Field of the invention The invention relates generally to rotating electrical machines. More particularly, the invention relates to cooling of an electrical machine.

Background

Rotating electrical machines can generate considerable heat, making cooling of an electrical machine challenging, especially in conjunction with large high power electrical machines. Additionally, in order to avoid excessive wear due to differential thermal expansion, it is important to cool the inner components such as a rotor as well as the outer components such as a stator and a casing. Cooling can be a challenge for electrical machines that are subjected to a wide range of ambient temperatures, humidity levels, and dust/dirt levels. In this kind of situations there is many times a need to arrange a closed circulation cooling so that the cooling fluid is separated from the ambient air.

An effective cooling for a stator of an electrical machine can be provided with liquid cooling but a rotor of the electrical machine has to be usually cooled with gaseous cooling fluid that can be, for example, air or hydrogen. Publication WO2008046817 discloses an electrical machine that comprises a stator, a rotor, and a casing which closes off the rotor from the stator, forming a seal. For efficient cooling, the stator has a liquid cooling device with a corresponding stator cooling circuit. The casing is arranged to form a part of the outer wall of the cooling circuit. The rotor of the electrical machine is, however, cooled with gaseous cooling fluid. A closed cir- culation cooling implemented with gaseous cooling fluid requires typically a heat exchanger for transferring heat from the gaseous cooling fluid to the ambient air. The physical size of this kind of gas-to-gas heat exchanger may be quite large and therefore the heat exchanger may require a considerable room. Furthermore, the large-size gas-to-gas heat exchanger may represent a significant portion of the to- tal costs of a system comprising the electrical machine. Summary

The following presents a simplified summary in order 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 criti- cal 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.

In accordance with the first aspect of the invention, there is provided a new elec- trical machine. The electrical machine according to the invention comprises:

- a rotor comprising blowing structures for moving gaseous cooling fluid when the rotor is rotating, and

- a stator comprising cooling channels for conducting cooling liquid, wherein the stator further comprises heat exchange structures for transferring heat from the gaseous cooling fluid to the cooling liquid, and the electrical machine further comprises guide structures for directing the gaseous cooling fluid moved by the rotor to the heat exchange structures and/or for directing the gaseous cooling fluid from the heat exchange structures back to the blowing structures of the rotor.

In the above-described electrical machine, the liquid cooling arrangement operates not only as a cooling system of the stator but also as a gas-to-liquid heat exchanger for the cooling arrangement based on the gaseous cooling fluid. Hence, there is no need for a gas-to-gas heat exchanger for transferring heat from the gaseous cooling fluid to the ambient air.

In accordance with the second aspect of the invention, there is provided a new system that comprises:

- a cooler device, e.g. a liquid-to-air heat exchanger or a liquid-to-water heat exchanger,

- an electrical machine according to an embodiment of the invention, and

- pumping devices for circulating the cooling liquid through the cooler device and the cooling channels of the stator of the electrical machine. A number of exemplifying embodiments of the invention are described in accompanied dependent claims.

Various exemplifying 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 embodiments when read in connection with the accompanying drawings.

The verb "to comprise" is used in this document as an open limitation that neither excludes nor requires the existence of unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.

