This invention relates to reluctance motors. In particular, the invention relates to two-phase reluctance motors. The invention also relates to electrical drive systems containing motors of the foregoing, kinds.

In the class of electrical machine generally referred to as variable reluctance electrical machines, high permeability ferromagnetic stationary and moving parts having pole surfaces separated by the smallest air gap consistent with mechanical clearance are arranged to form magnetic paths of low reluctance except for highly saturated constriction zones determined by the overlap between the poles, so that the magnetic flux is determined primarily by the position of the moving part and as little as possible by the intensity of the excitation current or currents. Constructions of variable reluctance motor having three or more phases, which are referred to subsequently as multiphase constructions of such machines, have the disadvantage however of inherently leading to structures which are more strongly excited in so-called "bell mode" than single-phase or

two-phase machines. In other words, multiphase variable reluctance motors are considerable sources of vibration and noise, especially in larger size machines. There is an inherent propensity towards noise and vibration in a stator which is relatively thin in the radial direction under the exciting action of only that one third

(three-phase) or one quarter (four-phase) of the poles which are energised at any one time. In addition, the number of switching elements required by a multiphase machine as herein defined adds to the cost and complexity of the system, especially at relatively low power ratings.

Thus there are compelling reasons for considering two-phase or single-phase machines, especially for power ratings under 100 KW, to secure improvements in respect of both noise and vibration, and also from the point of view of cutting down on the number of switching elements. In a two-phase motor, hammer blow is more evenly spread out around the periphery of the stator, with half of the poles energised at any one time, and "bell effect", i.e. ringing of and noise emanation from the excited motor structure, is substantially reduced.

• The present invention is therefore directed to a two-phase reluctance motor, but reduction in the number of phases brings with it certain additional problems requiring to be addressed to provide a satisfactory construction of two-phase machine, problems not present in multiphase constructions of reluctance motor as previously defined. The difficulties in question relate in particular to torque continuity and self-start, and revolve around the location of the limiting constrictions zone and the control of the cross-sectional area of the flux path during the passage of the rotor poles across the stator polefaces.

In a multiphase reluctance motor, the limiting constriction zone may be located at or adjacent to the overlapping poleface surfaces of one or both of the relatively displaceable parts of the machine by dimensioning the machine so that, throughout the working stroke of the

machine, the cross-sectional area of the ferromagnetic material in the flux path at the variable interface in the poleface region is less than the cross-section available to magnetic flux elsewhere in the flux path. The working stroke is determined by the extent of the relative mechanical displacement of the stationary and moving parts during which a substantially uniform rate of flux increase prevails, i.e. the displacement increment during which the cross-sectional area of the flux path of the constriction zone continues to increase. The working stroke terminates when increase in the cross-sectional area of the flux path ceases.

In multi-phase constructions of reluctance motor as previously defined, mechanical limitations constraining the provision of this increasing cross-sectional area of the constriction zone, and thereby the length of the working stroke, are not normally of major significance, since the proliferation of phases means that another phase is always ready to take over rotor drive according as the working stroke of a particular pair of co-operating rotor and stator poles comes to an end. Thus torque continuity is assured and the machine may be started in either direction from any rotor position.

Torque continuity and bidirectional self-start are not however inherent in the case of a reluctance motor having less than three phases. In U.S. Patent Specification No. 3,956,678 of Byrne et al, a construction applicable to a two-phase motor is described in which the arcuate extent of the rotor poles is extended, to provide a self-starting machine capable of generating continuous torque in a single preferred direction of rotation. In this machine, a working stroke of 90° per phase is achieved by making each rotor poleface arc 100°, while retaining a relatively conventional stator poleface arc of 50°. Thus the angular extent of the rotor poleface surface is approximately twice that of the stator poleface. In order to provide the required flux constriction zone at the variable poleface region interface, approximately one-half of the rotor poleface surface extending rearwardly from the leading poletip in the direction of

rotation is underlaid or backed by an iron depletion region, this region being defined by trapezoidal slots in the arrangement particularly described in the specification. This configuration enables a generally linear increase of flux with rotation to continue over virtually the full extent of pole relative displacement until the rotor pole portion of full or undepleted iron density comes into substantially full alignment with the stator pole, this being a trailing pole portion in the arrangement described. Magnetic saturation is thus confined to the neighbourhood of the mechanically variable interface or overlap between the stator and rotor poles. The region of depleted iron density forms the leading edge region of the rotor pole during normal operation of the motor described. As this region begins to overlap the stator pole, flux starts to increase in approximately linear dependence on rotor angular displacement. The depleted region is dimensioned so that when it comes into full alignment with the stator pole, the flux level is approximately one-half of its maximum value. Flux then continues to build up generally linearly towards its maximum level as further rotation of the rotor brings the trailing rotor pole portion of full or undepleted iron density into substantially full overlap with the rotor pole. This machine is self-starting in the direction corresponding to the regions of depleted iron density being leading edge regions of the rotor poles during rotor rotation.

