WO2009049625A1

WO2009049625A1 - A magnetic actuator and a valve comprising such an actuator

Info

Publication number: WO2009049625A1
Application number: PCT/DK2008/000365
Authority: WO
Inventors: Michael Birkelund
Original assignee: Danfoss A/S
Priority date: 2007-10-18
Filing date: 2008-10-16
Publication date: 2009-04-23

Abstract

A magnetic actuator (1) comprising a first pole part (2), a second pole part (3) movable relative to the first pole part (2), a coil (5) arranged circumferentialIy relative to the pole parts (2, 3), and a flux conductor guide (12) arranged between the second pole part (3) and the coil (5) and adapted to guide magnetic flux generated by the coil (5) to the second pole part (3). The flux conductor guide (12) and the second pole part (3) each define a cross sectional area which varies stepwise along a direction of movement of the second pole part (3) in such a manner that the stepwise varying part of the flux conductor guide (12) can be received within the stepwise varying part of the second pole part (3), or in such a manner that the stepwise varying part of the second pole part (3) can be received within the stepwise varying part of the flux conductor guide (12). Furthermore, a magnetic valve (9) having the actuator (1) arranged therein.

Description

A MAGNETIC ACTUATOR AND A VALVE COMPRISING SUCH AN ACTUATOR

FIELD OF THE INVENTION

The present invention relates to a magnetic actuator, in particular for use in a magnetic valve. More particularly, the present invention relates to a magnetic actuator which is particularly suitable for use in valves which are open in a non-energized state, i.e. in so-called 'normally open' (NO) valves.

BACKGROUND OF THE INVENTION

Magnetic valves are usually provided with a magnetic actuator comprising two opposing pole parts made from a magnetisable material, one of the pole parts being movable relative to the other pole part, and an electrical coil arranged circumferentially relative to the pole parts. When a current is supplied to the electrical coil, a magnetic field is induced in accordance with Ampere's law inside the coil, i.e. in the area where the pole parts are located. As a consequence, the pole parts act as electromagnets and their ends will be attracted to each other. Since one of the pole parts is movable relative to the other pole part, the pole parts will move towards each other, and a gap formed between the pole parts will thereby decrease, possibly closing completely. By connecting the movable pole part to a closing element of the valve in a suitable manner and in accordance with the valve type, it can be obtained that this attraction between the pole parts results in the opening or closing of the valve.

In magnetic valves of the normally open (NO) type, the stationary pole part is normally arranged between the movable pole part and the valve closing element, and the pole parts are preferably mechanically biased away from each other, e.g. by means of a spring, in order to bias the valve closing element towards an open state. When a current is supplied to the electrical coil, the movable pole part moves towards the stationary pole part against the biasing force, and the valve closing element is thereby moved towards a corresponding valve seat, thereby closing the valve.

It has turned out that it is difficult to transfer sufficient magnetic flux to the movable pole part to ensure that the operation described above can take place with a desired performance. In order to solve this problem a flux conductor in the form of a bushing or sleeve has previously been inserted between the coil and the movable pole part. Such a flux conductor reduces the flux losses in the device because it leads the magnetic flux in an efficient manner. However, using a flux conductor in the form of a bushing or sleeve has the consequence that a larger coil is required in order to accommodate the bushing or sleeve inside the coil. Thereby the size of the entire actuator as well as the manufacturing cost increases. One example of a magnetic actuator comprising a flux conductor in the form of a bushing or sleeve is disclosed in US 6,918,571.

SUMMARY OF THE INVENTION

It is, thus, an object of the invention to provide a magnetic actuator which is suitable for use in magnetic valves which are open in a non-energized state.

It is a further object of the invention to provide a magnetic actuator which can be operated with a higher performance regarding temperature and magnetic forces than similar prior art magnetic actuators.

