Abstract:
A combination of a torque-generating apparatus with an alternator/generator. The torque-generating apparatus comprises a first assembly ( 10, 25 ) including a generally cylindrical member of magnetically soft material and having a longitudinal axis, and a second assembly ( 2 ) arranged coaxially within the first assembly and including an electromagnetic winding ( 4 ). The first assembly ( 10, 25 ) and the second assembly ( 2 ) are rotatable relative to each other about the axis. Relative rotation between the first ( 10, 25 ) and second ( 2 ) assemblies induces a magnetic field which generates rotational torque between the first and second assemblies. Rectification means ( 31 ) is provided to convert alternating current electrical output from the alternator/generator ( 22 ) to provide D.C. current to the electromagnetic windings ( 4 ) of the torque generating apparatus to generate an electromagnetic braking effect.

Description:
This invention relates to a combination of a torque-generating apparatus with a generator/alternator. More particularly, but not exclusively, the invention relates to such a combination for down hole use in drilling applications. 
   BACKGROUND OF THE INVENTION 
   The presently preferred power generator in oilfield exploration or extraction applications is the mud alternator because it is powerless on the surface, and therefore safe to handle, and only generates power once down hole and spinning in the mud flow. However, one of the major problems faced by both the provider and end-user of drilling equipment has been the control of impeller speed of down hole mud alternators. 
   All mud alternators effectively run open loop and the only way to govern their speed is either to modify the flow rate of mud pumped past the impeller or to change the impeller. These methods are not always successful in controlling alternator speed and, hence, power output. An alternator that rotates too quickly generates too much voltage for the down hole apparatus and consequently causes the apparatus to overheat. If an alternator causes the apparatus to overheat for too long, the apparatus is likely to fail resulting in a costly interruption of the drilling programme. A loss of down hole power results in a loss of communication with the drilling apparatus and the entire drill string must be pulled from the well to effect repairs. For an extended reach hole, the extraction, repair and replacement can take in excess of two days and a drilling engineer therefore avoids excessive rotation speeds for the alternator if at all possible. 
   To ensure the alternator is always operated within safe limits, the alternator RPM or output voltage is transmitted to the surface at frequent intervals in order that the drilling engineer may monitor the performance of the alternator and effect alternator speed corrections by adjusting the mud pumping-rate. 
   The preferred method for avoiding excessive rotation speed for the alternator is to select a suitable impeller for the alternator during the planning stage of a drilling contract. Mud flow rates can vary widely, for example from 150 to 800 United States gallons per minute. One of the planning tasks is to establish the minimum and maximum mud flow rates for the task in order that an optimum impeller can be selected. The difficulty with this method is that an incorrect impeller is sometimes fitted to the alternator. The drilling engineer is then faced with the decision of not using the drilling apparatus until the correct impeller has been fitted, or risking damage to the drilling apparatus due to excessive alternator power output. 
   BRIEF SUMMARY OF THE INVENTION 
   There is therefore a requirement to be able to control alternator speed in order to avoid excessive power outputs. 
   Furthermore, present apparatus is vulnerable to the aggregation of mud and debris leading to clogging of the machinery and down hole failure. 
   U.S. Pat. No. 4,713,567 describes an electromagnetic brake device for a sports training apparatus which includes a rotor (first assembly) including a generally cylindrical member of magnetically soft material and a longitudinal axis, and a stator (second assembly) arranged coaxially within the rotor and including an electromagnetic winding. The rotor and the stator are rotatable relative to each other about the axis. Relative rotation between the rotor and the stator induces a magnetic field which generates rotational torque between the rotor and stator assemblies, and rectification means is provided to convert alternating current electrical output from the generator to provide D.C. current to the electromagnetic windings of the torque generating apparatus to generate an electromagnetic braking effect. 
   It is an object of the present invention to provide a combination of a torque generating apparatus with an alternator/generator capable of fulfilling at least some of the above requirements. 