The iron depletion method of developing a generally linear increase in the cross-sectional area of the flux path with increasing pole overlap in an extended pole two-phase self-starting reluctance motor lacks however the ability for precise control of flux build-up to be achieved, in that the iron depletion regions essentially define incremental stages in the changing flux path area, so that the build-up in the area of the flux path takes place in a stepwise manner, albeit within a generally linear overall envelope, rather than in a manner which is truly linear or amenable to profiling to meet a particular torque/angle characteristic. Because of this, difficulties remain in precisely tailoring the torque/angle characteristic of the machine to

provide a smooth torque output. It is an object of the present invention to provide an improved rotor construction for a machine of the foregoing kind which will facilitate such tailoring or adjustment of the machine performance.

In summary therefore, in order to overcome noise and vibration difficulties as well as to reduce the number of switching elements, it is desirable to consider a two-phase reluctance motor, as compared with a three or four-phase construction. The two-phase machine entails however certain further difficulties specific to this configuration, especially in respect of self-start, torque continuity, and smoothness of torque/angle characteristic. The present invention is particularly focussed therefore on improvements in these areas.

According therefore to the invention in a first aspect thereof, there is provided a two-phase variable reluctance electric motor comprising a stator having a plurality of salient stator poles, the number of stator poles being four or a multiple of four, a magnetising winding for each stator pole, each magnetising winding being either a first phase winding or a second phase winding and alternate stator poles having windings of different phases so that the stator poles neighbouring each stator pole carrying a winding of the first phase each carry a winding of the second phase and the stator poles neighbouring each stator pole carrying a winding of the second phase each carry a winding of the first phase, and a rotor having a plurality of rotor poles, the number of rotor poles being one-half of the number of stator poles, each rotor pole having a dimension between an outer poleface surface of the rotor pole and an inner poleface surface of the rotor pole which increases progressively along the rotor pole in a direction opposite to the direction of displacement of the rotor relative to the stator from a poletip region of the rotor pole which is a leading poletip region during displacement of the rotor relative to the stator, the progressive increase in said dimension being in accordance with a predetermined criterion so that a magnetic constriction is defined within said rotor pole substantially throughout

progressive overlap of said rotor pole and a cooperating energised stator pole from initial overlap of said poletip region of said rotor pole with said cooperating stator pole to substantially full overlap of at least a portion of said rotor pole defined between said outer poleface surface of the rotor pole and said inner poleface surface with said cooperating stator pole, said magnetic constriction representing a limiting constriction in the path presented to magnetic flux in operation of the motor. Thus, in the two-phase motor construction of the invention, in simple terms, the pole becomes thicker in the trailing direction from the poletip.

Preferably, said rotor pole dimension increases substantially linearly with displacement of said rotor pole relative to said cooperating stator pole, subject, however, to the necessary practical deviations from linearity and idealised shapes in general which are entailed in practical machine design, where real material properties are taken account of such as by computer modelling. The modifications involved are however typically small, from the idealised shapes and profiles developed on the basis of ideally saturable iron and by ignoring fringing flux. The variation in said dimension of the rotor pole is suitably selected to provide a desired torque/angle characteristic for the machine of the invention. Suitably also, said rotor pole is substantially homogeneous at least in said portion of the rotor pole defined between said outer poleface surface of the rotor pole and said inner poleface, said portion extending from said poletip region in said direction opposite to the direction of displacement of the rotor relative to the stator. By homogeneous is meant that the rotor pole is substantially continuous and of substantially uniform density in said portion extending from the poletip region, without any internal air spaces or hollow portions or any other features leading to variation in pole density, at least so far as its magnetic characteristics are concerned.

