Current axial flow pumps require a coupling mechanism between a pump and a motor which drives the pump. The motor is connected to the pump by a gearbox and a bearing. Each of these components must be manufactured separately and are then joined together. This required accurate alignment of all the components to close tolerances, making it very difficult to manufacture and assemble.

Conventional pumps which incorporate such a coupling mechanism increases the overall length of the pump and contributes to the weight and cost of the pump.

According to the present invention there is provided pumping apparatus comprising a Moineau pump and a centrifugal pump in combination, and a primemover for

providing rotational motion of an outer rotor of the Moineau pump.

Optionally, the centrifugal pump may be replaced by any pump of a given specific speed. Typically, such specific speed pumps include propeller, axial flow and mixed flow types, for example.

Typically, the Moineau pump comprises an outer and an inner rotor, wherein the lobe ratio (or number of teeth) of the inner and outer rotors are in the ratio of n: n+1, where n is any whole number.

Typically, the centrifugal pump is coupled to the Moineau pump at or near a fluid intake of the Moineau pump. This, when loaded, provides a higher than normal pressure fluid feed to the Moineau pump.

Alternatively, it may be mounted at or near a fluid outlet.

Typically, the outer rotor is rotatably mounted using an outer bearing.

The inner rotor may typically be mounted on a static central shaft which acts as a bearing. Alternatively, the inner rotor may be rotatably mounted using an integral shaft which rotates on a bearing.

Preferably, the lobe ratio of the Moineau pump is 1: 2 and the inner rotor defines a cantilever. Typically, the cantilever comprises three portions. Typically, the portions are a pumping profile, a support profile and a shaft profile. The pumping profile typically is an axial three dimensional sinusoid. Preferably, the sinusoid has a specific rolling diameter and most preferably has a rolling diameter which is one and a

half times the diameter of the inner rotor.

The support profile is typically a continuation of the pumping profile which extends axially in a two dimensional sinusoid. Typically, the shaft profile is a continuation of the support profile axially in one dimension only. Preferably, the shaft profile is a cylinder. Typically, the centre of the inner rotor shaft is also the centre of the rolling diameter.

Typically, the primemover is an electric motor.

Optionally, the primemover may comprise a hydraulic, pneumatic or an internal combustion engine.

According to a second aspect of the present invention there is provided a motor comprising an outer housing, an outer rotor mounted for rotation within the outer housing, an inner rotor mounted for rotation within the outer rotor, the inner rotor and outer rotor being of a Moineau construction.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which :- Fig. 1 is sectional view of a Moineau pump combined with a centrifugal pump; Fig. 2 is a view of a rotor assembly of Fig. 1; Figs 3a, b and c are sectional views through the line A-A in Fig. 2 showing 1: 2,2: 3 and 3: 4 lobe ratio Moineau pumps respectively; Fig. 4a is a section of an outer rotor of the 1: 2 lobed configuration of Fig. 1 showing the outer rotor axial section profile; Fig. 4b is a section of the inner rotor of the 1: 2 lobed configuration of Fig. 1 showing the inner rotor

axial section profile; Fig. 5a is a view showing a 1: 2 lobe ratio Moineau pump assembly, showing a solid shaft with a cantilever support; Fig. 5b is a plurality of views showing the interaction of the inner and outer rotors of Fig 5a as the outer rotor is turned through 360 degrees anti- clockwise; Fig. 6a is a front elevation of an inner rotor of a 1: 2 lobed Moineau pump, drilled axially to accept a static inner support; Fig. 6b is a sectional view of the inner rotor of Fig. 6a through the line A-A; Fig. 6c is a view of the inner rotor mounted on a static inner shaft; Fig. 6d is a view of a 1: 2 lobed inner rotor mounted on a rotatable inner shaft with a centrifugal rotor attached thereto; Fig. 6e is a view of a 1: 2 lobed inner rotor mounted on a static inner shaft showing the difference in the mounting of the centrifugal rotor; Fig. 7 is a section through the pump of Fig. 1 utilising the static inner support of Fig. 6c; Fig. 8a is a sectional view of the pumping apparatus of Fig. 1 modified to include a by-pass system for excess fluid, in normal operation; Fig. 8b is a sectional view of the apparatus of Fig. 8a showing the flow of fluid when the by-pass is operational; Fig. 8c is a sectional view of a pumping apparatus using a static inner rotor shaft; Figs 9a, b and c are a series of views of the apparatus of Figs 8a and b in progressive stages of mass transfer of fluid through a vertical conduit; Fig. 10a shows an end elevation, and a side elevation in the direction of arrow A, of the inner

