Vacuum cleaner with motor between separation stages

A vacuum cleaner having a main body and a dirt separator that is removably attached to the main body. The dirt separator includes a first dirt-separation stage, a second dirt-separation stage, and a vacuum motor for moving air through the first dirt-separation stage and the second dirt-separation stage. The vacuum motor has an impeller driven by an electric motor. The first dirt-separation stage is then located upstream of the impeller, and the second dirt-separation stage is located downstream of the impeller.

REFERENCE TO RELATED APPLICATION

This application claims priority of United Kingdom Application No. 1418791.8, filed Oct. 22, 2014, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a vacuum cleaner having a vacuum motor located between dirt-separation stages.

BACKGROUND OF THE INVENTION

A vacuum cleaner typically comprises a vacuum motor that pulls dirt-laden air through one or more dirt-separation stages. As the air is drawn through the vacuum cleaner, there is a pressure drop across each dirt-separation stage. Consequently, the suction power generated at the inlet of the vacuum cleaner is significantly less than the suction power generated by the unrestricted vacuum motor. The suction power at the inlet may be increased by employing a more powerful vacuum motor. However, this inevitably increases the cost, size, weight and/or power consumption of the vacuum cleaner.

SUMMARY OF THE INVENTION

The present invention provides a vacuum cleaner comprising a main body and a dirt separator removably attached to the main body, wherein the dirt separator comprises a first dirt-separation stage, a second dirt-separation stage, and a vacuum motor for moving air through the first dirt-separation stage and the second dirt-separation stage, the vacuum motor comprises an impeller driven by an electric motor, the first dirt-separation stage is located upstream of the impeller, and the second dirt-separation stage is located downstream of the impeller.

Since the second dirt-separation stage is located downstream of the impeller, the pressure drop associated with the second dirt-separation stage occurs downstream of the impeller. As a result, the air pressure at the inlet of the impeller is higher in comparison to a scheme in which both dirt-separation stages are located upstream of the impeller. Since the air pressure at the impeller inlet is higher, the impeller imparts a greater pressure rise to the air. This greater pressure rise may then be used to increase the flow rate, increase the separation efficiency and/or decrease the power consumption of the vacuum cleaner.

In view of the benefits of locating the second dirt-separation stage downstream of the impeller, one might be tempted to additionally locate the first dirt-separation stage downstream of the impeller. However, by locating the first dirt-separation stage upstream of the impeller, relatively coarse dirt, which might otherwise block, jam or damage the impeller, may first be removed.

The two dirt-separation stages form part of a common dirt separator that is removable from the main body. This then has the advantage that the dirt separator may be carried to a bin and the dirt collected by both separation stages may be emptied together in a single action. The vacuum motor also forms part of the dirt separator. Whilst this has the disadvantage of increasing the weight of the dirt separator, it has the advantage that a shorter and/or less tortuous path may be taken by the air moving from the first dirt-separation stage to the vacuum motor and/or from the vacuum motor to the second dirt-separation stage. As a result, flow losses are reduced.

The first dirt-separation stage may comprise a single cyclonic separator, and second dirt-separation stage may comprise a plurality of cyclonic separators. As a result, a relatively high separation efficiency may be achieved for the dirt separator. Furthermore, dirt may be removed without the need for a filter or other means that would require washing or replacing.

Where the first dirt-separation stage comprises a cyclonic separator, the cyclonic separator may have a central axis about which air within the cyclonic separator rotates. The vacuum motor may then comprise a rotational axis about which the impeller rotates, and the central axis and the rotational axis may be coincident. As a consequence, a relatively straight path may then be taken by the air as it moves from the first dirt-separation stage to the vacuum motor, thus reducing flow losses.

The second dirt-separation stage comprises a plurality of cyclonic separators arranged about the vacuum motor. By arranging the cyclonic separators around the vacuum motor, a relatively short and/or straight path may be taken between the outlet of the vacuum motor and the inlet of each of the cyclonic separators. As a result, relatively high speeds may be achieved for the air entering the cyclonic separators, thereby improving the separation efficiency.