In an especially favoured construction, said outer poleface surface of the rotor pole is a portion of a substantially cylindrical

surface and said inner poleface surface of the rotor pole is an underlying inwardly-directed surface of the rotor pole. Preferably said underlying inwardly-directed surface of the rotor pole is substantially concave. In this rotational configuration of machine according to the invention, the arcuate extent of the outer poleface surface of the rotor is suitably substantially equal to the pitch of the stator poles, but it may alternatively be either greater than or less than the stator pole pole pitch in variants of the invention.

In a motor according to the invention having four stator poles and two rotor poles, a portion of each rotor pole in a region of the pole remote from said leading poletip region may be of depleted density for magnetic flux. In a particular construction, the motor may comprise a multiplicity of laminations, said portion of depleted density of said each rotor pole being defined by the provision of at least one aperture in each of said laminations. Said portion of the rotor pole in a region of the pole remote from said leading poletip may also be profiled to provide a transition surface between the outer poleface surface of the rotor pole and a central shaft-mounting region of the pole. Thus in a machine in accordance with the invention, not only may .the leading poletip region be shaped to meet the objectives of the invention, but the rotor pole may also be further modified as to its magnetic density in its trailing region also.

The invention also extends to a motor substantially as described herein with reference to and as shown in any one or more of the accompanying drawings, as well as to a lamination for a motor substantially as described herein with reference to and as shown in any one or more of the accompanying drawings. In addition, the invention extends to an electrical drive system comprising a motor of the foregoing kind together with power supply/drive control means.

The invention will now be described having regard to the accompanying drawings, in which

Figure 1 is a diagrammatic rectilinear representation of two poles of a two-phase reluctance motor according to to the invention, showing the inter-relationship between stator poles and rotor profile in side sectional schematic representation,

Figure 2 shows the arrangement of Figure 1 in a basic rotational configuration, again in side sectional schematic representation,

Figure 3 is a diagrammatic representation of the geometrical basis on which an appropriately curved rotor profile for a rotary construction of motor according to the invention may be constructed, in side sectional schematic view,

Figure 4 is a sectional view, in the direction of the machine axis, of suitable stator and rotor cross-sections for an 8/4 pole machine, and

Figure 5 shows the invention as embodied in a 4/2 pole geometry.

Figure 1 shows a basic rectilinear configuration of reluctance machine in accordance with the invention in side sectional view, in order to illustrate the concept underlying the invention. As shown in Figure 1, two stationary or driving poles, 1 and 2, subsequently referred to as stator poles, each have a stator winding 3, 4 respectively, these stator windings being sequentially energised in operation of the system. A moving or driven member 5 is also subsequently referred to as a rotor pole, the terms stator and rotor being used for convenience and not necessarily implying any limitation to rotational constructions of machine. Each stator pole 1, 2 has a poleface 6 which is directed towards, at a spacing of small dimension, an outer poleface surface 7 of the rotor pole. On the opposite side of the rotor pole 5, in a direction away from the stator pole 1 or 2, the rotor pole 5 has an inner rotor pole face 8. The direction of

motion or drive of the rotor pole 5 relative to the stator poles 1 and 2 is indicated by arrow 9. Forward motion or drive of the reluctance motor rotor 5 is engendered by sequential energisation of stator windings 1, 2 etc. in known manner. The poleface dimension of stator pole 1 or 2 in the direction of travel 9 is indicated by the designation A-A ¹ , for pole 1, and B-B ¹ for pole 2. T designates the leading tip of the moving element or rotor pole 5, while dimension x denotes the spacing of tip T from pole 1 corner or edge A, where tip T first comes under stator pole 1. Pole 1 and winding 3 together define an "a" phase, while pole 2 and winding 4 define a "b" phase of the two-phase machine. The cross-sectional area available as a flux path is defined at any stage during advancing overlap of rotor pole 5 with stator pole 1 in the direction of arrow 9, by the dimension designated A-C in Figure 1, namely that defined by a line extending at right-angles to the inner poleface surface 8 of rotor pole 5 to meet the outer rotor poleface surface 7 at the location where it immediately underlies corner or edge A of stator pole 1.

