Patent Publication Number: US-6911752-B1

Title: Concatenated motor assembly

Description:
RELATED APPLICATIONS 
   This application is a divisional application of U.S. Non-Provisional Application Ser. No. 09/621,206 filed Jul. 21, 2000 now U.S. Pat. No. 6,468,058, and claims the benefit of U.S. Provisional Application Ser. No. 60/144,967 filed Jul. 21, 1999. 

   BACKGROUND OF INVENTION 
   1. Field of Invention 
   The present invention relates to the field of motors, and more particularly but not by way of limitation, to a concatenated motor assembly that includes a first stator inducing a current in a rotor adjacent the first stator for use in powering a second stator. 
   2. Discussion 
   A variety of systems are used to bring fluids from below ground to the surface in a well when the pressure is insufficient or it is beneficial for other reasons. One common method involves using a pumping system to draw fluids from the producing formation(s) to the surface for collection and processing. In one class of pumping systems, a submersible pumping unit is immersed in the well-bore fluids and driven to force fluids through production tubing to the earth&#39;s surface. Such pumping systems typically include an electric submersible motor (ESM), a submersible production pump with sealing portions to protect the motor from well-bore fluids, a gearbox, and a variety of other controls such as a variable speed drive (VSD). 
   In many pumping systems, centrifugal pumps are used but centrifugal pumps are not adequate in a number of circumstances. In particular, centrifugal pumps are typically inefficient at lower pump speeds. Alternatives to centrifugal pumping systems include positive displacement pumping systems, such as a progressive cavity pumping systems (PCS). During the start-up phase of the pumping system a higher torque is needed from a motor portion of the pumping system to drive the progressive pump portion of the pumping system. In order to provide the higher torque required at start-up, and to speed match the motor to the operating range of the progressive cavity pump portion of the system, the progressive cavity pumping system usually includes a gear reducer for increasing motor output torque and speed matching. 
   Typically, such gear reducers are positioned within the well-bore and thus are size constrained. Also, such gear reducers operate at speeds determined by a fixed ratio of the output speed of the motor, so motors of the progressive cavity pumping system generally need to be coupled with a variable speed driver to effect operation of the prior art progressive cavity pumping system over a range of speeds. Even when a variable speed drive is used, the gear reducers limit the range of speeds for operating the progressive cavity pump portion of a progressive pumping system, typically making higher production rates unavailable. Thus prior art progressive cavity pumping systems ordinarily fail to afford the flexibility necessary to pump fluids at both low and high flow rates. 
   Within a typical prior art progressive cavity pumping system, a motor coupled to a variable speed drive exhibits decreasing torque in response to an input from the variable speed drive for a lower rotational speed and show significant decreases in available torque for current supplied at frequencies below 30 Hertz. Additionally, the maximum torque transfer of a gearbox assembly within a typical prior art progressive cavity pumping system is limited by the gearbox size, specifically an available diameter for the gears of the gearbox; thus a well-bore diameter often limits the available horsepower of a typical prior art progressive cavity pumping system. Within a typical well-bore, the available horsepower of most progressive cavity pumping systems equipped with a gearbox and operating under a variable speed drive is limited to about 80 horsepower. Furthermore, the inclusion of a gearbox and a variable speed drive in a prior art progressive cavity pumping system add significantly to the cost of the system. 
   Variable speed drives (VSD) are often used in conjunction with a gearbox within a prior art progressive cavity pumping system to achieve a wider operating speed range but an alternative method is to use the VSD directly with an ESM to run the motor in a controlled low speed operation. However, a prior art progressive cavity pumping system with a variable speed drive coupled directly to a motor of the system typically has a limiting starting torque, which often proves to be insufficient for a system utilizing a progressive cavity pump that requires a starting torque of nearly 145% of the running torque of the system. Also, a prior art progressive cavity pumping system configured with a variable speed drive coupled directly to an electric submersible motor is horsepower limited and non-applicable to a number of submersible applications. 
   Therefore, challenges remain and a need persists for a cost competitive, progressive cavity pumping system compliant with high torque start-up demands placed on the system, while providing improved reliability for steady state operation of the pumping system. 
   SUMMARY OF INVENTION 
   A concatenated motor assembly includes a continuous rotatable shaft, a first motor module assembly, a motor coupling unit, and a second motor module assembly. The first motor module assembly cooperates with the continuous rotatable shaft and provides an induced current while rotating the continuous rotatable shaft. The motor coupling unit cooperates with the continuous rotatable shaft, communicates with the first motor module assembly, receives the induced current and provides power. The second motor module assembly communicates with the motor coupling unit, cooperates with the continuous shaft and uses the provided power to further rotate the continuous shaft. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a side elevational diagram, in partial cross-section, of an oil well having disposed therein an electrical submersible pumping system driven by a concatenated motor system constructed in accordance with the present invention. 
