Thomson coil actuator

Abstract

A Thomson coil actuator 100 comprises a disc 106 which is at least partially conductive and at least two conductive coils 104a,b arranged concentrically around an axis 120. When provided with an electric current by a driver circuit 108, the coils drive the disc in a first direction 122 along the axis. The coils do not overlap along the axis, and the driver circuit provides electric current to each coil independently and simultaneously. The driver circuit may comprise at least two capacitors, which each provide a respective current to a respective coil when discharged. Each conductive coil may be arranged in the same plane or in separate planes offset along the axis. The disc may be flat or stepped. The disc may be fixedly coupled to a rod 102 to form an axially movable assembly which is moved in the first direction by the coils. The rod may be coupled to a movable electric contact of a switching mechanism.

Description

Field This relates to a Thomson coil actuator, and an electrical switching apparatus 5 comprising said Thomson coil actuator. Background Thomson coil actuators are used in electrical switching apparatus, such as circuit breakers. In applications which require a rapid opening time, such as a hybrid circuit 10 breaker, high rated capacitors (e.g., high voltage and / or high capacitance) are needed. The rapid acceleration of the Thomson coil actuator plate (or disc) as a result of these high rated capacitors can lead to very large bending stresses within the disc. The lifetime of the disc, and thus of the Thomson coil actuator itself, may therefore be reduced. Moreover, there can be challenges in incorporating a higher rated capacitor 15 into electrical switching apparatus, both from a safety point of view and due to size constraints (since higher rated capacitors are larger and need to be charged to a higher voltage after every discharge). It is therefore desirable to provide a Thomson coil actuator which addresses some of 20 these challenges. Summary Disclosed herein is a Thomson coil actuator, and an electric switching apparatus comprising said actuator. 25 In a first aspect, a Thomson coil actuator comprises a disc which is at least partially conductive (also called herein a "conductive disc") and at least two conductive coils arranged concentrically around an axis, wherein the at least two conductive coils do not overlap along the axis. The coils are configured to drive the disc in a first direction 30 along the axis when an electric current is provided to the at least two conductive coils. A driver circuit is configured to provide an electric current to each of the at least two conductive coils independently, wherein the driver circuit configured to provide the at least two electric currents simultaneously. 35 By providing two or more conductive coils which are driven independently, but simultaneously, the required acceleration of the disc may be achieved whilst providing smaller circuitry for each individual coil. Moreover, the Thomson coil actuator may have a simple design. In particular, the multiple, concentric, and not overlapping, coils can be considered as a direct replacement for a single coil. In this way, the use separate coils and separate sources of electric current, controlled to act in synchronization, can allow for smaller circuitry for the same actuator performance. By reducing the size of the driver circuit, a smaller actuator may be provided. 5 In some implementations, the driver circuit comprises at least two capacitors. Each capacitor is configured to provide a respective electric current to a respective one of the at least two coils, wherein the driver circuit is configured to simultaneously discharge the at least two capacitors. Optionally, the driver circuit is further 10 configured to synchronize the charging of the at least two capacitors. By providing two or more conductive coils which are driven independently, but simultaneously, the required acceleration of the disc may be achieved with a lower rated capacitor (lower capacitance and / or voltage) than conventional actuators and with a simple design. In particular, these multiple, concentric, and not overlapping, coils can be considered as 15 a direct replacement for a single coil. In this way, the use of separate coils and separate sources of electric current, controlled to act in synchronization, can allow for a reduction in capacitor ratings and overall size for the same actuator performance. Moreover, by reducing the capacitor ratings, safety of the actuator may be improved. 20 As described herein the disc is substantially planar. The disc comprises a first surface and a second surface opposite the first surface, wherein the first surface of the disc is arranged to face the at least two conductive coils. Optionally, two or more second conductive coils are disposed on an opposite side of the disc from the at least two conductive coils, the two or more second coils configured to drive the disc in a second 25 direction opposite the first. In this way, the Thomson coil actuator may be used to both open and close an electrical switching apparatus. In some implementations, each conductive coil is arranged in a same plane. In other implementations, one or more of the at least two conductive coils are arranged in 30 separate planes offset along the axis. The arrangement of the conductive coils can be tailored to a specific application so as to reduce bending stresses within the disc. Even when the conductive coils are offset along the axis, they do not overlap each other along the axis. In this way, the arrangement described herein differs from previous arrangements having a multilayer coil comprising a plurality of overlapping coil layers. 