GB2081981A - Fractional horsepower ac motor having an oscillating permanent magnet armature - Google Patents

Abstract

A fractional horsepower oscillating a.c. motor e.g. for driving a toothbrush, has a cylindrical permanent magnet armature (4) and a stationary coil, e.g. composed of two equal part-coils (2,3), and orientated with its axis perpendicular to the axis of the armature. The turns of the coil are rectangular and the motor is provided with a tubular stator case (1) of rectangular cross- section surrounding the coil (2,3). The outer periphery of the coil (2,3) is adapted to the rectangular cross- section of the stator (1), and its planes of winding lie parallel with larger sidefaces of the stator (1). The stator may have a circular, or polygonal shape as in Figs. 6-11 (not shown). <IMAGE>

Description

SPECIFICATION Fractional horsepower ac motor having an oscillating permanent magnet armature The invention refers to a fractional horsepower a.c. motor having an oscillating permanent magnet armature, in particular for the oscillating drive of a toothbrush fastened to the armature shaft, with an armature which is a cylindrical, diametrically magnetised permanent magnet and having a stationary coil.

Known a.c. motors of this kind (French Patent No. 1470893) have rather complicated stator structures with several stator parts in double--T or Z form, where the bars on the inside form bent poles and surround the radially orientated middle parts of the stator coils.

In the case of a known electric toothbrush (West German Patent No. 1119819) the motor is supported in a caselike handle and exhibits an armature which oscillates at the frequency of the supply voltage and is fastened to a shaft onto which an interchangeable plug-on toi brush can be mounted. Upon connection of he a.c. motor to the a.c.

netword the plug-on toothbrush oscillates at the network frequency about the axis of the brush.

The oscillating armature of this known a.c.

motor is made in the form of a salient-pole rotor which exhibits a permanent magnet having soft iron pole shoes and is held by a return spring in its rest position. The stator exhibits two pole shanks forming poles and without windings, which are orientated in the direction longitudinal to the case and lie diametrically opposite one another; the pole shanks have arms prolonged beyond the region of the oscillating armature in the direction towards the rear end of the case, which are connected by a cross-yoke which carries the stator winding.

The object of the invention is to improve the motor efficiency as compared with known a.c. motors having oscillating armatures, in such a way that either approximately the same torque can be achieved at reduced power consumption or else a higher torque at approximately the same power consumption, and furthermore to design the construction of the motor in a particularly compact and simple way. The higher efficiency in particular allows the motor to operate at low voltage, that is, for example, at 20V, which in the very case of electric toothbrushes or else massage appliances is particularly advantageous for reasons of safety. Such a hand appliance can then preferably be fed from the network via a transformer and as regards its electrical insulation does not feed to fulful any strict regulations.

This problem is solved according to the invention in that the coil is orientated with its axis perpendicular to the armature axis and surrounds the permanent magnet, where its turns run essentially rectangularly, and that the stator is a tubular part of the case, coaxial with the axis of the armature and of magnetizable material, which encloses the coil.

This produces a simple, space-saving structure and an optimal degree of efficiency because the armature consists of a simple cylindrical permanent magnet which is surrounded over its whole length by the stationary coil which in turn lies inside the tubular case of the stator. With this compact arrangement having an armature coaxial with the stator case and a coil lying between them, which preferably takes up almost all the vacant space inside the stator-of course with the exception of the clearance required for the armature, the stray losses are restricted to a minimum. In addition this makes optimal use of the space taken by the coil and the stator.

The coil may preferably be composed of two essentially equal part-coils the plane of contact of which passes at least approximately through the axis of the armature and which may be connected in series or in parallel, depending upon the supply voltage employed.

In a preferred embodiment the tubular case of the stator has a rectangular cross-section having a long and a short pair of sidefaces, where the long sidefaces preferably form poles and thereby a geometry having variable reluctance dependent upon the position of the armature, without special pole shoes or projecting poles of some other shape being provided. In this way it can be achieved that the torque exerted upon the armature by the magnetic field of the coils is strengthened by the static torque origintating from the rectangular stator geometry, which tries to turn the armature so that the direction of magnetization of the permanent magnet is lying in parallel with the short sidefaces of the stator.

