The present invention relates to rotary electric machines, and more particularly to the rotors of such machines.

BACKGROUND

Asynchronous electric motors conventionally comprise a stack of magnetic laminations, with slots passing through this laminations stack. Aluminum is injected under pressure to form bars which are connected to the outside of the laminations stack by short-circuiting rings.

In order to improve electrical performance, it may prove advantageous to reduce the electrical resistivity of the bars still further by replacing the aluminum with copper. However, because the melting point of copper is far higher than that of aluminum, it becomes difficult to inject copper into the slots.

One known solution thus consists in introducing conducting copper bars into the slots and in injecting aluminum under pressure to fill the space left empty by the bars inside the slots.

Applications WO2011015494, WO2010100007, WO2009038678, US2011291516, WO2014067792, WO2011020788, WO2015071156, WO2012041943, JPH10234166, CN1925280, JPH11206080, JP2005278373, and JPH1028360 describe rotors of which the laminations stacks have slots accepting copper bars and into which aluminum is injected under pressure.

Application US 2015/288265 discloses a rotor comprising a stack of laminations and a cast structure. The latter comprises two end rings and a plurality of grooves around the laminations stack extending between these end rings.

Application DE 102010041796 discloses a rotor comprising a wire wound around copper bars in order to prevent the latter from oscillating as the aluminum is cast.

Application JPS56103953 discloses a rotor comprising copper bars inserted in slots of a laminations stack and short-circuiting rings made of aluminum cast on the ends of the bars, and in which the laminations are angularly offset about the axis of rotation of the rotor.

SUMMARY

One problem that arises when injecting aluminum is that of holding the bars in the corresponding slots. This retention needs not to hamper the creation of the rear short-circuiting ring.

The invention seeks to solve this problem, and achieves this using a method for manufacturing a rotor of a rotary electric machine, comprising a stack of magnetic laminations defining the slots in which bars are made from a first electrically conducting material are received, in which method a second electrically conducting material, different from the first, is injected using an injection machine, from the front of the laminations stack, the bars being held at their rear end against the pressure associated with the injection by a positioning tool that passes through a cavity used for forming a short-circuiting ring at the rear of the laminations stack.

By virtue of the invention, the bars are held axially in the slots during the injection without there being a need to provide specific arrangements in the region of the slots. The manufacture of the rotor can therefore remain simple. For example, thanks to the axial retention of the bars by the positioning tool during injection, there is no need to provide firm clamping of the bars in the laminations stack in order to immobilize them axially relative to said stack. It is possible to insert the bars into the slots with a relatively large clearance. Thus, there is no need to use precision-machined copper bars and it is possible to use bars in their as-extruded state. That avoids expensive machining operations. Neither is it necessary to provide elements attached to the bars in order to immobilize them relative to the laminations stack, such as wires wound around the bars, which complicate the production of the rotor. In addition, the insertion of the bars into the slots is made easier, because firm clamping is not needed, and the use of a powerful press to drive the bars into the slots is avoided, which press would otherwise be indispensable. Because the bars do not need to be firmly clamped in the slots, that also offers greater design freedom in the choice of the shape of the slots and the cross section of the bars, and shapes that optimize certain properties, such as, for example, electrical performance and/or slot-filling, can be selected more easily.

Preferably, the bars protrude beyond the laminations stack to come into contact with the positioning tool.

Preferably, the bars are in the as-extruded state, and are not machined along their length.

Preferably, the bars are housed in the slots with a clearance present between the bars and the slots. In particular, this clearance may be such that, in the absence of the positioning tool, the injection pressure would cause the bars to move axially within the stack. In other words, the clamping of the bars within the stack prior to the injection step is not firm enough for the bars to be able to withstand the pressure of the injection without moving relative to the stack, in the absence of the positioning tool.

The positioning tool can be withdrawn on the demolding of the rotor.

The positioning tool may comprise fingers each having a slightly conical shape converging toward the laminations stack.

Preferably, the first material is copper and the second material aluminum.

The rotor may comprise a ring made from a conducting material, preferably the first conducting material, to come into contact with the positioning tool. In that case, the number of fingers is, for example, lower than the number of bars.

Preferably, the magnetic laminations contain a magnetic steel, for example silicon steel, with its various grades. The thickness of each lamination is for example comprised between 0.35 mm and 0.65 mm.

A further subject of the invention is a rotor of a rotary electric machine, notably obtained by implementing the method defined hereinabove, comprising:a stack of magnetic laminations exhibiting slots,bars made from a first conducting material housed in the slots,a second material injected into the slots,short-circuiting rings cast with the second material injected into the slots, one of these rings having recesses opening to the outside and extending as far as the bars or as far as an annular-shaped insert resting on the bars.

