An electrical generator comprising a rotor and two stators

Abstract

The electric power generator includes a rotor and a plurality of stators, which are coaxially and concentrically arranged around a central axis. An inner stator separated by an inner air gap is provided on the radial inner side of the rotor, and an outer stator separated by an outer air gap is provided on the radial outer side of the rotor. The rotor includes a plurality of magnetic pole structures, which are configured to provide or generate a magnetic field having a plurality of magnetic poles. The cross-sectional thickness of the rotor is uneven, wherein: the inner surface of the rotor protrudes inward at the magnetic pole structure, and the inner air gap is uneven because the inner air gap is radially shorter at the magnetic pole structure and radially longer between the magnetic pole structures; and the outer surface of the rotor protrudes outward at the magnetic pole structure, and the outer air gap is uneven because the outer air gap is radially shorter at the magnetic pole structure and radially longer between the magnetic pole structures.

Description

Technical Field

[0001] The present invention relates to electrical generators, and in particular, the present invention relates to electrical generators having a rotor and two stators in a stator-rotor-stator configuration. Background Art

[0002] In a prior art electric power generator, a single rotor and a single stator are usually provided. As is well known, the rotor generates a magnetic field and rotates, and the rotating magnetic field induces current in a winding provided in the stator which remains stationary. In this configuration, the magnetic field from the rotor is directed to the stator in only one direction, that is, the magnetic field from the rotor is directed to the stator only radially inward or only radially outward.

[0003] The inventors are aware of generators with a dual stator, single rotor configuration. US6297575 discloses a three-layer structure in which the magnetic field source from the rotor is located between the two stators and is a common magnetic field source for the interaction of these stators. The present disclosure intends to use this structure to perform a generator function or a motor function or a combination of a motor function and a generator function.

[0004] However, the disclosure of US6297575 lacks technical specifications regarding air gaps. When the unit is operated as an electrical generator, the effects of air gaps and their interactions do not appear to be considered. The disclosure does not study the air gaps between different layers, especially in terms of power generation, although the air gaps may be critical to power generation parameters. The inventors consider this to be a shortcoming in US6297575 and wish to address this issue.

[0005] The present invention aims to address the above mentioned drawbacks of not optimizing the air gap to improve the performance and the quality of the generated electrical power. Summary of the invention

[0006] Therefore, the present invention provides an electric power generator comprising a rotor and a plurality of stators, wherein:

[0007] The rotor and the plurality of stators are coaxially and concentrically arranged around a central axis;

[0008] The plurality of stators include an inner stator, which is disposed radially inwardly of the rotor, and the inner stator and the rotor are separated by an inner air gap;

[0009] The plurality of stators include an outer stator, which is disposed radially outside the rotor, and the outer stator and the rotor are separated by an outer air gap;

[0010] Each of the inner stator and the outer stator has a respective winding configured to induce current therein;

[0011] the rotor comprises a plurality of pole structures configured to provide or generate a magnetic field having a plurality of poles, the magnetic field being configured, in use, to induce current in the windings; and

[0012] The cross-sectional thickness of the rotor is uneven, where:

[0013] The inner surface of the rotor protrudes radially inward at the pole structure, and the inner air gap is non-uniform because the inner air gap is radially shorter at the pole structure and radially longer between the pole structures; and

[0014] The outer surface of the rotor protrudes radially outward at the pole structure, and the outer air gap is non-uniform because the outer air gap is radially shorter at the pole structure and radially longer between the pole structures.

[0015] In other words, the outer surface of each pole structure may have a sharper curvature or a smaller radius of curvature than the displacement arc it describes. The inner surface of each pole structure may be flat or at least have a gentler curvature or a larger radius of curvature than the displacement arc it describes.

[0016] An electrical generator may have only a single rotor and exactly two stators.

[0017] The electric generator may include a back iron. The back iron may provide a return path and help close the magnetic circuit to prevent the magnetic field of the rotor from radiating outward. The back iron may be arranged around the outer stator. There may be a return path in the outer stator and the inner stator to complete the magnetic circuit. The inner return path may be located in the inner surface of the inner stator, and the outer return path may be in the outer surface of the outer stator.

[0018] It may be noted that an advantage of the present disclosure is that the magnetic field generated by the rotor is utilized both inside the rotor (utilized by the inner stator) and outside the rotor (utilized by the outer stator). This may result in a more efficient conversion of the magnetic field into electric current.