As shown in Figure 1, where the "a" phase 1, 3 is energised, the constriction limiting magnetic flux at the particular disposition of rotor. pole 5 with respect to stator pole 1 is the cross-section at

A-C. In order for excitation of the "a" phase 1, 3 to produce uniform force in the direction of motion 9 for each value of dimension x, i.e. the spacing between the tip T of rotor pole 5 and the initially encountered corner or edge A of stator pole 1, for all values of x up to the stator pole pitch length A-B, in accordance with the formula F = f &_. _. _. dA„/dx, see Byrne, J. V., "Tangential forces in overlapped saT, c pole geometries incorporating ideally saturable material", IEEE Transactions on Magnetics, MAG 8, 1972, 1, 2-9, it is necessary for the constriction at A-C to increase uniformly with x throughout the stroke of the rotor pole 5. For the purposes of explanation only, in the representation of Figure 1, the stator poleface dimension A-A ¹ is assumed to be equal to the interpole spacing A'-B, in other words, pole width is one-half pole pitch. Thus, as the constriction at A-C increases uniformly with x, it is required under these circumstances to

become equal to the limiting section A-A ¹ defined by the dimension of the stator pole surface 7 in the direction 9 of relative motion of rotor 5 and the stator as the tip T of the rotor pole 5 approaches the initially encountered pole corner or edge B of the "b" phase 2, 4. Thus the dimension of A-C, for all values of x, is equal to one-half of x, so that the required uniform force in the direction of motion is achieved by profiling the moving part or rotor pole 5 in the shape of a wedge, with an included angle of 30°. In defining cross-sections in terms of a dimension in the plane of the paper or representation, it will be evident that the element in question is therefore assumed to be of constant cross-section in the direction perpendicular to the paper, i.e. the cross-section shown is representative of all cross-sections. This assumption, while convenient for ease of representation and understanding, may not necessarily be maintained in totality in all practical constructions, although representing a preferred and normal configuration.

At the end of this elongated working stroke of the "a" phase, the moving element or rotor pole 5 is in an appropriate position, with tip T of the rotor pole approaching the initially encountered corner or edge B of stator pole 2, for the "b" phase winding 4 to be energised. An arbitrarily large array of alternate "a" and "b" phase poles may be used to extend the total "working stroke" of the device indefinitely in the linear direction of motion 9. A finite such array however, when wrapped around to define a cylindrical structure, to be depicted as a circle in cross-sectional axial representation, forms a rotating machine.

In Figure 2, the basic rectilinear configuration of Figure 1 is thus shown transformed into a rotary geometry. Stator poles 21 and 22 together with stator windings 23 and 24 again define "a" and "b" phases. The rotor pole 25 underlies the stator polefaces 26 and has an outer rotor poleface 27 facing stator poleface 26 and an inwardly-directed concavely-curved poleface surface 28, this corresponding to the wedge-defining inner surface 8 of Figure 1.

Surface 28 may alternatively be regarded as radially inwardly-directed or underlying. The direction of rotation of the rotor pole 25 relative to the stator poles 21 and 22 is indicated by arrow 29.

Stator pole span in the direction of rotation is again indicated by dimension A-A' and B-B'. Pole width is again one-half of pole pitch, so that the circumferential distance from A to B is twice that along the poleface from A to A', this being however an assumption for ease of representation and understanding and not essential to the invention. The magnetic constriction dimension is again indicated by A-C. The criterion determining the magnitude of this dimension is however in the rotational construction determined by the relationship between the radial dimension R of the machine, this being the distance betwen the axis of rotation and the radially inner poleface surface 26 of the stator poles 21 and 22, and the angular rotation of the rotor pole 25, this again being measured from the corner or edge A of stator pole 21 initially encountered by tip T of rotor pole 25. This relationship establishes therefore the limiting constriction dimension as equal to one-half of the radius multiplied by the angular displacement of the rotor in radians measured in terms of advance of tip T from corner or edge A of stator pole 21. Thus by adopting this formula, the rotor curved surface 28 is laid out in the direction T-C so that the flux constriction at A-C increases uniformly with angular displacement of tip T from datum location A as measured in radians and becomes equal to the poleface width A-A' as the pole tip T of rotor pole 25 approaches the corner or edge B of the succeeding "b" phase stator pole 22.

