ITVI20100220A1 - PERMANENT MAGNET GENERATOR WITH REDUCED COGGING EFFECT AND RELATED MAGNET - Google Patents

Description

PERMANENT MAGNET GENERATOR WITH REDUCED COGGING EFFECT AND RELATIVE MAGNET.

DESCRIPTION

Technical field

The invention relates to a permanent magnet generator with reduced cogging effect which comprises a stator with a plurality of tooth-shaped magnetic lamination packs with respective electrical windings arranged radially around the generator rotation axis which alternate with a plurality of slots with a relative opening and a rotor with a plurality of permanent magnets arranged in a radial shape around the rotation axis of the generator. The invention also concerns a relative magnet.

State of the art

Known permanent magnet generators have the disadvantageous cogging effect. The cogging torque is the typical braking moment that opposes the rotation of the rotor in permanent magnet motors (PMM) even in the absence of load.

It is very important to reduce this cogging torque in wind generators because it tends to greatly limit the starting of the blades at wind speeds lower than the nominal one (with significant loss of energy production) and also produces mechanical vibrations and noise.

For example, with a nominal wind speed of 10 m / s and a cogging of 10%, the blades begin to move with a rather high air speed: about 4 m / s, while with a cogging of 1% 2 are enough. m / s (the usable power varies with the cube of the air speed). A significant reduction in the cogging effect would result in a significant reduction in the wind speed necessary to start the wind generator.

Figure 1 schematically illustrates the cogging mechanism. The cogging is caused by the interaction between the magnets mounted on the rotor R and the anisotropy of the stator due to the opening of the slots A (variation of the magnetic reluctance). In fact, the highest value is found in the coincidence during rotation, between magnet edge B and slot A, while with edge B in the center of tooth D or in the center of slot A the cogging is zero, as can be seen from the cogging trend C represented in the graph below. The arrow M represents the direction of movement of the rotor R.

A minor variation of the magnetic reluctance during the rotation process will lead to a minor contribution to cogging, as well as the non-coincidence between the edges B of the various magnets and the opening A of the various slots of the stator.

Basically, the cogging is due to the variation of the magnetic energy of the field produced by the magnets with the angular position of the rotor R: formally the cogging is equal to the partial derivative of the coenergy with respect to the angular position.

Clearly an acceptable cogging value depends on the application. For example, in generators equipped with PMG (permanent magnet generators) cogging is not particularly important, so values close to 20% of the nominal torque are acceptable. In the wind field, cogging is a very important parameter of PMG: typical reference values can be considered from 3% to 5% of the nominal value, a value of 1% is considered extremely low for mass production generators . In wind turbines available on the market, the cogging effect is quite high.

Presentation of the invention

The object of the present invention is to realize a permanent magnet generator with a low cogging effect, and particularly a low power wind generator (micro wind) with a lower cogging effect than known generators. A further aim of the invention is to propose a generator in which the lowering of the cogging effect is not accompanied by a high loss of power. Another purpose of the invention is to create a permanent magnet that helps reduce the cogging effect with limited power loss.

The objects of the invention are achieved by a permanent magnet generator of the type mentioned at the beginning in which the air gap between the tooth-shaped magnetic lamination packs and the permanent magnets is variable and in which the permanent magnets have inclined edges. To have a reduction in cogging without penalizing too much the output voltage, the inclination (skewing) of the magnets is chosen with an angle value (in a radial direction) that varies between a minimum corresponding to the cogging period and half. of the induced hollow pitch. The combination of variable air gap and inclined magnets significantly reduced the cogging effect. The inclination of the magnets (skewing) makes the production of the rotor a little more demanding but this disadvantage is largely compensated by the very reduced cogging. The larger the tilt angle, the smaller the cogging effect. With this combination, cogging values lower than 1% of the maximum torque are reached.

