GB2333906A

GB2333906A - Austenitic/Martensitic rotor for a reluctance machine

Info

Publication number: GB2333906A
Application number: GB9802130A
Authority: GB
Inventors: Adam Alasdair Pride
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1998-01-30
Filing date: 1998-01-30
Publication date: 1999-08-04
Also published as: DE19903324A1; GB9802130D0; JPH11220856A

Abstract

The rotor is formed by laminating thin plates and the base material of the rotor is austenitic steel or martensitic steel, alternatively. When austenitic steel is used the rotor pole portions are formed by a rapid cooling enabling austenite to martensite transformation. To cool the rotor pole portions 19,20 of the rotor, cold shoes 31 filled with liquid nitrogen are pressed on both surfaces of the rotor. When martensitic steel is used, the rotor pole portions are formed by a rapid heating enabling martensite to austenite transformation electrically heated hot shoes. Shaped poles 35a 37a may be formed.

Description

TITLE Reluctance Machine DESCRIPTION Field of the invention The present invention relates to reluctance machines, and in particular to switched reluctance motors, variable reluctance machines, stepper motors and synchronous reluctance motors.

Prior Art One of the drawbacks of the reluctance machine, in general, is that the conventional toothed rotor geometry thereof suffers from relatively great windage loss, as known from US 5,053,666, for example. In addition, the increase in windage loss is proportional to the square of the speed, or greater, which results in the overall efficiency becoming so low at high speed that reluctance machines then become impracticable. This is widely recognized and there have been many attempts to make the rotor surface smooth so as to reduce windage loss.

Previously, use has been made of segments and/or fillers to fill the gaps between the teeth so as to produce a smooth rotor surface. The segments and/or fillers must be retained in some way to prevent them being thrown out by centrifugal force as the rotor rotates. Further, the segments and/or fillers must be non-magnetic, otherwise the rotor has no variable magnetic reluctance required for the reluctance machine configuration.

US 4,918,831 shows non-magnetic, high-resistance, segments welded between teeth. Welding the segments and the laminations together increases undesirable eddy current loss because the laminations are electrically connected.

US 5,111,096 shows that non-magnetic segments are preferably aluminium cast around magnetic segments. In ceneral, hi strength non-magnetic cast metals, or example nconel, have hich melting points which would deride the magnetic an electrical properties cf the rotor during the cast:nc of the segments. US 5,023,502 shows a filler material (e.c. Bakelite, nylon, between teeth retained by tangs and end rings. The tangs do not support the filler material fully. Retention of the filler material is nct reliable because of the fatigue cf the tangs, after the reluctance machine is driven for a long time.

An object of this invention is to reduce windace Icss of a rotor of a reluctance motor without segments and/or fillers to fill the caps between the teeth of the rotor.

The Invention According to the invention, the reluctance machine comprises: a cylindrical stator provided with plural pairs of opposing stator pole portions which project inward in the diametrical direction at regular intervals and which extend in the axial direction; a coil wound on the stator pole portions; a disk-shaped rotor provided with at least one pair of opposing rotor pole portions which extend outward in the diametrical direction at regular intervals on the rotor and which extend in the axial direction, and with non-magnetic portions disposed between the rotor pole portions, the rotor being formed by the lamination of thin plates; and a housing containing the stator and the rotor and supporting the rotor rotatably, wherein the base material of the rotor is austenitic steel, and the rotor pole portions are formed by rapid cooling enabling austenite to martensite transformation.

According to a further embodiment of the invention, the reluctance machine comprises: a cylindrical stator provided with plural pairs of opposing stator pole portions which project inward in the diametrical direction at regular intervals and which extend in the axial direction; a coil wound on the stator pole portions; a disk-shaped rotor provided with at least one pair of opposing rotor pole portions which extend outward in the diametrical direction at regular intervals on the rotor and which extend in the axial direction, and with non-magnetic portions disposed between the rotor pole portions, the rotor being formed by the lamination of thin plates; and a housing containing the stator and the rotor and supporting the rotor rotatably; wherein the base material of the rotor is martensitic steel, and the rotor pole portions are formed by rapid heating enabling martensite to austenite transformation.

The Drawings The invention is illustrated with reverence to Figures 1 te 8 of the accompanying drawings. In the drawings: Figure 1 is a schematic view cf a switched reluctance machine i accordance with the present invention; Figure 2 is a view of rotor and cold shoes for preparation cf the austenite to the martensite transformation in accordance with the present invention; Figure 3 is a view of the rotor, the cold shoes and hot shoes or preparation cf the austenite to the martensite transformation of another embodiment in accordance with the present invention; Figure d is a view of the rotor, the cold shoes and hot shoes or preparation of the austenite to the martensite transformation of another embodiment in acordance with the present invention; Figure 5 is a front view cf the rotor cf another embodiment i accordance with the present invention; Figure 6 is a front view of the rotor of another embodiment in accordance with the present invention; Figure 7 is a diagram of character between temperature and magnetic phase in accordance with the present inventicn; and Figure 8 is a schematic view of a synchronous reluctance machine in accordance with the present invention.

