GB2037092A - Electric motor - Google Patents

Abstract

A fractional-horsepower motor has two permanent magnet segments (11) whose thickness decreases linearly towards the ends of the segments. Each segment has two portions (15, 16) of differing remanence and coercive field strength for the purpose of increasing the resistance to demagnetisation by the transverse field of the armature, the portion (16) made from the higher coercive magnetic material (2) being disposed at the trailing end of the segment. Each segment is dimensioned such that the thickness 1M min at the two ends of the segment, and the position ( alpha 1), relative to the centre of the segment, of the location (17) at which the two segmental portions (15, 16) abut against one another, satisfy certain specified equations. <IMAGE>

Description

SPECIFICATION Electric motor The invention relates to electrical machines as is particularly concerned with fractional horsepower electric motors.

An electrical machine is already known which has a stator including at least two permanent magnets which are arranged diametrically opposite one another to form an air gap with a rotatable armature disposed therebetween, each of which permanent magnets comprises a substantially arcuate segment whose thickness decreases towards its two ends, and each segment being arranged such that the decrease in the thickness of the segment corresponds to an equal increase in the air gap.

In such known, permanently energised machines or motors, the air gap, increasing towards the ends of the segments, serves to suppress the magnetically generated sounds of the machine or motor. The increased tooth flux increases the specific force on the armature teeth, particularly in the case of permanent magnets having high magnetic values and thus a high energy density.

The air gap induction, increased as the energy density of the permanent magnets increases, at the same time results in a considerable, abrupt change of flux which results in a considerable increase in the sounds caused by slot ripple frequency. The technique of increasing the air gap towards the ends of the segments serves as an effective means of reducing these magnetically generated sounds to a normal level since the flux at the edges of the magnet is thereby decreased.

However, the stress on the permanent magnet in a machine or a motor, which leads to demagnetisation by the transverse field of the armature, increases linearly from the centre of the magnet to that edge of the magnet which trails in the direction of rotation of the armature. On the other hand, the capacity of the permanent magnet to withstand stress, that is to say, its resistance to demagnetisation by the transverse field of the armature, decreases as the thickness of the magnet decreases. This means that the resistance of the magnet to demagnetisation is decreased by the aforegoing measures for decreasing the magnetically generated sounds.

Permanent magnets are already known which, for the purpose of increasing the resistance to demagnetisation, have two segmental portions of magnetic materials of different magnetic properties, one of which segmental portions has a higher remanence and the other has a higher coercive field strength. Since the risk of demagnetisation exists particularly at the trailing edge of the permanent magnet, where it is intensified during starting at low temperature, since, as mentioned above, the transverse field of the armature is at a maximum at the trailing edge of the magnet, it has been the practice to dispose the segmental portion made from the higher coercive magnetic material, which, of course, has a smaller remanence than the rest of the magnetic material, at that end of the segment of the permanent magnet which forms the trailing edge of the magnet.In order to optimise the permanent magnet, the proportions of the volumes of the segmental portions have been chosen such that the ratio of the volume of the segmental portion comprising a higher coercive magnetic material to the total volume of the segment is equal to the ratio of the coercive field strength of the magnetic material of higher remanence to the coercive field strength of the magnetic material of higher coercivity.Permanent magnets of this kind are referred to as "two-component magnets In the machine or motor described initially, no useful result is obtained by attempting to compensate for the resistance of the permanent magnet to demagnetisation, reduced by the tapering of the permanent magnet towards the end thereof, by constructing the permanent magnet in the form of a two-component magnet described above, that is to say, by disposing a segmental portion of high coercive magnetic material at that end of the segment which forms the trailing edge of the magnet. It has been found that a permanent magnet of this kind is demagnetised at the boundary surfaces of the two magnetic materials, that is to say, at the locations at which the two segmental portions abut against one another.

In accordance with the present invention, there is provided a fractional-horsepower electric motor, having a stator including at least two permanent magnets which are arranged diametrically opposite one another to form an air gap with a rotatable armature disposed therebetween, each of which permanent magnets comprises a substantially arcuate segment whose thickness decreases towards its two ends, each segment being arranged such that the decrease in the thickness of the segment corresponds to an equal increase in the air gap, and each segment having two segmental portions made from magnetic materials of different magnetic properties, one of which magnetic materials has a higher remanence than the other and the other has a higher coercive field strength than the first, the segmental portion of higher coercive magnetic material being arranged at that end of the segment which forms the trailing edge of the magnet with respect to the relative movement between the permanent magnets and the armature, the thickness (1 M)s measured in a radial direction, at the two ends of each segment satisfying the equation:

and the location at which the two segmental portions of each segment abut against one another being spaced from the centre of that segment by an angular distance (oti) which satisfies the equation

where: w .I, = transverse flux of the armature, HG2 = limiting field strength of the higher coercive magnetic material of the segmental portion, B,2 = remanent induction of the higher coercive magnetic material of the segmental portion, fi = angular length of the segment y.0 = absolute permeability, 1 L ma, = air gap at the ends of the segment, 1,, = air gap at the centre of the segment, A1, = increase in the. air gap at the ends of the segment, HG1 = limiting field strength of the higher remanent magnetic material of the segment, and B,1 = remanent induction of the higher remanent magnetic material of the segment.

