A rotor for a track-bound vehicle electric machine, such a machine and a track-bound vehicle having such a machine
TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART
The present invention relates to a rotor for a track-bound vehicle electric machine configured to generate a traction force of a said vehicle, said rotor comprising:
• a rotor body comprising a laminated assembly of mutually electrically insulated annular plates of magnetic steel axially stacked with respect to an axis of rotation of the rotor, and
• a plurality of permanent magnets fixed with respect to said rotor body for defining magnetic poles of the rotor and having each a magnetic polarity direction being substantially perpendicular to said axis of rotation,
as well as a track-bound vehicle electric machine comprising a stator and such a rotor.
The invention relates to a rotor for electric machines which may operate as motor and/or generator.
An electric machine provided with such a rotor is by the use of a plurality of permanent magnets to produce the magnetic flux of the rotor in motor operation a synchronous motor called permanent macjnet synchronous motor. Such a motor has lower rotor losses compared to an induction motor and thus higher efficiency. It can also be built with a higher pole number, compared to an induction, motor, without sacrificing performance, so that it can have a higher torque capability, features making such an electric machine particularly interesting as a track-bound vehicle
electric machine for which a high torque capability with respect to a determined size of the motor is an important feature.
By using such a laminated assembly to build the rotor body the efficiency of the machine is increased thanks to a considerable reduction of eddy current losses in the rotor body compared to a cast rotor body.
It is an ongoing attempt to reduce the losses of a said track-bound vehicle electric machine so as to improve the efficiency thereof and by that increase the torque capability with respect to a determined size of such a machine, and the present invention is directed to obtaining this by reducing the losses associated with the rotor.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a rotor of the type defined in the introduction having at least some feature contributing to lower losses when used in a said electric machine compared to such rotors already known.
This object is according to the invention obtained by providing such a rotor, in which the rotor body corriprises a plurality of longitudinal slots extending in parallel with said axis of rotation and at a distance from a circumferential outer surface of the rotor body, said permanent magnets are buried in said rotor body by being arranged in said slots in the form of a package of two or more permanent magnets arranged in one layer side by side in the transversal and/or longitudinal direction of the slot with the permanent magnets arranged in the same slot having the same magnetic polarity direction being substantially perpendicular to said one layer formed by said package, and layers of electrically insulating material separate on one hand said permanent magnets of each said package with respect to each other and on the other outer surfaces of said permanent magnet
package with respect to portions of said rotor body plates defining the respective slot.
By arranging such slots for receiving the permanent magnets of the rotor these permanent magnets, then buried inside the rotor body, may be fixed with respect to the rotor body while creating magnetic fluxes extending so as to interact more efficiently with magnetic fluxes created in stator windings of the electric machine than should the magnets be attached to the peripheral surface of the rotor body. The buried magnet configuration also allows a greater freedom of choice in design parameter selection in optimization of the torque/speed of a track-bound vehicle electric machine, i.e. a traction motor. Design variations can give motors with characteristics ranging from those typical for rotors with surface mounted magnets to those typical of a synchronous reluctance motor. However, the most important contribution to a possible increased efficiency of an electric machine provided with a rotor according to the invention is achieved by arranging the permanent magnets in each slot not as one single bar magnet, but as a package of a plurality of magnets arranged side by side, which results in considerably lower losses than should a single permanent magnet of the same size as said package be used. It is crucial to electrically insulate said permanent magnets arranged side by side with respect to each other for obtaining this and to survive the thermal, electrical and mechanical ageing stresses that will be present in service. It is also of great importance -that the outer surfaces of the permanent magnets package is electrically insulated with respect to portions of the rotor body place defining the respective slot, since the permanent magnets may otherwise establish a metallic contact between adjacent plates of the rotor body resulting in increased losses. Said layers of electrically insulating material should then be as thin as possible while still obtaining reliable electrical insulation on the respective side of said package extending in parallel with said one layer for a maximum magnetic field created by the permanent magnets, whereas the
value of the thickness of the layers of electrically insulating material separating the permanent magnets and the package with respect to the rotor body plate and extending substantially perpendicularly to said one layer is of less importance as long as reliable electrical insulation is obtained.
