Patent Publication Number: US-2011058946-A1

Title: Fan and method for operating a fan

Description:
PRIOR ART 
     The invention is based on a fan and a method for operating a fan as generically defined by the preambles to the independent claims. 
     It is generally known in electric power tools to use fans, especially radial and semiaxial fans. Because of sometimes excessive idling rpm because of an overly low fan moment in idling, interfering vibration of the electric power tool can occur or be increased, which among other things reduces the brush service life of the electric motor and contributes to increased material fatigue. 
     The fan in an electric power tool performs the task first of cooling the thermally stressed components in the electric power tool, and second, of regulating the idling rpm of the motor by means of a suitable idling moment. In a universal motor, for instance, by means of a fan disposed on the armature shaft, the universal motor is protected against uncontrolled revving during idling. A low load moment is also created by supporting the armature shaft at the bearing points; in comparison to the load moment from the fan, however, the load moment from the bearing is low. In an electrical appliance, such as a vacuum cleaner, the air flow fed by the fan is employed, for instance being aspirated. 
     In idling, that is at high rpm without work being drawn at the spindle, the electric power tool is sufficiently well supplied with cooling air. At the same time, noise emissions and the vibration load on the component structure both increase because of the high rpm. 
     DISCLOSURE OF THE INVENTION 
     The invention is based on a fan, in particular a radial fan, having a fan hub and a rotational axis, in which a plurality of fan blades for deflecting an air flow are disposed around the rotational axis. 
     It is proposed that an adjusting device be provided for rpm-dependent adjustment of an air flow at the fan blades. 
     The air flow at the fan blades can be varied, both by the fan blades themselves and additionally, or alternatively, by suitable fixtures in the fan. 
     As a result, different goals can be attained. On the one hand, as a first alternative, with increasing rpm of the fan drive, it is possible by a suitable adjustment of the air flow to increase the moment of the fan to beyond the normal rpm-dependent moment characteristic resulting when adjustment is not done. On the other hand, as a second alternative, with decreasing rpm of the fan drive, it is possible by a suitable adjustment of the air flow to improve the flowthrough behavior of the fan and as a result, at low rpm, or in other words as a rule under heavy load, an increased ratio—compared to a normal rpm-dependent flowthrough characteristic without adjustment—can be established between the volume flow and the rpm. The concrete embodiment of the adjusting device may differ for the first and the second alternatives. 
     Fundamentally, the fan can have fan blades that are curved forward or backward, or uncurved fan blades. 
     Advantageously, the knowledge that the work conversion in radial fans, and thus the fan moment, depend primarily on the exchange of impetus between the fan flow and its blade geometry can be exploited. Preferably, the fan is used in an electric power tool or an electrical appliance, such as a vacuum cleaner, and is driven by the electric motor thereof. A strong deflection of the flow in the fan and a high air flow rate produce a high torque at the shaft of the motor (in this case the electric motor). Correspondingly, a slight flow deflection or a lesser air flow rate produces a lesser torque. The flow deflection can be varied by the fan blades themselves, or additional elements may be provided that vary the air flow as a function of rpm. The flow deflection by the fan blades can be accomplished passively, for instance via flexible regions of the fan blades, and/or via an active control mechanism that acts on the fan blades. 
     In the second alternative, it is favorable if the adjustment of the air flow is done such that at low rpm, for instance at an operating point under load of the tool/appliance cooled by the fan, a lesser flow deflection takes place than at higher rotary speeds, for instance in idling, specifically in such a way that at low rotary speeds, the greatest possible air flow is delivered. The flow deflection can be done for instance with fan blades, in particular with additional adjusting mechanisms in the fan blades. The flow deflection can be achieved by means of flaps, for instance. 
     With the increase in the rpm of the fan and thus of the motor shaft of the electric motor by which the fan is preferably driven, the centrifugal force acting on the fan structure increases as well. 
     It is also advantageous that the adjustment of the air flow, in the first alternative, can be done such that at high rpm an adequate load moment, which with an unvaried air flow would be less, acts on the motor shaft. In the second alternative, at lower rotary speeds, an increased ratio between the volume flow in the rpm can be established, for optimal cooling performance. Favorably, it can be possible to make the reduction in the volume flow less dependent on the rpm reduction of the electric motor. 
