An electric machine element

Abstract

An electric machine element (301) comprises a core structure (302) and a winding (310) having coil sides (303a, 303b, 304a, 304b) and end-windings (307) The winding comprises an even number of parallel branches (308a, 308b) each having a current supply point (322a, 322b). The current supply points of different ones of the parallel branches are at different locations in the winding to balance leakage currents (i) from the parallel branches via parasitic capacitances to suppress flux asymmetry and thereby to eliminate, or at least reduce, circulating bearing currents.

Description

[0001] An electric machine element

[0002] Technical field

[0003] The invention relates generally to rotating electric machines. More particularly, the invention relates to an electric machine element comprising a core structure and windings. The electric machine element can be, for example, a part of a stator of an alternating current “AC” machine. Furthermore, the invention relates to an electric machine.

[0004] Background

[0005] The problem of bearing currents in rotating electric machines has been identified a long time ago and different solutions are still under search. Bearing currents may lead to a malfunction of bearings and thereby significantly reduce the lifespan of rotating electric machines. Bearing currents are typically classified into circulating and non-circulating ones. Circulating bearing currents are still an issue, especially in large motors and generators, and must be addressed to ensure reliable and noninterrupted operation of an entire electric system. Known countermeasures against circulating bearing currents are briefly discussed below.

[0006] A known solution to reduce circulating bearing currents comprises precise magnetic circuits and well-centered geometry. In conjunction with earlier electric machines, when manufacturing techniques were not mature, imbalanced magnetic flux, which is the original reason for circulating bearing currents, could often occur due to imperfect geometry which led to the voltage being induced between shaft ends. In modem electric drive and generator systems, magnetic flux imbalance occurs due to stator current non-uniform distribution along the stator winding rather than because of magnetic circuit geometry. Thus, fine tuning of machine geometry can no longer eliminate circulating bearing currents. Another known solution against circulating bearing currents is to use shaft grounding carbon brushes via which currents can bypass bearings and / or other sensitive parts. The drawbacks of carbon brushes are a need for springs, a considerable size, a need for maintenance, a possible need to be installed outside an electric machine, and a sensitivity toenvironmental conditions especially to oil and dust, which may lead to contact loss. A third known approach is to use a ring with electrically conducting microfibers sticking out from the inner diameter of the ring. The ring is assembled to a stationary part of an electrical machine in such a way that the microfibers touch a shaft providing a path for currents, thus reducing currents through bearings and / or other sensitive parts. Rings of the kind mentioned above require extra space and act as a separate element of an electric machine, which makes the whole structure larger and more complicated. A fourth known approach is to use electrically conductive grease that allows charged particles to flow through bearings, which prevents the accumulation of significant amounts of charge on bearing rings. Consequently, no arc between the bearing rings with a strong discharging flow should take place. The disadvantages of conducting greases are lower lubricating properties compared to the regular bearing greases and a higher price. A fifth known approach is to use bearings which are electrically insulated from the frame of an electric machine. On one hand, an insulator layer between a bearing and the frame may need to be relatively thick to achieve sufficient impedance against capacitive coupling of high frequency currents which can be circulating bearing currents especially in systems driven with frequency converters having high switching frequency. On the other hand, it may be challenging to achieve sufficient mechanical strength when the insulator layer is thick. Hybrid bearings have inner and outer rings made of stainless steel but with rolling elements made of ceramic. Hybrid bearings can effectively block either galvanic and capacitive currents, but they are very expensive, and in addition, the mechanical strength of ceramic balls is lower than that of rolling elements made of steel. A sixth known approach is to use special modulation strategies in frequency converters to minimize common mode voltage, reduce du / dt values, and / or improve a harmonic composition of output voltage. This approach, however, does not allow full elimination of common mode voltage and other negative pulse width modulation “PWM” effects because of hardware limitations and / or a possible drop in converter efficiency. Also, this approach does not eliminate classical circulating bearing currents caused by a flux asymmetry. A seventh known approach is to apply special circuit layouts for converters to allow elimination of common mode voltage and improve the output voltage waveform, especially when three- or more levels converter is employed. However, a special circuit requiresdevelopment of a new converter topology and extra components, which leads to converter price growth and may also reduce efficiency.

[0007] Summary

[0008] The following presents a simplified summary to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

[0009] In this document, the word “geometric” when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric point, a straight or curved geometric line, a geometric plane, a non-planar geometric surface, a geometric space, or any other geometric entity that is zero, one, two, or three dimensional.

