Self-Starting electric brushless motor having permanent magnet and reluctance poles

A self-starting brushless electric motor having a first motor part (stator) (11) with poles arranged in a row, the poles constituting ferromagnetic poles (13S) or permanent-magnet poles (14A), a second motor part (rotor) (12) with poles arranged in a row, the poles having ferromagnetic poles (16A) or permanent-magnet poles and being arranged opposite the row of poles on the first motor part (11), wherein the motor part with salient ferromagnetic poles or, if both motor parts have salient ferromagnetic poles, at least one of the motor parts has a permanent-magnet pole, and also a magnetizing winding on the first motor part. The system of poles formed by the pole rows is magnetically asymmetrical in the direction in which the motor parts (11, 12) are movable in relation to each other.

This invention relates to a self-starting brushless electric motor of the
 type which comprises reluctance poles (ferromagnetic salient poles) at
 least on one of the two relatively moving motor parts and one or more
 permanent-magnet poles in the pole system.
 Self-starting brushless electric motors can be supplied with direct current
 pulses of a single polarity or with alternating current polarity. When
 motors with moderate shaft power are supplied electronically,
 direct-current pulse supply uses the least number of electronic switches
 and thus gives the lowest system costs for motor and supply electronics.
 On the other hand, for higher power, when the number of electronic
 switches in the supply electronics in the motor must anyway be increased,
 it may be advantageous to supply the motor with alternating current
 polarity so that electric power will be supplied to the motor during both
 half-periods, thus achieving more uniform torque development and reducing
 the electrical conduction losses in the winding.
 A single-strand brushless motor has only one winding supplied from a single
 external current source and provided on one of two or more parts which are
 rotatable or otherwise movable in relation to each other. Such a motor can
 be self-starting, i.e. develop a driving torque when at standstill in a
 predetermined direction, the preferential starting direction, only if this
 starting direction is inherent in the design of the motor. Self-starting
 in the preferential direction may be built into the motor by providing
 asymmetry in the soft-magnetic iron core, e.g. through the use of
 asymmetrical salient poles and/or asymmetrical permanent-magnet poles, or
 by providing auxiliary windings with no connection to external current
 sources, e.g. short-circuited current paths as in known shaded-pole
 motors. Such current paths can only conduct current under the influence of
 a varying magnetic field linked to these current paths. In order for
 current to flow in such current paths when the motor is stationary, the
 winding connected to an external current source must be supplied with a
 pulsed or alternating current.
 It can be shown theoretically that motors that are not provided with
 auxiliary windings but are nevertheless able to exert torque in any rotor
 position even when the motor winding is deenergized must always contain a
 permanent-magnet pole.
 In the following the description is limited to motors for rotary movement
 which have a first part, in the following called the stator, provided with
 a winding, and a second part, in the following called the rotor, 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 between
 the parts of the motor need not be rotary but may equally well be linear
 or a combination of rotary and linear, i.e. occur simultaneously about and
 along an axis of rotation.
 The function of the motor may be described as comprising work cycles which
 are repeated a given number of times for each revolution. At extremely low
 speed, e.g. when starting up from standstill, the work cycle for a motor
 designed to be supplied with DC pulses of one polarity consists of one
 part when the winding carries current and another part when the winding is
 currentless. For a motor designed to be supplied with current pulses of
 alternating polarity the work cycle consists of one part when the winding
 is supplied with current of one polarity, followed by a currentless part
 and thereafter a part when the winding is supplied with current of
 opposite polarity followed by another currentless part of the work cycle.
 In the currentless state the rotor must reach a starting position, i.e. a
 position in which the winding, if supplied with current, gives rise to a
 driving torque, namely a torque in the preferential direction of the
 motor, that is sufficiently high to overcome any frictional torque or the
 like in the motor and/or in the object driven by the motor. The torque
 generated in the motor through permanent-magnetic forces must maintain its
 direction and be of sufficient strength until the rotor reaches a position
 in which the winding can be energized. It will be understood that the
 demand for torque development in currentless state means that the motor
 must include at least one permanent-magnet pole.
 Motors operating in accordance with the principles described and exhibiting
 magnetic asymmetry in the pole system are known through WO90/02437 and
 WO92/12567. An object of the present invention is to obtain improvements
 in motors of the type represented by the motors in the aforesaid
 publications.
 This object is achieved by means of the arrangement of magnetically active
 stator and rotor elements (poles).
 Besides the opportunity of realizing constructionally alternative
 embodiments, 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 permanent-magnet poles that pulls the
 rotor of the currentless motor to the nearest starting position. Such
 improvement is advantageous in applications where high frictional torque
 may appear in the driven object, for example in shaft seals.
 Increasing torque appearing in a motor whose rotor is stationary in a
 starting position and whose winding is supplied with the highest current
 available. Such improvement is also advantageous in the situations
 mentioned in the preceding paragraph.