35 In some implementations, the first surface of the disc comprises at least two concentric, conductive, planar portions, wherein adjacent planar portions are offset from one another along the axis. In other words, the disc is stepped, with concentric planar portions arranged on different planes to form the first surface. By providing a stepped disc or plate, bending stresses on the disc can be reduced as compared to a disc with a flat surface (for a same acceleration). Lifetime of the actuator may therefore be improved. In other examples, adjacent planar portions are not offset 5 from one another along the axis but instead lie in the same plane. In some implementations, the first surface of the disc comprises a step, the step configured such that the disc has a first thickness at an outer edge portion of the disc and a second thickness at an inner portion of the disc. Optionally, the first surface 10 comprises a first conductive planar portion arranged at the outer edge portion of the disc and a second conductive planar portion arranged at the inner portion of the disc. The disc or plate can have a uniform cross section or uniform thickness (i.e. the first thickness of the first conductive planar portion is equal to the second thickness of the 15 second conductive planar portion). In other examples, the inner portion can be thicker than the outer portion; in other words, the disc is stepped, and is thicker at the inner portion (near the axis) than at the edge. A thicker inner portion can help to reduce bending stresses within the disc. The second thickness can therefore be equal to or greater than the first thickness, depending on the disc arrangement or geometry. 20 By providing a stepped disc or plate which is thicker towards the centre than at the edge, bending stresses on the disc can be reduced as compared to a disc with a flat surface (for a same acceleration). Lifetime of the actuator may therefore be improved. 25 Optionally, each planar portion of the disc is arranged to face a respective one of the at least two conductive coils. In some examples, each planar portion of the disc is arranged to face a respective one of the at least two conductive coils, and adjacent coils are offset from one another along the axis; where the plate is stepped also, adjacent coils are optionally correspondingly offset from one another along the axis 30 (i.e., the offset of the adjacent planar portions corresponds to the offset of the adjacent coils). In other words, the conductive coils can be stepped to mirror the surface of the disc. This can allow to evenly drive the stepped disc and maximise the transfer of driving force from the conductive coils. In other examples, each planar portion of the disc is arranged to face a respective one of the at least two conductive 35 coils, wherein adjacent coils lie in a same plane and are not offset from one another along the axis. In such an arrangement, the plate can be flat / generally planar across the whole surface, or stepped. Optionally, each step or offset is more than or equal to a thickness of the respective coil. Alternatively, each step or offset is less than a thickness of the respective coil. Optionally, the distance along the first direction between the first and second planar portions of the stepped disc is more than a thickness of one of the conductive coils. In 5 this way, the at least two conductive coils could have different thickness and still be arranged proximate the surface of the disc. In another example, the distance along the first direction between the first and second planar portions is less than or equal to a thickness of one of the conductive coils. By aligning the coils and the planar disc portions in this way, separation between the coils and the disc can be reduced, 10 improving acceleration by maximising the transfer of the driving force from the coils to the disc. In some implementations, the at least two conductive coils are formed from amorphous cables. This can achieve higher magnetization than e.g., copper wire, 15 allowing the capacitor rating to be reduced. In accordance with a second aspect, there is provided a Thomson coil actuator. The Thomson coil actuator comprises a conductive disc comprising at least two planar portions arranged concentrically around an axis, and at least two conductive coils 20 arranged concentrically around the axis and configured to drive the disc in a first direction along the axis when an electric current is provided to the at least two conductive coils. Each coil overlaps a respective planar portion along the first direction. The at least two conductive coils do not overlap along the first direction. There is an offset between adjacent planar portions, and a corresponding offset 25 between adjacent conductive coils. The actuator further comprises a driver circuit configured to provide an electric current to each of the at least two conductive coils independently, the driver circuit configured to provide the at least two electric currents simultaneously. 