EP0960465A1

EP0960465A1 - An electric machine

Info

Publication number: EP0960465A1
Application number: EP98904487A
Authority: EP
Inventors: Vilmos TÖRÖK
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-02-12
Filing date: 1998-02-12
Publication date: 1999-12-01
Also published as: WO1998036487A1; SE9700477L; SE9700477D0

Abstract

An electric machine comprises a soft-magnetic first part (11) having a plurality of salient poles (14) which are spaced-apart along a first pole row (R), a second part (16) comprising a plurality of pairs of permanent-magnetic poles of alternating polarities arranged in a second pole row (P) which extends along the first pole row (R) in confronting relation thereto, bearing means supporting the first and second parts (11, 16) for relative movement in the direction of the first and second pole rows (R, P), and means (19) for producing a magnetic field linking the first and second parts (11, 16) and having an even number of poles which are of alternating polarities and spaced-apart along the first and second pole rows (R, P), the magnetic field travelling along the first and second pole rows (R, P) and subtending a plurality of the poles of the first and second pole rows, the number N of salient poles (14) of the first pole row (R) subtended by the travelling magnetic field being at least four times the number n of pole pairs of the travelling magnetic field, and the number of pole pairs of the second pole row (R) subtended by the travelling magnetic field being said number N of salient poles (14) plus or minus the number n of pole pairs of the travelling magnetic field.

Description

An electric machine

This invention relates to electric machines and, more particularly to electric machines, especially motors, of the kind which comprises- a pair of relatively movable parts, one of them having a row of soft-magnetic salient poles magnetizable by a magnetic field linking the two parts and the other having a row of permanent-magnetic poles of alternating polarities .

A prior art electric machine of this kind, namely a rotational electric motor, is disclosed in US-A-5, 047, 680 and is characterized by a high torque density and thus by a high ratio of the nominal torque of the motor to the volume of the parts of the motor which play a role in the generation of the torque .

In many applications of electric machines a high torque density is an essential property from an economical and or technical point of view. In many of the common types of electric motors, notably d.c. motors, induction and synchronous a.c. motors, the torque density is limited primarily by the need to avoid excessive heating and to some extent also by the number of poles of the motor. In the case of smaller motors, the highest torque density can be achieved with synchronous motors of the permanent-magnetic field type fed from an electronic power supply.

A permanent-magnet motor of the type disclosed in US-A- 5,047,680, in which two winding coils magnetize three axially spaced rows of soft-magnetic salient poles simultaneously, meets the requirement for a high torque density without excessive heat development. However, such a motor does not readily lend itself to production using the mature techniques applied to mass production of induction motors, for example.

An object of the invention is to provide an electric machine which meets the requirement for a high torque density without excessive heat development and can be produced by existing mass production techniques .

The invention accordingly provides an electric machine com- prising a soft-magnetic first part having a plurality of salient poles which are spaced-apart along a first pole row, a second part comprising a plurality of pairs of permanent-magnetic poles of alternating polarities arranged in a second pole row which extends along the first pole row in confronting relation thereto, bearing means supporting the first and second parts for relative movement in the direction of the first and second pole rows, and means for producing a magnetic field linking the first and second parts and having an even number of poles which are of alternating polarities and spaced-apart along the first and second pole rows, the magnetic field travelling along the first and second pole rows and subtending a plurality of the poles of the first and second pole rows, the number N of salient poles of the first pole row subtended by the travelling magnetic field being at least four times the number n of pole pairs of the travelling magnetic field, and the number of pole pairs of the second pole row subtended by the travelling magnetic field being said number N of salient poles plus or minus the number n of pole pairs of the travelling magnetic field. ^'

In the electric machine according to the invention the tra- veiling magnetic field interacts with a permanent-magnetic interference field which results from the relative motion of the row of soft-magnetic salient poles, the first pole row, and the row of permanent-magnetic poles, the second pole row, because of the slight difference between the pole pitch of the first pole row and the pole-pair pitch of the second pole row. Because the poles of the travelling magnetic field are generally directed toward the nodes of the permanent-magnetic interference field, where the polarity of that field changes, a strong torque is developed between the row of permanent- magnetic poles and the row of soft-magnetic salient poles.

