GB2456860A

GB2456860A - Pump magnetic drive adjustment

Info

Publication number: GB2456860A
Application number: GB0822184A
Authority: GB
Inventors: Paul David Shepherd; Magnus Cooper; David John Williams
Original assignee: CONCENTRIC VFP Ltd
Current assignee: CONCENTRIC VFP Ltd
Priority date: 2008-01-22
Filing date: 2008-12-05
Publication date: 2009-07-29
Anticipated expiration: 2028-12-05
Also published as: GB0822184D0; GB2456860A8; GB2456860B; GB0801100D0

Abstract

A pump 10 comprises a pump rotor 12, in a pumping chamber 13, and a drive shaft 16 coupled together by a first coupling half 20 connected to the pump rotor 12 and a second coupling half 22 connected to the drive shaft 16, one of the coupling halves 20, 22 being provided with an element capable of generating a magnetic field and the other coupling half 20, 22 being provided with an electrically conductive material. The position of at least one of the coupling halves 20, 22 is adjustable relative to the other coupling half to vary the degree of interaction between the magnetic field and the electrically conductive material in accordance with fluid pressure acting on the moveable half 20, 22 or halves, of the coupling. The fluid pressure in an annular chamber 44, between the first and second coupling halves 20, 22, is controlled by a valve, this preferably controlling the size of a gap G and therefore the amount of slip between the first and second coupling halves 20, 22, which determines the speed of rotation of the pump rotor 12. The pump 10 may be part of an automobile engine cooling system, the pump speed being controlled in response to a measured engine temperature.

Description

Title: Pump

Description of Invention

The present invention relates to a pump, particularly but not exclusively to a pump for pumping coolant around an automotive engine.

The pump for pumping coolant around an automotive engine is typically mechanically driven via a direct mechanical connection with an output shaft from the engine the pump is used to cool. When the pump is driven in this way it will be appreciated that there is a direct correlation between the speed of operation of the pump, and the engine speed. It may, however, be desired to control the speed of the pump independently of the engine speed, and, in order to do this, it is known to connect the pump to the engine output shaft via a magnetic coupling. This idea is disclosed in US6007303, for example, and in the pump described in this document, the torque transmitted by the magnetic coupling, and hence the speed of operation of the pump, is varied by using a mechanical actuator to alter the width of the air gap between the two halves of the magnetic coupling.

According to a first aspect of the invention we provide a pump including a pumping part which is mounted for rotation in a pump chamber formed by a housing, rotation of the pumping part causing pumping of fluid within the pump chamber from an inlet provided in the housing to an outlet provided in the housing, a rotatable drive shaft, and a coupling by means of which rotational movement of the drive shaft about its axis may be transmitted to the pumping part to cause the pumping part to rotate, the coupling comprising a first half which is connected to the pumping part, and a second half which is connected to the drive shaft, one of the first half or second half being provided with an element capable of generating a magnetic field, and the other of the first half or second half being provided with an electrically conductive material, one or both of the first and second half of the coupling being movable relative to the other to vary the degree of interaction between the magnetic field generated by one half of the coupling with the electrically conductive material provided on the other half of the coupling in accordance with fluid pressure acting on the movable half or halves of the coupling, wherein the pump further includes a valve which is operable to control the fluid pressure acting on the movable half or halves of the coupling.

Preferably the or each of the movable half or halves of the coupling is movable generally parallel to the axis of the drive shaft.

The first and second half of the coupling may be separated by a gap, and one or both of the first and second half of the coupling movable to alter the separation of the first and second half of the coupling.

Alternatively, one or both of the first and second half the coupling may be movable so as to alter the proportion of the first half of the coupling which opposes the second half of the coupling.

Preferably the first half of the coupling is movable whilst the second half of the coupling is fixed such that movement of the second half of the coupling so as to vary the interaction between the first and second half of the coupling is not permitted. The movable half of the coupling may be movable in accordance with fluid pressure in the pump chamber. Preferably the first half of the coupling is movable to alter the interaction between the first and second half of the coupling in accordance with fluid pressure acting on the pumping part and/or the first half of the coupling.

Preferably the housing includes a separator portion which extends between the first and second halves of the coupling. This separator portion of the housing preferably provides a fluid-tight seal which substantially prevents fluid in the housing contacting the second half of the coupling. In this case, pressure release means may be provided to vary the pressure of fluid in a portion of the pump chamber between the separator portion of the housing and the first half of the coupling. The pressure release means may comprise a conduit which extends from the portion of the pump chamber between the separator portion of the housing and the inlet.