       FIG. 2  is a longitudinal cross section of the concatenated motor assembly of FIG.  1 . 
       FIG. 3  is a longitudinal cross section, cutaway view of one of the motor assemblies of FIG.  1 . 
       FIG. 4  is an enlarged cross section of a portion of the motor of FIG.  3 . 
       FIG. 5  is a cutaway view of the motor coupling unit of FIG.  2 . 
       FIG. 6  is a schematic diagram of the electrical motor circuit showing the connections between the two motors of the concatenated motor assembly of FIG.  1 . 
       FIG. 7  is a graph of typical, actual torque versus motor speed for a typical progressive cavity pump. 
       FIG. 8  is a flow chart of the sequence of operation of the electrical submersible pumping system depicted in FIG.  1 . 
   

   DESCRIPTION 
   For purposes of disclosure and convenience of enablement, a pump system usage environment has been elected to aid in the understanding and presentation of the present invention and does not constitute a limitation on the uses of the present invention for alternate uses. Referring to the drawings in general and particularly to  FIG. 1 , depicted therein is a submersible concatenated pump system  10  constructed in accordance with the present invention. While the present invention will be described in relation to the embodiment shown in the appended drawings, it will be understood that the present invention can be adapted to other embodiments. 
   The concatenated pump system  10  is connected to a production tubing string  11  and supported thereby in a well-bore  12  and includes a power supply  13  supplying power through a power cable  14  to a concatenated motor assembly  15  used to drive a pump assembly  16 . The pump assembly  16  includes a pump  17  and a seal assembly  18  attached to the concatenated motor assembly  15 . The concatenated motor assembly  15  has a primary or control concatenation unit  20  and a secondary concatenation unit  22 . 
   As shown by  FIG. 2 , the primary concatenation unit  20  comprises a first motor module assembly  24  connected to a motor coupling unit  26 , while the secondary concatenation unit  22  includes a second motor module assembly  28  coupled to the motor coupling unit  26 . Optionally, the coupling unit  26  may be integrated with and made part of the first motor module assembly  24 . In either case, the coupling unit  26  provides both the mechanical and the electrical connection between the primary concatenation unit  20  and the secondary concatenation unit  22 . For the parts that are similar between the motor module assembly  24 , the coupling unit  26  and the motor module assembly  28 , the numbering convention will be designated by a common numeric character accompanied by the alpha character ‘A’ for the first motor module  24 , the alpha character ‘B’ for the second motor module  28 , and the alpha character ‘C’ for the motor coupling unit  26 . In a preferred embodiment, the power supply  13  (of  FIG. 1 ) supplies power to the primary concatenation unit  20  via the power cable  14  to a first power connection  30 A for application to the first motor module  24 . 
   Shown in greater detail by  FIG. 3 , the first motor module assembly  24  includes an elongated motor housing  32 A forming a first central bore  33 A (second central bore  33 B in the second motor module assembly  28 ) and enclosing a wound rotor induction motor  34 A. The wound rotor induction motor  34 A includes a stator portion  36 A adjacent the elongated motor housing  32 A and a rotatable wound rotor portion  38 A adjacent the stator portion  36 A. A rotatable shaft section  40 A is supported by a pair of first bearing assemblies  42 A, which in turn supports the wound rotor portion  38 A. The pair of first bearing assemblies  42 A (second bearing assemblies  42 B in the second motor assembly  28 ) are secured in place by the elongated housing  32 A. The rotatable shaft section  40 A includes a first spline  44 A for use in linking the first motor module assembly  24  to the coupling unit  26  and a second spline  46 A for use in linking the concatenated motor assembly  15  to additional concatenated motor assemblies of alternate embodiments. 
   In a preferred embodiment, the wound rotor induction motor  34 A is a three phase induction motor. As such, the stator portion  36 A is a three phase, Y-connected motor with three pairs of windings spaced 120° apart (not shown separately) that respond to applied voltages that have a 120-degree phase displacement. Applying three-phase power to the windings of the stator portion  36 A sets up a rotating magnetic field. Similar to the stator portion  36 A, the wound rotor portion  38 A has three pairs of Y-connected windings spaced 120° complimentary to the three pair of windings of the stator portion  36 A. The rotating magnetic field of the stator portion  36 A induces a magnetic field in the wound rotor portion  38 A by cutting through the three pairs of Y-connected windings resulting in an induced electromagnetic force (emf). The two fields interact and cause the wound rotor portion  38 A to turn in the direction of the rotating magnetic field of the stator portion  36 A and relative to the elongated motor housing  32 A. The current developed in the windings of wound rotor portion  38 A of the induction motor  34 A is passed to the coupling unit  26 , which collects the current and provides power to a stator portion  36 B of a second induction motor  34 B as shown in FIG.  2 . The frequency of the three phase power supplied to the second induction motor  34 B is determined by the frequency of the power supplied to the first induction motor  34 A and the rotational speed of the wound rotor portion  38 A relative to the stator portion  36 A. 
