GB2246244A - A permanent magnet rotor electrical machine having a secondary permanent magnetic circuit augmenting rotor flux - Google Patents

Abstract

The high speed electromagnetic motor/generator of the invention comprises a rotor having pole pieces 2A, 2B, a stator (8) surrounding at least a part of the rotor (1), an air gap separating the rotor (1) and the stator (8), and a permanent magnet (4) carried by the rotor to establish a primary magnetic circuit through the stator (8). The machine utilises a secondary magnetic circuit to augment the magnetic flux in the primary circuit, the secondary circuit comprising at least one permanent magnet (9) mounted externally of the rotor (1), and a magnetically permeable member (11) establishing a magnetic path externally of the rotor from one end of the rotor to the other. The poles 2A, 2B may be separated by a gap or barrier of non magnetic material. <IMAGE>

Description

TITLE: Dynamoelectric machines DESCRIPTION The invention relates to dynamoelectric machines, which may be electrical generator or motors. The machines according to this invention may take either the form of brushless DC or synchronous AC machines. Generally speaking the invention is most suited to machines capable of running at speeds of above 50,000 rpm and utilizing or producing several kilowatts of power.

Background Art The rotors of permanent magnet (PM) dynamoelectric machines are conventionally cons truc ted from permanent magnets which tend to have a low tensile strenqth and inhibit the maximum operating speed of tile machines.

At high rotational. speeds the tensile stresses indtlced in the rotor by the resultant centrifugal forres may exceed the limit permitted for the magnetic material and consequently y cause the machine to be damaged.

PM machines haye been proposed which are capable of running at very high speeds with low power ratings; and similarly PM machines have been proposed which are designed to run at relatively low speeds hut with bigh power ratings. Very few proposals have been made for machines that are capable of running at speeds in excess of 50 000 rpm at the same time as producing or consuming several kilowatts of power. One particular problem of such machines is that the centrifugal forces have tn be accommodated in the design of the rotor so that the rotors of high speed machines have been of small diamoter and therefore capable of utilizing only small magnets. This reduces the available magnetic flux, and thereby restricis the power of the machine.

One known way of increasing the amount of available magnetic flux and hence the power output of the machine is to ihtroduce into the rotor an axial flux which is split into two pole pieces separated by a space or by a non-magnetic filler to reduce magnetic Leakage. The conventional permanent magnet is fitted in the centre of the rotor, and the additional flux, introduced axially at the ends of the rotor follows the path of least reluctance and reinforces the flux induced by the permanent magnet. The axial flux at the ends of ttie pole pieces is provided by at least one DC winding. Such machines are known as hybrid PM machines, and the magnetic flux provided by the permanent magnet is referred to as the first magnetic circuit.The magnetic flux provided by the DC winding is referred to as the secondary magnetic circuit. Such machines are however significantly more expensive and complex than non-hybrid PM machines because of the need to provide the DC winding or windings and to control the current through that winding or those windings.

Summary o the Invention it is ah object of the invention to provide a dynamoelectric machine which avoids the above drawbacks.

The invention provides a dynamoelectric machine which comprises a rotor, a preferably laminated stator surrounding part of the rotor, an air gap separating the rotor and the stator and a permanent magnet carried by the rotor to establish a primary magnetic circuit through the stator, wherein means are provided for establishing a secondary magnetic circuit to augment the magnetic flux in the primary circuit, comprising at least one permanent magnet mounted externally of the rotor and a magnetimally permeable member establishing a magnetic path exterually of the rotor from one end of the rotor to the other The machine according to the invention is a variant of the hybrid PM machine discussed above, in which the secondary magnetic circuit is driven by a permanent magnet or magnets rather than by a DC winding.The rotor ie preferably strengthened by filling tie space between the poles with a non-magnetic material ,that is firmly attached to the poles, so that hoop stresses can be withstood. Alternatively the rotor may be made from a single piece of material that is treated to create the necessary zones of differing magnetic permeability.

By using one or more permanent magnets to drive the secondaty magnetic circuit the need for a DC winding, supply and control system is avoided. The invention allows the restrictions on diameter, lenqti and maqnetic field ih high speed dynamoelectric machines to be relaxed.