[0019] The outer air gap may be radially longer or wider than the inner air gap on average. The inner air gap may be 5 mm to 50 mm at its radial shortest point. The inner air gap may be 10 mm to 90 mm at its radial longest point. The outer air gap may be 20 mm to 100 mm at its radial shortest point. The outer air gap may be 30 mm to 165 mm at its radial longest point.

[0020] The inner stator may include teeth at its radially outer surface to accommodate its windings.Thus, the windings may be close to and opposite to the radially inner surface of the rotor.

[0021] The outer stator may include teeth at its radial inner surface to accommodate its windings.Thus, the windings may be close to and opposite to the radial outer surface of the rotor.

[0022] The corresponding teeth of the inner and outer stators may be equal or radially aligned; conversely, the corresponding teeth of the inner and outer stators may be unequal or radially offset.

[0023] The rotor may include intermediate support structures between adjacent pole structures to provide a constant or uninterrupted circumferentially extending rotor. The support structure may be a ferrous material to conduct or enhance the magnetic field. Conversely, the support structure may be a non-ferrous material that may prevent shorting of the magnetic field.

[0024] The inner stator may present a generally circular outer surface.Thus, the inner air gap may be generally uniform at its inner boundary (provided by the outer surface of the inner stator) and may be non-uniform at its outer boundary (provided by the inner surface of the rotor).

[0025] The outer stator may present a generally circular inner surface.Thus, the outer air gap may be generally uniform at its outer boundary (provided by the inner surface of the outer stator) and may be non-uniform at its inner boundary (provided by the outer surface of the rotor).

[0026] The advantages of non-uniform air gaps are that they can provide a better or more desirable output waveform. This can include a smoother waveform, less cogging, and / or less harmonic distortion, among other things. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 shows a schematic cross-sectional view of an electric power generator comprising a rotor and two stators according to the invention;

[0028] Figure 2 Shows Figure 1 an enlarged view of a portion of an electric power generator;

[0029] Figure 3 Shows Figure 1 Schematic flux density heat map of the generator;

[0030] Figure 4 Shows Figure 1 Schematic flux density heat map with threshold value of the generator;

[0031] Figure 5 Shows Figure 1 A graph of flux density between a stator and a rotor of a generator;

[0032] Figure 6 Shows Figure 1 A graph of flux density between an outer stator and a rotor of a generator;

[0033] Figure 7 Shows Figure 1 A graph of the phase voltage of the inner stator of the generator;

[0034] Figure 8 Shows Figure 1 A graph of the phase voltage of the outer stator of the generator;

[0035] Fig. 9 Shows Figure 1 A graph of the phase current of the inner stator of the generator;

[0036] Fig.10 Shows Figure 1 A graph of the phase current of the outer stator of the generator;

[0037] Fig.11 Shows Figure 1 A curve diagram of the torque of the inner stator of the generator;

[0038] Fig.12 Shows Figure 1 A curve diagram of the torque of the rotor of the generator; and

[0039] Fig.13 Shows Figure 1 Graph of the torque of the outer stator of the generator. DETAILED DESCRIPTION

[0040] Figure 1 to Figure 2 An electric power generator (referred to simply as a "generator") 100 according to the present invention is shown. The generator 100 has a central hub 102 defining a central axis at its radially inner center and a back iron 104 at its radially outer surface. The generator 100 also has many features commonly common to generators, such as bearings, hubs, frames, input drives, output wiring, etc., which are not shown, but those skilled in the art will understand that they may form part of the present invention.

[0041] The generator 100 has an inner stator 110, an intermediate rotor 120, and an outer stator 130. These can be considered as three concentric "layers" of the generator 100. The rotor 120 and the stators 110, 130 are coaxial about a central axis. The inner stator 110 is disposed radially inwardly of the rotor 120, and the inner stator 110 and the rotor 120 are separated by an inner air gap 112. The outer stator 130 is disposed radially outwardly of the rotor 120, and the outer stator 130 and the rotor 120 are separated by an outer air gap 132.