In order to make optimum use of this favourable effect, the axis of the coils must be orientated in parallel with the short sidefaces of the stator and the armature in its rest position must be held by suitable return means, for example, by a return spring, in a position in which the direction of the diametrical magnetic field of the armature is lying perpendicular to the axis of the coil and thereby in parallel with the long sidefaces of the stator.

A similar effect is obtained by a stator with octagonal cross-section with a pair of opposite sidefaces which are larger than the other sidefaces. The same effect can also be achieved if a cylidrical stator case is chosen, having two projecting stator poles lying diametrically opposite one another on its inner periphery.

In the case of a particularly simple embodiment of the motor in accordance with the invention the stator consists merely of a cylin drical sleeve without projecting poles; in this case the symmetrical stator geometry brings about a constant reluctance so that no additional static torque acts upon the armature.

The present invention will now be described by way of example with refence to the accompanying diagrammatic drawings in which only electrical components necessary for an understanding of the invention are shown and in which: Figure 1 is an exploded perspective view of a first embodiment of a motor according to the present invention; Figure 2 is a perspective view of the motor shown in Fig. 1 in an assembled state but with the stator shown cut away; Figure 3 is a perspective view of one of the part-coils of the motor shown in Figs. 1 and 2; Figure 4 is a perspective view of the other part-coil in partial cross-section; Figure 5 is a perspective view of the motor when assembled Figure 6 is an exploded perspective view of a second embodiment of motor according to the invention having a cylindrical stator case;; Figure 7 is a perspective view of the motor shown in Fig. 6 in an assembled state but with the stator shown cut away; Figure 8 is a perspective view of one of the part-coils of the motor shown in Figs. 6 and 7; Figure 9 is a perspective view of the other part-coil in partial cross-section; Figure 10 is a perspective view of the second embodiment of the motor when assembled; and Figure 11 is a cross-section through an octagonal two-part stator of a third embodiment of motor according to the invention.

With reference to Fig. 1, the stator 1 of the a.c. motor shown therein consists of a tubular part of the case having a rectangular crosssection, which case is made of magnetizable material and has long or respectively larger sidefaces 1 a and the short or respectively smaller sidefaces 1 b. The armature fastened to the armature shaft 5 consists of a cylindrical permanent magnet 4 having diametrical magnetization. The poles are indicated in Fig.

1 by the letters N and S. The armature shaft 5 may consist of magnetic material.

The permanent magnet 4 is surrounded over its whole length by a coil consisting of two part-coils 2 and 3, the axis of which runs perpendicular to the armature shaft 5 and perpendicular to the long sidefaces 1 a of the stator 1 and the direction of winding of which is indicated in Fig. 2 by arrows. The turns run essentially rectangularly and are adapted to the cross-section of the cylindrical permanent magnet 4, passing through the axis.The part coils 2 and 3 are constructed the same and rest against one another with their faces which point inwards and are orientated in parallel with their plane of winding, in a plane passing through the axis of the armature; the edges of the part-coil bodies, running diametrically to the armature, have in the centre approximately semi-circular recesses 2a, 2b and 3a, 3b respectively, which in the assembled state complete openings for the armature shaft 5 to pass through. The outer periphery of the two part-coils 2 and 3 is adapted to the inner periphery of the rectangular stator 1, which in this way holds the coils essentially positively, so that the rectangular coil openings 2c and 3c respectively lie in front of the long side faces 1 a of the stator. The inner periphery of the part-coils 2 and 3 is adapted to the cylindrical shape of the permanent magnet 4.In this way the permanent magnet 4 is surrounded by the part-coils 2 and 3 like a box and with only slight clearance, whereby the vacant space between armature and stator is in practice completely filled by the coilturns. If necessary the coil arrangement can also be made in such a way that they only partially fill the vacant space within the stator.

