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Title:
AN ACTUATOR, CIRCUIT BREAKER AND METHOD THEREFOR
Document Type and Number:
WIPO Patent Application WO/2011/144256
Kind Code:
A1
Abstract:
The present invention relates to an actuator comprising a flat coil (1), a magnetic armature (2) and an initial positioning element (3). The initial positioning element is arranged to keep the armature away from the flat coil (1) in an initial position, and the actuator is arranged such that a fault current through the flat coil is sufficient to, counteracting a force on the armature by the initial positioning element, magnetically attract the armature to the flat coil to a second position. A corresponding circuit breaker and method are also presented.

Inventors:
BECERRA, Marley (Klockartorpsgatan 34D, Västerås, S-723 44, SE)
Application Number:
EP2010/057068
Publication Date:
November 24, 2011
Filing Date:
May 21, 2010
Export Citation:
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Assignee:
ABB RESEARCH LTD (Affolternstrasse 44, Zurich, CH-8050, CH)
BECERRA, Marley (Klockartorpsgatan 34D, Västerås, S-723 44, SE)
International Classes:
H01H71/32
Domestic Patent References:
WO2000054292A1
Foreign References:
US2549323A
US4631508A
EP0147036A1
Other References:
None
Attorney, Agent or Firm:
AHRENGART, Kenneth (ABB AB, /Intellectual PropertyIngenjör Bååths Gata 1, T2 Floor E Västerås, S-721 83, SE)
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Claims:
CLAIMS

1. An actuator comprising a flat coil (1), a magnetic armature (2) and an initial positioning element, wherein the initial positioning element (3, 9) is arranged to keep the armature away from the flat coil (1) in an initial position, and the actuator is arranged such that a fault current through the flat coil is sufficient to,

counteracting a force on the armature by the initial positioning element, magnetically attract the armature to the flat coil to a second position.

2. The actuator according to claim 1, wherein the initial positioning element comprises a permanent magnet (3) arranged to keep the armature away from the flat coil (1) in the initial position.

3. The actuator according to claim 1, wherein the initial positioning element comprises a spring (9) arranged to keep the armature away from the flat coil (1) in the initial position. 4. The actuator according to claim 2, comprising a second permanent magnet (4) arranged in the centre of the flat coil .

5. The actuator according to any one of the preceding claims, wherein the magnetic armature has a hole in its centre, for guidance along a non-magnetic and non- conductive guide.

6. The actuator according to any one of claims 4 or 5 when dependent on claim 4, comprising a third permanent magnet (6) and a second magnetic armature (5), wherein the third permanent magnet (6) is arranged to keep the second magnetic armature (5) away from the flat coil (1) in an initial position, and the actuator is arranged such that a fault current through the flat coil is sufficient to, counteracting a force on the second armature by the third magnet, magnetically attract the second armature to be in contact with the flat coil in a second position, the third permanent magnet (6) and the second magnetic armature (5) being positioned on an opposite side of the flat coil (1) compared to the armature (2) and the initial positioning element (3) .

7. The actuator according to any of the preceding claims, wherein the armature is sectioned.

8. The actuator according to any of the preceding claims, comprising a multi-contact system, wherein the armature comprises a conductive bridge (8a, 8b, 8c) and a

permanent magnet (3a, 3b, 3c) for each gap of the multi- contact system.

9. The actuator according to any of the preceding claims, comprising a switch (7) for reset of the armature from the second position to the initial position.

10. A circuit breaker comprising the actuator according to any of the preceding claims wherein the circuit breaker is configured to be closed when the actuator is in the initial position and the circuit breaker is configured to be opened when the actuator is in the second position.

11. A method of circuit breaking using an actuator comprising a flat coil (1), a magnetic armature (2) and an initial positioning element, the method comprising the steps of: during normal current in the flat coil, keeping, using the initial positioning element, the armature away from the flat coil (1) in an initial position, and during a fault current in the flat coil (1), magnetically attracting the armature to the flat coil to a second position, counteracting a force on the armature by the initial positioning element. 12. The method according to claim 11, wherein: the initial positioning element comprises a first

permanent magnet (3) , a second permanent magnet (4) is arranged in the centre of the flat coil (1), and the actuator comprises a third permanent magnet (6) and a second magnetic armature (5) , wherein the method comprises the steps of: during normal current in the flat coil, keeping, using the first permanent magnet (3) the second magnetic armature (5) away from the flat coil (1) in an initial position, and during a fault current in the flat coil, magnetically attracting the second armature to the flat coil to a second position, counteracting a force on the second armature by the third magnet, wherein the third permanent magnet (6) and the second magnetic armature (5) are positioned on an opposite side of the flat coil (1) compared to the armature (2) and the initial positioning element (3) .