"Package" is here to be interpreted as an arrangement of said permanent magnets included therein tightly together, but they do not have to be secured to each other.
A rotor for an electric motor having permanent magnets arranged side by side on the outer periphery of the rotor body is known through US 2006/0097595 A1.
Furthermore, the permanent magnets of the rotor may have been obtained by introducing non-magnetized magnets in said slots and then activating them through magnetization after mounting in a rotor of a said electric machine, although it would in most cases be preferred and sometimes absolutely necessary to introduce already activated permanent magnets into said slots when manufacturing the rotor according to the invention.
Said layers of electrically insulating material may in a rotor according to the present invention be introduced into~said slots before the permanent magnets are inserted thereinto or all these layers may be applied to the surfaces of the permanent magnets before these are inserted into the slots. Combinations of these two methods of introducing permanent magnets and said layers of electrically insulating material into said slots when manufacturing a said rotor are possible, so that for instance permanent magnets arranged side by side may be insulated with respect to each other by being secured to each other as said package or a part of said package before introduction into a said slot, which has prior to said introduction been provided with layers of electrically insulating material covering the portions of the rotor body plates defining the slot.
According to an embodiment of the invention the rotor comprises said permanent magnets packages having > 2, especially > 4, permanent magnets arranged side by side in the transversal direction of the respective slot, and according to another embodiment of the invention said permanent magnets packages have 2-16, especially 4-10, permanent magnets arranged side by side in the longitudinal direction of the respective slot, which results in a considerable reduction of losses in the form of heat dissipation as a consequence of eddy current generation in the magnets with respect to the case of one single bar magnet arranged in said slot. The actual number of magnets arranged in each said slot depends on the specific rotor design and production parameters.
According to another embodiment of the invention said layers of electrically insulating material comprise a material having a heat resistance so as to obtain a long term electrical insulation of adjacent permanent magnets in a said package and of said permanent magnets with respect to the rotor plates for an operation temperature of said permanent magnets and rotor up to at least 200°C. By selecting a material for said electrical insulation with such a heat resistance it is ensured that the electrical insulation will be maintained over years of operation of a track-T5ound ve- t hide electric machine having such a rotor, since temperatures of 180°C, but seldom above 200°C, may often occur during such operation.
According to another embodiment of the invention said layers of electrically insulating material are attached to said permanent magnets. It is then suitable that these layers have been attached to the permanent magnets before insertion thereof into' said slots, and said layers are then preferably attached to said permanent magnets by adhesives, such as adhesive tapes, cyan-acrylate glues or epoxies. These layers are then preferably made of woven fibre glass, polymer films, such as polyester and
polyimide, glass reinforced polymer adhesive tapes, paper materials, such as polyaromatic polyamide or polyester non-woven materials, or non-organic materials, or combinations of these materials. Such an attachment of said layers of electrically insulating material to the permanent magnets may in some cases facilitate the insertion of the permanent magnets into said slot.
According to another embodiment of the invention the rotor comprises permanent magnets packages having permanent magnets arranged side by side in the transversal direction of the slot attached to each other as one rigid piece, which will facilitate the introduction of the permanent magnets into a said slot, when this rigid piece is created before insertion of the permanent magnets into the slot.
According to another embodiment of the invention the rotor comprises permanent magnets packages having at least one said piece also including permanent magnets arranged side by side in the longitudinal direction of the slot, which may under certain circumstances further simplify the introduction of said permanent magnets in a said slot.
