Permanent magnet rotor shaft assembly and method

This application claims priority to European Patent Application EP13306322 filed Sep. 26, 2013, the contents of which are herein fully incorporated by reference.

The present invention relates to a permanent magnet rotor shaft assembly for high speed electrical machines and a method of manufacture thereof.

An example of a known method for fabricating a permanent magnet rotor for an electrical machine is disclosed in U.S. Pat. No. 6,104,115 A. According to this method, permanent magnets are provided on a sleeve having a constant outer diameter whereas the inside of the sleeve is conical. A screen made essentially of a highly conducting material is then provided on the surface of the permanent magnets facing a stator of the electrical machine. An annular fiber reinforced support is fitted on the screen. The conical shape of the outside of an inner rotor support is complementary to the conical shape of the inside of the sleeve. Such a manufacturing process is not cost effective and the manufacturing and assembly time is not in accordance with serial production.

U.S. Pat. No. 4,741,094 A and document WO 98/34324 A1 disclose a method of making a two-pole magnet rotor for an electrical machine, comprising the steps of forming a cylindrical two-pole permanent magnet from either small blocks aligned with the same axis of magnetic polarity assembled in a solid overlaying relationship or a whole cylindrical block (WO 98/34324 A1); bonding a first end piece onto one end of the cylindrical two-pole permanent magnet; bonding a second end piece onto the other end of the cylindrical two-pole permanent magnet; and installing a thin cylindrical retaining hoop in interference fit fashion around the cylindrical two-pole permanent magnet and the portions of the first and second end pieces adjacent to the cylindrical two-pole permanent magnet. This method of manufacturing an assembly with two end shafts, a magnet cylinder made of a plurality of magnets assembled as a core, this core being axially compressed by end shafts and the set being radially compressed and fixed by a high tensile sleeve, is rather cost effective and is in line for serial production. However such a method remains difficult to put into practice in particular because it is difficult to insure a concentric alignment while inserting the sleeve.

U.S. Pat. No. 7,042,118 B2 further discloses a rotor construction method aiming at improving the concentric alignment when inserting a sleeve. According to such a method a plurality of hollow magnet discs are provided and a tie rod is placed in the central bores of the hollow magnet discs and fixed to end shafts. Such a method is not cost effective and the use of hollow magnet discs might be detrimental to the stiffness and efficiency of the assembly.

The technical problem to be solved is to provide a permanent magnet rotor shaft assembly for high speed electrical machines and a method of manufacture thereof which remedy the above-mentioned drawbacks.

More specifically the invention aims at improving the easiness of a manufacturing process with a reduced number of pieces arranged on a shaft.

The invention further aims at allowing the possibility of creating a rotor assembly providing a high stiffness permanent rotor shaft.

Another aim of the present invention is to have a simplified manufacturing process, enabling a lower cost and a high serial manufacturing process.

The invention is defined in the appended claims.

The invention more specifically relates to a permanent rotor shaft assembly for an electrical machine, comprising a permanent magnet cylindrical core having a longitudinal axis, the cylindrical core being axially compressed by first and second end shafts and being radially compressed by a sleeve made of a non-magnetic high strength metal, characterized in that at least one of the first and second end shafts comprises, in its portion facing the cylindrical core, a central shoulder head which cooperates with a mating central recess made in a central portion of a front face of the cylindrical core.

The permanent magnet cylindrical core may comprise rare earth magnets which may be made of NdFeBr or Sm2Co17 for example.

The sleeve may be made of a non-magnetic high strength metal such as Inconel, HasteHoy, Ti-6% Al-6% V-2% Sn, Ti-2.5% Cu.

The central shoulder head may be inserted in the mating central recess by tight fit assembly, slip joint assembly or glued assembly.

The first and second end shafts and the sleeve may be fixed on the permanent magnet cylindrical core by welding, adhesive or heat shrinking.

