The present invention relates to a slip ring for an electric machine.

This invention therefore finds its main application in the production of electric machines (alternators, belt starter generators and electric traction machines), in particular of electric machines for motor vehicles, preferably of the excited rotor type.

In the prior art, these slip rings are used to allow the (fixed) supply to excite the rotor (movable) windings and are normally made of plastic tubular elements, typically molded, to which the conductive elements, constituted by a pair of rings placed in sliding contact with the brushes of the electric machine and a series of conducting wires extending between the rings and the connecting ends of the windings, are bound.

One of the main problems in the manufacture of these components lies in the need to combine different materials (plastic and copper), ensuring the stability of the assembly at the speed and temperature reached by the alternator.

The aim of the present invention is therefore to make available a slip ring for an electric machine, able to overcome the drawbacks related to the above-mentioned know technique.

In particular, it is object of the present invention to provide a slip ring for an electric machine easy to manufacture and highly reliable.

More precisely, the object of the present invention is to provide a slip ring for an electric machine wherein the geometry of the components allows to compensate for deformations due to mechanical, thermal or electrical stress.

Said objects are achieved by a slip ring having the technical characteristics of claim 1 , i.e. a slip ring comprising an insulating element of tubular shape provided with a cylindrical body extending along its own central axis between a first end portion and a second end portion, at least two conductive elements each extending from a first end, placed in correspondence of the first end portion of the cylindrical body, and a second end, placed in correspondence of the second end portion of the cylindrical body, wherein each conductive element comprises at least one ring fitted on the first end portion of the tubular body and defining the first end, and a conductive wire (preferably having a circular or rectangular section) extending between the ring and a free end facing outwardly to the second end portion of the cylindrical body.

According to an aspect of the invention, each wire comprises at least one deformation compensation section housed inside the cylindrical body in order to reduce the thermal / mechanical stress due to the different response of the materials to the stresses.

These and other features, together with the relating advantages will become more apparent from the following exemplary, therefore non- limiting, description of a preferred, therefore not exclusive, embodiment of a slip ring for an electric machine according to what is shown in the appended figures, wherein:

- figure 1 is a schematic sectional view of a slip ring for an electric machine according to the present invention;

- figure 2 shows a perspective view of another embodiment of a slip ring for an electric motor according to the present invention;

- figure 3 shows a longitudinal sectional view of the slip ring of figure 2.

With reference to the appended figures, number 1 indicates a slip ring for an electric motor according to the present invention.

This slip ring 1 comprises an insulating element 2, of a tubular shape, and a plurality of conductive elements 6.

The insulating element 2 is preferably made of plastic material.

The insulating element 2 comprises a substantially cylindrical body 3 having an outer cylindrical surface 3a and an inner cylindrical surface 3b.

The inner cylindrical surface 3b preferably defines therein a housing which, when assembled, is fitted on the rotating shaft (not shown) of the rotor. The cylindrical body 3 therefore extends along its own central axis "A", in use corresponding to the rotating axis of the rotor, between a first end portion 4a and a second end portion 4b.

The second end portion 4b has a widened conformation with respect to the first one 4a, so as to allow the engagement on the rotating shaft.

More precisely, the second end portion 4b defines an annular shoulder 5 radially external with respect to the central axis "A".

Preferably, the conductive elements 6 extend each from a first end 6a. placed in correspondence of the first end portion 4a of the cylindrical body 3. and a second end 6b, placed in correspondence of the second end portion 4b of the cylindrical body 3.

More precisely, each conductive element 6 comprises at least one ring 7. 8, fitted on the first end portion 4a of the cylindrical body 3 and defining the first end 6a.

Thus, the slip ring 1 comprises a first 7 and a second ring 8 arranged in axial succession and placed side by side, fitted or co-molded around the cylindrical body 3. near the first end portion 4a.