30 The disc of the second aspect is stepped, with concentric planar portions arranged on different planes to form the first surface. By providing a stepped disc or plate, bending stresses on the disc can be reduced as compared to a disc with a flat surface (for a same acceleration). Lifetime of the actuator may therefore be improved. Moreover, the conductive coils are stepped to mirror the surface of the disc. This can 35 allow to evenly drive the stepped disc and maximise the transfer of driving force from the conductive coils. By aligning the coils and the stepped planar disc portions in this way, separation between the coils and the disc is reduced, improving acceleration by maximising the transfer of the driving force from the coils to the disc. In accordance with a third aspect, there is provided an electrical switching apparatus. The electrical switching apparatus comprises a first (fixed or stationary) electrical contact, a second (moveable) electrical contact, and the Thomson coil actuator of the 5 first or the second aspect. The Thomson coil actuator can drive the switching of the apparatus. In some implementations, the electrical switching apparatus comprises a vacuum interrupter, and the Thomson coil actuator drives the interrupter. In some other implementations, the electrical switching apparatus is a circuit breaker. Optionally the apparatus is a hybrid circuit breaker, and the Thomson coil actuator 10 drives the bypass relay. Optionally, the actuator of the first aspect and / or the second aspect further comprises a rod, the disc fixedly coupled to said rod to form an axially movable assembly. The at least two coils are configured to drive the movable assembly in the first direction 15 along the axis. The rod of the movable assembly is coupled to the second electrical contact in order to move the second electrical contact into or out of electrical contact with the first electrical contact when the at least two coils drive the movable assembly in the first direction along the axis. 20 In accordance with a fourth aspect, there is provided a method of operating a Thomson coil actuator of the first and / or second aspects. The method comprises simultaneously applying an electric current to each of the at least two conductive coils and inducing a first magnetic field in response to the applied electric current. The induced first magnetic field causes an eddy current within the disc. In response to the 25 eddy current, a second magnetic field is induced, the second magnetic field opposing the first magnetic field. The method further comprises repelling, as a result of the first and second magnetic fields, the disc away from the at least two conductive coils, thereby driving the disc in the first direction along the axis. 30 Features described with reference to the first aspect can be combined with the second aspect, and vice versa. Moreover, features of the first and / or second aspect can be combined in any suitable combination. List of Figures 35 The detailed description is with reference to the following Figures: Figure 1 is a schematic illustration of a conventional Thomson coil actuator; Figure 2 illustrates a Thomson coil actuator in accordance with the present disclosure; Figure 3 illustrates an example driving circuit for the actuator of Figure 2; Figure 4A illustrates the conventional planar coil of Figure 1 in cross section, and Figures 4B, 4C illustrate cross sections of examples of the conductive coils in accordance with the present disclosure; Figures 5A, 5B are schematic illustrations of the forces acting on a flat (Figure 5A) and 5 stepped (Figure 5B) disc; and Figure 6 is an example flowchart for operating an actuator as described herein. Detailed Description Figure 1 is a simplified view of an electrical switching apparatus comprising a io switching mechanism 20 (e.g., without limitation, a vacuum circuit breaker) and a Thomson coil actuator 10. The switching mechanism comprises a pair of separable electrical contacts (e.g., stationary electrical contact and movable electrical contact), which are separated by way of a rod 12 of the Thomson coil actuator 10. The rod 12 is coupled to the movable electrical contact in order to move the movable electrical 15 contact into and out of engagement with the stationary electrical contact. The Thomson coil actuator 10 comprises a disc 16 which is at least partially conductive (also called herein a "conductive disc") and at least one generally planar coil 14a,14b. The conductive disc 16 may be made of metal, optionally sheet metal. Although the 20 disc 16 may not be made entirely of a uniformly conductive component (and part of the disc may not be conductive), the disc 16 is structured so as to allow for proper operation in the switching apparatus. The disc 16 is fixedly coupled to said rod 12 to form an axially movable assembly. A driver circuit 18 functions to introduce electrical current into the coils 14a,14b to cause them to drive the disc 16 up and down, 25 thereby assisting the Thomson coil actuator 10 in closing and opening the electrical contacts of the mechanism 20, respectively. In particular, an electric current applied to coil 14a by the driver circuit 18 (optionally from discharge of a capacitor) induces a first magnetic field. The induced magnetic 30 field causes an eddy current with the disc 16. In accordance with Lenz's law, a second magnetic field is induced in response to the eddy current, the second magnetic field opposing the first magnetic field. These opposing magnetic fields repel the disc 16 away from the coil 14a and towards coil 14b, thereby driving the disc 16 and rod 12 down and opening the switching mechanism 20. Applying a current to coil 14b causes 35 the disc to move in the opposite direction, thereby closing the switching mechanism. One of coils 14a, 14b may be removed in order to provide for a single action switching apparatus, depending on the desired direction of movement of disc 16. A Thomson coil actuator of the present disclosure will now be described with reference to Figure 2. The underlying mechanism of the actuator of Figure 2 is the same as that described with reference to Figure 1. 