 Increasing, at least in certain embodiments, the air-gap 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 applications, e.g.
 in hand-held tools or other hand-held objects, but is also an economic
 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 coils form a single current circuit and may be connected
 in series and/or in parallel. When the supply electronics consist of
 several units operating in parallel, these may be connected each to its
 own coil or group of coils, as if they formed a single electrical circuit.
 Instead of supplying the winding with alternating current polarity a
 two-part winding can be used, the two winding halves being supplied with a
 single current polarity, but the winding halves having magnetically
 opposite directions.
 Ferromagnetic Salient Poles (Reluctance Poles)
 In most of the motors shown according to the invention, ferromagnetic
 salient poles, in the following also called reluctance poles, are to be
 found on the stator, alone or together with permanent-magnet poles.
 There may also be reluctance poles on the rotor, but preferably not mixed
 with permanent-magnet poles. A mixture of these pole types on the rotor
 can be contemplated but is normally not meaningful.
 The reluctance poles on both stator and rotor may be magnetically
 asymmetrical. For magnetically asymmetrical stator poles the asymmetry
 should be directed in the opposite direction to the preferential direction
 of motion of the motor, whereas on the rotor the asymmetry should be in
 the same direction as the preferential direction of motion.
 Alternatively or in addition, the reluctance poles on both stator and rotor
 may, however, show a certain magnetic asymmetry in the opposite direction
 to that described above without this making the motor inoperable.
 Permanent-magnet Poles
 Motors with only reluctance poles on the rotor must always be provided with
 a permanent-magnet pole on the stator. The permanent-magnet poles on the
 stator preferably are magnetically balanced, i.e. equal in number and size
 of both polarities.
 In certain cases it is an advantage if the permanent-magnet poles are
 asymmetrical.
 Motors with permanent-magnet rotor poles designed to be supplied with
 current pulses of a single polarity must always be provided with a
 permanent-magnet pole on the stator. If such permanent-magnet poles are
 asymmetrical in shape and have a main pole part and an auxiliary pole
 part, their main pole part may advantageously be displaced in the
 direction opposite to that of the auxiliary pole part, e.g. from a
 position it would have if the pole were symmetrical, consisting only of a
 main pole part.
 Motors with permanent-magnet rotor poles may lack permanent-magnet poles on
 the stator. Such motors are self-starting only if they are supplied with
 current pulses of alternating polarity. Such motors will then have more
 uniform torque development and higher average torque for given winding
 losses than the motors supplied with current pulses of a single polarity.
 The permanent-magnet poles, both symmetrical and asymmetrical, 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 geometric shapes. It is
 sufficient for the edges to consist of demarcation lines (demarcation
 zones) relating to the imprinted magnetic polarisation (in, for example, a
 permanent-magnet pole), 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 between the poles (only a magnetic pull in a direction transverse
 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
 magnetomotive 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
 preferential starting direction.
 In the starting position and the indrawn position, the auxiliary pole part
 extends at least to a point in the vicinity of the next pole (as seen in
 the relative preferential starting 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 (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 magnetically symmetric instead, 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 when observing magnetic 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
 helical 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 dimension and
 uniform radial magnetic polarization, magnetic pole asymmetry can result
 from the pole shape. For example, the leading and trailing ends of the
 pole may have different lengths in the axial direction of the motor. A
 similar effect can also be achieved by magnetically imprinting poles with
 a corresponding shape in a ring of permanent-magnetic material of uniform
 thickness. In this case the shape of the permanent-magnet ring has nothing
 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 a uniformly 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 extension 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 of rotation.
 Another method is to make the projection surface of the reluctance pole
 (the surface facing the air gap) symmetrical, but vary its radial distance
 from the axis of rotation, i.e. vary the width of the air gap along the
 pole surface, stepwise or continuously, in relation to an imagined
 (cylindrical) surface on the other motor part.
 A third method is to vary the magnetic saturation flux density along the
 pole surface. This can be achieved by using different magnetic materials
 for different parts of the salient pole, or it can be achieved by varying
 the filling factor of the laminated ferromagnetic poles, or by means of
 punched recesses, for example, 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.
 Of course several methods of achieving magnetic asymmetry can be used
 simultaneously. The choice of how to achieve asymmetry is usually
 dependent on a balance between the manufacturing costs of the actual motor
 and the cost of the supply electronics, 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 pole group 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 or on the rotor.
 A pole of a pole group on the stator is asymmetrically positioned if a
 rotor pole is moved through a distance longer or shorter than one-half
 rotor pole pitch when it is moved between 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 reluctance 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 corresponding manner, magnetic asymmetry resulting from asymmetric
 positioning of poles may also exist in the rotor. For example, in a pole
 row on a rotor comprising permanent-magnet poles of alternating 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 substantially equally spaced.
