The present invention refers to an electric motor with cooling circuit, in particular an axial-flow electric motor.

Electric vehicles, especially high-performance vehicles, have very powerful electric motors. As such, they dissipate a lot of heat, which must be disposed of. For example, high- power vehicles have electric motors with rated powers of hundreds of KW, hence the primary need to cool them. Especially for high-rpm motors the phenomenon of eddy currents can become relevant and contribute significantly to motor overheating.

In this regard, the Applicant has discovered that even the rotor, despite it cools with rotation, needs cooling, especially in an axial-flow motor.

The main object of the invention is to improve the present state of the art. The object is achieved by what is stated in the attached claims; and advantageous technical characteristics are defined in the dependent claims.

An electric motor is proposed comprising: a rotor that is rotatable about a rotation axis and equipped with a circular series of magnetic elements around said axis; a shaft that is integral with the rotor and extends along said axis, a stator comprising a seat for rotatably supporting the shaft, and windings arranged in a circular series around said axis, each winding for creating a magnetic field, with polar axis parallel to the rotation axis, through which to rotate the rotor thanks to the magnetic interaction with the corresponding circular series of magnetic elements of the rotor; wherein the rotor comprises internal channels for the circulation of a refrigerant. A method is also proposed for cooling an electric motor comprising: a rotor as defined above, a shaft as defined above, a stator as defined above, with the step of circulating a refrigerant fluid inside the rotor. Said internal channels may develop inside the rotor according to various path patterns, e.g. a serpentine or a spiral around the axis. According to a preferred variant, said internal channels extend into the rotor with polar symmetry with respect to said axis.

According to a further preferred variant, which ensures capillary coverage of the rotor’s volume and easy construction, said internal channels, in particular all of them, extend radially with respect to said axis. In particular, said channels extend radially with respect to said axis and comprise a radial delivery section and a radial return section for the fluid.

Each inner channel of the rotor comprises a fkuid inlet, from which it is supplied with refrigerant, and a fluid outlet from which it discharges heated fluid. The fluid inlet and outlet may be unique for a channel (the inlet and outlet of one channel carry fluid for that channel only) or common for many or all channels (the inlet and outlet carry fluid for more than one channel).

In general, the part of fluid circuit outside the rotor may be configured in any way.

According to a preferred variant, which simplifies the fluid circuit, the fluid inlets and outlets of the channels (e.g. all) are arranged (e.g. circularly) on a (e.g. cylindrical) surface in contact with or facing the shaft. This solution has the advantage of concentrating in a sufficiently narrow area the points of fluid injection into the rotor and the points of fluid drainage from the rotor, and is particularly effective if the rotor is preferably keyed onto the shaft. As an alternative or in combination, the fluid inlets and outlets of the channels (e.g. all) may be arranged to open onto the periphery or lateral surface of the rotor or to a side of the rotor orthogonal to said axis.

According to a more preferred variant, which simplifies the fluid circuit, the shaft comprises internal fluid channels configured to circulate fluid in the internal rotor’s channels. The shaft’s internal channels and the rotor’s internal channels form a circuit for fluid. In particular, the internal channels of the shaft have mouths marching with fluid inlets and outlets belonging to channels of the rotor, to simplify their coupling.

In particular, the outlets of the shaft’s channels are located on the side surface of the shaft, to facilitate coupling with the rotor’s channels if the rotor is keyed on the shaft. As an alternative or combination, the outlets are located at one end of the shaft. A first possibility to create the internal channels in the rotor would be by mechanically machining the rotor, which is a long and/or complicated thing to do. A simpler variant envisages that the rotor comprises two pieces that can be coupled together, and one of the two pieces comprises the internal channels. The other piece for example comprises the magnetic elements. Thus the creation of the channels can occur in a component of the rotor which can be easily machined individually.

In particular, to further simplify the machining process, the piece comprising the internal channels may be made up, in turn, of two, e.g. superimposed, elements or parts: one integrating the fluid delivery channels (from the outside of the rotor to the inside of the rotor) and one integrating the fluid return channels (from the inside of the rotor to the outside of the rotor).

More specifically, each of the two elements comprises a disc or ring with radial fins that extend in relief from a side of the disc or ring, so that the fins form open channels that run radially from the center to the perimeter of the disc or ring. The channels between the fins of one disc or ring carry fluid in one direction, and the channels between the fins of the other disc or ring carry fluid in the opposite direction. By coupling the two elements one on top of the other and fixing them in a circular seat of the rotor with a cover, one disc or ring closes the channels of the other and the cover closes the channels of the second.

The ring configuration is convenient if the elements are mounted on the rotor coaxially to the shaft.

The cooling fluid is e.g. water or oil or a synthetic fluid.

Preferably, said seat for the rotor is a cylindrical cavity of the stator.

