The main object of the invention relates to the rotors of electric motors for the traction drive of an electric vehicle, where the electric motor is directly cooled by dielectric coolant - oil.

EP 3 507 889 B1 discloses a rotor of an asynchronous electric motor, comprising a squirrel-cage and laminated rotor core cast in aluminum alloy and forming the filler body, where two shaft journals are used to fasten the filler body in place by a cooling duct interconnecting the shaft journals on both sides of the filler body. The oil for cooling a rotor, a stator, and a winding of the electric motor is introduced into the cooling duct, where a plurality of radial holes within the cooling duct if used to deliver the oil into the jacket-like cavity between the filler body and the cooling duct. Finally, the oil is released out of the rotor through the plurality of radial bores and/or radial passages on both sides of the rotor formed by the filler body and shaft journal.

In the known solutions, not all laminated rotor cores are cast in aluminum alloy to provide a filler body, thus for alternative embodiments, where filler body is not available or not an option for sake of electromagnetic design limitations, there is a lack of a solution for a directly cooled rotor of synchronous electric motors.

It is therefore the object of the invention to provide an improved or at least alternative embodiment of the apparatus described, in which disadvantages described are overcome.

This object is solved according to the invention by the object of independent claim 1 . Advantageous embodiments are the subject of the dependent claims. The present invention is based on the general idea to use an impeller on a rotor of an electric motor for traction drives in a function of a pump for propelling a coolant through the cooling system of the electric motor, and thereby to use a hollow shaft for receiving a laminated rotor core in place, wherein the coolant is released into the shaft preferably by a non-contacting stationary nozzle within an area of an inlet aperture, thus the coolant is introduced directly into the main cavity of the hollow shaft comprising a plurality of channels with outlet apertures in the area of an intake for the impeller. Advantageously, by cooling a stator and windings of the electric motor with impeller in a combination of the hollow shaft, a cooling performance and thus a continuous power of the electric motor is significantly increased, wherein a slip of the coolant is achieved by a gradual acceleration of the coolant to the target circumferential speed within the main cavity of the hollow shaft of the rotor, thus also the efficiency of the electric motor is essentially improved.

In the present invention, the compound term "first/second" is used to simplify the description. In the context of the present invention, the individual terms "first" and "second" of the compound term "first/second" are always linked with an "and/or". Thus, in the rotor, the "first" element and/or the "second" element may be present. In this case, the respective "first" element is exclusively associated with a further "first" element of the rotor or the first axial end of the rotor, and the respective "second" element is exclusively associated with a further "second" element of the rotor or the second axial end of the rotor. Where the individual terms "first" and "second" are not used in the composite term "first/second", they are to be understood in accordance with this usage. Moreover, in the present invention, the terms "axial" and "radial" always refer to the axis of rotation. A rotor of an electric motor comprises a hollow shaft and a rotor core. In an advantageous embodiment, the hollow shaft is a two-part hollow structure, comprising a hollow shaft core and corresponding supporting body, adapted to be assembled and adapted to receive the rotor core in place. In an advantageous embodiment, the hollow shaft is a cylindrical body comprising a first end and a second end on the opposite side of the hollow shaft. In an advantageous embodiment, the hollow shaft is assembled by a press-fit connection. Such a press-fit connection is easy to make and does not require any further fasteners. The shaft in an alternative embodiment is assembled by interference fit and/or transition fit in combination with welding. In an advantageous embodiment, the hollow shaft comprises an inlet aperture for introducing the coolant, and a datum feature for positioning the rotor core on the outer surface of the hollow shaft. For releasing the coolant out of the hollow shaft in an advantageous embodiment the hollow shaft comprises a plurality of channels and/or apertures and/or passages and/or bores, provided and/or created and/or manufactured by machining after assembling the shaft, at least in the area of one end of the hollow shaft. The rotor core comprises a cylindrical body further comprising a plurality of laminated sheets of electric steel, wherein the rotor core comprises a first adjacent front surface on a first side of the rotor core and a second adjacent front surface on the second - opposite - side of the rotor core. In an advantageous embodiment, the rotor core is installed on the hollow shaft by interference fit and/or transition fit, and or clamping nut. The rotor core in the advantageous embodiment further comprises a plurality of magnets and a resin for fixation of the magnets in the rotor core, wherein the rotor core further comprises an impeller with a plurality of blades to promote the flow of the coolant within an enclosure of the electric motor. In an advantageous embodiment, the channels for releasing the coolant out of the hollow shaft are in the area close to the outer peripheral edge of the hollow shaft, more precisely in the area of a press-fit connection between the hollow shaft core and corresponding supporting body, wherein the number of the channels is preferably equal to the number of the blades on the impeller, and wherein the position of the channels is aligned to a leading edge of the corresponding blades.

The inlet aperture can be arranged radially inwardly of the plurality of channels, and an inner wall of the main cavity can extend over a curved or inclined section towards the maximum diameter of the main cavity. In this way the coolant can be gradually accelerated to a maximum angular velocity in the cavity due to boundary flow of the coolant along the cavity wall.

