DESCRIPTION

Coil layout for a generator having tape conductors

Field of invention

The present invention relates to a coil layout for a electric generator having tape conductors, in particular a high- temperature superconducting (HTS) generator. The present in vention further relates to a method of providing a coil lay out in an electric generator having tape conductors, in par ticular in a high-temperature superconducting (HTS) genera tor. Particularly, but not exclusively, the present invention may be applied to a HTS generator in a wind turbine.

Art Background

In the above described technical field, it is known to use superconducting electric generators for wind turbines. The use of superconductors in wind turbines is attractive because it permits to reduce weight or to generate a larger amount of power. High-temperature superconducting (HTS) generators may be conveniently used in wind turbine applications, as they are characterized by a higher critical temperature for super conductivity (77K or lower) .

In electrical generators a coil layout having overlapping coils at the end-windings may be required. Contact interfer ences between the coil conductors at the end-windings have to be avoided.

In normal conducting electric generators this can be achieved easily by bending the coil conductors, which have a circular section, at the end-windings. In superconducting electric generators the bending is however made problematic by the section shape of typical superconducting conductors (in par ticular superconducting conductors used in HTS applications) , which are typically shaped as a tape, i.e. with a flat rec tangular section. This may produce a bending stress within Amendment Information the superconducting conductor, which could damage or even break the coil. A solution to the above problem is provided in WO 2017/024409, which do not involve any bending of the coil conductors. This may involve a complex coil geometry.

There may be therefore still a need for providing a supercon ducting electric generator including a coil layout, which al lows overlapping of the coils at the end-windings without generating a dangerous level of bending stress.

A further need is that of allowing the above described over lapping, at the same time avoiding complex coil geometries.

Summary of the Invention

This need is met by the subject matter according to the inde pendent claims. Advantageous embodiments of the present in vention are described by the dependent claims.

According to a first aspect of the invention there is provid ed an electric generator. The electric generator has a sta tor, a rotor and a plurality of superconducting coils, the rotor extending axially along a longitudinal axis between a first axial end and a second axial end, the rotor including a plurality of slots, the plurality of slots being circumferen tially distributed around a longitudinal axis of the rotor, each of the superconducting coils respectively comprising:

- two slot portions respectively housed in two slots of the rotor,

- two end-windings axially protruding from the rotor at the first axial end and a second axial end,

wherein the slot portions are shaped and positioned in the respective slots so that the end-windings of two circumferen tially adjacent coils overlap and are distanced in a radial direction orthogonal to the longitudinal axis. A slot portion of a first superconducting coil is housed in a respective first slot and a slot portion of a second superconducting coil is housed in a respective second slot circumferentially adjacent to the first slot. The slot portions of the first Amendment Information and the second superconducting coils have respective differ ent curvatures or are inclined with respect to each other in a longitudinal section including the longitudinal axis.

Advantageously, the arrangement of the coil in two adjacent slots provides radial clearance for the end-windings while avoiding complex coil geometries. The present invention may be applied to both integral-slot and fractional-slot electric generators. The present invention may be applied to electric generators having a concentrated or a distributed coil lay out. This invention can be efficiently adapted to a supercon ducting electric generator of a wind turbine.

According to a second aspect of the invention there is pro vided a method of providing a coil layout in an electric gen erator. The electric generator has a stator and a rotor, the rotor extending axially along a longitudinal axis between a first axial end and a second axial end, the rotor including a plurality of slots, the plurality of slots being circumferen tially distributed around a longitudinal axis of the rotor, the coil layout including a plurality of superconducting coils each respectively comprising:

- two slot portions respectively housed in two slots of the rotor,

- two end-windings axially protruding from the rotor at the first axial end and a second axial end.

According to the method the slot portions are inserted in the respective slots so that the end-windings of two circumferen tially adjacent coils overlap and are distanced in a radial direction orthogonal to the longitudinal axis. A slot portion of a first superconducting coil is inserted in a respective first slot and a slot portion of a second superconducting coil is inserted in a respective second slot circumferential ly adjacent to the first slot. The slot portions of the first and the second superconducting coils have respective differ ent curvatures or are inclined with respect to each other in a longitudinal section including the longitudinal axis. Amendment Information

Preferably, the rotor is made of steel, preferably ferromag netic steel.

