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EP-4131733-B1 - PERMANENT MAGNET EXTERNAL ROTOR FOR ELECTRIC MOTOR, ELECTRIC MOTOR COMPRISING SAID ROTOR AND METHOD FOR MANUFACTURING SAID EXTERNAL ROTOR

EP4131733B1EP 4131733 B1EP4131733 B1EP 4131733B1EP-4131733-B1

Inventors

  • MARIONI, ELIO

Dates

Publication Date
20260506
Application Date
20210806

Claims (13)

  1. Permanent magnet external rotor (1; 1') for an electric motor (10), comprising a cup-shaped body (2) provided with a bottom wall (20) and a circumferential side wall (21); at least one magnet (3; 3'), defining an annular configuration and fixed inside said cup-shaped body (2); and a ferromagnetic metal insert (4) arranged between said circumferential side wall (21) and said at least one magnet (3; 3') so as to define the closure of a magnetic circuit of the electric motor (10); characterized in that said metal insert (4) is defined by a helical spring fitted around the annular configuration defined by the at least one magnet (3; 3'); wherein the helical spring adapts elastically to said annular configuration onto which it is fitted while ensuring at the same time the continuity of material along the entire circumference of said annular configuration and the consequent closure of the magnetic circuit.
  2. External rotor (1; 1') according to Claim 1, wherein at least the side wall (20) of said cup-shaped body is made of a plastic material overmoulded on top of the at least one magnet (3; 3') and the metal insert (4).
  3. External rotor (1; 1') according to Claim 2, wherein said cup-shaped body (20) comprises a flange (22) and an internal shoulder (23) which axially confine said metal insert (4) at the opposite ends.
  4. External rotor (1; 1') according to Claim 3, wherein said flange (22) and said internal shoulder (23) cover, entirely or partly, the axial opposed ends of the at least one magnet (3; 3').
  5. External rotor (1; 1') according to one of the preceding claims, wherein the helical spring which defines the metal insert (4) has, in the rest configuration, an internal diameter (d) smaller than the external diameter (D) of the annular configuration defined by the at least one magnet (3; 3') such that said helical spring is elastically deformed when fitted onto said at least one magnet (3; 3').
  6. External rotor (1;1') according to one of the preceding claims, wherein the helical spring which defines the metal insert (4) has, in the rest configuration, windings packed together and a length substantially equal to the axial length of the at least one magnet (3; 3').
  7. External rotor (1; 1') according to one of the preceding claims, wherein the helical spring which defines the metal insert (4) has windings with a square or rectangular cross-section.
  8. External rotor (1; 1') according to one of the preceding claims, wherein said bottom (20) comprises a central hub (25) connected, by means of a solid wall (24) or radial arms, to said side wall (20).
  9. External rotor (1) according to one of the preceding claims, wherein said at least one magnet is formed by a single toroidal magnet (3), defining a plurality of poles.
  10. External rotor (1) according to Claim 9, wherein said toroidal magnet (3) is a ring made of plasto-ferrite.
  11. External rotor (1') according to one of Claims 1-8, wherein said at least one magnet is formed by a plurality of magnetic wedges (3') arranged next to each other so as to define an annular configuration.
  12. Electric motor (10) comprising an internal stator (30) provided with a plurality of poles and an external rotor (1; 1') according to one of the preceding claims, mounted rotatably on top of said internal stator (30), wherein said at least one magnet (3; 3') surrounds the poles with the arrangement of an air gap in between.
  13. Method for manufacturing a permanent magnet external rotor (1; 1') for an electric motor (10), comprising the steps of: providing a toroidal magnet (3) or a plurality of magnetic wedges (3') arranged next to each other so as to define an annular configuration; fitting a helical spring made of ferromagnetic metallic material around an outer cylindrical wall of said annular configuration defined by the toroidal magnet (3) or by the plurality of magnetic wedges (3') wherein said helical spring adapts elastically to said annular configuration onto which it is fitted while ensuring at the same time the continuity of material along the entire circumference of said annular configuration and the consequent closure of the magnetic circuit; overmoulding, on top of the toroidal magnet (3) or the magnetic wedges (3') and the helical spring, a cup-shaped body (2) at least partially made of plastic material, wherein said cup-shaped body (2) comprises a bottom (20) and a side wall (21) which surrounds externally the helical spring.

