GB2127232A - Magnetic motors - Google Patents

Abstract

An electric motor comprises a stator 10 having two acurate permanent magnets 12 and 13 arranged in an annular configuration but with gaps 14 and 15 between the ends thereof. The magnets are magnetised with one pole on the arcuate outer surface and with the other pole on the arcuate inner surface. Pole pieces 16 and 17 are located in the gaps 14 and 15 respectively, each pole piece having a field coil 20 and 21 respectively associated therewith. A rotor 11 is mounted concentrically with the stator 10 and carries a pair of permanent magnets each disposed with one radial face magnetized with one polarity. The rotor shaft carries a switching arrangement for electric current from a source 30 to the coils 20 and 21. On rotation of the rotor, each time one of its bar magnets approaches a position of suitable equilibrium adjacent a gap between the stator magnets 12 and 13, the current is supplied to the coils 20 and 21 so as magnetically to plug both gaps and hence render uniform the magnetic field in the region of the gap. After the rotor magnets have passed the gaps, the current supplied is discontinued and the rotor may continue to turn towards the next gap. <IMAGE>

Description

SPECIFICATION Magnetic motors This invention relates to a motor, operating on magnetic attraction and repulsion principles to generate useful output work.

Rotary magnetic motors operate on the principle that like magnetic poles repel one another and opposite poles attract. Atypical motorthus has a fixed element (the stator) and a movable element (the rotor) and by causing a magnetic field to rotate around one ofthe rotor and stator and maintaining a static magnetic field in the other of the stator and rotor, the rotor may be caused to move with respect to the stator. In a conventional design of such motor, the rotating field is generated electrically -that is to say, electric current is passed through windings on the stator or rotor in such a waythatthe resultant magnetic field rotates around the respective component.A less efficient mannerfor driving such a motor is to cause the magnetic field to exist for a part only of a complete rotation of the rotor, for example by switching the supply of electricitywhich generates the field, and to rely on the momentum ofthe rotorto continue the rotation thereof during such periods as the field is absent. It will be apparent that such a design of motor inevitably has a reduced power output for a given field strength,forthe rotor is being driven by repulsion and/or attraction of magnetic poles for a fraction only ofthe total operating time.

It is a general object of this invention to provide a magnetic motorableto produce useful output shaft work by generation of a suitable stator magnetic field for driving the rotor.

According to this invention, there is provided a motor comprising at leasttwo arcuate permanent magnets arranged so as to define an annular stator but with gaps between the magnets, each permanent magnet being magnetised in the same sense with one pole on the inner peripherythereofand the other pole on the outer peripherythereof, a rotor mounted for rotation within the stator and about the axis thereof, the rotor including at least one permanent magnet disposed to sweep the inner periphery ofthe stator and having the leading radial face magnetised with one pole and the trailing radial face magnetised with the other pole, magnetic means magneticallyto plug the gaps in the stator such that the magnetic lines of force within the stator correspond substantially to those of a continuous annular magnet magnetised with its poles on its cylindrical surfaces, and actuation means to cause the magnetic means magnetically to plug a gap each time the magnet ofthe rotor approaches that gap and to stop magnetically plugging that gap after the magnet ofthe rotor has passed that gap.

It will be appreciated that there is a uniform magnetic field within a continuous annular magnet magnetised with its poles on its inner and outer cylindrical surfaces. Consequently, a rotor mounted for rotation within such an annular magnet about the axisthereof and including at least one permanent magnet disposed to sweep the inner periphery of the annular magnet may rotate freely within the stator.

