WO1987004574A1

WO1987004574A1 - Electrical machine

Info

Publication number: WO1987004574A1
Application number: PCT/GB1987/000034
Authority: WO
Inventors: Edward Spooner
Original assignee: The University Of Manchester Institute Of Science
Priority date: 1986-01-21
Filing date: 1987-01-20
Publication date: 1987-07-30
Also published as: AU6896087A; GB2205452B; GB2205452A; GB8817246D0

Abstract

An electrical machine comprising a member (3) of magnetic material, and a stator (1) supporting windings which when energised generate magnetic fields in a gap defined between the member (3) and the stator (1). The stator (1) supports at least two windings each of which is energisable to generate a different number of magnetic pole pairs from the other winding or windings. At least one winding comprises a plurality of subwindings arranged to be energised by respective phases of a polyphase current so as to generate a moving magnetic field in the gap. A resultant magnetic field appears in the gap which is a moving magnetic field that is the sum of the magnetic fields resulting from energisation of each of the windings. The resultant distribution of attractive force between the two members (1, 3) may have a long wavelength compared to the winding pole pitches, which is of value in harmonic drive type devices, or may rotate at twice the angular frequency of a supply current, which enables high speed operation.

Description

ELECTRICAL MACHINE

The present invention relates to an electrical machine, and in particular to an electrical machine which can be used to drive a load or can be used as a generator driven by an appropriate engine.

Conventional electrical machines develop a useful output by means of an electro-magnetic interaction between relatively moving components. The nature of conventional electric motors and generators is such that it is a relatively easy matter to produce devices operating at high speeds, but low speeds are difficult to achieve. Accordingly where a high torque and low speed output is required it is conventional practice to connect a gear box to a rotor of the electrical device so as to obtain the necessary output from a device operating at a relatively high speed.

Gear boxes add significant cost and inertia to the complete systems of which they form part. In addition, where gear boxes are used which comprise mating sets of gears, backlash is a significant problem. To avoid the backlash problem it has been proposed to use harmonic drives such as are described in the article by John H. Carlson in the Journal "Machine Design" dated January 10, 1985, pages 102 to 106. Harmonic drives comprise three basic components, that is a first member which is flexible, a second member which is substantially rigid, and a device for mechanically deforming the first member so as to contact the second member. Generally the first and second members are cylindrical with the first member being mounted within the second. The first member is distorted by an elliptical rotor, often referred to as a wave generator, which is rotated by an electrical motor. The first member thus engages the second member at two regions spaced apart by 180°. The first and second members carry series of teeth with the first member having for example two less teeth than the second. Thus for every 180° rotation of the wave generator the rotation of the first member lags by one tooth relative to the second member.

Harmonic drives have proved attractive alternatives to conventional gear boxes in various applications where backlash is a problem but they do not overcome all the problems of inertia and extra cost associated with conventional electrical device and gear box combinations.

U.S. Patent Specification No. US-A-3169201 discloses an electromagnetic harmonic drive which uses a magnetic power source rather than a mechanical elliptical rotor to distort the first member. This overcomes the above-mentioned inertia problem but the magnetic power source is complex and cumbersome, requiring an array of sixteen pairs of circumferentially arranged solenoids which are independently energisable. The independently energisable solenoids are however capable of generating localised magnetic fields of relatively high flux density on opposite sides of the distortable first member to cause the member to distort sufficiently to enable a large force to be generated between the first and second members.

U.S. Patent Specification No. US-A-3548227 discloses an electromagnetic linear drive which relies upon similar principles to the known harmonic drives but achieves deformation of a tubular first member by generating a rotating magnetic field. The field is generated by a stator winding employing six or more salient poles. The stator winding is not described in detail, but appears to be such that the tubular member will be subjected to a deforming force having a wavelength equal to the pole pitch, that is 5 60° with six poles. For a large force to be generated between the deformable tubular first member and the surface it is adapted to engage it is necessary to generate a magnetic field of high flux density. It is also necessary however for the

_]__Q tubular member to be thin and for the spacing between adjacent regions of contact between the tubular member and the surface to be relatively large if the resistance to deformation of the tubular member is not to be too great. These requirements are _l~ difficult to achieve with a simple winding in which the pattern of the deforming force has a wavelength equal to the pole pitch of the magnetic field.

A further characteristic of conventional AC electrical machines is that the maximum rotor speed

20 is tied to the supply frequency. Thus it is generally accepted that rotor speeds in excess of 3000rpm cannot be achieved with a 50Hz_s supply. This limitation is a function of the maximum speed of rotation of a magnetic field in a conventional AC

25 electrical machine. It would be highly desirable to increase this speed in certain applications, e.g. generators driven by turbines, as turbines can be designed for greater efficiency if they can operate at higher speeds.