[0010] In accordance with the invention, there is provided a new electric machine element that can be, for example, a part of a stator or a rotor of an alternating current “AC” machine that can be e.g. an induction machine, a permanent magnet machine, an electrically excited synchronous machine, or a synchronous reluctance machine.

[0011] An electric machine element according to the invention comprises a core structure and a winding having coil sides and end-windings. The winding comprises an even number of parallel branches each having a current supply point. The current supply points of different ones of the parallel branches are at different locations in the winding to balance leakage currents from the parallel branches via parasitic capacitances to suppress flux asymmetry and thereby to eliminate, or at least reduce, circulating bearing currents.

[0012] The winding can be, for example, a winding for a radial-flux electric machine so that all the coil sides are on a stator-rotor airgap side of the core structure. The coil sides can be in slots of the core structure, or the winding can be an airgap winding. In this exemplifying case, circulating bearing currents are reduced by supplying currents to the parallel branches at the opposite axial ends of the electric machine element. Inthis document, a winding in which all the coil sides are on the stator-rotor airgap side of the core structure is called a non-drum winding.

[0013] For another example, the winding can be a drum-winding for a radial-flux electric machine so that first half of the coil sides are on the stator-rotor airgap side of the core structure and second half of the coil sides are on a yoke side of the core structure. In the case of the drum winding, circulating bearing currents are reduced by supplying currents, at an axial end of the electric machine element, to a coil side on the stator-rotor airgap side and, at the same axial end, to a coil side on the yoke side wherein the above-mentioned coil sides belong to different ones of the branches in parallel with each other.

[0014] A more detailed explanation about phenomena related to the reduction of circulating bearing currents will be presented later in this document.

[0015] In accordance with the invention, there is also provided a new electric machine. An electric machine according to the invention comprises a stator and a rotor rotatably supported with respect to the stator. At least one of the stator and the rotor comprises an electric machine element according to the invention.

[0016] Exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.

[0017] Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in conjunction with the accompanying drawings.

[0018] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features.

[0019] The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated.Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

[0020] Brief description of figures

[0021] Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which:

[0022] Figures 1a, 1b, and 1c illustrate schematically an electric machine element according to the prior art and presented in this document to describe phenomena related to circulating bearing currents,

[0023] Figures 2a, 2b, and 2c illustrate schematically an electric machine element according to the prior art and presented in this document to describe phenomena related to circulating bearing currents,

[0024] Figures 3a, 3b, and 3c illustrate schematically an electric machine element according to an exemplifying and non-limiting embodiment,

[0025] Figure 4 illustrates schematically an electric machine element according to an exemplifying and non-limiting embodiment, and

[0026] Figures 5a, 5b, and 5c illustrate schematically an electric machine element according to an exemplifying and non-limiting embodiment.

[0027] Brief description of phenomena related to circulating bearing currents as a prelude to description of exemplifying and non-limiting embodiments Figures 1a, 1b, and 1c illustrate schematically an electric machine that comprises an electric machine element 101 according to the prior art. This electric machine is a radial-flux inner-rotor electric machine and is presented in this document to describe phenomena related to circulating bearing currents. The axial direction of the electric machine is parallel with the z-axis of a coordinate system 199. Figure 1a shows a section view where the geometric section plane is parallel with the yz-plane of the coordinate system 199. The electric machine element 101 is a part of a stator120, a rotor 114, and bearings 115a and 115b that are configured to rotatably support the rotor 114 with respect to the stator 113. The rotor 114 comprises a shaft 116 and an electromagnetically active part configured to interact with the stator 113.

[0028] The electric machine element 101 comprises a core structure 102, a winding 110 having coil sides and end-windings 107. In this example, the winding 110 is a nondrum winding where all coil sides are on a stator-rotor airgap side of the core structure 102. The coil sides are in slots of the core structure 102, but an airgap winding could also be possible. In figure 1a, two of the coil sides are denoted with references 103 and 104. The winding 110 can be for example a three-phase winding, and the electric machine can be e.g. an induction machine, a permanent magnet machine, an electrically excited synchronous machine, or a synchronous reluctance machine.

