This invention relates to a self-starting brushless electric motor of the type which comprises reluctance poles (ferromag- netic salient poles) at least on one of the two relatively moving motor parts and at least one permanent magnet in the pole system.

For convenience, the following description of the invention is limited to motors for rotary movement which have a first motor part, in the following called the stator, provided with a winding, and a second part, in the following called the ro¬ tor, arranged inside the stator and rotatable in relation thereto. It will, however, be appreciated that these two parts may exchange places, that the air gap separating the stator from the rotor need not be cylindrical but may equally well be flat or conical, and that the relative movement bet¬ ween the parts of the motor need not be rotary but may equal¬ ly well be linear or a combination of rotary and linear, i.e. occur simultaneously about and along an axis of rotation.

Electric motors which are designed for unidirectional rota¬ tion of the rotor and provided with a single-strand (single- phase) winding supplied with current through electronic drive circuitry require the least possible number of controlled power electronic circuit components in the drive circuitry. Prior art motors of this category include at least one per¬ manent magnet in their pole system to ensure that when the winding is deenergized the rotor will take a position, a starting position, also known as a detent position, in which upon energization of the winding it will be subjected to a magnetic force capable of moving the rotor in a predetermined direction, the preferential direction of rotation.

In many applications, such as in hand tools etc. , motors of the kind with which the invention is concerned can have a substantial friction or load torque when the winding is cur- rentless. It is then necessary for the rotor to have well-

defined starting or detent positions to which it is urged by the permanent magnet or magnets when the winding is deener- gized so that it will be reliably moved to such a position instead of stopping in a position in which the torque applied to it upon energization of the winding will be insufficient to set it in motion in the desired direction.

However, this requirement for a high detent torque applied by the permanent magnet or magnets should not be met in such a way that the winding requires a heavy starting current in order to overcome the detent torque. By arranging permanent magnets in the manner described in WO90/02437 and W092/12567 the magnetic field the winding produces when it is energized will oppose the magnetic field of the permanent magnets applying the detent torque so that the detent torque is sub¬ stantially reduced or even eliminated when the winding is energized.

In addition to reducing or eliminating the detent torque when the detent torque is no longer needed, the arrangement descri¬ bed in these publications results in a net torque contribu¬ tion from the permanent magnets. Besides, over the major por¬ tion of the path of the flux passing through the reluctance poles, the permanent magnets will oppose and reduce that flux and thereby allow for a reduction of the cross-sectional area of the stator iron in that portion and, consequently, the weight and cost of the motor.

If the load to be driven is not very heavy, a motor having only a single group of poles and an associated single winding coil may be adequate and optimal, because permanent magnets which are very powerful in relation to their weight are now available as a result of the development of the permanent- magnet technology in recent years. However, as a motor having a single permanent-magnet pole will have a nonsymmetrical magnetic circuit, it will not be possible just to remove one of the magnets of a prior art motor, e.g. of the design shown in W092/12567 and make the other magnet stronger.

The primary object of the invention is to provide a motor of the type represented by the motors disclosed in WO90/02437 and W092/12567 in which it is possible to have an odd number of permanent-magnet poles, e.g. a single permanent-magnet pole, on the motor part provided with the winding, the sta¬ tor, and still provide a favourable path for the flux pro¬ duced by the permanent-magnet pole or poles and/or excited reluctance poles.

This object is achieved by the arrangement of auxiliary pole means defined in the appended claims. The auxiliary pole means provides a separate return path through the stator by way of the rotor as a substitute for those poles associated with a winding coil which serve as return paths in the prior art motor.

Accordingly, the auxiliary pole means provides a favourable path (a path of adequate flux-carrying capacity) to the sta¬ tor for the magnetically unbalanced flux which exists because in the motor according to the invention there are relative positions of the motor parts in which a stator pole, a per¬ manent-magnet pole or a reluctance pole, is magnetically aligned with and passes flux to a rotor pole although there is no oppositely polarized stator pole belonging to the same winding phase which is also magnetically aligned with a rotor pole.

In the case of small motors according to the invention with reluctance poles on both motor parts, the reduction of the number of pole groups from two to only one may also result in an increased efficiency. This increase is due to the fact that the driving force applied to the movable part by a pole group increases progressively, rather than linearly, with the ampere turns acting on the pole group. Consequently, if all ampere turns act on a single pole group instead of being distributed between two pole groups, fewer ampere turns are required to produce a given driving force.

The principles of the present invention can be applied both to motors with single-strand or single-phase windings and to motors with multi-strand or polyphase windings. In the case of multi-strand or polyphase windings a pole group including a permanent-magnet pole may be provided for more than one winding strand or phase.

Besides the opportunity of providing a single pole group in¬ cluding a permanent-magnet pole (or such a pole group for each winding phase) the invention also offers the opportunity of increasing the force generated by the motor - torque in a rotating motor and linearly acting force or "tractive force" in a linear motor - in one or more respects:

- Increasing the torque generated by the permanent magnet or magnets pulling the rotor of the currentless motor to the nearest starting position. Such improvement is advan¬ tageous in applications where high frictional torque may appear in the driven object, for instance in shaft seals.

- Increasing the torque appearing in a motor whose rotor is stationary in a starting position and whose winding is supplied with the highest current available. Such impro¬ vement is also advantageous in the situations mentioned in the preceding paragraph.