₃0 It is an object of the present invention to provide an electrical machine which has a winding structure that enables the above problems to be obviated or mitigated.

According to the present invention, there is 5 provided an electrical machine comprising a member of magnetic material, and a stator supporting windings which when energised generate magnetic fields in a gap defined between the member and the stator, wherein the stator supports at least two windings each of which is energisable to generate a different number of magnetic pole pairs from the other winding or windings, and wherein at least one winding comprises a plurality of subwindings arranged to be energised by respective phases of a polyphase current so as to generate a moving magnetic field in the gap, the resultant magnetic field in the gap being a moving magnetic field which is the sum of the magnetic fields resulting from energisation of each of the windings. in the case of electrical machines of the harmonic drive type mentioned above, the present invention makes it possible for the pole pitch of each winding to be relatively short, which enables a relatively thin deformable member to transport the necessary flux between adjacent poles,- and yet the resultant pattern of force between the two members has a component with ^'a relatively long wavelength to which the deformable member offers little resistance.

In the case of electrical machines where a higher speed than that normally associated with conventional AC machines is desirable, the present invention makes it possible to generate a resultant pattern of force which contains a component rotating at twice the angular frequency of the AC supply and therefore to double the conventionally accepted maximum speed of such machines.

The term "subwinding" is used herein to identify a set of coils forming part of one winding which set is connected for example between one terminal of a polyphase system and another subwinding of the same winding or between one terminal of a polyphase system and another winding. Thus a conventional three phase winding of the type normally used in induction motors comprises three such subwindings. A conventional winding of this type is suitable for use in embodiments of the present invention.

Preferably two windings are provided each of which comprises a plurality of subwindings arranged such that if energised by respective phases of a polyphase current a moving magnetic field is generated in the gap.

Each phase of one winding may be connected in series with a corresponding phase of the other winding to a respective phase of a polyphase power supply, the phase sequence of the currents in the two windings being arranged to produce oppositely directed movement of the two field patterns. Alternatively each phase of one winding may be connected to a respective phase of a polyphase power supply, and the other winding may be connected to a DC power supply. In the latter case, the polyphase power supply may comprise a DC supply one terminal of which is connected to an array of switches and the other terminal of which is connected to the said other winding, the switches being actuable to connect each subwinding of the said one winding to the other terminal of the DC supply in series with the said other winding.

In one application of the present application, the member and/or the stator is deformable by the resultant magnetic field to bring the member and stator into contact at spaced regions of the member, the distance between points of contact along the surface of the member being different from the distance between those points of contact along the surface of the stator such that movement of the resultant magnetic field causes relative movement between the stator and the member. The member may be a deformable tubular rotor.

In another application of the invention, the member is tubular and defines a first thread having a start number equal to m positioned facing and coaxial with a second thread having a start number n defined by a movable element, m is not equal to n, m and n are equal to zero or an integral number, the first and second threads have the same thread pitch, means are provided for preventing relative rotation between the tubular member and the movable element, and the stator generates a rotating resultant force distribution which deforms the tubular member and/or the movable element such that at least one of (m-n) equally spaced circumferentially limited portions of one thread engages the other thread, rotation of the resultant force distribution causing rotation of the deformation to cause the movable element to move relative to the tubular member by a distance equal to the difference between the start pitches of the threads for each rotation of the deformation. The tubular member may comprise a deformable cylinder and the movable element may be a substantially rigid member, the tubular member being disposed between the stator and the movable element.

The term "thread pitch" is used herein to mean the axial distance between adjacent thread crests. The term "start pitch" is used herein to mean the axial distance between successive crests of the same continuous helix. The term "start number" is used herein to mean the number of separate helical grooves forming the thread. The convention is adopted that a right hand thread has a positive start member whereas a left hand thread has a negative start number. If the start number is zero, the "thread" is in the form of discrete annular circumferential grooves which are not interconnected.

In a still further application of the present invention, the member comprises a laminated rotor eccentrically mounted such that the gap width varies periodically around the machine, and two windings are provided which are energised to generate mmf waves rotating in opposite directions.

Preferably one winding has two more poles than the other, and each winding comprises coils the number of turns of which is in inverse proportion to the respective number of poles.