Although the invention will be exemplified below with refe- rence to a rotational motor having coaxial annular rows of poles separated by a cylindrical air gap, it will be appreciated that the invention can also be embodied in other machine configurations, such as in machines in which the rows of poles move relative to one another along a straight or arcuate path, in rotational machines in which the rows of poles are separated axially or by a barrel-shaped or conical air gap, and in machines in which the relative movement of the rows of poles occurs simultaneously about and along an axis of rotation. The invention may also be embodied in machines in which one of the pole rows is annular and performs a rotational movement while the other pole row, namely the pole row which is stationary with respect to the winding, only subtends a portion of the circumference of the annular pole row.

It will also be appreciated that although it is preferred to produce the travelling magnetic field by means of a winding, such as a polyphase winding of the type common in induction motors, it is basically possible within the scope of the invention to produce the travelling magnetic field by other means. For example, the travelling magnetic field may be produced by a permanent magnet system which is arranged to be moved along the first and second pole rows and direct a permanent-magnetic field through both pole rows.

The electrical machine according to the invention will be described in greater detail below with reference to the accompanying diagrammatic drawings which show two embodiments by way of example .

Fig. 1A is an end view of a first embodiment of a rotational motor in which a cylindrical rotor provided with soft-magne- tic salient poles is positioned in a wound cylindrical annular stator;

Fig. IB is a view similar to Fig. 1A and includes magnetic field lines representing the resulting travelling magnetic field which links the stator and the rotor;

Fig. IC is a developed view of a section of the pole rows in the motor of Fig. 1A as viewed from within the air gap (at the location indicated by an arrow IC in Fig. 1A) between the stator and the rotor and axially displaced from their working position but otherwise in the position in relation to each other that they assume in Fig. 1A;

Fig. ID is an axial sectional view of the motor of Fig. 1A;

Fig. 2A is an end view corresponding to Fig. 1A of a second embodiment of a rotational motor in which a cylindrical annular rotor encloses a wound cylindrical stator provided with soft-magnetic salient poles;

Fig. 2B is a view similar to Fig. 2A and includes magnetic field lines representing the resulting travelling magnetic field which links the stator and the rotor;

Fig. 2C is a view similar to Fig. IC showing the pole rows of the motor of Fig. 2A, the location of the illustrated section of the pole rows being indicated by an arrow IIC in Fig. 2A;

Fig. 2D is an axial sectional view of the motor of Fig. 2A;

Fig. 2E is a perspective view of the stator of the motor in Fig. 2A.

Fig. 2F is an end view of the stator of the motor in Fig. 2A with the winding diagrammatically indicated. Figs. 3A-3C are a series of developed views of the pole rows of the motor of Fig. 1A illustrating the relative positions of the row of salient rotor poles and the row of permanent- magnetic stator poles at three different stages of one elec- trie cycle, i.e. during one full revolution of the travelling magnetic field produced by the stator winding.

In the exemplary embodiments shown in the drawings, the machine according to the invention comprises three main parts: a laminated cylindrical soft-magnetic inner or first part which is provided with an annular first row of external salient poles (reluctance poles) , an intermediate or second part which comprises a cylindrical annular second row of radially polarized permanent-magnetic poles of circumferen- tially alternating polarities and coaxially surrounds the first pole row, and a laminated soft -magnetic cylindrical annular outer or third part which supports and coaxially surrounds the second part. The first part on the one hand and the second and third parts on the other hand are relatively rotatable, and the three parts are aligned as seen in side view (as seen in Figs. ID and 2D) so that the two pole rows confront one another across a narrow cylindrical air gap separating them.

Either the first or the third part serves as a stator provided with a polyphase winding producing a rotating magnetic field which links the stator and^' the rotor and passes through the intermediate or second part . In the embodiment shown in Figs. 1A-1D the inner or first part is the rotor while the outer or third part is the stator which carries the winding. In the embodiment shown in Figs. 2A-2F, on the other hand, the inner or first part is the stator while the outer or third part is the rotor.

Referring now to Figs. 1A-1D for a more detailed description of the motor according to the first embodiment, the inner or first part, i.e. the rotor, is designated by 11. It is secured to a rotor shaft 12 supported in bearings 13. Fourteen substantially uniformly spaced-apart salient poles (reluctance poles) 14, which are separated by grooves 15 defining axially extending or slightly skewed pole edges, form the first pole row R.

