GB2227793A

GB2227793A - Miniature axial fan

Info

Publication number: GB2227793A
Application number: GB9006293A
Authority: GB
Inventors: Siegfried Harmsen
Original assignee: Papst Motoren GmbH and Co KG
Current assignee: Ebm Papst St Georgen GmbH and Co KG
Priority date: 1985-11-08
Filing date: 1990-03-21
Publication date: 1990-08-08
Anticipated expiration: 2006-11-10
Also published as: GB9006293D0; GB2227793B

Abstract

A miniature axial fan (Figs 3, 4) has a central, impeller-driving electric motor 6, an impeller 2 secured to the rotor of the motor and surrounded by a casing having an inner wall 3 which is cylindrical for a substantial portion of its length and then diverges in the flow direction towards the outflow end A. The drive motor 6 is carried by webs, 5 and the number of impeller blades 7 differs from the number of webs 5. In the case of an impeller diameter at most 60 mm, the fan blades 7 (Fig 7) are irregularly distributed and constructed in such a way that adjacent the outflow end they have a large setting angle of 70 to 90 DEG , and, in a radial plan view on the rotation axis, the curvature of the blades from the inflow-side is initially limited and then becomes more marked towards the outflow-side blade edge. The webs 5 and casing (1) may be integrally formed with a flange 13 and tube (17) (Fig. 1) by moulding, for supporting the bearings for a shaft 12 of the motor rotor. The flange 13 may house commutator components for the motor 6. Various alternative motor and impeller hub constructions are disclosed as are further structural details of the moulded casing. A method of making the impeller blades is also referred to. <IMAGE>

Description

MINIATURE AXIAL FAN The invention relates to miniature axial fans of the type having a central, impeller-driving motor, the impeller being surrounded Dy a casing in which the inflow-side casing inner wall is cylindrical to beyond the axial centre of the casing and then diverges in the flow direction towards the outflow-side, the drive motor being carried by outflow-side webs, and the number of impeller blades differing from the number of webs.

In the case of such small axial fans, a low noise level and an adequate air capacity for the intended use are required in addition to the often necessary axial compactness. This is difficult to achieve in view of the small external dimensions, where a change of a few millimetres to one dimension in favour of one characteristic often has a serious effect on the other dimensions and therefore on all the characteristics.

European patent application 0100078 discloses an axial-flow fan suitable for the construction of compact axial fans with an impeller diameter of below 100 mm. The inventive combination of this application uses part of the means known therefrom, in conjunction with further measures which are especially effective in an axial fan of the aforementioned small size.

The object of the invention is to provide a very small, relatively compact axial fan having an impeller driven by a concentric coaxial drive motor constructed in such a way that it has a relatively good air capacity and low noise level.

According to the present invention, there is provided a miniature axial fan having a central, impeller-driving motor, the impeller being surrounded by a casing in which the inflow-side casing inner wall is cylindrical to beyond the axial centre of the casing and then diverges in the flow direction towards the outflow-side, the drive motor being carried by outflow-side webs, and the number of impeller blades differing from the number of webs, wherein in the case of an impeller diameter of at most 60 mm, the fan blades are constructed in such a way that on the outflow-side they have a large setting angle of 70 to 90"r and, in a radial plan view on the rotation axis, the curvature of the blades from the inflow-side is initially limited and then becomes more marked towards the outflow-side blade edge.

In the case of an impeller diameter of 45 mm- the said setting angle preferably is approximately 80 Usually the casing is square on the outflow side and preferably the increase in the flow cross-section increases in the flow direction towards the four contour corners of the square casing. It is especially preferred hat the said cross-sectional increase is continuous and, independently, that the casing has four webs at its outflow-cide, suitably arranged in the respective centres of the sides of the square contour.In the vicinity of the square corners fixing columns advantageously are connected to a square flange plate, said columns extending up to the entry plane and provided over their entire axial length with a respective through-bore, which bores consequently extend from the in. ow plane to the outflow plane.

Preferably, the number of blades cannot be diviced by the number of webs and suitably there are seven blades and four webs.