It is an even further object of the invention to provide a magnetic actuator comprising a flux conductor which is capable of transferring magnetic flux to pole parts of the actuator with an improved performance as compared to similar prior art magnetic actuators. According to a first aspect of the invention the above and other objects are fulfilled by providing a magnetic actuator comprising:

- a first pole part made from a magnetically conductive material,

- a second pole part made from a magnetically conductive material, said second pole part being arranged relative to the first pole part in a movable manner,

- a coil arranged circumferentially relative to the first pole part and the second pole part in such a manner that the second pole part can be moved relative to the first pole part in response to a magnetic field induced as a result of an electrical current passing through the coil,

- a flux conductor guide made from a magnetically conductive material, and being arranged between the second pole part and the coil, said flux conductor guide being adapted to guide magnetic flux generated by the coil to the second pole part,

wherein the flux conductor guide and the second pole part each define a cross sectional area which varies stepwise along a direction of movement of the second pole part in such a manner that the stepwise varying part of the flux conductor guide can be received in the stepwise varying part of the second pole part, or in such a manner that the stepwise varying part of the second pole part can be received in the stepwise varying part of the flux conductor guide.

The second pole part is arranged relative to the first pole part in a movable manner. Accordingly, relative movements between the first pole part and the second pole part are possible. In some cases the first pole part is stationary relative to an application, e.g. a magnetic valve, where the magnetic actuator is positioned, the first pole part in this case constituting a stationary pole part. In this case the second pole part is movable relative to the application wherein the magnetic actuator is positioned, i.e. the second pole part constitutes a movable pole part. However, as an alternative, the first pole part as well as the second pole part may be movable relative to the application where the magnetic actuator is positioned, as well as relative to each other. This will be described in further detail below. In any event, supplying current to the coil causes movements of the first pole part and/or of the second pole part, and these movements result in a desired operation of the actuator.

The first pole part as well as the second pole part is made from a magnetically conductive material, such as a ferromagnetic material, e.g. W 1.4105 or AISI 430.

The coil is arranged circumferentially relative to the first pole part and the second pole part. Furthermore, the coil is arranged in such a manner that the second pole part can be moved relative to the first pole part in response to a magnetic field induced as a result of an electrical current passing through the coil. Preferably, at least the second pole part, and most preferably also the first pole part, is arranged inside the coil. When a current is applied to the coil, a magnetic field is induced in accordance with Ampere's law. Thereby the first pole part as well as the second pole part will constitute electromagnets, and the end parts facing each other will accordingly attract each other, thereby causing relative movement between the first pole part and the second pole part.

The magnetic actuator further comprises a flux conductor guide arranged between the second pole part and the coil. The flux conductor guide is made from a magnetically conductive material, e.g. the same material which is used for the first pole part and/or the second pole part. Thus, the flux conductor guide is adapted to guide magnetic flux generated by the coil to the second pole part.

The flux conductor guide defines a cross sectional area which varies stepwise along a direction of movement of the second pole part. The second pole part also defines a cross sectional area which varies stepwise along the direction of movement of the second pole part.

In the present context the term 'stepwise¹ should be interpreted to mean that the cross sectional area varies abruptly at specific positions along the direction of movement of the second pole part, and that the cross sectional area is substantially constant between such positions along the direction of movement. Thus, the stepwise varying parts of the flux conductor guide and the second pole part have shapes which resemble stairs.

Furthermore, the stepwise varying parts of the flux conductor guide and the second pole part, respectively, are arranged relative to each other in such a manner that one of them can be received within the other. Thus, either the stepwise varying part of the flux conductor guide can be received within the stepwise varying part of the second pole part, or the stepwise varying part of the second pole part can be received within the stepwise varying part of the flux conductor guide. Accordingly, the stepwise varying parts of the flux conductor guide and the second pole part fit together, one inside the other. Due to the stepwise shapes of the flux conductor guide and the second pole part, the smallest distance between the flux conductor guide and the second pole part is substantially invariable during movements of the second pole part relative to the flux conductor guide. Thereby the conductance of the magnetic flux from the flux conductor guide to the second pole part does not vary significantly during such movements. Accordingly, a reliable flux conductance between the flux conductor guide and the second pole part can be ensured. Furthermore, the magnetic loss while conducting the magnetic flux from the flux conductor guide to the second pole part is minimised. This is very advantageous.