     Given a suitable adjustment in the first alternative, the result together with the centrifugal force and the fluid force, that is, the force that is exerted by the air flow, and given a suitable embodiment of the fan blades, leads to a suitable deformation of the fan blades and to an increase in the flow direction, to an increase in the fan moment, and to a reduction in the idling rpm. Because of the lesser blade deformation at the operating point (that is, at a lesser rpm), there are a lesser flow deflection and a lower fan torque, and thus better overall efficiency of an electric power tool in which the fan is disposed. 
     Favorably, a region that is deformable by centrifugal force and/or fluid force in at least one of the fan blades can form a component of the adjusting device. Preferably, the fan blade, along its longitudinal chord, can be connected at least in some regions to the fan hub, which is also known as the bottom plate of the fan blades, and can be separate from the fan hub in some regions. Alternatively or in addition, it is also conceivable that the fan blades can be connected in at least some regions on a fan cover disk that rotates with them and can be separate in some regions. The fan cover disk is located facing the fan hub in such a way that the fan blades are disposed between the fan cover disk and the fan hub. 
     Depending on the effect desired, various provisions are possible, which one skilled in the art will select to suit. In the case of a forward curvature of the fan blade, its region near the circumference can be connected to the hub and/or the fan cover disk. In the case of a rearward curvature of the fan blade, its region near the rotational axis and/or its region near the circumference can be connected to the fan hub and/or the fan cover disk. The attachment of the fan blades is preferably not continuous; instead, at least one interruption is provided. 
     Advantageously, in the unconnected region the fan blade can be embodied as more flexible than in the connected region. However, it can also be embodied flexibly throughout. 
     Because of the lesser blade deformation at the operating point (that is, at a lesser rpm), there are a lesser flow deflection and a lower fan moment, and thus better efficiency of an electric power tool or electrical appliance in which the fan is disposed. As a result of the utilization of the fluid and centrifugal forces, which are always present, additional fittings, such as an active mechanical adjusting mechanism in the electric power tool or electrical appliance, can be dispensed with. 
     In the preferred passive variable-rpm blade deformation in preferred radial fans, for regulating the operating performance of electric power tools, an adaptation of the fan characteristic of the radial fan can advantageously be achieved by means of passive blade deformation, utilizing the incident fluid and centrifugal forces for optimizing the operating performance of electric power tools. In a favorable way, in the first alternative, the idling rpm can be reduced, and as a consequence, component vibration can be lessened. This also increases the brush service life in the AC range and postpones fatigue in the material. Noise emissions from the preferred electric power tool can be reduced as well. 
     In addition to fan blades, movable elements which vary the air flow as a function of rpm can be provided on the fan hub and/or on the fan cover disk. This is especially preferred in the second alternative. 
     The fan blades can additionally be adjustable, or can also be embodied rigidly. These movable elements, preferably flaps, can change their position as a function of the centrifugal force of the fan. The flaps may be an integral component of the fan, or separate components that are disposed on the fan. Thus at low rpm, the embodiment can be such that an optimal air flow guidance takes place, with a corresponding high air flow. At higher rotary speeds, by greater deformation or adjustment of the flaps and correspondingly adequate blocking of the cross section and/or a favorable deflection of the flow, a lesser air flow can be delivered. 
     Favorably, alternatively or in addition, a separate device from the fan hub and/or the fan cover disk, for adjusting the fan blades, may be provided, for instance in a manner known in exhaust gas turbochargers or for the adjustment of the rotor blades of a helicopter. 
     It is even conceivable to combine the first and second alternatives with one another, if active air quantity control can be done by means of a regulator and/or controller. By active adjustment of the first alternative and/or of the second alternative, the flow geometry of the fan can be designed favorably for every instance of operation. For instance, the fan could first be operated by the first alternative, that is, at increased fan moment at high rpm. In a load situation (lesser rpm), the air quantity decreases, until such time as the motor temperature, after a brief time, has risen so much that more cooling air is needed. A switch could then be made over to the second alternative, which in the load situation makes a high cooling air flow possible. Expediently, an adjustment, for instance active adjustment, of the air flow at the fan blades and/or at movable flaps could then be provided, via actuators. 
     A method for operating a fan is proposed, in which an air flow at the fan blades is varied as a function of rpm in such a way that at high rotary speeds, a higher moment of the fan is established, compared to an rpm-dependent moment characteristic without adjustment. This is a preferred mode of operation in accordance with the first alternative. 
