Field of the Invention

The present invention relates to a squirrel-cage rotor for a high-speed electrical machine, and more particularly to a reinforced squirrel-cage rotor.

Background to the Invention

Induction rotors are often used in high-speed electrical machines, and a common type of induction rotor used in high-speed electrical machines is a squirrel-cage rotor.

Typically, squirrel-cage rotors comprise a core that is formed of thin laminated steel sheets. Bars of conducting material are embedded in the surface of the core, these bars linked at their ends by two conductive‘short-circuit’ rings which abut the laminated core. Together, these bars and the conductive rings form a cage of conducting material which may be known as a squirrel cage.

Squirrel-cage rotors for high-speed electrical machines are difficult to design. It is desirable to design the rotor diameter to be a large as possible to maximise the potential power generated by the machine. However, the larger the rotor of a high-speed electrical machine the larger the associated centrifugal forces. This can be problematic because at the rotational speeds used in high-speed electrical machines the centrifugal forces generated can cause the squirrel cage, and in particular the short-circuit rings, to fail. In many instances, the strength of the short-circuit rings limits the performance and size of the rotor.

This problem is exacerbated for squirrel cages produced via low-cost manufacturing techniques, such as casting, as cast squirrel cages tend to be weaker than squirrel cages assembled from individually fabricated components. However, fabricating squirrel cages via casting is desirable as it is a relatively low-cost and high-throughput method of manufacture compared to the post-fabrication assembly of individual parts.

Objects and aspects of the present claimed invention seek to alleviate at least these problems of the prior art.

Summary of the Invention According to a first aspect of the present invention, there is provided a squirrel-cage rotor for a high-speed electrical machine comprising: a rotor core comprising a plurality of channels, and a squirrel-cage comprising two short-circuit rings connected by a plurality of conducting members. The plurality of conducting members is housed within the plurality of channels, and each of the two short-circuit rings is proximate to one of a pair of opposing end faces of the rotor core. The squirrel-cage rotor of the present invention further comprises a reinforcement member. The reinforcement member abuts or lies adjacent to a first surface of at least one of the short-circuit rings, wherein the normal to said first surface is substantially perpendicular to the rotational axis of said squirrel-cage rotor.

The present invention is advantageous as the short-circuit ring is reinforced by the reinforcement member. The short-circuit ring is often the component of a squirrel cage rotor for high-speed machinery that fails under operational load. That is, at high operational speeds or larger rotor diameters the centrifugal forces generated are larger than the material of the short-circuit rings can withstand. In the present invention, providing a reinforcement member increases the strength of the short-circuit ring, ensuring a rotor of any given size can operate at greater rotational speeds without risk of failure. Therefore, the present invention can improve the performance and reliability of squirrel-cage rotors used in high speed electrical machines. Furthermore, this reinforcement facilitates fabricating the short- circuit ring from relatively low-cost methods of manufacture, such as casting, which can simultaneously reduce cost and improve performance.

In this specification, central and peripheral are used to describe the relative position of features with reference to the rotational axis of the present invention. In other words, central and peripheral are used to describe relative distance in a radial direction. For example, a central surface of a feature is closer to the rotational axis than a peripheral surface of that feature. However, describing a feature as central does not mean that it is necessarily closer to the rotational axis than a different non-related feature. For example, the central surface of a first feature may be further away from the rotational axis than the peripheral surface of a second feature. In many embodiments of the invention, the rotational axis of the rotor is also the longitudinal axis of the rotor.

Additionally, inner and outer are used to describe the relative position of features with reference to the midplane of the present invention, where the midplane lies in a direction substantially perpendicular to the rotational axis. That is, the inner surfaces of a feature are closer to the midplane that the outer surfaces of the same feature. Furthermore, the channels of the core can be grooves or recesses on the surface of the rotor core, or the channels can be cavities or holes within the rotor core. Preferably the rotor core is a laminated rotor core.

Preferably, the reinforcement member is substantially annular. Preferably, the reinforcement member is a ring. Preferably, the central aperture of the annular or ring-shaped reinforcement member has a radius substantially identical to the radius of the first surface of the short-circuit ring.

