FI20175653A1

FI20175653A1 - An electric machine

Info

Publication number: FI20175653A1
Application number: FI20175653A
Authority: FI
Inventors: Janne Turunen; Michiel Bosch
Original assignee: The Switch Drive Systems Oy
Priority date: 2017-07-05
Filing date: 2017-07-05
Publication date: 2019-01-06
Also published as: WO2019008220A1; FI20175653A; FI128650B

Abstract

A stator of an electric machine comprises windings (101), a ferromagnetic core structure (102), and a frame structure (103) surrounding the ferromagnetic core structure. The frame structure comprises a first channel (106) for conducting first cooling medium which is circulated through the airgap of the electric machine and a second channel (107) isolated from the first channel and suitable for conducting second cooling medium for receiving heat from the first cooling medium. The first channel is twisting so that the first channel is made longer and the cross-sectional flow area of the first channel is made smaller. Thus, the flow velocity in the first channel can be kept sufficiently high despite that the volume flow rate of the first cooling medium is restricted by the airgap. The higher flow velocity in the first channel improves the heat transfer from the first cooling medium to the second cooling medium.

Description

An electric machine

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Field of the disclosure

The disclosure relates generally to cooling of rotating electric machines. More 5 particularly, the disclosure relates to a stator of an electric machine. Furthermore, the disclosure relates to an electric machine.

Background

In a rotating electric machine, such as an electric motor or a generator, a magnetic flux is developed between the electromagnetically active parts of the rotor and the 10 stator of the electric machine. In a radial flux electric machine, the maximum torque is proportional to the product of the air-gap radius, the area of the air-gap surface, the magnetic flux density in the air-gap, and the linear current density in the air-gap surface of the stator. Hence, without increasing the mechanical size of the electric machine, the maximum torque can be increased by increasing the linear current 15 density because the magnetic flux density cannot be practically increased any more when the saturation point of iron has been exceeded. Increasing the linear current density increases, however, the resistive losses in the windings of the electric machine. Therefore, the cooling plays a significant role in the operation of the electric machine.

Many operating conditions require electric machines which have a closed construction in order to prevent water and/or impurities from getting inside an electric machine. A typical cooling arrangement of an electric machine having a closed construction comprises a cooling circulation inside the electric machine. The cooling medium of the internal cooling circulation is typically, but not necessarily, 25 air. The cooling medium of the internal cooling circulation delivers heat to cooling medium of a second cooling arrangement which in turn delivers heat for example to the ambient air or to another heat sink. The cooling medium of the second cooling arrangement can be for example air or water. Typically, the second cooling arrangement cools the stator of an electric machine whereas the internal cooling 30 circulation cools the rotor of the electric machine. In many electric machines of the

20175653 prh 05 -07- 2017 kind described above, the internal cooling circulation flows axially through the airgap of the electric machine and the rotor is cooled mainly via the airgap surface of the rotor. The heat transfer from the internal cooling circulation to the cooling media of the second cooling arrangement requires mechanical structures which act as a heat5 exchanger. In many applications, it is advantageous or it can be even necessary that a frame structure of an electric machine is acting as a heat-exchanger because an external heat-exchanger requires room and causes additional costs.

In many electric machines, the airgap is narrow in order to achieve desired electrical properties. For example, high speed electrical machines characterize in a compact 10 physical size, which typically leads to a narrow airgap between the rotor and the stator. When the above-discussed internal cooling circulation flows axially through a narrow airgap, this may lead to a situation where the volume flow rate, m³/s, of the internal cooling circulation is relatively low due to the small cross-sectional flow area in the airgap. Thus, mechanical structures which act as a heat-exchanger should be 15 capable of effectively transferring heat from the internal cooling circulation having a low volume rate to cooling medium of a second cooling arrangement. Especially in conjunction with electric machines where the frame structure is used as a heatexchanger, it may be challenging to provide a sufficiently effective heat transfer from an internal cooling circulation whose volume rate is low.

Summary

The following presents a simplified summary in order to provide 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 25 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.

In accordance with the invention, there is provided a new stator for an electric machine. A stator according to the invention comprises windings, a ferromagnetic core structure, and a frame structure surrounding the ferromagnetic core structure.

The frame structure comprises:

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- at least one first opening for receiving first cooling medium from a room comprising the end-windings and the airgap of the electric machine and at least one second opening for conducting the first cooling medium back to the room comprising the end-windings and the airgap, the first and second openings having an axial distance between each other,

- at least one first channel for conducting the first cooling medium from the first opening to the second opening, and

- at least one second channel isolated from the first channel and suitable for conducting second cooling medium for receiving heat from the first cooling medium.