The first dirt-separation stage may comprise a first dirt collector, the second dirt-separation stage may comprise a second dirt collector, and the first dirt collector may surround the second dirt collector. As a result, a relatively compact arrangement may be realised. The first dirt-separation stage is intended to remove relatively coarse dirt whilst the second dirt-separation stage is intended to remove relatively fine dirt. Since the first dirt collector surrounds the second dirt collector, a relatively large volume may be achieved for the first dirt collector whilst maintaining a relatively compact overall size.

The second dirt-separation stage may comprise a plurality of cyclonic separators and a plurality of channels, each channel extending from an outlet of the vacuum motor to an inlet of a respective cyclonic separator. The channels may then be used to avoid abrupt changes in the speed of the air as it moves from the vacuum motor to the cyclonic separators, thereby reducing flow losses. In particular, the channels may be used to ensure that the relatively high speed of the air exiting the vacuum motor is largely maintained on entering the cyclonic separators.

The inlet angle of each channel may be defined so as to minimise the incidence angle of the air entering the channel during normal use of the vacuum cleaner. As a result, flow losses are reduced. The absolute flow angle at which the air exits the impeller may be in excess of 30 degrees. Accordingly, each channel may have an inlet angle of at least 30 degrees.

Each channel may be substantially straight. Consequently there is no or relatively little turning of the air as it moves along the channel By contrast, if the air were forced to follow a tortuous path between the vacuum motor and the cyclonic separators, flow losses would be greater and thus the speed of the air entering the cyclonic separators would be slower.

The impeller may be a centrifugal impeller, which has the advantage of being able to achieve relatively high flow rates in relation to its size. Air then enters the vacuum motor in an axial direction (i.e. in a direction parallel to the rotational axis of the vacuum motor), and exits in a radial direction (i.e. in a direction normal to the rotational axis of the vacuum motor). Since the air exits in a radial direction, it is not necessary to turn the air exiting the impeller and thus flow losses are reduced. Furthermore, where the second dirt-separation stage comprises a plurality of cyclonic separators arranged around the vacuum motor, a relatively straight path may be established between the outlet of the vacuum motor and the inlet to each of the cyclonic separators, thereby further reducing losses.

At least part of the air discharged from the second dirt-separation stage may be used to cool the vacuum motor. As a result, the vacuum motor may operate at higher electrical power. At least part of the air discharged from the second dirt-separation stage may be pushed through the interior of the vacuum motor so as to cool one or more components of the electric motor. In particular, the air may flow over and cool an electrical winding and/or a power switch of the electric motor. As a result, the winding and the power switch are able to carry higher currents and thus the electric motor is able to operate at higher electrical power. In order to achieve this, the vacuum motor may comprise a first inlet, a first outlet, a second inlet and a second outlet. The first inlet is then located upstream of the impeller, whilst the first outlet is located downstream of the impeller and upstream of the second dirt-separation stage. The second inlet is then located downstream of the second dirt-separation stage, whilst the second outlet is located downstream of the second inlet. At least part of the air discharged from the second dirt-separation stage then enters the vacuum motor via the second inlet, flows over one or more components of the electric motor and exits the vacuum motor via the second outlet.

DETAILED DESCRIPTION OF THE INVENTION

The vacuum cleaner1ofFIG. 1comprises a main body2to which a dirt separator3is removably attached.

Referring now toFIGS. 2 to 7, the dirt separator3comprises a first dirt-separation stage4, a motor plenum5, a vacuum motor6, and a second dirt-separation stage7.