Figure 3 shows again in diagrammatic cross-sectional view looking in the direction of the axis of rotation of the machine of Figure 2, a manner of geometrical construction for the constriction-defining curved surface 48, also designated by T-C. For convenience of representation, the rotor 45 is in this instance presumed to be stationary, and successive positions of the "a" phase stator pole 41 are designated by references 41a, 41b and 41c. Again

for convenience of representation, only the righthand part of the stator pole in the vicinity of the A corner or edge is shown, the successive dispositions of this corner being designated by Al, A2 and A3 respectively. In order to define the radially inner or underlying concavely-curved surface 48 of the rotor pole 45, circular arcs are struck from the corner positions Al, A2, A3, in each case equal to one-half of the peripheral distances T-Al, T-A2 and T-A3. Thus the distance in question in each case is that extending along the convexly-curved outer poleface surface 47 of the rotor pole 45, and not the straightline distance between tip T and the reference points in question. The curve 48 (T-C) is then constructed to be tangential to each of these arcs, to thereby provide the desired profile of this surface 48. It will be appreciated that in a rotational construction, the inner surface 48 is tangential to the arcs constructed from the notional corner positions Al, A2 etc., this construction thus corresponding to the rightangle disposition of dimension A-C with respect to the underlying poleface surface in the rectilinear configuration of Figure 1.

Thus in a rotational configuration of two-phase motor in accordance with the present invention, the outer periphery 27 or 47 of a rotor pole 25, 45 extending rearwardly from the leading pole tip T, is defined by an arc of a circle, as viewed in sectional or end view. The inner periphery 28, 48 of the leading region of the pole in the direction of rotation 29 is defined by a generally crescent-shaped concavely-curved surface 28, 48, again as seen in end view in the representations of Figures 2 or 4. Typically, in a practical laminated pole construction, at least this leading region of the pole defined between the concavely-curved inner and outer convexly-curved surfaces is constructed to be fully homogeneous, in other words, there is no iron depletion or air space within the body of the leading or tip region. Thus proceeding rearwardly from the pole tip T, there is a progressive increase in the depth of the uniform homogeneous pole in the generally radially inward direction extending from its generally convex outer peripheral surface 27, 47, with increasing displacement

away from the leading pole tip T, this progressive increase in the depth of the pole taking place in a generally radial direction into the body of the pole throughout a leading pole region extending rearwardly from the leading poletip of the pole relative to the direction of rotation.

The cross-sectional area available as a flux path is defined at any stage during pole overlap, which proceeds in the direction of rotation indicated by arrow 29 in Figure 2, by the dimension designated by reference A-C in the rotor/stator relative dispositions of Figures 2 and 3. This dimension represents the spacing between the concave poleface surface 26, 46 of stator pole 21, 41, as defined by the stator poletip A first encountered by the leading pole tip T of rotor pole 25, 45 during rotation, and the nearest point on the inner or underlying or crescent-shaped concave rotor poleface surface 28, 48. This nearest point as seen in the cross-sectional representations of Figures 2 and 3, which in a practical construction corresponds to an axially-extending line in a three-dimensional machine, varies in location along surface 28, 48 as rotation continues, from its initial location at or near pole tip T, as indicated for position 41a of stator pole 41 in Figure 3. By appropriate selection of the rotor inner profile 28, 48, the build-up of flux path area may be arranged to take place in any desired manner and in particular to produce a substantially flat-topped torque/angle characteristic. The particularly favoured relationship between rotor pole displacement and constriction cross-section discussed with respect to Figures 1, 2 and 3 results in substantially uniform force or torque in the direction of motion of the rotor relative to the stator for all positions of rotor pole relative to stator pole throughout the entire elongated working stroke of the rotor pole, this corresponding to the pole pitch of the stator poles, in the conceptual arrangements described.

Figure 4 is a cross-sectional representation of a practical embodiment of machine including the invention, in which eight stator poles are provided, of which two 61, 62 are indicated, along with

windings 63 and 64. The machine has four rotor poles 65 spaced at 90° intervals. Figure 4 indicates how the iron and copper of such a machine may be laid out in practice. T-C again represents the constriction-defining profile of the underlying or concave inner rotor pole surface 68. At the end of the working stroke, dimension C-E indicates the constriction effective at this stage, when the trailing end E of the outer rotor pole surface 67 approaches the stator pole corner or edge A. An arc 71 centered on point C and of radius C-E then defines a minimum section for the root of the rotor pole 65, such that a further flux constriction is not formed in this region. In practice, the rotor pole trailing edge profile may adopt a configuration such as that designated by reference 72, so that the circumferential dimension C-F is of greater extent than the circumferential distance between point C and the theoretical or notional optimised trailing surface profile 71. Thus the practical trailing edge profile 72, extending between points E and F, in practice may lie outside the minimum profile 71 determined from theoretical considerations, and may be regarded as defining a transition surface between the outer poleface surface 67 of the rotor and a central shaft-mounting portion of the rotor lamination stack. Dimensioning of the stator iron is also required so that flux constrictions are avoided other than the limiting constriction desired within the rotor pole 65 along the line or section C-A and equating to the stator poleface dimension A-A'. To this end, it may be necessary for pole taper, for example, to be introduced into the stator poles 61 and 62 etc. The features of the eight-four pole arrangement of Figure 4 also apply to all other configurations of two-phase machine of greater pole number.

