Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K21/00—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets
  • H02K21/38—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with rotating flux distributors, and armatures and magnets both stationary
  • H02K21/44—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with rotating flux distributors, and armatures and magnets both stationary with armature windings wound upon the magnets
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K21/00—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets
  • H02K21/12—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with stationary armatures and rotating magnets
  • H02K21/22—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with stationary armatures and rotating magnets with magnets rotating around the armatures, e.g. flywheel magnetos
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K1/00—Details of the magnetic circuit
  • H02K1/06—Details of the magnetic circuit characterised by the shape, form or construction
  • H02K1/12—Stationary parts of the magnetic circuit
  • H02K1/14—Stator cores with salient poles
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K16/00—Machines with more than one rotor or stator
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K2201/00—Specific aspects not provided for in the other groups of this subclass relating to the magnetic circuits
  • H02K2201/06—Magnetic cores, or permanent magnets characterised by their skew
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K3/00—Details of windings
  • H02K3/04—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors
  • H02K3/28—Layout of windings or of connections between windings

Definitions

• 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 .
• a high torque density is an essential property from an economical and or technical point of view.
• 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.
• the highest torque density can be achieved with synchronous motors of the permanent-magnetic field type fed from an electronic power supply.
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 .
• the inner or first part is the rotor while the outer or third part is the stator which carries the winding.
• the inner or first part is the stator while the outer or third part is the rotor.
• the inner or first part i.e. the rotor
• the inner or first part is designated by 11. It is secured to a rotor shaft 12 supported in bearings 13.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• pole groups comprise more than two soft-magnetic salient poles
• 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.
• phase difference for the Xth space harmonic component is X times the phase difference for the fundamental component.
• 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.
• 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.
• 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.
• 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 may be constant as shown in Figs. 2A-2C, or constant only within each pole group.
• 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.
• 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.
• 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 .
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• demarcation lines demarcation zones
• winding coils 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) .
• 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.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Synchronous Machinery (AREA)
• Iron Core Of Rotating Electric Machines (AREA)

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• 1997
  • 1997-02-12 SE SE9700477A patent/SE9700477L/xx not_active Application Discontinuation
• 1998
  • 1998-02-12 EP EP98904487A patent/EP0960465A1/de not_active Withdrawn
  • 1998-02-12 WO PCT/SE1998/000247 patent/WO1998036487A1/en not_active Application Discontinuation

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