The above-mentioned first channel is twisting so that a flow-path constituted by the first channel between the first and second openings is at least two times longer, and more preferably three times longer, than the axial distance between the first and second openings. As the first channel is made longer and correspondingly the cross15 sectional flow area of the first channel is made smaller, the flow velocity, m/s, of the first cooling medium in the first channel can be kept sufficiently high despite that the volume flow rate, m³/s, of the first cooling medium is restricted by the airgap. The heat transfer area of the first channel is not sacrificed because the decrease in the contour length of the cross-sectional flow area is compensated for by the increase 20 in the length. The higher flow velocity in the first channel improves the heat transfer from the first cooling medium to the second cooling medium.

In a stator according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned first channel is twisting so that the first channel comprises axially directed first portions and circumferentially directed second 25 portions connecting the first portions in series so that the first cooling medium flows back and forth in the axial direction of the electric machine when being conducted by the first channel. As the first channel is in this exemplifying case arranged to conduct the first cooling medium back and forth in the axial direction, the first channel is longer and its cross-sectional flow area is smaller than in a case where a 30 corresponding channel is arranged to conduct cooling medium only in one axial direction.

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In accordance with the invention, there is provided also a new electric machine. An electric machine according to the invention comprises:

- a stator according to the invention, and

- a rotor rotatably supported with respect to the stator.

In an electric machine according to an exemplifying and non-limiting embodiment of the invention, the rotor comprises blower vanes for circulating the first cooling media via the airgap of the electric machine and via the at least one first channel of the frame structure of the stator.

An electric machine according to an exemplifying and non-limiting embodiment of 10 the invention has a closed construction so that the end-windings rooms, the airgap, and the at least one first channel of the frame structure constitute a closed room.

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

Various exemplifying and non-limiting embodiments of the invention both as to 15 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.

The verbs “to comprise” and “to include” are used in this document as open 20 limitations that neither exclude nor require the existence of unrecited features. 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.

Brief description of the figures

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:

20175653 prh 05 -07- 2017 figures 1a, 1b, and 1c illustrate an electric machine according to an exemplifying and non-limiting embodiment of the invention, and figure 2 illustrates an electric machine according to another exemplifying and nonlimiting embodiment of the invention.

Description of the exemplifying embodiments

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

Figure 1a shows a schematic section view of an electric machine according to an exemplifying and non-limiting embodiment of the invention. The section plane is parallel with the yz-plane of a coordinate system 199. The electric machine comprises a stator 100 according to an exemplifying and non-limiting embodiment of the invention and a rotor 122 rotatably supported with respect to the stator. The 15 rotational axis of the rotor 122 is parallel with the z-axis of the coordinate system

199. The electric machine can be for example an induction machine, a permanent magnet machine, a reluctance machine, or an electrically excited synchronous machine. The stator 100 comprises a ferromagnetic core structure 102 made of, or at least comprising, ferromagnetic material. In this exemplifying case, the 20 ferromagnetic core structure 102 comprises a yoke and stator teeth connected to the yoke. The ferromagnetic core structure 102 may comprise e.g. a stack of ferromagnetic sheets that are electrically insulated from each other and stacked in the axial direction, i.e. in the z-direction of the coordinate system 199. The stator 100 comprises windings 101 whose coil sides are located in the stator slots between 25 the stator teeth.

The stator 100 further comprises a frame structure 103 that comprises a channeling for enabling a cooling circulation inside the electric machine and for acting as a heatexchanger. The frame structure 103 comprises first openings for receiving first cooling medium from a room comprising the end-windings and the airgap of the 30 electric machine and second openings for conducting the first cooling medium back

20175653 prh 05 -07- 2017 to the room comprising the end-windings and the airgap. In figure 1a, one of the first openings is denoted with a reference 104 and one of the second openings is denoted with a reference 105. The first cooling medium can be for example air. The frame structure 103 comprises first channels for conducting the first cooling medium from the first openings to the second openings. In figure 1 a, one of the first channels is denoted with a reference 106. As illustrated with dashed line curves in figure 1a, the first cooling medium is circulated via the airgap, via the first openings of the frame structure, via the first channels of the frame structure, and via the second openings of the frame structure. The rotor 122 comprises advantageously blower 10 vanes 123 for circulating the first cooling media. The first channels of the frame structure, the first and second openings of the frame structure, and the room comprising the end-windings and the airgap constitute advantageously a closed room so as to prevent water and/or impurities from getting inside the electric machine.