Preferably the pumping part is an impeller.

The valve may be electrically operable.

The conductive material may be copper or copper based.

The element capable of generating a magnetic field may comprise a permanent magnet.

The first half may be provided with the conductive material, and the second half provided with the element capable of generating a magnetic field.

The half of the coupling provided with a conductive material may also be provided with a portion made from a magnetisable material such as cast iron.

According to a second aspect of the invention we provide a cooling system for an automotive engine, the cooling system including a pump according to the first aspect of the invention, an electrically operable valve which is operable to control the fluid pressure acting on the movable half or halves of the coupling, a temperature sensor, the electrically operable valve being connected to an electronic control unit which receives an input from the temperature sensor representative of a temperature of the engine and which is configured to operate the valve in accordance with the input from the temperature sensor.

According to a third aspect of the invention we provide a method of operating a cooling system for an engine, the cooling system including a pump including a pumping part which is mounted for rotation in a pump chamber formed by a housing, rotation of the pumping part causing pumping of fluid within the pump chamber from an inlet provided in the housing to an outlet provided in the housing, a rotatable drive shaft, and a coupling by means of which rotational movement of the drive shaft about its axis may be transmitted to the pumping part to cause the pumping part to rotate, the coupling comprising a first half which is connected to the pumping part, and a second half which is connected to the drive shaft, one of the first half or second half being provided with an element capable of generating a magnetic field, and the other of the first half or second half being provided with an electrically conductive material, one or both of the first and second half of the coupling being movable relative to the other, the pump further including a valve which is operable to control the fluid pressure acting on the movable half or halves of the coupling, wherein the method includes the step of operating the valve to the vary the degree of interaction between the magnetic field generated by one half of the coupling with the electrically conductive material provided on the other half of the coupling.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings of which FIGURE 1 shows a cross-section through a pump having an axially-aligned coupling according to the first aspect of the invention, FIGURE 2 shows a schematic illustration of an engine cooling system including the pump shown in Figure 1, and FIGURE 3 shows a cross-section through a pump having a radial coupling, according to the first aspect of the invention.

Referring now to Figure 1, there is shown a pump 10 including a pumping part 12, which in this example is an impeller, which is mounted for rotation about axis A in a pump chamber 13 formed by a housing 14, rotation of the impeller 12 causing pumping of fluid within the pump chamber 13 from an inlet 14a to an outlet 14b provided in the housing 14. Rotation of the impeller 12 is driven by a rotatable drive shaft 16, which is coupled to the impeller 12 via a magnetic coupling 18. The coupling 18 comprises a first half 20 which is secured to the impeller 12, and a second half 22 which is secured to the drive shaft 16.

The impeller 12 is of generally conventional construction, and has a front surface 24 which comprises a generally frusto-conical portion 24a and a generally cylindrical portion 24b which extends into the pump chamber 13 from around the central aperture formed at the apex of the frusto-conical portion 24a. The impeller 12 also has a generally flat, annular back surface 26, which lies in a plane generally perpendicular to the axis of rotation A of the impeller 12, the impeller 12 being arranged such that the back surface 26 is located between the front surface 24 and the magnetic coupling 18. The outer diameter of the back surface 26 is substantially the same as the largest diameter of the frusto-conical portion of the front surface 24, and the front surface 24 is oriented relative to the back surface 26 such that the separation of the front 24 and back 26 surfaces increases from the outer diameter of the frusto-conical portion 24a to the cylindrical portion 24b. A plurality of radial fins are provided between the front surface 24 and back surface 26 of the impeller 12.

The housing 14 is shaped to provide a volute 15 which extends around the outer periphery of the front 24 and back 26 surfaces of the impeller 12, the outlet 14b being provided in the wall of the volute 15. The inlet 14a is provided so as to direct incoming fluid into the cylindrical portion 24b of the front surface 24 of the impeller 12. A portion of the housing 14 lies closely adjacent to, but not touching, the front surface 24 of the impeller 12 to provide a degree of separation between the volute 15 and the inlet 14a.

The first half 20 of the coupling 18 comprises an annulus made from an electrically conductive material, which in this example is copper but could equaiiy be another conductive material such as aluminium, mounted on a backing made from a magnetisable material, in this example, iron. The annulus has substantially the same inner and outer diameter as the back surface 26 of the impeller 12, and is mounted against and secured to the back surface 26 of the impeller 12.