Increasing, at least in certain embodiments, the airgap power of the motor for given heat losses, thereby giving a smaller and economically more favourable motor for a given purpose, which may be a great advantage when low motor weight is of importance for certain types of appli¬ cations, e.g. in hand-held tools or other hand-held ob¬ jects, but is also an economical advantage in general provided an unavoidable cost increase in the supply electronics does not cancel the effect.

The magnetically active elements in the motor of relevance to the invention are as follows:

Coils on the stator

In principle the coil of each winding strand or phase forms part of a single electrical circuit.

Ferromagnetic salient poles (reluctance poles)

In most of the motors according to the invention shown in the appended drawings, ferromagnetic salient poles, in the following also called reluctance poles, are to be found on the stator, alone or together with permanent- magnet poles.

Preferably, the rotor has only reluctance poles.

The reluctance poles on both stator and rotor may be mag¬ netically asymmetrical. For magnetically asymmetrical sta¬ tor poles the asymmetry should be directed in the oppo¬ site direction to the preferential direction of motion of the motor, whereas on the rotor the asymmetry should be in the preferential direction of motion.

Alternatively or in addition, the reluctance poles on both stator and rotor may, however, show a certain mag- netic asymmetry in the opposite direction to that de¬ scribed above without this making the motor inoperable.

Permanent-magnet poles

Motors with only reluctance poles on the rotor must al¬ ways be provided with a permanent-magnet pole on the sta¬ tor.

In certain cases it is an advantage if the permanent- magnet pole or poles is/are asymmetrical.

The permanent-magnet poles, both symmetrical and asymme¬ trical, may have skewed ends or edges, i.e. edges running

at an angle to the direction of the rotor axis. In some cases such skewing of the edges of the permanent-magnet poles may be extremely beneficial to the function of the motor. Such skewed edges need not be embodied in geome- trie shapes. It is sufficient for the edges to consist of demarcation lines (demarcation zones) relating to the im¬ printed magnetic polarisation (in, for instance, a perma¬ nent magnet), i.e. they are imprinted when the permanent- magnet poles are magnetized.

These demarcation lines for zones with the same magnetic polarization may run other than linearly without the function of the motor being greatly affected.

The magnetic asymmetry can be achieved in several ways within the scope of the invention and the appended claims and some of them will be explained below.

As in the prior art motors, the magnetic asymmetry aims at building the preferential starting direction into the motor, but the magnetic asymmetry in motors according to the present invention also serves other purposes.

Basically, an additional purpose of the magnetic asymmetry as utilized in the present invention is to extend what is herein termed the pull-in distance. This is the distance over which a pole, a permanent-magnet pole or a magnetized reluctance pole, on one of the motor parts is capable of attracting a pole on the other motor part sufficiently to cause the two poles to be pulled towards one another from a first stable position, such as the indrawn position, to the next stable position, such as the starting position, in which they are mutually aligned magnetically and, accordingly, no magnetic pull force in the direction of relative movement exists bet- ween the poles (only a magnetic pull in a direction trans¬ verse to that direction) .

During this pull-in motion the permeance between the two poles or, in other words, the magnetic flux passing between them (assuming that the magnetomomotive force is constant) should increase steadily to a maximum value occurring when the poles are magnetically aligned. An extension of the pull- in distance thus calls for a lowering of the mean value of the rate of flux change over the pull-in distance.

Such a lowering can be accomplished by means of magnetic asymmetry, e.g. by providing on at least one of the poles an additional pole part extending in the relative preferential starting direction so that the pole will have a main pole part and an auxiliary pole part which determines the prefe¬ rential starting direction.

In the starting position and the indrawn position, the auxi¬ liary pole part extends at least to a point in the vicinity of the next pole (as seen in the relative preferential start¬ ing direction) on the other motor part and it may even slightly overlap that pole. However, an overlapping portion of the auxiliary pole part must not carry as much flux per unit length of overlap (as measured circumferentially) as overlapping portions of main pole parts.

Assuming that in a rotary motor chosen by way of example both the leading ends and the trailing ends of both the stator poles and the rotor poles extend axially, magnetic asymmetry of a stator pole could in most cases in principle be observed in the following way. The rotor of the motor is replaced with a homogenous ferromagnetic cylinder of the same diameter as the rotor and the flux density in the air gap is measured along an axially extending line on the cylinder surface as the cylinder is rotated to move the line in the preferential direction of rotation past the pole. A graph showing the measured flux density (as averaged over the length of the line) versus the angular position of the line relative to the pole would rise, more or less steadily or in more or less distinct steps, from a point near zero at the leading end of

the pole, to a roughly constant value under the main portion of the pole and then decline steeply at the trailing end. If the pole were instead magnetically symmetrical, the graph would be symmetrical and resemble a Gaussian curve.

With suitable modifications the above-described principle is applicable also in other cases, such as for observing magne¬ tic asymmetry of a rotor pole or a pole whose leading and trailing ends do not extend axially. For example, where the ends of the pole are skewed so that they extend along a heli¬ cal line, the observation can be made with the measurement of the flux density taking place along a correspondingly skewed line.

