Rotor body for a permanent magnet machine

Field of invention

The present invention relates to the field of permanent magnet electrical machines . Particularly, but net exclusively, the present invention relates to permanent magnet electrical generators for direct drive wind turbines .

Art Background

A permanent magnet electric machine , such as an electric generator installed in a wind turbine , typically comprises a rotor which rotates about a stator around a central rotational axis . Stator and rotor are separated from each other by an airgap, circumferentially extended around the rotational axis . In a direct drive wind turbine , a permanent magnet electric generator is used, which has the generator rotor directly attached to the wind rotor .

In the above defined technical field, it is known to build an electric generator having a rotor body usually made of sheet metal by welding on both the inside and outside through the complete longitudinal length . Pockets are afterwards milled on the inside of the generator rotor house for the placement of permanent magnets .

This manufacturing process based on a welding process is becoming more and more challenging and expensive by the permanently increasing si ze of Wind Turbine Generators .

Additionally, the rotor body is made of steel , and this leads to eddy current losses and heat generation during operation, especially in case of electrical generators including a frac- tional slot concentrated winding . This generated thermal energy increases the temperatures of the permanent magnets . The temperature increase reduces the generated flux density and hence the performance of the generator . Further, higher permanent magnet operating temperatures increase the required heavy rare earth metal content ( and hence the price ) of the permanent magnets .

It is therefore desirable to provide ef ficient and cost- ef fective constructional features of a rotor body for a permanent magnet machine for avoiding the above-mentioned inconveniences .

Summary of the Invention

This obj ective may be solved by the permanent electrical machine according to the independent claims . Advantageous embodiments of the present invention are described by the dependent claims .

According to a first aspect of the present invention, a rotor for an electrical machine is provided . The rotor extends along a longitudinal axis between a drive end and an axially opposite non-drive end . The drive end is attachable to a torque input . The rotor comprises :

- a rotor body extending axially between the drive end and the non-drive end and extending in a radial direction orthogonal to the longitudinal axis between an inner side and an outer side ,

- plurality of permanent magnets attached to the inner side a circumferentially distributed about the longitudinal axis , wherein the rotor body includes a plurality of lamination sheets stacked along the longitudinal axis .

According to a second aspect of the present invention a method of manufacturing is provided for manufacturing a rotor for an electrical machine . The rotor extends along a longitudinal axis between a drive end and an axially opposite non-drive end, the drive end being attachable to a torque input . The method comprises the steps of :

- manufacturing a plurality of permanent magnets ,

- manufacturing a plurality of lamination sheets , each lamination sheet having a lamination inner border and a lamination outer border,

- stacking the plurality of lamination sheets along the longitudinal axis for forming a rotor body extending axially between the drive end and the non-drive end and extending in a radial direction orthogonal to the longitudinal axis between an inner side and an outer side , the stacked lamination inner borders forming the inner side and the stacked lamination outer borders forming the outer side ,

- attaching the permanent magnets to the inner side of the rotor body .

The above-described rotor may be implemented in an electric generator installed in a wind turbine , in particular in a direct drive wind turbine .

The laminations structure of the rotor body according to present inventions allows eliminating expensive machining processes on the inner side ( for example for the placement of the permanent magnets ) and/or on the outer side of the rotor body . Features may be provided on the lamination sheets which after the stacking provides the features which are required on the rotor . For example , according to possible embodiments of the present invention, the rotor body may include a plurality of tensioning bolts for applying a compression to the plurality of lamination sheets along the longitudinal axis , the tensioning bolts being inserted in respective tensioning holes , each bolt and hole extending axial between the drive end and the non-drive end . In such embodiments , holes may be provided on the lamination sheets , which after the stacking form the tensioning holes . Using a laminated structure substantially reduce the eddy current losses in the rotor body in operation leading to a reduction in production losses , a reduction in the permanent magnet operating temperature and an increase in the flux density .

According to embodiments of the present invention, the tensioning bolts and holes are provided on the outer side of the rotor body . The outer side of the rotor body comprises a plurality of radial protrusions axially extending between the drive end and the non-drive end, each of tensioning holes being at least partially housed in one of the radial protrusions . This may lead to an improved cooling on the outer side , this further reducing the permanent magnet operating temperature . Reduce temperatures of the rotor may also reduce air gap variation due to thermal expansion . Reducing the permanent magnet operation temperature could lead to a reduction of the heavy rare earth metal content of the permanent magnets .

According to further embodiments of the present invention, the inner side may include a plurality of attaching means for attaching the plurality of permanent magnets to the rotor body . This attaching means may include seats for permanent magnet modules . Alternatively, the inner side may include a plurality of axial cavity extending between the drive end and the non-drive end for housing the plurality of permanent magnets . This allows a direct assembly of the permanent magnets to the laminated rotor body, without using baseplates . This can eliminate the baseplate losses leading to a further reduction of the overall losses .