rotor of Fig. 5a; Fig. 10b is an end elevation, and a side elevation in the direction of arrow B, of the inner rotor of Fig.

10a; Figs lla, b and c shows various samples of relationships between pitch length and the length of the support profile for the inner rotor of Fig. 5a; Figs 12a and b are vertical and horizontal sections respectively through the outer rotor from a 1: 2 lobed configuration having an inner diameter d and an outer diameter 4d; Figs 12c and d are the same views as Figs 12a and 12b with the inner rotor installed ; Fig. 12e is an end elevation showing the relationship of diameters and dimensions of the profiles regarding the inner and outer rotors; Fig. 13a is a side elevation of an inner and outer rotor of a Moineau pump with a 1: 2 lobe configuration and a 1: 4 inner to outer rotor diameter relationship; Fig. 13b is a sectional end elevation of the inner and outer rotors of Fig. 13a; Fig. 13c is a section of the inner and outer rotors of Fig. 13a in the direction of line A-A; Fig. 13d is a sectional plan view of the inner and outer rotors of Fig. 13b in the direction of line B-B; Figs 14a to f are a plurality of views of the 1: 2 lobed Moineau type pump, showing various arrangements of bearings, seals, stator and an electric primemover; Figs 15a and 15b are end and sectional elevation views respectively of a propulsion unit according to the present invention embodying a ceramic bearing and a variable outlet orifice; Figs 16 and 17 show a comparison between a propeller-type propulsion unit and a propulsion unit according to the present invention; Fig. 18 is a section through a modified pump of

Fig. 1 utilising a hollow inner rotor; Fig. 19 shows a single pump rotor embodiment of the invention; and Fig. 20 is a sectional view of a motor utilising the configuration of the present invention.

Referring in the first instance to Fig. 1, there is shown a pumping apparatus, generally designated 10, which comprises a Moineau pump 12 and a centrifugal pump 14 in combination.

The Moineau pump 12 is driven by an electric motor which has a stator 16 and a rotor 18, in keeping with conventional notation and operates in the conventional manner. It will be appreciated that any type of motor could be used and the electric motor of this embodiment is by way of example only.

The rotor 18 is coupled to an outer rotor 20 of the pump 12. As the rotor 18 is rotated by the electromagnetic force of the electric motor, the outer rotor 20 is caused to rotate in synchronisation with the rotor 18 of the electric motor. Rotation of the outer rotor 20 causes a subsequent rotation of an inner rotor 22.

As will be appreciated by those skilled in the art, Moineau pumps operate on the basis that the outer rotor 20 and the inner rotor 22 come into sliding contact at certain points along the length of the pump 12. The number of points at which they contact is governed by the lobe ratio of the inner rotor 22 to the outer rotor 20. The lobe ratio of the Moineau pump 12 shown in Fig. 1 is 3: 4, as will be appreciated by those skilled in the art.

Referring now to Figs 3a to c, a number of different lobe ratios are shown as examples. Fig. 3a shows a lobe ratio of 1: 2 whereas Figs 3b and 3c show 2: 3 and 3: 4 ratios respectively.

