GB2093278A

GB2093278A - Electric Motor

Info

Publication number: GB2093278A
Application number: GB8125400A
Authority: GB
Original assignee: Kollmorgen Technologies Corp
Current assignee: Kollmorgen Technologies Corp
Priority date: 1980-09-02
Filing date: 1981-08-20
Publication date: 1982-08-25
Also published as: GB2093278B; DE3133626A1; SE8105161L; JPS5771256A; FR2489620B1; IT8149191A0; FR2489620A1

Abstract

In an electric motor, each pole piece has an opening, preferably of a diamond shape, filled with a slug 63 of aluminum. Said opening and said slug align the magnetic flux lines in the pole pieces under motor load (Fig. 6, not shown) and serve for the extraction of heat. Contraction on cooling of molten Al, or other non- magnetic or low magnetic permeability material poured into the opening, ensures protuberances urge together the laminations (Fig. 3, not shown). <IMAGE>

Description

SPECIFICATION Flux Forcing Motor The present invention relates to electric motors, and more particularly, to an electric motor wherein the stator is constructed for the removal of heat along the axial direction while restraining a circumferential displacement of the magnetic flux during rotation of the rotor.

Electric motors are constructed in numerous configurations to accommodate the many situations in which they are deployed. In one form of motor having the permanent field applied by the stator magnets, permanent magnets such as those of a ferrite material are placed around a pole piece to provide a strong magnetic field. In order to insure that a maximum amount of the flux of the permanent magnets passes through the rotor, the stator may be formed of pole pieces each of which has ferrite magnets disposed along three sides of the pole pieces while a fourth side is curved to mate with the rotor.

A problem arises in that the ceramic, or glasslike, form of the ferrite material is a poor conductor of heat with the result that heat developed within the motor does not have a direct path of egress in the radial direction towards the outer case of the motor, the lack of the direct path limiting the maximum thermal load acceptable and further endangering the motor through a short excessive rise in the temperature.

A further problem attendant to the high field strength imparted by the ferrite magnets in the subject arrangement lies in the instability of the direction of the magnetic field in that its direction depends on both the physical structure of the rotor and the stator. A changing geometry of the relative positions of the structural elements of the rotor and the stator occurs upon rotation of the rotor relative to the stator with the result that the magnetic lines of force are displaced periodically from their preferred direction and location within each of the pole pieces. As a result, the motor suffers from a reduction of torque and a reduction of efficiency of operation from that which could be obtained if the magnetic lines of force were to remain in place.

The aforementioned problems are overcome, and other advantages are provided by an electric motor constructed in accordance with the present invention by the inclusion of a high reluctance path in each of the stator pole pieces. The path is provided by the removal of magnetizable material from a pole piece leaving a void therein, this being followed by the introduction into the void of the pole piece of a slug of material which is nonmagnetic or has a low magnetic permeability. The slug extends transversely of the magnetic field. In addition, the material of the slug has a higher thermal conductivity than the material of the pole piece for the extraction of heat from within the pole piece, the heat including that produced within the rotor and radiated therefrom into the stator.The slug may have an outer cylindrical surface in a preferred embodiment of the invention; it is provided with a cross sectional shape having sides which diverge from a vertex near the rotor. The diverging sides of the slug generally follow the lines of flux during a no-load condition of operation of the motor and, in a preferred embodiment of the invention, give a generally diamond shape to the cross section of the slug. The slug is located within the pole piece along an axial plane which bifurcates the magnetic field and thereby constrains the field to remain stationary so as not to undergo a tangential displacement around the rotor during rotation thereof.

As the slug passes through the pdle piece, the surface of the slug and of the void are sufficiently close together to provide for thermal conductivity between the materials of the pole piece and the slug. The pole piece is conveniently formed of a stack of laminations, each of which has been stamped to provide a void. The resulting void extends longitudinally through the pole piece and has the aforementioned cross sectional shape.

Preferably, intimate thermal contact between the slug and the pole piece is attained by constructing the slug of a material, such as aluminum, which has a lower melting point than the iron or mild steel of which a pole piece is usually constructed.

The slug is formed in situ by pouring molten aluminum into the void of the pole piece, after which the aluminum cools and hardens to form the slug. The slug extends beyond the periphery of the voids in the end laminations of the stack of laminations so that, upon the cooling of the aluminum, the slug contracts and thereby produces a compressive force which urges the laminations together tightly. The ends of the slug are then ground to mate with the end portions, or bells of the motor housing, the bells serving as heat sinks for withdrawal of the heat from the slugs. The ends of the slug are placed close to, or preferably pressed against, the bells to provide thermal conductivity between the slug and the bells. Thus, the heat of the stator pole pieces is conducted via the slugs to the bells, whereupon the heat radiates from the motor to provide the requisite cooling of the motor.