Preferably, the stepwise varying parts of the flux conductor guide and the second pole part are arranged relative to each other in such a manner that the parts defining a substantially constant cross sectional area are positioned corresponding to each other, in such a manner that at least a portion of such parts remain positioned with an overlap with respect to each other when the second pole part performs movements which can be expected during normal operation. In this case such overlaps define regions where the distance between the flux conductor guide and the second pole part are very small, and where the magnetic flux can thereby be reliably conducted from the flux conductor guide to the second pole part as described above, regardless of movements performed by the second pole part during normal operation.

The stepwise varying parts of the flux conductor guide and the second pole part may each define at least two different cross sectional areas. As described above, each of the cross sectional areas defined by the flux conductor guide and the second pole part, respectively, may be regarded as a 'step¹ of a 'stair'. Each step defines a flux path in which only very low losses are introduced due to the small distance between the flux conductor guide and the second pole part. By increasing the number of steps defined by the flux conductor guide/second pole part, the flux density present at each step can be reduced, and it is thereby possible to prevent saturation of the magnetically conductive materials of the flux conductor guide and the second pole part. On the other hand, a low number of steps ensures that the parts of the flux conductor guide/second pole part defining a substantially constant cross sectional area are relatively large, thereby allowing relatively large movements of the second pole part without risking that the 'constant area' parts are moved away from each other. Thus, the number of steps should preferably be chosen in such a manner that due consideration is taken to both of these issues. It has been found by the inventors of the present invention that at least two steps, preferably at least three steps, is suitable for magnetic actuators which are to be used in magnetic valves. It should be noted that the number of steps defined by the flux conductor guide and the number of steps defined by the second pole are not necessarily equal.

The stepwise varying part of the flux conductor guide may define a protruding part and the stepwise varying part of the second pole part may define a recess adapted to receive the protruding stepwise varying part of the flux conductor guide. According to this embodiment, the stepwise varying part of the flux conductor guide can be received within the stepwise varying part of the second pole part.

Alternatively, the stepwise varying part of the second pole part may define a protruding part and the stepwise varying part of the flux conductor guide may define a recess adapted to receive the protruding stepwise varying part of the second pole part. According to this embodiment, the stepwise varying part of the second pole part can be received within the stepwise varying part of the flux conductor guide.

According to one embodiment, the stepwise varying part of the flux conductor guide may define a protruding part as well as a recess, and the stepwise varying part of the second pole part may define a protruding part as well as a recess, and the recess of the flux conductor guide may be adapted to receive the protruding part of the second pole part, and the recess of the second pole part may be adapted to receive the protruding part of the flux conductor guide. According to this embodiment, the stepwise varying part of the flux conductor guide and the stepwise varying part of the second pole part are 'interleaved'. This has the consequence that there are more paths available for the magnetic flux being conducted from the flux conductor guide to the second pole part, and thereby an even more reliable flux conductance can be obtained, and the losses introduced are even further decreased. This is very advantageous.

The magnetic actuator may further comprise a shading ring arranged on the first pole part or on the second pole part. It is known that when a magnetic actuator is used in a magnetic valve which is driven by means of AC currents, a so-called AC valve, the noise level as well as the wear on the movable parts of the valve is very high. It is also known that positioning a shading ring on one of the pole parts solves these problems. A shading ring is a ring made from an electrically conductive material. When such a ring is arranged in one of the pole parts it provides a second magnetic field having a phase which is different from the phase of the main magnetic field used for providing relative movements of the first and second pole parts, the main magnetic field being provided by the coil. The presence of this second magnetic field ensures that the total magnetic force will never go to zero. This total magnetic force has a magnitude which is able to hold the first pole part and the second pole part together. Thus, in the case that the magnetic valve is provided with a biasing spring, the total magnetic force should be higher than the force of the biasing spring. Accordingly, noise and wear arising from the first pole part and the second pole part repeatedly hitting each other is thereby reduced. Accordingly, providing the magnetic actuator with a shading ring as described above renders the magnetic actuator suitable for use in AC valves.