In a 1: 2 ratio configuration, as in Fig 3a, the inner rotor 22 is an axial three-dimensional sinusoid or a spiral. Any vertical section of the spiral gives a circle which has a centre offset from the centre 24 of the inner rotor shaft 26. The outer rotor 20 has a slot 28 which spirals in a similar manner to that of the inner rotor 22, the inner rotor 22 being engaged in the slot 28. The dimensions of the inner 22 and outer 20 rotors are optional.

In a 1: 2 ratio configuration, most of the dimensions of the inner 22 and outer 20 rotors 20 are governed by the diameter d of the inner rotor 20 cross-sectional profile. The slot 28 has two semi-circular ends which are connected by a substantially straight portion. The distance between the centres of the semi-circular sections which define radiused ends is d, that is the same as the inner rotor diameter. The radius of the curves is d/2 giving the slot 28 an over-all length of 2d.

Similarly, the inner rotor 22 as shown in Fig. 4b has the radius of the circles (which are cross-sections of the spiral) as d/2. As previously mentioned, the centre of this circle is offset from the centre of the inner rotor shaft 26, the distance being labelled as e (the eccentricity) and is expressed in terms of d as d/4.

As will be appreciated by those skilled in the art, as the centre of the spiral of the inner rotor 22 is

offset by a value e from the inner rotor shaft centre 24, the inner rotor 22 has a rolling diameter of 1.5d.

The principle of the Moineau pump 12 with a lobe ratio of 1: 2 is best described with reference to Figs 5a and 5b. In Fig. 5b, the numbered sequence of drawings shows the outer rotor 20 rotating one revolution anti- clockwise.

The outer rotor 20 is rotated by the rotor 18 of the electric motor. As the outer rotor 20 rotates, the inner rotor 22 rotates due to the inter-engagement of the inner 22 and outer 20 rotor profiles. The 1: 2 lobe ratio could be viewed as a standard gearing, where the inner rotor 22 has one elongated tooth and the outer rotor 20 has two. At certain points along the length of the rotors 20,22, the rotors 20,22 will come into sliding contact with one another. Hence, the electric motor drives the outer rotor 20 which in turn causes the inner rotor 22 to rotate due to the inter- engagement of the rotors 20,22.

As can be seen from the sequence of drawings in Fig.

5b, one revolution of the outer rotor 20 causes two revolutions of the inner rotor 22 as would be expected from a 1: 2 ratio.

Figs 6a to c show the configuration of the inner rotor 22 and also a static inner rotor shaft 26 of a 1: 2 lobed Moineau pump. In particular, Fig. 6a shows the offset of the inner rotor shaft 26 in comparison to the centre line of the spiral, when the inner rotor is supported by a static shaft.

A number of options for mounting the inner rotor 22 in the pumping apparatus 10 are available. Firstly, as

shown in Figs 6b and c, the inner rotor 22 is mounted onto a static shaft 26. The static shaft 26 is then mounted within the apparatus of Fig. 7 which will be described hereinafter, using an internal bearing surface 23, as best shown in Fig. 6b.

Alternatively, the inner rotor 22 is mounted on a rotatable inner support shaft 27 using a bearing 30 (see Figs 8a and 8b), the inner shaft 27 being rotatably mounted as part of the pumping apparatus 10.

If a cantilever support system is used, which will be described hereinafter with reference to Figs 10a and lOb, the shaft 27 is not required as the cantilever support provides the shaft. The cantilever support is generally for use with a 1: 2 lobed Moineau configuration.

Fig. 6d shows the inner rotor 22 of a 1: 2 lobed Moineau pump with a centrifugal rotor 21 of the centrifugal pump 14 attached thereto. In this embodiment, the inner rotor 22 is mounted in the apparatus 10 using the rotatable shaft 26 as will be described hereinafter.

The centrifugal rotor 21 is attached to the inner rotor 22 using a key 27 on the inner rotor 22. A boss 25 is then removably attached to the front end of the rotor 22 using a screw thread, thereby securing the centrifugal rotor 21 in place.

