[0001] The invention concerns the realization of a circuit for the impulse-rate feeding
of armoured electromagnets and of electromagnets fed by such circuit. The invention
is particularly efficient when applied to electromagnets having small and medium traction
fields of force.
[0002] It will be pointed out here that electromagnets are electromechanical devices which
are used as driving units in different fields of application, such as automations,
armour-plated doors, actuators, etc.
[0003] The currently known constructions are electromagnets having a cylindrical mobile
core which are usually fed with direct current, or electromagnets fed with alternate
current, but they are built with a frame and with a lamellar pack. In both of the
just-mentioned constructions, the electromagnet coil is fed at a fixed voltage.
[0004] The distinctive and characteristic elements of each magnet are the maximum traction
force which the electromagnet can exert in relation to the stroke of its mobile core
and the maximum number of driving movements that can be performed within a predetermined
period of time.
[0005] It will be pointed out that in the following description, the expression "electromagnet
in closed position" will indicate the condition wherein the mobile core is drawn at
the end of its stroke and the aircore is zero, while the expression "electromagnet
in open position" will indicate the condition wherein the electromagnet is being fed
and the mobile core must perform its stroke.
[0006] It is known that the current circulating within the coil of the electromagnet causes
a heat dissipation, which besides wasting energy, also determines the limits in the
performance of the electromagnet itself.
[0007] It will also be pointed out that, while the current required to move the core during
the traction phase and to exert a certain force is significant, the current necessary
to keep the magnet in its closed position is considerably reduced.
[0008] For instance, in the case of electromagnets, which are fed with alternate current,
the current required when the electromagnet is in its closed position is 10 - 15 times
lower than the current necessary to move the core during the traction phase. In fact,
when the electromagnet is fed with alternate current, the current is higher when the
electromagnet is in its open position, than when the electromagnet is it closed position,
since the inductance value is low when the electromagnet is open and high when the
electromagnet is closed.
[0009] Therefore, it can be said that, in the case of electromagnets which are fed with
alternate current, a current drop will occur when the mobile core closes, due to the
reactance change in the electromagnet circuit.
[0010] Even if there is self-limitation of the current in the closed position, the electromagnet
which is fed with alternate current presents some serious construction limitations,
due to the fact that the magnetic circuit must be built with laminated iron, in order
to reduce the induced currents and, as a consequence, losses in the magnetic circuit;
some riveting is also required to mechanically secure the lamellae, so that the construction
is very expensive. During the operation of the electromagnet vibrations may easily
occur and, in order to avoid the occurrence of said vibrations, it becomes necessary
to insert into the magnetic circuits same adequate short-circuit windings.
[0011] All told, the alternate-current electromagnet presents considerable constructive
as well as operative problems.
[0012] In the electromagnets of the cylindrical type which are fed with direct current,
there are no vibrations since there are no voltage variations during the feeding process,
but the main limit of the direct-current electromagnets consists in the fact that
the current passing through the winding is practically constant during the entire
operation, that is both during the opening and the closing of the electromagnet.
[0013] Since during the closing process the required current is high, and since said current
remains unchanged during the entire closing time of the electromagnet, it can be clearly
understood that the direct current electromagnets are suited to work in not-all-too
hard cycles.
[0014] The purpose of the invention is that of obtaining a simply constructed electromagnet
with reduced dimensions and high-level operations both concerning the traction capacity
and the number of cycles which it can withstand.
[0015] Another purpose is that of obtaining an electromagnet fed by an alternate-current
source, wherein the coil is passed through by high current in the opening phase, so
that said electromagnet may develop a considerable traction force and that, at the
same time, the current may be reduced to the minimum necessary to maintain the closed
position, in order to avoid energy waste and damaging high temperatures which limit
its functional performance. That is, the proposed construction should guarantee a
high ratio between the traction current and the closing current.
[0016] Yet another purpose is that of making an electromagnet available, the construction
of which is to be considerably simplified both from the mechanical and the electrical
point of view, in comparison with electromagnets giving equal performances.
[0017] All above-mentioned purposes are reached by an armoured electromagnet which, in accordance
with the patent claims, includes a metal, preferably cylindrical, structure, consisting
of a cylindrical container, inside which there is an axial opening wherein the mobile
core, which is also cylindrical, slides and wherein said coil is characterized in
that it is fed by an implulse-rating current with variable intensity supplied by an
electronic circuit, fed with alternate current.
[0018] According to the invention the terminals of the coil of the electromagnet are connected
with the nodes of a diode bridge which is fed through a normally closed electronic
switch, connected parallel to a capacitor.
[0019] When the feeding circuit is run through by alternate current, the electronic switch
which is normally closed, represents a short circuit and the bridge is, therefore,
fed by all the alternate current. Because of this a rectified and pulsating current
will operate in the electromagnet coil; this will be the maximum current and, therefore,
the maximum possible traction force of the electromagnet will develop. Since the normally
closed electronic switch has a delayed opening, when said switch is run trough by
current, it will open after a certain programmable period of time and at the terminals
of the diode bridge a reduced voltage will operate due to the voltage drop which will
operate in the capacitor placed in parallel to the feeding source of the bridge. The
voltage which is reduced at the terminals of the rectifying bridge will cause the
coil to be fed at a reduced current. The current drop is also promoted by some special
construction expedients concerning the cylindrical geometry of the electromagnet,
the choice of a type of iron having a special magnetic permeability and the reduction
of the air cores of the yoke between the core and the botton of the external cylindrical
housing.