Each winding may have three subwindings for energisation by a three phase current, first subwindings of both windings being connected in series, and second and third subwindings of one winding being connected in series with third and second subwindings of the other winding. With such an arrangement, an electrical machine results which has an operating speed twice that of conventional machines using single windings.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:- Figs. 1 and 2 are simplified end views of components of an embodiment of the present invention in the de-energised and energised conditions respectively;

Fig. 3 is an axial section through an embodiment of the present invention of the type generally illustrated in Figs. 1 and 2;

Fig. 4 illustrates waveforms representing the magnetic fields and resulting deforming force generated in the embodiment of the invention illustrated in Fig. 3;

Fig. 5 is a simplified end sectional view of a further embodiment of the invention incorporating a passive inner stator;

Figs. 6 and 7 illustrate two further embodiments of the invention in which linear relative motion is generated;

Fig. 8 illustrates a simple circuit for obtaining a stepper operation with an embodiment of the invention of the type illustrated in Figs. 1 to 4;

Fig. 9 illustrates a stepper drive combining two arrangements of the type described with reference to Fig. 8;

Fig. 10 is a schematic illustration o^'f a rotary to linear motion converter of a type to which the present invention can be applied; Fig. 11 is a schematic illustration of a glandless actuator incorporating a motion converter of the type described with reference to Fig. 10 and an electrical machine in accordance with the invention; Fig. 12 illustrates the electrical machine of

Fig. 11 in greater detail;

Fig. 13 illustrates an electrical machine in accordance with the invention and suitable in particular for operation as a high speed generator; Fig. 14 illustrates the interconnection of windings of the machine of Fig. 13 and a three phase power supply;

Figs. 15a to 15c schematically illustrate three alternative configurations for the machine of Fig. 13 operating as a generator; and

Fig. 16 schematically illustrates an arrangement for operating the machine of Fig. 13 as a motor.

Referring to Figs. 1 and 2 of the accompanying drawings, the illustrated embodiment of the invention comprises a laminated outer stator 1 which supports stator windings (not shown) mounted in slots 2 which extend all around the inner circumference of the stator. Only four of the slots are shown to simplify the drawings. The detailed structure of the windings is described below. A laminated inner rotor 3 is mounted to rotate about axis 4 within the stator 1. The rotor 3 is thin in the radial direction. The rotor need not be laminated in applications where the frequency of alternation of the currents in the windings is low.

When the windings are de-energised the rotor 3 assumes a simple circular cross section as shown in Fig. 1 and as a result there is no contact between the rotor 3 and stator 1. When the windings are energised however the rotor 3 assumes the elliptical configuration as shown in Fig. 2 and as a result the rotor 3 and stator 1 come into contact in two regions spaced apart by 180° about the axis 4. The winding is so arranged that the radial force attracting the rotor to the stator is distributed periodically so as to achieve the two illustrated regions of contact. Alternatively the winding may be so arranged that three or more regions of contact or lobes may be established. Assuming that the elastic deformation of the rotor is sufficient to close the gap between the rotor and stator, torque applied between the rotor and stator will be reacted at the areas of contact. Teeth may be formed on the outer surface of the rotor and the inner surface of the stator to avoid relative slippage or if the surface contact pressure is sufficient the torque may be reacted by suitable engaging friction surfaces.

Changing the current in the stator windings changes the magnetic field distribution and in turn the pattern of deformation of the rotor. _^ In particular, by suitably arranging the windings, a balanced set of polyphase alternating currents fed into the winding will produce a rotating pattern of constant amplitude. The pattern of elastic deformation will rotate with the field and the positions of the contact zones will also progress around the stator in synchronism with the current.

In the course of a given number (p) of cycles of the alternating current, the field pattern and contact zones move through one revolution around the stator. Provided that no slip occurs at the contact zones, then each contact zone will travel the same linear distance around the rotor periphery as around the stator bore. Since the stator radius (r) and the rotor radius in the undeformed condition differ by the radial thickness (g) of the airgap, one revolution of the contact zones around the stator corresponds to a greater linear distance than one revolution around the rotor periphery and the relative angular position of the rotor and stator change as the field rotates. If the deformation of the rotor comprises bending with negligible extension of the mean radius (although it is recognised that some extension may be unavoidable) then for every revolution of the field, the rotor must turn by g/(r-g) revolutions in the same direction unless slip occurs. Alternatively, if teeth are formed on the mating surfaces (S on the stator and R on the rotor) then the rotor will rotate by (S-R)/R revolutions whilst the field rotates once with respect to the stator.