[0029] An exemplifying path of current supplied to the winding 110 is depicted with line 108 in figure 1a. The current supplied to the winding is denoted as i + Ai. As the current is alternating “AC” current which may contain higher harmonic components, a part of the current leaks away from the winding 110 to the rotor 114 and to the core structure 102 mainly via parasitic capacitances between the winding 110 and the rotor 114 and via parasitic capacitances between the winding 110 and the core structure 102. The part that leaks out is denoted as Ai. In figure 1a, a dash-and-dot line 117 depicts non-circulating bearing current which leaks capacitively from the winding 110 to the rotor 114 and flows through the bearings and the frame to the ground. A dotted line 118 depicts stator ground current which leaks capacitively from the winding 110 to the core structure 102 and flows through the frame 120 to the ground. The total ground current is the part Ai that leaks out from the winding 110 via the parasitic capacitances.

[0030] Figure 1b shows a section view of the electric machine element 101 and the rotor 114 so that the section is taken along a line A1-A1 shown in figure 1a, and figure 1c shows a section view of the electric machine element 101 and the rotor 114 so that the section is taken along a line A2-A2 shown in figure 1a. The geometric section planes related to figures 1b and 1c are parallel with the xy-plane of the coordinate system 199. At the location of the line A1-A1 , the current in the upper coil side is i +Ai and the current in the lower coil side is i because the part Ai has leaked out from the upper and lover coil sides. Thus, the magnetomotive force “MMF” around a geometric circle 121a is proportional to Ai. At the location of the line A2-A2, the current in the upper coil side is i + Ai / 2 and the current in the lower coil side is i + Ai / 2 because the half of the part Ai has leaked out in the upper coil side and it is assumed here that no significant leakage takes place in the end-windings 107. Thus, the magnetomotive force around a geometric circle 121b is substantially zero. Therefore, as a net effect, a circulating magnetic flux in the core structure 102 is generated. As the current is alternating “AC” current, the circulating magnetic flux is altering its direction between clockwise and counterclockwise directions. The shaft 116, being an electric conductor and located in the center of the circulating AC magnetic flux, will experience an induced electromotive force “EMF” according to the Faraday’s induction law. This induced EMF, or voltage, becomes a root cause of circulating bearing currents. If even a part of the shaft 116 has the voltage induced on it, like in this example, the potential difference propagates between the shaft ends causing current that circulates via the shaft 116 and closes a current loop via the bearings 115a and 115b and the frame 120. In figure 1a, the circulating bearing currents are depicted with a dashed line 119.

[0031] Figures 2a, 2b, and 2c illustrate schematically an electric machine that comprises an electric machine element 201 according to the prior art. This electric machine is a radial-flux inner-rotor electric machine and is presented in this document to describe phenomena related to circulating bearing currents. The axial direction of the electric machine is parallel with the z-axis of a coordinate system 299. Figure 2a shows a section view where the geometric section plane is parallel with yz-plane of the coordinate system 299. The electric machine element 201 is a part of a stator 213 of the electric machine. Furthermore, the electric machine comprises a frame 220, a rotor 214, and bearings 215a and 215b configured to rotatably support the rotor 214 with respect to the stator 213. The rotor 214 comprises a shaft 216 and an electromagnetically active part configured to interact with the stator 213.

[0032] The electric machine element 201 comprises a core structure 202, a winding 210 having coil sides and end-windings 207. In this example, the winding 210 is a drumwinding where first half of the coil sides are on a stator-rotor airgap side of the corestructure 202 and second half of the coil sides are on a yoke side of the core structure. In figure 2a, two of the coil sides on the stator-rotor airgap side are denoted with references 203 and 204, and two of the coil sides on the yoke side are denoted with references 205 and 206. The coil sides on the stator-rotor airgap side are in slots of the core structure 202, but an airgap winding could also be possible. Correspondingly, the coil sides on the yoke side are in slots of the core structure 202, but these coil sides could be mechanically supported in some other way. The winding 210 can be for example a three-phase winding, and the electric machine can be e.g. an induction machine, a permanent magnet machine, an electrically excited synchronous machine, or a synchronous reluctance machine.

[0033] An exemplifying path of current supplied to the winding 210 is depicted with line 208 in figure 2a. The current supplied to the winding is denoted as i + Ai. As the current is AC current which may contain higher harmonic components, a part of the current leaks away from the winding 210 to the rotor 214 and to the core structure 202 mainly via parasitic capacitances between the winding 210 and the rotor 214 and via parasitic capacitances between the winding 210 and the core structure 202. The part that leaks out is denoted as Ai. In figure 2a, a dash-and-dot line 217 depicts non-circulating bearing current which leaks capacitively from the winding 210 to the rotor 214 and flows through the bearings and the frame 220 to the ground. A dotted line 218 depicts stator ground current which leaks capacitively from the winding 210 to the core structure 202 and flows through the frame 220 to the ground. The total ground current is the part Ai that leaks out from the winding 210 via the parasitic capacitances.

