BACKGROUND TO THE INVENTION AND PRIOR ART

The present invention relates to an arrangement for cooling an electrical machine according to the preamble of claim 1.

Hybrid vehicles usually comprise an electrical machine which serves as prime mover during running of the vehicle and as generator at times when the vehicle is being braked. The electrical machine is usually situated within a housing or the like.

Whether running as prime mover or as generator, the electrical machine is subject to temperature rise. Temperature rise in an electrical machine is substantially confined to specific components, e.g. its stator windings. Conventional cooling of electrical machines in hybrid vehicles is usually by air or water led past an external surface of the housing which encloses the electrical machine. Heat transfer between the warm components of the electrical machine and the external surface of the housing is usually small. This may be due to the warm components being situated relatively far from the external surface of the housing or to their being surrounded by air, which has a very low thermal conductivity. Conventionally cooled electrical machines therefore run great risk of becoming overheated.

US 2005/0206251 refers to an arrangement for cooling an electrical machine which is situated in a housing. The electrical machine comprises in a conventional way a stator and a rotor which is mounted on a rotatable shaft. In this case, oil is used as cooling agent. The oil is led via a duct situated within the rotatable shaft to a location radially internal to the electrical machine. The oil is led thereafter into a radial duct in the hub of the rotor. The radial duct ends with apertures which lead the oil to two lateral surfaces of the rotor from which the oil is flung radially outwards by the centrifugal force which occurs when the rotor rotates. The oil thereupon comes into contact with protruding portions of the rotor windings and the stator windings. The rotor windings and the stator windings thus provide effective cooling. SUMMARY OF THE INVENTION

The object of the invention is to propose an arrangement which makes very good cooling of components of an electrical machine possible in a relatively simple way and at low cost.

This object is achieved with the arrangement of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 1. In this case the gearbox oil is used as cooling agent to cool at least one component of the electrical machine. The arrangement therefore comprises an extra oil loop connected to the oil system which lubricates the gearbox. The extra oil loop has here an extent such that the oil comes into substantially direct contact with at least one component of the electrical machine which needs cooling. The oil thus provides very effective cooling of that component. After the cooling of the component, the oil is led back and mixed with oil circulating in the oil system which lubricates the gearbox. In this case, oil already present in an existing oil system is used to cool at least one component of the electrical machine. The oil system which lubricates the gearbox comprises with advantage an oil pump which circulates the oil in the oil system, and an oil cooler for cooling the oil. The oil loop is preferably so arranged that the existing oil pump can also be used to circulate the oil through the extra oil loop and the oil cooler in order to cool the oil before it is circulated through the extra oil loop. Very good cooling of said component of the electrical machine can in this case be provided substantially entirely by appropriate configuration of the constituent oil ducts of the extra oil loop. Cooling of at least one component of the electrical machine can in this case be achieved in a relatively simple way and at low cost.

According to an embodiment of the present invention, the extra oil loop has an extent such that the oil comes into contact with at least one coil end of the stator windings. The stator windings often have coil ends situated at a distance from the housing. In addition, the space between the coil ends and the housing is occupied by air, which is a very poor heat conductor. The coil ends are therefore normally subject to very poor cooling when the electrical machine is cooled in a conventional way. They therefore risk warming to a very high temperature. In this case this is prevented by the oil which circulates in the extra oil loop coming into direct contact with the coil ends. The oil thus provides very effective cooling of the coil ends. According to an embodiment of the present invention, the extra oil loop has an extent such that the oil comes into contact with at least one surface of the rotating rotor. The oil thus provides the rotor with effective cooling. With advantage, the extra oil loop has an extent such that the oil comes into contact with a surface of the rotor which is so oriented that the oil is flung radially outwards from said surface towards a coil end. The stator's coil ends are normally disposed radially externally to the rotor's lateral portions. Oil which reaches the rotor's lateral portions is flung substantially radially outwards by centrifugal force when the rotor rotates. A large proportion of the oil thus comes into contact with the radially externally situated coil ends of the stator.