An additional feature of the invention is the use of a jig for magnetizing the ferrite elements while holding the ferrite elements in their respective positions about the pole pieces. The jig includes coils and electrical circuitry for the introduction of large currents in the coils for magnetizing the ferrite elements in situ. By use of the jig, the stator can be constructed without the necessity for handling the magnetized permanent magnets, thereby avoiding the danger that two such magnets may be damaged by the high attractive forces which have been known to cause magnets to smash into each other and shatter.

The~aforementioned features and other aspects of the invention are described in the following description, taken in connection with the accompanying drawings wherein Fig. 1 is a stylized perspective view of an electric motor embodying the invention, a portion of an end, or bell, of the motor being cut away to show a portion of the stator; Fig. 2 is an end view of the motor of Figure 1 with the bells thereof removed, Fig. 2 showing the components of the stator positioned about the rotor; Fig. 3 is a sectional view of the motor taken along the line 3-3 in Fig. 2, the bells and brushes of the motor being shown in phantom;; Figures 4, 5 and 6 each show a lamination of a pole piece located alongside the rotor for demonstrating the effect of the high reluctance path upon the direction and location of magnetic lines of force, Figure 4 showing a no-load field pattern which is symmetrical with, and without the high reluctance path, Figure 5 showing an asymmetrical field pattern resulting from full load without the high reluctance path, and Figure 6 showing a symmetrical pattern resulting from full load with the high reluctance path; and Fig. 7 is a diagrammatic view of a jig utilized in the manufacture of the motors for magnetizing the ferrite elements in situ.

Referring to Figure 1,2 and 3, a motor 20 is constructed in accordance with the invention and includes a circumferential frame 22 with bells 26 secured thereto by nuts 28 and bolts 30. The bolts 30 have the form of tie rods and pass completely through the motor 20 to connect with the bells 26 at the opposite ends of the frame 22.

The motor may be supported by a bracket (not shown) secured by screws to tapped holes in the bell 26. A rotor shaft 34 protrudes through the bells 26. A portion of bell 26 is cut away in Figure 1 to show elements of a stator 35, namely, a side magnet 36 supported between neighbouring pole pieces 38 of the stator 35, and a top magnet 40 secured to the top of a pole piece 38. The pole pieces are located circumferentially around a rotor 44.

During the assembly of the motor 20, the pole pieces 38, the top magnets 40 and the side magnets 36 of the stator 35 are held within their respective positions by a jig (not shown) as the sections of the frame 22 are welded together with welds 46. Tension within the frame 22 tightly hoids the elements of the stator 35 in their respective positions. As shown in Fig. 2, teeth 50 protrude outwardly from the sides of the pole pieces 38 for locating the side magnets 36 within their respective positions. In each corner of the arrangement of Fig. 2, there is a passage 51 between the top magnets 40 and a side magnet 36 which provides adequate space for the passage of bolt 30 for securing the bells 26 of Figures 1 and 3.

The rotor 44 is supported by its shaft 34, the ends thereof passing through apertures 53 in the bells 26, shown in phantom in Fig. 3. The rotor 44 includes a circumferential array of teeth 54 spaced apart by channels 56. A commutator 58 is secured about the rotor shaft 34. Eiectrical excitation of windings 60 of the rotor 44 is obtained via the commutator 58 and brushes 62, also shown in phantom. The windings 60 are located within the channel 56. Currents within the windings 60 induce the magnetic poles of the rotor 44 at the sites of the teeth 54.

In accordance with the invention, the side magnets 36 and the top magnets 40 are made from a ferrite material or other material which is readily magnetized, the magnets 36 and 40 being placed about each of the pole pieces 38 to provide an intense magnetic flux which passes through the respective pole pieces 38 and into the rotor 44. In addition, a slug 63 is provided within each pole piece 38 for extracting heat therefrom which heat is entrapped by virtue of the thermal insulation properties of the ferrite material of the side magnets 36 and the top magnet 40. The heat within the pole pieces 38 includes heat coupled thereto by radiation from the rotor 44. The slugs 63 are formed of a nonmagnetic material such as aluminum which has a low permeability, to retain the high reluctance path to the flux provided by a void into which the slug 63 is inserted.The material of the slugs 63 has a high thermal conductivity to remove the heat.

With reference to Fig. 4, a lamination 64 of a pole piece 38 has a void 66 for the passage of a slug 63 as was seen in Figures 2 and 3. The void 66 has a generally diamond shape with a main axis parallel to the axis of rotation of the rotor 44.