Preferably, the first surface of the short-circuit ring has an inner surface which is proximate to the rotor core. In this case, the short-circuit ring further comprises an outer surface which is removed or distal from the rotor core. Preferably, the outer surface opposes the inner surface. Preferably, the reinforcement member abuts or is adjacent to the inner surface of the short-circuit ring. The inner surface of the first surface, and in particular, the peripheral side of the inner surface, has been found to be particularly prone to failures or deformation during operation of the rotor in a high-speed electrical machine. As such, ensuring the reinforcement member abuts or is adjacent to the inner surface of the short-circuit ring has been found to be beneficial in preventing the squirrel cage and its rotor from failing. Preferably, the reinforcement member abuts or is adjacent to the outer edge.

Preferably, the reinforcement member abuts or lies adjacent to the majority of the first surface. Preferably, the reinforcement member abuts or lies adjacent to the entirety of the first surface. Preferably, the reinforcement member is complementary to the first surface. Preferably, the reinforcement member encircles the short-circuit ring. Preferably, the reinforcement member completely encloses the short-circuit ring.

Preferably, the first surface is curved. Preferably, the first surface is a peripheral surface of the short-circuit ring.

Alternatively, the first surface is an internal surface of the short-circuit ring or a central surface of the short-circuit ring. In these embodiments, the reinforcement member may be located partially or completely within the short-circuit ring. One way of manufacturing such an embodiment would be to cast the short-circuit ring around the reinforcement member.

Preferably, the reinforcement member additionally abuts or lies adjacent to a second surface of the short-circuit ring. Preferably, the second surface defines a plane that is substantially parallel to the plane defined by the first surface. Preferably, the second surface lies in a plane substantially perpendicular to the rotational axis of the squirrel-cage rotor. Preferably, the first surface of the short-circuit ring and the second surface of the short-circuit ring are directly connected to one another. Preferably, the reinforcement member abuts the area where the first surface of the short-circuit ring and the second surface of the short-circuit ring are directly connected to one another.

Preferably, the reinforcement member abuts or lies adjacent to the majority of the second surface. Preferably, the reinforcement member abuts or lies adjacent to the entirety of the second surface. Preferably, the reinforcement member is complementary to the second surface.

Preferably, the reinforcement member comprises a first portion and a second portion, where, in use, the first portion abuts or is adjacent to the first surface of the short-circuit ring and the second portion abuts or is adjacent to the second surface of the short-circuit ring. Preferably, the first portion occupied a plane substantially perpendicular to the plane occupied by the second portion. Preferably, the first portion and the second portion are integrally formed. Preferably, the peripheral surface of the second portion is flush with the rotor core.

Preferably, the short-circuit ring is a single piece. Preferably, the short-circuit ring is cast metal.

Preferably, the squirrel cage is integrally formed. Preferably, the squirrel cage is a single piece. Preferably, the squirrel cage is a cast squirrel cage.

One potential drawback associated with integrally formed or cast metal squirrel-cages is that the short-circuit ring is weaker and more prone to failure than squirrel-cages made from several individual components assembled post-fabrication. However, it can be beneficial to provide the squirrel-cage and short-circuit ring as cast metal, as it enables the squirrel-cage and the short-circuit rings to be manufactured by high-volume and high-throughput processes such as casting and die casting. Additionally, integrally formed squirrel cages do not require post-fabrication assembly, which lowers costs and manufacturing time. The present invention reinforces the squirrel cage and counteracts its relative weakness, which enables cast squirrel cages to be more widely used. Preferably, the squirrel-cage comprises a first material, and the reinforcement member comprises a second material. Preferably, the first material and the second material are different materials.

Preferably, the melting point of the second material is greater than the melting point of the first material. Preferably, the melting point of the second material is at least 100 K greater than the melting point of the first material. Preferably, the melting point of the second material is at least 200 K greater than the melting point of the first material. Preferably, the melting point of the second material is at least 400 K greater than the melting point of the first material.

Preferably, the tensile strength of the second material is greater than the tensile strength of the first material. Preferably, the tensile strength of the second material is at least 10 % greater than the tensile strength of the first material. Preferably, the tensile strength of the second material is at least 25 % greater than the tensile strength of the first material. Preferably, the tensile strength of the second material is at least 50 % greater than the tensile strength of the first material.