The first pole part may be movable in response to a magnetic field induced as a result of an electrical current passing through the coil or to a force from a servo system. According to this embodiment, the first pole part as well as the second pole part is movable.

The magnetic actuator may further comprise mechanical biasing means adapted to mechanically bias the first pole part and the second pole part in a direction away from each other. According to this embodiment, the first pole part and the second pole part are positioned as far away from each other as possible when the actuator is in a non-energized state. When the actuator is energized, the first pole part and the second pole part are moved towards each other against the force provided by the mechanical biasing means.

The mechanical biasing means may comprise a spring, preferably a compressible spring, but a torsion spring may alternatively be used.

According to one embodiment, the stepwise varying parts of the flux conductor guide and the second pole part may define a distance between neighbouring steps, said distance being longer than a representative distance which the second pole part travels during normal operation. As mentioned above, it can hereby be ensured that the 'steps' are not moved apart during normal operation of the magnetic actuator. Accordingly, for each step a region is maintained which defines a very small distance between a portion of the flux conductor guide and a portion of the second pole part. Magnetic flux can be conducted via these regions with only very low losses being introduced.

According to a second aspect of the invention the above and other objects are fulfilled by providing a magnetic valve comprising:

- a valve seat, - a valve closing element being movable between a position in which it abuts the valve seat, thereby closing the valve, and positions in which it does not abut the valve seat, thereby allowing fluid to pass the valve, and

- a magnetic actuator according to the first aspect of the invention, the first pole part or the second pole part of said magnetic actuator being connected to the valve closing element, the valve thereby being opened or closed in response to movements of the pole part connected to the valve closing element.

The valve closing element may be movable between only two positions, i.e. one in which the valve is closed and one in which the valve is open. Alternatively, the valve closing element may be movable between a plurality of positions, e.g. continuously, the plurality of positions defining various degrees of opening of the valve.

Since the valve according to the second aspect of the invention comprises a magnetic actuator according to the first aspect of the invention, the advantages described above also apply to the valve. Thus, the valve may be operated in such a manner that it is open in a non-energized state, it is capable of transferring magnetic flux to the pole parts in a reliable manner, the valve thereby being operable in a reliable manner and with low losses, etc.

The valve may be of a kind which is open in a non-energized state, i.e. a so-called normally open (NO) valve. In this case, it is preferably the second pole part which is connected to the valve closing element, i.e. movements of the second pole part causes the valve to open and close.

As an alternative, the valve may be of a kind which is closed in a non- energized state, i.e. a so-called normally closed (NC) valve. In this case the first pole part is preferably arranged in a movable manner relative to the remaining parts of the valve, and the first pole part is preferably connected to the valve closing element. Accordingly, the valve is, in this case, preferably opened and closed in response to movements of the first pole part.

It should be noted that a person skilled in the art would readily recognise that any features described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Figs. 1 and 2 are cross sectional views of a prior art magnetic actuator for use in a valve of a normally open type, in a non-energized and an energized state, respectively,

Fig. 3 is a cross sectional view of a valve having the magnetic actuator of Figs. 1 and 2 arranged therein,

Figs. 4 and 5 are cross sectional views of a prior art magnetic actuator for use in a valve of a normally closed type, in a non-energized and an energized state, respectively,

Figs. 6 and 7 are cross sectional views of a magnetic actuator according to a first embodiment of the invention, for use in a valve of a normally open type, in a non-energized and an energized state, respectively, Figs. 8 and 9 are cross sectional views of a valve of a normally open type and having a magnetic actuator according to a second embodiment of the invention arranged therein, in a non-energized and an energized state, respectively, and

Figs. 10 and 11 are cross sectional views of a magnetic actuator according to a third embodiment of the invention, for use in a valve of a normally closed type, in a non-energized and an energized state, respectively.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross sectional view of a prior art actuator 1 for use in a valve of a normally open type. The actuator 1 is shown in a non-energized state, i.e. a valve having the actuator 1 arranged therein would be open.