     In accordance with the second alternative, a method for operating a fan is proposed in which an air flow at the fan blades is varied as a function of rpm in such a way that at low rotary speeds, a greater ratio between volume flow and rpm is established than at high rotary speeds. 
     The invention can advantageously be employed in radial fans or semiaxial fans, which axially aspirate an air flow and radially or semiaxially output cooling air. The invention can also be employed in axial fans, which axially aspirate an air flow and axially output it. 
     The invention can be used especially advantageously for electric power tools having motors or universal motors that have a fan, or in general in fans that are used for cooling motors. However, since electric power tools typically have a fan, the invention, such as the principle of passive blade deformation, can advantageously be employed in any electric power tool with a suitably high motor rpm, given suitable fan geometry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Drawing 
       Further advantages will become apparent from the ensuing description of the drawings. In the drawings, exemplary embodiments of the invention are shown. The drawings, description and claims include numerous characteristics in combination. One skilled in the art will expediently consider the characteristics individually as well and put them together to make useful further combinations. 
         FIGS. 1   a - 1   c  show a perspective view of a rearward-curved preferred fan with a plurality of fan blades ( FIG. 1   a ), a detailed view with internal attachment of one fan blade to the fan hub ( FIG. 1   b ), and a detailed view with external attachment of one fan blade to the fan hub ( FIG. 1   c ); 
         FIGS. 2   a ,  2   b  show a perspective view of a forward-curved preferred fan with a plurality of fan blades with external attachment to a fan hub ( FIG. 2   a ) and a detailed view with external attachment of one fan blade ( FIG. 2   b ) to a fan hub; 
         FIGS. 3   a - 3   c  show an effect of a resultant force on a rearward-curved fan blade with internal attachment ( FIG. 3   a ) and with an external attachment ( FIG. 3   b ) to a fan hub, and an effect of a force on a forward-curved fan blade with external attachment to a fan hub ( FIG. 3   c ); 
         FIGS. 4   a ,  4   b  show a plan view on a detail of a preferred fan with movable flaps ( FIG. 4   a ) and a side view of a movable flap at various rotary speeds ( FIG. 4   b ). 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     In the drawings, functionally identical or identically acting elements are each identified by the same reference numerals. The figures are schematic illustrations of the invention. They do not show specific parameters of the invention. Furthermore, the figures show only typical features of the invention and are not intended to limit the invention to the features shown. 
     For explaining the invention,  FIGS. 1   a - 1   c  and  2   a ,  2   b  show a fan  10  with a fan hub  18 . A plurality of fan blades  12  for deflecting an air flow is disposed on the fan hub  18  about a rotational axis  14 . The inflow direction of the air flow is in the axial direction in the center region  16  of the fan hub  18 . A shaft, not shown, is disposed at the center of the center region  16 . For the sake of simplicity, no cover disk is shown, since it would conceal the fan blades  12 . However, it is understood that it can be present and can be connected to the fan blades  12  in a way comparable to what is described below for the fan hub  18 . A fan cover disk of this kind is known in the most various versions, for instance with or without exit-side deflection of the flow in the circumferential region of the cover disk. The cover disk may also be solidly connected to the fan blades  12  and rotate with them. The fan blades  12  can also be fully attached in their length to the cover disk and attached only partly to the fan hub  18 , so that they protrude radially past the fan hub  18 . 
       FIGS. 1   a  through  1   c  show rearward-curved fan blades  12 , while forward-curved fan blades  12  are shown in  FIGS. 2   a ,  2   b . The direction of rotation is clockwise, as an example. 
     According to the invention, an adjusting device  26 ,  28  for rpm-dependent adjustment of an air flow is provided at the fan blades  12 . The fan blades  12  can be connected to the fan hub  18  in the region  16  near the center of the fan hub  18 , or in the region of its circumference. The unconnected regions of the fan blades  12  can deform under the influence of centrifugal force and of the fluid force, that is, the pressure exerted by the air flow. Advantageously, these regions can be embodied in flexible fashion. 
     A region  26   a  (near the circumference) or  28   a  (near the center) of the fan blades  12  that is deformable by centrifugal force and fluid force forms a component of the adjusting device  26 ,  28 . The fan blades  12 , along their longitudinal chord  20 , can be connected at least in some regions to the fan hub  18  and separate at least in some regions from the fan hub  18 . 