The frame structure 103 comprises second channels which are isolated from the first channels and which are suitable for conducting second cooling medium for receiving heat from the first cooling medium. In figure 1a, one of the second channels is denoted with a reference 107. In this exemplifying case, the second channels are substantially circumferential and the second channels are located, in 20 a radial direction, between the first channels and the ferromagnetic core structure 102. Thus, the second cooling medium is arranged to receive heat from the first cooling medium and from the ferromagnetic core structure 102, too. The second cooling medium can be for example water. The stator may further comprise a piping interface 121 for connecting the second channels of the frame structure to an 25 external system for circulating the second cooling media.

In the exemplifying electric machine illustrated in figure 1a, the frame structure 103 comprises a frame element 118 and a cover element 119 surrounding the frame element. The frame element 118 comprises fins on an outer surface of the frame element and the cover element is in contact with the edges of the fins so as to form 30 the first channels. Figure 1 c shows a view of a section taken along a line A-A shown in figure 1 a. In figure 1 c, one of the above-mentioned fins is denoted with a reference

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120. The frame structure 103 can be cost effective, because the frame element 118 can be manufactured by e.g. casting.

Figure 1 b shows a side view of the electric machine so that the cover element 119 is presented as a section view. In figure 1b, one of the fins on the outer surface of the frame element 118 is denoted with a reference 130, one of the first openings for receiving the first cooling medium from the room comprising the end-windings and the airgap is denoted with a reference 108, one of the second openings for delivering the first cooling medium back to the room comprising the end-windings and the airgap is denoted with a reference 109, and two of the first channels are denoted with references 110 and 111. The flow of the first cooling medium is depicted with dashed line curves. As illustrated in figure 1b, each of the first channels is twisting so that a flow-path constituted by the first channel between the respective first and second openings is at least two times longer than the axial distance D between the first and second openings. As the first channels are made longer and correspondingly the cross-sectional flow areas of the first channels are made smaller, the flow velocity, m/s, of the first cooling medium in the first channel can be kept sufficiently high despite that the volume flow rate, m³/s, of the first cooling medium is restricted by the airgap. The heat transfer area of the first channel is not sacrificed because the decrease in the contour length of the cross-sectional flow area is compensated for by the increase in the length. The ability to maintain higher flow velocity in the first channels improves the heat transfer from the first cooling medium to the second cooling medium.

In the exemplifying electric machine illustrated in figures 1a-1c, each first channel is twisting so that the first channel comprises axially directed first portions and circumferentially directed second portions connecting the first portions in series so that the first cooling medium flows back and forth in the axial direction of the electric machine when being conducted by the first channel. In figure 1 b, two of the axially directed portions of the first channel 110 are denoted with references 112 and 113 and one of the circumferentially directed second portions of the first channel 110 is denoted with a reference 114. It is however also possible that the first channels are arranged to be twisting in a way different from that illustrated in figure 1b. Figure 2 shows a side view of an electric machine according to another exemplifying and

20175653 prh 05 -07- 2017 non-limiting embodiment of the invention. In this exemplifying case, a frame element 218 of a stator 200 comprises helical fins on its outer surface so that the frame element 218 and a cover element 219 constitute helical first channels. One of the first channels is denoted with a reference 206. In figure 2, the cover element is 5 presented as a section view so that the section plane is parallel with the yz-plane of a coordinate system 299.

In the exemplifying electric machine illustrated in figures 1a-1c, each of the first openings is on a wall of a first end-winding room of the electric machine and each of the second openings is on a wall of a second end-winding room of the electric 10 machine. In figure 1a, the first end-winding room is denoted with a reference 115 and the second end-winding room is denoted with a reference 116. Thus, the electric machine comprises an asymmetric cooling arrangement where the first cooling medium is guided into the room comprising the end-windings and the airgap from one end of the electric machine and guided out from the opposite end. In many 15 cases, an asymmetric cooling arrangement of the kind described above may result in a significant temperature difference between the end windings, especially when the volume flow rate of the cooling medium is relatively low. In the exemplifying electric machine illustrated in figures 1a-1c, the end-winding in the end-winding room 115 is warmer than the end-winding in the end-winding room 116 because the 20 first cooling medium is warmed up in the end-winding room 116 and in the airgap prior to arriving at the end-winding room 115. It is worth noting that the abovedescribed principle of using twisting channels for improving heat transfer from an internal cooling circulation is also applicable in a symmetrically cooled electric machine where cooling medium of the internal cooling circulation is guided into both 25 end-winding rooms and the cooling medium is guided out via one or more radial channels of a stator core structure. A symmetrical cooling arrangement may, however, increase the complexity of the machine structure, especially when water jacket cooling is utilized for stator cooling.