With this robust rotor design, a small air gap of less than 10 mm can be achieved which is quite unusual for a supercon ducting rotor due to cryogenic constraints. This results in a high flux density in the air gap.

Furthermore, the electric generator may be an integral-slot electric generator.

Alternatively, the electric generator may be a fractional- slot electric generator.

Furthermore, the electric generator may have a distributed or a concentrated coil layout.

Furthermore, the electric generator comprises a cooling sys tem, the cooling system comprising:

at least a first cooling unit for cooling the rotor,

at least a second cooling unit for cooling the superconduct ing coils, the second cooling unit being thermally connected to the first cooling unit, the first cooling unit providing a hot source for the second cooling unit,

wherein the first cooling unit is connected to an ambient en vironment, providing a hot source for the first cooling unit, a first cooling medium being circulated in the first cooling unit (101), the first cooling medium exchanging thermal ener gy with the ambient environment in the first cooling unit, and

the second cooling medium is circulated in the second cooling unit, the second cooling medium exchanging thermal energy with the first cooling medium in the second cooling unit. Amendment Information

The first cooling medium is a mixture of water and a composi tion which is fluid at a temperature of -50°C or lower, al ternatively -40°C or lower, preferable being a mixture of wa ter and glycol or water and ethanol, a mixture thereof or any other coolant.

Alternatively, the temperature of the first cooling medium is between -50 °C and -30 °C; alternatively the first tempera ture (Tl) being between -30 °C and -10 °C.

Furthermore, the cooling system comprises a pressure control vessel for controlling the pressure of the second cooling me dium, the pressure control vessel being interposed between the second cooling unit and the superconducting coils.

Furthermore, a vapor phase and a liquid phase of the second cooling medium are housed in the pressure control vessel.

Furthermore, the electric generator of any of the previous claims, wherein the air gap between rotor and stator of less than 15 mm, preferably less than 10mm.

Furthermore, the rotor is made of steel, preferably ferromag netic steel.

All the above described embodiments apply to both the appa ratus and the method of the present invention.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited. Amendment Information

Brief Description of the Drawing

Figure 1 shows a schematic section of a wind turbine in cluding an electric generator and a cooling sys tem according to the present invention,

Figure 2 shows a schematic partial cross section view of the rotor of the electric generator of Fig. 1, taken according to the section line II-II of Fig ures 3, 4 and 5.

Figure 3 shows a schematic longitudinal section view of the rotor of a first embodiment of the electric generator of Figs. 1 and 2.

Figure 4 shows a schematic longitudinal section view of the rotor of a second embodiment of the electric generator of Figs. 1 and 2.

Figure 5 shows a schematic view of a cooling system for the rotor of the present invention,

Figure 6 shows a longitudinal view of another embodiment of the rotor of the electric generator of Fig. 1. Figure 7 shows a schematic, partial cross view of the em bodiment of the rotor of Fig. 6.

Detailed Description

The illustrations in the drawings are schematic. It is noted that in different figures, similar or identical elements are provided with the same reference signs.

Figure 1 shows a wind turbine 1 according to the invention. The wind turbine 1 comprises a tower 2, which is mounted on a non-depicted fundament. A nacelle 3 is arranged on top of the tower 2. The wind turbine 1 further comprises a wind rotor 5 having two, three or more blades 4 (in the perspective of Figure 1 only two blades 4 are visible) . The wind rotor 5 is rotatable around a rotational longitudinal axis Y. When not differently specified, the terms axial, radial and circumfer- Amendment Information ential in the following are made with reference to the rota tional axis Y.

The blades 4 extend radially with respect to the rotational axis Y. The wind turbine 1 comprises a direct drive electric generator 11. According to other possible embodiments of the present invention (not represented in the attached figures) , the present invention may be applied to any other type of di rect drive machine with either internal or external rotor.

The wind rotor 5 is rotationally coupled with the rotor ei ther directly, e.g. direct drive or by means of a rotatable main shaft 9. A schematically depicted bearing assembly 8 is provided in order to hold in place the main shaft 9 and the rotor 5. The rotatable main shaft 9 extends along the rota tional axis Y. The direct drive electric generator 10 in cludes a stator 20 and a rotor 30. The rotor 30 is rotatable with respect to the stator 20 about the rotational axis Y.