     In the case of a rearward curvature of the fan blade  12 , an attachment  30  of its regions  28   a  near the rotational axis ( FIGS. 1   a  and  1   b ) or a (partial) attachment  32  of its regions  26   a  near the circumference ( FIG. 1   c ) on the fan hub  18  is provided. 
     In the case of a forward curvature of the fan blades  12 , an attachment  32  of their regions  26   a  near the circumference to the fan hub  18  is provided, as can be seen in  FIGS. 2   a ,  2   b . In the vicinity of the outer circumference, the fan blades  12  are connected (attachment  32 ) to the fan hub  18 , while the region near the center of the fan blades  12  is detached from the fan hub  18 . 
     Depending on the whether the fan  10  is optimized to an increased fan moment at high rotary speeds or increased overall efficiency of the electric power tool or electrical appliance (first alternative), or to an increased ratio between the cooling air flow and the rpm at lesser rotary speeds (second alternative), the fan blades  12  and the fan  10  should be designed correspondingly suitably so as to attain the applicable goal in combination with the rpm-dependent deformation of the fan blades  12  or with an expedient adjusting mechanism in the second alternative. 
     When there is little space in the tool, additional fixtures for adjusting the fan blades or air flow can be dispensed with. Adapting the geometry to adapt the flow guidance can be done passively by an intentional, variable-rpm blade deformation on the basis of the elasticity of the material of the radial fans, utilizing fluid and centrifugal forces. If space is adequate, active blade adjustment (mechanical adjusting mechanism) can in principle also be done. With the increase in the rpm, the centrifugal force acting on the fan structure rises. Along with the fluid force, and given suitable blade attachment to the fan hub and/or fan cover disk, this leads to a deformation of the fan blades and an increase in the flow deflection, an increase in the fan moment (beyond the normal rpm-dependent moment characteristic), and the reduction of the idling rpm. As a result of the lesser blade deformation of the operating point (lower rpm), a lower fan moment (lesser flow deflection) and better overall efficiency of the electric power tool result. 
       FIGS. 3   a  through  3   c  show a position of the fan blades  12  at equal rotary speeds and under the influence of centrifugal and fluid forces. Position  12   a  corresponds to undeformed fan blades  12 . The deformation of the fan blades  12  in the unconnected regions is clearly visible; a rearward-curved fan blade  12  with internal attachment, that is near the center, to the fan hub, not shown, in  FIG. 3   a  and with an external attachment, that is, near the circumference, to a fan hub in  FIG. 3   b  is shown.  FIG. 3   c  shows a forward-curved fan blade with external attachment to the fan hub, not shown. 
     In addition to the fan blades  12 , which can optionally be rigid or deformable, movable elements  40  can be provided on the fan hub  18 , which vary the air flow as a function of rpm. This variant is especially preferred in the second alternative.  FIG. 4   a  shows a plan view on a preferred fan  10  (the fan blades are not shown), in which the elements  40 , embodied for instance as flaps, are provided in the manner of tongues stamped halfway out of the fan hub  18 .  FIG. 4   b  shows a side view of the fan  10 . It can be see how at low rotary speeds, an air flow L 1  is relatively weakly deflected by the element  40  embodied as a flap. In this state, the air flow L 1  is delivered optimally. 
     At higher rotary speeds, the element  40   a  embodied as a flap lifts somewhat away from the fan  10  and deflects the air flow L 2  more strongly than at lower rotary speeds. In this case, the flap causes the air to leave the fan  10  sooner. 
     With the provisions shown for the first alternative, the idling rpm of an electric power tool or electrical appliance can advantageously be reduced. This can be achieved by means of a greater deflection of the air flow in the idling mode of the electric power tool. At the same time, in the operating points of the tool or appliance (lower rpm), good overall efficiency can be attained. 
     The mode of operation of the second alternative can be explained in conjunction with a vacuum cleaner, as an example. In the normal operating mode, the fan of a vacuum cleaner delivers a certain quantity of air, which should be as large as possible to make good vacuuming possible. If the vacuum cleaner sticks to a surface an air flow can no longer be aspirated, then the rpm of the fan rises, since air is no longer being delivered and power is no longer being demanded from the driving electric motor. The operating behavior of the fan in this range of high rotary speeds in the second alternative now has the effect that the suction of the vacuum cleaner is weaker, and the vacuum cleaner can easily be freed from the surface. The rpm then drops again, since air can be aspirated again. As a result of the operating behavior at lesser rpm in the second alternative, the air flow can increase again, and vacuuming can continue in the normal fashion.