The intermediate or second part is designated by 16 and comprises a ring of twenty-six substantially uniformly spaced- apart permanent-magnetic poles of circumferentially alternating polarities. These permanent-magnetic poles form the second pole row P. Accordingly, the surface of the second pole row which confronts the salient poles 14 of the first part across the air gap 17 exhibits a number of north-south pole pairs which equals the number of salient poles 14 minus one. The second part 16 preferably is a solid cylindrical sleeve of permanent-magnetic material on which the poles are formed by conventional magnetizing techniques.

The outer or third part, the stator, on the inner periphery of which the second part 16 is secured, is designated by 18. It closely . resembles a conventional induction motor stator and may be produced by means of the kind of automated production apparatus which is conventionally used for mass production of such stators.

The stator includes a three-phase winding 19 positioned in stator slots 20 and arranged to produce a two-pole rotating magnetic field for the excitation of the salient poles 14 of the rotor 11. The north-south axis of this field is designated by F in Fig. IB, in which magnetic field lines are inclu- ded to show, at a selected stage in the excitation cycle, the resulting rotating field, that is, the field resulting from the superpositioning of the rotating two-pole field produced by the winding and the permanent-magnetic interference field caused by the relative displacement of the salient rotor poles and the permanent-magnetic poles 16N, 16S.

At this stage the rotating magnetic field is orientated toward those two diametrically opposite regions of the annu- lar permanent-magnetic pole row P where the salient poles 14 of the rotor 11 are directed generally toward the inter-pole portions of that pole row.

As will be described in greater detail below with reference to Figs. 3A-3C, the permanent-magnetic interference field in the air gap 17 between the row P of permanent-magnetic poles and the row R of salient poles 14 will change through a full cycle for an angular movement of the rotor 11 corresponding to a single pole pitch of the row R of salient poles. Accordingly, to cause the rotor to perform a full revolution, the two-pole magnetic field produced by the winding 19 has to rotate through a number of revolutions equal to the number of poles of the pole row R, that is, fourteen revolutions.

A comparison of the motor of Figs. 1A-1D with a four-pole permanent-magnet synchronous motor of conventional design with a similar stator shows that the frequency of the current energizing the stator winding is 14/2=7 times higher for the motor of Figs. 1A-1D while the shaft torque is about 3 times higher. It follows, therefore, that the gain in shaft torque requires a proportionally greater increase of the frequency. Within certain limits the higher frequency means no major problem as it can be accomplished at no or only an insigni- ficant additional cost by means of known electronic power supplies .

An advantage of a motor constructed in accordance with this invention is also that it has a higher ratio of shaft torque to moment of inertia than conventional permanent-magnet motors and is therefore suitable in applications requiring rapid acceleration of the motor. A further advantage is the robustness of the rotor made up of steel laminations only.

In the embodiment shown in Figs. 2A-2F, the soft-magnetic inner or first motor part, the stator, is designated by 31. Twelve salient poles 32 form an annular row R of poles separated by axially extending grooves 33A and slots 33B. The slots 33B accommodate the three-phase winding 34.

The intermediate or second part, the part having an annular row P of permanent-magnetic poles is designated by 35, the individual north and south poles of the eleven pole pairs constituting the pole row P being designated 35N and 35S.

The outer or third, likewise soft-magnetic part, the rotor, to which the second part 35 is secured, is designated by 36. As shown in Fig. 2D the third part 36 is secured to a cup- shaped hub 37 which is rotatably supported by a central shaft 38. This shaft is journalled in a tubular member 39 which is provided on a base plate 40 and supports the stator 31.

As is best shown in Figs. 2A and 2E, the salient poles 32 are provided in pairs on six radial projections 41 on the stator 31. These projections 41 are bifurcated by the grooves 33A so that each projection forms two salient poles 32. Each pair of adjacent projections 41 is surrounded by one of the two coil sections which belong to each of the three phases U, V and W of the winding 34. This is shown in Fig. 2F from which it is also seen that the two coil sections of each phase are posi- tioned around diametrically opposite projections 41 and that the phases are angularly displaced 120° relative to one another. As will be explained below, the above-described arrangement of the winding coils is advantageous.

In many types of polyphase machines, such as three-phase induction machines, in which the stator winding produces a rotating magnetic field, the circumferential width of each winding coil usually is only slightly less than a pole pitch, e.g. 5/6 of the pole pitch. This is because wide coils, while they undesirably entail long inactive end turns, are favourable in respect of the generation of the magnetomotive force by the winding. Whenever the coil width is larger than one- third of the pole pitch, and this is usually the case, the winding coils belonging to different phases must overlap. In certain polyphase machines, however, such as stepper motors, there is no need to generate a rotating magnetic field through the interaction of coils belonging to different phases and the phase currents are controlled independently of one another. In such machines the winding coils do not overlap so that each coil is only associated with a single group of soft-magnetic salient poles.