Referring now to Fig. 3, this shows an axial section through an embodiment of the invention of the type schematically illustrated in Figs. 1 and 2. The stator 1 comprises a stator structure 5 supporting a laminated stator core 6 and stator windings 7. The stator structure 5 also supports a tubular extension 8 in which the rotor 3 is supported on bearings 9. The rotor comprises an output shaft 10 from which a flange 11 extends to a deformable transmission tube 12 supporting a laminated rotor 13. A protective cover 14 is provided with a dust seal 15 on the shaft 10. When the windings 7 are de-energised there is no contact between the radially outer surface of the laminated rotor 13 and the radially inner surface of the stator core 6. When the windings 7 are energised however contact regions are established spaced apart around the axis 4 by 180O.

In order to develop the high force between rotor and stator necessary for substantial deformation of the rotor, it is necessary to produce a magnetic field of high flux density. However it is necessary also for the rotor to be radially thin if it is not to offer too great a resistance to deformation. The combination of high flux density in the airgap with a thin rotor restricts the choice of pole pitch to small values because of the inability of magnetic steel for example to operate at flux densities exceeding approximately 1.8 Tesla. With a simple polyphase winding the resulting pattern of rotor deformation would have a wavelength equal to the pole pitch of the magnetic field pattern. The resistance of even a very thin rotor to such short wavelength deformation is high. This problem is overcome in accordance with the present invention by providing more than one winding each with a relatively short pole pitch, the windings generating respective magnetic fields which interfere to produce a resultant magnetic field with an associated force distribution containing a component of relatively long wavelength.

Referring to Fig. 4, this shows waveforms illustrating the effect of combining two separate simple three phase windings each of short pole pitch in accordance with the present invention. The individual magnetomotive force (mmf) patterns of the two component windings are shown in Figs. 4a and 4b where it can be seen that their pole pitches differ slightly. In the example of Fig. 4, one winding has twelve poles distributed around the stator and the other has eight. The combined mmf pattern as shown in Fig. 4c has two regions where the component fields add supportively and two regions where they tend to cancel. The pattern of radial force resulting without deformation of the rotor is at each point proportional to the square of the mmf and is shown in Fig. 4d. Saturation of the stator and rotor will modify the pattern of Fig. 4c somewhat. The deformation of the rotor reduces the airgap in the regions of high force and increases it in the regions of low force. If all the coils of each phase of the winding are connected in series, the mmf pattern will remain undisturbed by rotor deformation but the differences in total flux density between the regions where the mmfs support and cancel will be reinforced. The differences in radial force between those regions will be further reinforced because force varies as the square of flux density. Fig. 4e illustrates the resulting force pattern in the presence of deformation.

By combining two simple windings of different pole number, their individual pole pitches may be kept small so that a thin rotor may be employed to transport flux between adjacent poles but the force distribution contains a long wavelength component to - 13 -

which the rotor offers little elastic resistance.

The rotor must be connected to the driven load (or driving engine if the machine is working as a generator) by a system which can transmit torque whilst permitting easy radial distortion. Fig. 3 as described above employs a length of thin-walled tube to achieve the desired characteristics.

In the above description it has been assumed that the stator is rigid and the rotor deforms elastically into contact with the stator. Alternatively the rotor could be made rigid and the stator made to deform, or both members could be deformable. With either of these alternative arrangements however deformation of the stator may endanger the stator winding insulation. It has also been assumed that the stator is external to the rotor. The alternative of an external rotor is entirely practical however and for some applications may be preferred. As a further alternative, Fig. 5 illustrates an arrangement using an inner passive stator 16, an outer wound stator 17, and a rotor tube 18. The stator 17 comprises two windings arranged to act cooperatively as described with reference to Fig. 4. The rotor tube makes mechanical contact with the inner stator 16. This arrangement allows a metal transmission tube 18 to be used directly in frictional contact with the passive stator 16. Alternatively the configuration may be inverted using an inner wound stator and an outer passive stator provided that the rotor is also inverted to avoid magnetic flux passing radially through the structural tube and inducing eddy currents. Teeth may be more conveniently used for developing torque in this arrangement because it avoids the contact between - 14 -

magnetic laminated components.

The above described embodiments of the invention are rotary machines. Linear embodiments of the invention are also possible however if the flexible member equivalent to the rotor is generally planar but is subjected to a wave-like deformation. Such an arrangement is shown in Fig. 6 and comprises a fixed stator 19 with a movable member comprising a framework 20 and a deformable member 21. The stator 19 incorporates slots 22 only two of which are shown and which receive suitable windings (not shown) . The windings act cooperatively as described with reference to Fig. 4.