13. The method of circuit breaking according to claim 11 or 12, comprising the step of: bi-stably fixing the armature (s) at the flat coil by a second permanent magnet in the centre of the flat coil.

Description:
AN ACTUATOR , CIRCUIT BREAKER AND METHOD THEREFOR FIELD OF INVENTION

The present invention relates to actuators in general and more particularly to actuators with fast motion

requirements, e.g. for electrical switching applications.

BACKGROUND

Actuators are utilized e.g. in circuit breakers

throughout electrical power distribution networks for protecting the networks from damage caused by e.g.

overload or short circuits. Circuit breakers have at least one contact point with a fixed and a movable contact piece, the actuator directly or indirectly interacts with the movable contact piece to open the contact point if the fault current exceeds a pre-set tripping threshold. The circuit breaker has to be opened (tripped) as quickly as possible upon detection of a fault current, and this in turn puts high demands on the actuator operating the circuit breaker. The opening of contacts of the circuit breaker has to be performed rapidly and reliably. Once the breaker is tripped, the contacts should remain in open position by a latching mechanism which keeps the movable assembly in the open position of the contacts in order to reach galvanic isolation of the faulty part of the circuit. The reset of the breaker to the closed position after the fault is cleared is performed by means of an extra unlatching mechanical system. Other solutions for keeping the contact at open position after tripping is by setting a bistable operation of the actuator by using mechanical or magnetic means. While mechanical bistable systems (based on springs) are complicated and unreliable for small distance between the initial and final position, the magnetic systems can use permanent magnets on the moving piece which in turn increases its mass and reduces its acceleration for the same actuator driving force. The importance of fast tripping is particularly relevant for DC systems, since an arc is not quenched naturally as in AC systems.

Fast and reliable actuators can also be used in various mechanical applications where speed and accuracy are important.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a fast actuator which can be used in tripping devices where high speed and small displacements are required. This object, among others, is according to the present invention attained by an actuator, circuit breaker and a method therefore, respectively, as defined by the

appended claims.

According to a first aspect of the present invention it is provided an actuator comprising a flat coil, a

magnetic armature and an initial positioning element. The initial positioning element is arranged to keep the armature away from the flat coil in an initial position, and the actuator is arranged such that a fault current through the flat coil is sufficient to, counteracting a force on the armature by the initial positioning element, magnetically attract the armature to the flat coil to a second position. In this way the weight of the armature can be decreased and the circuit breaker tripping speed of the actuator can be increased.

The initial positioning element may comprise a permanent magnet arranged to keep the armature away from the flat coil in the initial position.

The initial positioning element may comprise a spring arranged to keep the armature away from the flat coil in the initial position. The actuator may comprise a second permanent magnet arranged in the centre of the flat coil, whereby a fast and robust bi-stable actuator is achieved.

By providing the magnetic armature of the actuator with a hole in its centre, for guidance along a non-magnetic and non-conductive guide, the robustness of the actuator is improved and its weight is further reduced which in turn increases the speed of the actuator.

By utilizing both sides of a flat coil, the actuator comprises a third permanent magnet and a second magnetic armature, wherein the third permanent magnet is arranged to keep the second magnetic armature away from the flat coil in an initial position, and the actuator is arranged such that a fault current through the flat coil is sufficient to, counteracting a force on the second armature by the third magnet, magnetically attract the second armature to be in contact with the flat coil in a second position, the third permanent magnet and the second magnetic armature being positioned on an opposite side of the flat coil compared to the armature and the initial positioning element. For reduced sensitivity against eddy currents the

armature may be sectioned.

By having a multi-contact system in the actuator and providing the armature with a conductive bridge and a permanent magnet for each gap of the multi-contact system, the actuator can be used in a circuit breaker for current limitation in systems with higher voltage DC applications. The flat coil is connected in series with the circuit to be protected and the multi-contact system. Advantageously, the actuator comprises a switch for reset of the actuator from the second position to the initial position. The switch can be driven by mechanical gears or by electromagnetically .