According to another embodiment of the invention the rotor comprises layers of electrically insulating material separate from said permanent magnets in the form of sheets arranged between adjacent permanent magnets of a said package and/or between these permanent magnets and said portions of the rotor body plates defining the respective slot. Such sheets may preferably be inserted into the respective slot before the introduction of the permanent magnets thereinto and are then preferably mechanically strong but flexible and provided with slippery surfaces. It is then also preferred that the material is stiff enough so that no wrinkles are formed when inserting the magnets.
According to another embodiment of the invention the rotor comprises at least one said sheet arranged in a said slot be-
tween said permanent magnets package and said portions of the rotor body plates and defining the respective slot while forming a tube-like electrically insulating structure receiving said permanent magnets package. Such a tube-like electrically insulating structure may be inserted into the slot before the magnets or together with the magnets while being wrapped therearound.
According to another embodiment of the invention said slots are provided with grooves extending in the longitudinal direction of the slot in opposite walls of the slot each receiving a said sheet in the form of a rigid strip separating permanent magnets of said package arranged side by side in the transversal direction of the slot. Such strips may be used for guiding permanent magnets when inserting these into a said slot, and these magnets may then be coated by an electrically insulating material, so that said strips have mainly the function of facilitating the introduction of said magnets into the slots.
According to another embodiment of the invention said sheets of electrically insulating material are made of flexible films of polyester, polyimide or other polymer, flexible papers of polyaro-matic polyamide, flexible composite materials, such as laminations of the previous two material types, such as polyester-film/polyaromatic polyamide-paper laminate, or wove,n~glass fabric coated with fluoropolymers. Such material used for said sheets are preferred when the sheets are introduced into a rotor slot before the magnets, since the sheets may then be given the properties desired for this, such as mechanical strength, flexibility, low friction with respect to the magnets and low tendency to form wrinkles.
According to another embodiment of the invention' said sheets are substantially rigid, such as in the form of polymer-reinforced fibre glass laminate. Such sheets are preferred in a rotor for a track-bound vehicle electric machine, since thin layers of polymer, such as polyester having glass fibres therebetween, may
form slippery surfaces promoting the movement of magnets during insertion thereof into said slots, and the glass fibres will ensure a long term insulation of adjacent magnets with respect to each other by maintaining a distance between these magnets, although the polymer layers of the laminate may thermally decompose at the highest operation temperatures of a said rotor.
According to another embodiment of the invention at least said layers of electrically insulating material separating the outer surfaces of said permanent magnets package with respect to portions of said rotor body plates defining the respective slot are made of a material with mechanical resistance to abrasion, so that the risk of a degraded electric insulation resulting from the introduction of said permanent magnets into a said slot may be avoided.
According to another embodiment of the invention said layers of electrically insulating material separating said outer surfaces of a said permanent magnets package with respect to portions of said rotor body plates defining the respective slot are made of a slippery, low friction material, such as glass fabric coated with fluoropolymers.
The invention also relates to a track-bound vehicle electric machine comprising a stator and a rotor according to the present invention as well as a track-bound vehicle having a driving arrangement for generating a traction force of the vehicle including at least one electric machine having a stator and a rotor according to the present invention. The advantageous features and the advantages of such an electric machine and such a vehicle appear clearly from the above discussion of the different embodiments of a rotor according to the present invention.
Further advantages as well as advantageous features of the invention will appear from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.