Because the rotor is in a state of neutral equilibrium, the rotor may come to rest at any angular position with respect to the stator.

if nonsuch an annular magnet is divided into at least two separate parts, there is a magnetic discontinuity in the region of the two gaps between the two magnet parts and a rotor mounted as described above will always cometo restjuxtaposed to one ofthe two gaps. This rest position, where the rotor naturally will stop, is thus a stable position. Byensuringthatthe permanent magnet of the rotor has one pole on the leading radial face thereof (considering the 'normal' or intended sense of rotation) and the other pole on the opposed radial face thereof, the rotorwill come to rest at a stable position where the rotor is adjacent but displaced from one ofthe gaps, depending upon the relative senses of magnetisation of both the rotor magnet and the annular magnet parts.If the rotor is turned to a similar position but on the other side of a gap, itwill be apparentthatthis position is unstable andtherotorimmediatelywill be driven backtothe above-mentioned stable position.

The magnetic machine ofthis invention operates generally on theforegoing principle; by arranging magneticallyto plug the gaps in a discontinuous stator each time the rotor approaches a stable position, the statorwill appear magnetically to be a continous annular magnet and the rotorwill therefore pass freely through the position which otherwise would have been stable. However, eachtimethe rotor is at a position of unstable equilibrium and hence where the rotor is able to be driven by the field of the stator, the magnetic plugging ofthe gaps is stopped sothatthestatoronce moreappearsto bediscon- tinuous.

The plugging of gaps in the stator may be achieved in a variety of ways, either electrically or by using permanent magnets. One preferred manner four plugging the gaps is to provide soft-iron or other highly permeable pole pieces in each gap, each pole piece having substantially the same radii of curvature as the inner and outersurfaces of the two arcuate permanent magnets and having an angular extent substantially equal to the width ofthe gaps between the permanent magnets, each pole piece having a pole shank around which is wound a field coil. Forthis preferred arrangementthe actuation means may switch a supply of currentto each field coil as appropriate, so as to ensure that the gap is magnetically plugged at the appropriate time.To achieve this, the rotor may carry a slip ring having insulating and conducting portions suitably disposed with respect to the gaps in the stator and the position of the permanent magnet on the rotor, such that a pair of brushes wiping the slip ring will be placed into and out of electrical communication atthe appropriate times.

Onepossibleoperating modefortheabove-de- scribed arrangement isto energise the field coil of a pole piece each time the gap isto be plugged The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

magnetically, such thatthe magnetic field generated by the electric current flowing through the field coil is ofthe same strength and polarity as the field of the arcuate permanent magnet, at least when considerec from within the stator. Then, each time the magnet of the rotor approaches the above-mentioned unstable position, the supply of current to the coil may be discontinued such that the magnetic field generated by the coil collapses and the stator once more appears to be discontinous.

An alternative mode of operation for the abovedescribed preferred embodimentisto rely on the induced magnetism inthesoft-iron pole pieces.

Provided that the gap is sufficiently small and thatthe pole piece fits sufficiently closely to the arcuate permanent magnets each side of the gap, the induced magnetism in the pole piece will correspond very closelyto the magnetic field ofthe two arcuate permanent magnets, and thus the stator will appear continuous. However, each time the gap is to be unplugged -that is to say, the stator is to appear discontinuous-electriccurrent may be caused to flow through the field coil associated with the pole piece in such a sensethatthe magnetic field created by the currentflow is in exact opposition to the induced magnetism,thereby simulating an air gap in the stator.

The above-described alternative mode of operation may be modified, where the width ofthe gaps, the permeability of the pole pieces and other factors are appropriatelyselectedsuchthatthe magnetism induced in a pole piece as the rotor passes thereover is ofthe same polarity as that produced by energisation ofthe coil of that pole piece; for this modification, the coil of a pole piece would be energised as the rotor sweeps thereoverso that energy will be imparted to the rotor by virtue ofthe resultant repulsion of like poles.

An alternative manner four magnetically plugging the gaps in the stator is to employ permanent magnets which are moved into and outofthe gaps, at the appropriate times. Forsuch a case, each permanent gap-plugging magnet should be of arcuate form with the same inner and outer radii of curvature as the arcuate permanent magnets forming the main part of the stator, and the plugging magnets should moreover be polarised in the same sense as the main stator magnets, across the radial width thereof. The arcuate extent of each plugging magnet should be substantiallythe same as the arcuate width of the gap which is to be plugged, but each plugging magnet should be an easy sliding fit in the gap.