[0042] Each of the inner stator 110 and the outer stator 130 has a respective winding 114, 134 configured to induce current therein. Accordingly, the rotor 120 includes a plurality of pole structures 122, 124 configured to provide or generate a magnetic field having a plurality of poles, the magnetic field being configured to induce current in the windings 114, 134 in use. Each pole may include a magnetic body 122 and an embedded field winding 124, so that the pole structures 122, 124 are electromagnetic in this example (although in other examples, the pole structures 122, 124 may be permanent magnets). The rotor 120 includes an intermediate support structure 126 to provide structural support between the pole structures 122, 124.

[0043] The rotor 120 is generally annular, but not uniformly annular; more specifically, the cross-sectional thickness of the rotor 120 is not uniform. This is shown in the enlarged portion of the generator 100. Figure 2 The inner surface of the rotor 120 protrudes radially inward at the pole structures 122, 124, so the inner air gap 112 is non-uniform because it is radially shorter (indicated by arrows 112.1) at the pole structures 122, 124. The inner air gap 112 is radially longer (indicated by arrows 112.2) between the pole structures 122, 124, i.e. at the intermediate structure 126 - or radially aligned with the intermediate structure 126.

[0044] The outer air gap 132 has a similar configuration, although it is wider / radially longer on average than the inner air gap 112. More specifically, the outer surface of the rotor 120 protrudes radially outward at the pole structures 122, 124, so the outer air gap 132 is non-uniform in that it is radially shorter at the pole structures 122, 124 (indicated by arrows 132.1) and radially longer at the intermediate structure 126 between the pole structures 122, 124 (indicated by arrows 132.2).

[0045] The distance can be within the following ranges as indicated in Table 1:

[0046] characteristic scope The shortest distance (112.1) of the inner air gap (112) 5mm to 50mm The longest distance (112.2) of the inner air gap (112) 10mm to 90mm The shortest distance (132.1) of the outer air gap (132) 20mm to 100mm The longest distance (132.2) of the outer air gap (132) 30mm to 165mm

[0047] Table 1

[0048] The shape of the magnetic body 122 shows the magnetic flux density. Figure 3 It is also clear in FIG. 1 that the magnetic body 122 has a flat inner surface (which is considered to be an inward protrusion relative to a more conventional arc-shaped curved inner surface, which would be concentric with the central axis and equidistantly spaced). In other words, in use, the inner surface of the magnetic body 122 has a displacement arc that is smaller than that described by the inner surface.

[0049] Similarly, the outer surface of the magnetic body 122 is more curved (or has a smaller radius of curvature) than a more conventional constantly curved outer surface; this is considered to be convex because the curvature is tighter than the displacement arc described by the outer surface of the magnetic body 122.

[0050] The inventors have conducted simulations and tests on the generator 100. Figures 3 to 11 The results of the simulations are summarized in . The specific parameters of the simulations are provided in Table 2:

[0051]

[0052]

[0053] Table 2

[0054] One of the advantages of the generator 100 is that it produces an electrical output (ie, the voltage and current characteristics of the output) having low total harmonic distortion (THD) without requiring any correction, regulation, or rectification circuitry.

[0055] Figure 3 to Figure 4 Magnetic flux density heat maps 202 , 204 are shown of the generator 100 in use. Figures 5 to 11 Various graphs 206 to 224 showing characteristics of the generator 100:

[0056] Graph 206 shows the flux density between the inner stator 110 and the rotor 120;

[0057] Graph 208 shows the flux density between the outer stator 130 and the rotor 120;

[0058] Graph 210 shows the phase voltage of the inner stator 110;

[0059] Graph 212 shows the phase voltage of the outer stator 130;

[0060] Graph 214 shows the phase current of the inner stator 110;

[0061] Graph 216 shows the phase current of the outer stator 130;

[0062] Graph 220 shows the torque of the inner stator 110;

[0063] Graph 222 shows the torque of rotor 120; and

[0064] Graph 224 shows the torque of the outer stator 130 .

[0065] While the specific values ​​of the graphs 206 - 224 may not be directly related, the shapes of the graphs 206 - 224 are related, illustrating the relative smoothness of the various output characteristics.

[0066] The inventors note that the present disclosure may have other advantages and features. The magnetic field of both the inner and outer surfaces of the rotor 120 is utilized. The magnetic flux reading after FFT in the inner air gap 112 may be between 0.3T and 0.99T, and for the outer air gap 132, may be between 0.2T and 0.95T.