According to an embodiment of the present invention, the extra oil loop has an extent such that the oil is led through said bearings. The oil which is led through the extra oil loop thus comes into direct contact with the mutually movable surfaces of the bearing by which the rotor is arranged for rotation in the housing. The bearing is thus provided with very good cooling and lubrication during operation of the electrical machine. The rotor is with advantage fastened by a splined connection on the rotatable shaft and the oil in said extra oil loop is arranged to come into contact with the splined connection. A splined connection provides the rotor with secure fastening on the rotatable shaft. In this case the supply of the oil in the extra oil loop provides good lubrication and cooling of the splined connection.

According to an embodiment of the present invention, the extra oil loop is adapted to leading the oil through an internal space in the housing which contains the electrical machine and the oil is arranged to accumulate within the internal space in an amount such that an oil level is created whereby the oil is in contact with at least some portion of the stator windings. When an appropriate oil level is created in the internal space which contains the electrical machine, the oil will cover a lower portion of the stator and its windings. The oil accumulated there provides further cooling for these components.

According to an embodiment of the present invention, the extra oil loop is adapted to receiving oil from the oil system of the gearbox at a first location situated within the rotatable shaft. The rotatable shaft extends with advantage both through the electrical machine and through the gearbox. This makes it possible to use the rotatable shaft to lead oil to the electrical machine from the oil system of the gearbox. In this case an initial part of the extra oil loop may take the form of a duct extending in an axial direction within the rotatable shaft.

According to an embodiment of the present invention, the extra oil loop is adapted to leading oil back to the ordinary cooling system at a second location situated in an internal space which is adapted to containing the gearbox. The gearbox oil normally accumulates in such an internal space in the gearbox housing in an amount such that the oil comes into contact with the movable components of the gearbox. The extra oil loop may comprise an aperture situated between the first space and the second space in the housing, which aperture has a lower edge surface situated at a height level which defines the oil level in the first space. For the oil to run across from the second space to the first space, the oil level in the first space needs to be higher than the lower edge surface of the aperture. Situating the lower edge surface of the aperture at an appropriate height level makes it possible for a desired oil level to be maintained in the first space which contains the electrical machine.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below by way of example with reference to the attached drawings, in which:

Fig. 1 depicts an electrical machine which is cooled and lubricated by oil, and Fig. 2 depicts in more detail the extra oil loop which leads the oil through the

electrical machine.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Fig. 1 depicts a housing 1 arranged to enclose an electrical machine 2 and an undepicted gearbox. The housing 1 comprises a first housing component 1 a and a second housing component lb which are connected by bolts or similar fastening means. A rotatable shaft 3 extends substantially centrally through the housing 1. The shaft 3 is adapted to being connected to an undepicted shaft of a combustion engine via an adjustable clutch. The housing 1 has a first internal space 4 which surrounds the electrical machine 2 and a second internal space 5 which is adapted to containing the gearbox. The electrical machine 2 comprises in a conventional way a rotor 6 and a stator 7. The rotor 6 comprises a rotor hub 6a fastened on the shaft 3 by a splined connection 6b. The rotor hub 6a supports a rotor core 6c. The rotor 6 comprises also a bearing element 6d to which the rotor 6 is connected by two bearings 8. The two bearings 8 in this case are two ball bearings situated at an axial distance from one another. The stator 7 comprises a stator core 7a fastened in an appropriate way on the inside of the housing 1. The stator core 7a comprises the stator windings 7b.

The undepicted gearbox is lubricated and cooled by an oil system with a circulating oil. A schematically depicted oil pump 9a is adapted to circulating the oil in the oil system from the second internal space 5 which surrounds the gearbox to a

schematically depicted oil cooler 9b. The oil cooler may be cooled by air or by a coolant. After the oil has been cooled in the oil cooler 9b, it is led into the rotatable shaft via at least one radially disposed oil duct 10. From the radial oil duct 10, the oil is led to an axially disposed oil duct 1 1 which has a substantially central extent within the shaft 3. The axial oil duct 11 comprises a first duct element 1 a which is part of the oil system which cools the gearbox. The first duct element 1 la leads cooled oil to the second internal space 5 which contains the gearbox. The oil accumulates in the second internal space 5 in an amount such that an oil level B is maintained in the second internal space 5 which surrounds the gearbox. The oil in the second internal space 5 is used to cool and lubricate the components of the gearbox.