In the embodiment of the attached figures the rotor 30 is ex ternally arranged with respect to the stator 20. According to other embodiments (not shown) the configuration is radially inverted and the rotor 30 is externally arranged with respect to the stator 20. The following detailed description of the rotor 30 applies to either an internal or an external rotor 30.

Figure 2 schematically shows a partial and schematic section al view of the rotor 30, orthogonal to the longitudinal axis Y. The rotor 30comprises a rotor iron core 31 and extends ax ially along the longitudinal axis Y between a first axial end 22 and a second axial end 23 (as shown in figures 3 to 5) and includes a plurality of slots (six slots 51a, 52a, 53a, 51b, 52b, 53b in the non-limiting embodiment of Figure 2) circum ferentially distributed around the longitudinal axis Y and alternating with a plurality of teeth 60 along circumferen tial direction X, orthogonal to the longitudinal axis Y.

Along the circumferential direction X, the six slots 51a,

52a, 53a, 51b, 52b, 53b are ordered in series as a first slot Amendment Information

51a, a second slot 52a, a third slot 53a, a fourth slot 51b, a fifth slot 52b and a sixth slot 53b.

The rotor iron core 31 comprises cooling pipes 58, which are configured to carry a cooling medium 58a, as better detailed in the following. The cooling pipes 58 may also be placed at other locations, which would provide sufficient thermal con tact to the rotor iron core 31.

A plurality of superconducting coils (three coils 41, 42, 43 in the non-limiting embodiment of Figure 2) is arranged in the slots 51a, 52a, 53a, 51b, 52b, 53b. The superconducting coils 41, 42, 43 are arranged according to a distributed coil layout. The superconducting coils comprise superconducting conductors shaped as a tape, i.e. having a flat rectangular section where one dimension is significantly greater than the other .

According to the different embodiments of the present inven tion, the greater dimension of the conductor section may be oriented parallel or orthogonal to the circumferential direc tion X.

A first superconducting coil 41 comprises:

- two slot portions 41a, 41b respectively housed in two re spective slots (the first slot 51a and the fourth slot 51b in the non-limiting embodiment of Figure 2),

- two end-windings 41c axially protruding from the rotor 30at the first axial end 22 and a second axial end 23.

A second superconducting coil 42 comprises:

- two slot portions 42a, 42b respectively housed in two re spective slots (the second slot 52a and the fifth slot 52b in the non-limiting embodiment of Figure 2),

- two end-windings 42c axially protruding from the rotor 30at the first axial end 22 and a second axial end 23.

A third superconducting coil 43 comprises: Amendment Information

- two slot portions 43a, 43b respectively housed in two re spective slots (the third slot 53a and the sixth slot 53b in the non-limiting embodiment of Figure 2),

- two end-windings 43c axially protruding from the rotor 30at the first axial end 22 and a second axial end 23.

The above described winding layout of the superconducting coils 41, 42, 43 may be repeated along the circumferential axis X beyond the first slot 51a and the sixth slot 53b.

The above described winding layout determines an overlapping of the end-windings 41c, 42c, 43c.

According to different embodiment of the present invention (not shown) a different number of superconducting coils may be provided. According to different embodiment of the present invention (not shown) the superconducting coils are housed in the rotor 30according to any other winding layout which caus es an overlapping of the end-windings. To avoid interference between the end-windings 41c, 42c, 43c the slot portions 41a, 41b, 42a, 42b, 43a, 43b are shaped and positioned in the re spective slots 51a, 51b, 52a, 52b, 53a, 53b so that the end- windings 41c, 42c, 43c of two circumferentially adjacent coils 41, 42, 43 overlap and are distanced in a radial direc tion R orthogonal to the longitudinal axis Y and to the cir cumferential direction X.

Details in regard to the cooling vessel for the superconduct ing coils 41, 42, 43 are shown in figure 7.

Two different embodiments are respectively shown in Figures 3 and 4 , where the slot portions 41a, 42a, 43a of the supercon ducting coil 41, 42 43 and the first, second and third slots 51a, 52a, 53a are shown superposed to one another in a longi tudinal section including the longitudinal axis Y.

In the first embodiment of Figure 3, the slot portions 41a, 42a, 43a have respective different curvatures. Particularly, the slot portion 41a of the first superconductive coil 41 is curved with a convex shape pointing towards the longitudinal Amendment Information axis Y, the slot portion 42a of the second superconductive coil 42 is straight and the slot portion 43a of the third su perconductive coil 43 is curved with a convex shape pointing away from the longitudinal axis Y.