Preferably, the fatigue strength of the second material is greater than the fatigue strength of the first material. Preferably, the fatigue strength of the second material is at least 10 % greater than the fatigue strength of the first material. Preferably, the fatigue strength of the second material is at least 25 % greater than the fatigue strength of the first material. Preferably, the fatigue strength of the second material is at least 50 % greater than the fatigue strength of the first material.

Preferably, the Young’s modulus of the second material is greater than the Young’s modulus of the first material. Preferably, the Young’s modulus of the second material is at least 10 % greater than the Young’s modulus of the first material. Preferably, the Young’s modulus of the second material is at least 25 % greater than the Young’s modulus of the first material. Preferably, the Young’s modulus of the second material is at least 50 % greater than the Young’s modulus of the first material.

Preferably, the squirrel cage comprises copper. Preferably, the squirrel-cage consists of copper. Squirrel cages fabricated comprising or consisting of copper tend to be weaker than their aluminium equivalents. As such, the present invention enables a copper squirrel-cage of a given size to be used at speeds which were previously unobtainable. Preferably, the squirrel cage comprises aluminium. Preferably, the squirrel-cage consists of aluminium.

Preferably, the reinforcement member comprises a metal or alloy. Preferably, the reinforcement member consists of a metal or alloy. Preferably, the reinforcement member comprises steel. Preferably, the reinforcement member consists of steel. Preferably, the reinforcement member comprises high-strength steel. Preferably, the reinforcement member consists of high-strength steel. Preferably, the reinforcement member comprises titanium. Preferably the reinforcement member consists of titanium.

Preferably, the reinforcement member comprises a non-magnetic material. Preferably, the reinforcement member consists of a non-magnetic material.

Alternatively, the reinforcement member comprises a magnetic material. Preferably, the reinforcement member consists of a magnetic material.

Preferably, the radius of the short-circuit ring is less than the radius of the rotor core. Preferably, the distance between the peripheral surface and the central surface of the reinforcement member is the same as the difference in the radius of the short-circuit ring and the rotor core. Preferably, the radius of the reinforcement member is substantially the same as the radius of the rotor core. Preferably, the peripheral surface of the reinforcement member and the peripheral surface of the rotor core are flush when the rotor is assembled.

Preferably, the rotor core abuts the short-circuit ring. Preferably, the outer surface of the rotor core abuts the inner surface of the short-circuit ring.

Preferably, the reinforcement member is retained in position by an interference or friction fit. Preferably, the interference fit is with the short-circuit ring.

Preferably, the squirrel-cage rotor comprises two reinforcement members. Preferably, each of the two short-circuit rings abuts or lies adjacent to one of the reinforcement members. Preferably, each of the two reinforcement members comprises the optional features described herein. Preferably, the squirrel-cage rotor comprises two substantially identical reinforcement members. Preferably, the squirrel-cage rotor comprises two reinforcement members that are mirror images of one another. In this way, both of the short-circuit rings of the squirrel-cage rotor are reinforced. Preferably, the short-circuit ring further comprises an end ring, wherein the end ring extends from the short-circuit ring in a direction parallel to the rotational axis of the squirrel cage rotor. Preferably, the end ring is integrally formed with the squirrel cage. Preferably, the outer surface of the short-circuit ring and the peripheral surface of the end ring are both abutted by a reinforcement member.

According to a second aspect of the present invention, there is provided a turbogenerator comprising a squirrel-cage rotor as described herein.

Detailed Description

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows an angled view of the squirrel-cage rotor in accordance with a first embodiment of the present invention;

Figure 2 shows a cross-section of the squirrel-cage rotor of Figure 1 , where the cross- section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor;

Figure 3 shows a cross-section of the squirrel-cage rotor in accordance with a second embodiment of the present invention, where the cross-section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor;

Figure 4 shows a cross-section of the squirrel-cage rotor in accordance with a third embodiment of the present invention, where the cross-section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor;

Figure 5 shows a cross-section of the squirrel-cage rotor in accordance with a fourth embodiment of the present invention, where the cross-section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor;

Figure 6 shows a cross-section of the squirrel-cage rotor in accordance with a fifth embodiment of the present invention, where the cross-section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor; Figure 7 shows a cross-section of the squirrel-cage rotor in accordance with a sixth embodiment of the present invention, where the cross-section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor; and

Figure 8 shows a cross-section of the squirrel-cage rotor in accordance with a seventh embodiment of the present invention, where the cross-section has been taken in a radial plane which is parallel to and passes through the rotational axis of the squirrel-cage rotor.