According to a specific embodiment, the first and second end shafts further comprise cylindrical tracks for mechanical bearings.

Such cylindrical tracks for mechanical bearings may be located on a stepped portion of the first and second end shafts.

Alternatively or in addition the first and second end shafts may further comprise cylindrical stack iron laminations for magnetic bearings.

Such cylindrical stack iron laminations for magnetic bearings may be located on a stepped portion of the first and second end shafts.

Preferably, in the permanent rotor shaft assembly according to the invention, a central shoulder head is provided in each of the first and second end shafts, in its portion facing the cylindrical core, a mating central recess is provided in each of the central portions of the front faces of the cylindrical core, and the central shoulder heads are respectively mounted in the mating central recesses.

The permanent magnet cylindrical core may form a two-pole permanent magnet or a four-pole permanent magnet.

The invention further relates to a rotary high speed electrical machine having tip speeds up to 300 m/s, characterized in that it comprises a permanent rotor shaft assembly as defined above.

The invention further relates to a method for making a permanent rotor shaft assembly for an electrical machine, comprising the steps of:

forming a permanent magnet cylindrical core having a longitudinal axis,

bonding a first end shaft onto one end of the permanent magnet cylindrical core,

bonding a second end shaft onto the other end of the permanent magnet cylindrical core,

installing a sleeve made of a non-magnetic high strength metal in interference fit fashion around the permanent magnet cylindrical core and the portions of the first and second end shafts adjacent to the permanent magnet cylindrical core, so that the cylindrical core be axially compressed by the first and second end shafts and be radially compressed by the sleeve,

characterized in that it further comprises the steps of:

providing at least one central shoulder head in the first and second end shafts, in its portion facing the cylindrical core, providing in a central portion of a front face of the cylindrical core, at least one mating central recess, which is adapted to cooperate with the at least one central shoulder head, and mounting the at least one central shoulder head in the at least one mating central recess, before inserting the sleeve made of a non-magnetic high strength metal in interference fit fashion around the permanent magnet cylindrical core and the portions of the first and second end shafts adjacent to the permanent magnet cylindrical core.

According to a preferred embodiment, a central shoulder head is provided in each of the first and second end shafts, in its portion facing the cylindrical core, a mating central recess is provided in each of the central portions of the front faces of the cylindrical core, and the central shoulder heads are respectively mounted in the mating central recesses, before inserting the sleeve made of a non-magnetic high strength metal in interference fit fashion around the permanent magnet cylindrical core and the portions of the first and second end shafts adjacent to the permanent magnet cylindrical core.

The central shoulder head may be inserted in a mating central recess by tight fit assembly, slip joint assembly or glued assembly.

The first and second end shafts and the sleeve may be fixed to the permanent magnet cylindrical core by welding, adhesive or heat shrinking.

Due to the provision of central shoulder heads and corresponding mating central recesses, the invention more specifically allows an easy concentric alignment of the first and second end shafts with the permanent magnet cylindrical core while inserting the sleeve. Moreover it increases the stiffness on the set once assembled.

FIG. 1 is a schematic longitudinal section of a permanent magnet rotor shaft assembly for a high speed electrical machine according to a first embodiment of the invention;

FIG. 2 is a schematic longitudinal section of a permanent magnet rotor shaft assembly for a high speed electrical machine according to a second embodiment of the invention; and

FIG. 3 is a schematic exploded view of the permanent magnet rotor shaft assembly of FIG. 2 .

The present invention will be described in connection with preferred embodiments which are given by way of examples.

A typical arrangement of a first embodiment of the invention is illustrated in FIG. 1 which shows a permanent rotor shaft assembly 100 for an electrical machine, more specifically for high speed applications which can reach tip speeds up to 300 m/s. The assembly 100 comprises a permanent magnet cylindrical core 101 having a longitudinal axis X-X′. The cylindrical core 101 is axially compressed by first and second end shafts 102 A, 102 B and is radially compressed by a sleeve 104 made of a non-magnetic high strength metal. The set of elements 101 , 102 A and 102 B forms the overall rotor shaft.