The first dirt-separation stage4comprises a cyclonic separator10and a dirt collector11. The cyclonic separator10and the dirt collector11are defined by an outer wall12, an inner wall13, a shroud14, and a base15. The outer wall12is cylindrical in shape and surrounds the inner wall13and the shroud14. The inner wall13is generally cylindrical in shape and is arranged concentrically with the outer wall12. The upper part of the inner wall13is fluted, with the flutes providing passageways along which dirt separated by the cyclonic separators40of the second dirt-separation stage7are guided to a further dirt collector42. The shroud14is located between the outer wall12and the inner wall13and comprises a mesh through which air is permitted to pass.

The upper end of the outer wall12is closed off by a wall of the second dirt-separation stage7. The lower ends of the outer wall12and the inner wall13are closed off by the base15. The outer wall12, the inner wall13, the shroud14and the base15thus collectively define a chamber. The upper part of this chamber (i.e. that part generally defined between the outer wall12and the shroud14) defines a cyclone chamber16, whilst the lower part of the chamber (i.e. that part generally defined between the outer wall12and the inner wall13) defines a dirt-collection chamber17. The first dirt-separation stage4therefore comprises a cyclonic separator10and a dirt collector11located below the cyclonic separator10.

The outer wall12includes an opening (not shown) that serves as an inlet to the first dirt-separation stage4. The space between the shroud14and the inner wall13defines a passageway18that is closed at a lower end and is open at an upper end. The upper end then serves an outlet for the first dirt-separation stage4.

The motor plenum5is located above the first dirt-separation stage4and serves to connect fluidly the outlet of first-dirt separation stage4with the inlet of the vacuum motor6.

The vacuum motor6comprises a housing20, an impeller21, and an electric motor22. The impeller21is a centrifugal impeller that is driven by the electric motor22. The housing20is generally cylindrical in shape, is closed at a front end and is open at a rear end. The impeller21and the electric motor22are then housed within the housing20such that the impeller21is adjacent the front end.

The housing20comprises a first inlet25located upstream of the impeller21, a first outlet26located downstream of the impeller21, a second inlet27located downstream of the first outlet26, and a second outlet28located downstream of the second inlet27. The first inlet25comprises a circular opening located in the front end of the housing20. The first outlet26comprises an annular opening formed around the side of the housing20. The second inlet27comprises a plurality of apertures that are again formed around the side of the housing20. The second inlet27is located rearward of the first outlet26, which is to say that, relative to the first outlet26, the second inlet27is located further towards the rear of the housing20. Finally, the second outlet28comprises a plurality of apertures that are defined between the open rear end of the housing20and the electric motor22.

The first inlet25is aligned with the inlet of the impeller21, whilst the first outlet26is aligned with and surrounds the outlet of the impeller21. In being a centrifugal impeller, air enters the impeller21in an axial direction (i.e. in a direction parallel to the rotational axis) and exists in a radial direction (i.e. in a direction normal to the rotational axis). Consequently, air enters the vacuum motor6via the first inlet25in an axial direction, and exits the vacuum motor6via the first outlet26in a radial direction. As explained below, air discharged from the second dirt-separation stage7re-enters the vacuum motor6via the second inlet27, flows over and cools components of the electric motor22, and exits via the second outlet28.

The vacuum motor6is mounted within the second dirt-separation stage7by means of an axial mount29and a radial mount30. Both mounts29,30are formed of an elastomeric material and act to isolate the second dirt-separation stage7and thus the remainder of the dirt separator3from the vibration generated by the vacuum motor6. The axial mount29is attached to the front end of the housing20and abuts a wall of the second dirt-separation stage7so as to form a seal. During use, the axial mount29deforms to absorb vibration of the vacuum motor6in an axial direction, i.e. in a direction parallel to the axis of rotation of the vacuum motor6. The radial mount30is attached to the side of the housing20and comprises a sleeve31that surrounds the housing20, a lip seal32located at one end of the sleeve31, and a plurality of ribs33that extend axially along the sleeve31. The radial mount30abuts a wall of the second dirt-separation stage7such that the lip seal32forms a seals against the wall, whilst the ribs33are crushed slightly. During use, the ribs33further deform to absorb vibration of the vacuum motor6in a radial direction, i.e. in a direction normal to the axis of rotation of the vacuum motor6.