[0034] Figure 2b shows a section view of the electric machine element 201 and the rotor 214 so that the section is taken along a line A1-A1 shown in figure 2a, and figure 2c shows a section view of the electric machine element 201 and the rotor 214 so that the section is taken along a line A2-A2 shown in figure 2a. The geometric section planes related to figures 2b and 2c are parallel with the xy-plane of the coordinate system 299. At the location of the line A1-A1, the current in the coil side 203 is i + Ai, the current in the coil side 205 is i + Ai / 2 because a part Ai / 2 has leaked out from the coil sides 203 and 205, the current in the coil side 206 is i + Ai / 2, and the currentin the coil side 204 is i because a part Ai / 2 has leaked out from the coil sides 206 and 204. Thus, at the location of the line A1-A1 , there is a greater magnetomotive force to generate magnetic flux in the clockwise direction in the core structure 202 than in the counterclockwise direction. At the location of the line A2-A2, the current in the coil side 203 is i + 3Ai / 4 because a part Ai / 4 has leaked out from the coil side 203, the current in the coil side 205 is i + 3Ai / 4 because it is assumed here that no significant leakage takes place in the end-windings 207, the current in the coil side 206 is i + Ai / 4 because a part 3Ai / 4 has leaked out from the coil sides 203, 205, and 206, and the current in the coil side 204 is i + Ai / 4 because it is assumed here that no significant leakage takes place in the end-windings 207. Thus, at the location of the line A2-A2, there is a greater magnetomotive force to generate magnetic flux in the clockwise direction in the core structure 202 than in the counterclockwise direction. Therefore, in both the locations of the lines A1-A2 and A2-A2, there is a greater magnetomotive force to generate magnetic flux in the clockwise direction in the core structure 202 than in the counterclockwise direction. As a net effect, a circulating magnetic flux in the core structure 202 is generated. As the current is alternating “AC” current, the circulating magnetic flux is altering its direction between clockwise and counterclockwise directions. This causes circulating bearing currents. In figure 1a, the circulating bearing currents are depicted with a dashed line 219.

[0035] As illustrated with the above-presented examples, circulating bearing currents may occur even if no asymmetry is present in the geometry of magnetic circuits.

[0036] Description of exemplifying and non-limiting embodiments

[0037] The specific examples provided in the description below should not be construed as limiting the scope and / or the applicability of the invention. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

[0038] Figures 3a, 3b, and 3c illustrate schematically a radial-flux electric machine that comprises an electric machine element 301 according to an exemplifying and nonlimiting embodiment. The axial direction of the electric machine is parallel with the z-axis of a coordinate system 399. Figure 3a shows a section view where thegeometric section plane is parallel with the yz-plane of the coordinate system 399. The electric machine element 301 is a part of a stator 313 of the electric machine. Furthermore, the electric machine comprises a frame 320, a rotor 314, and bearings 315a and 315b configured to rotatably support the rotor 314 with respect to the stator 313. The rotor 314 comprises a shaft 316 and an electromagnetically active part configured to interact with the stator 313. Figure 3b shows a section view of the electric machine element 301 and the rotor 314 so that the section is taken along a line A1-A1 shown in figure 3a, and figure 3c shows a section view of the electric machine element 301 and the rotor 314 so that the section is taken along a line A2-A2 shown in figure 3a. The geometric section planes related to figures 3b and 3c are parallel with the xy-plane of the coordinate system 399.

[0039] The electric machine element 301 comprises a core structure 302, a winding 310 having coil sides and end-windings 307. In this example, the winding 310 is a nondrum winding where all coil sides are on a stator-rotor airgap side of the core structure 302. In figure 3a, four of the coil sides are denoted with references 303a, 303b, 304a, and 304b. In this exemplifying case, the core structure 302 has teeth and slots between the teeth, and the coil sides are in the slots. In figure 3b, one of the teeth of the core structure 302 is denoted with reference 311. It is, however, also possible that an electric machine element according to an exemplifying and nonlimiting embodiment comprises an airgap winding.