In the case of permanent-magnet poles with uniform radial di¬ mension and uniform magnetic polarization radially or trans¬ versely to the air gap, magnetic pole asymmetry can result from the pole shape. For example, the leading and trailing end of the pole may have different lengths in the axial di- rection of the motor. A similar effect can also be achieved by magnetically imprinting poles with a corresponding shape in a ring of permanent-magnet material of uniform thickness. In this case the shape of the permanent-magnet ring has no¬ thing to do with the magnetic pattern or "magnetic shape".

Magnetic pole asymmetry can also be achieved by providing a permanent-magnet pole with different radial dimensions at the leading and trailing ends, respectively, (i.e. by giving the air gap at the pole a width that varies in the direction of the relative movement of the motor parts) but giving it uni¬ formly strong magnetization over its entire volume.

Several methods can of course be used simultaneously in order to achieve magnetic asymmetry for the permanent-magnet poles.

There are also several ways of achieving magnetic asymmetry for salient ferromagnetic poles, the reluctance poles. One method is to arrange the surface of such a pole facing the

air gap asymmetrically with regard to its dimension (width) in the axial direction of the motor, in which case the entire pole surface may be situated at the same radial distance from the axis or rotation.

Another method is to make the projection surface of the re¬ luctance pole (the surface facing the air gap) symmetrical, but vary its radial distance from the axis of rotation, i.e. vary the width if the air gap along the pole surface, step- wise or continuously, in relation to an imagined (cylind¬ rical) surface on the other motor part.

A third method is to vary the magnetic saturation flux den¬ sity along the pole surface. This can be achieved by using different magnetic materials for different parts of the re¬ luctance pole, or it can be achieved by varying the filling factor of the laminated reluctance poles, or by means of punched recesses, for instance, below the actual pole surface (so that the actual pole surface appears to be homogenous) , or by varying the radial dimension of an auxiliary pole part such that it will have a shape resembling the profile of the curved beak of a bird when viewed in the direction of the axis of rotation.

Of course several methods of achieving magnetic asymmetry can be used simultaneously. The choice of how to achieve asymme¬ try is usually dependent on a balance between the manufactur¬ ing costs of the motor and the cost of the supply electro¬ nics, since the choice of the type of asymmetry may affect the size of the power electronic switch elements included in the supply electronics.

As will become apparent, in motors embodying the invention magnetic asymmetry may characterize not only an individual pole of a group of poles which are associated with a common winding coil such that all poles of the group are subjected to the magnetic field produced upon energization of the coil. It may also characterize the group of poles and then not only

by virtue of magnetic asymmetry of one or more individual poles but also by virtue of an asymmetrical positioning of an individual pole within a pole group.

A pole of a pole group on the stator is asymmetrically posi¬ tioned if a rotor pole is moved through a distance longer or shorter than one-half rotor pole pitch when it is moved bet¬ ween a position in which it is magnetically aligned with that stator pole and the next adjacent position in which any pole on the rotor is magnetically aligned with a stator pole of a different pole type or, in the case of a stator having only permanent magnets, a pole of different polarity.

In other words, a permanent-magnet pole, for example, on the stator is asymmetrically positioned with respect to a reluc¬ tance pole in the same or a different pole group if a rotor pole traverses a distance which is longer or shorter than one-half rotor pole pitch when the rotor moves between a position in which a rotor pole is magnetically aligned with that permanent-magnet pole, i.e. is in the starting position, to the next following or next preceding position in which a rotor pole - which may be any rotor pole - is in an indrawn position.

In a pole row comprising permanent magnet poles of alternat¬ ing polarity, the north-pole permanent-magnet poles may be displaced in either direction from a central position between the south-pole permanent-magnet poles with all like poles sub¬ stantially equally spaced.

It should be noted in the context of the present invention that a pole group (pole unit) may comprise a single pole or a plurality of poles associated with a magnetizing coil.

The invention will now be described in more detail with refe¬ rence to a number of exemplifying embodiments shown schemati¬ cally in the accompanying drawings.

Figure 1A shows an end view of a first embodiment of a rota¬ ting motor.

Figure IB is a developed view of a section of the rows of poles in the motor viewed from within the air gap and axially displaced from their working position, but otherwise in the position in relation to each other that they assume in Figure 1A;

Figure IC shows the motor in the same way as Figure 1A, with the permanent-magnetic field lines in the stator and rotor included;

Figure ID is a longitudinal sectional view of the motor of Figure 1;

Figure IE is a view similar to Figure IB but shows the stator and rotor poles in the same axial position and displaced to a different relative angular position and also includes a graph representing the magnetic attraction force acting between a stator pole and a rotor pole.

Figures 2A and 2B are views corresponding to Figures 1A and IB showing a second embodiment;

Figures 3A and 3B are views corresponding to Figures 1A and IB showing a third embodiment;

Figure 4 is a view corresponding to Figure 1A and shows a fourth embodiment;

Figures 5A and 5B are views corresponding to Figures 1A and IB showing a fifth embodiment;

Figures 6A, 6B and 6C are views corresponding to Figures 1A, IB and IC showing a sixth embodiment, namely a two-phase motor.

Figures 7A to 7D are developed views resembling Figure IB of pole combinations which differ from each other in respect of the shape and placement of a permanent-magnet pole on the stator.