The windings are energised so as to cause the central portion of the flexible member 21 to adopt a wave-like configuration. This results in an area of contact 23. By adjustment to the current supplied to the windings supported by the stator 19 ^'the area of contact can be moved along the stator and this will result in a relative movement between the stator 19 and the framework 20 equal to the difference between the length of the upper surface of the stator 19 and the length of the flexible member 21 with which it comes into contact. in the arrangement of Fig. 6 it is assumed that the framework 20 is supported on appropriate low friction bearings so as to maintain small gaps between the undeformed ends of the flexible member 21 and the stator 19. Fig. 7 illustrates a further embodiment of the invention similar to that of Fig. 6 but incorporating a double-sided stator 19 so as to provide extra areas of contact 23.

In large-scale versions of the machine in any of the configurations described above it will be possible to produce deformations of large amplitude. In such cases, it is relatively easy to cut small teeth in the mating surfaces and the maximum torque is then not restricted to the effects of friction.

In all the configurations discussed the stators carrying windings each carry two separate sets of coils arranged for different pole numbers. Several options exist for connecting these windings together and to a power supply:

10 1) Each winding may be a standard three-phase winding arranged to provide opposite direction of movement of field with each phase of the first winding connected in series with the corresponding phase of the second winding and then to the power

_L5 supply. The windings thus carry identical current and develop a well defined magnetic field distribution. The angular speed of rotation of the deformation wave is then 2w/(P2^~P_]_) where w is the angular frequency of the electrical supply and P_]_ and

20 ?2 ^are -■-- numbers of pole pairs of the two windings.

2) One winding can carry a three-phase set of alternating currents, with the second winding carrying direct current. Such an arrangement is described below with reference to Fig. 8. The speed

25 of rotation of the deformation wave is then w/(P2~Pι) •

3) Either winding configuration (1) or (2) above may be employed with a variable frequency supply to give variable speed operation.

4) To obtain a stepping operation, the winding may 0 be supplied by a set of switches to progressively change the currents therethrough. Two, three or any larger number of phases may be used. For example, as shown in Fig. 8, if three phase windings 24 are used and the first is supplied with direct current and the ₃₅ second with switched current from switches Si to S6, then each change of switch state will move the deformation wave by 60°/(P2~Pι). If the machine uses teeth meshing between stator and rotor, S teeth on the stator and R teeth on the rotor, then the output motion will be 60(S-R)/S(P2~Pι) • The machine may thus be employed as a stepper motor of extremely fine resolution.

To achieve a stepper drive of extreme resolution and rapid rotation two compliant-iron steppers of the type described with reference to Fig. 8 may be used in such a way that the motion of the output shaft is the sum of the motion of the two steppers, e.g. as shown in Fig. 9. Referring- to Fig. 9, an output shaft 25 supports brushes and slip rings 26 to feed current to a first machine comprising stator 27, fixed to the output shaft 25, windings 28 and rotor 29 mounted on a transmission tube 30.- -• The tube 30 is connected to a rigid flange 31 mounted on bearings 32. The tube 30 also supports the. rotor 33 of a second machine comprising stator 34 and windings 35, the stator 34 being fixed to a support surface 36. Thus the second machine rotates the flange 31 relative to the surface 36, and the first machine rotates the "stator" 27 relative to the flange 31. By arranging that the number of teeth of one machine differs slightly from the number of teeth of the other the machine moves by slightly different amounts for one switch state change.

For very accurate positioning it is possible by choosing the appropriate number of steps delivered by each stepper to position the output shaft with a resolution equal to the difference between the angular resolution of the two individual steppers. For rapid movement of the shaft between stations the two machines may be stepped with moderate frequency in the appropriate phase sequence so that their motions add constructively.

By way of example, if each machine gives elliptical deformation, and the second machine has a diameter of 300 mm at the stator bore, carries 900 stator teeth, and 898 rotor teeth, its angular resolution is 60°.2/900.2= .0 arc min ^~~ 240 sec. If the first machine has a stator diameter of 299.3 mm, carries 898 stator teeth, and 896 rotor teeth, its resolution is 240.5345 arc sec. Thus the combination can position the output shaft with a resolution of 0.53 arc. sec.

If each motor is driven with 50Hz current, then their speeds are 6.666 rpm and 6.682 rp . The output shaft may thus rotate at 13.35 rpm which is equivalent to sweeping through 544000 resolution elements per second. With conventional stepper motors a figure of 1000 is difficult to achieve. The freedom from backlash ensures that this system would need no additional equipment to produce a full positioning system.

In a typical application for a system such as an astronomical^' telescope drive, it is necessary at present to incorporate a set of absolute position encoders on a set of drive shafts in a highly complex arrangement of gears and motors in order that adequate angular resolution can be achieved in conjunction with reasonable sweeping speed and compensation for backlash.