[0040] The winding 310 comprises an even number of parallel branches. Figure 3a shows two parallel branches 308a and 308b. The branch 308a belongs to coils sides 303a and 304a, and the branch 308b belongs to coils sides 304b and 303b. The branch 308a has a current supply point 322a at a first axial end of the machine element 301 and the branch 308b has a current supply point 322b at a second, opposite axial end of the machine element 301. In this exemplifying case, the coil sides which belong to branches which are connected in parallel with each other are in same ones of the slots of the core structure 302 to eliminate or at least minimize a possible difference in created magnetomotive forces “MMF” between the branches.

[0041] To illustrate the effect of the above-presented winding arrangement to eliminate or at least reduce circulating bearing currents, we consider an exemplifying situationin which current supplied to the winding 310 is i + Ai, and thus current supplied to the branch 308a is i / 2 + Ai / 2 and, correspondingly, current supplied to the branch 308b is i / 2 + Ai / 2. As the current is alternating “AC” current which may contain higher harmonic components, a part of the current leaks away from the winding 310 to the rotor 314 and to the core structure 302 mainly via parasitic capacitances between the winding 310 and the rotor 314 and via parasitic capacitances between the winding 310 and the core structure 302. The above-mentioned Ai represents the part that leaks. In figure 3a, a dash-and-dot line 317 depicts non-circulating bearing current which leaks capacitively from the winding 310 to the rotor 314 and flows through the bearings and the frame to the ground. A dotted line 318 depicts stator ground current which leaks capacitively from the winding 310 to the core structure 302 and flows through the frame 320 to the ground. The total ground current is the part Ai that leaks out from the winding 310 via the parasitic capacitances.

[0042] Referring to figure 3b, currents at the location of the line A1-A1 shown in figure 3a are:

[0043] - the current in the coil side 303a is i / 2 + Ai / 2,

[0044] - the current in the coil side 304a is i / 2 because the part Ai / 2 has leaked out from the coil sides 303a and 304a,

[0045] - the current in the coil side 304b is i / 2 + Ai / 4 because the part Ai / 4 has leaked out from the coil side 304b, and

[0046] - the current in the coil side 303b is i / 2 + Ai / 4 because it is assumed here that no significant leakage takes place in the end-windings 307.

[0047] Referring to figure 3c, currents at the location of the line A2-A2 shown in figure 3a are:

[0048] - the current in the coil side 303a is i / 2 + Ai / 4 because the part Ai / 4 has leaked out from the coil side 303a,

[0049] - the current in the coil side 304a is i / 2 + Ai / 4 because it is assumed here that no significant leakage takes place in the end-windings 307,

[0050] - the current in the coil side 304b is i / 2 + Ai / 2, andthe current in the coil side 303b is i / 2 because the part Ai / 2 has leaked out from the coil sides 304b and 303b.

[0051] In the light of the above-mentioned currents, the resultant magnetomotive force “MMF” around a geometric circle 321a is Ai / 2 in the clockwise direction at the location of the line A1 -A1. The resultant MMF around a geometric circle 321b is Ai / 2 in the counterclockwise direction at the location of the line A2-A2. As can be noted, when the supply to the parallel branches is applied at the opposite axial ends of the electric machine element 301 , the distribution of the leakage current components in the parallel branches creates the resulting magnetic flux circulating in opposite directions at the different axial ends of the electric machine element 301. Thus, in different ends of the shaft 316, an EMF with opposite polarities and equal strength is induced, resulting in substantially zero total shaft voltage, meaning prevention or at least reduction of circulating bearing currents.

[0052] The winding 310 can be for example a three-phase winding, and the electric machine illustrated in figures 3a-3c can be e.g. an induction machine, a permanent magnet machine, an electrically excited synchronous machine, or a synchronous reluctance machine. Each of the phases may comprise the even number of the parallel branches so that the elimination or at least reduction of circulating bearing currents is done in each phase individually. The winding 310 may have two or more pole pairs so that, within each of the pole pairs, each of the phases has the even number of the parallel branches. Thus, the elimination or at least reduction of circulating bearing currents is done in each phase within each pole pair individually.

[0053] The core structure 302 may comprise, for example, ferromagnetic sheets stacked in the axial direction of the electric machine element, i.e. in the z-direction of the coordinate system 399, and electrically insulated from each other. It is also possible that the core structure 302 comprises ferrite and / or other suitable ferromagnetic material such as e.g. Somaloy®.