In an electric machine according to an exemplifying and non-limiting embodiment of 30 the invention, the wall of the first end-winding room 115 has one or more bypass openings connecting the first channels to the first end-winding room 115. In figure 1b, two of the bypass openings are denoted with a reference 117. As illustrated in figure 1b, the flow-path from the first opening 108 to the bypass openings 117 comprises two series connected and axially directed portions 112 and 113 of the first channel 110. Therefore, the first cooling medium is cooled down prior to being directed to the above-mentioned end-winding room via the bypass openings 117.

Thus, the bypass openings 117 supply cooled medium to the end-windings in the above-mentioned end-winding room. In a case where there are no bypass openings of the kind described above, the above-mentioned end-windings would be cooled only by medium which flows out of the air gap and which has been already warmed up.

The specific examples provided in the description given above should not be construed as limiting the applicability and/or the interpretation of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.

Claims

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What is claimed is:

1. A stator (100, 200) of an electric machine, the stator comprising windings (101), a ferromagnetic core structure (102), and a frame structure (103) surrounding the ferromagnetic core structure, wherein the frame structure comprises:

5 - at least one first opening (104, 108) for receiving first cooling medium from a room comprising end-windings and an airgap of the electric machine and at least one second opening (105, 109) for conducting the first cooling medium back to the room comprising the end-windings and the airgap, the first and second openings having an axial distance (D) between each other,

10 - at least one first channel (106, 110, 111,206) for conducting the first cooling medium from the first opening to the second opening, and

- at least one second channel (107) isolated from the first channel and suitable for conducting second cooling medium for receiving heat from the first cooling medium,

15 characterized in that the first channel is twisting so that a flow-path constituted by the first channel between the first and second openings is at least two times longer than the axial distance (D) between the first and second openings.
2. A stator according to claim 1, wherein the first channel comprises axially directed first portions (112, 113) and circumferentially directed second portions

20 (114) connecting the first portions in series so that the first cooling medium flows back and forth in an axial direction of the electric machine when being conducted by the first channel.
3. A stator according to claim 1, wherein a wall of a first end-winding room (115) of the electric machine has the first opening (104, 108) and a wall of a second end-

25 winding room (116) of the electric machine has the second opening (105, 109).
4. A stator according to claim 2, wherein a wall of a first end-winding room (115) of the electric machine has the first opening (104, 108) and a wall of a second endwinding (116) room of the electric machine has the second opening (105, 109).

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5. A stator according to claim 4, wherein the wall of the first end-winding room of the electric machine has one or more bypass openings (117) connecting the first channel to the first end-winding room, a flow-path from the first opening (108) via the first channel to each of the bypass openings comprises at least two of the series

5 connected and axially directed first portions (112, 113) of the first channel.
6. A stator according to any of claims 1 -5, wherein the frame structure comprises a frame element (118) and a cover element (119) surrounding the frame element, and the frame element comprises fins (120) on an outer surface of the frame element and the cover element is in contact with edges of the fins so as to form the

10 first channel.
7. A stator according to any of claims 1 -6, wherein the second channels (107) are substantially circumferential.
8. A stator according to any of claims 1 -7, wherein the second channels (107) are located, in a radial direction, between the first channel (106, 110, 111) and the

15 ferromagnetic core structure (102).
9. A stator according to any of claims 1 -8, wherein the stator comprises a piping interface (121) for connecting the second channel of the frame structure to an external system for circulating the second cooling media.
10. An electric machine comprising:

20 - a stator (100) comprising windings (101), a ferromagnetic core structure (102), and a frame structure (103) surrounding the ferromagnetic core structure, and

- a rotor (122) rotatably supported with respect to the stator, wherein the stator is according to any of claims 1-9.

25
11. An electric machine according to claim 10, wherein the rotor comprises blower vanes (123) for circulating the first cooling media via the airgap of the electric machine, via the first opening of the frame structure, via the first channel of the frame structure, and via the second opening of the frame structure.
12. An electric machine according to claim 10 or 11, wherein the following constitute a closed room: i) the room comprising the end-windings and the airgap, ii) the first and second openings of the frame structure, and iii) the at least one first channel of the frame structure.