The actuating means should be arranged to cause physical movement of each plugging mag net at the appropriate time, as the magnet of the rotor approaches a gap and as the rotor leaves the vicinity of a gap. Conveniently therefore the actuating means includes a slide rod which serves to support a plugging magnet, and an electromagnetic solenoid assembly adapted to cause sliding movement of the slide rod thus to movethe plugging magnet. The actuating means may further comprise means to supply powerto the or each solenoid at the appropriate time, which means includes a switching arrange ment suitably disposed next to the rotor shaft so as to be actuated at the appropriate intervals thereby.For example, a microswitch may be employed, there being camming projections provided on the rotor shaftto operate the microswitch as required.

Because the arcuate permanent magnets of the stator and the arcuate plugging magnets are all polarised in the same sense, the movement of a plugging magnetfrom a position remotefromthe statorto a positionwherethe plugging magnetisfully aligned with the stator magnets will require considerable force to effect. This is because the plugging magnet will have to be moved th rough a position where unlike poles (on the plugging magnet and on the stator magnets respectively) are adjacent, to a position where like poles are adjacent. However, a force-reducing mechanism may be provided, adapted to be operated simultaneously with the movement of a plugging magnet.For example, each plugging magnet may be associated with a force-reducing mechanism comprising a pair of bar magnets, magnetised acrosstheirwidth and arranged end-to-end but with a gaptherebetween,thusto some extent corresponding to the arrangement of the arcuate permanent magnets ofthe stator. A secondary plugging magnetthen may be mounted for movement with the stator gap-plugging magnet (primary plugging magnet), the secondary plugging magnet being arranged to move into and out of the gap between the two bar magnets as the primary plugging magnet performs similar movement.The secondary plugging magnet must however be magnetised in the opposite sense to that of the bar magnets, such that when fully aligned therewith, the south pole ofthe secondary plugging magnet lies adjacent the north poles of the two aligned bar magnets, and visa versa. It will be thus appreciated thatthe movement of the primary plugging magnet th rough the position where unlike poles are adjacent to a position where like poles are adjacent will occursimultaneouslywith movement of the secondary plugging mag net through a position where like poles are adjacentto a position where unlike poles are adjacent.Since the movement of the primary and secondary plugging magnets is mechanically identical but magnetically in complete opposition,theforces generated in the two systems will tend to cancel each other out and thus the force required to move the primary plugging magnet to a position where it completely plugs the gaps in the stator may be relatively small.

Instead of the above-described electromagnetic solenoid arrangement for moving the primary plugging magnet (and also the secondary plugging magnets, if provided), it would be possible to arrange a mechanical linkageforthis purpose, which linkage could include for example a cam provided on the rotor shaft which cam drives a cam followerthus to operate the actuating means.

In a preferred embodiment of this invention, the rotor comprises an arm mounted about its mid-point with a pair of bar magnets provided one on each end ofthearm,the magnets being magnetised such that the magnetic pole of the radial face leading on rotation ofthe rotor for each magnet is the same. Though in the foregoing, reference has been made exclusively to the stator having two arcuate permanent magnets there thus being two gaps provided between the stator magnets, it will of course be appreciated that the stator may have several arcuate permanent magnets with a like number of gaps therebetween.

Similarly, though the rotor preferably has one pair of poles each constituted by a bar magnet appropriately mounted on the rotor shaft, it may have two or more pairs of poles, equi-spaced around the rotor. In addiction, since a d.c. energised electromagnet may magnetically be identical two a permanent magnet, the permanent magnets referred to herein may be replaced by such d.c. energised electromagnets, as appropriate. For a case where the stator has a plurality of gaps, all the gaps may be plugged and unplugged simultaneously,though in certain cases, the force requirements may be reduced by plugging onlythose gapsthata rotor magnetatanygiven instant is about to pass thereover.