The motor has preferably two identical rotors, as defined above, which are integral to the same shaft. The rotors rotate adjacent to opposite sides of the stator. In this case the cylindrical cavity of the stator is a pass-through cavity in which the shaft is rotatably housed.

Preferably the motor comprises means to circulate a refrigerant fluid in said internal channels of the rotor, and/or a reservoir or tank of refrigerant fluid communicating with the circuit for fluid that carries refrigerant fluid to the channels of the rotor.

Further advantages will be clear from the following description, which refers to an example of a preferred embodiment of motor in which:

• Figure 1 shows an exploded view of a rotor of an electric motor;

• Figures 2 and 3 show vertical cross-sectional views of the electric motor.

Equal numbers in the figures indicate equal or substantially equal parts . In order not to crowd the drawings, equal elements are not all numbered.

Fig. 2 and 3 show an electric motor MC that comprises a shaft 10 integral with a rotor 50, both rotatable about an X axis

The motor MC preferably has polar symmetry around the X axis.

The shaft 10 coaxially passes through a stator 30 with which it couples through bearings 32 placed between the stator 30 and the shaft 10.

The stator 30 has a circular series of electric windings 34 which are arranged with polar symmetry around the X axis and are equipped with a ferromagnetic core. In use each winding 34 generates a magnetic field to rotate the rotor 50, which comprises a circular series of magnetic elements 52, e.g. permanent magnets, that cooperate with the magnetic field generated by the electric windings 34. This magnetic field has polar axis parallel to the X axis. The magnetic elements 52 are mounted on an outer disc 58.

To cool the rotor 50, it comprises internal channels for the circulation of a refrigerant fluid. The internal channels extend into the rotor with polar symmetry with respect to the X axis and are visible in Fig. 1.

The internal channels derive from the superposition of two elements 60, 70, each comprising a ring 62, 72 with radial fins 64, 74 which extend in relief from a side of the respective ring.

The fins 64, 74 form open channels that run radially from the center to the perimeter of the ring 62, 72.

The two elements 60, 70 are coupled one on top of the other and fixed inside a circular seat 80 of the rotor 50 with a cover 82, e.g. by bolts 84. In this way, the ring 72 closes the channels of the fins 64, while the cover 82 closes the channels of the fins 74.

The two rings 62, 72 are equal, while the seat 84 has a slightly larger diameter than the two rings 62, 72, to create an outermost circular space S that allows the passage of fluid from one ring to the other.

The ring configuration of the elements 60, 70 allows to mount them coaxially to the shaft 10 and to exploit the latter for the circulation of the fluid.

The channels created between the fins 64, 74 are configured to carry fluid in one direction and the opposite direction, respectively.

As it can be seen, once the assembly is completed, the channels extend radially with respect to the X-axis and thanks to their position being central to - and recessed in - the rotor 50, they allow a fluid circulation that can efficiently subtract heat from the rotor 50.

Each channel is supplied with refrigerant fluid from an inlet and discharges heated fluid through a fluid outlet. In the example shown, each channel between the fins 64, 74 has as inlet and outlet a circular space that substantially follows the circular edges of the rings 62, 72.

With reference to the cross-sections in Fig. 2 and 3, it can be seen that a fluid present at the smaller circular edge of the ring 62 can move away from the X axis by running through the channels between the fins 64 up to the outer perimeter of the ring 62. Flere it reverses its direction in correspondence of the space S, it arrives onto the ring 72, it passes inside the channels between the fins 74 and it returns towards the X axis up to the smaller circular edge of the ring 72. Along the way, the fluid has subtracted heat from the body of the rotor 50.

The channels in the rotor 50 by construction have inlets and outlets circularly arranged on a surface in contact with the shaft 10, i.e. the smaller edges of the rings 62, 72. Then the shaft 10 can be exploited to transport the cooling fluid. For this purpose, the shaft 10 comprises fluid-delivery internal channels 12 and fluid-return internal channels 14 which are configured to circulate fluid in the internal channels of the rotor 50. In particular, the channels 12, 14 have respectively superficial outlets 16, 18 on the side surface of the shaft 10. The outlets 16 match the smaller edge of the ring 62 and supply the channels between the fins 64, while the outlets 18 match the smaller edge of the ring 72 and drain fluid from the channels between the fins 74.

Said seals or O-rings 98 separate the fluid flow relative to the outlets 16, 18 isolating them. For example, the internal channels 12 for fluid delivery are a bundle of radial by-passes that reach the outlets 16 and branch out from a common central duct 12a coaxial to the X axis.

For example, the internal fluid-return channels 14 are a bundle of channels parallel to the X-axis and arranged with polar symmetry around the X-axis, from which radial by-passes 14a extend to reach the outlets 16.

The channels in the shaft 10 may also vary from what is illustrated.

The channels in the shaft 10 may also be exploited to circulate the refrigerant fluid inside the stator 30.