Referring to Fig. 10, the illustrated arrangement comprises a shaft 37 which it is desired to move in the direction of arrows 38. A flexible nut 39 which cannot be moved in the direction of arrows 38 is arranged around the shaft 37. The shaft 37 and flexible nut 39 are supported in a suitable frame (not illustrated) such that no relative rotation between the two components is possible.

The outer surface of the shaft 37 supports a cylindrical thread 40 and the inner surface of the flexible nut 39 supports a thread 41. The thread pitches of the threads 40 and 41 are the same but the internal diameter of the thread 41 is greater than the external diameter of the thread 40 such that when the nut 39 is in its free condition- there is not contact between the two threads. When it is desired to move the shaft 37 parallel to the arrows 38, the nut 39 is deformed such that a circumferentially limited portion of the thread 41 is pushed radially inwards into engagement with a facing portion of the thread 40. The deformation of the nut 39 is then rotated about the shaft axis and as a result the shaft 37 moves relative to the nut 39 by a distance equal to the difference between the start pitches of the two threads each time the deformation rotates through 360°. So long as a deformation of the nut 39 is maintained the shaft 37 and nut 39 remain locked together axially. Reversal of the direction of rotation of the deformation causes a reversal in the direction of movement of the shaft relative to the stationary nut 39. If the thread 40 has a start number equal to m, and the thread has a start number equal to n, then providing m is not equal to n rotation of a region of contact between the two threads will cause relative axial movement between the shaft and the nut. Up to (m - n) equally spaced regions of contact between the threads can be provided. For example, if m = 1 and n = 0, the nut 39 can be deformed so as to contact only one portion of the shaft. If however the thread 40 is a multi-start thread, for example if m = 2 and n = 0, then the nut 39 can be deformed so as to engage the thread 40 at two positions equally spaced apart about the thread axis. The greater the length of thread in engagement the greater is the force which can be transmitted to the shaft 37 but in turn a greater deformation force is required.

Fig. 11 illustrates a development of the basic structure illustrated in Fig. 10. In the arrangement of Fig. 11 a machine part 42 is equivalent to the shaft 37 of Fig. 10. A flexible nut 43 mounted on a deformable cylinder 44 is equivalent to the flexible nut 39 of Fig. 10. The machine part 42 rests upon a flange 45 and the deformable cylinder 44 is welded to pipe sections 46. The machine part 42 may be moved relative to the flange so as to permit fluid to flow between the pipe sections 46 or alternatively to seal the pipe. The machine part 42 is connected to the pipe structure by a flexible bellows 47 which enables the machine part 42 to move parallel to the axis of the pipes 46 but prevents significant rotation of the part 42 about the pipe axis. A winding structure for creating a rotating deformation of the cylinder 44 is schematically illustrated as component 48.

As in the case of the embodiment of Fig. 10, when a portion or portions of the cylinder 44 deform radially inwards the thread of the nut 43 is axially locked in engagement with the part 42. Rotation of the deformation about the pipe axis causes the part 42 to move parallel to the axis and thus the position of the part 42 relative to the flange 45 can be controlled. It will be seen that the mechanism which enables the position of the part 42 to be controlled is glandless, this being a very valuable feature in certain applications such as those found in a processing plant for hazardous fluids. Referring now to Fig. 12, reference numerals taken from Fig. 10 are used for equivalent components. In the arrangement of Fig. 12 the magnetic stator 48 is shown as defining an array of slots 49 only two of which are illustrated. Two three phase windings (not shown) are carried by the slots 49 and these windings are energised to cause the required deformation of the nut 39. The windings are arranged as described with reference to Fig. 4 to have a relatively short pole pitch but to produce a relatively long wavelength in the resultant force pattern.

Referring to Figs. 13 to 16, further embodiments of the invention are described which enable high speed operation of electrical machines. In the described embodiments, torque is developed by the same action as in a reluctance- motor, but unlike the reluctance motor, relative motion occurs between the rotor and the magnetic fields established by the stator. The shaft speed is tied to the supply frequency, however, whereas 3000rpm is the maximum speed available for an AC machine of conventional type operating with a 50Hz supply, the described embodiments of the invention may operate at βOOOrpm with a 50Hz supply. As in the case of the winding structure described with reference to Fig. 4, the machine illustrated in Fig. 13 comprises a stator 50 which carries two distinct three phase AC windings 51, 52 in slots 53, the windings being of conventional type but of differing pole number. Each is arranged to produce a rotating pattern of magnetic field with angular velocity given by the electrical supply angular frequency, w_s, divided by the number of pole pairs, p. The directions of rotation of the fields are arranged to be opposite and the amplitudes are preferably arranged to be equal. The mmf pattern produced by • each winding has a sinusoidally distributed component, F, rotating at its synchronous speed and given by:

^Fl = ^Fmax sin(p₁θ- Wgt+øx⁾

- ² = ^Fmax sin⁽p₂θ+ w_st+^₂ ⁾ φ is an arbitrary phase angle

The usual design methods may be employed to reduce the harmonic components to an acceptable level.