 It should be noted in the context of the present invention a pole group
 (pole unit) may comprise a single pole or a plurality of poles associated
 with a magnetizing coil.

Throughout the drawings, the polarity of the radially magnetized
 permanent-magnet poles is indicated by an arrow-head pointing towards the
 North-pole side of the magnet.
 Moreover, in all embodiments shown in the drawings, 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 FIGS. 1A-1C 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 FIG. 1D. The axis of rotation of the rotor is indicated by a
 small circle, designated 12A 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. Each pole group
 comprises two salient ferromagnetic poles 13S, also called reluctance
 poles, spaced from each other circumferentially with a permanent-magnet
 pole 14A arranged between them. The surfaces of these poles 14A facing the
 rotor are located on a cylindrical surface that is concentric with the
 axis of rotation 12A of the rotor.
 For each pole group the stator 11 is also provided with a coil 15 wound
 around the pole group and forming part of a common magnetizing winding.
 On the outside of the rotor 12, distributed uniformly around its periphery,
 are four salient ferromagnetic poles 16A, also called reluctance poles.
 The surfaces of these poles facing the stator are located on a cylinder
 that is concentric with the axis of rotation 12A, 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 an air gap 17 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 FIGS. 1A to 1D, all the poles 13S, 14A on the
 stator 11 and the poles 16A on the rotor 12 are located in the same plane
 perpendicular to the axis of rotation, so that during rotation all poles
 on the rotor pass over and interact with all poles on the stator. The
 motor may of course have several axially separated sets of pole groups
 arranged in this manner. Furthermore, instead of being located in closed
 paths or rows running peripherally around the rotor, the poles on each
 motor part may be arranged, for example, on helical paths.
 The pole surfaces on the reluctance poles 13S of the stator are
 magnetically symmetrical in the sense intended in this application. The
 significance of this is that if the rotor 12 were to be replaced by a
 homogenous ferromagnetic cylinder, the envelope surface of which coincides
 with the cylindrical surface on which the rotor poles 16A are otherwise
 located, a magnetic field would flow through the air gap 17 below and
 around the pole surfaces of the stator poles when the current was supplied
 to the winding 15, the magnetic field having such distribution that a
 diagram of the mean value of the magnetic flux density along a generatrix
 on said ferromagnetic cylinder surface, drawn as a function of the angular
 position of this cylinder in relation to the stator, would show symmetry
 of the same type as, for example, a Gaussian curve, i.e. mirror symmetry
 about a line perpendicular to the abscissa. Said symmetry means that the
 shape of the diagram is independent of which direction of rotation of the
 cylinder is defined as positive.
 On the other hand, the permanent-magnet poles 14A are magnetically
 asymmetrical since they have a protrusion 14' on the side facing against
 the direction of rotation of the rotor, said protrusion being caused by
 the poles 14A on this side having narrower breadth, i.e. dimension
 parallel to the axis of rotation 12A, than over the main part of the
 poles. The part of the permanent-magnet poles 14A 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
 14'.
 The reluctance poles 16A on the rotor 12 are also asymmetrical in
 corresponding manner since they are provided on their leading or
 counterclockwise side, the side directed in the direction of rotation with
 a protrusion 16' (auxiliary pole part) having a breadth less than that of
 the main part (main pole part) of the poles.
 As is evident from the above, the asymmetry in the poles can be achieved in
 ways other than those just described. One example of an alternative method
 is indicated in dash-dot lines in FIGS. 1A and 1B. In this alternative
 each pole has the same breadth across its entire axial and circumferential
 dimension, but at one side the pole surface is offset radially inwards so
 that the air gap 17 is larger at this side than over the main part of the
 pole.
 FIG. 1B shows a developed view of one of the pole groups of the stator and
 the rotor in FIG. 1A as viewed from within the air gap 17 and with the
 rotor poles displaced axially in relation to the stator pole group. The
 parallel dash-dot lines R and S indicate the direction of the relative
 movement between rotor and stator. The dash-dot lines R and S also are
 lines (alternatively described as paths, circles, or rows) along which the
 poles are deployed. 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 FIG. 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 13S tend to keep the rotor poles 16A in an attracted or
 indrawn position with the main pole parts opposite to the reluctance
 poles.
 When current is no longer supplied to the winding in this rotor position,
 then only the permanent-magnetic flux from the permanent-magnet poles 14A
 acts on the rotor to pull it further in the direction of the starting
 position. FIG. 1C shows the flux pattern of the permanent-magnet poles in
 this position.
 As is shown in FIGS. 1A-1D, in the indrawn position the auxiliary pole
 parts 16' of two of the rotor poles 16A, the upper right and the lower
 left rotor poles, extend up to the auxiliary pole parts 14' of the
 permanent-magnet poles 14 and preferably even overlap the auxiliary pole
 parts slightly. This relative position of the permanent-magnet poles and
 the auxiliary pole parts of the rotor poles is a position in which the
 magnetic attraction force the permanent-magnet poles 14 exert on these
 rotor poles, and hence the counterclockwise torque applied to the rotor 12
 by the permanent-magnet poles 14A, is at or near its maximum, the winding
 coils being currentless.