The second dirt-separator stage7comprises a plurality of cyclonic separators40, a plurality of channels41, a dirt collector42, and a cover43.

The cyclonic separators40are arranged in a ring about the vacuum motor6. Each cyclonic separator40is frusto-conical in shape and comprises a tangential inlet44, an air outlet45, and a dirt outlet46. The interior of each cyclonic separator40defines a cyclone chamber47. During use, air enters the cyclone chamber47via the tangential inlet44. Dirt separated within the cyclone chamber47is then discharged through the dirt outlet46whilst the cleansed air exits through the air outlet45.

Each channel41extends linearly from the first outlet26of the vacuum motor6to the inlet44of a respective cyclonic separator40. The cyclonic separators40are positioned relative to the vacuum motor6such that the inlet44of each cyclonic separator40is located roughly at the same level as the first outlet26. The height of each inlet44is greater than the height of the first outlet26. Accordingly, each channel41increases in height as it extends from the first outlet26to the inlet44. Additionally, the channel41decreases in width as it extends between the first outlet26and the inlet44. This then ensures that the cross-sectional area of the channel41is relatively constant along its length, the advantages of which are explained below.

As illustrated inFIG. 4, each channel41has a centreline50that extends from the inlet to the outlet of the channel41. Each channel41then has an inlet angle α defined by the intersection of (i) the tangent to the centreline50at the inlet of the channel41, and (ii) the radial axis51of the impeller21extending through the centre of the inlet of the channel41. The term ‘inlet angle’ is therefore used in the same manner as that employed in compressor technology when referring to the blades or vanes of a diffuser. For example, the inlet angle of a diffuser vane is defined as the angle between (i) the tangent to the camber line at the leading edge of the vane, and (ii) the radial axis of the impeller extending through the leading edge of the vane. The channels41of the second dirt-separation stage7resemble a channel diffuser. However, in contrast to a conventional channel diffuser, which seeks to decelerate the airflow in order to increase the static pressure, the channels41of the second dirt-separation stage7does not attempt to decelerate the airflow; the reasons for this are set out below.

Referring now toFIG. 5, the inlet angle α of each channel41is defined so as to minimise the incidence angle γ of the airflow. During normal use of the vacuum cleaner1, the flow rate of the airflow passing through the cyclonic separator3will vary, e.g. as the vacuum cleaner1is used on different surfaces. As the flow rate varies, so too does the absolute flow angle β of the airflow exiting the impeller21. For example, the flow rate may vary between 5 l/s and 15 l/s. At the lower flow rate, the vacuum motor6rotates at a higher speed due to the reduced load and thus the airflow exits the impeller21at a higher flow angle of, say, 65 degrees. At the upper flow rate, the vacuum motor6rotates at a lower speed due to the increased load and thus the airflow exits the impeller21at a lower flow angle of, say, 35 degrees. The average flow rate during normal use may be, say, 10 l/s resulting in an absolute flow angle of 50 degrees. The inlet angle α of each channel is therefore defined as 50 degrees so as to minimise the incidence angle γ.

The dirt collector42is defined by the inner wall13and the base15. More particularly, the interior space bounded by the inner wall13and the base15defines a dirt-collection chamber48. The dirt collectors11,42of the two dirt-separation stages4,7are therefore adjacent. Moreover, the dirt collector11of the first dirt-separation stage4surrounds the dirt collector42of the second dirt-separation stage7. As explained below, the first dirt-separation stage4is intended to remove relatively coarse dirt, whilst the second dirt-separation stage7is intended to remove relatively fine dirt. By having a first dirt collector11that is outermost and surrounds a second dirt collector42, a relatively large volume may be achieved for the first dirt collector11whilst achieving a relatively compact overall size for the dirt separator3.