According to the invention one ( 102 A) of the first and second end shafts 102 A, 102 B comprises, in its portion facing the cylindrical core 101 , a central shoulder head 103 A which cooperates with a mating central recess 106 A made in a central portion of a front face of the cylindrical core 101 .

The shoulder head 103 A insures the concentricity of the elements 101 and 102 A during the assembly process and provides support for the cylindrical magnet core 101 , whilst introducing additional stiffness to the set of assembled elements 101 , 102 A and 102 B once the mounting process is achieved.

The permanent magnet cylindrical core 101 comprises rare earth magnets such as NdFeBr or Sm2Co17.

The sleeve 104 is made of a non-magnetic high strength metal which may be advantageously chosen among Inconel, Hastelloy, Ti-6% Al-6% V-2% Sn, Ti-2.5% Cu.

The magnetization direction of the magnet 101 may be radial, diametral or constituted by a Halbach magnetization.

The polarity of the magnets of the core 101 can be 2 poles or 4 poles.

The central shoulder head 103 A is inserted in the mating central recess by tight fit assembly, slip joint assembly or glued assembly.

The first and second end shafts 102 A, 102 B and the sleeve 104 are fixed on the permanent magnet cylindrical core 101 by welding, adhesive or heat shrinking.

The sleeve 104 constitutes a hoop which radially compresses the permanent magnet cylindrical core 101 and the first and second end shafts 102 A, 102 B.

The first and second end shafts 102 A, 102 B axially compress the permanent magnet cylindrical core 101 .

According to a specific embodiment, the first and second end shafts 102 A, 102 B further comprise cylindrical tracks 105 A, 105 B for mechanical bearings.

The reference numerals 105 A, 105 B may also alternatively represent cylindrical stack iron laminations for magnetic bearings. Such cylindrical stack iron laminations may be made on the first and second end shafts 102 A, 102 B as an alternative for cylindrical tracks for mechanical bearings, but the first and second end shafts 102 A, 102 B may also comprise simultaneously in a not shown embodiment a combination of both cylindrical tracks for mechanical bearings (such as auxiliary bearings associated with magnetic bearings) and cylindrical stack iron laminations for magnetic bearings.

The tracks for mechanical bearings and/or the stack iron laminations for magnetic bearings may be located on a stepped portion 122 A, 122 B of the first and second end shafts 102 A, 102 B. Actually the elements 105 A, 105 B may be located at different levels (different area or different shoulder) of the end shafts according to the needs.

FIGS. 2 and 3 illustrate a second embodiment of the invention which relates to a permanent rotor shaft assembly 200 for an electrical machine, more specifically for high speed applications which can reach tip speeds up to 300 m/s. The permanent rotor shaft assembly 200 comprises a permanent magnet cylindrical core 201 having a longitudinal axis X-X′. The cylindrical core 201 is axially compressed by first and second end shafts 202 A, 202 B and is radially compressed by a sleeve 204 made of a non-magnetic high strength metal. The set of elements 201 , 202 A and 202 B forms the overall rotor shaft in a manner similar to the overall rotor shaft of the assembly 100 of the first embodiment.

However in the permanent rotor shaft assembly 200 according to the second embodiment of FIGS. 2 and 3 , a central shoulder head 203 A, 203 B is provided in each of the first and second end shafts 202 A, 202 B, in its portion facing the cylindrical core 201 , a mating central recess 206 A, 206 B is provided in each of the central portions of the front faces of the cylindrical core 201 , and the central shoulder heads 203 A, 203 B are respectively mounted in the mating central recesses 206 A, 206 B. Such arrangement of the second embodiment is otherwise similar to the first embodiment and the same materials, assembly means and optional features such as the additional tracks or stack iron laminations 105 A, 105 B of FIG. 1 may also be applied to the embodiment of FIG. 2 . The assembly 200 of FIG. 2 nevertheless constitutes a best mode of implementation of the invention and ensures an easy concentric alignment of the elements 202 A, 201 , 202 B, whilst inserting the sleeve 204 and confers an enhanced stiffness on the set of elements once assembled.