 At the same time, the spacing of the other two rotor poles 16, the upper
 left and the lower right rotor poles, from the permanent-magnet poles 14A
 is substantial so that the permanent magnet-poles only apply an
 insignificant clockwise torque to the rotor.
 Accordingly, the net counterclockwise torque applied to the rotor by the
 permanent-magnet poles 14A is capable of forcefully jerking the rotor 12
 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
 16A 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 each permanent-magnet pole
 14A and the rotor pole which it overlaps and with which it interacts
 increases steadily with increasing pole overlap so that a counterclockwise
 torque is exerted on the rotor 12 until the starting position has been
 reached.
 In the starting position each of the two first-mentioned rotor poles 16A is
 magnetically aligned with respectively the upper and the lower
 permanent-magnet pole 14A, a portion of the auxiliary pole part 16' on the
 leading end of the rotor pole 16A extending in the counterclockwise
 direction beyond the permanent-magnet pole and the trailing end of the
 rotor pole being positioned opposite to the auxiliary pole part 14' of the
 permanent-magnet pole. This is shown in FIG. 1F.
 FIG. 1E includes a graph representative of an exemplary embodiment of the
 motor shown in FIGS. 1A to 1D which shows the pull-in force F acting
 between the permanent-magnet poles 14 and the rotor reluctance poles 16 as
 a function of the overlap d of the leading end 16" of the rotor reluctance
 pole 16 and the corresponding edge 14" of the permanent-magnet pole 14A,
 during the pull-in movement from the indrawn position to the starting
 position. In the right-hand portion of FIG. 1E the indrawn position of the
 rotor reluctance pole 16A is shown as a cross-section. The view is looking
 outward through a cylindrical surface, centered on the rotation axis 12A,
 slightly smaller in diameter than the rotor. FIG. 1E shows the features of
 FIG. 1B, but overlapped.
 The graph shows the pull-in force F acting on the rotor reluctance pole 16
 for different amounts of overlap (positive d) and separation (negative d)
 between the auxiliary pole part 14' of the permanent-magnet pole 14A and
 the auxiliary pole part 16' of the rotor reluctance poles 16A in the
 indrawn position.
 From FIG. 1E it is apparent that if the overlap in the indrawn position is
 -1 mm, that is, if the leading end 16" of the reluctance pole 16 is spaced
 1 mm in the negative or clockwise direction from the permanent-magnet
 pole, the pull-in force is quite small. If the leading end 16" is opposite
 the end of the auxiliary pole part 14' (zero overlap), the pull-in force
 is substantially greater, and for a positive overlap of about 1 mm the
 pull-in force on the auxiliary pole part 16' 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 stator permanent-magnet pole 14 in
 conjunction with the asymmetry of the rotor reluctance pole 16 thus
 produces a dramatic increase of the initial value of the pull-in force in
 comparison with the case where only the rotor reluctance pole is
 asymmetrical as in the motor disclosed in WO92/12567. This increase of the
 pull-in force broadens the field of application of the motor according to
 the invention.
 From FIG. 1E it is also apparent that as the overlap then gradually
 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 16A reaches the starting position.
 Moreover, FIG. 1E shows that during the counterclockwise pull-in movement
 of a rotor reluctance pole 16A from the indrawn position to the starting
 position, the permanent-magnet pole 14A will exert a substantial
 attraction force on the rotor reluctance pole throughout a circumferential
 distance which is greater than the circumferential dimension of the
 permanent-magnet pole 14A: from a position in which the leading end 16" is
 opposite to or only slightly spaced in the clockwise direction from the
 end 14" of the auxiliary pole part 14' of the permanent-magnet poles 14A
 up to a point in which the leading end 16" 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 part 16' of all four rotor poles 16A
 will therefore be near a stator reluctance pole 13S 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 13S and the leading end 16"
 of the rotor reluctance pole 16A 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 reluctance pole behind the rotor pole).
 Accordingly, the net torque applied to the rotor by the stator reluctance
 poles 13S will act in the counterclockwise direction 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 opposes the magnetic flux produced by the
 permanent-magnet poles 14A so that the permanent-magnet poles do not
 substantially counteract the movement from the starting position.