Instead of two part-coils which facilitate the assembly of the motor, in principle only a one-part coil may also be provided.

The end faces of the stator 1 are closed by two rectangular flanges 6 and 7 adapted to the cross-section of the stator opening, and exhibiting in the centre bearing openings 8 and 9 respectively for the armature shaft 5 to pass through, and may likewise consist of magnetic material.

The motor described may typically have an axial length of about 55mm, a height of about 20mm and a width of about 1 5mm. The power consumption in operation at 50 Hz or at 60 Hz may typically lie bwteeen 1.3 and 1.6 W.

For reduction of eddy current losses it may be advantageous to provide in the walls of the stator 1 as shown in broken lines in Fig. 1, slits 10 which run in parallel with a plane orientated perpendicularly to the axis of the armature.

The motor described works as follows: the permanent magnet armature 4 in the rest position is forced by return means, for example, a return spring 20 surrounding the armature shaft 5 as indicated in Fig. 2, into a position which is shown in Fig. 2 and in which the direction of the diametrical magnetic field of the armature is lying perpendicular to the axis of the coil.If the part-coils 2 and 3, which may be connected in series or in parallel, depending upon the level of the supply voltage, are now energized by an alter nating current, the alternating magnetic field H generated cooperates with the magnetic moment m of the armature magnet 4 in such a way that an alternating torque is exerted upon the armature of the value M = m H sin a, where a is the angle between the direction of the magnetic field H and that of the armature magnetization and in the position of rest of the armature illustrated in Figs. 2 amounts to 90 . The armature thereby experiences an oscillating deflection out of its rest position, so that it oscillates at the frequency of the supply voltage.

The rectangular shape of the case 1 of the stator now confers two additional advantageous effects. Firstly this stator geometry for a given amount of copper in the wire of the coil allows the generation of a relatively high field in the direction axial to the coil. Secondly the magnetic circuit exhibits a reluctance which depends relatively strongly upon the angle between the axis of magnetization of the armature magnet 4 and the major axis of symmetry of the rectangular cross-section of the stator, since the two long or respectively large sidefaces 1 a of the stator 1 have the function of preferred poles.The action of this variable reluctance consists in the following: When the armature is deflected a little out of its rest position shown in Fig. 2, which forms an unstable balance, in the absence of a coil field a static torque acts upon it, which tries to turn the armature ir .0 that position in which the reluctance of le magnetic circuit is least; in this position which differs from the rest position by an angle of 90 , the direction of magnetization of the armature magnet 4 is aligned with the minor axis of symmetry of the cross-section of the stator. This static torque advantageously strengthens the normal torque generated by the coil field and thereby favours the oscillating motion of the armature.

This effect of strengthening the useful torque may also be described by the statement that in the expression specified above for the torque M the value m, that is, the magnetic moment of the armature magnet, depends upon the aforesaid angle a.

Under given electrical and magnetic conditions the value of the angle of oscillation of the armature depends upon the external loading and upon the tuning of the mechanical oscillating system, that is, upon the mass of the armature and the object fastened to it, upon the characteristic of the return means and upon the damping or friction respectively.

Under certain circumstances this angle may amount to 180 and possible even exceed this value. That is, if no measures are provided for limiting the angle of the armature and therefore the armature is free, the motor can behave as a rotating singlephase synchronous motor. But for the special application of the motor to the drive of a toothbrush the angle of oscillation of the armature and hence the oscillatory motion of the toothbrush about the axis of the brush is limited to an angle of about 60 or less.

The second embodiment of a motor in accordance with the invention in accordance with Figs. 6 to 10 differs from the first embodiment in that the stator 11 consists of a cylindrical sleeve, that correspondingly the outer periphery of the coil which is again formed from two equal part-coils 1 2 and 13, in adaptation to the inner periphery of the stator is cylindrical and that the flanges 1 6 and 1 7 at the ends, which again exhibit bearing openings 1 8 and 1 9 for the armature shaft 5 to pass through, are circular in adaptation to the cross-section of the openings in the stator 11. The armature consists again as in the case of the first embodiment of a cylindrical permanent 4 fastened onto the armature shaft 5 and having diametrical magnetization.