Brief description of figures

The exemplifying 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: figure 1 a shows a schematic side-section view of an electrical machine according to an embodiment of the invention, figure 1 b shows a detail of the electrical machine illustrated in figure 1 a, figure 1 c illustrates flow of cooling liquid and flow of gaseous cooling fluid in the electrical machine illustrated in figure 1 a, figure 2a shows a schematic side-section view of an electrical machine according to an embodiment of the invention, figure 2b shows a detail of the electrical machine illustrated in figure 2a, and figure 3 illustrates a system according to an embodiment of the invention. Description of embodiments Figure 1 a shows a schematic side-section view of an electrical machine according to an embodiment of the invention. The electrical machine comprises a rotor 101 and a stator 102. The rotor 101 comprises blowing structures for moving gaseous cooling fluid when the rotor is rotating. The gaseous cooling fluid can be for example air and its purpose is to cool the rotor and also the end-windings of the stator. In figure 1 a, the flow of the gaseous cooling fluid is illustrated with a closed line 106. The rotor comprises a hub 108, a rim part 109, and blower blades 1 10 supporting the rim part to the hub. The blower blades 1 10 are arranged to move the gaseous cooling fluid in an axial direction when the rotor is rotating. Thus, the blower blades represent the blowing structures. The above-mentioned axial direc- tion is the positive z-direction of the coordinate system 190. Also the negative z- direction of the coordinate system 190 is an axial direction. The rim part 109 of the rotor comprises the electromechanically active parts of the rotor. The electrical machine can be, for example, a permanent magnet synchronous machine in which case the rim part 109 of the rotor comprises permanent magnets. The electrical machine can be, for example, an electrically excited synchronous machine in which case the rim part 109 of the rotor comprises excitation windings. The electrical machine can be, for example, an asynchronous machine in which case the rim part 109 of the rotor may comprise a squirrel cage winding or slip-ring windings. The stator 102 of the electrical machine comprises cooling channels 104 for conducting cooling liquid. The cooling liquid is assumed to be received from an external system and, after the cooling liquid has been warmed up as a consequence of absorbing heat, to be delivered back to the external system. In figure 1 a, the ingress and the egress of the cooling liquid are illustrated with arrows 107. The sta- tor 102 comprises heat exchange structures 103 for transferring heat from the gaseous cooling fluid to the cooling liquid. The cooling channels 104 for the cooling liquid are formed by tangential grooves on the heat exchange structures and by the surfaces that are against the heat exchange structures. Thus, the cooling channels are tangentially directed, and figure 1 a that is a side-section view of the electrical machine shows cross-sections of the cooling channels. The shape of the heat exchange structures 103 is such that it allows the gaseous cooling fluid to flow in an axial direction, i.e. in the negative z-direction of the coordinate system 190. Figure 1 b illustrates one of the heat exchange structures seen along the arrow A shown in figure 1 a. As illustrated in figure 1 b, the heat exchange structures comprise cooling ribs 1 13 between which the gaseous cooling fluid is capable of flowing in the axial direction. As the cooling liquid flows in the tangential direction and the gaseous cooling fluid passes the heat exchange structures in the axial direction, crossflow heat transfer takes place in the heat exchange structures 103. Figure 1 c illustrates the flow of the cooling liquid and the flow of gaseous cooling fluid in the electrical machine illustrated in figure 1 a. The flow of the cooling liquid is illustrated with arrows 127 and the flow of the gaseous cooling fluid is illustrated with the closed line 106. In the exemplifying case illustrated in figure 1 a, the stator 102 comprises a cylindrical frame 1 14 arranged to support a stator core 1 15 and the heat exchange structures 103 so that the stator core is attached to the inner surface of the cylindrical frame and the heat exchange structures are attached to the outer surface of the cylindrical frame. The stator comprises a cylindrical covering 1 16 arranged to surround the heat exchange structures 103 and to form, together with the cylindrical frame 1 14 and the heat exchange structures 103, axially directed channels for the gaseous cooling fluid. The electrical machine comprises guide structures for directing the gaseous cooling fluid moved by the rotor to the heat exchange structures and/or for directing the gaseous cooling fluid from the heat exchange structures back to the blowing structures of the rotor. In the exemplifying case shown in figure 1 a, bearing shields 105 and the cylindrical covering 1 16 constitute the guide structures.

In the electrical machine described above and illustrated in figure 1 a, the liquid cooling arrangement operates not only as a cooling system of the stator but also as a gas-to-liquid heat exchanger for the cooling arrangement based on the gaseous cooling fluid. Hence, there is no need for a gas-to-gas heat exchanger for transferring heat from the gaseous cooling fluid to the ambient air.

Figure 2a shows a schematic side-section view of an electrical machine according to an embodiment of the invention. The electrical machine comprises a rotor 201 and a stator 202. The rotor 201 comprises blowing structures for moving gaseous cooling fluid when the rotor is rotating. The gaseous cooling fluid can be for example air and its purpose is to cool the rotor and also the end-windings of the stator. In figure 2a, the flow of the gaseous cooling fluid is illustrated with a closed line 206. The rotor comprises an electromechanically active part 220 that is supported to a hub 208. The rotor comprises axial channels 21 1 at the center portion of the rotor and radial channels 212 extending from the axial channels to an outer surface of the rotor. The axial and radial channels constitute the blowing structures of the rotor as the gaseous cooling fluid is moved in the axial channels by the centri- fugal force when the rotor is rotating. The negative and positive z-directions of a coordinate system 290 represent axial directions. The electrical machine can be, for example, a permanent magnet synchronous machine in which case the electromechanically active part 220 comprises permanent magnets. The electrical machine can be, for example, an electrically excited synchronous machine in which case the electromechanically active part 220 comprises excitation windings. The electrical machine can be, for example, an asynchronous machine in which case the electromechanically active part 220 may comprise a squirrel cage winding or slip-ring windings.