 The embodiments illustrated in the other figures are described only insofar
 as they differ from the embodiment shown in FIGS. 1A-1D. The same
 designations are used throughout for all embodiments, with the suffix
 letter A or S indicating asymmetrical or symmetrical. Unless otherwise
 stated, "symmetry" and "asymmetry" with regard to the poles relates 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 FIGS. 2A-2C differs from the motor in FIGS. 1A-1C only with
 regard to the stator poles. More specifically, one stator reluctance pole
 13A in each stator pole group is magnetically asymmetrical with an
 auxiliary pole 13' of the same type as the auxiliary pole 16' on the rotor
 poles 16A, whereas the other reluctance pole 13S and the permanent-magnet
 poles 14S are magnetically symmetrical. The auxiliary pole parts 13'
 amplify the above-described effect of the auxiliary rotor pole parts 16'
 when the winding coils are energized with the rotor poles in the starting
 position.
 The motor in FIGS. 3A, 3B also differs from the motor in FIGS. 1A-1C only
 with regard to the stator poles. In this case the stator reluctance poles
 13A and 13S in each stator pole group are the same as in FIGS. 2A, 2B,
 i.e. one is asymmetrical and the other is symmetrical. However, the
 permanent-magnet poles 14A are asymmetrical with an auxiliary pole 14' of
 the same type as the rotor auxiliary pole 16', with the asymmetry directed
 in the same direction as the asymmetry of the stator reluctance pole 13A.
 Consequently, asymmetry exists in all three pole types in this motor.
 FIGS. 2A, 2B and 3A, 3B, as well as some of the following figures, show
 that all poles of a certain type, whether reluctance poles or
 permanent-magnet poles, in a pole group need not necessarily be of the
 same kind in respect of symmetry or asymmetry. This is true for all
 embodiments.
 The motor in FIGS. 4A, 4B has two symmetrical stator reluctance poles 13S
 and one asymmetrical stator permanent-magnet pole 14A in each pole group
 and, as far as the stator 11 is concerned, therefore agrees with the motor
 in FIGS. 1A-1C. In this case, however, the rotor 12C is designed
 differently from the rotor 12 in the previous embodiments, partly since it
 has only permanent-magnet poles 18AN and 18AS, asymmetrical ones with
 auxiliary poles 18' facing the same way, and partly since these
 permanent-magnet poles are arranged without spaces around the periphery of
 the rotor body, adjacent poles having opposite polarities, N and S,
 respectively.
 In the motor in FIGS. 5A, 5B each stator pole group has only two reluctance
 poles, i.e. symmetrical reluctance poles 13S, and the stator 11 thus lacks
 permanent-magnet poles. The rotor 12C is similar to the rotor in FIGS. 4A,
 4B except that the permanent-magnet poles 18AN, 18AS are shaped slightly
 differently.
 In the motor in FIGS. 6A, 6B, as in the motor in FIGS. 2A-2B, the stator
 pole groups have one asymmetrical and one symmetrical reluctance pole 13A
 and 13S, respectively, combined with a symmetrical permanent-magnet pole
 14S, while the rotor 12C is the same as the rotor in FIGS. 4A, 4B.
 The motor in FIGS. 7A, 7B has stator pole groups of the same type as the
 motor in FIGS. 3A, 3B, i.e. with one asymmetrical and one symmetrical
 reluctance pole 13A and 13S, respectively, and an asymmetrical
 permanent-magnet pole 14A, and the rotor 12C is of the same design as in
 FIGS. 4A, 4B and 6A, 6B.
 The stator pole groups of the motor in FIGS. 8A, 8B have only asymmetrical
 reluctance poles 13A and thus no permanent-magnet poles, and the rotor is
 very similar to that in FIGS. 4A, 4B and 6A, 6B.
 In the motor in FIGS. 9A, 9B, stator pole groups are used which have a pole
 combination corresponding to that in the motor in FIGS. 7A, 7B--an
 asymmetrical reluctance pole 13A, a symmetrical reluctance pole 13S and an
 asymmetrical permanent-magnet pole 14A--together with symmetrical
 permanent-magnet poles 18SN, 18SS of alternating polarity on the rotor.
 Like the motor in FIGS. 8A, 8B, the motor in FIGS. 10A, 10B has stator pole
 groups with only reluctance poles, i.e. asymmetrical reluctance poles 13A
 with a relatively long auxiliary pole 13'. As in the motor in FIGS. 9A,
 9B, the rotor 12C has only magnetically symmetrical permanent-magnet poles
 18SN, 18SS, in this case however with partially skewed edges.
 FIGS. 11A, 11B show a motor in which the rotor 12D resembles the rotor 12C
 in FIGS. 1A to 1D except that it is provided with three asymmetrical
 reluctance poles 16A. In this case the stator 11D is circular and only
 provided with permanent-magnet poles, namely two diametrically opposite
 groups of asymmetrical permanent-magnet poles 14AN, 14AS, each group
 comprising two spaced poles of alternating polarity N and S, the pole
 pitch being one-half of the rotor pole pitch. Moreover, in this motor the
 winding coils 15D are supplied with current pulses of alternating
 polarity. The winding coils 15D differ from the winding coils of the
 preceding embodiments in that they are sectionally toroid-wound around the
 stator 11D.