Throughout the drawings, the polarity of the permanent-magnet poles which are magnetized transversely to the air gap, is indicated by an arrow-head pointing towards the north-pole side of the magnet.

Moreover, in all embodiments shown in the drawings except that shown in Figures 3A, 3B, the asymmetry of the stator and/or the rotor poles is directed such that the preferential starting direction of the rotor is counterclockwise.

The motor shown in Figures 1A-1D is a rotating motor, as are the other motors also, with a first motor part in the form of a laminated ferromagnetic stator 11 and a second motor part in the form of a laminated ferromagnetic rotor 12 journalled for rotary movement in the stator by suitable bearings as shown in Figure ID. The axis C of rotation of the rotor is indicated by a small circle or a dash-dot line and designated by C, and the preferential direction of rotation is indicated by an arrow (counterclockwise in all illustrated embodiments) .

The stator 11 has two diametrically opposite pole groups 13 and 14. The pole group 13 comprises two salient ferromagnetic poles 15S, also called reluctance poles, which are spaced from each other circumferentially, and a permanent-magnet pole 16A arranged between them. The other pole group 14 is identical with the pole group 14 except that it has no per¬ manent-magnet pole.

The surfaces of all poles 15S and 16A facing the rotor 12 are located on an imaginary cylindrical surface that is concent¬ ric with the axis of rotation 12A of the rotor.

For each pole group 13, 14 the stator 11 is also provided with a coil 17 wound around the pole group and forming part of a common magnetizing winding circuit, i.e. both winding coils are energized with the same pulsating direct current.

On the outside of the rotor 12, distributed uniformly around its periphery, are four uniformly spaced salient ferromag¬ netic poles 18A, also called reluctance poles. The surfaces of these poles facing the stator are located on an imaginary cylinder that is concentric with the axis of rotation C, a short distance from the cylinder containing the pole surfaces of the stator, so that the pole surfaces of the stator and those of the rotor form a cylindrical air gap 19 between them. The pole pitch of the rotor 12 corresponds to the spacing of the reluctance poles 13S within each pole group on the stator 11.

In the embodiment shown in Figure 1A to ID, all the poles 15S, 16A on the stator 11 are located in a first annular pole row, and the poles 18A on the rotor 12 are located in a se¬ cond annular pole row in the same plane perpendicular to the axis of rotation, so that during rotation all poles on the rotor sweep over and interact with all poles on the stator. The motor may of course have several axially separated pole rows or sets of pole groups arranged in this manner. Further¬ more, instead of being located in closed paths or rows runn¬ ing peripherally around the rotor, the poles on each motor part may be arranged, for instance, on a helical path.

Throughout the following description, a suffix letter S of the numerical references used in the drawings for reluctance and permanent-magnet poles indicates that the pole is magne¬ tically symmetrical. Similarly, a suffix letter A indicates that the pole is magnetically asymmetrical.

Accordingly, the pole faces on the reluctance poles 15S of the stator 11 are magnetically symmetrical in the sense in¬ tended in this application and as explained above.

On the other hand, the permanent-magnet poles 16A are magne¬ tically asymmetrical since they have a protrusion 16' on the side facing against the direction of rotation of the rotor, said protrusion being caused by the poles 16A on this side having narrower breadth, i.e. dimension parallel to the axis of rotation 12A, than over the main part of the poles, see Figure IB. The part of the permanent-magnet poles 16A with full breadth may be said to constitute a main pole part, while the narrower protrusion may be said to constitute an auxiliary pole part, designated in the figures by 16'.

The reluctance poles 18A on the rotor 12 are also asymme¬ trical in corresponding manner since they are provided on their leading or counterclockwise end, the side or end di- rected in the direction of rotation, with a protrusion 18'

(auxiliary pole part) having a breadth less than that of the main part (main pole part) of the poles, see Figure IB.

As is evident from the above, the asymmetry in the poles can be achieved in ways other than those just described. In one example of an alternative method each pole has the same breadth across its entire circumferential and axial dimen¬ sions, but at one end the pole surface is offset radially inwards so that the air gap is wider at this side than over the main part of the pole.

Figure IB shows a developed view of the pole group 13 of the stator 11 and a portion of the pole row of the rotor 12 in Figure 1A as viewed from within the air gap and with the rotor poles displaced axially in relation to the stator pole group. The parallel dash-dot lines R and S indicate the di¬ rection of the relative movement between rotor and stator, while the dash-dot line L perpendicular thereto represents the centre line between the stator poles. The position of the stator and rotor poles in relation to each other corresponds to the relative position shown in Figure 1A and is the stable position the rotor assumes in relation to the stator when current is supplied to the winding 15 so that the reluctance

poles 15S tend to keep the rotor poles 18A in an attracted or indrawn position with the main pole parts of the rotor poles opposite to the stator reluctance poles.

When current is no longer supplied to the winding coils 17 in this rotor position, then only the permanent-magnetic flux from the permanent-magnet poles 16A acts on the rotor 12 to pull it further in the direction of the starting position, i.e. counterclockwise.