The bottom of each cyclonic separator40projects into the dirt collector42such that dirt separated by the cyclonic separator40is discharged through the dirt outlet46and falls into the dirt-collection chamber48. As noted above, the upper part of the inner wall13is fluted. The flutes provide passageways along which the dirt separated by the cyclonic separators40is guided to the bottom of the dirt-collection chamber48.

The cover43overlies the cyclonic separators40and the vacuum motor6. The cover43acts to guide the cleansed air discharged from the cyclonic separators40to the second inlet27of the vacuum motor6. The lip seal32of the radial mount30forms an annular seal against the cover43such that all air discharged from the cyclonic separators40re-enters the vacuum motor6via the second inlet27. The cover43comprises a plurality of exhaust vents49located above the vacuum motor6. Air discharged from the vacuum motor6via the second outlet28is then exhausted from the dirt separator3and the vacuum cleaner1via the exhaust vents49.

During use, the vacuum motor6pulls dirt-laden air in through a suction inlet of the vacuum cleaner1. The dirt-laden air is then carried via ducting from the suction inlet to the dirt separator3. The dirt-laden air enters the first dirt-separation stage4via the inlet in the outer wall12. The dirt-laden air then spins within the cyclone chamber16causing relatively coarse dirt to be separated. The coarse dirt collects in the dirt-collection chamber17, whilst the partially cleansed air is pulled through the shroud14, up through the passageway18, and into the motor plenum5. From the motor plenum5, the partially cleansed air is pulled into the vacuum motor6via the first inlet25. The air is then discharged from the vacuum motor6via the first outlet26. The partially cleansed air is then pushed along the channels41of the second dirt-separation stage7and into the cyclonic separators40via the tangential inlets44. The partially cleansed air then spins within the cyclone chambers47causing relatively fine dirt to be separated. The fine dirt is discharged through the dirt outlet46and collects in the dirt-collection chamber48, whilst the cleansed air is discharged through the air outlet45. From there, the cleansed fluid is pushed into the vacuum motor6via the second inlet27. The cleansed air is then pushed through the interior of the vacuum motor6causing components of the electric motor22to be cooled. Finally, the cleansed, heated air is discharged from the vacuum motor6via the second outlet28and is exhausted from the vacuum cleaner1via the exhaust vents49in the cover43.

The first dirt-separation stage4is located upstream of the impeller21, whilst the second dirt-separation stage7is located downstream of the impeller21. Consequently, air is pulled through the first dirt-separation stage4but is pushed through the second dirt separation stage7. This arrangement contrasts with a conventional vacuum cleaner in which both dirt-separation stages are located upstream of the vacuum motor. As air passes through a dirt-separation stage, there is a pressure drop in the airflow. Since the second dirt-separation stage7is located downstream of the impeller21, the pressure drop associated with the second dirt-separation stage7occurs downstream of the impeller21. As a result, the pressure at the inlet of the impeller21is higher in comparison to a conventional arrangement in which both dirt-separation stages are located upstream of the impeller. Consequently, for the same shaft power generated by the electric motor22, a greater pressure rise is imparted to the air by the impeller21. This greater pressure rise may then be used to increase the flow rate, increase the separation efficiency and/or decrease the power consumption of the vacuum cleaner1, as will now be explained.

If the shaft power of the electric motor22and the separation efficiencies of the dirt-separation stages4,7are unchanged, the greater pressure rise generated by the impeller21will result in a higher flow rate through the vacuum cleaner1. As a result, greater suction power will be generated at the suction inlet of the vacuum cleaner1. Rather than increasing the flow rate, the greater pressure rise generated by the impeller21may instead be used to increase the separation efficiency of one or both of the dirt-separation stages4,7. As the separation efficiency of a dirt-separation stage increases, so too does the pressure drop associated with the stage. Accordingly, the greater pressure rise may be used to increase the separation efficiency of one or both of the dirt-separation stages4,7whilst maintaining the same flow rate through the vacuum cleaner1. Finally, rather than increasing the flow rate or the separation efficiency, the shaft power of by the electric motor22may be reduced so that the same flow rate and separation efficiency are achieved. As a result, the same cleaning performance is achieved but at a lower power consumption.