In the case of a square side length of 50 x 50 mm, the limited curvature preferably corresponds to a radius of curvature of approximately 60 mm and the marked curvature corresponds to a radius of curvature of approximately 15 mm.

The leading edges of the blades advantageously can be set back from the entry plane by a few millimetres and the trailing edges of the blades terminate an axial distance from the webs of a few millimetres. When the dimensions are as specified in above, the said spacings are 3 mrl from the entry place and 4 mm from the webs respectively.

The blades preferably are distributed in non-uniform manner around the circumference of the hub.

It also is preferred that the intake end of the casing inner wall has a radius of approximately 4 mm and, independently that the impeller hub has a diameter of at least half the diameter of the complete impeller.

Usually, the driving motor will be a brushless d.c. motor with an external rotor and a one or two-pdlse operation. The driving motor can be three-phrase and have a microprocessor control.

Preferably, in the vicinity of its seat, the impeller hub is tapered conically towards the entry plane. Usually, the impeller diameter is below 50 mm, suitably 40 mm.

In one preferred embodiment, the driving motor has a rare earth magnet and the diameter of the external rotor casing is reduced in step-like manner in the vicinity of the closed rotor bottom for the press fit reception of the impeller hub, the hub external diameter roughly corresponding to the external diameter of the rotor.

The invention is described hereinafter relative to embodiments and the attached drawings, in which: Figure 1 is a section, along section line I-I of Figure 2, of a casing for a fan in accordance with the present invention; Figure 2 is a plan view, in the direction of arrow II of Figure 1, of the casing of Figure 1.

Figures 1 and 2 which are twice natural size, i.e. a scale of 2:1, show a fan casing, in which can be inserted an impeller with a coaxial drive motor, as shown in Figures 3 or 4.

Figures 3, 4, and 5 show, in double the actual size, embodiments of fans of the present invention having external rotors of the drive motor and impeller hubs of different constructions. In each case, the commutating electronics are located in the flange.

Figure 6 shows an embodiment with an additional or alternative fixing of the plastic impeller to the external rotor casing by thermal compression. The commutating electronics are located on an annular support below the external rotor cap.

Figures 7 to 8 show details of the blade dimensioning for the impeller, e.g. according to Figure 3.

Figure 1 shoes an anyled longitudinal section through the casing of a miniature fan according to the invention. Concentric to rotation axis 10, a bearing tube 17 is constructed in stepped manner for the engagement of bearings and the positioning of a stator body. The bearing tube 17 is integral with a flange 13, to which are connected for webs 5 distributed at 90" angular steps. The webs 5 extend radially to the centres of respective sides of a square flange plate 15, from which a casing tube 4 having an inner wall surface 3 extends axially to the entry plane E.

Fixing columns 14 secured to flange plate 15 extend from exit plane A axially up to entry plane E and are located radially outside the casing tube 4. Fixing bores 16 extend concentrically in columns 14. Parts 17, 13, 5, 4,'l, 14, 15 are constructed integrally as a single injection moulding. The columns 14, which extend the full axial length of the fan and are constructed in solid form, give the fastening of the miniature fan excellent rigidity. According to this design, which only provides for one jointing plane inside and outside the flow channel, an inexpensive mould is provided. Outwardly divergent zones 18, 19 with divergent angles y , g at the exit plane A extend from the cylindrical part of the inner wall surface 3 to the exit plane A,. The angle is very small so that removal from the mould is ensured and in addition a minimal cross-sectional increase is obtained. The cylindrical part of the inner wall surface 3 must, for mrnufacturing reasons, have a lift-out bevel, at least in the injection moulded part, i.e said cylindrical part is only substantially cylindrical (cf. angle & ). The divergent zones 18 in the four corners of the plate 15 have much larger widening angle 8 are knoWn per sW from German patent 17 28 338. These zones 18 extend conically (or alternatively step-wise) to the exit plane A (from the cylindrical part of the flow channel).

Figure 2 shows the sloping wall of the divergent zones 18 from the outside (cf. reference numeral 27).

