GB2251028A

GB2251028A - Cooling electric motor with a radial-flow fan

Info

Publication number: GB2251028A
Application number: GB9027774A
Authority: GB
Inventors: William Roger Thomas
Original assignee: Black and Decker Inc
Current assignee: Black and Decker Inc
Priority date: 1990-12-21
Filing date: 1990-12-21
Publication date: 1992-06-24
Anticipated expiration: 2010-12-21
Also published as: GB2251028B; GB9027774D0

Abstract

An assembly, for example a garden string trimmer, comprising an electric motor (6) and a motor-cooling fan (20) connected at one end coaxially to its armature shaft (7) for directing a flow of air through and/or over the exterior of the motor, in which the fan is a radial flow fan having a backing plate (19) from which depend at right-angles, and towards the motor, a multiplicity of planar blades (22) orientated radially, the backing plate being dish-shaped and concave facing the motor whereby the longitudinal length of the blade edges (21) from which air exits the fan radially is substantially less than the maximum longitudinal length of the blades measured in a direction parallel with rotor shaft (7). <IMAGE>

Description

Electric Motor with a Cooling Fan This invention relates to assemblies comprising an electric motor and a motor-cooling fan, and to electric appliances incorporating such an assembly. The invention has particular, but not exclusive, reference to electric motor assemblies for powering appliances which, from time to time, may be used in an environment in which there is some risk of water entering the motor and giving rise to mechanical and/or electrical problems. Examples of such appliances are garden appliances, for example vegetation cutters which may be mowers or filament cutters, hedge trimmers etc.

It is common practice in such garden appliances to incorporate shrouds which limit to some extent the risk of water droplets entering the electric motor powering the appliance. It is also common practice to include a fan for drawing air over the motor to reduce heating up of the motor when the appliance is in use. The path of such cooling air frequently provides one or more entry points for water droplets and thus the shrouds must be located in positions in which they obstruct to some extent the flow of cooling air over the motor.

The positioning of shrouds thus becomes a compromise and this results in some loss of protection against the entry of water droplets and in some reduction in the rate of flow of cooling air over the motor for a given rate of power consumption.

The invention provides an assembly comprising an electric motor and a motor-cooling fan connected at one end coaxially to its armature shaft for directing a flow of air through and/or over the exterior of the motor, in which the fan is a radial flow fan having a backing plate from which depend at right-angles, and towards the motor, a multiplicity of planar blades orientated radially, the backing plate being dish-shaped and concave facing the motor whereby the longitudinal length of the blade edges from which air exits the fan radially is substantially less than the maximum longitudinal length of the blades.

The backing plate is capable of functioning as a shroud for the motor, but also, by virtue of its configuration, serves to increase the efficiency of the fan. We have found that the dish-shaped backing plate, by presenting a smoothly curved path for air flow, and by reducing the exit area for air leaving the blades radially, increases the efficiency of the fan. Thus, in comparison with a fan having a flat backing plate, the volume rate of air flow per unit rate of power consumption may be substantially greater. The rate of air flow for a given speed of rotation of the fan may then be predetermined by the radius of the blades and the extent, for example up to 20%, to which they project beyond the periphery of the backing plate.

The invention also provides an electric appliance incorporating within a housing an assembly in accordance with the invention defined above, in which the fan is positioned outside the housing so as to draw air longitudinally from the gap between the housing and the motor and/or longitudinally through the motor and to expel the air radially externally of the housing.

By way of example only, a string trimmer powered by an electric motor and embodying the invention will now be described in greater detail with reference to the accompanying scale drawings, in which: Figure 1 is a side elevation partly in section of the string trimmer; Figures 2A and 2B are side elevations partly in section and on a larger scale of part of the trimmer shown in Figure 1; and Figure 3 shows a set of explanatory graphs.

The string trimmer shown in Figure 1 is of conventional outward appearance and has a body 1 of clamshell construction formed to provide a handle 2 at one end and a motor housing 3 at the other end. The body 1 is made up of two clam-shell halves, one of which has been removed from Figure 1 in order to reveal components housed within the body.

As can be seen from Figure 1, the handle 2 extends away from the body 1 at an angle of about 100 degrees whilst the motor housing 3 is also angled slightly with respect to the central length of the body 1. This allows a user to handle the trimmer comfortably in use.

The handle 2 is contoured to provide a surface that is easily held in the user's hand. The handle accommodates a pivotally mounted lever 4 movement of which by the user operates an electric switch, shown in Figure 1 as block 5, which controls the supply of electric power to an electric motor 6 located in the housing 3.

The motor 6 is supported in the housing 3 by suitable internal ribs on the clam-shell halves. The rotational axis of the motor is vertical when the trimmer is in normal use.