5 As shown in Figure 2, the Thomson coil actuator 100 comprises a disc 106 which is at least partially conductive (conductive disc 106) and at least two conductive coils 104a (black circles), 104b (white circles). The at least two conductive coils are arranged concentrically around an axis 120 and configured to drive the disc 106 in a first direction 122 along the axis when an electric current is provided to the at least two io conductive coils. The at least two conductive coils do not overlap along the axis. In other words, the coils do not overlap in the first direction when the actuator 100 is viewed along the axis 120. The disc can be flat or it can be stepped, or it can be any other suitable shape. In this 15 example, the disc 106 is flat and comprises a first (generally planar) surface and a second (generally planar) surface opposite the first surface; the first surface of the disc is arranged to face the at least two conductive coils 104a, 104b. The first surface comprises a conductive portion of the disc. The conductive disc 106 may be made entirely of metal, optionally sheet metal, or of a conductive portion in combination 20 with a non-conductive (for example, electrically insulating) portion. Any suitable material may be used for the disc 106, as is known in the art. The actuator 100 further comprises a driver circuit 108 configured to provide an electric current to each of the at least two conductive coils 104a, 104b independently. 25 The driver circuit is configured to provide the at least two electric currents simultaneously. By providing two or more conductive coils 104a, 104b which are driven independently, but simultaneously, the same acceleration of the disc 106 may be achieved as in the arrangement of Figure 1, but with smaller driving circuitry 108 for each individual coil. By reducing the size of the driver circuit 108, a smaller 30 actuator may be provided. In specific examples (not illustrated in Figure 2), the driver circuit comprises at least two capacitors. Each capacitor is configured to provide a respective electric current to a respective one of the at least two coils, wherein the driver circuit is configured to 35 simultaneously discharge the at least two capacitors. The driver circuit 108 is further configured to synchronize the charging of the at least two capacitors. A suitable exemplary driver circuit 108 is described further below with reference to Figure 3. By providing two or more conductive coils which are driven independently, but simultaneously, the required acceleration of the disc may be achieved with a lower rated capacitor (lower capacitance and / or voltage) than the actuator of Figure 1, and with a simple design. In particular, the use of separate coils and separate sources of 5 electric current, controlled to act in synchronization, can allow for a reduction in capacitor ratings and overall size for the same actuator performance. Moreover, by reducing the capacitor ratings, safety of the actuator 100 may be improved. The actuator 100 of Figure 2 further comprises a rod 102. The disc 106 is fixedly io coupled to said rod 102 to form an axially movable assembly. The at least two conductive coils 104a, 104b are configured to drive the movable assembly in the first direction 122 along the axis 120. The rod 102 can be coupled to a moveable electrical contact of an electrical switching apparatus (not shown), as discussed with reference to Figure 1, and the actuator 100 can thereby be used to move the moveable electrical 15 contact into or out of electrical contact with a fixed or stationary electrical contact when the at least two coils 104a, 104b drive the movable assembly in the first direction 120 along the axis. The switching apparatus can be a vacuum interrupter, a bypass relay, or any other suitable switching mechanism. 20 The at least two conductive coils 104a, 104b are in place of coil 14a in Figure 1. With reference to Figure 2, two or more second conductive coils 110a (white circles), 110b (black circles) may also be provided in place ofcoil 14b. The second surface of the disc is arranged to face the second conductive coils 110a, 11b. The second surface can comprise another conductive portion of the disc, allowing the disc to be driven in a 25 second direction opposite the first direction 122. It will be understood that one set of conductive coils (either coils 104a, 104b or coils 110a, 110b) may be removed in order to provide for a single action switching apparatus, depending on the desired direction of movement of disc 106. 