Methods of manufacturing rotors having interfering sintered magnets and carbon filament sheaths for electric motors

Abstract

A method of manufacturing an electric-motor rotor may include providing a magnet of sintered magnetic material, the magnet having a first diameter; providing a sheath of composite material, the sheath having an inner diameter smaller than the first diameter; reducing the first diameter to a second diameter; fitting the sheath onto the magnet when the magnet has the second diameter; and letting the magnet with the sheath fitted thereon to interfere, allowing the sheath to exert inward pressure on the magnet. An electric-motor rotor may include a magnet of sintered magnetic material, the magnet having a first diameter; and a sheath of composite material, the sheath having an inner diameter smaller than the first diameter. The sheath may be associated with the magnet in such a way that the magnet with the sheath fitted thereon interfere so that the sheath exerts inward pressure on the magnet.

Claims

1. A method of manufacturing a rotor for an electric motor, the method comprising: providing a magnet of sintered magnetic material, the magnet having a first diameter; forming a radial holding sheath of composite material, the forming including winding at least one carbon filament around a chuck to form a wound carbon filament arrangement that is fixed in position by a layer of resin and has an inner diameter that is smaller than the first diameter; reducing the first diameter of the magnet to a second diameter; fitting the sheath onto the magnet concurrently with the magnet having the second diameter and the sheath being at room temperature; and letting the magnet with the sheath fitted thereon to interfere, thus allowing the sheath to exert inward pressure on the magnet.

2. The method of claim 1, wherein the reducing the first diameter of the magnet comprises: lowering a temperature of the magnet.

3. The method of claim 2, wherein the lowering the temperature of the magnet comprises: dipping the magnet into fluid at low temperature.

4. The method of claim 3, wherein the low temperature is a temperature lower than about 120 C.

5. The method of claim 3, wherein the fluid is liquid nitrogen.

6. The method of claim 3, wherein the low temperature is a temperature lower than 120 C.

7. The method of claim 3, wherein the low temperature is a temperature lower than 150 C.

8. The method of claim 3, wherein the fluid comprises liquid nitrogen.

9. The method of claim 1, wherein the fitting the sheath onto the magnet is carried out substantially without mechanical interference.

10. The method of claim 1, wherein the letting the magnet with the sheath fitted thereon to interfere comprises: heating the magnet and the sheath fitted thereon at room temperature.

11. The method of claim 1, wherein the providing the radial holding sheath of the composite material comprises: forming the sheath on a chuck and taking the sheath off the chuck.

12. The method of claim 1, wherein the reducing the first diameter of the magnet comprises: lowering a temperature of the magnet using fluid at low temperature.

13. The method of claim 12, wherein the low temperature is a temperature lower than 120 C.

14. The method of claim 12, wherein the low temperature is a temperature lower than 150 C.

15. The method of claim 12, wherein the fluid comprises liquid nitrogen.

16. The method of claim 1, wherein the fitting the sheath onto the magnet is carried out without mechanical interference.

17. The method of claim 1, wherein the fitting the sheath onto the magnet is carried out with very low interference.

18. The method of claim 1, wherein the letting the magnet with the sheath fitted thereon to interfere comprises: heating the magnet and the sheath fitted thereon to room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1.1 is a schematic side-plan view of a cylindrical core mounted on a central shaft and a radial holding sheath to be fitted onto the cylindrical core;

(2) FIG. 1.2 is an axonometric view of the components of FIG. 1.1;

(3) FIG. 1.3 is an enlarged cross-sectional view of the components shown in FIG. 1.1;

(4) FIG. 2 is an enlarged cross-sectional view of the cylindrical magnet, mounted on the central shaft, the cylindrical magnet having a smaller diameter so that the sheath may be fitted thereon;

(5) FIG. 3.1 is a schematic side-plan view of the step in which the sheath is fitted onto the cylindrical magnet;

(6) FIG. 3.2 is an axonometric view of the step shown in FIG. 3.1;

(7) FIG. 3.3 is an enlarged cross-sectional view of the step shown in FIG. 3.1;

(8) FIG. 4.1 is a schematic side view of the rotor according to the invention;

(9) FIG. 4.2 is an axonometric view of the rotor of the invention; and

(10) FIG. 4.3 is an enlarged cross-sectional view of the rotor according to the invention.

DETAILED DESCRIPTION

(11) The invention relates to a method of manufacturing a rotor 4 for an electric motor. The electric motor is not shown, but is an electric motor of the known type, for example a brushless electric motor. The electric motor typically also comprises a stator not shown in the various figures.

(12) The method comprises the initial step of manufacturing a cylindrical magnet 2 of sintered material mounted on a shaft 1. The shaft 1 may be in the form of a hollow shaft of metallic material, for example steel.