Fig. 6e shows the alternative arrangement where the inner rotor 22 is rotatably mounted onto a static inner shaft 27 which acts as an internal bearing. In this embodiment, the centrifugal rotor 21 is welded onto the static inner shaft 27. Note that there is no requirement for a boss to be screwed on to the static shaft 27 to hold the rotor 21 in position.

Turning now to Fig 7 there is shown an over-all view of the Moineau pump 12 for use with the present invention.

This particular embodiment uses a static inner rotor shaft 27. The inner rotor shaft 27 is non-rotatably mounted using two pins 32. Rotatably mounted on the shaft 27 is the inner rotor 22. The outer rotor 20 spirals around the inner rotor 22 as previously described.

In order to drive the outer rotor 20, the electric motor consists of a stator 16 and a rotor 18 as will be appreciated by those skilled in the art. The outer rotor 20 rotates on a series of bearings 34, two in this particular embodiment.

As the pump 12 is primarily to be used for pumping fluids, a plurality of seals 36 prevent fluid from entering the pump 12 and subsequently damaging the electric motor.

When the outer rotor 20 rotates, rotation of the inner rotor 22 is caused due to the 1: 2 gearing arrangement.

As can be seen from Fig. 7, a series of pockets 38 are created in 3-dimensions which allow fluid to be pumped from an inlet 40 to an outlet 42. The spiralling motion of the rotors 20,22 coupled with their configuration and lobe ratio, produces the pockets 38 which move longitudinally through the pump 12. This type of device is often referred to as a progressive cavity pumping device.

Referring again to Figs 1 and 2, the apparatus 10 of the present invention provides that a Moineau pump 12 has a centrifugal pump 14 coupled at the leading end of the Moineau pump 12. The centrifugal pump 14 is attached to the inner rotor shaft 26 of the Moineau

pump 14 using conventional means, such as welding or the use of a boss and keyway as described previously.

In the embodiment shown in Fig. 1, the centrifugal pump 14 is attached by way of a boss and keyway. The centrifugal rotor 21 (sometimes referred to as an impeller) is basically a wheel fitted with vanes. The purpose of the centrifugal pump 14 is to, in effect, create a supercharge for the Moineau pump 14.

Note that the centrifugal pump 14 is only one example of a specific speed pump as will be appreciated by those skilled in the art. Any specific speed pump may be used in place of the centrifugal pump as shown, and the present invention is not limited to such use.

In use, fluid flows axially towards the centrifugal rotor 21, is deflected by it, and flows out of the apertures between the vanes. The vanes produce a centrifugal acceleration (that is to say a change in direction and acceleration) of the fluid as it passes, thereby creating a higher pressure after the centrifugal pump outlet or diffuser 46.

The Moineau pump 12 is an example of a positive displacement pump, which operate on the principle of taking in fluid at a low pressure and expunging the same fluid at a higher pressure. Therefore, the effect of increasing the pressure at the diffuser 46, which feeds directly into the Moineau pump inlet 40 gives an increase in pressure at the Moineau pump outlet 42. It therefore follows that if the pressure at the inlet 40 can be increased, a similar increase in pressure at the outlet 42 can be expected. Hence, the centrifugal pump 14 creates an increase in pressure at the Moineau pump inlet 40, and has a function similar to that of a supercharger in a vehicle.

Referring now to Figs 8a and b, there is shown an alternative embodiment of the 3: 4 lobed configuration of Fig. 1. The apparatus in Figs 8a and b uses a bearing 30 on which the shaft 26 of the inner rotor 22 rotates. Here, the apparatus is generally the same as that shown in Fig. 1 with the addition of two ducts 48 which are in fluid communication with the volutes 46.

Fluid flow into the ducts 48 is controlled by a plurality of one-way valves 50.

In the normal mode of operation, as shown in Fig. 8a, fluid is drawn into the apparatus 10 as discussed previously. A vacuum is created at the diffuser 46 behind the centrifugal pump 14 which closes the non- return valves 50. Thus, all fluid which enters the centrifugal pump 14 passes through the Moineau pump 12 and is expelled at the outlets 42.