[0067] In conventional generator designs, harmonics increase as current increases; however, in the design of the present generator 100, good results are produced because the quality of the power produced is good in terms of harmonics. In this embodiment, when the current on the inner stator 110 is held constant, the harmonics are reduced (compared to conventional generator designs) when the current of the outer stator 130 (specifically, the windings 134 in the outer stator 130) is increased up to a certain point, at which the inner air gap 112 ranges from 15mm to 55mm at its shortest distance and ranges from 55mm to 95mm at its longest distance, and the outer air gap 132 ranges from 25mm to 55mm at its shortest distance and ranges from 60mm to 120mm at its longest distance, and when the current of the outer stator 130 is increased and the current of the inner stator 110 is held constant, the improvement is that the harmonics and total harmonic distortion of the voltage on the outer stator are reduced to at least 0.44%.

[0068] From this point on, the harmonics begin to increase as the current on the outer stator 130 increases. As the current in both the inner stator 110 and the outer stator 130 increases, the harmonics and the total harmonic distortion percentage in both the inner stator 110 and the outer stator 130 decrease. The reduction in harmonics in the outer stator 130 is more significant. On the other hand, as the current on the outer stator 130 decreases, the harmonics on the outer stator 130 increase, which is the opposite of conventional designs. All of this occurs when the current in the inner stator 110 is maintained at a constant value (rated current). At half the rated current on the outer stator, the harmonics are higher than at the rated current. When the current of both the inner stator 110 and the outer stator 130 increases, the harmonic and total harmonic distortion percentages in both the inner stator 110 and the outer stator 130 decrease, wherein the inner air gap 112 ranges from 25 mm to 65 mm at its shortest distance and ranges from 45 mm to 110 mm at its longest distance, and the outer air gap 132 ranges from 65 mm to 110 mm at its shortest distance and ranges from 94 mm to 140 mm at its longest distance, and when the current of both the inner stator 110 and the outer stator 130 is increased, the harmonic and total harmonic distortion of both the inner stator 110 and the outer stator 132 are reduced by at least 4%.

[0069] Another beneficial embodiment is that the outer stator 130 can operate at much higher currents than the current of a conventionally designed power generator without compromising the quality of the power. The current can be increased to more than 100% relative to the conventional design value, and this increases the output without compromising the quality of the power supply to the consumer. When the current is increased by 50%, the total harmonics on the outer stator 130 increase by up to 0.88%. The current in the outer stator 130 can still be increased by up to 100%, which will improve the performance of the generator 100 without compromising the quality of the voltage harmonics of the power generator, and when the current is increased by 100%, the harmonics are close to the 3% THD limit of the total harmonics on the outer stator, and the IEEE519 standard, which is an internationally accepted standard for harmonics, is 3%. The outer stator 130 can produce more power at the right quality level without changing the configuration.

[0070] The harmonics criteria are not a direct challenge to the outer stator 130, but the inner stator 110 reacts in an opposite way compared to the outer stator 130. At this level, the harmonics of the inner stator 110 may increase as the current increases, while the harmonics of the outer stator 130 may decrease as the outer stator 130 current increases. This is the case until the current of both the inner stator 110 and the outer stator 130 increases, which is completely unexpected, wherein the inner air gap 112 ranges from 25mm to 85mm at its shortest distance and ranges from 65mm to 100mm at its longest distance, and the outer air gap 132 ranges from 35mm to 55mm at its shortest distance and ranges from 63mm to 120mm at its longest distance, and when the current on the inner stator 110 decreases, the harmonics and total harmonic distortion in both the inner stator 110 and the outer stator 130 decrease by at least 3%.

[0071] The inner stator 110 reacts in the same manner as a conventional generator in that as the current increases, the harmonics also increase. The inner stator 110 can be designed for low voltages, and the outer stator 130 can be designed for higher voltages at higher currents at the same time and for the same configuration. In this operating configuration, the inner stator 110 can contribute as little as 13% of the total output of the generator, while the outer stator can contribute 87% of the electrical output. As the current of the outer stator 130 increases above a certain point, the harmonics on the inner stator 110 decrease; that is, as the current of the outer stator 130 increases by +100%, the harmonics on the inner stator 110 decrease by 10%, so this can indicate that the best operating point to keep the harmonics within acceptable standards is to operate the outer stator 130 at a higher current. This operation requires that the outer stator 130 be made larger so that larger conductors can be used on the outer stator 130 to carry more current. The inner stator 110 can also be made larger to accommodate the larger conductors. A surprising result is that when the inner stator 110 is offline and the outer stator 130 is loaded, the outer stator harmonics do not change.