In the second embodiment of Figure 4, the slot portions 41a, 42a, 43a are inclined with respect to each other. Particular ly, the slot portion 41a of the first superconductive coil 41 is inclined towards the first axial end 22, the slot portion 42a of the second superconductive coil 42 is straight and the slot portion 43a of the third superconductive coil 43 is in clined towards the second axial end 23.

Other embodiments (not shown) may be provided according to the present invention, provided that it is assured that, at the first axial end 22 and the second axial end 23 or at a longitudinal distance from the first axial end 22 and the second axial end 23, the end-windings are distanced along the radial direction R.

Figure 5 shows the superconducting coils 41, 42, 43 wound on the rotor iron core 31 and a cooling system 100. The cooling system 100 comprises a first cooling unit 101 for cooling the rotor iron core 31. The first cooling unit 101 is a heat ex changer where a first cooling medium 58a is circulated. Ac cording to possible embodiment of the invention, the first cooling medium 58a is a mixture of water and glycol. In the first cooling unit 101 the first cooling medium 58a exchanges thermal energy with the ambient environment, which represents the hot source for the first cooling unit 101. The first cooling unit 101 is connected to the cooling pipes 58 (Fig.

2) of the rotor iron core 31 through a first duct 105, deliv ering the mixture of water and glycol (or similar components) to the rotor iron core 31 at a first temperature Tl. Accord ing to possible embodiments of the present invention, the first temperature Tl is comprised between -50 °C and -30 °C. Particularly, the first temperature Tl may be comprised be- Amendment Information tween -30 °C and -10 °C. From the first duct 105 the first cooling medium 58a enters the cooling pipes 58 of the rotor iron core 31, which represents a cold source for the first cooling unit 101. Through the mixture of water and glycol at the first temperature T1 the rotor iron core 31 is kept at a desired low temperature, by removing the iron losses in the rotor iron core 31 and to allow the superconducting coils 41, 42, 43 to stay below the cryogenic temperature of 77 K. Indi rectly, also the stator 20 is cooled by the thermal exchange between the first cooling medium 58a and the rotor iron core 31, cooling power being transferred to the rotor 30 through the air gap between the stator 20 and the rotor 30 by connec tion and/or radiation.

From the cooling pipes 58 of the rotor iron core 31 the first cooling medium 58a is delivered back to the first cooling unit 101 through a second duct 106, delivering back the mix ture of water and glycol from the rotor iron core 31 at a second temperature T2, greater than the first temperature Tl. According to embodiments of the present invention, the second temperature T2 is 5 K to 15 K higher than the inlet tempera ture. The thermal exchange with the ambient environment per formed in the first cooling unit 101 permits to cool the mix ture of water and glycol to reach again the first temperature Tl . The cooling system 100 comprises a second cooling unit 102 for cooling the superconducting coils 41, 42, 43 below the cryogenic temperature of 77 K. The second cooling unit 102 is thermally connected to the first cooling unit 101, the latter providing a hot source for the second cooling unit 102. The second cooling unit 102 is an heat exchanger con nected to the first cooling unit 101 through a third duct 107, delivering the mixture of water and glycol to the second cooling unit 102 at a third temperature T3. According to a possible embodiment of the present invention, the third tem perature T3 has a value of -50 °C. According to possible em bodiments of the present invention, the third temperature T3 may have the same value of the first temperature Tl. From the third duct 107 the first cooling medium 58a enters the second Amendment Information cooling unit 102, which represents a second cold source for the first cooling unit 101. The mixture of water and glycol at the third temperature T3 exchange thermal energy with a second cooling medium 17a which is circulated in the second cooling unit 102. According to possible embodiments of the present invention, the second cooling medium 17amay be a gas with low boiling point like nitrogen (N) or helium (He) . In the second cooling unit 102 the second cooling medium 17a is cooled by exchanging thermal energy with the first cooling medium 58a. In particular, the second cooling medium 17a in the second cooling unit 102 may change its phase from the va por to the liquid phase. According to other embodiments of the present invention, the second cooling medium 17a may be sub-cooled and therefore the phase change does not occur.