[0054] Figure 4 illustrates schematically an electric machine element 401 according to an exemplifying and non-limiting embodiment. Furthermore, figure 4 illustrates schematically a rotor 414. The electric machine element 401 comprises a corestructure 402 and a winding 410 having coil sides and end-windings, wherein winding comprises an even number of parallel branches. In this exemplifying case, the winding 410 is a three-phase winding having two pole pairs. The regions for coils sides of different ones of phases R, S, and T are denoted with references R1+, R1-, S1+, S1-, T1+, T1-, R2+, R2-, S2+, S2-, T2+, T2-, where e.g. R1+ means positively directed coil sides of the phase R within pole pair 1 and e.g. T2- means negatively directed coil sides of the phase T within pole pair 2. It is to be noted that figure 4 is schematic and thus the core structure 402 may have slots in which the coil sides are located, i.e. the winding 410 does not need to be an airgap winding even if teeth and slots are not shown in figure 4.

[0055] The pole pairs 1 and 2 are connected in parallel so that the phases R, S, T of the parallel connected pole pairs constitute the above-mentioned parallel branches. For example, the winding of the phase R belonging to the pole pair 1 is in parallel with the winding of the phase R belonging to the pole pair 1. Each of the phases R, S, and T within the pole pair 1 has a current supply point at a first axial end of the machine element 410, and each of the phases R, S, and T within the pole pair 2 has the current supply point at the second, opposite axial end of the machine element. In figure 4, the current supply points at the first axial end of the machine element 410 are depicted with solid lines and the current supply points at the second axial end of the machine element 410 are depicted with dashed lines. In figure 4, the current supply point of the phase T in the pole pair 1 is denoted with reference 422a and the current supply point of the phase T in the pole pair 2 is denoted with reference 422b. In this exemplifying case, the three-phase winding is star connected, but a delta connection is also possible.

[0056] In general, the even number of the parallel connected pole pairs can be more than two in which case, within half of the pole pairs, each of the phases has its current supply point at the first axial end of the machine element, and, within another half of the pole pairs, each of the phases has its current supply point at the second, opposite axial end of the machine element. Thus, half of the pole pairs are supplied from one end and thus tend to generate a circulating bearing current in one direction whereas the other half of the pole pairs are supplied from the other one end andthus tend to generate a circulating bearing current in the opposite direction, i.e. shaft EMF components caused by different pole pairs cancel each other.

[0057] Figures 5a, 5b, and 5c illustrate schematically an electric machine that comprises an electric machine element 501 according to an exemplifying and non-limiting embodiment. The axial direction of the electric machine is parallel with the z-axis of a coordinate system 599. Figure 5a shows a section view where the geometric section plane is parallel with the yz-plane of the coordinate system 599. The electric machine element 501 is a part of a stator 513 of the electric machine. Furthermore, the electric machine comprises a frame 520, a rotor 514, and bearings 515a and 515b configured to rotatably support the rotor 514 with respect to the stator 513. The rotor 514 comprises a shaft 516 and an electromagnetically active part configured to interact with the stator 513. Figure 5b shows a section view of the electric machine element 501 and the rotor 514 so that the section is taken along a line A1-A1 shown in figure 5a, and figure 5c shows a section view of the electric machine element 501 and the rotor 514 so that the section is taken along a line A2-A2 shown in figure 5a. The geometric section planes related to figures 5b and 5c are parallel with the xy-plane of the coordinate system 599.

[0058] The electric machine element 501 comprises a core structure 502, a winding 510 having coil sides and end-windings 507. In this example, the winding 510 is a drumwinding where first half of the coil sides are on a stator-rotor airgap side of the core structure 502 and second half of the coil sides are on a yoke side of the core structure 502. In figure 5a, two of the coil sides on the stator-rotor airgap side are denoted with references 503 and 504, and two of the coil sides on the yoke side are denoted with references 505 and 506.

[0059] The winding 510 comprises an even number of parallel branches. Half of the parallel branches have current supply points at a first axial end of the machine element 501 and each of these current supply points is connected to a respective one of the coil sides on the stator-rotor airgap side and another half of the parallel branches have current supply points at the same first axial end of the machine element and each of these current supply points is connected to a respective one of the coil sides on the yoke side. Figure 5c shows two parallel branches 508a and 508b. The branch508a belongs to coils sides 504 and 506 and has a current supply point 522a connected to the coil side 504, and the branch 508b belongs to coils sides 503 and 505 and has a current supply point 522b connected to the coil side 503. In this exemplifying case, branches which are connected in parallel with each other are 180 electric degrees apart from each other to avoid circulating currents between the branches.