 Other combinations of poles on stator and rotor are also possible within
 the scope of the invention. Besides the pole arrangements illustrated and
 described above, the following list includes examples of pole arrangements
 falling within the scope of the invention.
 I. Symmetrical Reluctance Poles on the Stator
 A. Asymmetrical permanent-magnet poles on the stator (in symmetrical or
 asymmetrical placement)
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles FIG. 1
 b. Symmetrical reluctance poles
 2. Permanent-magnet poles on the rotor
 a. Asymmetrical permanent-magnet poles (arranged in symmetrical or
 asymmetrical pole row) FIG. 4
 b. Symmetrical permanent-magnet poles (arranged in symmetrical or
 asymmetrical pole row)
 B. Symmetrical permanent-magnet poles (in asymmetrical placement on the
 stator)
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles
 b. Symmetrical reluctance poles FIG. 12
 C. Lacks permanent-magnet poles on the stator
 1. Permanent-magnet poles on the rotor
 a. Asymmetrical permanent-magnet poles FIG. 5
 b. Symmetrical permanent-magnet poles arranged in asymmetrical pole row
 II. Asymmetrical Reluctance Poles on the Stator
 A. Symmetrical permanent-magnet poles on the stator
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles FIG. 2
 b. Symmetrical reluctance poles
 2. Permanent-magnet poles on the rotor
 a. Asymmetrical permanent-magnet poles FIG. 6
 B. Asymmetrical permanent-magnet poles on the stator
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles FIG. 3
 b. Symmetrical reluctance poles
 2. Permanent-magnet poles on the rotor
 a. Asymmetrical permanent-magnet poles FIG. 7
 b. Symmetrical permanent-magnet poles FIG. 9
 C. Lacks permanent-magnet poles on the stator
 1. Permanent-magnet poles on the rotor
 a. Asymmetrical permanent-magnet poles (arranged in symmetrical or
 asymmetrical pole row) FIG. 8
 b. Symmetrical permanent-magnet poles (arranged in symmetrical or
 asymmetrical pole row) Fig. 10
 III. Symmetrical Permanent-magnet Poles Only on the Stator
 A. Stator poles arranged in a symmetrical pole row
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles
 B. Stator poles arranged in asymmetrical pole row
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles
 b. Symmetrical reluctance poles
 IV. Asymmetrical Permanent-magnet Poles on the Stator
 A. Stator poles arranged in symmetrical Pole row
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles FIG. 11
 b. Symmetrical reluctance poles
 B. Stator poles arranged in asymmetrical pole row
 1. Reluctance poles on the rotor
 a. Asymmetrical reluctance poles
 b. Symmetrical reluctance poles
 FIGS. 12A and 12B show a motor which is also within the scope of the
 invention. All individual poles, both those on the stator and those on the
 rotor, are magnetically symmetrical. In this motor the object of the
 invention is instead achieved by magnetic asymmetry within the pole groups
 on the stator, namely by asymmetrical positioning of a symmetrical
 permanent-magnet pole between a pair of symmetrical reluctance poles of
 the pole groups. As the rotor is also provided with symmetrical reluctance
 poles, the motor shown in FIGS. 12A, 12B may be regarded as belonging to
 category I.B.1.b. in the above categorization.
 More particularly, the motor shown in FIGS. 12A, 12B comprises a stator 11
 having reluctance poles 13S similar to those shown in FIGS. 1A to 1D. The
 rotor 12 also resembles the rotor in FIGS. 1A to 1D, except that its poles
 16 are provided with auxiliary pole parts 16' both at the leading end and
 at the trailing end and these auxiliary pole parts are of lesser
 circumferential dimension.
 Each pole group comprises a rectangular magnetically symmetrical
 permanent-magnet pole 14 which is placed in an asymmetric or off-centre
 position adjacent to one of the two reluctance poles 13S. The two
 permanent-magnet poles 14 are connected to a common actuating mechanism 20
 including a lever 21. In operation of the motor the permanent-magnet poles
 14 are stationary in the selected off-centre position, but by shifting the
 lever 21 downwards from the position shown in full lines in FIG. 12A, the
 permanent-magnet poles 14 can be moved circumferentially from the
 illustrated off-centre position to a corresponding off-centre position
 (indicated in dash-dot lines in FIGS. 12A, 12B) adjacent to the other
 stator reluctance poles 13S to reverse the preferential direction of
 rotation of the rotor.
 The rotor 12 is shown with its poles 16 in the indrawn or attracted
 position. In this position the auxiliary pole parts 16' at the trailing
 end (assuming counterclockwise rotation of the rotor) of two of the rotor
 poles, the upper left pole and the lower right pole, is closely adjacent
 to one end of each permanent-magnet pole 14 and preferably there is a
 slight overlap between each permanent-magnet pole and the adjacent one of
 these rotor poles. The spacing of the opposite side of each
 permanent-magnet pole 14 and the other adjacent rotor pole is substantial.