   According to the present invention there is provided a combination of a torque-generating apparatus with an alternator/generator, the torque-generating apparatus comprising: a first-assembly including a generally cylindrical member of magnetically soft material and having a longitudinal axis, a second assembly arranged coaxially within the first assembly and including an electromagnetic winding, the first assembly and the second assembly being rotatable relative to each other about the axis, the arrangement being such that relative rotation between the first and second assemblies induces a magnetic field which generates rotational torque between the first and second assemblies, and rectification means is provided to convert alternating current electrical output from the alternator/generator to provide D.C. current to the electromagnetic windings of the torque generating apparatus to generate an electromagnetic braking effect, and characterised in that the torque-generating apparatus and the alternator/generator are positioned axially adjacent one another on a common shaft. 
   A “magnetically soft material” is a material which is not capable of being substantially permanently magnetised, but which becomes magnetised whilst in an externally applied magnetic field. 
   The first assembly may be a rotor assembly of the apparatus for producing rotational torque and the second assembly may be a stator assembly of the torque generating apparatus. 
   The second assembly may comprise a magnetically soft steel. 
   The first and second assemblies may be separated by a narrow gap. 
   The first assembly may be disposed so as substantially to surround the second assembly. 
   The first assembly may be substantially solid or may be formed from a plurality of laminations. 
   The second assembly may be substantially solid or may be formed from a plurality of laminations. 
   A number of generally longitudinal grooves may be provided to the inside surface of the cylindrical member of the first assembly. The grooves may be substantially parallel to the longitudinal axis of the first assembly. Alternatively, the grooves may be provided so as to form at least a partial helix around the longitudinal axis of the first assembly. 
   The second assembly may be provided with a number of pole pieces extending generally radially from the longitudinal axis thereof. The pole pieces of the second assembly may be provided with an electromagnetic winding, adjacent poles being magnetisable in opposite directions. Means may be. provided to control the degree of the magnetisation. Gaps between the pole pieces may be filled with a potting material. The surface of the second assembly may be covered with a layer of soft magnetic or non-magnetic material. 
   The first assembly may be provided with external rotation means such as an impeller adapted to rotate the first assembly, the impeller being adapted to be disposed in use within a moving fluid, the motion of the fluid acting upon the impeller so as to rotate the first assembly. 
   The alternator/generator may be provided with external rotation means, such as an impeller adapted to rotate the alternator/generator, the impeller may be adapted to be disposed in use within a moving fluid, the motion of the fluid acting upon the impeller so as to rotate the alternator/generator. The impeller may be an integral part of a magnet carrier of the alternator/generator. 
   The electrical output of the alternator/generator may be connected directly to the electromagnetic winding of the torque-generating apparatus or may be connected indirectly by way of alternator voltage regulation means to create the electromagnetic braking effect. 
   Where the electrical power of the alternator/generator is connected directly to the electromagnetic winding of the torque-generating apparatus, the electromagnetic braking effect may be modified by varying the resistance of the winding, for example with one or more external resistances or by changing the gauge of the winding wire, or by varying the gap between the first and second assemblies of the torque-generating apparatus. 
   Where the electrical output of the alternator/generator is connected indirectly to the electromagnetic winding of the torque-generating apparatus, the alternator voltage regulation means may function to provide a progressive braking effect and/or to effect braking at a predetermined set point. The predetermined set point may be determined by the voltage output of the alternator/generator. The predetermined set point may be variable. 