A rotor 54 is arranged within the stator so that the airgap between the rotor and the stator varies with angular position. The ratio between radial flux density and the mmf between stator and rotor thus varies also. This quantity is referred to as airgap permeance, P_g. The airgap permeance distribution will also contain, in addition to a constant component, a sinusoidally distributed component, P_q, - _t which in general will have p_g maxima and minima around the machine and which rotates with the physical rotation of the rotor at angular velocity W_r ^pg' 1 = ^pg'max^C0S(Pg^θ-Pg ^r+^r^

The shape of the rotor can be chosen to ensure that the harmonic components of the permeance distribution are acceptably low.

The principal magnetic flux density distribution, B, is found by multiplying the total mmf by the permeance to give: B = (F_x+F₂) P_g The energy, E, stored by virtue of the magnetic field in the airgap may be shown to be equal to the integral over the surface of the rotor of the quantity:

¹/ ⁽Fl+F2⁾²Pg which gives the component of interest as:

If we arrange for the number p_g to be 1 by suitably shaping the rotor and for pi to differ by 1 from p₂ then the integral is finite. If also the rotor angular velocity is 2w_s then the stored energy is also constant and is equal to:

E = τtRLP_g,_maxF_maχ2

The rotor will be subject to a torque tending to pull it toward the position which maximises the magnetic stored energy, but this position is constantly rotating at 2w_s. This is the basis of reluctance motor action. The torque is given by:

T = E/d g = Tf^RLPg,max^Fmax^{2 s}^g

Referring again to Fig. 13, it can be seen that the rotor 54 is cylindrical but is mounted eccentrically on a shaft 55 so that the airgap 56 varies periodically around the machine. The rotor is laminated because, unlike in a conventional reluctance machine, there exists relative motion between the rotor and the fields produced by the stator. The large hole 57 in the rotor is needed to ensure that the rotor is mechanically balanced about the shaft to avoid the vibration which would be caused if the eccentric shaft was left unbalanced and rotated at high speed. Although in the illustrated embodiment the rotor comprises an eccentrically mounted cylindrical member, the rotor may be of any shape which produces a suitable variation of permeance whilst remaining mechanically balanced.

In the illustrated embodiment, the two windings have four and six poles respectively, but any combination with a difference of two in pole number would be equally effective. The electrical connections to the windings is as shown in Fig. 14, whereby the phase sequence of the currents in the two windings give rotation of the mmf waves in opposite directions as desired. The windings each receive the same current but they are formed using coils with numbers of turns in inverse proportion to the respective pole number so that the amplitudes of the two mmf waves are the same. Some minor deviation from this simple relationship may be desirable to adjust for the small difference in winding factor.

The described machine is primarily of interest as a generator driven by a turbine. The higher the speed the greater is the achievable turbine efficiency. There is no starting torque so use of the machine as a motor is more difficult.

For operation as a high speed generator the arrangements of Fig. 15a-15c may be employed. In Fig. 15, reference numeral 58 indicates a 6000rpm turbine, reference numeral 59 indicates a generator of the type described in Fig. 13, reference numeral 60 indicates a load, reference numeral 61 indicates a three phase 50Hz supply, reference numeral 62 indicates power factor correction capacitors, and reference numeral 63 indicates an overexcited synchronous machine load.

In common with other forms of reluctance machine, the described embodiment of the invention absorbs reactive power from the -electrical supply regardless of whether it is working as a motor or as a generator. For operation as a motor, some form of starting equipment must be provided. The arrangement of Fig. 16 is one of many possibilities. A small induction motor 64 is used to accelerate the reluctance machine 65 up to its operating speed through a speed increasing gearbox 66. A conventional reluctance motor operating at say 3000rpm from a 50Hz supply may use a set of conductors mounted on the rotor to provide torque by induction motor action through the range from rest to full speed. However, the new machine cannot use this attractive method because induction motor action is available only up to half the full speed of this machine. Reference has been made above to conventional winding structures of the type familiar from induction motors. This should be sufficient to enable a skilled person to put the invention into effect by simply providing two windings rather than the conventional single winding. One such arrangement is however described below in some detail by way of further illustration.