Referring first to Figure 1, a reluctance machine iO according to the Invention is provided with z housIng 11 which is mad aluminium. Such a reluctance machine s a switched reluctance motor. in an inner bore Ila of the housing 11, a cylindrical stator 12 is disposed. The stator 12 is formed by laminating electromagneti steel plates and is fixed t the inner ore 11a of ~'-e housing t its cuter circumferential portion by hea. shr King.

The stator 12 is provided with three pairs of opposing stator pole portions 13a,13b; 14a,14b; 15a,15b which reject inward in the diametrical direction at regular intervals and which extend in the axial direction of an output shaft 18 On each pair of the stator pole portions 13a,13b; 14a,14b; 15a,15b, for example, on the pair of the stator pole portions 13a,13b, coils 16a,16b are wound and are connected in series with eac other. Coils (not shown are wound on each of the pairs of the stator pole portions (14a,14b and 15a,lSb) and connected in pairs in series. These coils are connected wit a drive circuit 21.

A disk-shaped rotor 17 which is formed by laminating electromagnetic steel plates 15) is fixed on the output shaft 18 which is rotatably supported on the housing 11. Thereby the rotor 17 is able to rotate with the output shaft 18 within the stator 12.

Since the lamination is in the form of a complete disk, the lamination has inherently high hoop strength and a rotor as a whole has a smooth surface for the minimum windage loss. Furthermore, the rotor 17 is provided with two pairs of opposing rotor pole portions 19a,19b; 20a,20b which extend outward in the diametrical direction at regular intervals on tne rotor 17 and which extend in the axial direction of the shaft 18. Each inner end of ther rotor pole portions 19a,19b; 20a,20b is continuously connected to a central portion 22 so as to allow magnetic flux to flow among the rotor pole portions 19a,19b; 20a,20b. Each of the rotor pole portions 19a,19b; 20a,20b aligns with each of the stator pole portions 13a,13b; 14a,14b; 15a,15b as the rotor 17 rotates, while maintaining a certain clearance therebetween.

A magnetic area 40 (the rotor pole portions 19a,19b; 20a,20b and the central portion 22) and non-magnetic areas (non-magnetic portion) 23 are formed at the cylindrical rotor 17 by so-called "rapid sub-zero cooling" enabling austenite to martensite transformation. The transformation is shown in Fig. 7 and is caused by cooling. The base austenitic steel, which is the base material of the rotor lamination, should preferably have a high nitrogen content ( > 0.5%) as these steels generally have high strength. The transformation should be as fast as possible. Because the laminations are thin, for example, 0.1 or 0.05 mm, so as to keep eddy current losses low at high speed, it is possible to cool a selected area (the magnetic area 40: the rotor pole portions 19a,19b; 20a,20b and the central portion 22) very rapidly. This ensures good definition of the magnetic and the non-magnetic areas 23.

Fig. 1 shows a rotation sensor providing position and angle of the rotor 17 signals but the position and the angle can be sensed via the coils.

"Switched reluctance" type stators are those in which the stator is constructed and energised so that the magnetic field rotates in distinct angular steps, as shown in Fig. 1. Such a stator is also employed in stepper motors although these are usually optimised to give many accurate angular steps rather than continuous rotation.

Methods for forming the rotor pole portions 19a,19b; 20a,20b is as follows: Method 1: As shown in Fig. 2, cold shoes 31 are filled wlth liquid nitrogen (cold medium) at 77K. Each outline of the contact ends 31a of the cold shoes 31 coincides the desired shape of the rotor pcle portions l9a,19b; 20a,20b and the central portion 22 cf the rotor 20. The cold shoes 31,31 are pressed on the both surfaces of the rotor 20 at the same time, so that the partial austeite to martensite transformation of the rotor 20 is achieved right away. As shown in Fig. 3, electrically heated hot shoes 32 may be combined and used with the cold shoes 31,31 to prevent the non-magnetic (austenitic) areas 23 of the rotor 17 becoming cold enough for the transformatIon to occur. As a result, precise definition of the magnetic area 40 and the non-magnetic areas 23 of the rotor 17 is achieved.

Method 2: The non-magnetic areas 23 of the rotor 17 are masked by a heat insulator 33, e.g. resin, on both surfaces of the rotor i7, as shown in Fig. 4. As a result, the rotor 17 is dipped in Liquid nitrogen 34, so that the partial austenite to martensite transformation of the rotor 17 is achieved against a non-masked area (the magnetic area 40) right away.