Figure IC shows the rotor moved from the indrawn position in Figure 1A to in the starting position, also called detent or parking position, in which one rotor pole 18A is magnetically aligned with the permanent-magnet pole 16A and thus is in a position in which the flux linking the two poles is at a maxi¬ mum. A portion of the auxiliary pole part 18' on the leading end of the rotor pole extends in the counterclockwise direc¬ tion beyond the permanent-magnet pole 16A and the trailing end of the rotor pole is positioned opposite to the auxiliary pole part 16' of the permanent-magnet pole. This is also shown in Figure IE.

From Figure IC is apparent that in the starting position of the rotor there is no favourable flux path along which the flux of the permanent-magnet pole 16A can pass diametrically to the opposite side of the stator as in the prior art mo¬ tors. To provide a return path for the permanent-magnet flux back to the permanent-magnet pole 16A, the stator 11 is pro¬ vided with a pair of diametrically opposite ferromagnetic auxiliary pole pieces 20 positioned between the two pole groups 13 and 14 such that in the starting position they are opposite to two diametrically opposite rotor poles 18A. These auxiliary pole pieces 20 have pole faces on the imaginary cy¬ lindrical surface containing the pole faces of the stator poles. As is shown by the flux pattern in Figure IC, the flux from the permanent-magnet pole 16A is split mainly in two re¬ turn paths extending through the auxiliary pole pieces 20 and

passed through the upper portion of the stator 11 back to the south-pole side of the permanent-magnet pole.

The stator pole pieces 20 may be integrally formed in the stator 11 or they may be separate pole pieces which are attached to the stator after the winding coils 17 have been inserted.

As is shown in Figs. 1A, in the indrawn position the auxili- ary pole part 18' of one of the rotor poles 18A, the upper right rotor pole, extends up to the auxiliary pole part 16' of the permanent magnet 16A and preferably even overlaps this auxiliary pole part 16' slightly. This relative position of the rotor reluctance pole and the permanent-magnet pole is a position in which the magnetic attraction force the perma¬ nent-magnet pole 16A exerts on the rotor pole, and hence the counterclockwise torque applied to the rotor 12 by the perma¬ nent-magnet pole 16A, is at or near its maximum, the winding coils being currentless.

At the same time, the spacing of the upper left rotor pole 18A from the permanent-magnet pole 16A is substantial so that the permanent-magnet pole 16A only applies an insignificant clockwise torque to the rotor.

Accordingly, the net counterclockwise torque applied to the rotor by the permanent-magnet pole 16A is capable of force¬ fully jerking the rotor counterclockwise from the indrawn position and turn it through an angle corresponding to one- half of the rotor pole pitch to bring the rotor poles 18A to the starting position (pull-in movement) .

Throughout the pull-in movement of the rotor from the indrawn position to the starting position the magnetic flux between the permanent-magnet pole 16A and the rotor pole 18A which it overlaps and with which it interacts increases steadily with increasing pole overlap so that a counterclockwise torque is

exerted on the rotor until the starting position has been reached.

Figure IE also includes a graph representative of an exemp- lary embodiment of the motor shown in Figures 1A to ID and shows the pull-in force F acting between the permanent-magnet pole 16A and the rotor reluctance pole 18A versus the overlap position d of the leading end 18'' of the rotor reluctance pole 18 during the pull-in movement from the indrawn position to the starting position. In the right-hand portion of Figure IE the indrawn position of the rotor reluctance pole 18A is shown in dash-dot lines.

The graph shows the pull-in force F acting on the rotor reluc- tance pole 18A for different amounts of overlap (positive and negative) between the auxiliary pole part 16' of the perma¬ nent-magnet pole 16A and the auxiliary pole part 18' of the rotor reluctance pole 18A in the indrawn position.

From Figure IE it is apparent that if the overlap in the in¬ drawn position is -1 mm, that is, if the leading end 18'' of the reluctance pole 18A is spaced 1 mm in the negative or clockwise direction from the end 16" of the permanent-magnet pole 16A, the pull-in force is quite small. If the leading end 18'' is opposite the end 16" of the auxiliary pole part 16' of the permanent-magnet pole 16A (zero overlap), the pull-in force is substantially greater, and with a positive overlap of about 1 mm, the pull-in force on the auxiliary pole part 18A' is at or near its maximum where it is three to four times the pull-in force for a negative overlap of about 1 mm. The asymmetry of the permanent-magnet pole 16A in con¬ junction with the asymmetry of the rotor reluctance pole 18A thus produces a dramatic increase of the initial value of the pull-in force in comparison with the case where only the ro- tor reluctance pole is asymmetrical as in the motor disclosed in W092/12567. This increase of the pull-in force broadens the field of application of the motor according to the inven¬ tion.

From Figure IE it is also apparent that as the overlap gradu¬ ally increases during the pull-in movement, the pull-in force remains approximately constant during the first portion of the pull-in movement, namely until the main pole parts begin to overlap. During the continued pull-in movement the pull-in force will first increase and then gradually drop to zero as the reluctance pole 18A reaches the starting position.

Moreover, Figure IE shows that during the counterclockwise pull-in movement of a rotor reluctance pole 18A from the indrawn position to the starting position, the permanent- magnet pole 16A will exert a substantial attraction force on the rotor reluctance pole throughout a circumferential dis¬ tance which is greater than the circumferential dimension of the permanent-magnet pole 16A: from a position in which the leading end 18'' is opposite to or only slightly spaced in the clockwise direction from the end 16' ' of the auxiliary pole part 16' of the permanent-magnet pole 16A up to a point in which the leading end 18'' is well past the permanent- magnet pole.