(13) FIG. 1.1 shows the shaft 1 and the magnet 2 mounted thereon. The right-hand part of FIG. 1 also shows a radial holding sheath. The sheath 3 forms a closed, annular, banding strip which, once mounted, exerts a pressure on the sintered magnet 2 made of magnetic material. This pressure is reduced as a result of the centrifugal force during high-speed rotation so as to prevent it from being deformed and/or destroyed. The pressure exerted by the sheath against the magnetic material opposes the high pressure due to the centrifugal force at a high/very high speeds of rotation.

(14) The sheath 3 is typically formed by winding a filament (or a plurality of filaments) around a chuck. Preferably, the sheath 3 comprises carbon filaments. The wound arrangement of filaments may be typically fixed in position by means of a layer of resin or other similar material. The sheath 3 thus forms a hollow cylinder, as also shown in FIG. 1.2.

(15) The cylindrical magnet 2 made of sintered material is constructed so as to have an outer diameter D1 greater than the inner diameter D0 of the sheath 3, as shown schematically in FIG. 1.3.

(16) The size of the thickness of the sheath 3 of carbon fiber (or other composite material) must be determined depending on the nominal rotational velocity of the rotor and the rotor diameter: the maximum tractional force exerted on the fiber yarnand therefore the choice of the thickness of the bandingare, in fact, dependent on these parameters.

(17) According to the invention, the holding sheath 3 is associated with the cylindrical magnet 2 so that this magnet exerts on the sheath a radial thrust directed inwards when the rotor 4 is not rotating.

(18) Advantageously, according to the invention, the magnetic material is therefore greatly compressed in the rest condition of the rotor.

(19) Subsequently, according to the invention, the initial diameter D1 of the cylindrical core 2 is reduced from the initial value to a second value D2<D1.

(20) In FIG. 2, the difference between the values D1 and D2 has been accentuated for greater illustrational clarity; however, as will be explained further below, the difference between D1 and D2 is, in reality, very small.

(21) Advantageously, according to the invention, the reduction in diameter of the cylindrical magnet from D1 to D2 is performed by means of lowering of the temperature of the magnet 2.

(22) In other words, the method according to the invention makes use of the different thermal expansion constant of the carbon fiber, iron and magnet as well as the good tensile strength of the carbon fiber and the good compressive strength of the sintered magnet.

(23) Preferably, the reduction in the temperature envisages dipping the magnet 2 in a low-temperature fluid.

(24) When subjected to a low temperature, the rotor as a whole (shaft and pole shoe with similar thermal expansion properties) contracts and the diameter is reduced. The reduction is generally comprised between 0.1 and 0.2%, for example about 60 m for a diameter of 40 mm.

(25) In order to obtain the results described, the temperature must be, for example, lower than 120 C. or 150 C.

(26) According to the invention, the fluid used to perform this lowering of the temperature is liquid nitrogen or similar fluids.

(27) Once the temperature has been reduced and consequently the diameter of the magnet 2 reduced from the value D1 to the value D2, the sheath 3 may be fitted onto the cylindrical magnet 2 without substantially exerting any force.

(28) In other words, the method according to the invention envisages a step of fitting the radial holding sheath 3 onto the magnet 2 when the magnet has the second diameter D2 (smaller than D1). When the magnet and the sheath are fitted onto each other, the diameter D2 is smaller than or the same as the diameter D0, the inner diameter of the sheath 3.

(29) Advantageously, according to the invention the step of fitting the sheath 3 is performed substantially without mechanical interference.

(30) In fact, advantageously, according to the invention, the fitting process is performed by means of thermal interference of the carbon fiber banding on the magnetic rotor made of sintered material (namely making use of the different thermal expansion coefficients of the carbon fiber and magnet).

(31) During the next step of the method according to the invention it is envisaged allowing the cylindrical magnet 2 with the sheath 3 fitted onto it to interfere such that the banding exerts inwardly directed pressures on the magnet.

(32) The magnetic material is thus greatly compressed in the rest condition; at the nominal speed the centrifugal force opposes the compressive forces imparted by the banding, producing a resultant force at the magnet/carbon fiber interface which continues to keep the magnet compressed, thereby avoiding tensile forces incompatible with the sintered material.

(33) This solves the problems associated with the fact of not being able to provide the carbon fiber with the necessary tension for opposing the tensile (centrifugal) forces and allows the sintered material to work in a permanent pressure situation.

(34) Advantageously, during heating, between the banding and the magnetic material of the rotor, compressive forces are produced such as to be greater than those which can be produced by winding the carbon fiber yarn using known technologies at room temperature.

(35) As a result of the invention described it is possible to mount, by means of thermal interference (namely by making use of the different thermal expansion coefficients of the carbon fiber and magnet), the carbon-fiber banding (already wound and fixed using special resins) on the magnetic rotor made of sintered material; this operation solves the problems associated with the fact of not being able to provide the carbon fiber with the necessary tension for opposing the tensile (centrifugal) forces and allows the sintered material to work in a permanent pressure situation.

(36) The invention as described makes use of the different thermal expansion coefficients of the carbon fiber, iron and magnet as well as the good tensile strength of the carbon fiber and the good compressive strength of the sintered magnet.