In view of the benefits in locating the second dirt-separation stage7downstream of the impeller21, one might be tempted to locate the first dirt-separation stage4downstream of the impeller21. This would then further increase the pressure at the inlet of the impeller21. However, locating the first dirt-separation stage4downstream of the impeller21would then expose the impeller21to all of the dirt that is drawn into the vacuum cleaner1. By locating the first dirt-separation stage4upstream of the impeller21, relatively coarse dirt, which might otherwise block, jam or damage the impeller21, is first removed from the airflow. The impeller21is therefore exposed only to relatively fine dirt carried by the partially cleansed air.

The vacuum motor6comprises a centrifugal impeller21, which has the advantage of relatively high flow rates in relation to its size. As a consequence of employing a centrifugal impeller, air enters the impeller21in an axial direction, and exits in a radial direction. The housing20includes an outlet26that surrounds the outlet of the impeller21. As a result, it is not necessary to turn the air exiting the impeller21within the housing20, thereby reducing flow losses. Additionally, the air exiting the vacuum motor6moves at relatively high speed, which as will now be explained has significant advantages for the separation efficiency of the second dirt-separation stage7.

The cyclonic separators40of the second dirt-separation stage7are arranged around the vacuum motor6. As a result, a relatively short and straight path is provided for the airflow as it moves from the first outlet26of the vacuum motor6to the inlets44of the cyclonic separators40. This then helps reduce flow losses that would otherwise arise if the airflow were forced to follow a tortuous path between the vacuum motor6and the cyclonic separators40. The first outlet26of the vacuum motor6is located roughly at the same level as the inlet44to each cyclonic separator40. In particular, the first outlet26lies in a plane that passes though the inlet44of each cyclonic separator40. As a result, there is relatively little turning of the air in an axial direction, thereby reducing flow losses.

The channels41help to ensure that the speed of the air exiting the vacuum motor6is maintained at the inlets44to the cyclonic separators40. To this end, each channel41is straight and has an inlet angle α that serves to minimise the incidence angle γ of the airflow during normal use of the vacuum cleaner1. Additionally, the cross-sectional area of each channel41is constant along the length of the channel41. As a result, the relatively high speed of the air exiting the vacuum motor6is largely maintained at the inlets44of the cyclonic separators40. This then has the advantage of improving the separation efficiency of the cyclonic separators40.

In a conventional vacuum cleaner having cyclonic separators located upstream of a vacuum motor, the air is typically accelerated at the inlets to the cyclonic separators, which act as nozzles for the airflow. The air discharged from the cyclonic separators then flows into a plenum, causing the airflow to decelerate. Finally, the air is again accelerated at the vacuum motor. The airflow is therefore subjected to abrupt changes in speed as the airflow moves between the cyclonic separators and the vacuum motor. However, with each abrupt change in speed, the airflow experiences flow losses. With the vacuum cleaner1of the present invention, the channels41act to prevent abrupt changes in speed as the airflow moves between the vacuum motor6and the cyclonic separators40, thereby reducing flow losses.

The cross-sectional area of each channel41is constant along its length. As a result, the speed of the airflow entering the cyclonic separators40is largely the same as that exiting the vacuum motor6. However, depending on the particular design of the cyclonic separators40(e.g. size, shape and number) as well as the speed of the airflow exiting the vacuum motor6, it may be desirable to either accelerate or decelerate the airflow. Accordingly, the cross-sectional area of each channel41may decrease or increase gradually along its length. Nevertheless, in contrast to the conventional vacuum cleaner described in the previous paragraph, the airflow does not undergo an abrupt change in speed on its path from the vacuum motor6to the cyclonic separators40.