[0060] To illustrate the effect of the above-presented winding arrangement to eliminate or at least reduce circulating bearing currents, we consider an exemplifying situation in which current supplied to the winding 510 is i + Ai, and thus current supplied to the branch 508a is i / 2 + Ai / 2 and, correspondingly, current supplied to the branch 508b is i / 2 + Ai / 2. As the current is alternating “AC” current which may contain higher harmonic components, a part of the current leaks away from the winding 510 to the rotor 514 and to the core structure 502 mainly via parasitic capacitances between the winding 510 and the rotor 514 and via parasitic capacitances between the winding 510 and the core structure 502. The above-mentioned Ai represents the part that leaks. In figure 5a, a dash-and-dot line 517 depicts non-circulating bearing current which leaks capacitively from the winding 510 to the rotor 514 and flows through the bearings and the frame 520 to the ground. A dotted line 518 depicts stator ground current which leaks capacitively from the winding 510 to the core structure 502 and flows through the frame 520 to the ground. The total ground current is the part Ai that leaks out from the winding 510 via the above-mentioned parasitic capacitances.

[0061] Referring to figure 5b, currents at the location of the line A1-A1 shown in figure 5a are:

[0062] - the current in the coil side 505 is i / 2 + Ai / 2,

[0063] - the current in the coil side 503 is i / 2 because the part Ai / 2 has leaked out from the coil sides 505 and 503,

[0064] - the current in the coil side 504 is i / 2 + Ai / 2, and

[0065] - the current in the coil side 506 is i / 2 because the part Ai / 2 has leaked out from the coil sides 504 and 506.Referring to figure 5c, currents at the location of the line A2-A2 shown in figure 5a are:

[0066] - the current in the coil side 505 is i / 2 + Ai / 4 because the part Ai / 4 has leaked out from the coil side 505,

[0067] - the current in the coil side 503 is i / 2 + Ai / 4 because it is assumed here that no significant leakage takes place in the end-windings 507,

[0068] - the current in the coil side 504 is i / 2 + Ai / 4 because the part Ai / 4 has leaked out from the coil side 504, and

[0069] - the current in the coil side 506 is i / 2 + Ai / 4 because it is assumed here that no significant leakage takes place in the end-windings 507.

[0070] As can be seen in figures 5b and 5c in the light of the above-mentioned currents, MMF in the counterclockwise direction in the core structure 502 is substantially the same as MMF in the clockwise direction at the location of the line A1-A1. Correspondingly, MMF in the counterclockwise direction in the core structure 502 is substantially the same as MMF in the clockwise direction at the location of the line A2-A2. Thus, there is substantially no magnetic flux circulating the rotor 514. The substantial absence of magnetic flux circulating around the rotor 514 results in eliminated, or at least reduced, shaft voltage, shaft current, and circulating bearing currents threat.

[0071] The winding 510 can be for example a three-phase winding, and the electric machine can be e.g. an induction machine, a permanent magnet machine, an electrically excited synchronous machine, or a synchronous reluctance machine. Each of the phases may comprise the even number of the parallel branches so that the elimination or at least reduction of circulating bearing currents is done in each phase individually. The winding 510 may have two or more pole pairs so that, within each of the pole pairs, each of the phases has the even number of the parallel branches. Thus, the elimination or at least reduction of circulating bearing currents is done in each phase within each pole pair individually.

[0072] In the exemplifying electric machine element 501 , the core structure 502 has teeth and slots between the teeth on the stator-rotor airgap side and the coil sides on thestator-rotor airgap side are in the slots. In figure 5b, one of the teeth on the statorrotor airgap side of the core structure 502 is denoted with reference 511. It is, however, also possible that an electric machine element according to an exemplifying and non-limiting embodiment comprises an airgap winding. In the exemplifying electric machine element 501 , the yoke of the core structure 502 has yoke teeth and yoke slots between the yoke teeth, and the coil sides on the yoke side are in the yoke slots. In figure 5b, one of the yoke teeth is denoted with reference 512. It is, however, also possible that the coil sides on the yoke side are mechanically supported in some other way.

[0073] In the exemplifying cases described above with reference to figures 3a, 3b, 3b, 4, 5a, 5b, and 5c, each of the electric machine elements according to the abovepresented exemplifying and non-limiting embodiments is a part of a stator of a radialflux inner-rotor electric machine. It is, however, also possible that an electric machine element according to an exemplifying and non-limiting embodiment is a part of a stator of a radial-flux outer-rotor electric machine, or a part of a rotor of a radial-flux inner-rotor electric machine, or a part of a rotor of a radial-flux outer-rotor electric machine. The above-described leakage current balancing principle based on parallel branches and suitably selected current supply points is applicable in many axial flux electric machines, too.