Advantageously, the optimization of one or more parameters is added to the measurement of the variable air gap and inclined magnets, including: the ratio between the number of slots and the pairs of poles, the polar expansion of the magnets with respect to the polar pitch in relation to the slot / pole ratio, the value of the air gap as a function of the opening of the slots, the height of the stack length to reduce the cogging effect. The adaptation must be a good compromise between the construction requirements and the performance (power) of the generator. The cave / pole ratio (c / 2p) is an important parameter that allows you to raise as much as possible the value of the fundamental cogging frequency which corresponds to the least common multiple between caves and poles. A combination of 18 slots and 16 poles, and more preferably 18 slots and 22 poles, proved to be particularly suitable with 144 and 198 cogging peaks per revolution. The polar expansion of the magnets with respect to the polar pitch can be optimally adapted to the ratio between slots and poles. The value of the air gap is a parameter that can be varied according to the opening of the slot in order to obtain a smaller variation of the magnetic reluctance when the magnet passes under the opening of the slot. The choice of slot opening and tooth width are preferably a compromise between cogging, magnetic saturation, Carter factor, dispersion inductance and an acceptable geometric dimension that allows automatic stator winding.

In a very advantageous variant of the invention, the variability of the air gap is achieved by the fact that the upper side and the lower side of the magnets define an internal curved surface and an external curved surface with the same direction of curvature that they describe in section transversal arcs of non-concentric circumferences and with respective different radii so that the distance between the two surfaces which represents the thickness of the magnet is variable in cross section and in which the arc of circumference identified by the radius of the internal curved surface in cross section which it is located in front of the tooth-shaped magnetic lamination packs is not concentric with the arc of circumference identified by the radius corresponding to the cylindrical surface described by the tooth-shaped magnetic lamination packs. In this way, a variable air gap is created in a simple and effective way from the point of view of reducing the cogging effect.

This particular structure of the magnets can also be adopted for non-inclined magnets, and this for systems with external or internal rotor.

The thickness of the magnet is therefore variable in the radial direction. They are magnets that in section are half way between a â € œbread loafâ € shape that is a â € œDâ € shape and a radial shape that is a â € œC ".

Advantageously, the ratio between the maximum air gap Î ”2 at the edges of the magnet and the minimum air gap Δ 1 at the center of the magnet is in the ratio 3 to 1, 1.

A very advantageous variant of the generator according to the invention, which made it possible to reduce the cogging to values below 1%, is a generator with an external rotor which has the following dimensions and dimensional ratios:

Table 1

Measure Value Value Parameter

nominal minimum maximum slot opening 3.2 mm 3.0 mm 3.5 mm central air gap 1.1 mm 0.8 mm 1.6 mm Side air gap ratio

1.5 1.1 3 Î ”2 and central air gap Δ 1

magnet internal radius 145 mm 88 mm 500 mm central magnet thickness 3.5 mm 3.00 mm 3.6 mm magnet width 16 p 25.3 mm 24.6 mm 26 mm magnet width 22 p 18.5 mm 17.8 mm 19.2 mm external stator radius 75 mm

In a very advantageous variant of the invention, the thickness of the magnet is constant in the median direction of the magnet so that the cross section follows a helical path. Preferably, the curved outer surface is a part of a cylindrical surface and is concentric to the radius of the cylindrical surface described by the magnetic lamination packs, while the curved inner surface of the magnet is not cylindrical. A constant thickness in the ZZ median direction (inclined with respect to the generator axis) guarantees a smaller cogging effect and a lower power loss than a magnet with variable thickness in its median Z direction.

Preferably, the magnet is defined by the fact that the maximum thickness remains constant along the midline ZZ which follows a helical path and decreases symmetrically towards the edges of the magnet. The fact that the cross-section follows a helical path also means that the individual cross-sectional areas following one another at different magnet heights in the helical path are essentially congruent.