The winding arrangement of the motor shown in Figs. 2A-2F takes advantage of the fact that rotating machines embodying the invention have a rotating stator magnetic field. Because the winding coils of the motor shown in Figs. 2A-2F are posi- tioned around two projections 41, that is, two groups of soft-magnetic salient poles 32, the amplitude of the total magnetomotive force is larger by a factor equal to the square root of three than for an alternative comparable arrangement in which each coil only surrounds a single group of soft- magnetic salient poles.

In motors in which the soft-magnetic salient poles are arranged in pole groups such that all poles in each group are subjected to the same magnetomotive force, it may be advan- tageous to place the poles within the group at circumferential distances from one another which are slightly different from the distance between neighbouring homopolar permanent- magnetic poles. This is because such a slight difference may substantially reduce the influence of, for example, the 5th and 7th space harmonics of the field generated by the row of permanent-magnetic poles which would otherwise generate a torque ripple of intolerable magnitude. Only a slight reduction of the torque generating capability results from arranging the soft-magnetic salient poles in this manner.

In the motor shown in Figs. 2A-2F, in which the difference between the centre-to-centre distance between neighbouring soft-magnetic salient poles 32 and the centre-to-centre distance between neighbouring homopolar permanent-magnetic poles is l/l2 of the last-mentioned distance, a reduction by almost 75% of the influence of the 5th and 7th space harmonics is achieved with a uniform pole pitch of the soft-mag- netic salient poles 32 throughout the circumference of the stator 31; the consequent reduction of the torque generating capability is only about 3%. If the pole groups comprise more than two soft-magnetic salient poles, there is a possibility to reduce the influence of more than two space harmonics or to reduce the influence of the 5th and 7th space harmonics more than in the case of two soft-magnetic salient poles in each pole group.

Generally speaking, when the group of soft-magnetic salient poles per pole group is two as in Figs. 2A-2F, the determination of the preferred centre-to-centre distance between neighbouring salient poles of the same pole group is based on the following rule:

For the suppression of the influence of the X-order space harmonic components of the permanent-magnetic flux generated by the second pole row P, the phase difference between these flux components in the two salient poles of each group should be about 180 electrical degrees so that these flux components cancel or almost cancel each other.

As is known, the phase difference for the Xth space harmonic component is X times the phase difference for the fundamental component. Thus, if it is desired to eliminate the influence of, for example, the 5th harmonic component of the permanent- magnetic flux on the operation of the motor, the phase difference should be 180/5 = 36 electrical degrees with respect to the fundamental component .

As the 7th space harmonic has generally the same undesirable influence on the torque ripple of a 3 -phase motor as the 5th space harmonic, both giving rise to six torque ripple periods in each period of the current supplied to the motor, a good compromise is achieved by attenuating both the 5th and the 7th space harmonics by approximately the same factor.

To this end, a phase difference of 30 electrical degrees with respect to- the fundamental component of the permanent- magnetic flux generated by the second pole row P can be chosen such that the respective flux components in the two salient poles of each group will have a phase shift of 150 electrical degrees with respect to the 5th space harmonic component and a phase shift of 210 electrical degrees with respect to the 7th space harmonic component. Both the 5th and the 7th space harmonic component will then be attenuated by a factor which is sin 15° « 0,26.

This phase difference between the fundamental components of the permanent-magnetic fluxes carried by the two soft-magnetic salient poles of a pole group can be achieved by making the circumferential centre-to-centre distance of the two poles of the pole group either 11/12 or 13/12 of the circum- ferential centre-to-centre distance between neighbouring homopolar permanent-magnetic poles. The choice between these two possibilities may be made with consideration given to other factors, such as the space required for the winding.

The above-explained basis for the choice of the spacing of the soft-magnetic salient poles can be used also when each pole group comprises more than two poles, the more general rule being that when it is desired to reduce the influence of a harmonic flux component of a certain order, the sum of the flux components of that order within the group should be brought as near zero as possible.