The rings 7, 8 are coaxial with the cylindrical body 3 and have an exposed annular surface 7a, 8a which, in use. is placed in sliding contact with the power brushes.

Preferably, the cylindrical body 3 has, in correspondence of its own first end portion 4a, a circumferential groove or recess 9 suitable to allow the engagement of the rings 7, 8.

Preferably, the groove or recess 9 has at least two circumferential edges defining two respective (axial) retaining radial shoulders 9a of the rings 7. 8.

In the preferred embodiment, the rings 7, 8 and the cylindrical body are co- molded. In this way, preferably, the cylindrical body 3 comprises, in correspondence of the first end portion 4a, a first and a second groove 9 being parallel to each other, each of which are counter-shaped to the first 7 or the second ring 8, respectively, for housing and locking them both radially and axially.

Moreover, the conductive elements 6 comprise at least one pair of conducting wires (preferably of circular or rectangular section) 10, 11 each extending starting from a first terminal portion 10a, 11a, constrained to a respective ring 7, 8 (and defining the first end 6a of the conductive element), and a second terminal portion 10b 11 b (or free end), projecting externally to the tubular body 2 near its second end portion 4b.

These conductive wires 10, 11 extend internally to the tubular body 3, between the outer cylindrical surface 3a and the inner cylindrical surface 3b.

In a first embodiment, the cylindrical body comprises at least a pair of at least partly axial channels extending between the first end portion 4a and the second end portion 4b, wherein the conductive wires 10, 11 are housed.

Alternatively, and preferably, the conductive wires 10, 11 are co-molded to the tubular body 3 and are completely embedded in the same.

In correspondence of the respective second end sections 10b, 11 b of the wires 10, 11 , the conductive elements 6 comprise two strips 12.

Said strips 12 are fixed to the second end portions 10b. 11 b (preferably welded) and inserted into the second end portion 4b of the cylindrical body 3 so as to protrude radially.

More precisely, these strips 12 define the radial projections of the annular shoulder 5.

In the preferred embodiment, the wires 10, 11 comprise at least one deformation compensation portion 10c, 11c.

Advantageously, in this way it is possible to compensate the different deformability of the wire with respect to the plastic material surrounding it, thus avoiding the occurrence of cracks in the cylindrical body 3. In some embodiments, this portion 10c, 11c comprises at least one coil or a spring-shaped portion, which allows the wire deformation to be absorbed.

Preferably, according to a preferred aspect of the present invention, the slip ring 1 comprises an excitation element 13 for a detection sensor of the number of revolutions around the central axis "A".

The excitation element 13 is constrained to (or integrated in) the insulating element. The detection sensor (not shown) is instead arranged in correspondence of the fixed part of the electric machine. For example, the sensor could be integrated into the power supply circuit of the electric machine, integral with the casing of the same and operate according to the principle of the Hall effect (Hall sensor).

Preferably, the cylindrical body 3 has an appropriately shaped housing seat for housing said excitation element 13.

In the preferred embodiment, the excitation element 13 is placed coaxially with said central axis "A", preferably in correspondence of the first end portion 4a of the cylindrical body 3.

Thus, said housing seat is preferably defined by an axial slot or recess formed in correspondence of the first end portion 4a along said central axis "A".

Preferably, said excitation element 13 is defined by a magnet or magnetic element 13a. More preferably, this magnet or magnetic element 13a has a cylindrical (or discoidal) shape and is engaged in said housing seat.

In the shown embodiment, the excitation element 13 is secured to the cylindrical body 3 (in particular in the housing seat) by gluing.

Alternatively, however, this excitation element could be integrated into the cylindrical body in another way, either by mechanical interference or, for example, by co-molding.

The invention achieves the intended objects and achieves important advantages. In fact, by realizing the slip ring by molding the insulating body around the conductive elements already placed and suitably shaped with the compensation section, it is possible to fix the different response of the materials to the thermal / mechanical stresses, simplifying, inter alia, the manufacture of the product.