According to other embodiments of the present invention, a plurality of insulations layers is interposed between the plurality of lamination sheets . This could further reduce the eddy current losses in the rotor body, thus further reducing the heat generation . A coating may further be provided at least over one of said inner side and outer side . This would seal potential leakages .

Brief Description of the Drawings

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 embodiment but to which the invention is not limited .

Fig . 1 shows a schematic section of a wind turbine including a rotor according to the present invention .

Fig . 2 shows a cross-sectional view of a permanent magnet machine including a rotor according to the present invention .

Fig . 3 shows an axonometric view of the rotor body of the permanent magnet machine of fig . 2 .

Fig . 4 shows a magni fied view of the detail IV of fig . 3 .

Fig . 5 shows a partial axonometric view of the rotor body of fig . 3 , showing further features according to the present invention .

Fig . 6 shows a partial sectional view of the rotor body of fig . 3 , showing further features according to the present invention . Detailed Description

The illustrations in the drawings are schematically . It is noted that in di f ferent figures , similar or identical elements are provided with the same reference signs .

Figure 1 shows a partial cross-sectional view of a wind turbine 1 including a permanent magnet machine 12 , i . e . an electrical generator, which includes a rotor 10 according to the invention . The wind turbine 1 comprises a tower 2 , which is mounted on a non-depicted foundation . A nacelle 3 is arranged on top of the tower 2 . The wind turbine 1 further comprises at least a wind rotor 5 having a hub and at least one blade 4 ( in the embodiment of Figure 1 , the wind rotor comprises three blades 4 , of which only two blades 4 are visible ) . The wind rotor 5 is rotatable around a rotational axis Y . The terms axial , radial and circumferential in the following are to be intended with reference to the longitudinal axis Y of rotation of the permanent magnet machine 10 . The blades 4 extend substantially radially with respect to the rotational axis Y . The permanent magnet machine 12 extends longitudinally between a drive end 21 connected to the wind rotor 5 and an opposite non-drive end 22 . The torque input from the wind rotor 5 is transmitted to the electrical generator at the drive end 21 . The permanent magnet machine 12 includes a stator 11 and a rotor 10 . The rotor 10 is rotatable with respect to the stator 11 about the rotational axis Y . In the embodiment of Figure 1 , the rotor 10 is radially external with respect the stator 11 and rotatable about the longitudinal axis Y . A circumferential air gap is provided between the stator 11 and the rotor 10 .

The wind rotor 5 may be rotationally coupled with the drive end 21 of the rotor 10 directly ( direct drive generator ) . Alternatively, according to other embodiments of the present invention, the wind rotor 5 is rotationally coupled with the rotor 10 by means of a rotatable main shaft and/or through a gear box . Figure 2 shows a partial cross-sectional view of the permanent magnet machine 12 including a plurality of permanent magnets 200 attached to the rotor 10 . The permanent magnets 200 are attached to a side of the rotor 12 which radially faces the stator 11 at an air gap 15 . Each of the permanent magnets 200 is attached to a rotor body 100 of the rotor 10 by means of a respective axial cavity 180 extending between the drive end 21 and the non-drive end 22 , as better clarified in the following . According to other embodiments of the present invention (not shown) , each of the permanent magnets 200 is attached to the rotor body 100 of the rotor 10 by means of the respective support or baseplate .

Figures 3 to 6 illustrate an embodiment of a rotor according to the present invention . The rotor 10 extends along the longitudinal axis Y between the drive end 21 and the axially opposite non-drive end 22 . The rotor 10 comprises a rotor body 100 extending axially between the drive end 21 and the non- drive 22 and extending in a radial direction orthogonal to the longitudinal axis Y between an inner side 110 and an outer side 120 . A coating may be provided over one or both the inner side 110 and the outer side 120 . Two annular plates

191 , 192 are respectively provided at the drive end 21 and at the non-drive end 22 . The first annular plate 191 attached at the drive end 21 is normally referred to as front plate . The second annular plate 192 attached at the non-drive end 22 is a brake disc, which may be used for applying a braking torque to the rotating rotor 10 . The rotor body 100 includes a plurality of lamination sheets 101 stacked along the longitudinal axis Y, between the drive end 21 and the non-drive-end 22 . The stack of lamination sheets 101 therefore axially extend from one to the other of the two annular plates 191 ,