Referring to Figures 1 and 2 there is depicted a first embodiment of the reinforced squirrel- cage rotor 10 in accordance with the present invention. The squirrel cage-rotor 10 is substantially cylindrical and comprises a squirrel-cage 12, rotor core 14 and a shaft 16. The rotor core 14 is formed from a stack of laminated magnetic sheets 18. The rotor core 14 and the stack of laminated magnetic sheets 18 are coaxially aligned with the rotational axis 24 of the squirrel-cage rotor 10.

The laminated magnetic sheets 18 are fabricated from electrical steel and are substantially planar and circular. The peripheral curved edge of the sheets 18 comprises an array of indentations 20, where all the indentations 20 are identically sized and evenly distributed around the peripheral radial edge of the laminated magnetic sheet 18. To form the cylindrical rotor core 14, the laminated magnetic sheets 18 are aligned and stacked together. The indentations 20 of each sheet 18 in the rotor core 18 stack are aligned to form channels 22. The channels 22 extend straight and continuously along the peripheral surface of the cylindrical rotor core 14 between its planar, circular, outer, end surfaces. Embodiments are envisaged where the channels 22 are slanted as they extend between the outer surfaces of the rotor core 14.

The shaft 16 is fixedly attached to the rotor core 14, such that the central axis of rotor core 14 and the central axis of the shaft 16 are coaxially aligned with the rotational axis 24 of the squirrel-cage rotor 10. In this way, the shaft 16 can be attached to external machinery such that, in use, the rotation of squirrel-cage rotor 10 inside a stator (not shown) can be used to generate an electrical current.

The squirrel cage 12 of the squirrel-cage rotor 10 comprises two short-circuit rings 26 and a number of conducting members 28. The short-circuit rings 26, and the squirrel cage 12 itself, are coaxially aligned with the rotor 10. Each of the two short-circuit rings 26 is substantially identical, and they are substantially circular annuluses. The short-circuit rings 26 have two curved surfaces: a peripheral surface and a central surface. The peripheral surface of each of the short-circuit rings 26 is at a constant distance from the rotational axis 24 around its circumference. This distance is slightly smaller than the radius of the rotor core 14. The central surface of the short-circuit rings is also at a constant radius from the rotational axis 24 around its circumference. The short-circuit rings 26 also comprise two planar opposing surfaces, an inner surface and an outer surface, such that the short-circuit rings 26 have a rectangular cross-section. In the squirrel-cage rotor 10, the inner face of the short-circuit rings 26 are adjacent to and abut the outer surface of the rotor core 14.

The short-circuit rings 26 are electrically and fixedly connected to one another by the conducting members 28. The conducting members 28 are housed within the channels 22, and each channel 22 has an associated conducting member 28. As such, the length of the conducting members 28 is substantially similar to the length of the rotor core 14. Also, the cross-section of conducting members 28 and the cross-section of the channels 22 are substantially similar, although the conducting members 28 are slightly smaller as they do not completely fill the channel 22. The peripheral surface of the conducting members 28 is offset from the peripheral edge of the rotor core 14. This offset is caused by the peripheral edge of the conducting members 28 being aligned with the peripheral edge of the short- circuit rings 26.

Both the short-circuit rings 26 and the conducting members 28 consist of a conducting material, in this case copper. In this embodiment, the short-circuit rings 26 and the conducting members 28 are formed by a casting process. In this process, the rotor core 14 is placed within a mould. Molten copper is poured in the mould to form the conducting members 28 and the short-circuit rings 26. In this way, the short-circuit rings 26 and conducting members 28 are provided as a single piece, and the squirrel cage 12 is integrally formed.