From the second cooling unit 102 the first cooling medium 58a is delivered black to the first cooling unit 101 through a fourth duct 108, delivering back the mixture of water and glycol to the first cooling unit 101 at a fourth temperature T4, greater than the third temperature T3. According to pos sible embodiments of the present invention, the fourth tem perature T4 may have the same value of the second temperature T2. The thermal exchange with the ambient environment per formed in the first cooling unit 101 permits to cool the mix ture of water and glycol to reach again the third temperature T3. According to the above described scheme, the second cool ing unit 102 can be designed without taking ambient tempera tures into account. Only the first cooling unit 101 has to be designed with sufficient thermal capacity to handle high am bient temperatures.

The cooling system 100 further comprises a pressure control vessel 110 for controlling the pressure of the second cooling medium 17a. The pressure control vessel 110 is interposed be tween the second cooling unit 102 and the superconducting coils 41, 42, 43 and houses both vapor and liquid phases of the second cooling medium 17a, optionally mainly liquid phas es. The second cooling unit 102 is connected to the pressure control vessel 110 through a fifth duct 109, delivering the Amendment Information liquefied second cooling medium 17a to the pressure control vessel 110. From the pressure control vessel 110 the second cooling medium 17a in the vapor phase is delivered back to the second cooling unit 102 through a sixth duct 111. The pressure control vessel 110 is connected to the superconduct ing coils 41, 42, 43 through a seventh duct 114, delivering the liquefied second cooling medium 17a to the superconduct ing coils 41, 42, 43 at a fifth temperature T5. According to a possible embodiment of the present invention, when the sec ond cooling medium 17a is nitrogen, the fifth temperature T5 has a value of -206 °C. According to other possible embodi ment of the present invention, the fifth temperature T5 may have different value depending on the nature of the second cooling medium 17a. For example, when the second cooling me dium 17a is helium, the fifth temperature T5 may be lower than -206 °C. In the seventh duct 114 a circulation pump 112 and a first electric connection 118 for the superconducting coils 41, 42, 43 are provided. Through the liquefied second cooling medium 17a at the fifth temperature T5 the supercon ducting coils 41, 42, 43 are kept below the cryogenic temper ature of 77 K. From the superconducting coils 41, 42, 43 the liquefied second cooling medium 17a is delivered black to the pressure control vessel 110 through an eighth duct 115, de livering back liquefied second cooling medium 17a to the pressure control vessel 110 at a sixth temperature T6, great er than the first temperature Tl. According to a possible em bodiment of the present invention, the sixth temperature T6 has a value of -197 °C. In any case the sixth temperature T6 is lower than the minimum temperature required to maintain the second cooling medium 17a in the liquid phase. In the eighth duct 115 a second electric connection 119 for the su perconducting coils 41, 42, 43 is provided.

Figure 6 shows a further embodiment of the rotor 30 having a rotor iron core 31, which is morphologically identical to the embodiment of the figure 2 to 5. In such embodiment, the su perconducting coils 41, 42, 43 are arranged according to a concentrated coil layout, where each slot is dedicated to Amendment Information conductors of one coil only. The slot portions of each super conducting coils 41, 42, 43 are housed in two respective ad jacent slots 51a, 51b; 52a, 52b; 53a, 53b of the rotor iron core 31.

Fig . 7 shows a schematic, partial cross section of the rotor 30 of figure 6.

Heat sinks 16are thermally linked to the respective supercon ducting coils 41, 42, which are arranged in the slots 51a, 51b, 52a, 52b.. Each heat sink 16 is configured to carry a second cooling medium 17a to cool the superconducting coils 41, 42. The second cooling medium 17a serves to cool the su perconducting coils 41, 42. The second cooling medium 17a is configured to set the superconducting windings 13 in a super conducting state. The heat sink 16 and the superconducting windings 41,42 are arranged in a thermal shielding 14 having a thermal insulating property. Such a material can be any ap propriate material having a thermal insulating property, preferably a vacuum insulation with radiation shielding lay ers .

In a broad interpretation, the heat sink 16 can be estab lished by the second cooling medium 17a as such. Alterna tively, as shown in the embodiment, the heat sink 16 compris es a support member 16, which supports the superconducting coils 41, 42, 43 and has a cooling channel 17, which is con figured to receive the second cooling medium 17a . The cool ing channel 17 has a circular cross section; however, the cooling channel 17 may likewise have a rectangle cross sec tion or any other cross section. Since the superconducting coils 41, 42 are usually made of a material, which can be set in a superconductive state, the superconducting coils 41, 42 are often made of a refractory material for example a type of ceramics without sufficient firmness. Since the superconduct ing coils 41, 42 are supported by the heat sink 16 which also can act as a support member, which is usually formed of a Amendment Information firmer material such as metal, the superconducting coils 41, 42 are protected and less likely damaged.