However, if the centrifugal pump 14 and the Moineau pump 12 are not hydraulically matched, that is to say the centrifugal pump 14 is drawing in more fluid than the Moineau pump 12 can cope with, then the vacuum at the diffuser 46 reduces and the non-return valves 50 open. This allows fluid to flow though the ducts 48 as shown in Fig. 8b and exit the apparatus 10 at secondary outlets 52. The inclusion of the non-return valves 50 is required if the capacity of the centrifugal pump 14 is greater than that of the Moineau pump 12.

Fig. 8c shows the same apparatus of that of Figs 8a and b but the inner rotor 22 of the Moineau pump 12 is coupled using a static inner shaft 26 as described above. Note that because there are no dynamic bearings on which the inner rotor 26 rotates, an overall reduction in cost is possible.

The apparatus 10 is shown in progressive stages of pumping in Figs 9a to c. Fig. 9a shows the apparatus 10 in a static condition, with the fluid 54 at a distance from the apparatus 10. Fig. 9b shows that the air within and in front of the apparatus 10 has been expelled through the outlet 42 and the fluid 54 has entered the eye of the centrifugal pump 14.

Fig. 9c shows the apparatus 10 in operation where the centrifugal pump 14 is providing an excess flow of water to the Moineau pump 12 and the fluid 54 is being discharged from the outlet 42. The non-return valves 50 are open in this particular embodiment and the ducts 48 are conveying excess fluid 54 to the secondary outlets 52.

Referring now to Figs l0a and b, there is shown in more detail the profile of the inner rotor 22 of a 1: 2 arrangement. The inner rotor 22 depicted in the drawings is formed as a cantilever. As the rotor 22 only has this one cantilever support then the number of bearings required for it to rotate on can be reduced to less than three; three being the conventional minimum number of bearings required for this type of application.

The inner rotor 22 comprises three distinct profiles; a shaft profile 56, a support profile 58 and a pumping profile 60. The pumping profile 60 is axially machined in three dimensions to be a spiralling sinusoid. The spiral is contained within a rolling diameter of 1.5d, where d is the diameter of the inner rotor 22, as previously explained.

The support profile 58 is a continuation of the sinusoid of the pumping profile 60 but in two-

dimensions only. That is to say that the support profile 58 does not spiral, as best seen in Fig. lOb.

The shaft profile 56 is a continuation of the support profile 58 in one dimension only, that is to say it is a cylinder.

Turning now to Figs lla to c, there is shown sample relationships between the axial length of the support profile 58 and pitch of the inner rotor 22.

It is found that the 1: 2 embodiment of inner rotor 22 has a pitch which is a number times the diameter d of the inner rotor 22 and the length of support profile 58 which is given by the same number times the eccentricity e. Note that the variable e has previously been defined as the eccentricity, that is to say the offset distance between the inner rotor 22 rolling diameter centre and the centre if the inner rotor shaft 24.

The profile of the inner rotor 22 allows one end to rotate freely. As such, only one bearing at the shaft profile 56 is required, thereby reducing the weight, cost and complexity of the design.

Figs 12a to e shows an alternative arrangement for the inner 22 and outer 20 rotors. In this example, the ratio of inner to outer rotor diameters is 1: 4. The cantilever support profile as described above, allows the inner and outer rotor diameters to be equal to and/or greater or less than 1: 2, providing that the inner rotor 22 has a specific rolling diameter.

This particular embodiment has an inner rotor pitch of 3d, a rolling diameter of 2.5d and a cantilever support of 0.75d, where d is the diameter of the shaft, as best

seen in Fig. 12e. It should be noted that in cases where the outer rotor diameter progresses beyond twice that of the inner rotor diameter d, it will be possible to operate at a reduced angular velocity and attain the same volumetric throughput for specific requirements.