   For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show schematically various embodiments of the present invention. The figures may not be to scale. In the drawings: 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of an embodiment of a combination according to the present invention; 
       FIG. 2  is an end view of the combination of  FIG. 1  looking in the direction of the arrow B; 
       FIG. 3  is a cross-sectional view of the combination of  FIG. 1  taken along the line A—A; 
       FIG. 4  is a more detailed view of a stator shown in  FIG. 1 ; 
       FIG. 5  shows two views of a stator pole piece; 
       FIG. 6  is a cross-section of the stator shown in  FIG. 4 ; 
       FIG. 7  is an end view of the stator shown in  FIG. 4 ; 
       FIG. 8  is a sectional view of a rotor shown in  FIG. 1 ; 
       FIG. 9  is an end view of the rotor of  FIG. 8 ; 
       FIG. 10  is an end view detail of the rotor and stator assembly; 
       FIG. 11  is a cross-sectional view of another embodiment of a rotor similar to that shown in  FIG. 1  showing a laminated structure; 
       FIG. 12  is an end view, with an end cap removed, of the rotor of  FIG. 11 ; 
       FIG. 13  is an end view detail, with an end cap removed, of a rotor and stator assembly incorporating the rotor of  FIGS. 11 and 12 ; 
       FIG. 14  is a schematic drawing of a rectifying means used in the combination of  FIG. 1 ; and 
       FIG. 15  is a schematic drawing of an alternative rectifying means incorporating an alternator voltage regulator device used in the combination of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a combination of a torque generating apparatus and alternator/generator suitable for use in drilling apparatus. The torque generating apparatus comprises a second assembly in the form of a cylindrical stator  2 . The stator  2  is shown in more detail in  FIGS. 4 to 7 . The stator is made of magnetically soft material and is provided with stator windings  4  arranged so that, when energised, the stator  2  is magnetised as discussed below. The stator is mounted on a fixed shaft  8 . Surrounding the stator is a first assembly in the form of a magnetically soft steel rotor  10 . The rotor  10  is mounted on the stator  2  by way of bearings  6 ,  11 , such as thrust bearings. The rotor/stator assembly is contained within a cylindrical housing  14  which may typically be a section of drill collar. The assembly is supported by an anchor  16  which is bolted to the housing. 
   A three phase alternator  22  is provided on the same shaft as the torque-generating apparatus. The alternator  22  and the torque-generating apparatus share a common rotor. 
   The output voltage of the alternator is connected via rectification means  31  to the torquer stator windings so that the torquer provides negative feedback in the form of progressive braking as the rotational speed of the joint rotor assembly increases or when the rotational speed of the rotor exceeds a predetermined limit. 
   The stator windings  4  of the torque generating apparatus are connected to windings of the alternator by way of access holes  19  formed in the core of each apparatus as shown in  FIG. 1 . 
   In use, the combination is disposed down hole, drilling mud being pumped down the hole in the direction indicated. The moving drilling fluid acts on the impeller  12  so as to rotate the rotor  10 . 
   Mud alternators, such as the illustrated alternator  22 , are generally three phase electrical rotating machines connected to a full wave rectifier  31  to convert the three alternating current waveforms into a single direct current supply. A typical circuit diagram is shown in  FIG. 14 . The impeller  12  is an integral part of a magnet carrier  24  of the alternator and spins at several thousand rpm in response to the flow rate of mud past the impeller. By electrically connecting the rectified output of the alternator  22  to the stator windings  4  of the torque-generating apparatus, an electromagnetic brake is created between the torque-generating apparatus and the alternator. Depending on the manner of the electrical connection, two different braking effects can be created as will be explained in more detail hereinafter. 
   Electrically connecting the stator windings  4  of the torque generating apparatus to the rectified output of the alternator  22  creates a progressive braking effect because the alternator  22  progressively energises the stator windings  4  with current. The greater the rectified alternator voltage, the greater the current flowing in the stator windings  4  and the greater the electromagnetic braking effect. The feedback loop created provides proportional control over the alternator output and the maximum speed of the alternator  22  is controlled irrespective of the profile of the impeller  12  and the mud flow rate. Such an electrical connection provides the more simple method of speed control within the alternator assembly and may be fine-tuned by varying either the resistance of the stator windings or the air/mud gap between the rotor and stator of the torque-generating apparatus. The resistance of the stator windings  4  may be modified by adding external resistors in series with the stator windings, or by changing the gauge of wire used to construct the stator windings. The total resistance, and hence the braking control achieved by either method, may be determined by calculation. Changing the air/mud gap involves a grinding operation to increase the gap between the rotor and stator of the torque-generating apparatus to reduce the braking effect. 