The shapes of the magnetic area 40 and the non-magnetic area 23 are freely designed and they depend cn the shape cf the cold shoe 31 or the resin-mask 33. Fig. @ shows a non-uniform rotor pole portions.

The rotor pole portions 19a,10b; 20a,20b comprises magnetic ion 3a 3Sb; 36a,36b and intermediate phase portion 37a,37b; 38a, 38b at the outward end thereof. The intermediate phase is a phase between the martensite and the austenite, as shown in Fig. 7. While the rotor 17 rotates, joust before the magnetic portions a,35b is aligned for example, with the stator pole portions 13a, 13b, the intermediate portions 37a,37b face to the stator pole portions 13a,13b. Since intermediate portions includes some martensite, some magnetic flux may flow through the intermediate portions 37a,37b before the magnetic portions 35a,35b align with the stator portions. Thus, the large rate of change of magnetic flux is prevented.

Fig. 6 shows so-called "Inverted 2-pole rotor". The shape inverts a conventional 2-pole toothed rotor shape by putting the magnetic areas 61,62 on the outside diameter of the rotor. Arrow 64 show the magnetic flux. Arrow 63 show an aligned position having low reluctance characteristics Arrow 65 shows an un-aligned position having high reluctance characteristics The magnetic area 40 and the ron-magnetic areas 23 may also be formed at the cylindrical rotor 17 by so-called rapid heating" enabling martensite to austenite transformation. The transformation is caused by heating and the base steel of the rotor 17 Is martensitic steel. In this case, the cold shoes 31 of Figs. 2 and 3 are replaced by hot shoes (not shown) and the hot shoes 32 of Fig.3 are no longer used.

During the operation of the reluctance machine, the torque is developed on the rotor pole portions 19a,19b; 20a,20b. As the rotor pole portions 19a,19b; 20a,20b are integral with the non-magnetic portions 23, the rotor 17 is torsicnally stiffer than the conventional rotor of the reluctance machine.

Conventionally, the non-magnetic portion cf the rotor was removed by stamping and scrapped. On the other hand, the non-magnetic portion 23 of the embodiment is not removed. That is, better utilisation cf the base lamination steel. The whole cf the rotor lamination area is used. Furthermore, the simple round disk lemination has a shorter edge length than in conventional reluctance machines, so thatlower stamping force is required, less degradation of the magnetic properties due to stamping is achieved, tool wear is reduced ano tool cost is lowere.

The above-mentioned rotor 17 can be also apply to synchronous motor 50, as shown in Fig. 8, which is also one of a reluctance machine.

The synchronous motor 50 employs , for example, a typical 3 phase, 4-pole, 2-layer winding. "Synchronous reluctance" type stators are those in which the stator is constructed and energized so that the magnetic field rotates as smoothly as possible. This type of stator is essentially the same as those used for induction machines, for example, the stator has many slots through which the winding is evenly distributed so as to give a sinusoidal magnetic field when energised from a sinusoidal supply.

Such a stator also covers some other type of machine which use a variable reluctance rotor. For example, "Inductor and flux-switch" where the rotor is energised across the air gap with a non-rotating source of magnetic flux and "Lundell" where the rotor is energised via slip-rings and so on are proposed.