Figs 13a to d show a variety of views of the 1: 2 lobed arrangement with an inner to outer rotor diameter ratio of 1: 4.

Referring now to Figs 14a to f there is shown a number of alternative arrangements of bearings, seals and the position of the electric motor or primemover.

Figs 14a to c have seals 62 at either end of the outer rotor 20 as shown. Note that the inner rotor shaft 26 is rotatable mounted using an inner rotor bearing 64 which is located at the same position in each of the figures. The position of the bearing 64 is dependent upon the use of the cantilever support structure as previously discussed. Note that the inner rotor 22 has a free end which results in one of the dynamic bearings being unnecessary.

The position of the electric motor which comprises a stator 16 and rotor 18 varies between a substantially central location in Fig, 14b, to a position where it is near the inner rotor bearing 64 in Fig 14a, to a position near the opposite end.

The outer rotor 20 rotates on a bearing 66, which can be placed in any of the positions shown in Figs 14a to c. These diagrams show the versatility of design which is available by using the cantilever support profile as previously described.

Figs 14e and f are similar arrangements to those shown in Figs 14a to c, except that the seals 62 are positioned near the cantilever support and substantially in the centre of the apparatus.

Figs 15a and 15b show an example of a marine propulsion unit which incorporates the present invention. Here, a 1: 2 ratio Moineau pump 12 is used. In this particular embodiment, a ceramic bearing 70 is used to support the cantilever profile. With this type of bearing, some of the fluid from the outlet 72 is fed back to the bearing 70 via a central duct 74 in the inner rotor 22 to lubricate the faces of the bearing 70 which are in sliding contact with one another. Note that the ceramic bearing 70 is supported by aerodynamic vanes 76 and that the intake 78 is parabolic.

The use of the ceramic bearing 70 obviates the necessity to use seals, although it may be desirable to seal off a chamber 80 in which the electric motor is housed.

The outlet 72 comprises a variable area nozzle 82. The nozzle 82 allows greater control of the thrust which the apparatus can supply.

Referring now to Figs 16 and 17 there is shown respectively a propeller propulsion device 100 and a marine propulsion device 110 according to the present invention. The propeller device 100 has a forward velocity of Vc, as shown by arrow 112 and a thrust having a velocity component value of Vj. The marine propulsion device 110 similarly has a craft velocity of Vc, as shown by arrow 114 and a thrust having a velocity component value of Vj.

As can be seen from the drawing, both devices 100,110 have an associated velocity acting in the opposite direction which is of the same magnitude as the craft velocity Vc, as shown by arrow 116.

However, the component of Vc acting in the opposite direction to the direction of the devices 100,110 produces internal drag on the propeller device 100.

Thus, the actual magnitude of the propulsion at the outlet of the device 100 is proportional to Vj minus Vc, as given by arrow 118.

The propulsion unit 110 of the present invention has, by the inherent design of the pumping system, very little or no internal drag. As a result, a pressure front is built-up in front of the device 110 and the thrust produced by the device 110 is therefore directly proportional to Vj, as shown by arrow 120.

Referring now to Fig. 18, there is shown a modified pump where in the inner rotor 122 is formed of a hollow member mounted on a static shaft 126. In this arrangement, there is defined a spiral path 128 which forms a dynamic seal between the static shaft 126 and the rotating inside skin 122A of the inner rotor 122.

Hence, the present invention provides a pumping apparatus which uses a particular ratio Moineau pump to drive a centrifugal pump. The Moineau pump can be of any lobe configuration. The centrifugal pump functions as a type of supercharger to provide a higher pressure input to the Moineau pump thereby increasing the efficiency of the Moineau pump and also the power output of it.

It will be appreciated by those skilled in the art that

the centrifugal pump is one type of rotor pump which may be coupled to the Moineau pump as described. Other types of Specific Speed rotors may be used. The centrifugal pump is used as an example only and the present invention is not limited to such use. In addition, the centrifugal pump may be positioned at the fluid outlet of the Moineau pump, as opposed to the fluid inlet.