Between the fixing columns 14 and the casing tube 4 are provided bridges 28, which impart the stability of the columns to the casing tube 4. On the entry side, the inner wall surface 3 has a rounded portion 31 with a radius of curvature of approximately 4 to 5 mm.

The casing of Figures 1 and 2 represents a parallelepiped of 50 x 50 x 25 mm. In the embodiment shown in Figures 1 and 2 possible inventive importance is attached to the combination of optimum flow channel, relatively great strength of the casing construction, econmomic manufacturability as a mass-produced product, together with the dimensions for this small size, so that importance may be attached to the dimensions for this small size, so that importance may be attached to the dimensions and proportions. The columns 14 constructed as cylindrical sleeves with through fixing bores 16 and thin bridges 28 extending over the entire axial casing length radially with respect of the thin casing ring 4 ensure optimum design for a simple mould.

In Figures 3, 4, and 5 bearing tube 17 and flange 13 are as shown in Figure 1. Stator plates are placed on the outer shoulder of bearing tube 17 and engage with said shoulder by insulating end plates. In known manner within the bearing tube is provided a pair of ball bearings, which are axially tensioned by a spring. A shaft 12 is mounted in the bearings and connected in non-rotary manner to an external rotor casing of the impeller hub. Figures 3 to 5 only differ with respect to this external rotor casing and the impeller hub. In all three cases, identical blades 7 are mounted on a hub 21 to give an impeller 2. The stator plates are the winding of a drive motor 6, whilst the commutating electronics are located axially within the flange 13.

In the case of such small axial fans it is necessary with such a relatively large drive motor 6 (i.e. a relatively large ratio of the impeller hub or drive motor diameter to the impeller diameter or the envelope of the blade ends) to make the radial blade dimensions relatively large (i.e. to design the drive motor with the hub providing a flow inner wall of small diameter). The flow inner wall is formed by the hub and the external rotor. In Figu-es 3, 4 and 5, it is necessary to find favourable dimensions to provide a given power requirement and a reliable fixing of the impeller blades 7 to the hub and the fan wheel to the external rotor, whilst still providing adequate air capacity.

The embodiment of Figure 3 uses a plastic-bound magnet (or ceramic magnet) of relatively large thickness and over which is draB a relatively thin, soft magnetid, cup-shaped cap 33. The cylindrical outer part 22 of impeller hub 21 completely surrounds cap 33. Positive fastening of the external rotor to the plastic hub is obtained by good anchoring of the cap to the hub resulting from injection moulding a thickened outer end 24 of the hub. The cup-shaped hub 21 and the blades 7 are integrally constructed as a single plastic injection moulding to provide the impeller 2.

The embodiment of Figure 4 has a short cylindrical part. 25 of an impeller hub 26, which engages over the external rotor of the drive motor 6 by a relatively short distance, prujecting roughly over a quarter of the axial length. The soft magnetic external rotor casing 33 is reduced in diameter in step-like manner at its base to provide a cylindrical outer surface which permits a press fit for the plastic hub 26; its external diameter roughly corresponding to that of the rotor casing. For otherwise identical motor dimensions, this construction permits a somewhat larger cross-section to be obtained by omission of the cylindrical outer wall 22 of the plastic hub 26. As in the case of Figure 3, the plastic hub 26 is invention moulded in one piece with the blades 7.Thus, the required flow cross-section increase is obtained by reduction of the drive motor diameter, including the impeller hub diameter.

If the fastening of the impeller to the external rotor is not satisfactory, it is possible, as shown in Figure 6, to provide hot-compressed pins for additionally, or alternatively, securing it to the base of the external rotor. In such a construction, cylindrical shoulder 25 could be omitted and it would then be possible to conically taper the impeller hub towards the entry plane E. Such a conical taper would bring a further improvement in the flow behaviour, particularly if externally the boundary casing wall from the inflow side was initially cylindrical, as known from EP-0100 078-A1 (EU-456).