Claims

CLAIMS 1. A reluctance machine, comprising: a cylindrical stator provided with plural pairs of opposing stator pole portions which project inward in the diametrical direction at regular intervals and which extend in the axial direction; a coil wound on the stator pole portions; a disk-shaped rotor provided with at least one pair of opposing rotor pole portions which extend outward in the diametrical direction at regular intervals on the rotor and which extend in the axial direction, and with non-magnetic portions disposed between the rotor pole portions, the rotor being formed by the lamination of thin plates; and a housing containing the stator and the rotor and supporting the rotor rotatably; wherein the base material of the rotor is austenitic steel, and the rotor pole portions are formed by rapid cooling enabling austenite to martensite transformation.
2. A reluctance machine according to claim 1, wherein a pair of cold shoes, filled with a cold medium, are pressed on both surfaces of each thin rotor plate so as to cool the rotor pole portions thereof.
3. A reluctance machine according to claim 2, wherein an outline of each contact end of the cold shoes coincides with the desired shape of the rotor pole portions.
4. A reluctance machine according to claim 2 or claim 3, wherein electrically heated hot shoes are combined with the cold shoes so as to prevent cooling of the non-magnetic portions of each thin rotor plate.
5. A reluctance machine according to claim 1, wherein the non-magnetic portions of each thin rotor plate are masked by a heat insulator on both surfaces thereof and each thin rotor plate is dipped in cold medium.
6. A reluctance machine comprising: a cylindrical stator provided with plural pairs of opposing stator pole portions which project inward in the diametrical direction at regular intervals and which extend in the axial direction; a coil wound on the stator pole portions; a disk-shaped rotor provided with at least one pair of opposing rotor pole portions which extend outward in the diametrical direction at regular intervals on the rotor and which extend in the axial direction, and with non-magnetic portions disposed between the rotor pole portions, the rotor being formed by the lamination of thin plates; and a housing containing the stator and the rotor and supporting the rotor rotatably; wherein the base material of the rotor is martensitic steel, and the rotor pole portions are formed by rapid heating enabling martensite to austenite transformation.
7. A reluctance machine according to claim 6, wherein a pair of heated hot shoes are pressed on both surfaces of each thin rotor plate so as to heat the rotor pole portions thereof.
8. A reluctance machine according to claim 7, wherein an outline of each contact end of the hot shoes coincides with the desired shape of the rotor pole portions.
9. A reluctance machine according to any preceding claim, wherein the rotor pole portions comprise a magnetic portion and an intermediate phase portion at the outward end thereof, the intermediate phase having characteristics of both martensite and austenite.
10. A reluctance machine according to any preceding claim, wherein the stator is formed by the lamination of electromagnetic steel plates.
11. A reluctance machine according to any preceding claim, wherein the thin plates of the rotor lamination have a nitrogen content greater than 0.5%.
12. A reluctance machine according to any preceding claim, wherein the plates of the rotor are of a thickness in the range of 0.05 - 0.lmm.
13. A reluctance machine as hereinbefore described with reference to Figures 1 to 5 and Figure ; cf the drawings.

13. A reluctance machine as hereinbefore described with reference to Figures 1 to 5 and Figure 6 of the drawings.

Amendments to the claims have been filed as foliows 1. A reluctance machine, comprising: a cylindrical stator provided with plural pars of opposing stator pole portions which project inward in the diametrical direction at regular intervals and which extend in the axIal direction; a coil wound on the stator pole portions; a disk-shaped rotor provided with at least one pair of opposing rotor pole portions which extend outward in the diametrical direction at regular intervals on the rotor ana which extent in the axia direction, and wth non-magnetic portions disposed between toe rotor pole portions, the rotor being formed by the lamination of thin plates; and a housing containing the stator and the rotor and supporting the rotor rotatably; wherein the base material or the rotor is austenitic steel, and the rotor pole portions are formed by rapid cooling enabling austenite to martensite transformation.

2. A reluctance machine according to claim 1, wherein a pair ol cold shoes, filled with a cold medium, are pressed on both surfaces of @ each thin rotor plate so as to coo the rotor pole portions thereof.

3. A reluctance machine according to claim 2, wherein an outline of each contact end of the cold shoes coincides with the desired shape of the rotor pole portions 4. A reluctance machine according to claim 2 or claim 3, wherein electrically heated not shoes are combined wit the cold shoes se as to prevent cooling of the non-magnetic portions of each thin rotor plate.

5. A reluctance machine according to claim 1, wherein the non-magnetic portions of each thin rotor plate are masked by a heat insulator on both surfaces thereof and 6. A reluctance machine comprIsing: a cylindrical stator provIded with plural pairs of opposing stator pole portions which project inward in the diametrical direction at regular intervals and which extend in the axial direction; a coil wound on the stator pole portions; a disk-shaped rotor provided with at least one pair of opposing rotor pole portions which extend outward in the diametrical direction at regular intervals or the rotor and which extend in the axial direction, and with non-magnetic portions disposed between the rotcr pole portions, the rotor being formed by the lamination of thin plates; and a housing containing the stator and the rotor and supporting the rotor rotatably; wherein the base material of the rotor is martensitic steel, and the non-magnetic portions are formed by rapid heating enabling martensite to austenite transformation.

7. A reluctance machine according to claim 6, wnerein a pair of heated hot shoes are pressed on both surfaces of each thin rotor plate so as to form the non-magnetic portions thereof.

8. A reluctance machine according to claim 7, wherein an outline of each contact end of the hot shoes coincides with the desired shape of the non-magnetic portions.

9 A reluctance machine according to any preceding claim, wherein the rotor pole portions comprise a magnetic portion and an intermediate phase portion at the outward end thereof, the intermediate phase having characteristics of both martensite and austenite.

10. A reluctance machine according to any preceding claim, wherein the stator is formed by the lamination of electromagnetic steel plates.

11. A reluctance machine according to any preceding claim, wherein the thin plates of the rotor lamination have a nitrogen content greater than 0.5%.

12. A reluctance machine according to any preceding claim, wherein the plates of the rotor are of a thickness in the range of C.5 - u.imm.