[0001] The present invention relates to an electromagnet for an automatic circuit breaker,
and in particular to an electromagnet for an automatic circuit breaker according to
the preamble of the first claim.
[0002] A circuit breaker is an electrical apparatus capable of interrupting or establishing
a flow of current in a circuit by means of suitable separable contacts; in particular,
the contacts are pressed together with a suitable force in a closed circuit breaker
condition, while they are separated by an insulation space in an open circuit breaker
condition. Circuit breakers are commonly provided with a quick-release mechanism which
increases the speed of a circuit breaker opening operation, so as to prevent the striking,
between the contacts which are at the point of detachment from each other, of an electrical
arc which is subsequently maintained by a violent phenomenon of ionization of the
medium (air, for example) lying between the detached contacts.
[0003] Circuit breakers of the automatic type include a protective device which automatically
activates the release mechanism, causing the circuit breaker to open, when an excess
current is present as a result of a short circuit. The short-circuit protection device
basically consists of an electromagnetic relay; the electromagnet has a fixed core
on which there is a winding through which the electrical current of the circuit breaker
passes and a moving element (the keeper or keeper) which moves under the action of
a magnetic field produced by the current in the winding.
[0004] Known electromagnets used in automatic circuit breakers have a cylindrical structure.
The electromagnet includes a fixed cylindrical core enclosed by an external shell
and a cylindrical keeper, coaxial with the fixed core, which moves with a translational
motion. The fixed core is covered by an insulating guide around which the winding
is disposed.
[0005] A disadvantage of known electromagnets is that they have rather large overall dimensions,
equal to the sum of the overall dimensions of the fixed cylindrical core, the insulating
guide and the winding. This is because the fixed cylindrical core (and the moving
keeper) must have a minimum diameter to ensure correct operation of the electromagnet,
and the insulating guide must have a minimum thickness to ensure sufficient mechanical
strength; and, in any case, the winding must have a rather large minimum cross-section
to reduce the heating action produced by the Joule effect by the current in the circuit
breaker.
[0006] Typically, known electromagnets have an overall dimension equal to the width of a
DIN module circuit breaker, namely 17.5 mm. These electromagnets cannot, therefore,
be used in circuit breakers of smaller sizes, for example with a width of half a DIN
module.
[0007] The object of the present invention is to overcome the aforesaid disadvantages. To
achieve this object, an electromagnet for an automatic circuit breaker as described
in the first claim is proposed.
[0008] Briefly, the present invention provides an electromagnet for an automatic circuit
breaker comprising a substantially flat open core and a winding disposed around the
core to create a magnetic field, in which the core has a first portion which is twisted
with respect to a remaining second portion about a line of development of the core,
such that the first portion is disposed in a plane intersecting the second portion,
the winding being disposed around the first portion.
[0009] The present invention also provides an automatic circuit breaker comprising this
electromagnet.
[0010] Further characteristics and the advantages of the electromagnet for an automatic
circuit breaker according to the present invention will become evident from the following
description of a preferred embodiment of the invention, provided for guidance and
without restriction, with reference to the attached figures, in which:
Fig. 1 is a sectional view of an automatic circuit breaker in which the electromagnet
according to the present invention can be used;
Fig. 2 shows schematically, in a perspective view, an embodiment of the electromagnet
according to the present invention.
[0011] With reference to Fig. 1 in particular, an automatic circuit breaker 100 of the magnetothermal
type comprising an insulating casing 105 is illustrated; on a left-hand side wall
and on a right-hand side wall of the insulating casing 105 there are provided, respectively,
an input terminal 110a and an output terminal 110b for connecting of the circuit breaker
100 to corresponding electrical cables of an external circuit (not shown in the figure).
[0012] The circuit breaker 100 includes a fixed contact 115a and a moving contact 115b;
the moving contact 115b is pressed against the fixed contact 115a in a closed condition
of the circuit breaker 100, while the moving contact 115b is separated from the fixed
contact 115a in an open condition of the circuit breaker 100. An external control
lever 120 enables the circuit breaker 100 to be opened and closed manually (or by
means of a suitable actuator).