In contrast to the known machines described above, the latter machine has the advantage that, without any change in the generation of sound by the machine or the motor, the resistance of the permanent magnets to demagnetisation by the transverse field of the armature is considerably increased. The design of the permanent magnets is optimised when the specified dimensions are borne in mind. The energy content of the magnets is used to a maximum extent without exceeding the limiting field strength of the two magnetic materials, that is to say, that field strength which, when exceeded, results in permanent demagnetisation.

The invention will be further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-section of the armature and magnetic frame of a fractional-horsepower motor in accordance with the invention; and Figure 2 is a graph showing the demagnetisation curves of the magnetic materials contained in the permanent magnets of Fig. 1.

Referring to Fig. 1, only the parts of the fractional-horsepower motor essential for understanding the invention are illustrated. A magnetic flux return ring 10 of ferromagnetic material is shown in Fig. 1 and is a part of a fixed magnetic frame. The inside of the magnetic flux return ring 10 carries two diametrically opposite permanent magnets 11. An armature 1 2 is arranged in the interior of the magnetic frame and normally carries the armature winding in grooves (not illustrated) distributed around the circumference. The armature 1 2 rotates in an anti-clockwise direction, as is indicated by the arrow a in Fig. 1.

Each permanent magnet 11 comprises a substantially arcuate segment 1 2. The thickness of each segment 13, measured in a radial direction, decreases towards the end of the segment.

The width of the air gap 1 4 formed between the armature 1 2 and the segments 1 3 has the value 1,0 in the centre of the segments, and the value 1, may at the end of the segments. The thickness of the two segments 1 3 is 1 M ma, in the centre of the segments, and 1 Mmi, at each of the segments. The segments 1 3 are constructed and disposed relative to the armature 1 2 such that the decrease in the thickness of the segments from 1 M max in the centre of the segment to 1M mi, at the end of each segment corresponds to an equal increase in the air gap 1,0 from the centre of the segments to 1 lax at the end oi the segments. Thus, the total of the air gap and the thickness of the segments is constant at every location on the segments 13, so that the following applies: 1Mmin+ 1 L max = const. (1), and 1M max+ 1L min = const. (2).

Thus, the two segments 13 are symmetrical with respect to their configuration. Each segment 13 has two segmental portions 15 and 16 made from magnetic materials of differing magnetic properties. As will be seen in Fig. 2, one magnetic material, indicated by the reference numeral 1 in Fig. 2, has a higher remanence than the other magnetic material indicated by the reference numeral 2 in Fig. 2. On the other hand, the magnetic material indicated by the reference numeral 2 has a higher coercive field strength than the magnetic material indicated by the index 1. The segmental portion 15 is made from a higher remanence magnetic material in accordance with the reference numeral 1 in Fig. 2.The segmental portion 16 of higher coercive magnetic material in accordance with reference numeral 2 in Fig. 2 is arranged at that end of the segment 13 which forms the trailing edge of the magnet with respect to the rotation of the armature (arrow a in Fig. 1).

The two segments 13 are dimensioned such that the thickness 1 lint measured in a radial direction, of the two ends of the segments satisfy the equation:

in which: w . Ie = Transverse flux of armature HG2 = Limiting field strength of the higher coercive magnetic material of the segmental portion 1 6 (Fig. 2) B.2 = Remanence induction of the higher coercive magnectic material of the segmental portion 16 (Fig. 2) ss = Angle of overlap of segment 13 (Fig. 1) y0 = Absolute permeability 1 Lmax= Air gap at the ends of the segments (Fig. 1).

The term "limiting field strength" refers to that field strength of the magnetic material up to which the magnetic material can be demagnetised without it being irreversibly demagnetised.

The limiting field strength is determined by the break in the function B F(H), as is shown in Fig.

2.

The thickness 1 M of the segment 13 decreases linearly from the centre of the segment to the ends of the segment. Following from what has been stated above, the air gap 1 L also decreases linearly from the centre of the segment to the ends of the segment.