[0074] The specific examples provided in the description given above should not be construed as limiting the scope and / or the applicability of the invention. It is to be noted that lists and groups of examples given in this document are non-exhaustive lists and groups unless otherwise explicitly stated.

Claims

What is claimed is:

1. An electric machine element (301 , 401 , 501 ) comprising a core structure (302, 402, 502) and a winding (310, 410, 510) having coil sides (303a, 303b, 304a, 304b, 503-506) and end-windings (307, 507), wherein the winding comprises an even number of parallel branches (308a, 308b, 508a, 508b, R1±, R2±, S1±, S2±, T1±, T2±) each having a current supply point (322a, 322b, 422a, 422b, 522a, 522b), characterized in that the current supply points of different ones of the parallel branches are at different locations in the winding to balance leakage currents (Ai) from the parallel branches via parasitic capacitances to suppress flux asymmetry.

2. An electric machine element according to claim 1, wherein the winding (310, 410) is a non-drum winding for a radial-flux electric machine such that all the coil sides (303a, 303b, 304a, 304b) are on a stator-rotor airgap side of the core structure, and half of the parallel branches (308a, R1±, S1±, T1±) have the current supply points (322a, 422a) at a first axial end of the machine element and another half of the parallel branches (308b, R2±, S2±, T2±) have the current supply points (322b, 422b) at a second, opposite axial end of the machine element.

3. An electric machine element according to claim 1 , wherein the winding (510) is a drum winding for a radial-flux electric machine such that first half of the coil sides (503, 504) are on a stator-rotor airgap side of the core structure and second half of the coil sides (505, 506) are on a yoke side of the core structure, and half of the parallel branches (508a) have the current supply points (522a) at a first axial end of the machine element and each configured to supply current to one of the first half of the coil sides (504) and another half of the parallel branches (508b) have the current supply points (522b) at the first axial end of the machine element and each configured to supply current to one of the second half of the coil sides (505).

4. An electric machine element according to any one of claims 1-3, wherein the winding has two or more phases (R, S, T) configured to produce a rotating magnetic field in response to supplying alternating currents to the phases.

5. An electric machine element according to claim 4, wherein each of the phases has the even number of the parallel branches.

6. An electric machine element according to claim 4, wherein the winding has two or more pole pairs and, within each of the pole pairs, each of the phases has the even number of the parallel branches.

7. An electric machine element according to claim 4, wherein the winding has an even number of parallel connected pole pairs so that the phases (R, S, T) of the parallel connected pole pairs constitute the parallel branches (R1±, R2±, S1±, S2±, T1±, T2±).

8. An electric machine element according to claim 7 when depending on claim 2, wherein, within half of the pole pairs, each of the phases (R1±, S1±, T1±) has the current supply point (422a) at the first axial end of the machine element, and, within another half of the pole pairs, each of the phases (R2±, S2±, T2±) has the current supply point (422b) at the second, opposite axial end of the machine element.

9. An electric machine element according to any one of claims 1-8, wherein the core structure has, on the stator-rotor airgap side of the core structure, teeth (311 , 511) and slots between the teeth, and the coil sides (303a, 303b, 304a, 304b, 503, 504) on the stator-rotor airgap side of the core structure are in the slots.

10. An electric machine element according to claim 9 when depending on claim 2, wherein all the coil sides (303a, 303b, 304a, 304b) belonging to the branches connected in parallel with each other are in same ones of the slots.

11. An electric machine element according to claim 3, wherein the branches connected in parallel with each other are 180 electric degrees apart from each other to avoid circulating currents between the branches connected in parallel with each other.

12. An electric machine element according to claim 3 or 9, wherein the core structure has, on the stator-rotor airgap side of the core structure, teeth (511) and slots between the teeth, and the first half of the coil sides are in the slots.

13. An electric machine element according to claim 3 or 12, wherein a yoke of the core structure has yoke teeth (512) and yoke slots between the yoke teeth, and the second half of the coil sides are in the yoke slots.

14. An electrical machine comprising a stator (313, 513) and a rotor (314, 514) rotatably supported with respect to the stator, wherein at least one of the stator and the rotor comprises an electric machine element (301 , 501 ) according to any one of claims 1-13.

15. An electrical machine according to claim 14, wherein the electric machine is a radial-flux inner-rotor electric machine, and the electric machine element (301 , 501 ) is a part of the stator (313, 513).

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