   Alternatively, as shown in  FIG. 15 , the braking characteristics may be changed from proportional to any other form of control by providing an alternator voltage regulator device  33  between the alternator  22  and the torque-generating apparatus. A switch mode controller is preferred to minimise losses within the alternator voltage regulator device and offers the possibility of dynamically changing the speed characteristics down hole via mud pulse telemetry. 
   An alternator voltage regulator device can be used to apply both linear and/or non-linear braking characteristics to the alternator by way of the stator windings of the torque-generating apparatus. A typical circuit, shown in  FIG. 15 , comprises an alternator voltage regulator in the form of a programmable switch mode power supply deriving power from the rectified alternator output and supplying programmed power output to the stator windings  4  of the torque-generating apparatus. A predetermined or programmable voltage-sensitive trip device activates the switch mode power supply. Below the trip point, the alternator runs open loop and there is no induction braking. Above the trip point, the loop is closed and induction braking is applied to the alternator. In this manner the set point between open loop and closed loop control can be programmed at any chosen location on the alternator speed curve and may be tuned to vary the output voltage according to the requirements of different customers. 
   By coupling the alternator to a torque-generating apparatus, the alternator no longer operates open loop and can safely be left unattended down hole to monitor its own output. Manufacturers of down hole alternators need only provide one system for all customers instead of several impeller/alternator pairs for differing output power demands and variable mud flow rates. A single combined alternator and torque-generating apparatus may be programmed to reproduce the output power profiles corresponding to different impellers by limiting the output power with induction braking. Such an arrangement reduces design and manufacturing costs, simplifies the operational needs of field engineers, and improves down hole reliability of the alternator and down hole instrumentation. 
     FIG. 2  is an end view of the apparatus of  FIG. 1 . It shows the fixed shaft  8 , anchor  16  and the impeller blades  12 . The direction of mud flow in  FIG. 2  is into the paper, causing rotation of the impeller blades. 
     FIG. 3  shows a cross-sectional view as indicated in  FIG. 1 . The assembly anchor  16  is shown in cross-section, bolted to the drill collar housing. The shaft  8  can be seen in cross-section. The view in  FIG. 3  is looking inwards into the assembly in the direction of the incoming mud, and the impeller blades  12  can be seen behind the assembly anchor  16 . 
     FIGS. 4 to 7  are more detailed views of the stator assembly. The stator  2  of the torque generating apparatus is a simple four-pole electromagnet which forms the electrical and mechanical centre of the machine. The stator  2  has a central shaft from which radially project four pole pieces  35 , as shown. The number of pole pieces need not be limited to four—any suitable number of pole pieces may be provided, larger machines requiring more pole pieces. 
   To prevent the stator  2  from being crushed by normal down hole drilling pressures, the gaps between the stator pole pieces may be filled with a high compressive strength material such as epoxy filler (not shown). This allows the stator  2  to maintain its shape and survive pressures in excess of 20,000 pounds per square inch. 
   The stator  2  of the torque generating apparatus is wound with high-temperature-resistant enamelled copper wire (not shown in  FIGS. 4 to 7 ) so as to produce alternate north and south magnetisation of the pole pieces. To preserve the integrity of the stator winding from the drilling mud, as shown in  FIGS. 1 and 10  a thin sleeve of soft magnetic or non-magnetic material  21  is machined to cover the stator windings. End cheeks provided on the stator receive the sleeve  21  and are welded thereto to seal the assembly. This seals the edges of the stator  2  and protects the contents from contamination. The covering, for example in the form of a cylinder, allows the stator poles to rotate with respect to the rotor  10  whilst maintaining close magnetic contact. A small magnetic gap is required to create the high output torque reactions from this machine. 