The vertex of the void 66 at the top of the pole piece 38 opens into the surface of the pole piece 38 to better define the paths of the magnetic flux.

Lines 68 show the flux in the presence of the high reluctance path provided by the void 66 under a condition of no-load. The magnetic field configuration is symmetrical. The symmetrical field pattern is also obtained under a no-load condition with a pole piece constructed without the high reluctance path. However, under a condition of full load, the high reluctance path is effective as will be seen in Figures 5 and 6.

Each lamination 64 is stamped from a plate of soft iron or mild steel, the latter being preferred in the construction of motors, to provide a magnetic path of high permeability between the rotor 44 and sources of the permanent magnetism, namely, the top magnets 40 and the side magnets 36. The rotor 44, including the teeth 54 thereof, is fabricated of the same material as the pole piece 38 for guiding the magnetic.flux of the windings 60. The void 66, due to the absence of the iron therein, provides a high reluctance path in which there is a negligible amount of magnetic lines of force. Due to the aforementioned low permeability of the aluminum slug 63, the insertion of the slug 63 within the void 66 of a stack of the laminations 64 of a pole piece 38 does not significantly alter the path of the magnetic flux.

Referring to Figures 5 and 6, there are shown laminations of pole pieces located alongside a rotor as in Figure 4. The physical structure of Fig.

5 is the same as the structure of Fig. 4 except that the void 66 providing the high reluctance path has been deleted in order to show a comparison of magnetic fields both with and without the high reluctance path. In Fig. 6, the physical structure is identical to that of Fig. 4. The magnetic fields in both Figures 5 and 6 are drawn for the condition of full load. The lines 70 of magnetic flux in Fig. 5 have been shifted tangentially along the surface of the rotor 44 in the absence of the high reluctance path. However, in Fig. 6, the symmetrical pattern of the flux lines 68 has been retained by the high reluctance path of the void 66. The void 66 prevents flux emanating from magnets to the right of the pole piece 38 from moving over and joining with flux emanating from magnets to the left of the pole piece 38 and vice versa.Thereby, the symmetrical flow of flux results for all positions of the rotor 44 relative to the stator35.

The slug 63, in addition to retaining the aforementioned high reluctance path, also serves two additional functions, namely, the function of providing the aforementioned thermally conducting path for the removal of heat which builds up in pole piece 38, as well as the function of holding the stack of laminations 64 together.

Both of these additional functions are attained by forming the slug 63 in situ. In the case of the fabrication of the slug 63 of aluminum, which material has a lower melting point than that of the iron or mild steel from which the pole pieces 38 are fabricated, a pole piece 38 is formed by stacking the laminations 64 in registration with each other and, then, pouring molten aluminum into the voids 66 of the respective laminations 64. Thereupon, after the cooling of the aluminum, the aluminum solidifies to form the slug 63. As seen in Fig. 3, the ends of the slug 63 envelop the edges of the void 66 at the end of a stack of the laminations 64 during the pouring of the molten aluminum to form a protuberance 76 at each end of the stack of laminations 64.The protuberances 76 grip the outer ends of the stack of the laminations 64 so that, upon a contraction of the slug 63 during the cooling thereof, the laminations 64 are urged together and tightly held in position relative to each other. The pouring of the aluminum into the voids 66 provides intimate thermal contact between the surface of the slug 63 and the individual laminations 64 of a pole piece 38 for the conduction of heat therefrom. Each of the bells 26 serves as-a heat sink. Contact between a slug 63 and the two bells 26 is attained by machining the protuberances 76 at opposed ends of the slugs 63 so as to make a flush fit between the protuberances 76 and the inner surfaces of the bells 26. Thereby, there is provided a path of thermal conductivity between the slugs 63 and each of the bells 26.The bolts 30 and the nuts 28 of Fig. 1 draw the bells 26 inwardly towards each other for applying pressure against protuberances 76 to insure the thermal conductivity at the point of contact between the bells 26 and the protuberances 76.

Referring to Fig. 7, there is seen a jig 80 comprising a set of electromagnets 82 which are positioned in registration with the top magnets 40 of the stator 35 during the assembly of the motor 20 of Fig. 2. Additional electromagnets 84 are placed within the center portion of the stator 35 in registration with the top magnets 40. The electromagnets 82 and 84 are coupled via wires 86, and via a sequencing circuit 88 to a charger circuit 90. The charger circuit 90 comprises, by way of example, a well known bank of capacitors (not shown) which is discharged to provide a sudden large current of short duration in the windings of the electromagnets 82 and 84 to generate momentarily an intense magnetic field of sufficient strength to magnetize the ferrite material of the top magnets 40 and the side magnets 36.The sequencing circuit 88 includes switches (not shown) which couple the charger circuit 90 alternately to the electromagnets 82 and to the electromagnets 84. Thereby, the charger circuit 90 is utilized first for imparting the magnetic field to the side magnets 36 and top magnets 40.