The thermal compression of the impeller hub in the end face of the external rotor cap in the manner shown in Figure 6 is advantageous in an additional or alternate manner, There can also be a riveting or bonding, so that in the reduced diameter zone, as snon in Figure 6, free space 3 is formed by a conical impeller hub outer contour, tapering towards the entry plane E. This is shown on the right-hand side of Figure 6.

If the impeller is made from fibreglassreinforced plastic such a construction is readily possible, the blades still holding with the necessary rigidity on a disc-like hub 56. If the impeller is a punched bent part, the disc-like hub 56, the disc-like hub is appropriately riveted to the drive motor rotor.

Figure 5 shows another embodiment, in which a further increase in the flow cross-section is achieved through a radially deeper shoulder. The external diameter of casing 50 at cylindrical step 52is reduced by, for example 50 to 80%. A relatively small ring part 53 of the hub provides an adequate cross-section not only to achieve a satisfactory press fit on the outer surface 54 of step 52, but also to make the outer contour of the plastic hub with its overlapping ring part 53 such that it has a conical surface 65 tapering towards the inflow plane E. If surface 65 extends axially over at least 1/3 of the flow channel length, this taper is very effective.

This minimum length can be achieved without problems in the case of a mass-produced product with the hereinafter described concept of Figure 5. In this construction less demands are made on the plastic carrying the one-piece impeller 2 with blades 7, ring part 53 and radial base wall 55, so that the plastic can be cheaper here. In the case of the embodiment of Figure 5, a rare earth alloy, such as e.g. samarium cobalt is used for the rotor magnets 57. As is known, such magnets require a much smaller volume, so that the permanent magnet in the casing 50 can be made much thinner in the radial direction which, for the same air gap (assuming the same magnetic conditions), leads to a further reduction in the external diameter of casing 50.This advantageously small external diameter of the driving rotor (in the case of the presently used samarium - cobalt permanent magnet solution) and the pronounced radial reduction of step 52 (i.e. with a ratio of the diameter of step 52 to the diameter of cylindrical part 50 of the external rotor casing of 0.5 to 0.8) leads to an effective conical taper of the impeller hub towards the entry plane E. In the embodiment of Figure 5 the same rotor as in Figures 3 or 4 can be used, so that the same air gap diameter again applies. In Figure 5, the complete wall thickness of the rotor plates with parts 50 and 57 is approximately 1 to 2 rnm, compared with approximately 3 to 4 mm in the embodiment of Figure 3, which means a diameter reju=tic of approximately 4 mm.This is very important with such small sizes (hub diameter approximately 30 mm), because the flow cross--ection is considerably improved. This concept according to Figure 5 is favourable for miniature fans with a central motor, particularly with external rotors, independently of the casing. This also applies for miniature, so-called "motor impellers" (i.e. a motor with a mounted impeller). Although not exclusively, it is particularly effective for rare earth rotor magnets (with or without cobalt). Weaker magnets mean a somewhat larger "hub" diameter.

The left-hand side of Figure 6 shows a somewhat different embodiment of a fan of the invention, where the motor hub external diameter is reduced. It is possible to see an injection moulded plastics impeller 2 with a hub 19a, carrying uniformly distributed blades 7 round its periphery. This is pressed over and appropriately fixed to the diameter-reduced hub part 70 of the external rotor casing 22. The external diameter of plastic hub 21 largely corresponds to the external diameter of rotor casing 22 rlose to its open end.

The advantages of the plastic impeller is that it leads to a less expensive axial fan. It is also clear that the external diameter of hub 21 is even smaller than would be the case if said hub engaged completely over the external rotor of the driving brushless d.c.

motor. Thus, the rotor with mounted impeller constructed according to Figure 6 is advantageouslty usable in radially very small axal fans, according to the present application. In the present range of 30 to 60 mm impeller diameter with a coaxial "hub" motor, a minimum reduction of the impeller hub diameter is very advantageous for the flow behaviour (air quantity/time and noise) Although Figure 6 shows that the central fixing part 32 is the bearing tube, it is obvious that in many cases the central fixing part could merely comprise the inside of the stator iron core 58. Thus, the stator core could be used for fixing purposes either for ball bearings 48, 48' or for friction bearings 49 for certain uses of the brushless d.c.

motor. In this case the circuit board 20 would be fixed to an appropriate point of the stator by pins.