The stator as shown in Fig. 13 can have the following basic structure. For example 36 slots can be provided in the stator to receive coils forming two three phase windings one defining four poles and the other defining six poles. Each winding comprises 36 coils, with one side of each coil in one slot and the other side of each coil in a different slot. Thus each slot receives four coils sides, two from one winding and two from the other. In the case of the four pole winding, each coil spans 90°, and thus, numbering the slots from 1 to 36, and the coils of the four pole winding from la to 36a, slot 1 contains the left hand side of coil la, slot 2 the left hand side of coil 2a, and so on. Slot 9 contains the left hand side of coil 9a and the right hand side of coil la, slot 10 contains the left hand side of coil 10a and the right hand side of coil 2a, and so on. Coil numbers la, 2a, 3a, 10a, 11a, 12a, 19a, 20a, 21a, 28a, 29a and 30a are connected in series and represent one subwinding which is driven by a respective phase of the supply current. In the case of the six pole winding, each coil spans 60°, and thus, numbering the coils of the six pole winding from lb to 36b, slot 1 contains the left hand side of coil lb, slot 2 the left hand side of coil 2b, and so on. Slot 6 contains the left hand side of coil 6b and the right hand side of coil lb, and so on. Coil numbers lb, 2b, 7b, 8b, 13b, 14b, 19b, 20b, 25b, 26b, 31b and 32b are connected in series and represent one subwinding which is driven by a respective phase of the supply current.

The stators of Figs. 1 to 3, 5, 9 and 12 can all have the same pattern as described above but repeated twice to form 8 and 12 pole windings in 72 slots.

Claims

1. An electrical machine comprising a member of magnetic material, and a stator supporting windings which when energised generate magnetic fields in a gap defined between the member and the stator, wherein the stator supports at least two windings each of which is energisable to generate a different number of magnetic pole pairs from the other winding or windings, and wherein at least one winding comprises a plurality of subwindings arranged to be energised by respective phases of a polyphase current so as to generate a moving magnetic field in the gap, the resultant magnetic field in the gap being a moving magnetic field which is the sum of the magnetic fields resulting from energisation of each of the windings.

2. An electrical machine according to claim 1, comprising two said windings each of which comprises ^a plurality of subwindings arranged such that if energised by respective phases of a polyphase current a moving magnetic field is generated in the gap.

3. An electrical machine according to claim 2, wherein each phase of one winding is connected in series with a corresponding phase of the -other winding to a respective phase of a polyphase power supply so that the travelling fields produced move in opposite directions.

4. An electrical machine according to claim 2, _ wherein each phase of one winding is connected to a respective phase of a polyphase power supply, and the other winding is connected to a DC power supply.

5. An electrical machine according to claim 4, wherein the polyphase power supply comprises a DC supply one terminal of which is connected to an array of switches and the other terminal of which is connected to the said other winding, the switches being actuable to connect each subwinding of the said one winding to the other terminal of- the DC supply in series with the said other winding.

6. An electrical machine according to any preceding claim, wherein the member and/or the stator is deformable by the resultant magnetic field to bring the member and stator into contact at spaced regions of the member, the distance between points of contact along the surface of the member being different from the distance between those points of contact along the surface of the stator such that movement of the resultant magnetic field causes relative movement between the stator and the member.

7. An electrical machine according to claim 6, wherein the member is a deformable tubular -roto .

8. An electrical machine according to any one of claims 1 to 5, wherein the member is ^ tubular and defines a first thread having a start number equal to m positioned facing and coaxial with a second thread having a start number n defined by a movable element, m is not equal to n, m and n are equal to zero or an integral number, the first and second threads have the same thread pitch, means are provided for preventing relative rotation between the tubular member and the movable element, and the stator generates a rotating resultant magnetic field which deforms the tubular member and/or the movable elemnt such that at least one of (m-n) equally spaced circumferentially limited portions of one thread engages the other thread, rotation of the resultant magnetic field causing rotation of the deformation to cause the movable element to move relative to the tubular member by a distance equal to the difference between the start pitches of the threads for each rotation of the deformation.

9. An electrical machine according to claim 8, wherein the tubular member comprises a deformable cylinder and the movable element is a substantially rigid member, the tubular member being disposed between the stator and the movable element.

10. An electrical machine according to any one of claims 1 to 5, wherein the member comprises a laminated rotor arranged such that the gap width varies periodically around the machine, and two windings are provided which are energised to generate mmf waves rotating in opposite directions.

11. An electrical machine according to claim 10, wherein one winding has two more poles than the other.

12. An electrical machine according to claim 3, 10 or 11, wherein each winding comprises coils the number of turns of which is in inverse proportion to the respective number of poles.