The invention further relates to a method for making a permanent rotor shaft assembly 100 or 200 for an electrical machine, comprising the steps of:

forming a permanent magnet cylindrical core 101 respectively 201 having a longitudinal axis X-X′,

bonding a first end shaft 102 A respectively 202 A onto one end of the permanent magnet cylindrical core 101 respectively 201 ,

bonding a second end shaft 102 B respectively 202 B onto the other end of the permanent magnet cylindrical core 101 respectively 201 ,

installing a sleeve 104 respectively 204 made of a non-magnetic high strength metal in interference fit fashion around the permanent magnet cylindrical core 101 respectively 201 and the portions of the first and second end shafts 102 A, 102 B respectively 202 A, 202 B adjacent to the permanent magnet cylindrical core 101 respectively 201 , so that the cylindrical core 101 respectively 201 be axially compressed by the first and second end shafts 102 A, 102 B respectively 202 A, 202 B and be radially compressed by the sleeve 104 respectively 204 .

More specifically, the method according to the invention further comprises the steps of:

providing at least one central shoulder head 103 A respectively 203 A, 203 B in the first and second end shafts 102 A, 102 B respectively 202 A, 202 B, in its portion facing the cylindrical core 101 respectively 201 ,

providing in a central portion of a front face of the cylindrical core 101 respectively 201 , at least one mating central recess 106 A respectively 206 A, 206 B, which is adapted to cooperate with the at least one central shoulder head 103 A respectively 203 A, 203 B, and

mounting the at least one central shoulder head 103 A respectively 203 A, 203 B in the at least one mating central recess 106 A respectively 206 A, 206 B, before inserting the sleeve 104 respectively 204 made of a non-magnetic high strength metal in interference fit fashion around the permanent magnet cylindrical core 101 respectively 201 and the portions of the first and second end shafts 102 A, 102 B respectively 202 A, 202 B adjacent to the permanent magnet cylindrical core 101 respectively 201 .

In the preferred embodiment illustrated in FIGS. 2 and 3 , a central shoulder head 203 A, 203 B is provided in each of the first and second end shafts 202 A, 202 B, in its portion facing the cylindrical core 201 , a mating central recess 206 A, 206 B is provided in each of the central portions of the front faces of the cylindrical core 201 , and the central shoulder heads 203 A, 203 B are respectively mounted in the mating central recesses 206 A, 206 B, before inserting the sleeve 204 made of a non-magnetic high strength metal in interference fit fashion around the permanent magnet cylindrical core 201 and the portions of the first and second end shafts 202 A, 202 B adjacent to the permanent magnet cylindrical core 201 .

The central shoulder head 103 A, 203 A or 203 B is inserted in a corresponding mating central recess 106 A, 206 A or 206 B preferably by tight fit assembly, slip joint assembly or glued assembly.

The first and second end shafts 102 A, 102 B respectively 202 A, 202 B and the sleeve 104 respectively 204 are preferably fixed to the permanent magnet cylindrical core 101 respectively 201 by welding, adhesive or heat shrinking.

Generally speaking, the invention provides a simplification in the manufacturing process, increases performance and reduces cost.

Although preferred embodiments have been shown and described, it should be understood that any changes and modifications may be made therein without departing from the scope of the invention as defined in the appended claims. Thus the features of the different embodiments may be combined. In particular the stepped portions 122 A, 122 B and the cylindrical tracks or stack laminations 105 A, 105 B of the embodiment of FIG. 1 may be implemented in the embodiment of FIGS. 2 and 3 .