Byway of example only, three specificembodi- ments ofthis invention will now be described in detail, reference being made to the accompanying drawings, in which Figure lisa diagrammatic plan view of a first embodimentofthis invention employing electromagnetic plugging ofthe stator gaps; Figure 2 is a diagrammatic sectional viewthrough the motor of Figure 1, taken on line ll-ll marked on that Figure; Figure 3 is a diagrammatic plan view of a second embodiment of this invention employing a mechanical gap plugging arrangement; Figure 4 is a diagrammatic sectional view taken on line IV-IV marked on Figure 3; and Figure 5 is a diagrammatic plan view of a third embodiment of this invention, having a four pole stator.

Referring initially to Figures 1 and 2, the first embodimentofthis invention illustrated there comprises a generallyannularstator 10 and a rotor 11 mounted for rotation within the stator 10, about the axis thereof. The stator 10 comprises two arcuate permanent magnets 12 and 13, each magnetised across its cylindrical faces, as illustrated in Figure 1.

Each statorpermanentmagnet 12 and 13 has an angular extent of slightly less than 1 80" such that there aretwogaps 14and 15 betweenthe magnets 12and 13. Each gap 14 and 15 has disposed therein in a pole piece 16and 17 respectively, each pole piece also being of arcuateform and having the same internal and external radii of curvature as the permanent magnets 12 and 13. The arcuate width of each pole piece 16 and 17 is such thatthe pole piece fits snugly in the associated gap 14 or 15.

Each pole piece 16 and 17 is made of a material having high magnetic permeability, such as dead mild steel, and has attached thereto a pole shank 18 and 19 respectively. Around the pole shanks are wound field coils 20 and 21, arranged in an electric circuit, as illustrated in Figure 1, such that the pole pieces 16 and 17 may beenergised when required.

The rotor 11 comprises a shaft 22 mounted in ball bearings 23 and having a pair of arms 24 projecting therefrom. Mounted on each arm is a permanent magnet 25, each permanent magnet being magne tised in the same sense, across its generally radial faces, as illustrated in Figure 1.

Also mounted on the rotor shaft 22 is a slip ring 26 having two conducting sectors 27 and two insulating sectors 28. A pair of brushes 29 sweep the cylindrical surface of the slip ring 26, and thus when both brushes contact a conducting sector of the slip ring, the electric circuit of Figure 1 is completed, such that electricity mayflowfrom the battery 30 through the field coils 20 and 21, thus magnetically polarising the pole pieces 16 and 17. The relative disposition of the brushes 29 and the insulating and conducting sectors of the slip ring 26 are set relative to the angular position ofthe arms 24, whereby electricity is caused to flow shortly after the rotor has moved from the position illustrated in Figure 1, in the direction indicated by arrow A.Thus, as the end ofthe magnets 25 of the rotor pass overthe pole pieces 16 and 17, the coils are energised and remain so until the magnets 25 ofthe rotor have moved to such a position in the direction of arrow A thatthe magnets 25 once more completely overlie the stator permanent magnets 12 and 13. Thus, each coil 20 and 21 is energised twice on each full turn ofthe rotor, for a duration corresponding to the time taken forthe rotor magnets 25to sweep over the pole pieces 16and 17.

In the motor of Figures 1 and 2, were the pole pieces 16and 17absent,therotorwouldcometorestin substantially the position illustrated in Figure 1,this position thus being completely stable. On the other hand, were the stator a continuous annular magnet, the rotorwould beablefreelyto rotatetherewithinand would not display any particular stable position. The gap-plugging pole pieces 16 and 17togetherwith associatedfield coils 20 and 21 serveto ensurethe simulation of a continuous annularstatorwhenever the rotor approaches such a stable position, this being acheived by causing current to flow through the field coils.The strength of the magnetic field thus provided by each pole piece should be such that as viewed from within the stator, the stator appears as a substantially continuous annular permanent magnet.