In a variant of the invention, the inner curved surface and the outer curved surface are parts of cylindrical surfaces and the thickness of the magnet varies in the median direction of the magnet (Z). A magnet with variable thickness in the axial direction is obtained for example by cutting a permanent starting magnet with a variable air gap according to an angle with respect to the central axis of the magnet which defines the greater thickness; and therefore the edges of the magnet will be inclined at the same angle with respect to the central axis of the starting magnet. Since the thickness of the starting magnet is variable (maximum in the center, minimum at the edges), with this inclined cut the final magnet is obtained with a variable thickness by sectioning it parallel to the edges or to the new median line Z of the same.

In a very advantageous variant of the invention, the packs of tooth-shaped magnetic laminations have shaped slots that are located one in front of the other at the height of the slot opening with extension along the entire height of the pack of magnetic laminations shaped like a tooth and in which magnetic rods according to shape can be inserted to close said opening of the slot. This closing system is more practicable and more effective as regards the lowering of the cogging than the known method in which ferrite plates are inserted directly on the windings inside the slots. The shaped slots in the sheet packs are not linked to generators with variable air gap and / or with inclined magnets. Obviously they can also be applied in generators with straight magnets and with constant air gap or in generators with variable air gap and with straight magnets or in generators with inclined magnets and with constant air gap, etc.

Advantageously, the chocks are produced from a composite of insulating resin and iron powder. Preferably, the iron content is higher than 50%.

The preferred variant of the generator according to the invention is that in which the rotor is located outside the stator. But the inverse system is equally conceivable, in which the rotor turns inside the stator.

In an advantageous variant of the generator according to the invention, the height of the magnetic lamination packs is variable. The cogging, like the power, varies according to the height of the stack (stack length) and therefore with regard to the power to be obtained there is a certain cogging value which is greater the higher the power is. With a 42 mm pack you get a power of 700 W in continuous service at 415 rpm with a nominal torque of 16.1 Nm, a maximum deliverable power of about 1000W (415 rpm) with a torque of 23 Nm. cogging of 0.20 Nm.

With a package of 20 mm and a power of 450 W in continuous service at 520 rpm with a nominal torque of 8.26 Nm and a maximum deliverable power of 520 W with a torque of 9.55 Nm the cogging torque is 0.12 Nm. Higher parcels are also conceivable.

A very important aspect of the invention is the variant in which the generator according to the invention is a wind generator with a power between 400 W and 5 kW and a torque between 9 and 120Nm. As stated above, wind generators particularly feel the cogging effect with low wind speeds. Particularly satisfactory as a material for permanent magnets is NdFeB (neodymium iron boron) with a residual induction value equal to Br = 1,2Ã · 1.35 T, He (KA / m)> 850 and BHmax (KJ / m <3> )> 240.

Stator windings of the concentrated double-layer type, not superimposed with a fractional number of slots per pole and per phase q = 3/8 or q = 3/11, with a winding factor K = 0.945 or K = 0.902 have proved particularly suitable for a generator according to the invention.

Combining all the above advantages and choosing the dimensions in the field as indicated you get a low power (1000 W), 415 rpm (corresponding to a torque of 23 Nm) permanent magnet mini wind generator with part sizes active not exceeding 170 mm in diameter and 42 mm in length, the cogging does not exceed 0.20 Nm (approximately 1% of the nominal).

Another aspect of the invention also relates to a permanent magnet with variable air gap in which the upper side and the lower side of said magnets define an internal curved surface and an external curved surface with the same direction of curvature which describe in cross-section arcs of non-concentric circumferences and with respective different radii so that the distance between the two surfaces which represents the thickness of the magnet is variable in cross section and which has inclined edges. Advantageously, the distance between the two curved surfaces is constant in the median direction of the magnet so that the cross section follows a helical path.

In another variant of the invention the inner curved surface and the outer curved surface are parts of cylindrical surfaces and the thickness of the magnet varies in the median direction of the magnet.