When the winding coils 15 are again energized with the rotor poles in the starting position, the auxiliary pole parts 18' of all four rotor poles 18A will therefore be near a stator reluctance pole 15S ahead of it as seen in the direction of rotation of the rotor. On the other hand, the spacing of the trailing end of each rotor pole from the stator reluctance pole behind it is substantial. The magnetic attraction in the counterclockwise direction between the stator reluctance pole 15S and the leading end 18'' of the rotor reluctance pole 18A behind it will thus be heavily predominant over the magnetic attraction in the clockwise direction exerted on the trailing end of the same rotor pole by the next stator reluctance pole (i.e. the stator reluctance pole behind the rotor pole) .

Accordingly, the net torque applied to the rotor by the sta¬ tor reluctance poles 15S will act in the counterclockwise di¬ rection and will be high so that the motor will be capable of

starting against a considerable load. Again, the magnetic flux produced as a consequence of the energization of the winding coils 17 opposes the magnetic flux produced by the permanent-magnet pole 16A so that the permanent-magnet pole does not substantially counteract the movement from the starting position.

The embodiments illustrated in the other figures are descri¬ bed only insofar as they differ from the embodiment shown in Figures 1A-1D. The same designations are used throughout for all embodiments with the suffix letters A or S indicating asymmetrical or symmetrical. Unless otherwise stated, "symme¬ try" and "asymmetry" with regard to the poles relate to their magnetic symmetry or asymmetry rather than their geometrical symmetry or asymmetry (which may or may not correspond to the magnetic symmetry or asymmetry) .

The motor in Figures 2A and 2B differs from the motor in Fi¬ gures 1A-1C only with regard to the design of the stator 11 and the number of poles of the rotor 12.

More specifically, the stator has only one pole group 13, which is identical with the pole group 13 of Figure 1A-1D except that the permanent-magnet pole 16S is magnetically symmetrical and one of the reluctance poles is asymmetrical.

Moreover, the stator 11 itself forms a circumferentially ex¬ tended auxiliary pole 20 diametrically opposite to the pole group. This auxiliary pole 20 subtends an angle of about 180 degrees so that at least two of the five reluctance poles 18A of the rotor are always opposite the auxiliary pole.

The motor in Figures 3A and 3B differs from the motor in Figures 2A to 2C only in that both reluctance poles 15S of the stator are symmetrical as in Figure 1A and the permanent- magnet pole 16 is asymmetrically positioned with respect to the two reluctance poles 15S and in that the rotor reluctance poles 18S are provided with auxiliary pole parts 18' both at

the leading end and the trailing end. The circumferential di¬ mension of these auxiliary pole parts 18' is smaller than that of the auxiliary pole parts in the preceding embodi¬ ments.

The permanent-magnet pole 16 is connected to an actuating mechanism 21 including a lever 22. In operation of the motor, the permanent-magnetic pole 16 is stationary in a selected off-centre position, such as that shown in full lines near the left reluctance pole 15S, but by shifting the lever 21 downwards from the position shown in full lines the perma¬ nent-magnet pole can be shifted to a corresponding off-centre position, indicated in dash-dot lines in Figures 3A and 3B, on the other side of the line of symmetry between the reluc- tance poles 15S to reverse the preferential direction of ro¬ tation of the rotor.

Accordingly, in this embodiment the magnetic asymmetry in the pole system of the motor is achieved through asymmetry within the pole group 13, resulting from an asymmetical positioning of the permanent-magnet pole, rather than through asymmetry of one or more of the individual poles of the pole group.

In the motor shown in Figure 4, the stator 11 likewise com- prises only one pole group 13 similar to that shown in

Figures 1A to IE, except that the permanent-magnet pole is symmetrical. However, in this case the stator 11 is a so- called segment stator and accordingly only subtends a portion of the rotor circumference. The end portions of the stator 11 form auxiliary pole pieces 20 providing return paths for the magnetic flux from the reluctance poles 15S and the perma¬ nent-magnet pole 16S.

Figures 5A and 5B show a motor in which the rotor 12 resemb- les the rotor 12 in Figure 2A. In this case the stator 11 is only provided with permanent-magnetic poles, namely a group 13 of four evenly spaced asymmetrical permanent-magnet poles 16AN, 16AS of alternating polarities N and S on their sides

facing the air gap 19, the pole pitch being one-half of the rotor pole pitch. Moreover, in this motor the winding coils 17 are supplied with current pulses of alternating polarity.

Apart from the pole arrangement the stator 11 in Figure 5A resembles the stator 11 in Figure 2A and it accordingly com¬ prises an auxiliary pole 20 which is diametrically opposite to the pole group 13 and subtends an angle of about 180 de¬ grees.

Magnetical asymmetry on the stator pole group 13 of the motor in Figure 5A can be provided by displacing, for example, both permanent-magnet poles 16AS circumferentially in one direc¬ tion or the other from the illustrated central position to an off-centre position relative to the other permanent-magnet poles 16NS without changing the spacing of the the permanent- magnet poles 16AS from one another.