 Accordingly, in the illustrated indrawn rotor position, the magnetic
 attraction between each permanent-magnet pole 14 and the rotor pole 16
 ahead of it, as seen in the direction of rotation of the rotor, will be
 heavily predominant over the magnetic attraction between the
 permanent-magnet pole and the rotor pole behind it. When the current in
 the winding coils 15 is switched off with the rotor in the illustrated
 position, the permanent-magnet poles 14 therefore will pull the rotor
 clockwise to the starting position.
 In the starting position the auxiliary pole parts 16' on the leading end of
 the rotor poles 16 will be closely adjacent to, and preferably slightly
 overlap, the two stator reluctance poles 13S ahead of them. When the
 winding coils 15 are then again energized, these reluctance poles can
 therefore forcefully jerk the rotor in the counterclockwise direction away
 from the starting position as described above with reference to FIGS. 1A
 to 1E.
 FIGS. 13A to 13D are views corresponding to FIGS. 1B, 2B, 3B etc. and serve
 to further elucidate the concept of magnetically symmetrical and
 asymmetrical positioning of a permanent-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
 reluctance pole combination which is the same in all figures and similar
 to that in FIGS. 1A and 1B. All four figures show the rotor reluctance
 poles 16A in the starting position, that is magnetically aligned with the
 permanent-magnet pole 14S (FIG. 13A) or 14A (FIGS. 13B to 13D), the
 winding associated with the stator pole group being currentless.
 In the starting position the force exerted on the rotor reluctance pole 16
 by the permanent-magnet pole 14S, 14A 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 FIG. 13A, which is included for comparison and shows a symmetrical pole
 configuration corresponding to that shown in WO92/12567, the
 permanent-magnet pole 13S completely overlaps the main pole part 16A of
 the rotor reluctance pole 16. FIGS. 13B-13D show different stator pole
 configurations according to the invention which are asymmetrical by virtue
 35 of asymmetrical shape of the permanent-magnet pole (FIGS. 13B-D) and/or
 by virtue of asymmetrical positioning thereof (FIG. 13D).
 In FIG. 13D the stator pole group consisting of the poles 13S and 14A is
 asymmetrical both by virtue of pole asymmetry of the permanent-magnet pole
 14 and by virtue of a slightly asymmetric positioning of that pole
 (towards the left stator reluctance pole 13S), resulting in a slightly
 asymmetrical aligned position of the rotor reluctance pole 16A between the
 two stator reluctance poles 13S. Consequently, the distance the rotor pole
 16A traverses when it moves from the position in which it is magnetically
 aligned with the right stator reluctance pole to the position in which it
 is magnetically aligned with the permanent-magnet pole 14A is slightly
 longer than the distance it traverses from the last-mentioned position to
 the position in which it is magnetically aligned with the left reluctance
 pole 13S.
 Although magnetic asymmetry on the rotor resulting from asymmetrical
 positioning of poles is not shown in the drawings, such asymmetry, alone
 or in combination with asymmetry of individual poles, such asymmetry in
 the stator-rotor pole system is also possible. For example, in rotors of
 the kind shown in FIGS. 4A, 4B to FIGS. 10A, 10B, in which
 permanent-magnet poles of one polarity alternate with permanent-magnet
 poles of the other 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 electrical-steel plates as is indicated in FIG. 1D
 (where the thickness of the plates is heavily exaggerated for clearness of
 illustration).
 In the portions of the plates which form the stator reluctance poles 13S or
 13A, every second stator plate 11A is slightly reduced such that the
 curved plate edges 11B facing the air gap 17 are offset radially outwardly
 relative to the neighbouring plates, see FIG. 1A and the lower portion of
 FIG. 1D. In other words, only every second plate 11C extends up to the air
 gap 17 while the intervening plates 11A end short of the air gap 17. A
 similar reluctance pole design is or may be provided in the stator and or
 the rotor of all those motors in which both the stator and the rotor are
 provided with reluctance poles.
 This thinning out of the plate stack at the pole face of the reluctance
 poles serves to ensure 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 is not 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 magnetic induction swing (the interval in 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.
 FIGS. 14A and 14B show a modified technique for thinning out the reluctance
 poles at the pole faces. This modified technique, which is suitable for
 motors running at elevated operating frequencies, is not limited to the
 single-phase motors described above but is generally applicable to all
 motors having reluctance poles both on the stator and the rotor. For
 example, motors of the kinds disclosed in WO90/02437 and WO92/12567 can
 have reluctance poles of the stator and/or the rotor designed according to
 the modified technique.
 Increased motor speeds require increased operating frequencies of the
 current supply for the motor. However, increased operating frequencies are
 accompanied by increased iron losses. One technique for avoiding the
 increase of the iron losses consists in using thinner plates for the
 laminations, but if the plate thickness is reduced it may be difficult or
 impossible to use automatic production equipment. Another technique
 consists in reducing or shortening two out of every three plates, but this
 technique is in most cases unsatisfactory.