Dr awinfl Figure 1 is a perspective view, partly cut away, showing schematically the structure of one dynamoelectric machine according to the invention; Figure 2 is a partial section taken along the line 1-1 of Figute 1, illustrating the flux distribution through the rotor and the stator; Figure 3 le a schematic perspective view of reievant parts of a machine similar to that of Figure 1.

illustrabing the magnetic flux path; Figure 4 is a perspective view, partially in section illustrating one possible rotor structure of a dynamoelectric machine according to the invention; Figure 5 is a section taken along the line II-II of Figure 4; Figure 6 is a section similar to that of Figure 5 but through a modified rotor structure; Figure 7 is a perspective view, partly cut away and partly in section, of the rotor and stator orally of the dynamoelectric machine of Figures l and 2;; Figures 8 and 9 are axial sections through two alternative structures for the end mountinqs for the rotor and the linking of the secondary magnetic circuit with the rotor; Figure 10 is a cross section through an alternative end mounting structure; and Figures 11 and 12 2 are perspective views of alternative magnetically permeable structures for establishing the secondary magnetic circuit.

Referring first to Figure l the dynampelectrir machine illustrated comprises a rotor 1 comprising two pole pieces 2t and 2B rotatably mounted on bearings 10. The division between the pole pieces 2A and 2B is illustrated in Figure 1 as a single line depicting a sinusoidal junctioti, but in this respect F.gure l is only schematic and in fact the two pole pieces are magnetically separated from each other along that sinusoidal junctiol by a gap or a barrier 3 of non-magnetic material as illustrated in Figures 2 and 7.Furthermore, non-sinusoidal jlnctions may be provided as will be described in greater detail below.

Around the rotor 1 is R stator 8 which spans the jundlich between the pole pieces 2A and 2B. Atound the stator 8 is a jacket 15 of non -magnetic material which is provided for cooling purposes and may be formed with passages carrying cooling fluid. Around the cooling jacket JS is a cylindrical sheath 11 of magnetically permeable material having at its ends inturned flanges which contact a pair of cylindri ca] permanent magnets 9 which are mounted outwardly of the bearings 10 and which establish the secondary magnetic circuit as will be described below. The whole is surrounded by a nou-magnetie outer jacket 14 which may if desired be formed with further cooling passages (not shown). A secondary air gap 7 is formed between each magnet 9 and the associated pole piece 2A or 2B which it surrounds.

Figure 1 also illustrates a pair of rotor end pieces 13 which complete the rotor structure, and shows very schematically as a seguence of dotted line arrows the flux path through the rotor and secondary magnetic circuit. That flux path is illustrated in greater detail in Figures 2 and 3.

The pole pieces 2t and 2B carry at their mid section a permanent magnet 4 oriented with its poles diametrically apart across the rotor 1, between the pole pieces 2A and 2B. Figure 2 illustrates the respective radial and axial components of magnetic flux created by the permanent magnet 4 and the secondary magnetic circuit comprising the permanent magnets 9 and the magnetically permeable sheath 31. The flux path is better illustrated in Figure 3 which i111lstrates the magnetically permeable sheath 11 ar being a li-shaped member to one 5 i '.o of the rotor and stator assembly.

It is to be tinderstood that tie functioti of the magnetically permeable member is simply to complete the magnetic flux path from one of the permanent magnets 9 to the other in a manner analogous to the magnetically permeable bar or keep which is conventionally used to bridge the opposite poles of a horse-shoe magnet.

Discussion of the possible alternative shapes of the magnetically permeable member 11 is given in greater detail below. From Figure 3 it will be seen that the magnetic flux induced in the secondary magnetic circuit, and indicated as broad arrows, follows the path of minimum magnetic reluctance through the pole pieces 2A and 2B turninq through 90 to complement the diametrically oriented magnetic flux from the rotor tn the stator as created by the permanent magnet 4. All other significant features of Figure 3 are as illustrated in Figure 1 except that in Figure 3 the junction between the pole pieces 2A and 2B is shown as square and not sinusoidal.This square-ended junction is better illustrated in Figure 4 which shows that flats 16 are cut into the rotor assembly so as to produce additional reluctance torque (see also Figure 5) . Figure 6 shows a modification of this structure in which the flat flanks 16 have been filled with the non-magnetic material 3. This serves to reduce noise and vibration.