A further improvement of the structure comprises equipping the motor of Figure 6 with the fan casing 37, central fixing part 32, flange 30 and webs 5 moulded as a single plastic part. Thus, in accordance with the present invention an internal motor structure for a brushless d.c. motor is shown with an electronic drive system nd r.p.m. control circuit fixed within the motor to a component board in such a way that it is possible to obtain in stepwise manner a smaller diameter at the closed end of the external rotor hub than at the open end thereof. Such a stepwise diameter reduction makes it possible to use such a motor for axial fans with a larger cross-section on the air intake side thereof, par cularly in a fan with smaller dimensions, as well s in those cases where it is important to supply larger air quantities under higher pressure.

Figure 7 shows a partial development of the impeller hub 2, particularly according to Figure 3.

Blades 7 are non-uniformly distributed around the circumference of impeller hub 2. The gap 75 (here approximately 3 mm) is varied in order to reduce noise. The flow direction is indicated by arrow 74.

The leading edges 71 of blades 7 are displaced by a first axial distance 61 from entry plane E in the flow diPectron.. For example, in the embodimenfi according to Figure 3, distance 61 is 3 mm and the leading edge has a radius of approximately 0.6 mm. The final third of blade 7 is tapered and at the trailing edge 72 has a thickness of 0.4 mm. The trailing edge 72 is set back from the inner web edge 59 of webs 5 by a distance 62, preferably 4 xm (cf. Figure 3). The axial extent 63 of impeller 2 (cf. Figure 3) is approximately 20 mm.The inlet angle r at the inflow side formed between the tangent to the radial outside of blade edge 71 and the inflow plane E is in the range 25 to 45 . The setting angle d at the outflow side, formed between the tangent to the radial outside of blade edge 72 and outflow plane A is 70 to 90 , preferably 80".

Figure 8 shows blade 7 as a part bent in one plane (e.g. a stamped or punched). Blade 7 is constructed with different radii R1 and R2 on either side of an axis 76 having an angle of inclination of approximately 450 with respect to the blade root. The diameters of the two curvature cylinders on either side about axis 76 are for 2.R 1 = 120 mm and 2.R 2 = 30 mm. In radial plan view of blade 7 from the leading edge 71, there is initially a limited curvature R1 of the blade and this then changes to a greater curvature R2.

Figure 3 shows a further independently important economically advantageous method and structure to fix the rotor 2 on the shaft 12 by merely plastics injection moulding. The soft-iron cap 33 with its inner axially bent collarlike rim 133 is there completely imbedded in the plastic. The internal surface 134 of said collar has a distance of about 0.5 to about 2 mm to the shaft 12, preferably 0.6 trim.

This distance of gap 137 is filled with plastics and the collar or rim is partly perforated, so that the plastic part 135 surrounds and penetrates the rim or collar 133. The gap is as small as possible so that plastic material, when injected, penetrates the gap.

Because of heat problems the gap should not be larger than 1 to 2 mm. Said cylindrical collar surrounding said shaft is fixed with the rotor in any suitable way.

Figure 4 shows a known, more costly method where a separate, additional metal piece is necessary between shaft and collar.

The method of Figure 3 is an important feature independent from the type of fan or structure of the rotor housing.

In Figure 3 te internal rim of the rotor holding reinforcement element 33 is punched and bent in one step with the whole cup-like element 33.

It will be appreciated that the invention is not restricted to the particular details described above with reference to the drawings but that numerous modifications and variations can be made without departing from the invention as defined in the following claims. Further, the novel features disclosed herein can be employed separately or in any combination in other fans as appropriate.