[0013] A quick-release mechanism 125 is housed inside the insulating casing 105. When the
circuit breaker 100 is closed manually by moving the control lever 120, the release
mechanism 125 is set (cocked), thus putting a spring 130 under pressure; when the
circuit breaker 100 is opened, the release mechanism 125 is made to trip (releasing
the spring 130) such that the moving contact 115b is quickly separated from the fixed
contact 115a. The opening of the circuit breaker establishes an electrical arc between
the contacts 115a, 115b, which is guided, by a switching electrode 135, towards a
group of extinction strips 140.
[0014] The circuit breaker 100 also includes a protective device consisting of a bimetallic
strip 145 and an electromagnet 150. The bimetallic strip 145 (consisting of two materials
whose coefficients of thermal expansion are very different from each other) is connected
electrically to the input terminal 110a and, through a flexible conducting braid 155,
to the moving contact 115b. The electromagnet 150 consists of a fixed core 160, around
which there is an insulated winding 165, and a moving keeper 170, kept separate from
the fixed core 160 by means of a return spring 175, as described in detail below.
The winding 165 is connected electrically at one end to the fixed contact 115a and
at the other end to the output terminal 110b.
[0015] When the circuit breaker is closed (with the moving contact 115b pressed against
the fixed contact 115a), the current flowing through the circuit breaker 100 passes
through the bimetallic strip 145 and the winding 165 of the electromagnet 150. If
the circuit breaker 100 is subjected to a prolonged overload, the circuit breaker
current heats the bimetallic strip 145 and deforms it in such a way that it acts on
the release mechanism 125, causing the opening of the circuit breaker 100 (thermal
protection). In the case of a short circuit, however, the circuit breaker current
(which is very high) passing through the winding 165 magnetizes the fixed core 160,
which attracts the moving keeper 170 which acts, by means of a suitable mechanism
(not shown in the figure), on the release mechanism 125, causing the instantaneous
opening of the circuit breaker 100 (magnetic protection).
[0016] The electromagnet according to the present invention can, however, also be used in
automatic circuit breakers having a different structure, for example those with two
or more pairs of terminals (and a corresponding number of electromagnets), without
any thermal protective device, with a different release mechanism, and the like.
[0017] With reference now to Fig. 2 (the elements previously represented in Fig. 1 are identified
by the same reference numbers), the fixed core 160 of the electromagnet 150 consists
of a flat strip of ferromagnetic material with a weak coercive field, typically soft
iron. The fixed core strip 160 is developed along a line 205 having a generally U-shaped
configuration, with a smaller side 210 from whose ends a left-hand larger side 215a
and a right-hand larger side 215b extend perpendicularly.
[0018] The left-hand larger side 215a is twisted with respect to a remaining part of the
strip 160 (the smaller side 210 and the right-hand larger side 215b) about the line
of development 205. The winding 165 is disposed on the left-hand larger side 215a
(or at least on part of it).
[0019] The present invention can, however, also be produced with the strip having a different
configuration, for example a semi-circle or, ultimately, even rectilinear; alternatively,
a flat core, cut in a U-shape for example, with one larger side twisted and the winding
disposed around the other larger side, is used. More generally, a substantially flat
open core is provided, having a first portion which is twisted with respect to a remaining
second portion about a line of development of the core, such that the first portion
is disposed in a plane intersecting the second portion, the winding being disposed
around the first portion.
[0020] The electromagnet according to the present invention occupies less space, approximately
as much as the height of the second portion, since the overall dimension of the winding
is contained (at least partially) within this portion. This solution enables an electromagnet
of small dimensions to be produced, while retaining the operational characteristics
required for its operation. In particular, the electromagnet according to the present
invention may easily be housed in a small automatic circuit breaker, in particular
one with a small width. This result is obtained in an extremely simple way, with a
structure which can easily be mass-produced at low cost.
[0021] Advantageously, as in the example shown in the figure, the portion 215a around which
the winding 165 is disposed is an end portion of the core 160; this enables the core
160 to be made by a single bending operation (although the use of a non-end portion,
requiring a further bending operation, is not excluded). Preferably, the left-hand
larger side 215a is rotated through 90° about the development line 205, such that
it becomes perpendicular to the right-hand larger side 215b. This ensures that the
overall dimension of the winding is contained completely within the right-hand larger
side 215b, as a result of which the dimensions of the electromagnet are reduced to
a minimum.