In a practical embodiment of 4/2 pole machine, in other words, one having four stator poles and just two rotor poles, it becomes necessary to further modify the theoretical rotor pole construction to compensate for geometrical limitations inherent with this minimum number of poles in a two-phase machine. As shown in Figure 5, the theoretical constriction-defining profile T-C, representing the inner poleface surface 88 of rotor pole 85, extends radially inwards to a

degree which is unacceptable for practical engineering reasons. If the inner poleface surface 88 is carried through to point C, then the rotor pole root becomes unworkably close to the shaft centre or axis of rotation 94 of the machine. If surface 88 is carried inwards to this extent, there is not enough cross-section of iron in the centre body of the rotor to enable the rotor laminations to be mounted on shaft 93 in such a manner as to provide a practical, mechanically-viable construction. In addition, the rotor trailing surface 92 becomes almost continuous with the outer surface 87 of the rotor pole. Thus because of these factors, only a limited portion of the curve T-C, for example T-C, can be used in defining the constriction, which thus becomes A-C in place of the theoretical or idealised A-C.

The geometry shown provides a 50° stator pole arc with 100° rotor pole arc. The point C is then chosen so that the profile T-C controls flux only over the first half or 50° of the working stroke. As shown in Figure 5, the rotor is thus in a transition disposition, in which the constriction has just reached the value A-C at the end of the first half of the working stroke. For further motion, in other words through the remaining 50° of rotor pole displacement, the constriction is then defined between the points C and A as they move further apart, and this constriction of now continually increasing dimension will be non-1inearly related to mechanical displacement unless some modifying adaptations are effected. Thus without modification, the effect on the torque/angle characteristic of the machine would be an initial rise in torque after this transition point, followed by a serious torque shortfall towards the end of the working stroke.

To increase the linearity of the torque/angle curve for displacement after the transition point and to make the torque therefore more dependent in a linear manner on continuing relative displacement of the rotor with respect to the stator, holes or slots are provided in the rotor pole in a trailing portion or region of the pole. In the exemplary arrangement of Figure 5, the constriction

identified as A'-C, which would be effective at 80° of the working stroke, would be excessively large. By providing a single circular hole 95 within the body of the rotor pole 85, new flux constrictions are defined, indicated by dotted lines extending from point C to hole 95 and from hole 95 to point A ¹ . The combined length of these replacing constrictions is selected to be shorter than the constriction A'-C by any desired amount, which is dependent on or governed by the hole diameter. Thus in summary, a constriction of lesser dimension than the constriction A'-C, which would be effective in the absence of the hole 95, is achieved by the provision ^' of the hole 95. Additional holes or slots may be put in position to achieve increasingly closer approximations to a straight line constriction/ displacement relationship in this minimum rotor pole configuration of the invention. It will be appreciated that the holes 95 are punched in the individual laminations forming the rotor stack, preferably but not necessarily, in each lamination, and also preferably but not necessarily, each hole is axially aligned with the corresponding holes of the remainder of the apertured or punched laminations.

Further modifications directed to further improving torque/angle or force/distance characteristics in rotational and linear constructions of the motor or machine of the invention include blunting the leading edge of the rotor, which may also be accompanied by slight skewing of the rotor lamination stack, as part compensation for the magnetic effects of the blunting. Alternatively, skewing may be employed in conjunction with the sharp rotor tip of the representations shown. A non-magnetic filler material may be used to give the motor a smooth cylindrical shape, the interpole spaces in the rotor being filled up with a suitable magnetically inert material. In this manner, siren-type noise and windage losses may be reduced. The phase coils may also be connected in parallel rather than in series to inhibit unbalanced radial forces on the rotor, which latter may engender noise and vibration. The performance of a motor according to the invention may be further enhanced by the use of magnetic materials having higher saturation flux-density and squarer B-H characteristics

than conventional silicon iron. A suitable material is cobalt iron alloy.