30 Optionally, the at least two conductive coils are formed from amorphous cables. In other examples, copper wire can be used, and / or any other suitable material for forming conducting coils. The use of amorphous cables for the conductive coils can achieve higher magnetization than e.g., copper wire, allowing the capacitor rating to be even further reduced (since less current is required to achieve the same strength of 35 magnetic field). Safety of the actuator may therefore be further increased, and a smaller actuator may be provided. With reference to Figure 3, an exemplary driver circuit 108 for synchronized capacitor charging and discharging is described. Synchronized discharging of energy sources (such as capacitors) can be controlled using multiplexer switches or any other suitable configuration. The specific circuit 108 of Figure 3 is an example only, and within this 5 example circuit the 2 channel monostable multivibrator shown in Figure 3 is optional; the driver circuit 108 of Figure 3 will still function in the absence of this component. Moreover, any other switching method and driver circuit 108 can be used instead of that shown in Figure 3. io In this particular example of Figure 3, driver circuit 108 comprises two capacitors 300a, 300b, each configured to drive a respective coil 104a, 104b. An example sequence of operation is as follows: 1. Relayl, Relay2, Relay3, Relay4 ON (Relay5, Relay6 and Relay? OFF): Charging of capacitors 300a, 300b; 15 2. Relayl, Relay2, Relay3, Relay4 OFF: Isolation of DC source; 3. Relay5, Relay6 ON: Capacitors 300a, 300b can discharge instantaneously when the MOSFET is turned ON; 4. Relay? ON for 2ms: Multivibrator turned ON, Octocoupler and MOSFET ON, Capacitor discharge through Thomson Coil 104a, 104b to provide electrical 20 current and drive disc; 5. All relay OFF. The arrangement of the conductive coils 104a, 104b and disc 106 will now be described further with reference to Figures 4A to 4C. 25 An example of one of the generally planar coils 14a, 14b of Figure 1 is illustrated in the cross section of Figure 4A. Figure 4A shows a disc 16 having a conductive portion (or being entirely conductive), where at least part of the conductive portions of disc 16 are arranged proximate to a planar coil 14. Only one coil is shown. 30 In accordance with one or more examples of the present disclosure, and as discussed with reference to Figure 2, this conventional single coil 14 of Figure 4A can be replaced by at least two conductive coils 104a, 104, as shown in the cross sections of Figures 4B and 4C; a first conductive coil 104a is represented by black circles, and a 35 second conductive second coil 104b is represented by white circles. The optional second coils 110a, 110b described with reference to Figure 2 are not shown here. As in Figure 2, the second conductive coil 104b is closer to an inner edge of the disc (i.e. the portion of the disc proximate the axis 120) and the first conductive coil 104a is closer to the outer edge of the disc 106. The conductive disc 16 of Figures 1, 4A is here replaced by disc 106 (as in Figure 2). 5 The disc 106 comprises a first surface 160 arranged to face the at least two conductive coils 104a, 104b. As discussed above, the at least two conductive coils 104a, 104b are arranged concentrically around an axis (not shown) and configured to drive the disc 106 in a first direction 122 in accordance with the mechanism described with reference to Figures 1 and 2. The at least two conductive coils do not overlap io along the axis (e.g., do not overlap along the first direction 122). In this way, the arrangement of Figures 2, 4B, 4C is distinct from previous arrangements which use multilayer coils comprising a plurality of overlapping coil layers. In particular, by providing at least two coils, each driven independently but simultaneously, a simple design can be provided which is easy to control, and which provides a smaller and 15 safer actuator with an improved lifetime. Moreover, by providing power to each coil simultaneously, the disc can be evenly accelerated away from the coils and acceleration of the disc can be maximized, leading to an actuator with improved performance over asynchronous power supply. 20 In the example of Figure 4B, the conductive coils 104a, 104b are arranged in the same plane, as shown in Figure 4B. The disc 106 is generally planar. In particular, the first surface of the disc comprises a planar conductive portion and the coils 104a, 104b are disposed in a plane which is substantially parallel to the plane of the disc such that the conductive coils are arranged proximate to the planar conductive portion 25 of disc 106. This arrangement can provide the above discussed advantages of a smaller driver circuitry, including a smaller capacitor. Splitting the coils into two or more separately driven coils can also reduce the bending stress within the plate as compared to the arrangement of Figure 1 (see Table 1 below). 30 In the example of Figure 4C, the conductive coils 104a, 106b are arranged in separate planes which are offset along the axis, as shown in Figure 4C. In this example the inner coil 104b is offset further along the first direction 122 than the outer coil 104a, but any other offset arrangement may be provided. In this specific example, the first surface 160 of the disc 106 comprises at least two concentric, conductive, planar 35 portions 106a, 106b, wherein adjacent planar portions are offset from one another along the first direction 122. The first surface comprises a first conductive planar portion 106a arranged at the outer edge portion of the disc and a second conductive planar portion 106b arranged at the inner portion of the disc. Each planar portion 106a, 106b of the disc is arranged to face a respective one of the at least two conductive coils 104a, 104b. In other words, the coils and planar portions are correspondingly offset. This can allow to evenly drive the disc along its surface and maximise the transfer of driving force from the conductive coils. Moreover, bending 5 stresses on the disc can be reduced as compared to a disc with a flat surface (for a same acceleration), as shown in Table 1 below. Lifetime of the actuator may therefore be improved. The arrangement of Figure 4C can also be thought of as having a "step". In other io words, the disc 106 is stepped, with the first portion 106a being at a first level and the second portion 106b being at a second, lower, level. The stepped disc can be configured such that the disc has a uniform thickness when viewed in cross section, or the thickness of the disc may vary across the disc. In this specific example of Figure 4C, the thickness of the disc in the first conductive planar portion 106a is the same 15 thickness as in the second conductive planar portion 106b. However, in other examples, the stepped disc is configured such that the disc has a first thickness tl at an outer edge portion of the disc and a second thickness t2 at an inner portion of the disc. In some implementations, the first thickness is less than the second thickness, such that the disc is stepped and is thicker at the centre or inner potion than at the 20 outer edge of the disc, which can reduce the bending stress within the disc, but any other arrangement is possible. By providing a stepped disc or plate, bending stresses on the disc can be reduced as compared to a disc with a flat surface (for a same acceleration). Lifetime of the 25 actuator may therefore be improved. Moreover, as shown in Figure 4C, each planar portion 106a, 106b of the disc is arranged to face a respective one of the at least two conductive coils 104a, 104b; adjacent coils are correspondingly offset from one another along the first direction. In other words, the conductive coils can be stepped to mirror the surface of the disc. This can allow to evenly drive the stepped disc and 30 maximise the transfer of driving force from the conductive coils. Optionally, as shown in the example of Figure 4C, each step or offset is more than or equal to a thickness of the respective coil (e.g., the top of surface 160 can be seen above the coils 104b in the cross section). . In other words, the distance along the 35 first direction 122 between the first and second planar portions is more than or equal to a thickness of the respective coil 104b which is opposite the planar portion 106b. In this way, the at least two conductive coils could have different thickness and still be arranged proximate the surface 160 of the disc. By aligning the coils and the planar disc portions in this way, physical separation between the coils and the disc may be reduced, improving acceleration by maximising the transfer of the driving force from the coils to the disc. In other examples, each step or offset can be less than a thickness of the respective coil, such that the distance along the first direction 122 5 between the first and second planar portions is less than a thickness of the respective coil 104b which is opposite the planar portion 106b. Any suitable thickness of disc and / or coil can be used, and the offset or step can be any suitable size or dimension. In other examples, not shown here, the plate may be flat or generally planar and the io coils may be offset from one another along the axis. In other words, the first surface of the disc comprises at least two concentric, conductive, planar portions, wherein adjacent planar portions lie in a same plane and are not offset from one another along the axis; each planar portion of the disc is arranged to face a respective one of the at least two conductive coils, and adjacent coils are offset from one another along the 15 axis. In another example, not shown here, the plate may be stepped and the coils are generally planar. In other words, the first surface of the disc comprises at least two concentric, conductive, planar portions, wherein adjacent planar portions lie in a same plane and are not offset from one another along the axis; each planar portion of the disc is arranged to face a respective one of the at least two conductive coils, and 20 adjacent coils lie in a same plane and are not offset from one another along the axis. Table 1 shows example results from modelling (using COMSOL models) of the three exemplary arrangements. Example 1 corresponds to the arrangement shown in Figure 4A, with a single coil and single power source: one capacitor of 15000 uF and 400V. 25 Example 2 corresponds to the arrangement shown in Figure 4B, with a flat disc, a split coil and a split power source: one capacitor of 7500 uF and 350V, one capacitor of 7500 uF and 300V, flat disc. Example 2 corresponds to the arrangement shown in Figure 4C, with a stepped disc, split coil and a split power source: one capacitor of 7500 uF and 350V, one capacitor of 7500 uF and 300V. Displacement of the disc 16, 30 106 at 0.5 ms after capacitor discharge was measured, and the maximum surface von Mises stress in the disc determined. Table 1: example results from modelling (using COMSOL models) Example 1 Example 2 Example 3 Displacement of disc (mm) 0.5 ms after discharge 1.83 1.89 1.84 Maximum surface von Mises stress (N / m2) 7.87X107 5.06X107 3.73X107 From Table 1, it can be seen that Example 3 results in the lowest bending stress, with Example 1 having the highest bending stress. Displacement is comparable for all 5 examples. This shows that the use of two or more conductive coils and a split power source (e.g., two or more capacitors) is capable of producing the required acceleration of the disc 106. An improved Thomson coil actuator may therefore be provided. The specific sizes of the capacitors, and the dimensions of the steps or offsets for the io disc 106 can be selected or determined based on the specific application of the Thomson coil actuator and the properties of the disc. In particular, it is