[0022] The moving keeper 170 consists of a plate (of soft iron), whose width is preferably
equal to the width of the right-hand larger side 215b, which pivots about a free end
of the right-hand larger side 215b, in such a way as to form a gap 220 between itself
and the free end of the left-hand larger side 215a. In particular, the right-hand
end of the moving keeper 170 has a fork 225 having an upper branch 230a and a lower
branch 230b. The branches 230a, 230b terminate in projections 235a, 235b that project
towards the interior of the fork 225 to retain the moving keeper 170. In particular,
the fork 225 is snap-fitted on the free end of the right-hand larger side 215b, in
which an upper saddle 240a and a lower seating 240b are provided. The inner portion
of the projections 235a, 235b is shaped in the form of a knife-edge, so that it acts
as a fulcrum for the rotation of the moving keeper 170 about the right-hand larger
side 215b. The left-hand end of the moving keeper 170 is fixed to the return spring
175 and is typically provided with an anti-locking pin (not shown in the figure) of
non-magnetic material, projecting from an inner surface of the moving keeper 170 in
such a way that, even when the moving keeper 170 is attracted by the fixed core 160,
the gap 220 is not eliminated (to prevent the residual magnetism present in the fixed
core 160 from keeping the moving keeper 170 attracted even in the absence of current
in the winding 165, without requiring a return spring 175 of excessive size).
[0023] The moving keeper described above is particularly simple and effective, and enables
the available space to be used in the best way. The present invention is, however,
also suitable for use with a different moving keeper, for example one hinged to the
magnetic core, and the like.
[0024] Obviously, a person skilled in the art may make numerous modifications and changes
to the electromagnet for an automatic circuit breaker described above, in order to
meet contingent and specific requirements, all such modifications and changes being
contained within the scope of protection of the invention, as defined by the following
claims.
1. Electromagnet (150) for an automatic circuit breaker (100), comprising a substantially
flat open core (160) and a winding (165) disposed around the core (160) to create
a magnetic field,
characterized in that
the core (160) has a first portion (215a) which is twisted with respect to a remaining
second portion (210, 215b) about a line of development (205) of the core (160), such
that the first portion (215a) is disposed in a plane intersecting the second portion
(210, 215b), the winding (165) being disposed around the first portion (215a).
2. Electromagnet (150) according to Claim 1, in which the first portion (215a) is an
end portion of the core (160).
3. Electromagnet (150) according to Claim 1 or 2, in which the first portion (215a) is
substantially perpendicular to the second portion (210, 215b).
4. Electromagnet (150) according to any one of Claims 1 to 3, further comprising a moving
element (170) operated by the magnetic field, the moving element (170) pivoting about
a first free end of the core (160) to form a gap (220) between itself and a second
free end of the core (160).
5. Electromagnet (150) according to Claim 4, in which the core (160) has a generally
U-shaped configuration with a first (215a) and a second (215b) larger side, the first
portion consisting of the first larger side (215a) and the moving element (170) pivoting
about a free end of the second larger side (215b).
6. Electromagnet (150) according to Claim 5, in which the moving element (170) consists
of a plate whose width is substantially equal to the width of the second larger side
(215b).
7. Electromagnet (150) according to Claim 5 or 6, in which the moving element (170) terminates
at one end in a fork (225) which is snap-fitted on the free end of the second larger
side (215b).
8. Electromagnet (150) according to Claim 7, in which the fork (220) has a first and
a second branch (235a, 235b), each terminating in a projection (235a, 235b) that projects
towards the interior of the fork (220) and interacting with a corresponding seating
(240a, 240b) formed on the second larger side (215b) to retain the moving element
(170).
9. Electromagnet (150) according to Claim 8, in which each projection (235a, 235b) has
an inner portion shaped in the form of a knife-edge which acts as a fulcrum for the
moving element (170).
10. Automatic circuit breaker (100) comprising a quick-release mechanism (125) to open
the circuit breaker (100), and the electromagnet (150) according to any one of Claims
1 to 9 to operate the release mechanism (125) in a short-circuit situation.