This structure is efficient and lightweight while providing sufficient cooling capability to maintain superconductivity in the superconducting coils 41, 42 during operation.

The rotor 30 comprises the additional cooling pipes 58 which are configured to carry a first cooling medium 58a being preferable different from the second cooling medium 17a. In the embodiment, the additional cooling pipes 58 are placed within the rotor iron core 31. It can also be placed at other locations which would provide sufficient thermal contact to the stator core also. While the second cooling medium 17a is provided to cool the superconducting coils 41, 42 below a first temperature, the first cooling medium 58a is provided to cool the rotor 30 below a second temperature, wherein the first temperature is lower than the second temperature. While the superconducting coils 41, 42 are superconducting at a temperature which is equal to or lower than the first temper ature, the superconducting coils 41, 42 may be not supercon ducting at a temperature which is equal or higher than the second temperature. Such superconducting coils 41, 42 are al so called as HTS (High Temperature Superconductor) .

The superconducting coils 41, 42 can have a superconductivity of the second order. Superconductors of the second order are, up to the so-called "lower critical magnetic field", in the so called Meissner phase and behave like a superconductor of the first order. With higher magnetic fields, magnetic field lines in the form of so-called flux tubes can penetrate the material before the superconducting state at an "upper criti cal magnetic field" is completely destroyed. The magnetic flux in a flux tube is usually equal to the magnetic flux quantum.

The first temperature can be -196°C or lower, preferably -206°C or lower. The second cooling medium 17a can be liquid Amendment Information nitrogen (LN or LN2) , liquid helium ( ⁴ He or ³ He) or any other coolant which is suitable for cooling below the first temper ature .

The second temperature can be -20°C or lower, preferably -50°C or lower. The first cooling medium 58a can be water, glycol, ethanol, a mixture thereof or any other coolant which is suitable for cooling below the second temperature.

Since the rotor 30 is relatively large, for example for the wind turbine 1 having an output power higher than 1 MW, it comprises a plurality of laminated stator segments. The lami nated stator segments can minimize eddy currents. Alterna tively, the rotor 30 can be made as single piece, for example for a smaller wind turbine having a smaller output power.

In order to move the first and second coolants, two pumps (not shown) are provided which are configured to pump the first and second coolants through the heat sink 16 and the additional cooling pipes 58, respectively. Furthermore, the heat sink 16 and the additional cooling pipes 58 are connect ed to one or more heat exchangers (not shown) to achieve heat exchange between the first and second coolants and the ambi ent or another medium. Such heat exchange is already known in the state of the art and does not need to be described fur ther. Alternatively, convective flow could be used without pumps .

Advantageously, the present invention is capable to cool the rotor 30 to eradicate a heating effect of iron losses caused by variable magnetic flux inductions in the laminated stator segments of the rotor 30 and to sufficiently cool the super conducting coils 41, 42, 43 to make them superconducting.

In addition, a two-stage cooling is implemented to cool the rotor 30 via the expanded cooling pipes 58 with a wa

ter/glycol solution (or a similar coolant) to cool the rotor 30 and remove iron loss during operation. The superconducting Amendment Information coils 41, 42, 43 are then further cooled by the adjacent heat-sink 16 incorporating the cooling channel 17 which car ries liquid nitrogen (LN2) as second cooling medium 17a or another suitable coolant to further cool the superconducting coils 41, 42, 43 to the required operating temperatures. The heat-sink 16 removes any losses from the superconducting coils 41, 42, 43 in operation and is wrapped in the thermal shield 14 made of a material to minimize radiation heating of the superconducting coils 41, 42from the surrounding rotor 30.

With this robust rotor design, a small air gap between rotor 30 and stator 20 of less than 15mm, preferably less than 10mm can be achieved which results in a high flux density in the air gap.

Preferably, the rotor 30 is made of steel, preferably ferro magnetic steel.

The stator 20 of figure 1 comprises preferably superconduct ing coils