According to a second aspect of the present invention, it is presented a circuit breaker comprising the actuator according to the first aspect, wherein the circuit breaker is configured to be closed when the actuator is in the initial position and the circuit breaker is configured to be opened when the actuator is in the second position

According to a third aspect of the present invention, it is presented a method of circuit breaking using an actuator comprising a flat coil, a magnetic armature and an initial positioning element. The method comprises the steps of: during normal current in the flat coil,

keeping, using the initial positioning element, the armature away from the flat coil in an initial position, and, during a fault current in the flat coil,

magnetically attracting the armature to the flat coil to a second position, counteracting a force on the armature by the initial positioning element. In this way, the opening of the multi-contact system takes place with reduced bouncing, providing galvanic isolation of the faulty part of the circuit. The initial positioning element may comprise a first permanent magnet, a second permanent magnet may be arranged in the centre of the flat coil and the actuator may comprise a third permanent magnet and a second magnetic armature. The method may then comprise the steps of: during normal current in the flat coil, keeping, using the first permanent magnet the second magnetic armature away from the flat coil in an initial position, and during a fault current in the flat coil, magnetically attracting the second armature to the flat coil to a second position, counteracting a force on the second armature by the third magnet, wherein the third permanent magnet and the second magnetic armature are positioned on an opposite side of the flat coil compared to the

armature and the initial positioning element. The method of circuit breaking may comprise the steps of: bi-stably fixing the armature (s) at the flat coil by a second permanent magnet in the centre of the flat coil, for improved robustness.

Further features and advantages of the present invention will be evident from the following description.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more fully understood from the detailed description of embodiments given below and the accompanying figures, which are given by way of illustration only, and thus, are not limitative of the present invention, wherein:

Fig. 1 schematically illustrates a perspective view an armature and a flat coil of an actuator. Figs. 2A-B schematically illustrates an initial and a second position of an actuator according to an embodiment of the present invention based on permanent magnets, shown in a side view.

Fig. 3 schematically illustrates a sectioned armature, shown in a side view.

Fig. 4 schematically illustrates an initial position of an actuator according to an embodiment of the present invention, shown in a side view.

Fig. 5 schematically illustrates a fault condition of the actuator shown in Fig. 4.

Fig. 6 schematically illustrates a position between the initial position and the second position of the actuator shown in Fig . 4.

Fig. 7 schematically illustrates a second position of the actuator shown in Fig. 4.

Fig. 8 schematically illustrates an actuator having two armatures according to an embodiment of the present invention, shown in a side view.

Fig. 9 schematically illustrates a second position of an actuator according to an embodiment of the present invention based on springs, shown in a side view.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the following description, for purpose of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present

invention. However, it will be apparent for a person skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed

description of well-known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details.

A first embodiment of a circuit breaker comprising an actuator according the present invention will now be described with reference to Figs. 1, 2A, 2B. Fig 2A shows the actuator with an armature in an initial position and Fig 2B shows the actuator with the armature in a second position. It is to be noted that the actuator could equally well be applied in various other contexts; the actuator is not limited to the use of a circuit breaker. The actuator comprises a flat coil 1, a magnetic armature 2, a first permanent magnet 3 and a second permanent magnet 4. It is to be noted that the flat coil 1 can be any coil with a generally flat composition.

The magnetic armature 2 is bi-stably held in an initial position against an initial positioning element in the form of the first permanent magnet 3. When the actuator of the circuit breaker is in this initial position, the circuit breaker is in a closed position and the actuator is used to close an electrical circuit. Due to e.g. a fault condition the current in the flat coil 1 is

increased and the magnetic armature 2 is attracted to a second position against the flat coil 1. The magnetic armature 2 is bi-stably held in the second position by the second permanent magnet 4 positioned in the centre of the flat coil 1. In this second position the circuit breaker and its actuator is in an opened position and the actuator is used to break an electrical circuit.

The magnetic armature 2 may have a hole in its centre, for stable guidance of the magnetic armature 2 between the bi-stable positions, along a non-magnetic guide, such as a plastic rod.

The circuit breaker may comprise a switch 7 , such as a mechanical gear mechanism to be used for reset of the actuator from the second, open, position to the initial, closed, position. As seen in Fig 2B, if a user presses the switch 7 downwards, the magnetic armature 2 will be pushed away from the second position by the flat coil 1 back to the initial position. Optionally, a switch reset spring 11 is provided such that the switch 7 is returned to an upper position after usage such that no force, e.g. due to gravity, on the switch 7 is applied to the

magnetic armature 2 when in its initial position. Any force on the magnetic armature may compromise operational speed when the actuator is activated.

This embodiment is advantageously used without a magnetic core around the flat coil, due to the large magnetic field produced during a fault condition. In addition, the magnetic force of the actuator is a function of the current of the electrical circuit, and not of the

derivative thereof as in solutions based on Thomson coils. Due to the utilization magnetic properties of the

armature, no permanent magnets are needed on the moving part of the actuator, and its weight can thus be reduced, and the fastness of the tripping can hence be increased. A further advantage of utilization of a magnetic armature according to the present invention, is that since no mechanical springs are needed, the distance between the initial and second position of the magnetic armature can be very small, typically less than 5 mm, even down to about 2 mm if applied to low voltage DC breakers.