[0072] The fact that there are two air gaps 112, 132 means that two dispersed layers of magnetic poles can be used. Therefore, this arrangement enables both surfaces of the rotor 120 to be used to generate electricity. Utilizing the magnetic field on both surfaces of the magnetic pole structures 122, 124 can be an advantage of the present disclosure. This previously unused magnetic field is on the surface of the shaft attached to the rotor surface in conventional designs, which technically has the same size and the same characteristics as the other surface being utilized. In conventional designs, the generator has one rotor and one stator, which leads to the following situation: the other surface of the magnetic pole, even if it has all the characteristics of the surface being utilized, the other surface will never be utilized.

[0073] The air gaps 112, 132 and the magnetic flux may play an important role because they affect the performance and the quality of the electricity generated by the generator 100. The inner air gap 112 may be between 5 mm and 50 mm at the center of the pole and increase to between 10 mm and 90 mm at the end of the pole, and the outer air gap 132 may be between 20 mm and 100 mm at the center of the pole and increase to between 35 mm and 165 mm at the end of the pole. The FFTBr value (magnetic flux) reading at the midpoint of the inner air gap 112 between the poles on the inner stator and rotor may be between 0.3 T and 0.99 T, and for the outer air gap 132, may be between 0.2 T and 0.95 T. Because of the increase in the outer air gap 132, the magnetic flux reading may be very low compared to the magnetic flux reading of the inner air gap 112. Low flux readings result in low voltage generation.

[0074] To address this low voltage generation, the diameter / size of the outer stator 130 can be increased, which increases the voltage generated by more windings 134, and also by increasing the outer stator slot size (depth), and using larger size conductors or using more windings 134. Even though the magnetic field travels a long distance to reach the outer air gap 132, the overall size of the outer stator 130 makes up for the low magnetic flux density. The inner stator 112 can generate a low voltage, while the outer stator 130 can generate a higher voltage, which improves the performance and flexibility of the generator 100. The generator 100 can generate a higher voltage on both the inner stator 110 and the outer stator 130, and the two stators 110, 130 can also generate a low voltage, which is similar to almost degrading the generator when compared to the higher voltage on the stator. (Low voltage is generally considered to be a voltage below 1000V, and high voltage is considered to be a voltage above 1000V). Comparing low voltage to high voltage, higher voltage can generate more electricity than low voltage without changing the configuration. This design may be more suitable for low fuel cost applications especially using renewable energy sources such as wind, hydro and geothermal energy.

[0075] The generator 100 can have many characteristics for commercial success based on its low voltage harmonics, which enables the generator 100 to produce higher levels of current and therefore generate a large amount of electricity according to its size. Affordable and better quality electricity has long been needed. This embodiment provides a solution to that unmet market demand. This level of power quality brings many economic benefits and safety benefits to users of electrical energy and also to the transmission and distribution of electricity. It also brings benefits to harmonically sensitive electrical equipment. Compared with the generators of the prior art, the high level of power output drives a higher power / mass (kW / kg) ratio. This ability of the generator 100 reduces the capital expenditure cost in cents / kilowatt. In the presence of two stators 110, 130 and a single rotor 120 working together to achieve the generation of low voltage harmonics, low harmonics become an unexpected superior effect.

[0076] As the current on the outer stator 130 increases, the harmonics of the inner stator 110 and the outer stator 130 decrease, compared to conventional generators, where the harmonics increase as the current of a single stator increases. Another comparative advantage is that there is a significant reduction in the input power into the generator 100 when the inner stator 110 is loaded and then the outer stator 130 is loaded. As the current on both the inner stator 110 and the outer stator 130 increases, the voltage harmonics of the two stators 110, 130 continue to decrease.

[0077] Terms

[0078] 1. A three layer electric power generator with a rotor between two stators, wherein the air gap between the inner stator and the inner surface of the pole rotor is between 5mm and 50mm at the center of the pole and increases to between 10mm and 90mm at the end of the pole, and the outer stator air gap between the outer surface of the rotor and the inner surface of the outer stator is between 20mm and 100mm at the center of the pole and increases to between 35mm and 130mm at the end of the pole, and the FFT Br value (magnetic flux) reading at the midpoint of the inner air gap between the inner stator and the pole on the rotor is 0.3T and 0.7T reading, and for the outer air gap between the outer surface of the pole on the rotor and the inner surface of the outer stator, it is between 0.2T and 0.6T.