Further important features and advantages of the invention are apparent from the subclaims, from the drawings, and from the accompanying figure description based on the drawings.

It is understood that the above features and those to be explained below can be used not only in the combination indicated in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

Preferred embodiments of the invention are shown in the drawings and will be explained in more detail in the following description, wherein identical reference signs refer to identical or similar or functionally identical components.

It shows, each schematically

Fig. 1 an isometric view of a rotor for an electric motor according to the invention in an advantageous embodiment;

Fig. 2 a side view of the rotor for the electric motor according to the invention in the advantageous embodiment; Fig. 3 a sectional view of the rotor for the electric motor according to the invention in the advantageous embodiment;

Fig. 4 a detailed sectional view of the rotor for the electric motor according to the invention in the advantageous embodiment;

Fig. 5 an isometric view of the hollow shaft according to the invention in the advantageous embodiment, here shown in an exploded view.

Fig. 1 shows an isometric view of an overmolded rotor for an electric motor according to the invention in an advantageous embodiment. The rotor comprises a hollow shaft and a rotor core 2. The hollow shaft comprises a shaft core 1 A and a supporting body 1 B, wherein an inlet aperture 1 AO is used for introducing a coolant into a main cavity 7 of the hollow shaft and wherein a plurality of channels 1 C are provided in the area of a junction surface between the shaft core 1 A and supporting body 1 B. The main cavity 7 ensures homogeneous pressure distribution and uniform coolant discharge, preferably via all outlet apertures 3. The rotor core 2 comprises an impeller 2A with a plurality of blades 4, wherein the position of the channels 1 C outlet apertures 3 in advantageous embodiment is aligned with a leading edge of the blades 4. The blades 4 are preferably curved.

As shown in figs. 1 and 4, the outlet apertures 3 are directed radially outward and are arranged circumferentially between two adjacent blades 4. Regarding figs. 3, 4 and 5 the channels 1 C have an axial area 5 and a radial area 6 adjoining it, wherein the outlet apertures 3 being located at the end of the radial area 6. Fig. 2 shows a side view of the rotor for the electric motor according to the invention in the advantageous embodiment, where a plane for sectional view A-A is indicated.

Fig. 3 shows a sectional view A-A of the rotor for the electric motor according to the invention in the advantageous embodiment, where an area of detailed view B is indicated. In advantageous embodiment the hollow shaft, more precisely the supporting body 1 B comprises a datum feature 1 D for positioning the rotor core 2 and impeller 2A in regards to the position the channels 1 C for releasing the coolant out of the hollow shaft. Advantageously, the performance of the impeller and thus cooling of the electric motor is significantly increased.

According to Fig. 3, the outlet apertures 3 are arranged only at the upper end of the main cavity 7, whereby an impeller 2 can also be arranged at the lower end. In purely theoretical terms, outlet apertures (not shown) could also be provided at the lower end, which cause coolant to flow out of the main cavity 7, preferably in the axial direction between two adjacent blades 4 of the impeller 2. From there, the coolant can then be conveyed further due to the centrifugal force and the blades 4. This could result in further improved cooling and, associated with this, an increase in the performance of the electric motor.

The inlet aperture 1 AO is arranged radially inwardly of the plurality of channels 1 C, and an inner wall 9 of the main cavity 7 extends over a curved or inclined section 8 towards the maximum diameter of the main cavity 7. In this way the coolant can be gradually accelerated to a maximum angular velocity in the main cavity 7 due to boundary flow of the coolant along the inner wall 9. Fig. 4 shows a detailed sectional view of the rotor for the electric motor according to the invention in the advantageous embodiment, wherein the channels 1 C for releasing the coolant out of the hollow shaft and the datum feature 1 D are clearly visible.

The formation of the channels 1 C at the supporting body 1 B makes it comparatively easier to manufacture the channels 1 C, since they do not have to be manufactured as closed channels, but the channels 1 C are only formed by the assembly of 1 B with the shaft core 1 A. Thus, only a partial geometry of the channels 1 C has to be manufactured in the supporting body 1 B. This makes it generally easier to manufacture the supporting body 1 B. The channel 1 C is formed by the support body 1 B and the shaft core 1 A.

Fig 5 shows an isometric view of the hollow shaft according to the invention in the advantageous embodiment in an exploded view, wherein a details of channels 1 C structure in form of a groove on the supporting body/shaft 1 B are clearly visible, and wherein the surface for press-fit connection utilization between the shaft core 1 A and matching surfaces on the supporting body 1 B are evident. In addition or as an alternative to the press connection, an adhesive connection can of course also be used.

All parts of the rotor for the electric motor, in particular, the hollow shaft within the description is shown and described in the best mode embodiment. However, deviating shapes/sizes/distributions are also conceivable, wherein the coolant is preferably an oil, an air, or a mixture of oil and air.