FIELD OF THE INVENTION The present invention relates to magnetic anisotropy in rotating electrical machines, i.e. both motors and generators, in particular a method for

manufacturing an induction rotor. The invention also relates to a corresponding induction motor or generator. BACKGROUND OF THE INVENTION

Rotary induction or asynchronous motors are AC electric motors, where an electric current in the central, rotating rotor is provided by electromagnetic induction from a magnetic field in the surrounding fixed stator. Therefore, no mechanical commutation is required for the central rotor as with synchronous motors i.e. DC motors. The electric current in the rotor of an induction motor is required to give a resulting torque in combination with the rotating magnetic field from the stator. Inductions motors play a major part in modern motor application ranging from small motors, such as actuators for various kind of displacement and pumps. Inductions motor are often the fixed speed type motor, but variable speed is possible with variable frequency drive.

Induction rotors can generally be of the squirrel-cage or wound rotor type, though other variants are known. Three phase squirrel-cages induction motors are often used in industrial applications for their stable operation, whereas single phase squirrel-cage induction motors may be useful for smaller applications.

Induction rotors are typically manufactured using laminates of so-called electrical steel sheets with a relative high content of silicon, e.g. 3%, to promote magnetic susceptibility. The laminates also control or reduce unwanted Eddy currents in the rotor.

Due to the high rotational frequency of the rotor during operation it is of importance that the mounting of the rotor is very stable and precise, e.g. in the center of the stator. However, it is also important for operational stability that the rotor itself has a high degree of symmetry, both with respect to the dimensions and mechanical properties, but also with respect to the magnetic properties during operation. Hence, an improved method for method for manufacturing rotating induction machines would be advantageous, and in particular a more efficient and/or reliable induction machine would be advantageous.

OBJECT OF THE INVENTION

It is a further object of the present invention to provide an alternative to the prior art.

In particular, it may be seen as an object of the present invention to provide a method for manufacturing rotating induction machines that solves and/or mitigate the above mentioned problems of the prior art with the anisotropic magnetic properties of the rotor and/or stator.

SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method for

manufacturing an induction rotor of the squirrel-cage type, the induction rotor being arranged for forming an induction motor when fitted in an associated stator with a plurality (N) of poles positioned at an inner circumference of the stator, the method comprising :

-providing a rotor core with a plurality (M) of rotor bars extending radially outwards from a central shaft of the rotor, and

-providing a conducting metal structure around the rotor core, the conducting metal structure forming a closed circuit in a cage-like pattern, wherein the rotor core comprises a first plurality (O) of laminates, the outer shape of the laminates forming the rotor bars when the laminates are assembled to form the rotor core, each laminate comprising a magnetic susceptible electrical steel sheet, at least a sub-set (P) of the first plurality (O) of laminates having an anisotropic magnetic susceptibility, the anisotropic magnetic susceptibility being with respect to a symmetric cylindrical magnetic susceptibility around the central shaft of the rotor, wherein the method further comprising :

-rotating relatively to each other a second plurality (Q) of laminates of the said sub-set (P) in the rotor core around the central shaft of the rotor so as to, at least partly, compensate for the anisotropy in the magnetic susceptibility in the said sub-set (P) of laminates when assembling the first plurality (O) of laminates to form the said rotor core. The invention is particularly, but not exclusively, advantageous for obtaining an improved method for manufacturing rotors for induction motors in a simple, yet effective manner. Thus, the effect of magnetic anisotropy in the laminates of the rotor may be reduced, optionally eliminated, by implementing the present invention.

In a second aspect of the invention, the invention relates to an induction rotor manufactured according to the first aspect. In a third aspect, the invention relates to an induction motor, or an induction generator, manufactured according to the first aspect, and/or comprising a rotor according to the second aspect.

The invention is particularly, but not exclusively, advantageous for obtaining an improved rotor having improved stability during operation in an induction motor. In particular, the rotor may have less magnetic offset when operated in an induction motor. More, there may be reduced axial forces along the rotor shaft, i.e. the so-called "knocking" may be reduced.