In summary, the central concept of the invention involves control of the torque-angle characteristic of a machine of the kind to which the invention relates by wedge-shaped profiling of the leading part of the rotor poles, the underlying character of which may be best appreciated and comprehended having regard to the basic rectilinear configuration of Figure 1 and the 8/4 pole configuration of Figures 2 and 3. In these particular variants, the torque angle characteristic may be entirely controlled by the profiling of the leading portion of the rotor poles, without any further constructional adaptation or modification of the rotor.

However, in the case of a geometry in which the minimum possible number of poles are used, namely four rotor poles and two stator poles, wedge-shaped profiling alone is not achievable in a practical construction to such an extent as to ensure control of the torque-angle characteristic throughout the full angular extent of the working stroke of one phase of the machine, on account of mechanical constraints arising out of the compact constructional package of such a machine. The extreme curvature of this geometry constricts the iron cross-section available to flux in the rotor. Thus, a 4/2 pole machine requires further modification from a wrapped-around version of a rectilinear machine, this being achieved by modifying the trailing region of the rotor pole to overcome the difficulty arising out of the leading wedge of the rotor pole having to be curtailed because of the shortage of iron section in the radially inner region of the rotor. This enables the control of flux with angle in the later stages of the working stroke to be regulated, at a time when the leading wedge has ceased to be effective in the constrained environment of the 4/2 pole construction.

As also previously noted, the pole arcs and rotor profiles discussed in the present description and shown in the Figures are given

for the purposes of a simple explanation, involving ideally saturable iron and the neglect of fringing flux. In the design of practical machines, real material properties are taken account of such as by computer field modelling and this will necessarily lead to modifications, typically however relatively small, of the idealised shapes shown in the present Figures.

In summary however, in essence, the machine of the invention uses extended-arc broadly crescent-shaped rotor poles, at least in the leading portions of the poles, in which the profile of the crescent is shaped such that any desired generally linearly varying constriction with respect to angular displacement may be achieved in simple and advantageous manner, and in particular without the use of iron depletion and punched laminations in this leading portion of the rotor pole, with a view to providing a substantially linear torque/angle curve. Thus the invention provides, in a novel manner, for the constriction cross-section of the iron of the rotor in the flux path to increase uniformly over an extended working stroke phase, for continuity of torque in a single direction of rotation. The constriction does not occur in the air gap overlap zone of the poles, but along a notional surface within the crescent or wedge-shaped leading pole portion. This generally wedge or crescent-shaped rotor leading edge profile may be defined such as by computer field computation to achieve in particular substantial torque uniformity over the working stroke of each phase. In this, deviations from idealised theory created by fringing flux, the non-ideally saturating capabilities of the iron, and so on, which prevent constriction cross-section variation from representing a precise measure of torque, may all be overcome. Small air gaps are favoured in the machine of the invention, so that the iron will be driven into saturation at the constriction.

Two-phase reluctance motors of the present kind, in which the number of rotor poles is one-half of the number of stator poles, may be provided in four stator pole/two rotor pole configurations, as well as

8/4 and 12/6 configurations. In all embodiments, the machines may make use of economic power controllers using only two fast semiconductor switches. A multiplicity of circuit topologies are possible. The essential requirement is provision for connection of each phase winding to a power source to energise it, and then subsequently its connection to a power sink to de-energise it. A dissipating power sink may be used, or the power sink may act to route the residual magnetically-stored energy back to the power source. Pairs of tightly coupled windings may be used on each pole, one for energisation and the other for de-energisation.

The present invention in combination with a controller of the foregoing kind is especially suited to applications where the power source is represented by a low voltage battery. These samples of favoured areas of use include auxiliary drives in military and other heavy vehicles, electric vehicles, materials handling equipment, automotive auxiliaries, such as unidirectional radiator fan drives, and golf carts and lawnmowers.

In addition to the deviations from the idealised profile shown in the exemplary embodiments of the invention depicted in the drawings, in practical constructions, the base or transition region of the wedge or crescent-shaped profile may also be rounded off where it merges with the central region of the rotor, to smooth the torque angle characteristic of the machine and eliminate torque peaks. This smoothly-shaped rotor pole construction, which is suitably homogeneous in the wedge region extending from the pole tip, also eases problems of analysis of magnetic flow paths as compared with constructions in which iron depletion is used. The invention thus provides an arrangement which is both simpler to manufacture than previous machines of this kind and also yields an especially elegant magnetic structure and one readily amenable to analysis.