The actuator 1 comprises a first pole part 2 and a second pole part 3. The first pole part 2 is arranged in a stationary manner relative to the remaining parts of the actuator 1 , while the second pole part 3 is arranged in a movable manner relative to the remaining parts of the actuator 1. A biasing spring 4 mechanically biases the second pole part 3 in a direction away from the first pole part 2. A coil 5 is arranged circumferentially about the pole parts 2, 3.

The actuator 1 is operated in the following manner. When it is desired to close the valve having the actuator 1 arranged therein, an electrical current is supplied to the coil 5. Thereby a magnetic field is induced in accordance with Ampere's law. Due to this magnetic field, each of the pole parts 2, 3 acts as an electromagnet and the ends of the pole parts 2, 3 facing each other are attracted towards each other, thereby moving the second pole part 3 in a direction towards the first pole part 2 against the force applied by the biasing spring 4. Thereby valve closing element 6 is moved into abutment with valve seat 7, thereby closing the valve. Fig. 2 shows the actuator 1 in this position.

In order to ensure that sufficient magnetic flux is transferred to the second pole part 3 to allow the second pole part 3 to move as described above, the actuator 1 is further provided with a flux conducting sleeve 8. This has the disadvantage that the actuator 1 becomes relatively bulky for a given performance, since the flux conducting sleeve 8 must be accommodated within the coil 5.

Fig. 3 is a cross sectional view of a valve 9 having the actuator 1 similar to the one shown in Figs. 1 and 2 arranged therein. The actuator 1 shown in Fig. 3 has a shading ring 15 arranged on the first pole part 2. As described above, the shading ring 15 provides a second magnetic field during operation of the valve 9, and this second magnetic field ensures that the total magnetic force will never go to zero. The total magnetic force has a magnitude which is able to hold the first pole part 2 and the second pole part 3 together when the valve 9 is operated using AC currents. Accordingly, the pole parts 2, 3 are prevented from repeatedly hitting against each other during operation, and the noise level as well as the wear on the moving parts are considerably reduced.

Fig. 4 is a cross sectional view of a prior art actuator 1 for use in a normally closed valve. The actuator 1 comprises a first pole part 2 and a second pole part 3. The first pole part 2 as well as the second pole part 3 is arranged in a movable manner relative to each other and relative to the remaining parts of the actuator 1. A biasing spring 4 mechanically biases the first pole part 2 as well as the second pole part 3 in a direction towards a valve seat 7. Thereby the valve having the actuator 1 arranged therein is closed in a non-energized state. In Fig. 4 the actuator 1 is shown in this state. A coil 5 is arranged circumferentially about the first pole part 2 and the second pole part 3.

The first pole part 2 is connected to a valve closing element 6 in such a manner that movements of the first pole part 2 cause the valve to open or close.

The actuator 1 is operated in the following manner. When it is desired to open the valve having the actuator 1 arranged therein, a current is supplied to the coil 5. Thereby a magnetic field is induced in accordance with Ampere's law. In response to this magnetic field, the first pole part 2 and the second pole part 3 act as electromagnets, and the ends of the pole parts 2, 3 which face each other are attracted to each other. As a consequence, the pole parts 2, 3 move towards each other. Thereby distance part 10 causes pilot valve 11 to open, and the pressure of the fluid running through the valve causes the first pole part 2 to move in a direction towards the second pole part 3, thereby pushing the second pole part 3 further in this direction against the force applied by biasing spring 4. This causes valve closing element 6 to move out of abutment with the valve seat 7, and the valve is thereby opened. Fig. 5 shows the actuator 1 in this state.