[0020] It is obvious that if the opening of the electronic switch has been programmed for
a period of time sufficient to allow the closing of the mobile core of the electromagnet,
immediately after the closing a minimum current will operate which will be sufficient
to keep the electromagnet in the closed position. Since the closing time will amount
to a few hundredths of a second, it can easily be understood that the maximum current
running through the coil will circulate for a very short period of time, so that the
coil can withstand high currents without suffering any damage.
[0021] Tests performed on a prototype have led to the conclusion that the ratio between
the current with the electromagnet in the closed position and the current with the
electromagnet in the open position in the construction according to the invention
easily reaches the value of 1:30.
[0022] Thus the advantage of the invention becomes obvious, since, as had previously been
pointed out, the ratios between the closing current and the opening current in the
known direct current devices reach a value of 1:1 and the alternate-current ones reach
a maximum value of 1:10-15.
[0023] It will be pointed out that the dimensioning of the electromagnet will be dictated
by the opening current necessary to develop in the coil a magnetic force exceeding
the re-calling force applied to the mobile core. As far as the current keeping the
closed position is concerned, it is reduced because of the zero air-core.
[0024] In the substance, while it is necessary to bring into play high currents to close
the electromagnet, only reduced currents are necessary to keep it in its closed position
and that is why the advantage offered by the device according to the invention becomes
even more significant, since the ratio between the closing current and the opening
current is in the range of 1:30. As a consequence of the drastic reduction of current
when the magnet is in its closed position, it follows that the holding position of
the mobile core can be kept for an indefinite period of time, without any negative
consequence for the device.
[0025] It will also be pointed out that the combination of the above-mentioned electromagnet
characteristics, together with the mechanical characteristics concerning the compactness
and the construction simplicity of the electromagnet, make the electromagnet according
to the invention particularly advantageous when it is used in the robotic field and,
in general, in the field of motion-mechanisms, where high performances and minimum
dimensions are required.
[0026] Other advantages and characteristics will be better understood from the description
of a preferred from of execution of the invention, which is given by way of explanation
only, but is not meant to limit the scope of the invention and is illustrated in the
enclosed tables of drawing, wherein:
- Fig. 1 represents a view of the electromagnet according to the invention;
- Fig. 2 is a cross-section of the electromagnet of Fig. 1;
- Fig. 3 shows the plan of the feeding circuit of the electromagnet according to the
invention.
[0027] With reference to the above-mentioned Figures, it can be observed that the electromagnet
consists of a cylinder-shaped housing made by cutting a normal tube available on the
market, into which a coil 2 is inserted. Said coil 2 is anchored on the ring-shaped
projection 3 of lid 4 closing one end of the housing. Another lid 5 closes the bottom
part of the electromagnet. The mobile core 6 is a solid metal cylinder which is inserted
into the central opening of coil 2 and, as can be observed in Fig. 2, it can slide
within the space comprised between the bottom of lid 4 and the C-shaped stroke-stop,
indicated with 8, which is connected with housing 1.
[0028] Over the electromagnet, on top of lid 4, there is a box 9 containing the electromagnet
circuit which feeds coil 2.
[0029] As can be observed in Fig. 3, coil 2 of the electromagnet is fed at the terminals
20 and 21 of the diode-bridge.
[0030] At the terminals A and B of the circuit feeding the diode-bridge an alternate current
is applied and when the closing of the manual switch 30 is achieved, since the electronic
switch 40 is normally closed, all the tension present between the terminals A and
B wil set on the terminals 22 and 23 of the diode-bridge.
[0031] Because of the effect of the diode bridge, coil 2 is run through by a rectified pulsating
current which is approximately equal to the voltage applied to the terminal 22 and
23 of the diode bridge, devided by the resistance of coil 2 of the electromagnet.
After the mobile core 6 has closed on bottom 3 of lid 4, the electronic switch 40
will open, and its opening causes a voltage drop at the terminals of capacitor 31.
[0032] Thus the purpose of feeding the electromagnet according to the invention at different
current rating is fulfilled with the previously described advantages.
[0033] The electronic switch 40 which is normally closed and has a delayed opening, will
not be further described, since it is a device known to people who are knowledgeable
in this field.
[0034] It will then be understood that, by adjusting the delayed opening for the period
of time sufficient for the mobile core 6 to run the closing stroke of the electromagnet,
it is possible to limit the circulation of the maximum current within the coil to
the strictly necessary period of time, while during the entire period in which the
electromagnet remains in its closed position, the current is reduced to a minimum,
which allows to limit to a minimum the effects of the circuit heating, caused by current
circulation and with the consequence of reducing to a minimum the dimensions of the
circuit.
[0035] Here it is also necessary to point out the simplicity of construction of the electromagnet
according to the invention which, because it is fed with a rectified current, can
be made from a tube or, in any case, of a compact iron material, without any need
for magnetic lamellar constructions in order to avoid spurious currents due to the
circulation of alternate currents.
[0036] In reality the housing of electromagnet 1 is a tube which can readily be found on
the market; it is cut at its ends and closed with two simple, easily manufactured
caps. Moreover, the electronic circuit shown in Fig. 3, adequately miniaturized, is
placed in box 9 on top of the electromagnet, so that a compact construction with minimum
overall dimensions is obtained.
[0037] It can be understood that the proposed construction not only offers electrical and
mechanical operational advantages, but it is also manufactured at considerably lower
costs in comparison with the costs of electromagnets offering equivalent performances,
manufactured with the technique known up to now.