When the rotor has reached a positionwherethe current is turned off, just as the rotor magnets 25 have passed over a gap, the position of the rotor is unstable inthatthe rotor is subjected totorque-generating forces by the opposition ofthe poles ofthe rotor and the stator. This torque causes the rotorto rotate round to a position (as illustrated in Figure 1), which would be stable were the coils not energised, but as the rotor approaches that position the pole pieces are ener gised again such that the rotor may pass over the gaps. In this way, the rotor will rotate continuously generating useful output shaft work.

Verybriefly,theelectro-magneticoperation of the above-described embodiment will now be explained.

Considering initially a continuous annular stator, it will be appreciated that a certain energy input to the rotor must be provided if rotation ofthe rotor is to be maintained, in orderto overcome frictional drag of the bearings, windage and soon: let that energy input be m watts.In the case of a discontinuous stator, as described above, a particular magnetic field strength must be generated bythe coils wound on the pole piece shanks in orderto make the stator appear as a continuous annularstator: this field strength can be generated by p Ampere-turns-that is a current i being passed through a coil of n turns such that n-i=p. Now, the electrical power inputto such a coil is v-i watts, where is the impressed voltage across the coil, and sothepowerconsumed maybeexpressedasw = p.v/n. As thefield requires field strength p is constant, and where the voltage is held constant, the power consumed is solely a function of the coil design: the greaterthe number of turns, the smallerthe required power for a given field strength. By making n sufficiently large, one may be able to achieve the condition where m w, thus 10 giving nett useful output workfrom the motor, after taking into account the electrical energy input.

Referring now to Figures 3 and 4, there is illustrated a second embodiment of this invention which is similarto that illustrated with reference to Figures 1 and 2, and like parts are given like reference characters: such parts will not be described in detail again here.

The embodiment of Figures 3 and 4 differs from that of Figures 1 and 2 inthatthe gaps 14 and 15 in the stator are plugged mechanically, ratherthan electrically. Such gap-plugging is achieved by providing a pair of primary plugging permanent magnets 31 and 32, each of generally similar shape to the pole pieces 16and 17 of the embodiment of Figure 1,but magnetised across their arcuate faces, in the same sense as the stator permanent magnets 12 and 13.

Each primary plugging magnet 31 and 32 is carried by a support 33, slidably mounted for movement in directions of arrow B (Figure 4.). A spring 34 serves to bias each plugging magnet 31 and 32 radiaily outwardlyto a position where the magnets 31 and 32 are clearofthestatormagnets 12 and 13.

Each support 33 also carries a secondary plugging magnet35, disposed for sliding movement between a pair of linear bar magnets 36 and 37. Unlike the primary plugging magnets 31 and 32 the secondary plugging magnets 35 are polarised in the opposite sense to that of the associated bar magnets 36 and 37.

Provided around a part ofthe support 33 is a solenoid coil 38, a soft-iron armature (not shown) being mounted on the support 33 for attraction by the solenoid coil 38, when the coil is energised from a battery 39. Control of the flow of electric current through the coils 38 is effected by a microswitch 40, there being a pair of operating projections 41 provided at appropriate positions on the rotor shaft 22.

The operation ofthe motor of Figures 3 and 4 is closely similarto that of Figures 1 and 2, exceptthat the gaps inthe statorare plugged by moving the primary gap-plugging permanent magnets 31 and 32 into the illustrated positions, each time a rotor magnet 25 approaches a gap in the stator, when turning in the direction of arrow B. This movement is performed by energising the solenoid coils 38, on closing of the microswitch 40 by a projection 41. As soon as a rotor magnet 25 has passed over a gap, the microswitch 40 is turned off, so the coils 38 are de-energised and the magnets 31 and 32 are moved radially away from the stator 10, bythe action of springs 34.