In the drawings:
Fig 1 is a very schematic side elevation view illustrating an electric machine having a stator and a rotor according to an embodiment of the present invention,
Fig 2 is a simplified perspective view illustrating a permanent magnet package being a part of a rotor according to an embodiment of the invention,
Fig 3 is a simplified perspective view illustrating how a part of a permanent magnet package may be introduced into a slot in a rotor according to an embodiment of the present invention,
Fig 4 is a view illustrating the cross-section of a slot for permanent magnets in a rotor according to an embodiment of the invention,
Fig 5 is a perspective view illustrating how strips of electrically insulating material are introduced between magnets, either directly or by utilizing a system with grooves as in a slot shown in Fig 4 and permanent magnets are introduced into the slot, and
Fig 6 is a very simplified cross-section view illustrating how insulating material in the form of a polymer-reinforced fibre glass laminate or glass reinforced polymer adhesive tape may be used to form an electrical insulation
of adjacent permanent magnets with respect to each other or magnets with respect to slot surface.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Fig 1 shows schematically in an axial end view some of the main parts of a track-bound vehicle electric machine having a rotor according to the present invention. This electric machine has a stator 1 with a stator body 2 with a stator winding (not shown) wound therearound and configured to electrically create a plurality of stator poles disposed around the inner periphery of the stator body. Said stator winding is received in radial slots 3 in the stator body, extending over the entire length of this body. 48 slots are in this way uniformly distributed around the inner periphery of the stator body 2, so that 12 said slots belong to one stator pole in the case of a four-pole electric machine as in the embodiment shown in Fig 1. The total number of slots and number of slots per pole may of course be any other conceivable.
The electric machine further comprises a rotor 4 with a rotor body 5, which as the stator body is made of a laminated assembly of mutually electrically insulated annular plates of magnetic steel axially stacked with respect to an axis of rotation 6 of the rotor for keeping eddy current losses in these bodies at low levels. The rotor body 5 is rigidly connected with respect to rotation to a rotor axle 7, which is to be connected to the wheel axle of a vehicle, possibly through a gear box.
The rotor body 5 comprises a plurality of longitudinal slots 8, 9 extending in parallel with said axis of rotation 6 of the rotor body. It is shown in Fig 1 how four such slots are arranged per pole of the rotor, two 8 with a smaller cross-section and two 9 with a larger cross-section. The slots 9 have in the present case, which is an illustrative example of one possibility of many, a rectangular cross-section of approximately 10 x 55 mm and a
length of approximately 300 mm. Permanent magnets 10, here with a cross-section of 10 x 13 mm and a length of 37 mm, are arranged in each said slot in the form of a package of permanent magnets arranged in one layer, in one plane, side by side in the transversal and longitudinal direction of the slot with permanent magnets arranged in the same slot having the same magnetic polarity direction being substantially perpendicular to said one layer formed by said package, and the sides of the north N and south S poles of these magnets are indicated for two slots 8, 9 in Fig 1. Eight permanent magnets are arranged in an end to end relationship in the longitudinal direction in each such permanent magnets package, and two permanent magnets are arranged side by side in the transversal direction of a slot 8 and four in that direction of a slot 9.
Fig 2 illustrates the appearance of a said permanent magnets package 11 arranged in a slot 9 with a larger cross-section. Suitable materials for the permanent magnets are for instance NeFeB and SmCo.
Layers 12, 13 of electrically insulating material separate on one hand said permanent magnets of each said package with respect to each other (layers 12) and on the other outer surfaces of said permanent magnets package with respect to prortions of said rotor body plates defining the respective slot (layers 13). The layers 12 are arranged for reducing the losses by dividing the "bar magnets" into several smaller magnets, and the thickness of these layers is not crucial as long as they reliably insulate the magnets with respect to each other and may well be in the order of 1 mm, whereas the thickness of the layers 13 arranged on the magnetic pole sides of the magnets has to be as thin as possible while ensuring proper electrical insulation for obtaining a maximum magnetic field generated thereby and may then have a double-sided thickness of less than 0.5 mm, here about 0.2 mm.