192 . Insulations layers may be interposed between the plurality of lamination sheets 101 , in such a way that in the stack of lamination sheets 101 each lamination sheet 101 is axially interposed between two insulation layers . The lamination sheets 101 may be assembled by applying a staggering . Each of the lamination sheets 101 has a lamination inner border 111 and a lamination outer border 121 . The plurality of stacks lamination inner borders 111 forms after the stacking of the lamination sheets 101 the inner side 110 of the rotor body 100 . The plurality of stacks lamination outer borders 121 forms after the stacking of the lamination sheets 101 the outer side 120 of the rotor body 100 . The rotor 10 further comprises a plurality of permanent magnets 200 attached to the inner side 100 a circumferentially distributed about the longitudinal axis Y . The inner side 110 includes a plurality of attaching means for attaching the plurality of permanent magnets 200 to the rotor body 100 . In embodiment of the present invention (not shown) , permanent magnets are incorporated in permanent magnet modules , each permanent magnet module including at least one permanent magnet and at least one baseplate to which the permeant magnet is attached, and which is used for attaching the permanent magnet module to the inner side 110 of the rotor body 100 . In other to provide the coupling between the rotor body 100 and the permanent magnet module , the inner side 110 may include a plurality of attaching means for attaching the permanent magnet modules to the inner side 110 . The attaching means may be in the form of axial extended cavities for housing respective protrusions provided on the baseplate . Cavities and protrusions may have a dovetail sectional shape . According to the embodiment shown in the attached figures ( in particular, figure 2 and 5 ) , a plurality of axial cavity 180 may be provided at the inner side 110 , the axial cavities 180 extending between the drive end 21 and the non-drive end 22 for housing a plurality of permanent magnets 200 , without using a baseplate . The plurality of permanent magnets 200 are inserted and housed in such cavity radially protruding towards the air gap 15 . Each cavity is obtained providing a cover 181 attached to the inner side 110 of the rotor body 100 , the cavity 180 having the same cross-sectional shape of the magnet . The cover 181 may be integral with rotor body 100 . According to other embodiments of the present invention, the cover 181 may be assembled onto the inner side 110 of the rotor body 100 to create . The rotor body 100 may include a plurality of tensioning bolts 150 for applying a compression to the plurality of lamination sheets 101 along the longitudinal axis Y . Alternatively or additionally, the plurality of lamination sheets 101 may be welded together under axial pressure . After the plurality of lamination sheets 101 are stacked and compressed along the longitudinal axis Y, one or more welding may be applied to the lamination sheets 101 , in order to keep them under axial compression . The axial compression provided through the tensioning bolts 150 and/or the welding permits to conveniently transmit the torque input to the stack of lamination sheets 101 comprised in the rotor body 110 . The tensioning bolts 150 are inserted in respective tensioning holes 160 , each bolt 150 and hole 160 extending axial between the drive end 21 and the non-drive end 22 . The tensioning holes 160 may extend also through the two annular plates 191 , 192 so that they may be fixed to the plurality of stacked lamination sheets 101 by means of the plurality of tensioning bolts 150 . According to the embodiment shown in the attached figures , the tensioning bolts 150 and holes 160 are provided on the outer side 120 of the rotor body 100 . According to other embodiments (not shown) the tensioning bolts 150 and holes

160 may be provided on another position between the inner side 110 and the outer side 120 . The outer side 120 of the rotor body 100 may comprise a plurality of radial protrusions

161 axially extending between the drive end 21 and the nondrive end 22 , each of tensioning holes 160 being at least partially housed in one of the radial protrusions 161 .

As shown in figure 6 , another plurality of axial holes 170 may be alternatively or additionally provided, which extend across two or more lamination sheets 101 . Such holes are used for housing respective strengthening pins 155 for preventing circumferential relative rotation between the lamination sheets 101 . The strengthening pins 155 may be pressed inside the axial holes 170 with a small interference , in order to prevent circumferential relative rotation between the lamination sheets 101 on which they are active . Each pin 155 may extend axially along all the stacked lamination sheets 101 . Alternatively, each pin 155 may extend axially along a portion of the stacked lamination sheets 101 and abut axially between two lamination sheets 101 not having any hole aligned with such pin 155 , thus preventing axial relative movements between the pin 155 and the lamination sheets 101 . In the exemplary embodiment shown in figure 6 , three pins 155 are shown, two of which extend axially across 8 lamination sheets and a one across 4 lamination sheets . According to other embodiments (not shown) any other number of pins 155 may be used, each extending across two or more lamination sheets 101 .

According to other embodiments (not shown) , the rotor body 100 may include other types of axial strengthening elements for preventing circumferential relative rotation between the lamination sheets 101 . The axial strengthening elements 101 may be inserted in respective axial holes extending across two or more lamination sheets of the plurality of lamination sheets 101 .

The features of the rotor body 100 may be provided on each of the lamination sheets 101 , so that after the stacking process such features are obtained on the rotor . Holes and covers be provided on the lamination sheets 101 , which after the stacking form the tensioning holes 160 and the covers 181 , respectively .