Figures 6A to 6C show a motor according to the invention which may be regarded as a two-phase version of the motor shown in Figures 2A and 2B. Its stator 11 comprises two diametrically opposite pole groups 13U and 13V which are identical and substantially similar to the pole group 13 of Figure 2A. Each pole group comprises a pair of symmetrical reluctance poles 15S and a symmetrical permanent-magnet pole 16S positioned centrally between the reluctance poles and is associated with a winding coil 17U and 17V belonging to different phases ϋ and V of the stator winding.

The winding coils 17U and 17V are energized with pulsed direct currents in phase opposition.

Moreover, the stator has a pair of diametrically opposite auxiliary pole pieces 20 which are positioned between the pole groups.

The rotor 12 is provided with seven uniformly spaced asymme¬ trical reluctance poles 18A. When the reluctance poles 15S of

the upper pole group 13U are magnetically aligned with a pair of adjacent rotor poles 18A as shown in Figure 6A so that these rotor poles are in an indrawn position, the permanent- magnet pole 16S of the opposite pole group 13V is magneti- cally aligned with another rotor pole 18A. No other rotor pole is aligned with a stator pole. More generally, whenever a reluctance pole of a pole group on the stator is magneti¬ cally aligned with a rotor pole, no other rotor pole is mag¬ netically aligned with a reluctance pole of a different pole group, and whenever a permanent-magnet pole of a pole group on the stator is magnetically aligned with a rotor pole, no other rotor pole is magnetically aligned with a permanent- magnet pole of a different pole group.

Figure 6C shows the rotor turned from the position of Figure 6A through an angle corresponding to one-half of the rotor pole pitch, so that a rotor pole 18A is magnetically aligned with the permanent-magnet pole 16S of the upper pole group 13U. This figure also shows the flux pattern which is set up when the winding coil 15U of the upper pole group 13U is energized with the rotor in the said turned position.

In the motor shown in Figures 6A to 6C, the permanent-magnet poles 16S may also be placed in an off-centre or asymmetrical position as described with reference to Figures 3A, 3B.

As is apparent from the drawings, whenever a reluctance pole or permanent-magnet pole of a pole group on the stator is magnetically aligned with a rotor pole, the auxiliary pole means 20 presents to the flux passing from that stator pole to the rotor pole the most favourable path for returning the flux to the stator.

Combinations of poles on the stator and the rotor other than those specifically shown in the drawings and described here are also possible within the scope of the invention. Besides the pole arrangements illustrated and described above, the

following list (which is not exhaustive) includes examples of pole arrangements falling within the scope of the invention.

I. Symmetrical reluctance poles on the rotor

A. Only permanent-magnet (PM) poles on the stator

1. Symmetrical permanent-magnet poles arranged in asymmetrical pole row

2. Asymmetrical PM poles B. Reluctance poles and PM poles on the stator

1. Asymmetrically placed poles in a pole group Fig. 3

2. Symmetrically placed poles in pole groups 2.1 Symmetrical reluctance poles a. Asymmetrical PM poles 2.2 Asymmetrical reluctance poles a. Symmetrical PM poles b. Asymmetrical PM poles

II. Asymmetrical reluctance poles on the rotor

A. Only PM poles on stator

1. Symmetrical PM poles

2. Asymmetrical PM poles Fig. 5

B. Reluctance poles and PM poles on the stator

1. Asymmetrically placed poles in a pole group

2. Symmetrically placed poles in pole groups

2.1 Symmetrical reluctance poles a. Symmetrical PM poles Fig. 4,6 b. Asymmetrical PM poles Fig. 1

2.2 Asymmetrical reluctance poles a. Symmetrical PM poles Fig. 2 b. Asymmetrical PM poles

Figures 7A to 7D are views corresponding to Figures IB, 2B, 3B etc. and serve to further elucidate the concept of magne¬ tically symmetrical and asymmetrical positioning of a perma¬ nent-magnet pole between a pair of reluctance poles on the

stator. These four figures show four different shapes of the permanent-magnet pole together with a stator and rotor reluc¬ tance pole combination which is the same in all figures and similar to that in Figures 1A and IB. All four figures show the rotor reluctance poles 18A in the starting position, that is, magnetically aligned with the permanent-magnet pole 16S (Figure 7A) or 16A (Figures 7B to7D) , the winding associated with the stator pole group being currentless.

In the starting position the force exerted on the rotor re¬ luctance pole 18A by the permanent-magnet pole 16A or 16S is zero in the circumferential direction but in response to any deviation of the rotor pole from the aligned position the attraction force between the permanent-magnet pole and the rotor pole will develop a circumferential component tending to return the rotor pole to the starting position.

In Figure 7A, which is included for comparison and shows a symmetrical pole configuration corresponding to that shown in W092/12567, the permanent-magnet pole 15S completely overl DS the main pole part of the rotor reluctance pole 18A. Figures 7B to 7D show different stator pole configurations according to the invention which are asymmetrical by virtue of asymme¬ try of the permanent-magnet pole. In Figure 7D the stator pole group consisting of the poles 15S and 16A is asymmetri¬ cal both by virtue of pole asymmetry of the permanent-magnet pole 16A and by virtue of a slightly asymmetrical positioning of that pole (towards the left stator reluctance pole 15S) , resulting in a slightly asymmetrical aligned position of the rotor reluctance pole 18A between the two stator reluctance poles 15S.