Claims

1. A miniature axial fan having a central, impeller-driving motor, the impeller being surrounded b a casing in which the inflow-side casing inner wall is cylindrical to beyond the axial centre of the casing and then diverges in the flow direction towards the outflow-side, the drive motor being carried by outflow-side webs, and the number of impeller blades differing from the number of webs, wherein in the case of an impeller diameter at most 60 mm, the fan blades are constructed in such a way that on the outflow-side they have a large setting angle of 70 to 90 , and, in a radial plan view on the rotation axis, the curvature of the blades from the inflow-side is initially limited and then becomes more marked towards the outflow-side blade edge.

2. A miniature axial fan as claimed in Claim 1, wherein said setting angle is approximately 80e in the case of an impeller diameter of 45 mm.

3. A miniature axial fan as claimed in Claim 1 or Claim 2, wherein the casing is square on the outflow-side.

4. A miniature axial blower as claimed in Claim 3 wherein the increase in the flow cross-section increases in the flow direction towards the four Contour corners of the square casing.

5. A miniature axial fan as claimed in Claim 4, wherein said cross-sectional increase is continuous.

6. A miniature axial fan as claimed in any one of Climb 3 to 5, wherein the casing has four webs at its outflow- ide.

7. A miniature fan as claimed in Claim 6, wherein said webs are arranged in the respective centres of the sides of the square contour.

8. A miniature axial fan as claimed in Cl - 6, wherein in the vicinity of the square corners fixing columns are connected to a square flange plate, said columns extending up to the entry plane and provided over their entire axial length with a respective through-bore, which bores consequently extend from the inflow plane to the outflow plane.

9. A miniature axial fan as claimed in any one of the preceding claims, wherein the number of blades cannot be divided by the number of webs.

10. A miniature axial fan as claimed in Claim 9, wherein there are seven blades and four webs.

11. A miniature axial fan as claimed in any one of the preceding claims wherein in the case of a square side length of 50 x 50 mm, the limited curvature corresponds to a radius of curvature of approximately 60 mm and the marked curvature corresponds to a radius of curvature of approximately 15 mm.

12. A miniature axial fan as claimed in any one of the preceding claims, wherein the leading edges of the blades are set back from the entry plane by a few millimetres and the trailing edges of the blades terminate an axial distance from the webs of a few millimetres.

13. A miniature axial fan as claimed in Claim 12, wherein the dimensions are as specified in Claim 10 and the said spacings are 3 mm from the entry place and 4 mm from the webs respect y.

14. A miniature axial fan as claimed in any one of the preceding claims, wherein- the blades are distributed in non-uniform manner around the circumference of the hub.

15. A miniature axial fan as claimed in any one of the preceding claims, wherein the intake end of the casing inner wall has a radius of approximately 4 mm.

16. A miniature axial fan as claimed in any one of the preceding claims, wherein the impeller hub has a diameter of at least half the diameter of the complete impeller.

17. A miniature axial fan as claimed in any one of the preceding claims, wherein the driving motor is a brushless d.c. motor with an external rotor and a one or two-pulse operation.

18. A miniature axial fan as claimed in any one of the preceding claims, wherein the driving motor is three-phrase and has a microprocessor control.

19. A miniature axial fan as claimed in any one of the preceding claims, wherein the driving motor has a rare earth magnet and the diameter of the external rotor casing reduces in step-like manner in the vicinity of the closed rotor base for the press fit reception of the impeller hub, the hub external diameter roughly corresponding to the external diameter of the rotor.

20. A miniature axial fan as claimed in any one of the preceding claims, wherein, in the vicinity of its seat, the impeller hub is tapered conically towards the entry plane.

21. A miniature axial fan as claimed in any one of the preceding claims, wherein the impeller diameter is below 50 mm.

22. A miniature axial fan as claimed in Claim 1, wherein the housing is substantially as hereinbefore described with reference to and as shown in Figures 1 and 2.

22. A miniature axial fan as claimed in Claim 21, wherein the impeller diameter is 40 mm.

23. A miniature axial fan substantially as hereinbefore described with reference to and as shown in any one of the drawings.