The motor 6 has an output shaft 7 which carries a trimmer head 8 housing a spool 9 of string which, in use, extends radially outwardly from the head 8 through an aperture 10 in a wall 11 of the head.

Detachably secured to the lower (as seen in Figures 1 and 2) part of the housing 3 is a protective guard 12 within which the effective cutting length 13 of the string is rotated in a horizontal plane when the trimmer is in use. The forward portion 14 of the guard 12 projects only a short distance from the housing 3 to permit the cutting length 13 to cut vegetation as the trimmer is moved thereover.

The guard 12 has a central aperture 15 into which the lower (as seen in Figures 1 and 2) part of the housing 3 extends and through which the output shaft 7 passes.

Round the aperture 15 is formed a depending annular wall 16 which extends for a short distance towards the head 8. As can be seen from Figure 2B, the lower edge of the wall 16 is bevelled as at 17, the bevel facing inwardly towards the output shaft 7.

The trimmer head 8 comprises an open-topped cylindrical container 119 with a domed underside for periodic contact with the ground in order to "bump-feed" extra string, as will be explained below. The string supply spool 9 fits rotatably over the motor spindle 7 against which it is biassed upwardly by a compression spring 27 housed in the domed underside of the container 119. The container 119 with the spool 9 inside is removably fitted by means of a bayonet coupling to the underside of a backing plate 18 of a motor-cooling fan secured to the spindle 7. The fan serves to close the open-topped container 8, and to hold it-for rotation with the spindle 7.

Bump-feed mechanisms in string trimmers are wellknown, for example from U.S. Patent No. 4134204 (Perdue), and the example given here need not be described in detail.

Briefly, in normal use the entire head 8 is rotated at high speed by the motor, causing the end 13 of the string to exert centrifugally a pull on the stored length of string wound on the spool 9 and hence a torque on the spool. The spool 9 is prevented from rotating relative to the remainder of the head 8 by interengaging teeth 25, 26 on the opposed surfaces of the fan backplate 18 and the spool 9 respectively. In order to feed an extra length of string whilst the head is still rotating, the head 8 is bumped on the ground, causing the container 8 to shift axially towards the motor against the compression spring 27 bias, and releasing the teeth 25, 26 from interengagement.The consequential feed of string terminates after a predetermined angle of relative rotation has occurred to cause corresponding teeth 28, 28a on the underside of the spool to engage respective teeth 29, 29a in the base of the container 119. Lifting the trimmer from the ground to complete the "bump" frees the lower set of teeth 28, 28a and allows the spool to rotate through one further increment until teeth 25, 26 re-engage. The two sets of teeth thus constitute an escapement mechanism.

The motor-cooling fan comprises, in this example, twelve equi-angularly spaced, radially-orientated, planar blades 22 depending at right-angles from the backplate 18.

The surface 19, 24 of the backplate facing the motor is dish-shaped, in this example approximately partwtoroidal, and thus concave facing the motor. At its periphery, however, it has a smoothly-curving lip 20 directed at its edge generally transverse to the axis of the motor and the fan. The fan is thus a radial flow fan. At small radii, the edge 30 of the blade is inclined at about 60 to the axis; this changes abruptly to a straight edge 23 transverse to the axis, which edge extends to a right-angle corner at the periphery of the blade. The longitudinal edge 21 of the blade, from which air exits the fan radially, is straight and parallel to the axis; in this example, it coincides with the periphery 20 of the backplate.

Thus the effective length of each blade, in the longitudinal (axial) direction, varies with the radius. It increases with radius from zero to a maximum, over the base portion of the dish shape, and it then decreases to about 40% at the periphery 20. Thus the area over which air exits the fan radially, transcribed by the edges 21, is reduced by about 60% from what it would be were the backplate flat.

The fan draws air through air inlets 27 in the clamshells just above the upper end of the motor 6, and partly through the space 32 between the motor 6 and the clam-shell housing 3, but mainly through the interior of the motor itself, generally in a longitudinal direction to funnel into and pass through a central circular aperture 31 of a diameter substantially less than that of the opening 15 in the guard, in a base wall of the housing 3. This air has been heated by the motor and is expelled by the fan externally of the motor housing 3 and into the space under the guard 12, in which the air pressure is substantially atmospheric.The air flows through a passageway defined by the backing plate surfaces 24, 19, 20, by the base wall of the housing 3 surrounding the aperture 31, and to some extent by the annular wall 16; the passageway is rotationally symmetric about the axis, and its cross-sectional area (transverse to the general direction of air flow) at the exit 31 of the housing is substantially equal to that at the periphery of the fan (where the passageway is defined by the lower edge of the annular wall 16 and the periphery 20 of the backing plate. The annular area over which air exits the fan radially, determined by the longitudinal (axial) length of the edges 21, is approximately one half of the cross-sectional area of the passageway.The periphery 20 of the backing plate lies just within the inner surface of the annular wall 16, and the transition on each blade 22 from the inclined edge 30 to the transverse edge 23 occurs at approximately the same radius as that of the aperture 31 through which air flows.