A permanent magnet package of the rotor as shown in Fig 2 may have all the permanent magnets attached to each other as one rigid piece by the use of an adhesive combined with or having itself the feature of a material with electrically insulating properties therebetween, but it also conceivable to have only some or none of the magnets of the package attached to each other. It is for instance possible to have only the permanent magnets arranged side by side in the transversal direction of the slot attached to each other as one rigid piece 14 as shown in Fig 3. A tube 15 of electrically insulating material is here formed by cutting a flexible such material to desired dimensions and then roll it to such a tube, which has been inserted into a slot 9. The material of said tube 15 is then preferably mechanically strong and has "slippery" surfaces to promote the movement of the piece 14 of magnets during insertion. Several such pieces 14 of four magnets attached to each other and mutually separated by an electrically insulating layer 12 may then be successively inserted into the tube 15 and moved into the desired position in the slot 9. Non-magnetic tools with the approximate cross-section of the slot, a length greater than the rotor length are inserted into the slot from the opposite end and are used to provide reverse pressure on the magnets and to provide an indication of how far into the slot the magnets are inserted. End plates not shown may then be applied to both sides oTthe rotor body for keeping the magnets in place inside the rotor body.
Fig 4 illustrates a possible cross-section of a said slot 9 according to another embodiment of the invention, in which the slot 9 is provided with grooves 16, 17 extending in the longitudinal direction of the slot in opposite walls 18, 19 of the slot each receiving a sheet 20 of electrically insulating material in the form of a rigid strip separating permanent magnets of said package arranged side by side in the transversal direction of the slot. Such sheets may be there primarily for guiding the permanent magnets when introduced into the slot 9 and these magnets may be externally coated by a layer of electrically insulating material
for obtaining the electrical insulation of adjacent magnets with respect to each other. It is shown in Fig 5 how such permanent magnets 10 may be individually introduced into the slot 9 while being received in a said tube 15 and guided by such sheets 20 to form a permanent magnets package once introduced into the rotor slot 9. The sheets 20 may either be received in grooves as shown in Fig 4 or be simply arranged between magnets without any such grooves.
Fig 6 illustrates schematically a preferred material for being used as an electrically insulating layer, here between two magnets 10, 10'. This material may be used as a said strip 20 and is a polymer-reinforced fibre glass laminate or glass reinforced polymer adhesive tape having two films 21, 22 of a polyester and glass fibres 23 sandwiched therebetween. The strip 20 may also have only one said film 21, 22. The polyester layer has a low friction with respect to the magnets when inserted into the slot as shown in Fig 5, but this material may not withstand the high temperatures that may occur during operation of an electric machine for which the rotor is intended. However, the glass fibres 23 function as spacers and ensures proper electrical insulation of the permanent magnets 10, 10' also after thermal decomposition of the layers 21 and 22. Furthermore, the glass fibres make th.e strips sufficiently rigid for the guidance of the magnets when introduced into the slots without forming wrinkles.
The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the scope of the invention as defined in the appended claims.
Two or more magnets may side by side in the transversal direction of a said slot be secured to each other while forming a rigid piece and several such pieces be arranged side by side in said
transversal direction while being separated by one or more strips of the type shown in Fig 4 and 5.

WE CLAIMS
1. A rotor for a track-bound vehicle electric machine configured
to generate a traction force of a said vehicle, said rotor com
prising:
• a rotor body (5) comprising a laminated assembly of mutually electrically insulated annular plates of magnetic steel axially stacked with respect to an axis of rotation (6) of the rotor, and
• a plurality of permanent magnets (10) fixed with respect to said rotor body for defining magnetic poles of the rotor and having each a.magnetic polarity direction being substantially perpendicular to said axis of rotation,
characterized in that the rotor body comprises a plurality of longitudinal slots (8, 9) extending in parallel with said axis of rotation (6) and at a distance from a circumferential outer surface of the rotor body, that said permanent magnets (10) are buried in said rotor body by being arranged in said slots (8, 9) in the form of a package (11) of two or more permanent magnets arranged in one layer side by side in the transversal and/or longitudinal direction of the slot with the permanent magnets arranged in the same slot having the same magnetic polarity direction being substantially perpendicular to said "one layer formed by said package, and that layers (12, 13, 15, 20) of electrically insulating material separate on one hand said permanent magnets (10) of each said package with respect to each other and on the other outer surfaces of said permanent magnets package with respect to portions of said rotor body plates defining the respective slot.