The second half 22 of the coupling 18 comprises an element capable of generating a magnetic field which in this embodiment of the invention is a plurality of permanent magnets which form sectors of a generally annular array arranged about the axis of rotation B of the drive shaft. It should be appreciated that the element capable of generating a magnetic field could equally comprise a solenoid or coils of conductive wire to which an electric current is provided.

It should also be appreciated that the first half 20 of the coupling 18 could equally be provided with the permanent magnets and the second half 22 with the conductive annulus.

The impeller 12 is mounted on a generally cylindrical shaft 28 a first end 28a of which extends through the central aperture in the back surface 26 towards the front surface 24 of the impeller 12. The impeller 12 is provided with a cylindrical bearing surface 30 which surrounds and engages with the shaft 28 and supports the impeller 12 whilst permitting rotation of the impeller 12 about its axis A around the shaft 28 and lateral movement of the impeller 12 along the shaft 28. There is a stop 32, which has a larger diameter than the remainder of the shaft 28, provided at the first end 28 of the shaft 28. The stop 32 engages with the bearing surface 30 of the impeller 12 so as to prevent the impeller 12 from sliding off the first end 28a of the shaft 28.

The second end 28b of the impeller support shaft 28 extends from the pump chamber 13 through an aperture provided in the housing 14 and is mounted in a support structure 34 which is secured to the housing 14, in this example by means of bolts 36. Movement of the shaft 28 with respect to the support structure 34, and hence the pump housing 14 is thereby substantially prevented.

The support structure 34 also provides a bearing 38 for the drive shaft 16 and the second half 22 of the coupling 18. The bearing 38 encloses a generally cylindrical space in which is mounted the drive shaft 16. The bearing 38 supports the drive shaft 16 whilst permitting the drive shaft to rotate in the bearing 38 about its longitudinal axis B. The impeller support shaft 28 and the bearing 38 are arranged such that their longitudinal axes coincide. As a result, the axis of rotation A of the impeller 12 and the axis of rotation of the drive shaft B also substantially coincide.

The magnets 23 of the second half 22 of the coupling 18 are mounted on support 40 which includes a circular side wall 40a which extends around the support structure 34, and a generally circular end cap 40b which is secured to the drive shaft 16. The side wall 40a encloses a generally cylindrical space and has a first end on which the magnets 23 are mounted, and a second end which is closed by the end cap 40b. The end cap 40b is secured to the drive shaft 16 such that the longitudinal axis of the support 40 coincides with the axis of rotation B of the drive shaft 16. Thus, when the drive shaft 16 rotates about axis B, the magnets 23 rotate about the centre of the circle on which they are arranged. The outer diameter of this circle corresponds to the outer diameter of the first half 20 of the coupling, and the support 40 is oriented such the side wall 40a extends from the end cap 40b towards the first half 20 of the coupling 18. As such, the magnets 23 are directly adjacent the copper disc and are attracted to the back-iron of the first half 20 of the coupling 18.

The first 20 and second 22 half of the coupling 18 are, however, separated by a gap G which extends between the two halves of the coupling generally parallel to the axis of rotation of the drive shaft B. It will be appreciated that sliding movement of the impeller 12 along the impeller support shaft 28 will cause the width of this gap to be varied.

A non-conductive, non-magnetisable membrane 42 is provided in the gap to close the aperture in the housing 14 through which the impeller support shaft 28 extends. The membrane 42 is secured to the impeller support shaft 28 and the housing 14, such that there is a fluid-tight seal provided between these parts, and fluid flow into and out of the pumping chamber 13 is permitted via the inlet 14a and outlet 14b only. By virtue of the use of a magnetic coupling, no mechanical connection is required between the impeller 12 and the drive shaft 16, and the pump chamber 13 can be sealed without the need to provide a rotary seal.

The proximity of the magnets 23 to the copper annulus of the first half 20 of the magnetic coupling 18 ensures that relative movement of the magnets 23 and the copper annulus 20 causes electrical eddy currents to be set up in the copper annulus 20. These eddy currents produce a magnetic field which interacts with the magnetic field produced by the magnets 23, and produces a force which acts on the magnets 23 and hence produces a torque which acts to reduce the difference in speed between the two halves 20, 22 of the coupling 18.