The stator 202 of the electrical machine comprises cooling channels 204 for conducting cooling liquid. The cooling liquid is assumed to be received from an exter- nal system and, after the cooling liquid has been warmed up as a consequence of absorbing heat, to be delivered back to the external system. In figure 2a, the ingress and the egress of the cooling liquid are illustrated with arrows 207. The stator 202 comprises heat exchange structures 203 for transferring heat from the gaseous cooling fluid to the cooling liquid. The cooling channels 204 for the cooling liquid are formed by axially directed elongated cavities on the heat exchange structures and by the surfaces that are against the heat exchange structures. Thus, the cooling channels are axially directed, and figure 2a that is a side-section view of the electrical machine shows side-sections of the cooling channels. The shape of the heat exchange structures is such that it allows the gaseous cooling fluid to flow in the axial direction. Figure 2b shows a section view of one of the heat exchange structures. The section is taken along the plane A shown in figure 2a. The plane A is parallel to the xy-plane of the coordinate system 290. As illustrated in figure 2b, the heat exchange structures comprise cooling ribs 213 between which the gaseous cooling fluid is capable of flowing in the axial direction. Figure 2b also illustrates cross-sections of the cooling channels 204. As the cooling liquid flows in the axial direction and the gaseous cooling fluid passes the heat exchange structures in the axial direction, counterflow heat transfer can be arranged to take place in the heat exchange structures 203. Figure 2a illustrates a situation in which the counterflow heat transfer takes place because the flowing di- rections of the cooling liquid and the gaseous cooling fluid are opposite to each other in the heat transfer structures 203.

In the exemplifying case illustrated in figure 2a, the stator 202 comprises a cylindrical frame 214 arranged to support a stator core 215 and the heat exchange structures 203 so that the stator core is attached to the inner surface of the cylin- drical frame and the heat exchange structures are attached to the outer surface of the cylindrical frame. The stator comprises a cylindrical covering 216 arranged to surround the heat exchange structures 203 and to form, together with the cylindrical frame 214 and the heat exchange structures 203, axially directed channels for the gaseous cooling fluid. The electrical machine comprises guide structures for directing the gaseous cooling fluid moved by the rotor to the heat exchange structures and/or for directing the gaseous cooling fluid from the heat exchange structures back to the blowing structures of the rotor. In the exemplifying case shown in figure 2a, bearing shields 205 and the cylindrical covering 216 constitute the guide structures.

Figure 3 illustrates a system according to an embodiment of the invention. The system comprises a cooler device 301 that can be, for example, a liquid-to-air heat exchanger or a liquid-to-water heat exchanger. The system comprises an electrical machine 302 comprising a rotor and a stator. The rotor of the electrical machine 302 comprises blowing structures for moving gaseous cooling fluid when the rotor is rotating. The stator of the electrical machine 302 comprises cooling channels for conducting cooling liquid. The stator further comprises heat exchange structures for transferring heat from the gaseous cooling fluid to the cooling liquid. The electrical machine 302 further comprises guide structures for directing the gaseous cooling fluid moved by the rotor to the heat exchange structures and/or for directing the gaseous cooling fluid from the heat exchange structures back to the blow- ing structures of the rotor. The system further comprises pumping devices 303 and appropriate pipes for circulating the cooling liquid through the cooler device 301 and the cooling channels of the stator of the electrical machine 302. The pumping device 303 may comprise a pump and a separate electrical motor arranged to drive the pump. It is also possible that the pump is connected to the shaft of the electrical machine 302.

In a system according to an embodiment of the invention, the cooling channels of the stator of the electrical machine are tangentially directed and the shape of the heat exchange structures of the stator allows the gaseous cooling fluid to flow in the axial direction so as to provide crossflow heat transfer in the heat exchange structures. In this case, the electrical machine can be, for example, according to what is illustrated in figure 1 a.

In a system according to an embodiment of the invention, the cooling channels of the stator of the electrical machine are axially directed, the shape of the heat exchange structures of the stator allows the gaseous cooling fluid to flow in the axial direction, and the pumping devices are arranged to move the cooling liquid in the axially directed cooling channels against the flowing direction of the gaseous cooling fluid so as to provide counterflow heat transfer in the heat exchange structures. In this case, the electrical machine can be, for example, according to what is illustrated in figure 2a. The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the embodiments described above.