Magnetic asymmetry on the stator may be achieved also when all stator poles are permanent-magnet poles of alternating polarity. The permanent-magnet poles of one polarity may be collectively displaced in either circumferential direction from the central position between adjacent permanent-magnet poles of the other polarity, the poles within each set of

like-polarity poles still being substantially evenly spaced- apart.

In all embodiments illustrated in the drawings, the stator and the rotor are laminated from thin electric steel plates as is indicated in Fig. ID (where the thickness of the plates is heavily exaggerated for clearness of illustration) .

In those motors which have reluctance poles both on the sta- tor and on the rotor, the portions of the plates which form the stator reluctance poles or, every second stator plate 11A is slightly reduced such that the curved plate edges 11B facing the air gap are offset radially outwardly relative to the neighbouring plates, see Figures 1A and the lower portion of Figure ID. In other words, only every second plate lie ex¬ tends up to the air gap while the intervening plates 11A end short of the air gap.

As described in W092/12567, this thinning out of the plate stack at the pole face of the reluctance poles serves to en¬ sure that the change of the flux in the air gap between the stator and rotor reluctance poles that takes place as the rotor reluctance poles move past the stator reluctance poles is proportional to the change of the pole overlap area. In other words, they serve to ensure that the flux density in the pole overlap area is substantially constant as long as the flux change will not be limited by magnetic saturation in a different region of the magnetic circuit so that the torque developed by the interaction of the poles will be as uniform as possible.

Magnetically, the effect of the reduction or shortening of the reluctance pole portion of every second plate is a 50% lowering of the averaged value of the saturation flux density across the pole face serving the purpose of reducing the mag¬ netic induction swing (the interval within which the flux density varies over an operating cycle of the motor) in the

bulk of the laminated stack, where the predominant part of the iron losses arise.

In accordance with a modified technique for thinning out the reluctance poles at the pole faces which is suitable for mo¬ tors running at elevated operating frequencies, all plates, or the plates which extend up to the air gap, are provided with recesses which constrict the cross-sectional area of the plate presented to the magnetic flux in the pole and thereby contribute to a lowering of the flux density for which the pole becomes magnetically saturated at the pole face.

The recesses should be distributed substantially uniformly over the cross-section of the plate. They may take the form of holes, i.e. openings which are not open to the air gap, or they may take the form of openings which communicate with the air gap, preferably via narrow passages. Wide passages are undesirable because they give rise to eddy currents in the faces of the reluctance poles of the other motor part.

Instead of stacks of thin laminations or iron plates a compo¬ site body of iron powder or flakes embedded in a plastic ma¬ trix may be used.

Alternative embodiments

Motors according to the invention comprise pole groups with windings and may be shaped for rotary or linear movement.

The rotating motors may have 1. An air-gap surface that is disc-shaped or cylindrical, conical etc. , in principle any shape of surface that a generatrix rotating about a stationary axis can describe.

2. External rotor.

3. The difference between the pole number in the stator and rotor, respectively may be arbitrary, e.g. in the case of segmented stator(s) .

4. In a motor with a cylindrical air-gap surface and an in¬ ternal rotor, several pole groups may be arranged so that they are axially spaced. A motor may consist of several such "motor discs". These discs may have individually closed flux paths or may be interconnected by axially directed flux paths. Examples of such an arrangement can be found in W090/02437. The various "motor discs" can be magnetized by common coils for two "motor discs" for instance.

5. For motors with axial flux connection between "motor discs", the winding may consist of a cylinder coil sur¬ rounding the axis of rotation (an example of such an arrangement is shown in WO90/02437) . In this case, for instance, the rotating part may contain the pole types which would otherwise have been stationary, and vice versa.

6. The shape and/or distribution of the poles in a motor, e.g. the reluctance poles, may be chosen so that noise and vibration generated by varying magnetic forces be¬ tween stator and rotor are reduced as far as possible. Examples of known measures of this type are skewed pole edges or a slightly uneven distribution of the poles along the periphery of the rotor or a certain difference between the pole pitch in a pole group on the stator and the pole pitch on the rotor. Various measures can also be combined.

7. In motors with two, or some other even number of reluc¬ tance poles in each group on the stator and no permanent- magnet poles on the stator, the soft-magnetic stator yoke part which in the embodiments shown runs in the middle of the pole group, can be eliminated without affecting the magnetic function of the motor. The mechanical function of said stator yoke parts as spacers may be replaced with non-magnetic spacer means.

8. It will be understood that the coils shown in Figures 1-6 for magnetizing the pole groups can also be arranged dif¬ ferently, e.g. as coils of transformer type surrounding the yokes between the pole groups. The stator yoke may also be divided thereby enabling pre-wound coils to be used. Alternatively, for a yoke shaped as a section of a toroid, the coil may be sectionally toroid-wound around the yoke. It may also be economically advantageous in small motors, for instance, to replace two yokes that connect two pole groups together, with a single yoke with doubled cross-sectional area and have a single coil sur¬ rounding the yoke. Such arrangements are known in small shaded-pole motors and DC motors.

10. To enable the use of only a single electronic switching element in motors supplied with current pulses of a single polarity, field energy can be returned to the DC source by means of a feedback winding wound in parallel with the operating winding, as described in WO90/02437.