As already indicated, a machine, such as a motor, according to the invention based on the principles of the motor shown in Figs. 2A-2F may comprise more than two salient or reluctance poles per pole group. More particularly, the machine may have in the first pole row (R) 6p groups of salient poles with each such pole group having Y salient poles 32, Y being 2 or a greater integer and p being the number of pole pairs of the travelling magnetic field produced by the polyphase winding. The second pole row P then comprises M x p pairs of permanent-magnetic poles of alternating polarities, M being (Y x 6) ±1.

The centre-to-centre distance of neighbouring salient poles 32, i.e. the pole pitch of the second pole row, may be constant as shown in Figs. 2A-2C, or constant only within each pole group. Alternatively, the salient poles within each group may be non-uniformly spaced-apart such that when a given salient pole of the group is magnetically aligned with a permanent-magnetic pole of the second pole row P, at least one different salient pole within the same group has a centre-to centre distance from the nearest permanent-magnetic pole of the same polarity which is one twelfth of the centre- to centre distance between neighbouring homopolar permanent- magnetic poles, i.e. of the pole-pair pitch of the second pole row. This distance feature minimizes the sum of the dis- turbing influences of the 5th and the 7th order space harmonic flux components of the second pole row P on the torque developed by the machine. If there are three or more salient poles in a pole group, a proper positioning of the additional salient poles may be instrumental for reducing the influences of higher order space harmonic flux components on the torque.

In servo system applications, smooth motor motion at very low speeds is often required, and suppression of the influence of the 5th and 7th space harmonics in the magnetic field of the second pole row P is essential. Uniform spacing of the reluctance poles 32 of the first pole row R as illustrated in Figs. 2A-2C is ideal for achieving such suppression.

The embodiment shown in Figs. 2A-2F having two reluctance poles between any two neighbouring winding slots 33B also is advantageous in that the manufacturing cost can be minimized thanks to the reduction of the number of slots to the minimum required, namely a single slot per pole per phase, thereby allowing for the use of mature mass production technologies, developed mainly for fan motors having an external rotor.

For applications where the motor inertia is not critical, the external rotor -design as shown in Figs. 2A-2F offers the advantage of maximum torque capability for a given motor volume. This advantage primarily results from the increased diameter of the air gap compared to an internal-rotor machine having the same overall dimensions. An additional advantage is to be seen in the excellent cooling of the external rotor; this advantage is especially important when temperature sensitive permanent magnets are used in the rotor.

Figs. 3A-3C are three sequential representations of the pole rows R and P of the motor shown in Figs. 1A-1D and illustrate the permanent-magnetic interference field at three different stages, i.e. three different relative positions of the pole rows R and P .

In the upper portion of each of Figs. 3A-3C the moving row R of salient poles is shown as viewed from the air gap 17, the fourteen salient poles 14 being shaded in accordance with the amount by which they are overlapped by the twenty-six permanent-magnetic poles of the stationary pole row P; that por- tion of each salient pole 14 which is overlapped by a north pole is shaded by means of small circles while that portion which is overlapped by a south pole is shaded by dotting. The lower portion of each figure shows the two pole rows R and P in side view, the north and south poles of the pole row P being shaded by small circles and dotting, respectively. For convenience of illustration, the salient poles 14, which are numbered from 1 to 14 , are shown as being of the same width as the gaps 15 separating them.

In the interval between the first stage (Fig. 3A) and the third stage (Fig. 3C) , the row R of salient poles of the rotor has moved to the right relative to the row P of permanent-magnetic poles through a distance which corresponds to two-thirds of a pole pitch of the salient poles. After a further movement of one-third of a pole pitch from the position shown in Fig. 3C, the total relative movement will correspond to a full pole pitch, and the relative position of the pole rows and hence also the permanent-magnetic interference pattern, will correspond to that shown in Fig. 3A, the only difference being that each salient pole will have taken the position the salient pole ahead of it had in Fig. 3A.

Accordingly, the permanent-magnetic interference field will change through one cycle for a relative movement of the pole rows R and P corresponding to a full pole pitch of the row R of salient poles, that is, it will be repeated fourteen times during a full rotor revolution.

Figs. 3A-3C also show the sinusoidal fundamental wave K of the two-pole rotating magnetic field produced by the stator winding 19 and the likewise sinusoidal fundamental wave I of the two-pole permanent-magnetic interference pattern.