For a typical DC breaker the magnetic force of the first permanent magnet on the magnetic armature in its initial position is set to guarantee low contact resistance of the multi-contact system in the closed position, the magnetic force of the flat coil for a normal operating current of several tens of amperes on the magnetic armature is less than about 1 N, the magnetic force of the second permanent magnet on the magnetic armature in its initial position is about a few Newton, and the gravitation force on the magnetic armature is negligible in this context. The magnetic armature is in this example a 1 mm thick disc made of a sectioned steel sheets with high magnetic permeability, low conductivity, low mass density and very high saturation flux density. During a fault condition, when the current in the flat coil increases rapidly, the magnetic force of the flat coil on the magnetic armature can reach more than 100 N for a peak current of 2000 A, which is sufficient for the low weight armature to be moved very fast from the initial position to the second position, and the electrical circuit is thus broken by the circuit breaker. Fig. 9 schematically illustrates a second position of an actuator according to a second embodiment of the present invention based on springs, shown in a side view. In this embodiment, the initial positioning element is in the form of a spring 9. During normal operation, the spring keeps the armature 2 in the initial position by providing a repulsing force, away from the flat coil. When a fault condition occurs and a fault current flows in the flat coil 1, the resulting attracting magnetic force is sufficient to counteract the repulsive force of the spring 9, whereby the armature is brought to the flat coil. It is to be noted that the spring 9 may comprise several suitably placed springs for added stability.

The second permanent magnet 4, for bi-stably holding the magnetic armature in the second position is not necessary for the fast circuit breaking, which is illustrated in a third embodiment of the present invention in Fig. 3. This embodiment of the present invention illustrated in Fig. 3 further comprises a sectioned armature 2 for reduced effects of eddy currents therein.

A fourth embodiment of an actuator according to the present invention is illustrated in Figs. 4-7. This actuator is identical to that of the first embodiment described above, apart from the following. The multi-contact system has three gaps. The magnetic armature 2 has low conductivity and is provided with three electrically conductive bridges 8a, 8b and 8c, each bridging a gap of the multi-contact system. A permanent magnet is positioned at each gap, 3a, 3b and 3c. The actuator is in Fig. 4 illustrated in its initial position with the magnetic armature 2 positioned at the three permanent magnets 3a, 3b and 3.

With a fault condition the current in the flat coil 1 rapidly increases and the magnetic force of the flat coil 1 on the magnetic armature 2 increases, which is

illustrated in Fig. 5 wherein the electrically conductive bridge 8a over the permanent magnet 3a is shown just leaving the initial position against the permanent magnet 3a. As the magnetic armature 2 moves nearer the flat coil 1 the force driving the magnetic armature towards flat coil 1 increases.

In Fig. 6 the magnetic armature 2 is shown between the initial position and the second position, and the arc discharges are illustrated between the bridges 8a, 8b and 8c and the multi-contact system. The force on the magnetic armature 2 is even greater now, since the distance to the flat coil has decreased and the distance to the three permanent magnets 3a, 3b and 3c has

increased. As this distance increases, the summation of the arc voltage across each contact pair increases until it reaches a value equal or larger than the supply voltage, forcing a zero crossing of the current in the circuit . In the second position, shown in Fig. 7, the arc

discharges are quenched, and since the current in the flat coil thus is zero the magnetic force of the flat coil on the magnetic armature is zero. The magnetic armature is kept in the second position by another permanent magnet 4 in the centre of the flat coil 2. In this way the actuator has two bi-stable positions, the initial position and the second position.

A switch 7, as disclosed in Fig 2, may be used to reset the circuit breaker to the initial position. A fifth embodiment of an actuator according to the present invention is illustrated in Fig. 8. This actuator is identical that of the first embodiment described above, apart from the following.

The actuator comprises an additional magnetic armature 5 and another permanent magnet 6, keeping the additional magnetic armature 5 bi-stably fixed in the initial position. The additional magnetic armature 5 and the associated permanent magnet 6 are positioned an opposite side of the flat coil 1, compared to the magnetic

armature 2 and its associate permanent magnet 3. By having the permanent magnet 6, bi-stably fixing the additional magnetic armature 5 in the initial position, reversely magnetized compared to the permanent magnet 3, two electrical circuits can be protected with a single flat coil.

This fifth embodiment of the present invention may also be combined with the fourth embodiment of the present invention described above.

It will be obvious that the present invention may be varied in a plurality of ways. Such variations are not to be regarded as departure from the scope of the present invention. All such variations as would be obvious for a person skilled in the art are intended to be included within the scope of the present invention as defined by the appended claims.