As noted above, the inlet angle α of each channel41is ideally defined so as to minimise the incidence angle γ. The inlet angle will therefore depend on the absolute flow angle β of the airflow exiting the vacuum motor6, which in turn depends on the design of the impeller21and the speed of rotation of the electric motor22. Since the vacuum motor6forms part of the dirt separator3and the dirt separator3is removable from the main body2, it is desirable to employ a vacuum motor6that is relatively compact and light in weight. In order to achieve a relatively compact size and light weight whilst achieving the desired flow rate, relatively high speeds of rotation are likely. Accordingly, the absolute flow angle at which the air exits the impeller21is likely to be in excess of 30 degrees. Each channel41would then have an inlet angle α of at least 30 degrees.

The vacuum motor6is located directly above the first dirt-separation stage4. Additionally, the central axis of the cyclonic separator10(i.e. the axis about which air rotates within the cyclone chamber16) and the rotational axis of the vacuum motor6are coincident. As a result, a relatively short and straight path is taken by the air as it moves from the first dirt-separation stage4to the vacuum motor6, thereby reducing flow losses.

The two dirt-separation stages4,7form part of a common dirt separator3that is removable from the main body2. This then has the advantage that the dirt separator3may be removed, carried to a bin and the dirt collected by both separation stages4,7may be emptied together in a single action. For example, the base15may pivot relative to the outer wall12in order to empty both dirt-collection chambers17,48. The vacuum motor6also forms part of the dirt separator3. Whilst this has the disadvantage of increasing the size and weight of the dirt separator3, it has the advantage that a shorter and less tortuous path is taken by the air when moving from the first dirt-separation stage4to the vacuum motor6and when moving from the vacuum motor6to the second dirt-separation stage7. As a result, flow losses are reduced.

The first dirt-separation stage4comprises a cyclonic separator10, which has the advantage that relatively coarse dirt may be removed without the need for a filter or other means that would require washing or replacing. Nevertheless, the first dirt-separation stage4may comprise alternative means, such as a washable filter, for removing dirt that would otherwise block, jam or damage the impeller21.

The cleansed air discharged from the second dirt-separation stage7is pushed through the interior of the vacuum motor6and is used to cool components of the electric motor22. In particular, the cleansed air flows over and cools electrical windings34and power switches35that are used to control the flow of current through the windings34. As a result, the electric motor22is able to operate at higher electrical power. If the vacuum cleaner6as a whole were located upstream of the second dirt-separation stage7and if the air drawn through the vacuum motor6were used to cool the electric motor22, the fine dirt carried by the airflow may damage or otherwise shorten the lifespan of the electric motor22. For example, the dirt may clog bearings or cover thermally-sensitive electrical components. By locating the second dirt-separation stage7downstream of the impeller21but upstream of the electric motor22, the advantages outlined above regarding a greater pressure rise may be achieved whilst simultaneously cooling the electric motor22using relatively clean air.

All of the air discharged from the second dirt-separation stage7is pushed through the interior of the vacuum motor6. This then has the advantage of maximising cooling since all of the available air is returned through the vacuum motor6. Nevertheless, the path through the vacuum motor6may be relatively restrictive. It may therefore be desirable to push only a part of the airflow through the vacuum motor6. The remainder of the airflow would then bypass the second inlet27and instead flow along the outside of the vacuum motor6. This may be achieved, for example, by adapting the radial mount30such that the lip seal32forms only a partial seal. By pushing only part of the airflow through the interior of the vacuum motor6, the flow rate should increase owing to the less restrictive path formed by the bypass. If cooling of the electric motor22is not a concern or can be achieved by other means, pushing the air from the second dirt-separation stage7through the interior of the vacuum motor6may be avoided altogether. Alternatively, if the housing20is made of metal or some other material having a high thermal conductivity then it may be possible to achieve sufficient cooling of the electric motor22by passing the air along the outside of the vacuum motor6.