Within the context of the present invention, it is to be understood that the term magnetic susceptibility may be represented by related or similar measures, e.g. a relative measure of magnetic susceptibility, or a relative, or an absolute, measure of magnetic permeability, μ. Within the context of the present invention, it is to be understood that the first plurality comprises said sub-set (P) of laminates, and further that the subset (P) comprises the second plurality (Q) of laminates. Thus, the second plurality Q may be a further subset of the subset P. In a particular embodiment, the first plurality (O), the subset (P) and the second plurality (Q) may be the same group of laminates i.e. when all the laminates of the rotor (potentially) have a significant anisotropy in their magnetic susceptibility, and all the laminates are therefore rotated relative to each other according to the present invention.

In a particular advantageous embodiment, rotating of the second plurality (Q) of laminates of the said sub-set (P) may also, at least partly, reduces an effective axial offset of the magnetic center of the rotor core when assembling the first plurality (O) of laminates to form the said rotor core. It is expected that this could potentially improve the stability of the rotor, and in consequence the durability of the corresponding motor or generator with a rotor according to the invention.

Preferably, the second plurality (Q) of laminates of the said sub-set (P) may be rotated relative to each other evenly around the entire circumference of the rotor core to optimize the compensation of the undesired anisotropy in the magnetic susceptibility in the said sub-set (P) of laminates when assembling the first plurality (O) of laminates to form the said rotor core, e.g. each of the laminate in the second plurality (Q) may be rotated approximately 360 degrees divided with the number of laminates to be rotated, provided of course that such a rotation is possible for the specific rotor. Shifting to the nearest possible rotational displacement may be an option.

Preferably, the second plurality (Q) of laminates of the said sub-set (P) may be rotated relative to each other by a natural number of rotor bars when assembling the first plurality (O) of laminates to form the said rotor core in order to have a method of manufacturing that is relatively easy to implement.

Typically, two or more adjacent laminates within the second plurality (Q) of laminates may be rotated together around the central shaft of the rotor when assembling the first plurality (O) of laminates to form the said rotor core e.g. 5, 10, 15, 20, or 25 adjacent laminates may rotated together depending for example on the total number of laminates in the rotor.

More preferably, two or more adjacent laminates within the second plurality (Q) of laminates are rotated less than 180 degrees relative to each other around the central shaft of the rotor when assembling the first plurality (O) of laminates to form the said rotor core, more particularly less than approximately 170, 160, 150, 140, 130, 120, 110, 100, 90, 80, 70, 60, 50, 40, 30, 20, or 10 degrees relative to each other. It is to be understood that both directions of rotations may be applied within the context of the present invention.

Beneficially, at least part of the conducting metal structure around the rotor core along the central shaft of the rotor may be skewed relative to the central shaft, i.e. a so-called skewed induction rotor.

Advantageously, the first plurality (O) of laminates may be manufactured in cold- rolled strips. More advantageously, the first plurality (O) of laminates may be manufactured in non-grain oriented electrical steel. Alternatively, the first plurality (O) of laminates may be manufactured in grain oriented electrical steel, both kinds of electrical steel being useful for laminate manufacturing.

The invention may relate in any of the above aspects to an induction motor, preferably of the wound-type or the squirrel-cage type, preferably of the skewed squirrel-cage type. Generally, any induction motor type may be contemplated for beneficial application with the present invention. Thus, induction motor types of the cage induction motor, such as squirrel cage induction motor, or an induction motor with secondary cage (squirrel cage) winding(s). For more types of induction motors, the skilled person is referred to the IEC 60034-1, which is hereby incorporated by reference in its entirety.

Thus, the wound-rotor induction motor may be an induction motor with secondary polyphase coil winding(s). Other motor types include slip-ring induction motor or brushless wound-rotor induction motor. More particularly, repulsion start induction motor or repulsion induction motor may be considered. As the skilled person would readily understand, the teaching and principle of the present invention can be applied both for induction motors and for induction generators depending on the circumstances. The first, second, and third aspect of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in more detail with regard to the

accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible

embodiments falling within the scope of the attached claim set.