   An important feature of the apparatus is the use of electromagnetic advantage to minimise the power demands of the stator  2 . By its nature, this induction machine relies upon high rates of change of magnetic flux to effect braking in the rotor. High operating efficiency will therefore be achieved at high rotor RPM. 
     FIGS. 8 to 10  show the rotor  10  of the torque generating apparatus in more detail. The rotor  10  consists of a simple steel cylinder having grooves  20  machined to the inside surface. The grooves  20  perform two important functions. They allow the rotor  10  and stator  2  to maintain close magnetic contact and at the same time allow a sufficient flow of drilling mud through the annular space between the rotor and stator. This aids lubrication of the rotor bearings  6  and  11  and allows dissipation of heat. 
   The grooves  20  also prevent mud particles from aggregating within the annular space and clogging the apparatus. If the annular space were too small, mud particles would become trapped due to low mud-flow velocities. The mud particles would quickly aggregate, binding the stator  2  and rotor  10  and causing a down hole failure. In conventional down hole electrical apparatus, like alternators, which use a permanent magnet rotor, failure frequently occurs due to mud material becoming trapped and clogged within the space between the rotor and stator. The clogging problem is compounded by both soft and hard magnetic particles that circulate within the mud. Once trapped by the strong magnetic fields within the permanent magnet rotor, the magnetic particles capture non-magnetic mud particles, accelerating clogging. The present apparatus avoids this type of failure by providing a more generous space between rotor and stator (due to the grooves) in the area of the torquer and by being composed of soft magnetic material which does not trap particles to the same extent as a permanent magnet. 
   An important feature of the torque generating apparatus is the use of electromagnetic advantage and a rotor  10 , preferably solid, to dissipate waste heat from work done by the apparatus. The induced currents circulating in the rotor  10  would give rise to I 2 R heating in the rotor raising its working temperature. However, because the rotor  10  is manufactured from a magnetically soft material, its performance is unaffected by this temperature rise. It can therefore operate in temperatures much higher than the current limit of 180 C., without any loss of performance. In theory, the rotor  10  alone can operate at temperatures up to the Curie temperature of the permanent magnets. 
   According to another realisation of the rotor  10  of the torque generating apparatus, not shown in the figures, the grooves  20  are formed with a small flute or spiral twist along their length. In this way, every rotation of the rotor  10  produces a small pumping effect, pumping mud and contaminant particles through the apparatus. These features would expel hard and soft magnetic particles which would otherwise become trapped by the permanent magnets of the alternator windings. 
   The pole pieces of the stator  2  and the protruding portions of the rotor inner surface are disposed so as to correspond, being aligned (in the case of a four-pole apparatus) every quarter turn of the rotor  10 . As discussed, the number of pole pieces and protruding portions may be varied to suit a particular application. Although the rotor  10  and stator  2  in this example are formed of magnetically soft steel, any suitable soft magnetic material may be employed. Similarly, the protective coating of the stator  2  may be made of ferrous or non-ferrous material. 
   Normal use of the torque generating apparatus may erode the inner surface of the rotor  10  and/or the protective coating of the stator  2 . This would cause a gradual loss of output torque. The apparatus is, however, easy and economical to repair, as any mechanical errors may be easily corrected by welding, machining and/or grinding the relevant part. 
     FIGS. 11 to 13  show an alternative rotor  25  in more detail. The rotor  25  consists of a laminated steel cylinder having a number of conductors  29  running the length of the rotor  25 . The conductors  29  are connected at each end of the rotor  25  by means of a conductor end cap  27 . The arrangement of conductors  29  and end caps  27  form what is known as a squirrel cage conductor winding. 
   The conductors  29  and conductor end caps  27  consist of rods and plates of beryllium copper, which has a similar electrical resistivity to aluminium but is a stronger material, resistant to mechanical abrasion and chemical attack from drilling muds. 
   A close magnetic contact is still maintained between the rotor  25  and the stator  2 . 
   It should be understood that the stator which can be in a solid or laminated form can be used in conjunction with either a solid or laminated rotor.