and that during the transportation becomes larger than the
case of FIGS. 3A and 3B, resulting in more safety
transportation.
FIGS. 5A and 5B show another embodiment in which the lifting magnet
is controlled to exert a smaller attractive force than its
allowable capacity for limiting the weight of loads to a
predetermined value, for example, the number of sheets of
magnetizable material such as steel to be transported to a
predetermined value, thereby assuring attraction of a predetermined
number of such sheets and safety transportation thereof. In this
case, during the attraction mode operations, the excitation is set
at such a value H.sub.5 that the magnetic flux density reaches
point i lying below the saturation area on its hysteresis curve
and, during the lifting mode operation, reduced to zero thereby
changing the magnetic flux density to B7 at point j with which the
lifting magnet lifts loads. During the transportation mode
operations, the excitation is once increased to H.sub.1 thereby
changing the magnetic flux density through point i to H.sub.1 at
point a lying within the saturation area and then again reduced to
zero thereby returning the magnetic flux density to a value B.sub.1
corresponding to its remanence, which value B.sub.1 is larger than
the value B.sub.7 presented during the lift mode operations,
whereby the lifting magnet holds strongly loads by a larger
magnetic force during the transportation mode operations than
during the lift mode operations.
Now referring to FIG. 6 showing a circuit diagram of a system for
use with the method according to the present invention, a power
circuit 1 is connected at its input side to a power source (not
shown) and its output side to a contactor circuit 2, which is
adaptable to supply an exciting current to a lifting magnet 5 by
means of a power cable which is supported by a cable reel 3 and
connected to the lifting magnet through a cable connector 4. If a
plurality of such lifting magnets are utilized, a branch box 6 is
employed for connecting the power cable to them. The power circuit
1 may include a rectifier means for converting AC power to DC power
if an AC power source is utilized. The contactor circuit includes a
plurality of contactors for changing the connection of a resistor
circuit 9 thereby controlling the exciting current supplied to the
lifting magnet, a controller 7 adaptable to control the operation
of the contactors for selectively changing the exciting current
depending on the operating modes of attraction, lift,
transportation and release of loads, and an alarm 8 for warning
failure of the power supply. The exciting current may be changed
automatically in a predetermined time sequence by a timer means
which additionally provided in the controller or, alternately,
manually by the controller.
According to the present invention, once attracted loads are lifted
by a lifting magnet with a magnetic flux density not larger than
its remanence under that situation and transported with a larger
magnetic flux density than the above remanence. Hence it is capable
of carrying out transportation of the loads more safely and
reliably than the conventional method as described previously and
also capable of strongly holding the loads even at failure of the
power supply during the transportation. Further by applying a
smaller exciting current than that applied during the attraction
mode during the transportation, the transportation becomes more
safe and reliable, and by applying an exciting current
substantially equal to that applied during the attraction mode for
a short time in the transportation mode and then applying a smaller
exciting current as above mentioned during the remaining period,
the lifting magnet exhibits a magnetic flux density corresponding
to a given point on its saturation hysteresis curve whereby more
safe transportation is assured.
* * * * *