Brief description of the drawings

The invention is now illustrated with the help of the attached drawing tables where:

- fig. 1 illustrates in a schematic representation the cogging effect according to the state of the art;

- fig. 2 shows a detail of the stator and rotor of a wind generator according to the invention in section;

- fig. 3 shows in enlarged form a detail of fig. 2;

- figs. 4a-4f show an inclined magnet with variable air gap and variable thickness along the median line according to the invention in various views, and in particular: fig. 4a is an isometric view of the magnet, fig. 4b is a top view of the magnet, figs. 4c and 4d are side views of the magnet, FIG. 4e shows the section of the magnet along the line A1-A1 of fig. 4c, while fig. 4f shows the section of the magnet along the line B1-B1 of fig. 4b; - figs. 5a and 5b illustrate the cutting of an essentially rectangular shaped magnet to produce the variable air gap inclined magnet according to Figs. 4a-4f (fig. 5a in a top view and fig. 5b in a side view), while figs. 5c and 5d show sections of the magnet along the lines C1-C1 (fig. 5c) and D1-D1 (fig. 5d) of fig. 5a;

- figs. 6a-6g illustrate an inclined magnet with variable air gap and constant thickness along the median line according to the invention in various views, and in particular: fig. 6a is a top view of the magnet, figs. 6b and 6e are side views of the magnet, FIG. 6c shows a section of the magnet along the line A2-A2 of fig. 6b, figs. 6d, 6f and 6g show sections of the magnet along the lines C2-C2 (fig. 6d), D2-D2 (fig. 6f) and B2-B2 (fig. 6g) of fig. 6a;

- fig. 7a illustrates the helical cut to obtain the magnet according to figs. 6a-6g, while fig. 7b shows the circled detail of fig. 7a in enlarged form;

- fig. 8 shows in schematic form the position of the inclined magnet with variable thickness along the median line with respect to the starting magnet with variable air gap seen from above;

- fig. 9a shows a sectional detail of a rotor according to the invention, the lamination packs of which are provided with shaped slots for receiving correspondingly shaped magnetic rods and fig. 9b shows a detail of fig. 9a in enlarged form with a magnetic rod inserted in a slot.

- figs. 10a and 10b illustrate the inclination of a magnet according to the invention.

Description of preferred examples of execution

In fig. 2 A detail of a wind generator is shown in section. The external rotor 2 and the internal stator 4 are observed. The rotor 4 has a plurality of packs of magnetic laminations shaped like teeth 6 arranged radially around the axis of rotation 8 of the rotor 4. The external radius of the stator which corresponds to the surface of the packs of magnetic laminations shaped like teeth 6 is indicated with Rs. Each tooth-shaped magnetic lamination pack 6 has an electric winding 10. The tooth-shaped magnetic lamination packs 6 alternate with an equally large plurality of slots 12. The part of the tooth-shaped magnetic laminations packs 6 which lies at in front of the permanent magnet poles 16 it is in the shape of an anvil (â € œTâ € section) whose sides extend from time to time on a neighboring hollow 12. In this way a rather narrow opening 14 of the slot is realized. The permanent magnet poles 16 are located on the inner side of the rotor and are arranged radially around the rotation axis 8. The inner radius of the rotor casing which coincides with the outer radius of the magnets is called Rr. Each individual magnet 16 has an internal side facing the stator which is arched following the trend of an arc of a circle whose center 18 is not concentric with the center 8 of the stator 4. The corresponding radius is ̈ referred to as Ri. The radius Rr is smaller than the radius Ri; and furthermore Rr and Ri do not have the same center. This eccentricity is responsible for the fact that the air gap 20 is not constant, that is, it is variable, as it is better highlighted in fig. 3.