The following sequence for magnetizing the top magnets 40 and the side magnets 36 may be used: First energizing the electromagnet 82, top magnets 40 and side magnets 36 are magnetized. Subsequently energizing electromagnet 84, side magnets 36 are additionally magnetized. If so desired, using electromagnets 82 and 84 and energized top magnets 40 and, to a lesser degree, side magnets 36. Alternatively, pairs of magnets 82 and 84 may be energized sequentially for maximizing the magnetism of top magnets 40.

It is to be understood that the above described embodiment of the invention is illustrative only and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiment disclosed herein, but is to be limited only as defined by the appended claims.

Claims

1. An electric motor comprising a stator structure with pole pieces of magnetic material and one or more permanent magnets in contact with each pole piece and a source of magnetic lines of force, said source impressing a magnetic field through said pole pieces and into the rotor of said motor, said source including magnet means at least partly enveloping at least three sides of said pole pieces and having a thermal conductivity lower than that of said pole piece, characterized in that the pole pieces are provided with openings transversing said pole pieces with their main axis being substantially parallel to the axis of rotation of the said rotor; said voids or openings in said pole pieces being filled with a slug of a material which is non magnetic or has a low magnetic permeability, said material having a thermal conductivity which is comparatively high compared to the thermal conductivity of the permanent magnet material; said slug extending beyond the surface of said pole pieces and being in thermal contact with a heat-sink external to said pole pieces.

2. The electric motor as claimed in claim 1, characterized in that said slug has a cross section in the form of a diamond.

3. The electric motor as claimed in claim 2, wherein a diagonal of said cross section is oriented normally to the axis of rotation of the rotor.

4. The electric motor as claimed in any preceding claim characterized in that the vertex of said diamond shape is near the rotor and the sides of the said slug diverge from said vertex and generally follow the lines of flux during a no-load condition of operation of the motor.

5. The electric motor as claimed in claim 1, characterized in that said pole pieces are formed of a stack of laminations oriented transversely of said slug and said slug extending transversely of the lamination along an outer surface thereof to exert a compressive force thereon for holding said lamination in said stack.

6. The electric motor as claimed in any preceding claim further comprising tie rod means for tightening said heat sink against said slug.

7. The electric motor as claimed in any preceding claim comprising a pole piece of magnetic material, a heat sink and a thermally conductive non-magnetic slug providing a heat conducting path between said pole piece and said sink, said slug being positioned within said pole piece for directing lines of magnetic flux therethrough and extending exteriorly of said pole piece in a direction transverse to said flux for contacting said heat sink, characterized in that the ends of the slug are ground to mate with the end portions or bells of the motor housing, said bells serving as a heat sink for withdrawal of the heat from the slugs.

8. The electric motor as claimed in claim 7, characterized in that a vertex of said cross section adjacent ari inner surface of said pole piece is rounded for directing said lines of flux.

9. The electric motor as claimed in claim 8, characterized in that it further comprises top and side magnets enveloping said pole piece for impressing a magnetic field thereto.

10. The electric motor as claimed in claim 9, characterized in that said magnets are formed of a ferrite material having lower thermal conductivity than the conductivity of said pole piece member.

11. The electric motor as claimed in claim 7, characterized in that it further comprises a second pole piece spaced apart from said first mentioned pole piece, said first and second pole pieces having teeth extending into the space between the said pole pieces for locating a magnet therebetween.

12. The motor as claimed in claim 1 characterized in that it comprises further heat sinks, said heat sinks being positioned on opposite ends of the stator and tie rod means coupled between said heat sinks for urging them against said non-magnetic slug members.

13. The electric motor as claimed in claim 1, characterized in that said magnet means further comprise magnets located alongside said pole pieces and wherein said pole pieces include a tooth projecting outwardly from a side of said pole pieces for locating said magnets.

14. A method for forming the stator of the electric motor of any preceding claim, characterized by the following steps: forming laminations having an opening therein; stacking said laminations; pouring - poring molten non-magnetic material into the holes formed by the openings in the laminations of the said stack; "cooling said stack and said molten material to provide a slug in said stack; and shaping the ends of said slug to mate with a heat sink.