The magnitude of the torque produced by the interaction of the magnetic fields depends on the width of the gap G -the closer together the two halves 20, 22 of the coupling 18 are the stronger the torque. As a result, if the drive shaft 16 is rotating at a constant speed, the speed of rotation of the impeller 12 can be varied by varying the width of the gap G. Increasing the width of the gap G reduces the speed of the impeller 12, and decreasing the width of the gap G increases the speed of the impeller 12.

The width of the gap G is altered by sliding of the impeller 12 along the impeller support shaft 28, such movement being caused by the forces acting on the impeller 12 arising from the fluid pressure acting on the impeller 12 and the interaction between the two halves 20, 22 of the magnetic coupling 18.

When the impeller 12 rotates about the impeller support shaft 28, fluid at the inlet 14a is drawn into the cylindrical portions 24b of the front surface 24 of the impeller 12, and flows around the fins through the space between the front 24 and back 26 surfaces of the impeller 12 into the volute 15. The rotation of the impeller 12 propels the fluid around the volute 15 until it is expelled at the outlet 14b. As a result of the progressively increasing diameter of the volute, the velocity of the fluid is converted to static pressure, so the fluid becomes pressurised. This process causes the impeller 12 to be subjected to a static pressure of the pressurised fluid in the pump chamber 13, and a force acting on the front surface 24 of the impeller acting to push the impeller 12 towards the second half 22 of the coupling 18 caused by the momentum of the fluid flowing into the pump chamber 13 via the inlet 14a.

There is a gap between the first half 20 of the magnetic coupling 18 and the membrane 42, which forms an annular chamber 44 within the pump chamber 13, and dynamics of the fluid movement in the pump chamber 13 are such that fluid within this chamber is pressurised to a higher pressure than fluid at the pump inlet. As a result of the pressurised fluid within this chamber, there is a resultant force acting on the impeller 12 pushing it away from the second half 22 of the magnetic coupling.

The attractive force between the magnets 23 and the back-iron acts to draw the impeller 12 towards the second half 22 of the magnetic coupling 18 whilst the inductive repulsion between the magnets 23 and the magnetic fields produced by the eddy currents in the copper disc act to push the impeller 12 away from the second half 22 of the magnetic coupling 18. The magnitude of both of these forces increases with decreasing separation of the two halves 20, 22 of the coupling 18.

It will be appreciated that, in operation, the impeller 12 will move along the impeller support shaft 28 until either these forces cancel each other out, l,e, until the net resultant force on the impeller 12 is zero, or, if the forces acting to push the impeller 12 away from the second half 22 of the magnetic coupling 18 are strong enough, until the impeller 12 engages with the stop 32.

It will also be appreciated that by varying any of these forces, the width of the gap G can be varied, and in this example, this is achieved by providing a passage 46 which extends through the housing 14 from the annular chamber 44 between the first half 20 of the magnetic coupling 18 and the membrane 42 to the inlet 14a. Thus, this passage 46 provides a release for fluid pressure in the annular chamber 44, and therefore a means for reducing the width of the gap G. A valve (not shown), in this example an electrically operated solenoid valve, is provided to control flow of fluid through this passage 46, and, as such, the valve is operable to vary the width of gap G, and hence the torque transmitted by the magnetic coupling 18. The valve may, for example be located in the port 53 shown in the bottom right of Figure 1.

The pump 10 is adapted to be incorporated into a cooling system for an automotive engine, as shown in Figure 2, and to pump cooling fluid around the engine. In this case, the drive shaft 16 is connected to an output shaft of the engine by conventional mechanical transmission means.

At least one temperature sensor is provided in the engine, and this is / these are connected to an electronic control unit which receives an input from the or each sensor representing a temperature of the engine. The valve is connected to the electronic control unit so that the electronic control unit can control operation of the valve.

If the control unit determines that the temperature of the engine is higher than desired, it is programmed to open the valve to permit flow of fluid from the annular chamber 44 to the inlet 14a, thus reducing the fluid pressure in the annular chamber 44 and the width of the gap G. The resulting reduction in the width of the gap G causes the torque transmitted by the magnetic coupling to increase, i.e. the degree of slip between the drive shaft 16 and the impeller 12 to decrease, and the speed of rotation of the impeller 12 to increase. As the speed of rotation of the impeller 12 increases, the speed at which coolant is pumped round the engine increases. The engine is therefore cooled more effectively, and the temperature of the engine decreases.