Fig. 3 shows in enlarged form the detail highlighted with E in fig. 2. The air gap 20 which is located between a magnet 16 and a pack of magnetic laminations shaped like a tooth 6 is not constant along the whole width I of the magnet 16. Due to the eccentricity of the ray Ri, the central air gap 20 located between the center of the magnet 16 and the center of the pack of magnetic laminations shaped like a tooth 6 corresponds to a distance Î "1 which is less than the respective lateral distances Î" 2 (lateral air gaps) which are located at the edges of the magnet 16. The geometry in section of the magnet 16 shows a â € œDâ € which in the center has a thickness b1 and on the sides a smaller thickness b2.

Fig. 4a shows in an axonometric view an inclined magnet 116 with a variable air gap in which the thickness in the median direction of the magnet is not constant. The recesses 117c serve to secure the magnet 116 in the rotor. The magnet seen from above is shown in fig. 4b. The width of the magnet is defined by the lengths 11. Compared to the length (height) I2 of the magnet, the magnet is inclined at an angle of inclination at. Figs. 4c and 4d show the magnet 116 in side views, seen from one side 117a or 117b respectively (Fig. 4a). The magnet 116 is essentially a curved parallelepiped in which the curvatures follow different non-concentric R1 and R2 radii which results in a variable thickness in the transverse direction of the magnet 116, in fact, the thicknesses s1 and s2 are different. Fig. 4e shows the magnet 116 in section along the line A1-A1 of FIG. 4d. The thickness along the A1-A1 line is constant. Fig. 4f shows the section of the magnet 116 along the line B1-B1 of fig. 4b. The width 11 is constant in the axial direction of the magnet. Edge thicknesses s3 and s4 are different from center thickness s5.

Fig. 5a shows the production of the inclined magnet with variable air gap 116 from a magnet with a base surface, seen from above, essentially rectangular, indicated by the dashed line 115 with a starting width of 1100. Along the width the thickness of the rectangle varies in symmetrical shape with respect to the central line X, as can be seen for example in the side view of the magnet of fig. 5b. While the thickness is constant along the line X and along each line parallel to the line X. The cutting lines 113 indicate where the â € œthe rectangleâ € 115 is cut to obtain the inclined magnet 116. It is obvious that a cut following the lines 113 results in a magnet 116 in which the thickness varies in the median direction Z, and therefore also along the inclined edges 119 of the magnet. This fact is highlighted in figs. 5c and 5d which show sections through the magnet 116 following respectively the lines C1-C1 (fig. 5c) and D1-D1 (fig. 5d) of fig. 5a. The thicknesses s6 and s8, as well as the thicknesses s7 and s9, are distinguished. Fig. 6a shows in a top view an inclined magnet with variable air gap 216 in which the thickness is instead constant in the median direction ZZ and along the inclined edges. Figs. 6b and 6e show the side views of the magnet relative to the sides 217a and 217b. The width of the magnet 216 is represented by 1201 (Figs. 6d-6g). The spokes R11 and R12 are different and not concentric thus creating a variable thickness of the magnet in cross section. Fig. 6c shows a section of the magnet along the line A2-A2 of fig. 6b. The length of the magnet is designated with I202. Figs. 6d, 6f and 6g show different cross sections of the magnet 216, FIG. 6d the section along the line C2-C2 of fig. 6a, fig. 6f the section along the line D2-D2 of fig. 6a and fig. 6g the section along B2-B2 of fig. 6a. The thicknesses on the edges of the magnet are all the same and named with s10, even the central thicknesses s11 are all the same.

Fig. 7a illustrates how a magnet with constant thickness can be made in the direction of its median ZZ. Fig. 7b enlarges the interested part of fig. 7a. The outer surface of magnet 216 is cylindrical with a given radius R12. The section follows a helical path E. The individual sections form areas of equal size and shape.