Figure 1 is a schematic perspective view of a rotating electrical machine, e.g. an induction motor, according to the present invention, Figure 2 is a schematic perspective view of an induction rotor of the squirrel-cage type,

Figure 3 is a schematic side-view of an induction rotor showing the laminate structure of the rotor core,

Figure 4 is a schematic perspective drawing for implementing the method for manufacturing an induction rotor according to the present invention,

Figure 5 is cut-out extended section along a rotor shaft for demonstrating the magnetic offset in the prior art, and

Figure 6 is cut-out extended section along a rotor shaft for demonstrating the magnetic offset in rotor according to the present invention. DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 is a schematic perspective view of a rotating electrical machine 100, e.g. an induction motor, according to the present invention. The induction motor is preferably of the squirrel-cage type, where the induction rotor 10 is rotatably arranged in relation to a corresponding stator 20 with a plurality N of poles positioned at an inner circumference of the stator. The rotor has a plurality M of rotor bars extending radially outwards from a central shaft 15 of the rotor, and may provide a conducting metal structure around the rotor core, the conducting metal structure forming a closed circuit in a cage-like pattern, preferably a squirrel-cage. For further introduction to principles and operation of electrical machines, the skilled person is referred to Electrical Machines, Drives, and Power Systems, by Theodore Wildi, Prentice Hall, 2002, which is hereby incorporated by reference in its entirety.

Figure 2 is a schematic perspective view of an induction rotor 10 of the squirrel- cage type. Along the rotor shaft 15 different axial positions are indicated with letters A, B, C, D, and E, which will be used in connection with the results shown in Figure 7 below. The induction rotor comprising a rotor core with a plurality M of rotor bars extending radially outwards from a central shaft of the induction rotor, and a conducting metal structure 17 (only schematically indicated with dashed lines) around the rotor core, preferably the conducting metal structure forms a closed circuit in a cage-like pattern. Alternatively, a wound rotor configuration may be implemented with the present invention.

Figure 3 is a schematic side-view of an induction rotor 10 showing the laminate structure of the rotor core. The rotor core comprises a first plurality O of laminates 16, the outer shape of the laminates forming the rotor bars when the laminates are assembled to form the rotor core, each laminate comprising a magnetic susceptible electrical steel sheet as the skilled person in electrical machines will understand.

Figure 4 is a schematic perspective drawing for implementing the method for manufacturing an induction rotor according to the present invention. As shown in Figure 3, the rotor core comprises a first plurality O of laminates, each laminate comprising a magnetic susceptible electrical steel sheet. The sub-set P, here six laminates 16 as shown, can have an anisotropic magnetic susceptibility with respect to a symmetric cylindrical magnetic susceptibility around the central shaft 15 of the rotor 10, cf. Figures 1-2.

The method is then particularly in that a rotation is performed within the subset P. In the illustrative drawing, a second plurality Q of laminates, 16a, 16b, and 16c, the second plurality Q being a further subset of the subset P, are rotated around the central shaft of the rotor so as to, at least partly, compensate for the anisotropy in the magnetic susceptibility in the said sub-set (P) of laminates when assembling the first plurality (O) of laminates to form the said rotor core. In one example, this may be done by keeping laminate 16c fixed and relatively rotating laminate 16b an angle V, and rotating laminate 16a the double angle i.e. 2*V (schematically indicated by arrow A) during the manufacturing. The angle V could be approximately 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170 or 180 degrees relative to each other.

Figure 5 is cut-out extended section along a rotor shaft for demonstrating the magnetic offset found in the prior art rotors. The rotor shaft is along the central horizontal axis going through figure. The figure shows how the so-called magnetic gravity point may be offset from an ideal configuration, both in the upper scenario and the lower scenario. In the upper scenario, the center of the magnetic pole in the rotor is in Position 1, whereas in the lower scenario, the center of the magnetic pole in the rotor is in Position 2. In Position 1, the center is to the left of the balanced centerline, and in Position 2, the center is to the right of the balanced centerline. When rotating the rotor, the change between e.g . Position 1 and Position 2 may cause axially displacement of the center of the magnetic pole. During operation of the induction motor, particularly a skewed induction motor of the squirrel cage type, this may result in periodic axial movement because of the pole pass, which is believed at least partly to be due to the magnetic anisotropy of the laminates in the rotor core. It has been found by the inventor that the period of this axial movement can be described as a multiple of the slip frequency in the induction motor. Figure 6 is cut-out extended section along a rotor shaft for demonstrating the magnetic offset in a rotor according to the present invention. Thus, the invention reduces an effective axial offset of the magnetic center of the rotor core when assembling the first plurality O of laminates to manufacture the rotor core. Thus, by rotating the laminates one, or more, rotor slots, one or more times, along the lamination stack, there is obtained an advantage. It results in less offset of the magnetic gravity point as schematically shown in the Figure. In Figure 6, the earned balanced area is shown as the patterned areas. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.