 It is also an object of the present invention to provide a reluctance pole
 design which can be adapted for motors operating at elevated frequencies
 without it being necessary to resort to any of the above-described
 techniques.
 In accordance with this aspect of the invention, the desired reduction of
 the induction swing at increased operating frequencies is achieved in a
 reluctance pole of the type shown in FIGS. 1A-1D by providing recesses in
 those plates which extend up to the air gap, which recesses 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.
 In FIGS. 14A and 14B the modification is exemplified for the reluctance
 poles of the stator of the motor shown in FIGS. 1A-1D, namely the stator
 reluctance pole 13S to the right in the upper stator pole group. FIG. 14A
 shows the shortened reluctance pole portion of one plate 11A while FIG.
 14B shows the full-length reluctance pole portion of the neighbouring
 plate 11C. In the region near the air gap 17 this portion is provided with
 three recesses 11D in the form of elongate openings which have a closed
 contour and thus are not connected with the curved edge 11E facing the air
 gap 17. The three recesses are uniformly distributed along the length of
 the curved edge.
 When designing the recessed portion of the plates, the following empirical
 equation
EQU .DELTA.B.sub.2 =.DELTA.B.sub.1 (f.sub.1 /f.sub.2).sup.1/1.2
 is helpful. In this equation, .DELTA.B and f represent respectively the
 induction swing and the operating frequency, the indices 1 and 2 denoting
 two different operating conditions. As is immediately apparent from the
 equation, an increased operating frequency with unchanged iron losses
 calls for a reduction of the induction swing which is less than directly
 proportional to the increase of the operating frequency. For example, a
 doubling of the operating frequency requires a reduction of the induction
 swing to 56% of its previous value for the iron losses to remain
 unchanged. At elevated operating frequencies it becomes possible to adjust
 the iron and copper losses so that they become approximately equal which
 is optimal for torque generation. Consequently, the flux density may be
 chosen higher than the flux density corresponding to unchanged iron
 losses.
 Although the above-described recessing of the iron plates lowers the
 saturation flux density at the reluctance pole faces, a substantial
 increase in air-gap power for the same motor size may be achieved because
 the motor speed can be increased more than the motor torque must be
 decreased.
 Naturally, the recessing of the reluctance pole portions of the plates in
 accordance with the principle described above can be applied to motors in
 which the reluctance pole portions of all plates extend up to the air gap
 as shown in respect of the plates 11C in FIGS. 1D and 14B. If desired, the
 recessing may be different for neighbouring plates.
 Alternative Embodiments
 The following alternative motors are assembled from 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 cylindrical, conical, disc-shaped, 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 cylindrical air-gap surface and internal rotor, several
 pole groups may be arranged so that they are connected together by
 electrical steel laminations in the same plane (as in the embodiments
 illustrated in the drawings). A motor may consist of several such "motor
 discs" along a common axis of rotation.
 These discs may alternatively be designed without individually closed flux
 paths and are instead connected by axially directed flux paths. Examples
 of such an arrangement can be found in W090/02437. The "motor discs" can
 be magnetized by common coils for two "motor discs" for example.
 5. For motors with axial flux connection between "motor discs", the winding
 may consist of a cylinder coil surrounding the axis of rotation (an
 example of such an arrangement is shown in WO90/02437). In this case, for
 example, the rotating part may contain the pole types which would
 otherwise have been stationary, and vice versa.
 6. Motors in which the stator does not have both reluctance poles as well
 as permanent-magnet poles, and thus only one pole type, can be modified
 within the scope of the invention by having the stator poles exchange
 places with the rotor poles. Examples of such cases are the motors in
 FIGS. 5A, 5B, 8A, 8B and 10A, 10B. In these motors, thus, the reluctance
 poles on the stator can be replaced with permanent-magnet poles
 corresponding to those on the rotor, and the permanent-magnet poles on the
 rotor can be replaced with reluctance poles corresponding to those on the
 stator. FIGS. 11A, 11B show such a modification of the motor in FIGS. 8A,
 8B.
 7. 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 between 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.
 8. In motors with two, or some other even number of reluctance 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.
 9. It will be understood that the coils shown in FIGS. 1 to 12 for
 magnetizing the pole groups can also be arranged differently, e.g. as
 coils of transformer type surrounding the yokes between the pole groups,
 or as shown in FIG. 11A. The stator yoke may also be divided thereby
 enabling pre-wound coils to be used. It may also be economically
 advantageous in small motors, for example, to replace two yokes that
 connect two pole groups together, with a single yoke with doubled
 cross-sectional are and have a single coil surrounding 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 feedback winding wound in
 parallel with the operating winding, as described in WO90/02437.