Fig. 8 illustrates in another way how the magnet, with variable thickness in its median direction, can be made from a magnet similar to the â € œbread-loafâ € type with a base surface, seen from above, rectangular, that is, in which the thickness of the magnet varies symmetrically with respect to the X axis. In the case of the variable thickness magnet in the median direction, the magnet 116 (solid line in bold) assumes a position inside the â € œ starting rectangleâ € in which the Z axis that defines the median direction is inclined with respect to the central X axis.

In the case of the magnet with constant thickness in the median direction ZZ, the cross section (fig .6g) describes a helical path E (fig. 7a and 7b) in which the axis of the helix Y coincides with the helix Y axis of the external cylindrical surface of the magnet having radius R12.

Fig. 9 shows another measure applicable to a wind generator to lower the cogging effect. In the upper part of the packs of magnetic laminations shaped like teeth 306, in proximity to the hollow opening, there are shaped slots 307 which extend along the entire height of the packs. Correspondingly shaped magnetic rods 309 can be inserted in these slots 307 between two packs of laminations (Fig. 9b which illustrates a detail of Fig. 9a in enlarged form). These wedges magnetically close the openings of the slots 314 and thus contribute considerably to the lowering of the cogging effect.

Figs. 10a and 10b explain in a general way (ie regardless of the choice of the other parameters of the magnet) the concept of the inclination of the magnets.

Fig. 10b defines the angle of inclination a of a magnet with inclined edges according to the invention. The angle of inclination corresponds to the angle between an inclined edge (parallel to the median line M) and the height of the magnet. Fig. 10a shows the magnet of fig. 10b seen from the V direction. The inclination of the magnet is chosen with an angle value (in the radial direction) that varies between a minimum corresponding to the cogging period and half of the induced hollow step. That is, the arc of circumference AC with radius RC obtained from the projection on a radial surface (the dashed line represents the limitation of the radial plane of a radial surface) of the two extremes P1 and P2 of the same edge of a magnet, is equal to a value between the cogging step and half the value of the circumference with radius RC divided by the number of slots in the stator. The cogging step with 18 slots and 22 poles corresponds to 1/11 of the slot step, while it corresponds to 1/8 with 18 slots and 16 poles.

This inclination of the magnets interpreted in an axial sense, involves inclinations that vary as the stack length varies: the shorter the pack is, the more the axial inclination increases with the same percentage of pitch inclination. This can be seen in table 2 below where, as an example, the values of the angle a have been entered as a function of the height L of the lamination pack and of some pitch inclination fractions expressed as a percentage.

Table 2

to o

L stator mm 0I CN

20% groove step 33% p1 groove step 50% groove step

42 ~ 7 ° ~ 12 ° ~ 18 °

25 ~ 12 ° ~ 28 °

The following table 3 compares the characteristics of the two variants of inclined variable air gap magnets:

Table 3

Magnet type features

fig.

2 fig. 4a-f, fig. 5a-d fig. 6a-g, fig. 7a, 7b 116 216

Variable re-air gap Rs si si inclined magnets si external surface

magnet Rr R2 cylindrical R12 cylindrical internal surface

magnet Ri R1 cylindrical R1 1 non-cylindrical magnet thickness

along midline Z-axis variable Z-axis constant concentricity

internal surfaces (Ri)

and external (Rr) of the Rr- R2-R1 R12-R11 magnet Ri non-concentric non-concentric concentricity

internal surfaces

magnet (Ri) ed

external (Rs) of the teeth Ri- Rs-R1 Rs-R11 stator Rs non-concentric non-concentric concentricity

external surfaces

magnet (Rr) ed

external (Rs) of the teeth Rr- R2-Rs R12-Rs stator Rs concentric concentric The invention has achieved the aim of realizing a permanent magnet generator with low cogging effect (<1%).

In particular, a low power wind generator has been proposed with a reduced cogging effect compared to known generators. The inclined magnet proposed achieves the reduction of the cogging effect without reducing the power too much.

During the execution phase, further modifications or executive variants not described above may be made to the permanent magnet generator and to the magnet object of the invention. Should such modifications or variations fall within the scope of the following claims, they must all be considered protected by this patent.