Fig. 6 is an actuator 1 according to a first embodiment of the invention for use in a valve of the normally open type. The actuator 1 comprises a first pole part 2 and a second pole part 3. The first pole part 2 is arranged in a stationary manner relative to the remaining parts of the actuator 1 and the second pole part 3 is arranged in a movable manner relative to the remaining parts of the actuator 1. A biasing spring 4 mechanically biases the second pole part 3 in a direction away from the first pole part 2, thereby keeping valve closing element 6 away from valve seat 7 when the actuator 1 is in a non-energized state. Fig. 6 shows the actuator 1 in this state. A coil 5 is arranged circumferentially relative to the first pole part 2 and the second pole part 3.

The second pole part 3 has a cross sectional area which varies stepwise in the direction of movement of the second pole part 3. Thus, the second pole part 3 has a 'stair-like' shape. In the embodiment shown in Fig. 6 the second pole part 3 defines three steps, i.e. three different cross sectional areas. The actuator 1 comprises a flux conductor guide 12 which also has a cross sectional area which varies stepwise in the direction of movement of the second pole part 3. The flux conductor guide 12 is arranged and positioned in such a manner that is can receive the stair-like part of the second pole part 3 when the actuator 1 is in a non-energized state, i.e. the stepwise varying part of the second pole part 3 fits into the stepwise varying part of the flux conductor guide 12.

The actuator 1 is operated in the following manner. When it is desired to close the valve having the actuator 1 arranged therein, a current is supplied to the coil 5. Thereby a magnetic field is induced in accordance with Ampere's law, and the first pole part 2 and the second pole part 3 act as electromagnets as described above. Accordingly, the second pole part 3 moves towards the first pole part 2, thereby moving the valve closing element 6 into abutment with the valve seat 7, thereby closing the valve.

Fig. 7 shows the actuator 1 in such an energized state. It can be seen from Fig. 7 that the steps of the stepwise varying part of the second pole part 3 have not been pulled completely out of the corresponding step of the stepwise varying part of the flux conductor guide 12. Thereby it is ensured that, during the entire movement of the second pole part 3 in order to close the valve, regions exist where the distance between the second pole part 3 and the flux conductor guide 12 is very small. Accordingly, at least one 'flux path' from the flux conductor guide 12 to the second pole part 3, which introduces very low losses, is available to the magnetic flux during the entire movement. Furthermore, due to the stepwise variation, this 'smallest distance' is the same throughout the movement. Thus, magnetic flux is conducted from the coil 5 to the second pole part 3, via the flux conductor guide 12 with high performance and with very low losses. This is very advantageous.

Fig. 8 is a cross sectional view of a valve 9 of a normally open type. The valve 9 has a magnetic actuator 1 according to a second embodiment of the invention arranged therein. In Fig. 8 the valve 9 is shown in a non- energized state, i.e. it is open. The valve 9 and the actuator 1 shown in Fig. 8 are operated essentially as described above.

In the actuator 1 shown in Fig. 8 the second pole part 3 is shaped as a stair-like protruding part with a stair-like recess 13 formed therein. Similarly, the flux conductor guide 12 is shaped as a stair-like recess with a stair-like protruding part 14 arranged therein. The recess 13 of the second pole part 3 is shaped and sized to receive the protruding part 14 of the flux conductor guide 12. Thus, magnetic flux can be conducted from the flux conductor guide 12 to the second pole part 3 via the stepwise varying parts as described above with reference to Figs. 6 and 7, and/or it can be conducted via the protruding part 14. Thereby it is even further ensured that a good conductance path is available to the magnetic flux.

Fig. 9 shows the valve 9 of Fig. 8 in an energized state, i.e. in a closed state.

Fig. 10 is a cross sectional view of a magnetic actuator 1 according to a third embodiment of the invention. The actuator 1 of Fig. 10 is suitable for use in a valve of the normally closed type. In Fig. 10 the actuator 1 is shown in a non-energized state. The actuator 1 shown in Fig. 10 is operated essentially as described above with reference to Figs. 4 and 5. However, in the actuator 1 of Fig. 10 the second pole part 3 as well as the flux conductor guide 12 is shaped in a stair-like manner. Thereby the advantages described above with reference to Figs. 6 and 7 are also obtained here.