The squirrel-cage rotor 10 further includes two reinforcement members 30. The reinforcement members 30 are annular and have a rectangular cross-section which has a constant area and shape around the ring. The reinforcement members 30 have a curved peripheral surface, a curved central surface, a planar outer surface and a planar inner surface, where the radius of the central surface of the reinforcement members 30 is substantially the same as the radius of the peripheral surface of the short-circuit rings 26, such that they abut. Or in other words, the reinforcement members 30 are dimensioned such that the radii of the central apertures of the reinforcement members 30 are substantially similar to the radius of the curved surfaces of the short-circuit rings 26 which are parallel to the rotational axis 24.

In this embodiment, the reinforcement members 30 and their central surfaces are dimensioned such that they abut the entirety of the peripheral surfaces of the short-circuit rings 26 but do not extend beyond these surfaces. As such, the reinforcement members 30 are considered to encircle the short-circuit rings 26. The thickness of the reinforcement members 30 is such that their peripheral surfaces have radii substantially similar to the radius of the peripheral surface of the rotor core 14 and that their peripheral surfaces are flush.

The short-circuit rings 26 and reinforcement members 30 are dimensioned to be substantially similar in a direction parallel to the rotational axis 24. As such, the inner edges of the short-circuit rings 26 and the inner edges of the reinforcement members 30 are flush as they lie in the same plane. Moreover, the outer edges of the short-circuit rings 26 and the outer edges of the reinforcement members 30 are flush as they lie in the same plane.

The reinforcement members 30 consist of a material which is less likely to deform than the short-circuit rings 26 under the centrifugal forces present during operation of the high-speed machine. For example, the reinforcement members 30 may be made of a material with greater strength, a higher melting point, greater fatigue strength or greater Young’s modulus than the material of the short-circuit rings 26. Here, the reinforcement members 30 consist of non-magnetic high-strength steel whereas the squirrel cage 12 is fabricated from cast copper.

The reinforcement members 30 are attached to the squirrel-cage rotor 10 by placing the preformed reinforcement members 30 into the mould used to cast the squirrel cage 12. As such, the short-circuit rings 26 are cast and formed within the reinforcement members 30. Hence, the peripheral surfaces of the short-circuit rings 26 are complementary to the central surfaces of the reinforcement members 30. This creates a strong and robust fit between the reinforcement members 30 and the short-circuit rings 26.

Referring to Figure 3 there is depicted a second embodiment of a squirrel cage rotor 110 in accordance with the present invention. The rotor core 114, the shaft 116, the laminated magnetic sheets 118, the indentations 120, the channels 122, the rotational axis 124, and the conducting members 128 of the second embodiment are all substantially identical in structure and purpose as the equivalent features in the first embodiment of a squirrel cage- rotor 10.

The squirrel cage 112 of the second embodiment differs from the first embodiment as the short-circuit rings 126 of the second embodiment have a relatively larger radius than the first embodiment, such that the short-circuit rings 126 have the same radius as the rotor core 114. As such, the peripheral surface of the short-circuit ring 126 and the peripheral surface of the rotor core 114 are substantially flush.

The reinforcement members 130 are substantially similar to the first embodiment, in that its central surface is complementary to and abuts the entirety of the peripheral surface of the short-circuit ring 126. Also, in that, the inner surfaces and outer surfaces of the reinforcement members 130 are flush with the inner surfaces and outer surfaces of the short-circuit rings 126. The distance between the central surfaces and the peripheral surfaces of the reinforcement members 130 is larger in this embodiment. That is, the reinforcement member 130 is thicker in the radial direction, and it may afford greater reinforcement and resistance to deformation compared to radially thinner reinforcement members.

Referring to Figure 4 there is depicted a third embodiment of a squirrel cage rotor 210 in accordance with the present invention. The rotor core 214, the shaft 216, the laminated magnetic sheets 218, the indentations, the channels 222, the rotational axis 224, and the conducting members 228 of this third embodiment are all substantially identical in structure and purpose as the equivalent features in the first and second embodiment of the present invention. The squirrel cage 212 and the short-circuit rings 226 are substantially identical to the equivalent features of the second embodiment.