24. A fan having a direct drive motor and an external rotor motor housing, said housing being fixed with a plastic fan wheel, a central coaxial part of said fan wheel, directly contacting the shaft of the motor or a rotor part and imbedding a collar-type metal sheet of a reinforcement holding means of said plastic fan wheel. 25. Each and every novel feature disclosed herein.

Amendments to the claims have been filed as follows 3. A riniature axial fan having an impeller, a central, impeller-driving motor, the impeller being surrounded by a housing having an inner wall which is cylindrical from an inflow-side of the housing to beyond the axial centre of the casing and then diverges in the flow direction towards an outflow-side of the housing, the drive motor being carried by outflow-side webs, and the number of impeller blades differing from the number of webs, wherein the outflow-side of the housing is square and in the vicinity of the square corners fixing columns are connected to a square flange plate, said columns extending up to the entry plane and provided over their entire axial length with a respective through-bore, which bores consequently extend from the inflow plane to the outflow plane, and said housing, webs, flange plate and columns being integrally constructed.

2. A miniature axial fan as claimed in Claim 1, wherein the drive motor is carried on an axially extending bearing tube having an integral flange to which said webs are integrally connected.

3. A miniature axial fan as claimed in Claim 2, wherein said housing, webs, flange plate, columns, bearing tube and flange are a single injection moulding.

4. A miniature axial fan as claimed in any one of the preceding claims, wherein the flow cross-section of the fan increases in the flow direction towards the four contour corners of the square outflow-side of the housing.

6. A miniature axial fan as claimed in any one of the preceding claims, wherein the housing has four webs at its outflow-side.

7. A miniature fan as claimed in Claim 6, wherein said webs extend radially to the respective centres of the sides of the outflow-side of the housing.

8. A miniature axial fan as claimed in any one of the preceding claims, wherein the number of blades cannot be evenly divided by the number of webs.

9. A miniature axial fan as claimed in Claim 8, wherein there are only seven blades and only four webs.

10. A miniature axial fan as claimed in any one of the preceding claims, wherein the outflow-side of the housing is a 50 x 5n m square, and in a radial plan view towards the rotation axis, the curvature of the blades from the inflow-side corresponds to a radius of curvature of approximately 60 r::m and then becomes more marked towards the outflow-side blade edge to correspond to a radius of curvature of approximately 15 mm.

11. A miniature axial fan as clarified in any one of the preceding claims, wherein the leading edges of the blades are set back from the entry plane and the trailing edges of the blades terminate an axial distance from the webs.

12. A miniature axial fan as claimed in Claim 11, wherein the dimensions are as specified in Claim 10 and the leading edge of the blades are set back 3 mm from the entry plane and the trailing edge of the blades terminate 4 mm in an axial direction from the webs.

13. A miniature axial fan as claimed in any one of the preceding claims, wherein the blades are distributed in non-uniform manner around the circumference of the hub.

14. A miniature axial fan as claimed in any one of the preceding claims, wherein the inflow-side of the housing inner wall has a radius of approximately 4 mm.

15. A miniature axial fan as claimed in any one of the preceding claims, wherein the impeller hub has a diameter of at least half the diameter of the complete impeller.

16. A miniature axial fan as claimed in any one of the preceding claims, wherein the driving motor is a brushless d.c. motor with an ex vernal rotor and a one or two-pulse operation.

17. A miniature axial fan as claimed in any one of Claims 1 to 15, wherein the driving motor is threephrase and has a microprocessor control.

18. A miniature axial fan as claimed in any one of the preceding claims, wherein the driving motor has a rare earth magnet and the diameter of the external rotor casing reduces in step-like manner in the vicinity of the closed rotor base for the press fit reception of the impeller hub, the hub external diameter roughly corresponding to the external diameter of the rotor.

19. A miniature axial fan as claimed in any one of the preceding claims, wherein, in the vicinity of its base, the impeller hub is tapered conically towards the entry plane.

20. A miniature axial fan as claimed in any one of the preceding claims, wherein of the complete impeller diameter is less than 50 mm.

21. A miniature axial fan as claimed in Claim 20, wherein the said diameter is 40 mm.