[0079] 2. An electrical generator as claimed in clause 1 wherein the improvement is that the rotor is smaller than a conventional rotor for a comparable electrical output.

[0080] 3. An electrical generator as described in clause 1, wherein the improvement is that the size of the stator is larger than the size of a conventional stator.

[0081] 4. An electric generator according to claim 1, wherein the inner air gap is between 5mm and 50mm at the center of the pole and increases to between 10mm and 90mm at the end of the pole, and the outer stator air gap is between 20mm and 100mm at the center of the pole and increases to between 35mm and 130mm at the end of the pole, wherein the improvement is that the harmonics on the outer stator only decrease as the current in the outer stator increases.

[0082] 5. An electric generator according to clause 1, wherein the inner air gap is between 5 mm and 50 mm at the center of the pole and increases to between 10 mm and 90 mm at the end of the pole, and the outer stator air gap is between 20 mm and 100 mm at the center of the pole and increases to between 35 mm and 130 mm at the end of the pole, and wherein the improvement is that, starting from the rated current point in the outer stator alone, the harmonics on the inner stator decrease as the current increases.

[0083] 6. An electrical generator according to clause 2, wherein the improvement is that the stator windings are wound one after another in a spiral form.

[0084] 7. A three-layer electric power generator according to clause 1, characterised in that the outer stator can be operated at a higher current and still maintain the correct harmonic level.

[0085] 8. A three-layer electric power generator according to clause 1, characterized in that the inner stator can generate a low voltage and the outer stator can generate a higher voltage, or the inner stator and the outer stator can generate a high voltage.

[0086] 9. A three-layer electric generator according to claim 1, wherein the inner air gap ranges from 5mm to 50mm at its shortest distance and ranges from 10mm to 95mm at its longest distance; and the outer air gap ranges from 20mm to 140mm at its shortest distance and ranges from 30mm to 165mm at its longest distance.

Claims

1. An electric power generator comprising a rotor and a plurality of stators, wherein: The rotor and the plurality of stators are coaxially and concentrically arranged around a central axis; The plurality of stators include an inner stator, the inner stator being disposed radially inward of the rotor, the inner stator and the rotor being separated by an inner air gap; The plurality of stators include an outer stator disposed radially outside the rotor, the outer stator and the rotor being separated by an outer air gap; Each of the inner stator and the outer stator has a respective winding configured to induce current therein; The rotor comprises a plurality of pole structures configured to generate a magnetic field having a plurality of poles, the magnetic field being configured, in use, to induce a current in the winding; intermediate support structures between adjacent pole structures, the intermediate support structures being configured to provide a rotor with a constant or uninterrupted circumferential extension; an outer return path in the form of a back iron disposed about the outer stator, the outer return path being configured to provide a return path and help close a magnetic circuit radiating outwardly from the magnetic field of the rotor; an inner return path in the inner stator to complete the magnetic circuit, the inner return path being located in an inner surface of the inner stator; and The radial thickness of the rotor is non-uniform, wherein: the inner surface of the rotor protrudes radially inwardly at the pole structures so that the inner air gap is non-uniform in that the inner air gap is radially shorter at the pole structures and radially longer between the pole structures, wherein the inner air gap ranges from 25 mm to 65 mm at its shortest distance and ranges from 45 mm to 110 mm at its longest distance; and the outer surface of the rotor protrudes radially outward at the pole structures so that the outer air gap is non-uniform in that the outer air gap is radially shorter at the pole structures and radially longer between the pole structures, wherein the outer air gap ranges from 65 mm to 110 mm at its shortest distance and ranges from 94 mm to 140 mm at its longest distance; and The magnetic flux readings in the inner air gap are between 0.3T and 0.99T, while the magnetic flux readings in the outer air gap are between 0.2T and 0.95T.

2. The electric power generator according to claim 1, wherein: The outer air gap is radially longer on average than the inner air gap.

3. The electric power generator according to claim 1, wherein: The inner stator presents a circular outer surface, and the outer stator presents a circular inner surface.

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