The reinforcement members 230 of the squirrel-cage rotor 210 are also substantially similar to the second embodiment. That is, the reinforcement members 230 are annular, and their central surface abuts and is complementary to the whole of the peripheral surfaces of the short-circuit rings 226. The outer surfaces of the reinforcement members 230 are also flush with the outer surfaces of the short-circuit rings 226 as is described for the first and second embodiments. However, in this embodiment, the reinforcement members 230 are differently dimensioned to the short-circuit rings 226 in the direction parallel with the rotational axis 224 of the rotor 210. The reinforcement members 230 extend past the inner surfaces of the short-circuit rings 226 and towards the midplane of the squirrel-cage rotor 210, such that the inner surfaces of the reinforcement members 230 are adjacent to and abut the peripheral surface of the rotor core 214. In this embodiment, the inner surfaces of the short-circuit rings 226 are reinforced both by the rotor core 214 which prevents the bulging towards the centre and by the reinforcement members 230 which extend over the peripheral side of the inner surfaces of the short-circuit rings 226 and prevent deformation in a radial direction.

Referring to Figure 5 there is depicted a fourth embodiment of a squirrel-cage rotor 310 in accordance with the present invention. The squirrel-cage 312, the rotor core 314, the shaft 316, the laminated magnetic sheets 318, the indentations, the channels 322, the rotational axis 324, the short-circuit rings 326 and the conducting members 328 of the fourth embodiment are all substantially identical in structure and purpose as the equivalent features in the second and third embodiments of the present invention.

The squirrel-cage rotor 310 also comprises two reinforcement members 330. The reinforcement members 330 are annular and have a cross-section that is substantially L- shaped. The reinforcement members 330 comprise two portions, a radial portion 332 and an end portion 334. The radial portions 332 are complementary to and abut the whole of the peripheral surfaces of the short-circuit rings 326. The end portions 334 are complementary to and abut the whole of outer surfaces of the short-circuit rings 326. The radial portions 332 and the end portions 334 meet at a right angle to give the reinforcement members 330 a substantially L-shaped cross-section. The reinforcement members 330 also abut the edge where the outer surface and the peripheral surface of the short-circuit ring 326 meet. The short-circuit rings 326 are prevented from deformation outwardly or radially by the reinforcement members 330. In this way, the reinforcement member 330 completely encloses the short-circuit ring 326.

Referring to Figure 6 there is depicted a fifth embodiment of a squirrel-cage rotor 410 in accordance with the present invention. The squirrel-cage 412, the rotor core 414, the shaft 416, the laminated magnetic sheets 418, the indentations, the channels 422, the rotational axis 424, the short-circuit rings 426 and the conducting members 428 of the fifth embodiment are all substantially identical in structure and purpose as the equivalent features in the first and fourth embodiments of the present invention.

In this embodiment, the squirrel-cage rotor 410 comprises annular end rings 436. The end rings 436 are connected to and extends from the outer surfaces of the short-circuit rings 426. The end rings 436 are integrally formed with the short-circuit rings 426 and consist of the same conducting material. The end rings 436 are fabricated concomitantly with the short- circuit rings 426 when the squirrel cage 412 is cast. The end rings 436 are coaxially aligned with the rotor 410, rotor core 414, shaft 416 and short-circuit rings 426. The radii of the central surfaces of the end rings 436 are larger than the radii of the central surface of the short-circuit rings 426. However, the radii of the peripheral surfaces of the end rings 436 are smaller than the radii of the peripheral surface of the short-circuit rings 426. In other words, the end rings 436 are thinner in the radial direction than the short-circuit rings 426. Two separate areas of the outer surfaces of the short-circuit rings 426 are not connected to the end rings 436, a central area and a peripheral area, which accommodate first reinforcement members 438 and a second reinforcement members 440, respectively.

The first reinforcement members 438 and the second reinforcement members 440 are both annular rings of a material that is higher in strength than the squirrel cage 412. For example, non-magnetic steel. Both the first reinforcement members 438 and the second reinforcement members 440 have a planar inner surface and a planar outer surface. The first reinforcement members 438 have smaller radii than the second reinforcement members 440 and, as such, they can alternatively be described as the central reinforcement members 438 and the peripheral reinforcement members 440, respectively.