In this example, the longitudinal length of each blade 22 has a maximum of 12mm at its central region, and a value of 4mm at its outer edge 21, although this latter could be reduced to 3mm. The annular area over which air is expelled radially, directly from the fan, is thus one third, or alternatively one quarter, that which would be the case were the backing plate flat and the outer edges 21 the maximum length of 12mm.

The longitudinal gap between the blade edges 23 and the surface 32 of the base wall of the housing 3 is 7mm; other dimensions in Figures 2A and 2B are to scale, although of course some elements such as the string 13 are drawn diagrammatically.

The annular wall 16 together with the backing plate 18 together form a shroud which reduces the extent to which water droplets can enter the motor via the aperture 15. The outer, downward facing surface of the backing plate, being serpentine in section and turning away from the motor at its peripheral lip 20, assists in guarding against the ingress of droplets.

We have found that the reduction, by 60% or more, of the exit area for air flow from the fan surprisingly increases its efficiency, i.e. it reduces the power consumption but, with the same rotational speed and the same blade radius, maintains the same volumetric air flow rate. In other embodiments of the invention, the backing plate projects radially beyond the periphery of the blades, for example by 20%, and this increases the air flow whilepreserving the same increased efficiency.

With reference to Figure 3, the variation of vacuum pressure, fan input power and efficiency is plotted as a function of volumetric air flow rate. The unit of flow rate is cubic feet per minute (1 CFM = 28.4 litres per minute); that of pressure is inches of water (one atmosphere = 407 inches of water); and that of power is Watts; and efficiency is represented as a percentage. The graphs interpolate from actual measurements, taken both with a fan as shown in Figures 2A and 2B, i.e. with a dishshaped backing plate (graphs A) and with a fan having a flat backing plate (graphs B). At rotational speeds for normal operation of a string trimmer under no load, the air flow rate was 400 litres per minute, about 14 C, and in this region the efficiency is about 34% with, but only 23% without, the dish-shape embodying the invention.

Claims

1. An assembly comprising an electric motor and a motor-cooling fan connected at one end coaxially to its armature shaft for directing a flow of air through and/or over the exterior of the motor, in which the fan is a radial flow fan having a backing plate from which depend at right-angles, and towards the motor, a multiplicity of planar blades orientated radially, the backing plate being dish-shaped and concave facing the motor whereby the longitudinal length of the blade edges from which air exits the fan radially is substantially less than the maximum longitudinal length of the blades.

2. An electric appliance incorporating within a housing an assembly in accordance with Claim 1, in which the fan is positioned outside the housing so as to draw air longitudinally from the gap between the housing and the motor and/or longitudinally through the motor and to expel the air radially externally of the housing.

3. An electric appliance according to Claim 2, in which the motor armature shaft projects from the housing through an aperture therein whose diameter is substantially less than that of the fan, whereby air from within the housing is drawn axially through the aperture into the fan over a central region and is then expelled radially beyond the edge of the aperture.

4. An assembly or an electric appliance according to any preceding claim, in which the backing plate is parttoroidal, so that the longitudinal active length of the blades is greatest at an intermediate radius or range of radii and decreases smoothly towards the axis and towards the periphery of the fan.

5. An assembly or an electric appliance according to any preceding claim, in which the peripheral edge of each of the blades, from which the air exits the fan, is straight and parallel to the axis of the fan.

6. An assembly or an electric appliance according to any preceding claim, in which the periphery of the backing plate is at the same radius as the peripheral edge of each blade.

7. An assembly or an electric appliance according to any of Claims 1 to 5, in which the periphery of the backing plate is at a radius about 20% less than that of the peripheral edge of each blade.

8. An assembly or an electric appliance according to any preceding claim, in which the backing plate has a peripheral lip directed generally transverse to the axis of the motor and the fan.

9. An electric appliance according to any preceding claim, in which the housing and backing plate together define a passageway for the flow of cooling air, which passageway is generally rotationally symmetric about the axis, and whose cross-sectional area at the exit of the housing is substantially equal to that at the periphery of the fan.

10. An electric appliance according to Claim 9, in which the annular area over which air exits the fan radially is approximately one half of the said passageway cross-sectional area.

11. An electric appliance according to any preceding claim, in the form of a string trimmer, comprising a rotary head for the string cutting element driven by the electric motor, the head carrying the fan.

12. An assembly comprising an electric motor and a motor cooling fan, substantially as described herein with reference to the accompanying drawing.

13. An electric appliance, substantially as described herein with reference to the accompanying drawing.

14. An electric string trimmer, substantially as described herein with reference to the accompanying drawing.