Furthermore, the present invention provides a marine propulsion unit which has very little or no internal drag, giving a higher thrust output of similarly rated conventional devices coupled with an increase in efficiency. Note that it is possible to use the present invention in all types of fluid and hence the present invention may be used to propel aircraft through air for example, and should not be limited to marine, or submarine, applications.

With reference to Fig. 19 there is shown a single pump rotor embodiment of the invention.

In this embodiment a single pump rotor 190 is provided with a profiled inner chamber 191 which is lined with an elastomer seal 192. The elastomer seal 192 spirally contacts the surface of a static shaft 194 to define a sliding pumping seal. Rotation of the pump rotor 190 which is attached to a motor rotor 195 effects the necessary pumping action.

Referring now to Fig. 20, there is illustrated a mud motor having design features based on pumping apparatus herebefore described.

Conventional MWD (Measurement While Drilling) mud motors are limited in speed due to their construction.

Conventional design of MWD mud motors results in vibration due to the natural eccentricity between the stator and rotor which limits rotational speed and rate of penetration. Furthermore, conventional design places the MWD instrumentation distant from the drill bit and affects the accuracy of the measurements being taken.

As shown in Fig. 20, an MWD mud motor 200 comprises an inner rotor 201 mounted for rotation within an outer rotor assembly, the inner rotor and outer rotor assembly being located within an external housing 210 constituted by drill tubing. The outer rotor assembly rotates within the outer housing on bearings 208.

The outer rotor assembly comprises a first outer rotor 203 which interacts within the inner rotor 201 in the same manner as has been described hereabove in relation to the pumping apparatus.

The outer rotor assembly further comprises a second outer rotor 205 which is mechanically coupled to the first outer rotor 203 and carries a drill bit 206.

The inner rotor 201 is mounted for rotation upon an eccentrically mounted static inner bearing 220. The inner bearing 220 is in the form of a cantilever mounted shaft fixed to a support arrangement 222 in the form of a"spider"mounted within the external housing 210. The support bearing 220 may be retained within a suitable spigot mounted on the spider by means of a fixing pin 223.

The external housing 210 is provided with sample windows 230 through which the required information is communicated to acoustic data telemetry (ADT)

transducers 240 carried on the outer surface of the second outer rotor 205.

The ADT transducers are thus arranged to rotate past the windows 230 and as they do so, they gather the required information from the part of the hole being drilled at a particular depth at that instant. This information is transformed into acoustic signals which are relayed via the steel of the drill tubing to a receiver on the parent installation which will interpret them.

In use, mud is pumped from the parent installation into a mud entry 250 of the motor 200, thus causing the inner and outer rotors to rotate, whereafter the mud exits through a mud exit formed on the outer rotor 205.

In the embodiment described, the inner rotor 201 has a single lobe and the outer rotor 203 has two lobes resulting in the inner rotor 201 rotating at twice the speed of the outer rotor 203.

As the motor rotates, the mud exit hole 260 will shut off mud flow once in every revolution of the outer rotor 205. This will cause a momentary pressure pulse to travel back up the mud to the parent installation where it will be detected. The number of these pulses received per minute will represent the revolutions per minute of the outer rotor 205 and half the revolutions per minute of the inner rotor 201. In the current drawing the outer drill bit carrying rotor will be in synchronism with the speed of the outer rotor 205.

Whilst the outer rotor assembly is described as being formed of a first outer rotor 203 and a second outer rotor 205, these elements can be formed integrally.

Furthermore, the design permits additional instrumentation to be located in the static support bearing 220.

The mud motor described provides the advantage of reduced motor vibration permitting increased rotational speeds and improved penetration as well as increased instrumentational accuracy by extended positioning of the MWD instrumentation closer to the drill bit.

Modifications and improvements may be made to the foregoing without departing from the scope of the present invention.