Itwill be appreciated thatthe movement ofthe primary plugging magnets 31 and 32 to their illustrated position requires a considerable force, because each magnet is moved through a position where unlike poles are adjacent, to a position where like poles are adjacent. However, the secondary plugging magnets 35 together with the associated bar magnets 36 and 37 are provided significantly to reduce that force. This is achieved by arranging the polarity ofthe secondary plugging magnets 35with respect to the associated bar magnets 36 and 37to be in the opposite sense to that of the primary plugging magnets 31 and 32 with respect two the stator permanent magnets 12 and 13.Thus, as a primary plugging magnet is moved through a position where unlike poles are adjacent, the secondary plugging magnet is moved through a position where like poles are adjacent: the forces on a support 33 generated by the two plugging magnets 31 (or32 and 35 are thus in precise opposition and largely cancel one another.

In order properlyto simulate the electrical plugging action described in detail above with respect to Figures 1 and 2, it is of course importantthatthe field strength ofthe primary plugging magnets 31 and 32 is such that the magnetic field ofthe stator, when having the primary plugging magnets 31 and 32 in the illustrated position, appears magnetically to be a continuous annular permanent magnet Provided that this is so, the operation of the motor described above will be substantially identical to that ofthe motor of Figures 1 and 2.

Referring now to Figure 5, there is shown diagrammatically a further embodiment ofthis invention, somewhat similar to that illustrated in Figures 1 and 2, but intended for energisation with an alternating current supply.

The motor of Figure comprises a stator 50 with four arcuate permanent magnets 51 arranged in an annularformationwith gaps52therebetween. In each gap 52 there is a pole piece 53, the pole pieces being linked in pairs as shown by soft iron cores, around which are provided energising coils 54. As in the previous arrangements, the arcuate magnets are magnetised with the inside arcuate faces with one magnetic pole and the outside arcuate faces, with the other pole.

A rotor 55 is mounted for rotation concentrically within the stator 50, the rotor comprising two permanent magnets 56, as in the first-described embodiment. However, no switch-gear is associated with the rotor: instead the coils 54 are connected to an a.c.

supply when the motor isto run and the rotor then turns at a speed synch ronised to the a.c. waveform.

The operation ofthisthird embodiment of motor is essentiallythesame as that described above, except that the energisation of the pole-pieces takes place automatically in sympathy with the a.c. supply, the rotor then turning at a synchronous speed. This therefore obviates the need for switch-gear or mechanical pole moving arrangements. Moreover, the relatively long physical length ofthe coils permits the construction thereof with very high inductances, so achieving high ampere-turns characteristics, which coils thus have low energy requirements for a given magnetic flux density necessaryto obtain the gap plugging in the described manner. Thus, each time a rotor magnet approaches a gap, the current su pply to the coils should be such that in the region ofthatgap there is a uniform flux pattern, but as soon as the rotor magnet has passed that gap, the magnetic flux of the pole piece dies away so permitting drive to be imparted to the rotor. Additional drive is impartedto the rotor as a gap is plugged by virtue of the opposed end ofthe energised coil being ofthe opposite polarity to that where a gap is being plugged, so repelling (or attracting, as appropriate) the other permanent magnet ofthe rotor.

Claims (15)

1. A motor comprising at least two arcuate permanent magnets arranged so asto define an annular stator but with gaps between the magnets, each permanent magnet being magnetised in the same sensewith one pole on the inner peripherythereof and the other pole on the outer periphery thereof, a rotor mounted for rotation within thestatorand aboutthe axisthereof,the rotor including atleastoneperma- nent magnet disposed to sweep the inner periphery of the stator and having the leading radial face magnetised with one pole and the trailing radial face magnetised with the other pole, magnetic means magneticallyto plug the gaps in the stator such that the magnetic lines of force within the statorcorres- pond substantially to those of a continuous annular magnet magnetised with its poles on its cylindrical surfaces, and actuation means to cause the magnetic meansto plug a gap each time the magnet ofthe rotor approaches that gap and to stop magnetically plugging that gap after the magnet ofthe rotor has passed that gap.