desirable to select the dimensions of the first and second planar portions 106a, 106b such that the bending moment at the centre of the disc is less than the bending moment experienced with a flat disc. In this way, bending stresses can be reduced by 15 providing a stepped plate as compared to a flat plate. This determination is discussed below with reference to Figures 5A, 5B. Figure 5A illustrates the bending moment M of a flat disc 106 (as a free body diagram considering a static cantilevered arrangement), w is the uniformly distributed load, 20 and F is the equivalent point load. I is the total length of the disc. Figure 5B illustrates the bending moment Ml of a stepped disc 106 (as a free body diagram considering a static cantilevered arrangement), wl, w2 is the uniformly distributed load on each planar portion, and Fl, F2 is the equivalent point load (where F = Fl + F2). I is the total length of the disc and n is a factor varying from 0 to 1 (0 and 1 25 corresponding to a flat disc), a is the inner radius of the coil 104 nearest the centre of the disc. Force F = wl * (nl ■■■■ a) + w2 * (I ■■■■ nl) n2l2 — a2 I2 — n2l2 Bending moment Ml = wl *------F w2 *------- 30 For maximum acceleration, the force F should be maximised, and for minimum bending stress the bending moment M, Ml should be minimised. An analytical model and optimization process can be used to maximize F and minimise Ml to determine suitable parameters n for the stepped disc 106. By using a stepped plate optimized in this manner, the step geometry can be optimized to increase the flexibility of the disc and reduce von Mises bending stresses. Moreover, the width of each respective 5 conductive coil 104a, 104b can be selected or chosen to correspond to the width of each step (e.g., to provide the stepped arrangement illustrated in Figure 4C). The total thickness of the disc (when viewed in cross section) and the height / thickness of any steps or offsets can be determined in any suitable manner, including but not io limited to, simulations or optimization functions. It will be understood that there is a trade-off between reduced bending stresses due to the use of a step (optionally with a thicker central portion) and reduced acceleration of the disc due to increased disc mass (because of the increased thickness of the disc). The step size, thickness of the disc, and how the thickness varies across the disc, can be determined or selected for a 15 given application based on these factors. It will also be understood that the arrangement of Figure 4C and the calculations described with reference to Figures 5A, 5B can be expanded to any number of steps (i.e., there may be three planar portions, each offset along the axis, four planar 20 portions, more than four planar portions, etc). For a given number of stepped portions on the disc 106 there can be a corresponding number of conductive coils, or a different number of conductive coils. For example, each planar portion of the disc 106 may correspond to two or more conductive coils, optionally three or more conductive coils. In other examples, there may be a flat disc and three or more conductive coils, 25 optionally four or more conductive coils. Where there are more than two conductive coils, some of the conductive coils may share a power source. For example, if there are three conductive coils, there may only be two capacitors (such as capacitors 300a, 300b) and the driver circuit 108 may 30 be configured appropriately. The number of power sources can be configured or determined based on the overall number of conductive coils and the geometry of the disc. By using multiple, non-overlapping, coils with separate power sources having lower ratings, as discussed herein, a smaller and safer actuator can be provided. Moreover, the width of the respective coils can be designed in accordance with the 35 principles discussed with reference to Figures 5A, 5B in order to reduce bending stressed in the disc. Lifetime of the device may therefore be improved. In accordance with Figure 6, a method of operating a Thomson coil actuator 100 as described herein is provided. The method comprises (block 610) simultaneously applying an electric current to each of the at least two conductive coils and inducing a first magnetic field in response to the applied electric current. The induced first 5 magnetic field causes (block 620) an eddy current within the disc; in other words, the method comprises causing, by the induced first magnetic field, an eddy current within the disc. In response to the eddy current, a second magnetic field is induced (block 630), the second magnetic field opposing the first magnetic field; in other words, the method comprises inducing, in response to the eddy current, a second magnetic field, io the second magnetic field opposing the first magnetic field. The method further comprises repelling (block 640), as a result of the first and second magnetic fields, the disc away from the at least two conductive coils. The disc is thereby driven in the first direction along the axis. 15 As discussed herein, by driving the disc in the first direction, a moveable electrical contact of an electrical switching apparatus comprising the actuator 100 may be moved into or out of contact with a fixed / stationary electrical contact. The actuator 100 may therefore open and / or close the electrical switching apparatus. Optionally, the electrical switching apparatus is a hybrid circuit breaker, and the actuator 100 is 20 configured to drive the bypass relay of said circuit breaker.