As is apparent from the shading of the salient poles in Figs. 3A-3C, the pattern of the permanent-magnetic interference field exhibits a plurality of north-south pole transitions within each cycle. These transitions are not seen in the representation of the fundamental wave I of the permanent- magnetic interference field wave included in the figures but can be represented by a wave of shorter wavelength superposed on the fundamental wave. Each such transition will contribute to the torque which is developed between the row R of salient poles and the row P of permanent-magnetic poles under the action of the rotating magnetic field produced by the winding 19.

Figs. 3A-3C are generally representative also of the perma- nent-magnetic interference field produced in operation of the motor shown in Figs. 2A-2F, a difference being that each complete revolution of the twentytwo-poled rotor 31 will produce only eleven cycles of the permanent-magnetic interference field in the stator winding.

In motors in which the soft-magnetic salient poles confront roughly every second interpole area of the row of permanent- magnetic poles, the torque-generating capability is limited by the magnetic flux which leaks through the interpole spaces of the row of soft-magnetic salient poles and affect those interpole areas of the row of permanent-magnetic poles which do not confront the soft-magnetic salient poles. A reduction of this magnetic leakage flux, and a consequent increase of the torque-generating capability, can be achieved by magnetic shielding of said interpole spaces, e.g. by placing blocks of so-called high-temperature superconducting material (a mate- rial which is superconducting at temperatures near or above the boiling point of nitrogen) in the interpole spaces of the row of salient poles. Such blocks can be made from particles of superconducting material, glued together by means of a nonconducting material, such as a plastics material.

In the illustrated embodiments of the invention the poles of both pole rows R and P are shown as rectangular poles the longitudinal edges of which are perpendicular to the direction of relative movement. However, these edges may also be skewed such that they run at an angle to the direction of relative movement. In some cases such skewing of the edges of the permanent-magnetic 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, i.e. they are imprinted when the permanent-magnetic 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.

Moreover, in motors whose dimension in the direction parallel to the air gap and perpendicular to the direction of relative motion is short in relation to the pole pitch of the winding- induced magnetic field, it may be advantageous to arrange the winding coils so that they encircle the stator yoke instead of encircling a plurality of teeth or a plurality of soft- magnetic salien-t poles. The advantage of such a coil arrangement is a reduction of the total amount of copper in the winding and or a reduction of the end-winding overhang.

Motors with axial or conical air gap surfaces may consist of two stator parts with winding means and a rotor placed between stator parts and having poles on both sides facing the air gaps formed with the stator. The rotor does not need to be equipped with flux return path (yoke) .

A variant of this type of motor may be equipped with a third stationary stator part with associated winding means and placed between the other two stator parts and having air gap surfaces on two sides . Two rotor parts on a common shaft are arranged in the two interspaces between every two stator parts. The . third stator part does not need to be equipped with a flux return path (yoke) .

It is understood that such a motor may also be provided with a fourth stator part, similar to the third one, and a third rotor part, so that the motor will have six air gaps but only two flux return paths, while in a conventional motor design two flux return paths, one on the stator part and one on the rotor part, are associated with every air gap.

It is understood that the above principle of building multi- rotor motors may be extended to any chosen number of rotors .

Claims

1. An electric machine comprising a soft-magnetic first part (11, 31) having a plurality of salient poles (14,32) which are spaced-apart along a first pole row (R) , a second part (16, 35) comprising a plurality of pairs (16N-16S, 35N-35S) of permanent-magnetic poles (16N,16S, 35N,35S) of alternating polarities arranged in a second pole row (P) which extends along the first pole row (R) in confronting relation thereto, bearing means supporting the first and second parts (11,31, 16,35) for relative movement in the direction of the first and second pole rows (R,P), and means (19, 34) for producing a magnetic field linking the first and second parts (11,31, 16,35) and having an even number of poles (N,S) which are of alternating polarities and spaced-apart along the first and second pole rows (R,P), the magnetic field travelling along the first and second pole rows (R,P) and subtending a plurality of the poles of the first and second pole rows, the number N of salient poles (14,32) of the first pole row (P) subtended by the travelling magnetic field being at least four times the number n of pole pairs (N-S) of the tra- veiling magnetic field, and the number of pole pairs (16N- 16S, 35N-35S) of the second pole row (P) subtended by the travelling magnetic field being said number N of salient poles (14, 32) plus or minus the number n of pole pairs (N-S) of the travelling magnetic field.