2. A motor according to claim 1, wherein the magnetic means includes a soft-iron or other highly permeable pole piece in each gap, each pole piece having substantially the same radii or ofcurvature as the inner and the outer surfaces of the two arcuate permanent magnets and having an angular extent substantially equal to the width ofthe gaps between the permanent magnets, and each pole piece having a pole shank around which is wound a field coil.

3. A motoraccording to claim 2, wherein the actuation means switches a supply of current to each field coil as appropriate, to ensure that the gap is magnetically plugged attheappropriatetime.

4. A motor according to claim 3, wherein the rotor carries a slip ring having insulating and conducting portions suitably disposed with respectto the gaps in the stator and the position ofthepemanentmagneton the rotor, and there is a pair of brushes wiping the slip ring which brushes come into and out of electrical communication atthe appropriatetimes.

5. A motor according to claim 3 or claim 4, wherein the brushes and slip ring serve to switch the supply of current so asto energise the field coil of a pole piece each time the associated gap is to be plugged magnetically, whereby the magnetic field generated by the electric current flowing through the field coil is ofthe same strength and polarity as the field of the arcuate permanent magnet, as considered from within the stator.

6. A motor according to claim 3 or claim 4, wherein the brushes and slip ring serve to switch the supply of currentso asto energisethefield coil of a pole piece wheneverthe gap is not be be plugged magnetically, whereby the magnetic field generated by the electric currentflowing through the field coil is of the same strength but ofthe opposite polarity as the field of the arcuate permanent magnet, the gap being plugged magnetically by induced magnetism in the pole piece.

7. A motor according to claim 1, wherein permanent magnets are provided magnetically to plug the gaps in the stator, which permanent magnets are moved into and out ofthe gaps, at the appropriate times.

8. A motor according to claim 1, wherein each permanent gap-plugging magnet is of arcuate form with the same inner and outer radii of curvature as the arcuate permanent magnets forming the main part of the stator, the plugging magnets being polarised in the same sense as the main stator magnets.

9. A motor according to claim 8, wherein the actuating means includes a slide rod which serves to support a gap-plugging magnet, and an electromagnetic solenoid assembly adapted to cause sliding movement of the slide rod thus move the plugging magnet.

10. A motor according to claim 9, wherein the actuating meansfurthercomprises means to supply powertothe or each solenoid atthe appropriatetime, which means includes a switching arrangement suitably disposed next to the rotor shaft so as to be actuated thereby at appropriate intervals.

11. A motor according to claim 9 or claim 10, wherein each plugging magnet is associated with a force-reducing mechanism comprising a pair of bar magnets, magnetised across their width and arranged end-to-end butwith a gap therebetween, and a secondary plugging magnet mounted for movement with the stator gap-plugging magnet (primary plugging magnet), the secondary plugging magnet being arranged to move into and out ofthe gap between the two bar magnets asthe primary plugging magnet performs similar movements and the secondary plugging magnet being magnetised in the opposite sense to that of the bar magnets.

12. A motor according to claim 8, wherein a mechanical linkage is provided to move the primary plugging magnets, which linkage includes a cam provided on the rotor shaftand which cam drives a camfollowerthusto operate the actuating means.

13. A motor according to any of the preceding claims, wherein the rotor comprises an arm mounted about its mid-pointwith a pair of bar magnets provided one on each end of the arm, the magnets being magnetised such that the magnetic pole of the radial face leading on rotation ofthe rotorforeach magnet is the same.

14. A motor according to any ofthe preceding claims wherein the stator has four separate arcuate permanent magnets disposed in an annular configuration with gaps between adjacent ends ofthe magnets.

15. A motor substantially as hereinbefore de scribedwith reference to and as illustrated in Figures 1 and 2 or in Figures 3 and 40r in Figures 5 ofthe accompanying drawings.