Figures 4 to 6 show the rotor pole pieces 2A and 2B and the permanent magnet 4 as being hollow, with an axial centre hole 6. The centre hole 6 is used for cooling purposes, but equally the rotor structure could have been solid as in Figures 1 to 3.

Figure 7 better illustrates the sinusoidal division between the pole pieces of Figure 1. In Figure 7 the stator 8 is shown ds axially sectioned to permit the rotor structure to be seen. Only l pole piece 2A is shown. and the non-magnetic barrier 3 and the permanent magnet 4 are shown as axial sectioned A non-magnetic strengthening sleeve 7 doeS in fact extend the entire length of the rotor assembly; but is shown in Figure 7 as having been cut away to the rotor's end to enable t component parts of the rotor B8embly to be more clearly seen. In Figure 7 there is no cooling bore 6, and the permanent magnet 4 is in the form of a solid cylinder.It is held axially In position in a cehtral passage through the pole pieces 2A and 2R (not illustrated) by locating plugs 5 shown dotted. The secondary magnetic circuit is not shown in Figure 7, and has been omitted simply to enable tyke rotor construction to be shown.

Figure 8 illustrates one possible arrangement for the junctioh of the ends of the magnetically permeable member il and the rotor 1. Only the end portion of the magnetieally permeable member 11 is shown, with the remainder being broken away. Figure 8 shows how an annular permanent magnet 9 is fitted around the rotor 1, with the secondary air gap being clearly shown between the rotor and the magnet. The bearing 10 is mounted slightly inboard of the magnet 9, and the screen 14 provides protection from the ingress of dirt etc.

Figured 9 and lo show an alternative arrangement for the mounting of the magnetically permeable member tl. In these Figures the rotor 1 is omitted but it will be seen that the bearing 10 is disposed similarly to the arrangement in Figure 8, but that four discrete permenant magnets 9 are arranged radially outwardly from the rotor 1. It will be appreciated from Figures 9 and 10 that the permanent magnet or magnets 9 can be positioned anywhere along the magnetically permeable e member 11, so long as the magnetically permeable member cooperates with that permanent magnet or magnets to establish a magnetic path externally of the rotor from one end of the rotor to the other.

Figures ii and 12 illustrate two different configurations for the magnetically permeable member 11 . In Figure 11 it is shown as comprising a cylindrical portion IlA and two end plates 11B, aii being made from magnetically permeable material and being assembled to provide the sheath generally as shown in Figure 1. Figure 12 shows how an equivalent secondary magnetic circuit can be provided by four magnetically permeable rods tlA and two end plates 11B.

Figures ii and 12 do not illustrate the position of the permanent magnet or magnets 9, but it will be understood that the magnet or magnets 9 may be located anywhere along the flux path defined by the magnetically permeable member ii.

Claims (9)

1. A dynamoelectric machine which comprises, d totor, å stator surrounding part of the rotor, an air gap separating the rotor and the stator, and d permanent magnet carried by the rotor to establish a primary magnetic circuit through the stator, wherein means are provided for establishing a secondary magnetic circuit to augment the magnetic flux in the primary circuit, comprising at least one permanent magnet mounted externally of the rotor and a magnetically permeable member establ1thing a magnetic path externally of the rotor from one end of the rotor to the other.

2. A machine according to claim I, wherein the permanent magnet or magnets of the secondary magnetic circuit, together with the magnetically permeable member, comprises the stationary support for bearings for the rotor.

3. A machine according to claim 1 or claim 2, wherein the one br more permanent magnets of the secondary magnetic circuit Snd the magnetically permeable member establish a magnets path to the rotor outboard o-F bearings for the rotor.

4. A machine according to claim 3, wherein the one or more permanent magnets of the secondary magnetic circuit and the magnetically permeable member establish a magnetic path to the rotor immediately outboard of the rotor bearings.

5. A machine according to any preceding claim, wherein the rotor comprises two shaped magnetic portions separated by a non-magnetic barrier portion.

6. A machine according to any preceding claim, wherein the totor comprises two pole pieces of magnetically permeable material, between which is mounted at least one permanent magnet.

7. A machine according to any preceding claim, wherein a reinfotcing sleeve of non-magnetically permeable material is formed around the rotor.

8 machine according to any preceding claim, wherein the rotor is hollow with an axial cooling passage formed therein.

9. A dynamoclectric machine having primary and secondary magnetic circuits substantially as described herein with reference to the drawings.