With the exception of the external contour of the coil the two part-coils 1 2 and 1 3 are constructed, orientated and arranged in the stator 11 exactly as in the case of the first embodiment.

In order to achieve the advantageous effect of an additional static torque which, as explained with the aid of the first embodiment, originates in the case of the motor there described from the rectangular cross-section of the stator, in the case of the second embodiment as in Fig. 6 two separate stator pole pieces 14 and 1 5 of magnetizable material are provided, which lie diametrically opposite one another and rest against the inner periphery of the stator 11 or respectively are fastened to it.According to Fig. 6 these two stator pole pieces 14 and 1 5 have the shape of segments of a hollow cylinder and are adapted to the contour of the openings 1 2c and 1 3c respectively in the part-coils 1 2 and 1 3 respectively so that in the assembled state of the motor they engage in these openings in the coil.These two stator poles 14 and 1 5, projecting radially inwards, which may also be made in any other way or fastened or moulded onto the inner periphery of the stator, bring about in practice the same effect of a static torque as has been described with the aid of the first embodiment and has the result that the armature magnet 4 as soon as it is deflected a little out of its rest position shown in Fig. 7 experiences a torque which tries to turn it by 90 out of its rest position and which is superimposed upon the torque generated by the coil field. Besides this effect of a variable reluctance the projecting stator poles bring about a strengthening of the coil field.

By waiving the aforesaid effect of a static torque an a.c. motor in accordance with the invention may also exhibit a stator which consists only of a cylindrical case 11 (Fig. 6) and exhibits no projecting stator poles. This construction of a motor is particularly simple and has the advantage of a neutral balance as regards the rotation of the armature.

Fig. 11 shows the stator 21 with octagonal cross-section of a further embodiment of a motor in accordance with the invention, in the case of which, analogues to the example given in Fig. 1, the outer peripheries of the two halves of the coil (not shown) are adapted to the octagonal internal periphery of the stator and advantageously take up practically all the vacant space between stator and armature. In order to facilitate production and assembly of the motor, in the example given in Fig. 11 two equal box-like halves of the stator 21' and 21" are provided, which are joined in a plane which at least approximately passes through the longitudinal axis of the stator 21 and are fixed together in any manner at all, which can be, for example, with the aid of flanges adapted to the octagonal crosssection, which, like the flanges as in Fig. 1, form the end faces of the Stator.The two opposite side faces 21a of the stator 21 are larger than the other sidefaces and the axis of the coil is orientated perpendicular to these large sidefaces, so that equal advantageous effects are produced, as explained for the rectangular stator 1 shown in Fig. 1 with its pair of long sidefaces 1 a and its pair of short sidefaces 1 b. The distance between sidefaces orientated perpendicular to the large sidefaces 21a may generally be between 10 and 50%, preferably about 30% greater than the distance between the large sidefaces 21 b. This applies also to the measurement of the rectangular stator shown in Fig. 1.

An octagonal form of stator as in Fig. 11 brings with it the additional advantages that the motor can be particularly well and spacesaving accommodated in a grip-type casing with oval cross-section, such as used typically for electric toothbrushes, and that the maximum radial distance between armature and stator-sleeve is reduced by comparison with the diagonal dimensions in the case of a rectangular stator; in this way the volume of air or copper within the magnetic circuit is reduced and thus the field is enlarged. In addition, by amplifying the static torque a more favourable characteristic curve of the torque as a function of the armature angle is obtained.

Of course, also in the examples as shown in Figs. 1 and 6, the stator 1 or 11 can be composed of two halves, analogues to the stator 21 according to Fig. 11.