The first reinforcement members 438 are positioned such that their peripheral surfaces abut and are complementary to the central surfaces of the end rings 436. Additionally, the central surfaces of the first reinforcement members 438 are proximate the central surfaces of the short-circuit rings 426. The inner surfaces of the first reinforcement members 438 are complementary to and abut the outer surfaces of the short-circuit rings 426. The outer surfaces of the first reinforcement members 438 are flush with the outer surfaces of the end rings 436.

The second reinforcement members 440 are positioned such that their central surfaces abut and are complementary to the peripheral surfaces of the end rings 436. The peripheral surfaces of the second reinforcement members 440 are adjacent to and flush with the peripheral surfaces of short-circuit rings 426. The inner surfaces of the second reinforcement members 440 are complementary to and abut the outer surfaces of the short- circuit rings 426. The outer surfaces of the second reinforcement members 440 are flush with the outer surfaces of the end rings 436.

Since the first reinforcement members 438 and the second reinforcement members 440 both abut and are complementary to the outer surfaces of the short-circuit rings 426, they reinforce the short-circuit rings 426. In this embodiment, the inner surface of the short-circuit ring 426 is reinforced by the rotor core 414, and the short-circuit ring 426 is reinforced on both the inner and outer sides.

Referring to Figure 7 there is depicted a sixth embodiment of a squirrel-cage rotor 510 in accordance with the present invention. The squirrel-cage 512, the rotor core 514, the shaft 516, the laminated magnetic sheets 518, the indentations, the channels 522, the rotational axis 524, the short-circuit rings 526 and the conducting members 528 of the sixth embodiment are all substantially identical in structure and purpose as the equivalent features in the first, fourth and fifth embodiments of the present invention. This embodiment also comprises end rings 536 which are substantially identical to the equivalent feature of the fifth embodiment.

Additionally, the first reinforcement members 538 and the second reinforcement members 540 are similar to the fifth embodiment, with the notable exception that the outer surfaces of the first reinforcement members 538 and the outer surfaces of the second reinforcement members 540 are connected by third reinforcement members 542. As such, on each side of the rotor 510 a first reinforcement member 538, a second reinforcement member 540 and a third reinforcement member 542 is integrally formed as a single piece.

The third reinforcement members 542 are substantially annular. The third reinforcement members each have an inner surface, and an outer surface, wherein the inner surfaces of the reinforcement members 542 abut the outer surfaces of the end ring 536.

As such, the short-circuit rings 526 are reinforced by the first reinforcement members 538 and second reinforcement members 542 directly as described for the fifth embodiment. The third reinforcement members 542 act to help secure the first reinforcement members 538 and second reinforcement members 542 in place and transfers stress and strain between them to spread the load evenly. Thus, the third reinforcement members 542 improve the reinforcement of the short-circuit rings 526.

Referring to Figure 8 there is depicted a seventh embodiment of the squirrel-cage rotor 610 in accordance with the present invention. The squirrel-cage 612, the rotor core 614, the shaft 616, the laminated magnetic sheets 618, the indentations, the channels 622, the rotational axis 624, the short-circuit rings 626 and the conducting members 628 of the squirrel-cage 610 are all substantially identical in structure and purpose as the equivalent features in the first embodiment of the present invention. The squirrel-cage 610 also comprises two reinforcement members 630. In a similar way to the first embodiment, the inner surface of the reinforcement members 630 of this embodiment abuts the outer surface of the rotor core 614. Furthermore, the radius of the peripheral surface of the reinforcement member 630 is substantially identical to the radius of the peripheral surface of the rotor core 614, such that these two surfaces are substantially flush and aligned.

The reinforcement members 630 of this squirrel-cage rotor 610 also comprises two portions, a radial portion 632 and an end portion 634, in a manner similar to the fourth embodiment. The radial portions 632 are complementary to and abut the whole of the peripheral surfaces of the short-circuit rings 626, and the end portions 634 are complementary to and abut the whole of outer surfaces of the short-circuit rings 626. Furthermore, the radial portions 632 and the end portions 634 meet at a right angle to give the reinforcement members 630 a substantially L-shaped cross-section. The reinforcement members 630 also abut the edge where the outer surface and the peripheral surface of the short-circuit ring 626 meet. The short-circuit rings 326 are prevented from deformation outwardly or radially by the reinforcement members 630. In this way, the reinforcement member 630 completely encloses the short-circuit ring 626.