Claims (10)

CLAIMS 1) Permanent magnet generator with reduced cogging effect comprising a stator (4) with a plurality of packs of tooth-shaped magnetic laminations (6; 306) with respective electrical windings (10) arranged radially around the axis of rotation (8 ) of the generator which alternate with a plurality of slots (12; 312) with a relative opening (14; 314) and a rotor (2) with a plurality of permanent magnets (16; 116; 216) arranged in a radial shape around the € ™ rotation axis (8) of the generator in which the air gap (20) between said packs of tooth-shaped magnetic laminations (6; 306) and said permanent magnets (16; 116; 216) is variable and in which said magnets permanent (16; 116; 216) have sloping edges. 2) Generator according to claim 1) characterized by the fact that said magnets (16; 116; 216) are variable air gap magnets in which the upper side and the lower side of said magnets define an internal curved surface and an external curved surface with the same direction of curvature that describe in cross section arcs of non-concentric circumferences and with respective radii (R1.R2; R11.R12) so that the distance between the two surfaces which represents the thickness of the magnet is variable in cross section and in which the arc of circumference identified by the radius (Ri; R1; R11) of the internal curved surface in cross section that is in front of the tooth-shaped magnetic lamination packs (6) is not concentric with the arc of circumference identified by the radius (Rs) corresponding to the cylindrical surface described by the tooth-shaped magnetic lamination packs (6). 3) Generator according to claim 2) characterized in that the thickness (s10, s11) of the magnet is constant in the median direction of the magnet (ZZ) so that the cross section follows a helical path. 4) Generator according to claim 3) characterized by the fact that said magnet (216) is defined by the fact that the maximum thickness (s11) remains constant along the median line ZZ which follows a helical path and decreases in a symmetrical form towards the edges of the magnet. 5) Generator according to claim 2) characterized in that the internal curved surface and the external curved surface are parts of cylindrical surfaces and that the thickness (s1-s9) of the magnet varies in the median direction of the magnet (Z). 6) Generator according to claim 5) characterized by the fact that said magnet (116) is a cutout of a starting permanent magnet (115) with variable air gap defined by the fact that the maximum thickness is found along the central axis of the magnet (X) and decreases symmetrically towards the edges of the magnet and in which the inclined edges of the cut-out magnet have an angle of inclination with respect to the central axis (X) of the starting magnet. 7) Generator with permanent magnets according to any one of the preceding claims, characterized by the fact that said packs of tooth-shaped magnetic laminations (306) have shaped slots (307) which are located one in front of the other at the height of the Opening of the slot (314) with extension along the entire height of the pack of magnetic laminations shaped like a tooth and in which magnetic rods (309) can be inserted accordingly shaped to close said opening of the slot (314). 8) Generator according to any one of the preceding claims characterized by the fact that said generator is a wind generator with a power between 400 W and 5 kW and a torque between 9 and 120Nm. 9) Permanent magnet generator according to any one of the preceding claims characterized in that the rotor (2) is located outside the stator (4). 10) Permanent magnet with variable air gap (116, 216) in which the upper side and the lower side of said magnets define an internal curved surface and an external curved surface with the same direction of curvature which describe in cross section arcs of non-concentric circumferences and with respective radii (R1, R2; R11.R12) different so that the distance between the two surfaces which represents the thickness of the magnet is variable in cross section characterized by the fact that it has inclined edges, 11) Permanent magnet with variable air gap (216) according to claim 11) characterized in that the distance between the two curved surfaces is constant in the median direction (ZZ) of the magnet so that the cross section follows a helical path. 12) Permanent magnet with variable air gap (116) according to claim 10) characterized in that the inner curved surface and the outer curved surface are parts of cylindrical surfaces and that the thickness (s1-s9) of the magnet varies in the median direction of the magnet (Z). By assignment.