The two segmental portions 15 and 16 are in contact with one another at a common location 1 2. This abutment location 17 is spaced from the centre of the segment by an angular distance a, which satisfies the equation

in which 1Lo = Air gap in centre of segment A1,= Increase in air gap at the ends of the segment HG1 = Limiting field strength of the higher remanence magnetic material of the segmental portion 1 5 (Fig. 2) B,1 = Remanence of the higher remanence magnetic material of the segmental portion 15 (Fig. 2), and, in accordance with what has been stated above, the following applies:: M + 1 M = 1 M min + 1 L max = const = lMmia+(lLO+AlL) (5) With the above-described dimensioning of the two segments 13, the following results for the volume V2 of the segmental portion 16 relative to the total volume Vgas of each individual segment 13:

in which

The above-described fractional-horsepower motor having the specially dimensioned permanent magnet 11 will be described again with reference to a numerical example: Let the following be assumed: w .Io = 4130 A ss=140 1 LO = i0.05 cm Al1 =0.15cm HG1 = 230 kA/m 2300 A/cm Bri = 400 mT = 4000 G HG2 = 500 kA/m = 5000 A/cm BI2=230 mT=2300 G In accordance with equation (3), these assumed values result in

for the thickness 1M mi, at the ends of the segment 1 3.

The following thus results in accordance with equation (5).

1,+ 1M=0.696 cm=const.

In accordance with equation (4), the abutment location 17 is spaced from the centre of the segment by an angular distance a, which is

In accordance with the equation (7), th,e following ensues for the segment 13 thus dimensioned:

and, in accordance with equation (8)

In accordance with equation (6), the following then applies to the volume V2 of the segmental portion 1 5 relative to the total volume Vga5 of the segment 15:

Thus, the volume V2 of the segmental portion 1 6 is 30% of the total volume of each segment 13, and the volume V, of the segmental portion 1 5 is 70% of the total volume of each segment 13.

It has been found that, with the same sound level from the motor, the permanent magnet 11 dimensioned in accordance with the above numerical example exhibits twice the resistance to demagnetisation by the transverse field of the armature of a magnet of equal dimensions which is made from a single material. It will be appreciated that this enormous increase in resistance of the magnet is obtained at the expense of a slight loss of induction which, however, bears no compression to the advantage obtained.This loss of induction occurs by virtue of the fact that the magnetic material of the segmental portion 1 6 has a smaller remanence than the magnetic material of the segmental portion 15, that is to say, an abrupt decrease in induction occurs at the location 1 7 at which the two segmental portions 15, 1 6 abut against one another.

Consequently, the effective remanence induction of the permanent magnet 11 of Fig. 1 is somewhat smaller than the remanence of a magnet which is made from an homogeneous magnetic material which has the same remanence as the magnetic material of the segmental portion 1 5. In the numerical example given above, this effective remanence induction of the permanent magnet 11 is approximately 89% compared with a magnet which is made from a single material and which is of equal dimensions and has a high remanence of 400 mT, that is to say, the loss of induction is 11%.

The advantages of the above-described fractional-horsepower motor, whose permanent magnets 11 are dimensioned as described above, have already been mentioned initially. These advantages are: low magnetic sound, high resistance of the permanent magnets to demagnetisation by the transverse field of the armature, and a minimum volume of the permanent magnets.

Claims (3)

1. A fractional-horsepower electric motor, having a stator including at least two permanent magnets which are arranged diametrically opposite one another to form an air gap with a rotatable armature disposed therebetween, each of which permanent magnets comprises a substantially arcuate segment whose thickness decreases towards its two ends, each segment being arranged such that the decrease in the thickness of the segment corresponds to an equal increase in the air gap, and each segment having two segmental portions made from magnetic materials of different magnetic properties, one of which magnetic materials has a higher remanence than the other and the other has a higher coercive field strength than the first, the segmental portion of higher coercive magnetic material being arranged at that end of the segment which forms the trailing edge of the magnet with respect to the relative movement between the permanent magnets and the armature, the thickness (1M), measured in a radial direction, at the two ends of each segment satisfying the equation:

and the location at which the two segmental portions of each segment abut against one another being spaced from the centre of that segment by an angular distance (a,) which satisfies the equation

where:: w 1x = transverse flux of the armature, HG2 = limiting field strength of the higher coercive magnetic material of the segmental portion, Br2 = remanent induction of the higher coercive magnetic material of the segmental portion, ss = angular length of the segment, = = absolute permeability, 1 L max = air gap at the ends of-the segment, 1Lo = air gap at the centre of the segment, AlL = increase in the air gap at the ends of the segment, HG1 = limiting field strength of the higher remanent magnetic material of the segment, and Bri = remanent induction of the higher remanent magnetic material of the segment.

2. An electric motor as claimed in claim 1, in which the volumes of the segmental portions are determined such that the ratio of the volume (V2) of the segmental portion made from the higher coercive magnetic material to the total volume (Vges) of the segment satisfies the equation:

3. An electric motor substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.