Similarly, if the control unit determines that the temperature of the engine is lower than desired, it is programmed to close the valve to prevent flow of fluid from the annular chamber 44 to the inlet 14a, thus increasing the fluid pressure in the annular chamber 44 and the width of the gap G. The resulting increase in the width of the gap G causes the torque transmitted by the magnetic coupling to decrease, i.e. the degree of slip between the drive shaft 16 and the impeller 12 to increase, and the speed of rotation of the impeller 12 to decrease. As the speed of rotation of the impeller 12 decreases, the speed at which coolant is pumped round the engine decreases, the engine is therefore cooled less, and the temperature of the engine increases.

The electronic control unit can control the extent to which the valve is open, which in turn determines the pressure in the annular chamber 44, until an equilibrium pressure is reached at which the impeller speed is such that the desired degree of engine cooling is achieved.

An alternative embodiment of the invention is illustrated in Figure 3, in which the two halves of the coupling 18 are configured such that one half of the coupling is predominantly surrounds the other half (radial alignment'), such that a proportion of one half of the coupling opposes the other half. Referring now to Figure 3, there is shown a pump 110 having broadly the same components as pump 10. Components of pump 110 corresponding to those in pump 10 have been numbered with the same number as that used in Figure 1, with the addition of a preceding 1'. Rather than repeating the description of the common components of pump 110, a description of those parts that differ follows.

Impeller 112 is of a similar construction to impeller 12, having a front surface 124 which comprises a generally frusto-conical portion 124a and a generally cylindrical portion 124b which extends into pump chamber 113. Additionally, impeller 112 includes an outer wall 124c which is disposed around and projects outwardly from the perimeter of frusto-conical portion 124a. Outer wall 124c lies directly adjacent an inner wall of housing 114 so as to limit the amount of fluid that can bypass the edge of the impeller 112.

Impeller 112 further includes a generally-cylindrical flange 127 which extends from the perimeter of back surface 126 so as to receive second half of the coupling 122. The radially outer surface of the flange 127 lies directly adjacent the inner wall of the housing 114 so as to limit the amount of fluid that can pass from the high pressure region of the volute 115 into the gap G. Impeller 112 is mounted on a generally cylindrical shaft 128 for rotation about the shaft 128. First end 128a of shaft 128 is capped by end stop 133, which is mounted in a support structure 135. The support structure 135 extends from the housing 113 between the two halves 120, 122 of the coupling 118. It will be appreciated, therefore, that in this example, the support structure 135 provides the fluid tight seal between the two halves 120, 122 of the coupling 118, in addition to providing support for the first end 128a of the impeller shaft 128.

Second end 128b of impeller support shaft 128 extends from the pump chamber 113 through an aperture provided in the housing 114. A further end stop 137 is provided within housing 114, disposed adjacent inlet 11 4a and surrounding shaft 128, so as to limit the movement of impeller 112 towards inlet 114a. Cylindrical thrust bearings 135 are mounted in the central aperture of the impeller 112 through which the shaft 128 extends, on at either end of the impeller, the thrust bearings 135 providing both a bearing surface supporting rotation of the impeller 112 about the shaft 128, and a bearing surface which engages with either one of the end stops 133, 137 depending on the position of the impeller 112 relative to the shaft 128.

Inlet 114a is formed in a spiral configuration around generally cylindrical shaft 128, the opening of the inlet facing the opening in the front surface 124 of impeller 112. Fluid enters chamber 113 by flowing through inlet I 14a, causing it to enter the chamber with a radial motion. The fluid is drawn into the front of the impeller 112, and is forced through the fins of the impeller 112 under pressure resulting from its rotation. The fluid leaves the impeller 112 and enters volute 115, from where fluid leaves the chamber 113 via outlet 11 4b in the housing 114.

The partial seal formed between wall 124c and the inner wall of housing 114 assists in maintaining a pressure differential between the fluid between inlet 114a and outlet 114b, and ensures that a maximum amount of high pressure fluid in the volute 115 leaves the chamber 113 via the outlet 11 4b, rather than flowing back to the fluid at lower pressure around the inlet 11 4a.

The first half 120 of coupling 118 comprises an annulus made from electrically conductive material, which is secured to the inner surface of flange 127 on the impeller 112. The second half 122 of the coupling 118 comprises a generally annular array of magnets disposed around the outer wall of generally-cylindrical rotor element 125. Second half 122 of the coupling 118 lies radially within first half 120 of the coupling when the impeller 112 lies towards the left in Figure 3, i.e. nearest the first half 120 of the coupling 118 so that a large proportion of one half opposes the other, resulting in a strong coupling between the two halves.