The preferred application of the a.c. motor in accordance with the invention is the building into the handlelike case of an electric toothbrush having interchangeable plug-on toothbrushes; because of the relatively low power consumption such an electric toothbrush can then be supplied at low voltage via a transformer from the network, so that in practice no problems arise of electrical insulation. But the motor in accordance with the invention is also suitable for other purposes, for example, for massage appliances and for any application in the technical fields which rapid periodic motion plays a part such, for example, as in small pumps.

The invention is not restricted to the embodiments described but as regards the construction and assembly of the individual parts, especially as regards the cross-section of the stator, which may also be oval, square or generally polygonal, and as regards the coils, allows of manifold variants.

Claims (14)

1. A fractional horsepower a.c. motor having an oscillating permanent magnet armature for the oscillating drive of a device attached to the armature shaft, and a relatively stationary coil, characterised in that the armature is a cylindrical diametrically magnetized permanent magnet, the coil is orientated with its axis perpendicular to the longitudinal axis of the armature and surrounds the permanent magnet, the winding turns of the coil defining rectangular shapes and the stator comprises a magnetisable tubular portion of a case which encloses the coil and which is arranged coaxial with the axis of the armature.

2. A motor as claimed in claim 1, in which the coil comprises two at least approximately equal part-coils which rest against one another at faces orientated substantially parallel with their plane of wind in a plane passing at least approximately through the axis of the armature, the edges of the part-coil bodies running approximately diametrically to the armature defining at their centre semicircular recesses through which the armature shaft can pass.

3. A motor as claimed in claim 2, in which the part-coils are connected in series or in parallel depending upon the level of the supply voltage.

4. A motor as claimed in any one of claims 1 to 3, in which the tubular stator defines a square, rectangular or octagonal cross-section, and the axis of the coil is orientated in parallel with one pair of opposite sidefaces of the stator.

5. A motor as claimed in claim 4, in which the stator has a rectangular or octagonal cross-section in which the sidefaces of one pair of opposed sidefaces are larger than the other sides and form poles, the axis of the coil being orientated perpendicular to the large sidefaces.

6. A motor as claimed in claim 5, in which the distance between the sidefaces of the stator orientated perpendicular to the larger sidefaces is between 10% and 50% larger than the distance between the larger sidefaces.

7. A motor as claimed in any one of claims 1 to 3, in which the tubular stator defines a cylinder.

8. A motor as claimed in claim 7, in which the cylindrical stator has stator poles projecting diametrically opposite one another from stator's inner periphery and lying along an axis which is orientated at least approximately in parallel with the axis of the coils.

9. A motor as claimed in claim 8, in which the stator poles are separate members each in the form of segments of a hollow cylinder, and which members engage in substantially rectangular openings formed at the sides of the coil or respectively part-coils.

10. A motor as claimed in any one of claims 1 to 9, in which the outer periphery of the coil or respectively part-coils is adapted to the shape of the inner periphery of the stator and the winding turns take up approximately all the vacant space between armature and stator.

11. A motor as claimed in any one of claims 1 to 9, in which the endfaces of the tubular stator are closed by flanges the shape of which is adapted to the cross-sectional shape of the stator and which define bearing openings for the armature shaft.

1 2. A motor as claimed in claim 1, in which the tubular part of the case of the stator provided with slits for the reduction of eddy currents which slits are arranged to run in parallel with a plane orientated perpendicular to the axis of the armature.

1 3. A motor as claimed in any one of claims 1 to 12, in which in the rest position of the armature is determined by return means, the diametrical magnetic field of the armature lying substantially perpendicular to the axis of the coil.

14. A motor as claimed in any one of claims 1 to 13, in which the tubular stator is composed of two box-like halves which rest against each other in a plane which passes at least approximately through the longitudinal axis of the stator.

1 5. A fractional horsepower a.c. motor substantially as hereinbefore described with reference to Figs. 1 to 5 or Figs. 6 to 10 or Fig. 11 of the accompanying drawings.