Fig. 11 shows the actuator 1 of Fig. 10 in an energized state.

Claims

1. A magnetic actuator (1) comprising:

- a first pole part (2) made from a magnetically conductive material,

- a second pole part (3) made from a magnetically conductive material, said second pole part (3) being arranged relative to the first pole part (2) in a movable manner,

- a coil (5) arranged circumferentially relative to the first pole part (2) and the second pole part (3) in such a manner that the second pole part (3) can be moved relative to the first pole part (2) in response to a magnetic field induced as a result of an electrical current passing through the coil (5),

- a flux conductor guide (12) made from a magnetically conductive material, and being arranged between the second pole part (3) and the coil (5), said flux conductor guide (12) being adapted to guide magnetic flux generated by the coil (5) to the second pole part (3),

wherein the flux conductor guide (12) and the second pole part (3) each define a cross sectional area which varies stepwise along a direction of movement of the second pole part (3) in such a manner that the stepwise varying part of the flux conductor guide (12) can be received within the stepwise varying part of the second pole part (3), or in such a manner that the stepwise varying part of the second pole part (3) can be received within the stepwise varying part of the flux conductor guide (12).

2. A magnetic actuator (1) according to claim 1 , wherein the stepwise varying parts of the flux conductor guide (12) and the second pole part (3) each define at least two different cross sectional areas.

3. A magnetic actuator (1) according to claim 1 or 2, wherein the stepwise varying part of the flux conductor guide (12) defines a protruding part and the stepwise varying part of the second pole part (3) defines a recess adapted to receive the protruding stepwise varying part of the flux conductor guide (12).

4. A magnetic actuator (1) according to claim 1 or 2, wherein the stepwise varying part of the second pole part (3) defines a protruding part and the stepwise varying part of the flux conductor guide (12) defines a recess adapted to receive the protruding stepwise varying part of the second pole part (3).

5. A magnetic actuator (1) according to any of the preceding claims, wherein the stepwise varying part of the flux conductor guide (12) defines a protruding part (14) as well as a recess, and the stepwise varying part of the second pole part (3) defines a protruding part as well as a recess (13), and wherein the recess of the flux conductor guide (12) is adapted to receive the protruding part of the second pole part (3), and the recess (13) of the second pole part (3) is adapted to receive the protruding part (14) of the flux conductor guide (12).

6. A magnetic actuator (1) according to any of the preceding claims, further comprising a shading ring (15) arranged on the first pole part (2) or on the second pole part (3).

7. A magnetic actuator (1) according to any of the preceding claims, wherein the first pole part (2) is movable in response to a magnetic field induced as a result of an electrical current passing through the coil (5).

8. A magnetic actuator (1) according to any of the preceding claims, further comprising mechanical biasing means adapted to mechanically bias the first pole part (2) and the second pole part (3) in a direction away from each other.

9. A magnetic actuator (1) according to claim 8, wherein the mechanical biasing means comprises a spring (4).

10. A magnetic actuator (1 ) according to any of the preceding claims, wherein the stepwise varying parts of the flux conductor guide (12) and the second pole part (3) defines a distance between neighbouring steps, said distance being longer than a representative distance which the second pole part (3) travels during normal operation.

11. A magnetic valve (9) comprising:

- a valve seat (7),

- a valve closing element (6) being movable between a position in which it abuts the valve seat (7), thereby closing the valve (9), and positions in which it does not abut the valve seat (7), thereby allowing fluid to pass the valve (9), and

- a magnetic actuator (1) according to any of the preceding claims, the first pole part (2) or the second pole part (3) of said magnetic actuator (1) being connected to the valve closing element (6), the valve (9) thereby being opened or closed in response to movements of the pole part (2, 3) connected to the valve closing element (6).

12. A magnetic valve (9) according to claim 11 , wherein the valve (9) is of a kind which is open in a non-energized state.

13. A magnetic valve (9) according to claim 11 , wherein the valve (9) is of a kind which is closed in a non-energized state.