The magnitude of the torque produced by the interaction of the magnetic fields depends on the width of the gap G -the closer together the rotor 125 and impeller 112 are, the closer the alignment of the two halves 120, 122 of the coupling I 18, and therefore the stronger the torque. As a result, if the drive shaft 116 is rotating at a constant speed, the speed of rotation of the impeller 112 can be varied by varying the width of the gap G. Increasing the width of the gap G reduces the speed of the impeller 112, and decreasing the width of the gap G increases the speed of the impeller 112.

The pressure in annular chamber 144 can be regulated by means of valve 153 in the housing 114, which in the example comprises a piston 151 mounted in a cylindrical chamber 158 in the housing 114. A bore 146 extends between the cylinder 154 and the portion of the pump chamber 113 adjacent the gap G between the two halves of the coupling 118. The piston 151 is movable between a first position, in which the bore 146 is blocked, and a second position in which bore 146 is unblocked. Movement of the piston 151 in the cylinder 154 may be driven by a solenoid mechanism which may comprise an electromagnetic coil, for example.

When piston 151 is in its first position, and bore 146 is blocked, despite the partial seal provided by the outer surface of the flange 127 and the inner wall of the housing 114, some leakage of fluid occurs from the high pressure volute into the gap G between the two halves 120, 122 of the coupling 118.

Thus fluid pressure builds up in the gap G, which pushes the impeller 112 away from the second half 122 of the coupling 118, i.e. towards the right of Figure 3. This reduces the proportion of the conductive ring forming the first half 120 of the coupling 118 which directly opposes the magnets of the second half 122 of the coupling 118. This reduces the degree of interaction of the magnetic filed produced by the magnets with the conductive ring 120, and therefore reduces the torque transmitted by the coupling 118, so that the speed of rotation of the impeller 112 reduces relative to the speed of rotation of the drive shaft 116.

When the piston 151 in its second position, aperture 146 is opened, and fluid pressure in the gap G can be released by means of flow of fluid from the gap G to the relatively low pressure inlet 114a. This causes the impeller 127 to move towards the second half 122 of the coupling 118, i.e. to the left of Figure 3. This increases the proportion of the conductive ring 120 which directly opposes the magnets 122, increases the degree of interaction of the magnetic field produced by the magnets with the conductive ring, and hence increases the torque transmitted by the coupling 118 so that the rotational speed of the impeller 112 increases. When moved between the first and second positions, piston 151 partially blocks bore 146, allowing a restricted flow of fluid from the annular chamber 144 to the inlet 11 4a, which causes the impeller 112 to move to an intermediate position.

A probe 155 may be provided in order to take readings of heat and / or pressure in annular chamber 144, for example to allow for automatic operation of the valve 53 to regulate the pressure in annular chamber 144.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A pump including a pumping part which is mounted for rotation in a pump chamber formed by a housing, rotation of the pumping part causing pumping of fluid within the pump chamber from an inlet provided in the housing to an outlet provided in the housing, a rotatable drive shaft, and a coupling by means of which rotational movement of the drive shaft about its axis may be transmitted to the pumping part to cause the pumping part to rotate, the coupling comprising a first half which is connected to the pumping part, and a second half which is connected to the drive shaft, one of the first half or second half being provided with an element capable of generating a magnetic field, and the other of the first half or second half being provided with an electrically conductive material, one or both of the first and second half of the coupling being movable relative to the other to vary the degree of interaction between the magnetic field generated by one half of the coupling with the electrically conductive material provided on the other half of the coupling in accordance with fluid pressure acting on the movable half or halves of the coupling, wherein the pump further includes a valve which is operable to control the fluid pressure acting on the movable half or halves of the coupling.

2. A pump according to claim 1 wherein the or each of the movable half or halves of the coupling is movable generally parallel to the axis of the drive shaft.

3. A pump according to claim I or claim 2 wherein the first and second half of the coupling are separated by a gap, and one or both of the first and second half of the coupling is / are movable to alter the separation of the first and second half of the coupling.

4. A pump according to claim I or claim 2 wherein the first and second half the coupling are movable so as to alter the proportion of the first half of the coupling which opposes the second half of the coupling.