Claims

1. A Thomson coil actuator (100), comprising:a disc (106) which is at least partially conductive;5 at least two conductive coils (104a, 104b) arranged concentrically around anaxis (120) and configured to drive the disc in a first direction (122) along the axis when an electric current is provided to the at least two conductive coils, wherein the at least two conductive coils do not overlap along the axis; anda driver circuit (108) configured to provide an electric current to each of the at io least two conductive coils independently, the driver circuit configured to provide the at least two electric currents simultaneously.

2. The actuator of claim 1, the driver circuit comprising at least two capacitors (300a, 300b), each capacitor configured to provide a respective electric current to a15 respective one of the at least two coils, wherein the driver circuit is configured to simultaneously discharge the at least two capacitors.

3. The actuator of claim 2, the driver circuit further configured to synchronize the charging of the at least two capacitors.

204. The actuator of any preceding claim, wherein:each conductive coil is arranged in a same plane; orone or more of the at least two conductive coils are arranged in separate planes offset along the axis.

255. The actuator of any preceding claim, wherein the disc comprises a first surface and a second surface opposite the first surface, and wherein the first surface of the disc is arranged to face the at least two conductive coils.30 6. The actuator of claim 5, the first surface of the disc comprising at least twoconcentric, conductive, planar portions (106a, 106b), wherein adjacent planar portions are offset from one another along the axis.

7. The actuator of claim 5, the first surface of the disc comprising at least two35 concentric, conductive, planar portions (106a, 106b), wherein adjacent planar portions lie in a same plane and are not offset from one another along the axis.

8. The actuator of claim 6 or claim 7, wherein each planar portion of the disc is arranged to face a respective one of the at least two conductive coils, and wherein adjacent coils are offset from one another along the axis.

9. The actuator of claim 8 when dependent on claim 6, wherein the offset of the 5 adjacent planar portions corresponds to the offset of the adjacent coils.

10. The actuator of claim 6 or claim 7, wherein each planar portion of the disc is arranged to face a respective one of the at least two conductive coils, and wherein adjacent coils lie in a same plane and are not offset from one another along the axis.io11. The actuator of any preceding claim, wherein the at least two conductive coils are formed from amorphous cables and / or copper wires.

12. The actuator of any preceding claim, further comprising a rod (102), the disc 15 fixedly coupled to said rod to form an axially movable assembly, wherein the at least two coils are configured to drive the movable assembly in the first direction along the axis.

13. An electrical switching apparatus comprising:20 a first electrical contact;a second electrical contact; andthe Thomson coil actuator of claim 10,wherein the rod is coupled to the second electrical contact in order to move the second electrical contact into or out of electrical contact with the first electrical contact 25 when the at least two coils drive the movable assembly in the first direction along the axis.

14. A method of operating a Thomson coil actuator of any of claims 1 to 12, comprising:30 simultaneously (610) applying an electric current to each of the at least twoconductive coils;inducing a first magnetic field in response to the applied electric current;causing (620), by the induced first magnetic field, an eddy current within the disc;35 inducing (630), in response to the eddy current, a second magnetic field, thesecond magnetic field opposing the first magnetic field;repelling (640), as a result of the first and second magnetic fields, the disc away from the at least two conductive coils, thereby driving the disc in the first direction along the axis.19

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