2. A machine according to claim 1, in which the first part (31) has twelve salient poles (32) which are uniformly spaced-apart along the first pole row (R) and form six pole groups separated by slots (33B) , a polyphase winding (34) is accommodated in the slots

(33) of the first part (31) and adapted to produce a two-pole magnetic field travelling along the first and second pole rows (R, P) , the number of permanent-magnetic pole pairs (35N-35S) of the second part (35) is different from twelve by unity, a soft-magnetic third part (36) extending along the first and second pole rows (R,P) is provided on that side of the second pole row (P) which is remote from the first pole row (R) , the second part (35) being stationary with respect to the third part, the bearing means support the first part (31) and the assembly comprising the second and third parts (35,36) for rotation.

3. An electric machine comprising a soft-magnetic first part (11) having a plurality of salient poles (14) which are spaced-apart along a first pole row (R) , a second part (16) comprising a plurality of pairs (16N- 16S) of permanent-magnetic poles (16N,16S) of alternating polarities arranged in a second pole row (P) which extends along the first pole row (R) in confronting relation thereto, a soft-magnetic third part (18) extending along the first and second pole rows (R,P) on that side of the second pole row (P) which is remote from the first pole row (R) , the second part (16) being stationary with respect to the third part (18) , bearing means supporting the first part (11) and the assembly comprising the second and third parts (16,18) for relative movement in the direction of the first and second pole rows (R,P), and a polyphase winding (19) on the third part (18) for producing a magnetic field linking the first part (11) and the assembly comprising the second and third parts (16,18) and having an even number of poles (N,S) which are of alternating polarities and spaced-apart along the first and second pole rows (R,P), the magnetic field travelling along the first and second pole rows (R,P) and subtending a plurality of the poles of the first and second pole rows, the number N of salient poles (14) of the first pole row (P) subtended by the travelling magnetic field being at least four times the number n of pole pairs (N-S) of the travelling magnetic field, and the number of pole pairs (16N-16S) of the second pole row (P) subtended by the travelling magnetic field being said number N of salient poles (14) plus or minus the number n of. pole pairs (N-S) of the travelling magnetic field.

4. An electric machine comprising a soft-magnetic first part (31) having a first pole row (R) comprising 6p pole groups separated by slots (33B) , each pole group having Y salient poles (32) , p being a positive integer and Y being a positive integer greater than 1, a second part (35) comprising M times p pairs (35N-35S) of permanent-magnetic poles (35N,35S) of alternating polar- ities arranged in a second pole row (P) which extends along the first pole row (R) in confronting relation thereto, M being an integer different from Y times six by unity, a soft-magnetic third part (36) extending along the first and second pole rows (R,P) on that side of the second pole row (P) which is remote from the first pole row (R) , the second part (35) being stationary with respect to the third part (36), bearing means supporting the first part (31) and the assembly comprising the second and third parts (35,36) for relative movement in the direction of the first and second pole rows (R,P), and a polyphase winding (34) accommodated in the slots (33B) of the first part (31) for producing a magnetic field travelling along the first and second pole rows (R,P) and linking the first part (31) and the assembly comprising the second and third parts (35,36), the travelling magnetic field having two times p poles (N,S) which are of alternating polarities and spaced-apart along the first and second pole rows (R,P) .

5. An electric machine according to claim 4, characterised in that the salient poles (32) of each pole group are spaced- apart along the first pole row (R) such that when a first salient pole of the group is aligned with a permanent- magnetic pole (35N, 35S) of the second pole row (P) , at least one second pole in the same pole group is at a centre-to- centre distance from the nearest homopolar permanent-magnetic pole (35N, 35S). which is one-twelfth of the centre-to-centre distance between neighbouring homopolar permanent-magnetic poles of the second pole row.

6. An electric machine according to claim 4 or 5, characterised in that the salient poles (32) of the first pole row (R) are uniformly spaced-apart throughout the length of the first pole row.

7. An electric machine according to any one of claims 4 to 6, characterised in that the winding (34) comprises a plurality of coils distributed along the length of the first pole row (R) , each coil surrounding at least one pair of adjacent pole groups .

8. An electric machine according to any one of claims 4-7, characterised in that the centre-to-centre distance of any two soft-magnetic salient poles (32) within each pole group is proportioned to the pole-pair pitch of the second pole row (P) such that the influence of the space harmonic flux compo- nents of the second pole row (P) on the torque of the machine is minimized for either one given order of space harmonic flux component or for the sum of two such components.

9. An electric machine according to any one of claims 1 and 3 to 8, characterised in that the bearing means (13) supports the first and second parts (11,31, 16,35) for relative rotation.