2. A rotor according to claim 1, characterized in that it com
prises said permanent magnets packages having > 2, especially
> 4, permanent magnets arranged side by side in the transversal
direction of the respective slot.
3. A rotor according to claim 1 or 2, characterized in that it comprises said permanent magnets packages having 2-16, especially 4-10, permanent magnets arranged side by side in the longitudinal direction of the respective slot.
4. A rotor according to any of the preceding claims, characterized in that said layers (12, 13) of electrically insulating material comprise a material having a heat resistance so as to obtain a long term electrical insulation of adjacent permanent magnets in a said package and of said permanent magnets with respect to the rotor body plates for an operation temperature of said permanent magnets and rotor up to at least 200°C.
5. A rotor according to any of the preceding claims, characterized in that said layers (12, 13) of electrically insulating material are attached to said permanent magnets.
6. A rotor according to claim 5, characterized in that said layers (12, 13) of electrically insulating material are attached to said permanent magnets by adhesives, such as adhesive tapes, cyan-acrylate glues or epoxies.
7. A rotor according to claim 5 or 6, characterized in that said layers (12, 13) of electrically insulating material are" made of woven fibre glass, polymer films, such as polyester and poly-imide, glass reinforced polymer adhesive tapes, paper materials, such as polyaromatic polyamide or polyester non-woven materi- . als, or non-organic materials, or combinations of these materials.
8. A rotor according to any of claims 5-7, characterized in that it comprises permanent magnets packages having permanent magnets arranged side by side in the transversal direction of the slot attached to each other as one rigid piece (14).
9. A rotor according to claim 8, characterized in that it com
prises permanent magnets packages (11) having at least one
said piece also including permanent magnets arranged side by
side in the longitudinal direction of the slot.
10. A rotor according to any of the preceding claims, characterized in that it comprises layers (12, 13, 15, 20) of electrically insulating material separate from said permanent magnets in the form of sheets arranged between adjacent permanent magnets of a said package and/or between these permanent magnets and said portions of the rotor body plates defining the respective slot.
11. A rotor according to claim 10, characterized in that it comprises at least one said sheet arranged in a said slot between said permanent magnets package and said portions of the rotor body plates and defining the respective slot while forming a tube-like electrically insulating structure (15) receiving said permanent magnets package.
12. A rotor according to claim 10, characterized in that said
' slots (9) are provided with grooves (16, 17) extending in the
longitudinal direction of the slot in opposite walls (18, 19) of the slot each receiving a said sheet in the form of a rigid~strip (20) separating permanent magnets of said package arranged side by side in the transversal direction of the slot.
13. A rotor according to claim 10 or 11, characterized in that
said sheets (12, 13, 15) of electrically insulating material are
made of flexible films of polyester, polyimide or other polymer, flexible papers of polyaromatic polyamide, flexible composite materials, such as laminations of the previous two material types, such as polyester-film/polyaromatic polyamide-paper laminate, or woven glass fabric coated with fluoropolymers.
14. A rotor according to any of claims 10-12, characterized in that said sheets are substantially rigid, such as in the form of a polymer-reinforced fibre glass laminate (20).
15. A rotor according to any of the preceding claims, characterized in that at least said layers (13, 15) of electrically insulating material separating the outer surfaces of said permanent magnets package with respect to portions of said rotor body plates defining the respective slot are made of a material with mechanical resistance to abrasion.
16. A rotor according to claim 15, characterized in that said
layers (13) of electrically insulating material separating said
outer surfaces of said permanent magnets package with respect
to portions of said rotor body plates defining the respective slot
are made of a slippery, low friction material, such as glass fabric
coated with fiuoropolymers.
17. A track-bound vehicle electric machine comprising a stator
(1) and a rotor (4) according to any of claims 1-16.
18. A track-bound vehicle having a driving arrangement for gen
erating a traction force of the vehicle including at -i-east one
electric machine having a stator (1) and a rotor (4) according to
any of claims 1-16.