Description

Field of application The present invention relates, according to its more general aspect, to a permanent magnet external rotor, in particular for a synchronous electric motor, and to an electric motor comprising such a component. The invention also relates to a method for manufacturing said external rotor. The technical field of the invention therefore concerns electric machines. Electric motors with permanent magnet external rotors may find a useful application in various industrial sectors; more specifically, the present invention has a preferred application in the industrial sector of large-size electric household appliances, for example in the motors for operating the drum and/or the fans in washing machines or in laundry drying machines. Prior art Electric motors with a permanent magnet rotor are known in the art and at present are used in a plurality of applications, including the operation of components and pumps in electric household appliances. The conventional configuration of these like other motors comprises an internal rotor rotating inside a fixed stator; in some cases, however, it is preferred to use an external, cup-shaped, rotor which surrounds a central stator. This solution has proved for example to be particularly advantageous, allowing a substantial reduction in the dimensions, during the manufacture of a motor unit which operates fan and drum of laundry drying machines. The external rotor of these electric motors, in particular in the sectors such as that of electric household appliances in which the small dimensions and production costs of the components are critical, may be advantageously made from a plasto-ferrite toroidal magnet which defines a plurality of magnetic poles; alternatively, it is possible to use a plurality of ferrite magnetic wedges arranged next to each other so as to define a ring. The ring is inserted inside the aforementioned cup-shaped structure, which may be formed as a cap made of plastic material. It is obviously indispensable to insert a metal ring on the outside of the magnets, in order to close the magnetic circuit and allow operation of the electric machine. The operation of joining together the metal ring and magnets is a critical step in the production process since it is required to ensure close contact between the two elements in order to guarantee correct operation of the motor. It is therefore necessary to design extremely precisely the dimensions of the metal ring which must be able to fit perfectly on top of the toroidal magnet or the plurality of magnets arranged next to each other. This constraint on the tolerances is particularly onerous, also in view of the ratio between the diameter and axial length of the two elements to be joined together. Moreover, the joining step itself is also costly in technological and time-related terms. It is pointed out that the metal ring considered in the prior art is structurally rigid owing to its closed-sleeve configuration; on the other hand, it is not possible to make the part more elastic by designing it as an open metal band since there is the risk of the magnetic circuit being interrupted in the region of the opening if the gap is too big. A further difficulty arises more specifically with regard to the manufacture of the external rotor using a plurality of magnetic wedges. In fact, during moulding the injected liquid risks displacing the individual wedges, resulting in an irregular form in the region of the air gap, this being unacceptable for correct operation of the electric machine. For this reason, nowadays the more costly variant with a continuous ring made of plasto-ferrite is preferred, this requiring precise and costly machine-grinding of the cylindrical inner surface. Documents US 2018/026849 A1, DE 10 2010 051 264 A1, US 3,581,394 A, WO 2004/040737 A1 disclose rotors for electric motors according to the prior art. The technical problem underlying the present invention is therefore that of devising an alternative embodiment of the external rotor which allows simplification of the production process and reduction of the manufacturing costs, in particular avoiding the need to provide a metal component with very precise dimensional tolerances. Summary of the invention The technical problem described above is solved by a permanent magnet external rotor for an electric motor according to claim 1. It is pointed out that the annular configuration may be defined by a single toroidal magnet, for example a plasto-ferrite ring defining a plurality of poles or, alternatively, by a plurality of magnetic wedges arranged next to each other. The solution to the technical aforementioned technical problem consists in the original use of a helical spring, namely a spirally wound metal body which adapts elastically to the annular configuration of the at least one magnet onto which it is fitted, but which on the other hand ensures the continuity of material along the entire circumference and the consequent closure o