The oil system of the gearbox is in this case provided with an extra oil loop depicted in more detail in Fig. 2. The extra oil loop comprises a second duct element 1 lb of the axial oil duct 1 1 in the shaft 3. When the oil from the radial duct 10 reaches the axial oil duct 1 1, part of the oil is led through the first duct element 1 la and the remainder of the oil is led through the second duct element l ib and hence into the extra oil loop. The oil is led in the second duct element 1 lb to a radially disposed duct 12 in the shaft 3. The duct 12 has an aperture which ends substantially radially inside the electrical machine 2. The oil is led out from the duct 12 to a space 13 which extends annularly round the shaft 3. The space 13 is defined by the shaft 3 and a stationary portion 14 which is connected to the housing 1. The oil in the space 13 comes into contact with the splined connection 6b. The oil provides here lubrication and cooling for the splined connection 6b. The oil is led from the space 13 into a duct 15 which is defined by an internal portion of the rotor hub 6a and said stationary portion 14. The stationary portion 14 comprises an aperture so that the oil in the duct 15 can be led to a space 16 situated between the two bearings 8. The oil in the space 16 is led in opposite directions through the respective bearings 8. The oil thus cools and lubricates the bearings 8. After it has passed through the respective bearings 8, the oil reaches two substantially radial flow passages 17, 18 situated on opposite sides of the rotor's bearing element 6d. The oil is led here substantially radially outwards until it meets the protruding coil ends 7b i of the stator windings 7b. Finally, the oil reaches the housing's internal wall surface, where it runs downwards and accumulates on a bottom surface in the first internal space 4. An oil level A is formed in the first internal space 4. The oil is led thereafter from the first internal space 4 to the second internal space 5 via an aperture 19. The aperture 19 has a lower edge surface 19a which provides assurance that a certain oil level A is maintained in the first internal space 4. The oil accumulating on the bottom in the first internal space 4 is in contact with the portions of the stator 7 and the windings 7b which are situated in the lower part of the first internal space 4. When it leaves the first internal space 4 via the aperture 19, the oil leaves the extra oil loop. The oil which has passed through the extra oil loop is mixed with oil in the second internal space 5 which cools the gearbox.

According to the present invention, oil from the oil system of the gearbox is thus used to cool and lubricate selected components of the electrical machine 2. Oil from the ordinary oil system of the gearbox is therefore led in this case through an extra oil loop. The oil is led in the extra oil loop along a path such that it comes into contact with the components of the electrical machine 2 which primarily need cooling and, where necessary, lubrication. These components are thus provided with very good cooling and lubrication. During operation of the electrical machine 2, the oil in the oil loop comes into contact inter alia with, and hence lubricates and cools, the splined connection 6b. The oil is thereafter led through the two bearings 8, providing them with effective cooling and lubrication. The oil in the extra oil loop comes thereafter into contact with the rotor 6, leading to the latter being lubricated and cooled. Finally, the oil in the extra oil loop comes into direct contact with the stator windings 7b and, in particular, their coil ends 7b j. During operation of the electrical machine 2, the stator windings 7b and, in particular, their coil ends 7b i may become heated to a high temperature. In this case the oil in the extra oil loop comes into direct contact with the coil ends 7b i, providing them with very effective cooling. Maintaining an oil level A in the first internal space 4 promotes further cooling of the stator and its windings 7b by the oil present there. Moreover, in this case part of the air within the housing 1 is replaced by oil. Oil has significantly better thermal conductivity than air, thus assisting the transfer of heat from the components of the electrical machine to the external surface of the housing 1 by the oil and hence further promoting the cooling of the electrical machine 2.

The extra oil loop thus receives oil from the ordinary oil system in the duct 11 in the shaft 3 and returns the oil, after cooling and lubrication of the electrical machine 2, to the second internal space 5 situated downstream of the duct 11 with respect to the intended direction of oil flow in the ordinary oil system. The oil in the oil loop is thus led parallel with and in the same direction as the oil in the ordinary oil system. Hence the ordinary oil pump 9a can be used for leading the oil through the extra oil loop. The oil in the oil system is cooled in the oil cooler 9b after it has left the second internal space 5. The fact that the inlet duct 1 lb of the extra oil loop is situated substantially immediately downstream of the oil cooler 9b makes it possible for cooled oil to be received in the oil loop.

The invention is in no way limited to the embodiment to which the drawing refers but may be varied freely within the scopes of the claims.