5. A pump according to any preceding claim wherein the first half of the coupling is movable whilst the second half of the coupling is fixed such that movement of the second half of the coupling so as to vary the degree of interaction between the first and second half of the coupling is not permitted.

6. A pump according to any preceding claim wherein the movable half of the coupling is movable in accordance with fluid pressure in the pump chamber.

7. A pump according to any preceding claim wherein the first half of the coupling is movable to alter the interaction between the first and second half of the coupling in accordance with fluid pressure acting on the pumping part and I or first half of the coupling.

8. A pump according to any preceding claim wherein the housing includes a separator portion which extends between the first and second halves of the coupling.

9. A pump according to any one of claims 6, 7 and 8 wherein pressure release means are provided to vary the pressure of fluid in a portion of the pump chamber between the separator portion of the housing and the first half of the coupling.

10. A pump according to claim 9 wherein the pressure release means comprises a conduit which extends from the portion of pump chamber between the separator portion of the pump housing and the first half of the coupling and the inlet.

11. A pump according to claim 1 and claim 10 wherein the valve is operable to permit or prevent flow of fluid through the conduit.

12. A pump according to claim 11 wherein the membrane provides a fluid-tight seal with the housing which substantially prevents fluid in the housing contacting the second half of the coupling.

13. A pump according to any preceding claim wherein the pumping part is an impeller.

14. A pump according to claim I wherein the valve is electrically operable.

15. A pump according to any preceding claim wherein the conductive material is copper.

16. A pump according to any preceding claim wherein the first half of the coupling is provided with the magnetisable material, and the second half of the coupling is provided with the permanent magnet.

17. A pump according to any preceding claim wherein the half of the coupling provided with the conductive material is also provided with a portion made from a magnetisable material.

18. A cooling system for an automotive engine, the cooling system including a pump according to any preceding claim, an electrically operable valve which is operable to control the fluid pressure acting on the movable half or halves of the coupling, a temperature sensor, the electrically operable valve being connected to an electronic control unit which receives an input from the temperature sensor representative of a temperature of the engine and which is configured to operate the valve in accordance with the input from the temperature sensor.

19. A method of operating a cooling system for an engine, the cooling system including a pump including a pumping part which is mounted for rotation in a pump chamber formed by a housing, rotation of the pumping part causing pumping of fluid within the pump chamber from an inlet provided in the housing to an outlet provided in the housing, a rotatable drive shaft, and a coupling by means of which rotational movement of the drive shaft about its axis may be transmitted to the pumping part to cause the pumping part to rotate, the coupling comprising a first half which is connected to the pumping part, and a second half which is connected to the drive shaft, one of the first half or second half being provided with an element capable of generating a magnetic field, and the other of the first half or second half being provided with an electrically conductive material, one or both of the first and second half of the coupling being movable relative to the other, the pump further including a valve which is operable to control the fluid pressure acting on the movable half or halves of the coupling, wherein the method includes the step of operating the valve to the vary the degree of interaction between the magnetic field generated by one half of the coupling with the electrically conductive material provided on the other half of the coupling.

20. The method of claim 20 wherein the or each of the movable half or halves of the coupling is movable generally parallel to the axis of the drive shaft.

21. The method of claim 19 or claim 20 wherein the first and second half of the coupling are axially aligned and separated by a gap, and the separation of the first half of the coupling from the second half of the coupling is altered by varying the fluid pressure acting on the first half of the coupling.

22. The method of claim 19 or claim 20 wherein the first and second half of the coupling are radially aligned, and the extent of the alignment of the first and second half of the coupling is altered by varying the fluid pressure acting on the first half of the coupling.

23. The method of claim 21 or claim 22 wherein the degree of interaction between the first and second half of the coupling is altered by operating a valve to control flow of fluid from a portion of the pumping chamber between the first half of the coupling and a portion of the housing between the first half of the coupling and the second half of the coupling.

24. The method of claim 23 wherein the valve is operable to control flow of fluid from a portion of the pumping chamber between the first half of the coupling and a portion of the housing between the first half of the coupling and the second half of the coupling to the pump inlet.

25. A pump substantially as hereinbefore described with reference to and / or as shown in the accompanying drawings.

26. A cooling system for an engine substantially as hereinbefore described with reference to and I or as shown in the accompanying drawings.

27. A method of operating a pump substantially as herein before described with reference to and / or as shown in the accompanying drawings.

28. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.