   Returning to  FIG. 2 , the coupling unit  26  includes a rotatable shaft section  40 C with a first spline  44 C and a second spline  46 C while the second motor module  28  includes a rotatable shaft section  40 B with a first spline  44 B and a second spline  46 B. Collectively, the rotatable shaft sections  40 A,  40 B and  40 C combine to form a continuous rotatable shaft  49  via a coupling between their respective splines, i.e., spline  44 A coupled with spline  46 C, and spline  44 C coupled with spline  46 B. 
   The wound rotor induction motor  34 A, a portion of which is shown in  FIG. 4 , has the cylindrically shaped rotor portion  38 A attached to the continuous rotatable shaft  49  of the concatenated pump system  10 , which rotates within the cylindrically shaped stator portion  36 A. The rotor portion  38 A is made up of a series of rotor segments  50 A separated by oil bearings  52 A. The stator portion  36 A is made up of steel laminations  54 A and brass laminations  56 A including a series of stator windings  58 A running through the laminations that are coupled to the power conductor  30 A, causing rotation of the rotor portion  38 A within the wound rotor induction motor  34 A, in a manner well known in the art. As will be appreciated by those skilled in the art, stator windings  58 A will typically be wound and connected in groups depending upon the design of the stator portion  36 A, the number of poles in the wound rotor induction motor  34 A, and the desired speed of the wound rotor induction motor  34 A. 
   In a preferred embodiment the second motor module  28  has substantially the same construction as the first motor module  24  described above. However, the second motor module  28  having substantially the same construction as the first motor module  24  is not a limitation on the scope of the invention. Dissimilar construction of the second motor module  28  relative to the first motor module  24 , for example an induction motor absent a wound rotor portion, is embodied within the scope of the present invention. 
   The concatenated pump system  10  has the motor coupling unit  26 , shown in  FIG. 5 , which along with the first motor module assembly  24  forms the control concatenation unit  20 , as shown in FIG.  2 . The motor coupling unit  26  includes bearings  64  disposed between the rotatable shaft sections  40 C of the continuous rotatable shaft  49  and an elongated coupling housing  70 . Included in the motor coupling unit  26  is a portion referred to as slips  72 . The slips  72 , as shown in  FIG. 5 , have two parts: the stationary outer slips (stator)  74  serve to collect the current developed in the windings of the wound rotor portion  38 A of the induction motor  34 A, and to provide the collected current as power to the stator portion  36 B of the second induction motor  34 B; inner slips (rotor)  76  rotate within the outer slips  74  and serve to receive the current developed in the windings of the wound rotor portion  38 A of the induction motor  34 A. 
   In a preferred embodiment, the slips  72  have a construction similar to the induction motor  34 A wherein the inner slips  76  are built in a manner similar to the rotor portion  38 A and the outer slips  74  are built in a manner similar to that of the stator portion  36 A, including a series of windings running through the outer slips  74 , in a manner well known in the art. As will be appreciated by those skilled in the art, these windings will typically be wound and connected in groups depending upon the design of the motor coupling unit  26 . 
   The inner slips  76  are supported by the continuous rotatable shaft  49 , and the inner slips  76  rotate at the same rotational velocity as the wound rotor portion  38 A of the induction motor  34 A during operation of the concatenated pump system  10 . The outer slips  74  are connected to the second motor module  28  through a connector  78 . The inner slips  76  rotate past the outer slips  74  at a slip ring connector  82 . 
     FIG. 6  is a schematic of a preferred embodiment of the concatenated motor assembly  15 . The stator portion  36 A of the induction motor  34 A shows that windings  84 A are three phase, Y-connected pairs of windings spaced 120 degrees apart. The windings  84 A are responsive to applied voltages that have a 120-degree phase displacement. Likewise, the rotor portion  38 A of the induction motor  34 A shows that windings  86 A are three phase, Y-connected pairs of windings spaced 120 degrees apart. The windings  86 A respond to a rotating magnetic field, developed when current is applied to the stator portion  36 A of the induction motor  34 A. The response of the windings  86 A to the rotating magnetic field cutting through the windings  86 A is to generate a current. The current generated has a frequency offset from the frequency of the current supplied to the stator portion  36 A, is modulated by the rotational speed of the continuous rotatable shaft  49 , and has a voltage phase substantially the same, with a slight time shift, as the phase of the voltage supplied to the stator portion  36 A. 
   The first rotor portion  38 A is mechanically connected to the inner slips  76  via the continuous rotatable shaft  49 . The rotation of the inner slips  76  relative to the outer slips  74  induces a power output from the slips  72  used to power stator windings  84 B of induction motor  34 B of the second motor module  28  that is frequency dependent on the rotational speed of the continuous rotatable shaft  49 . This use of concatenation creates the concatenated motor assembly  15  displaying a resultant “third motor” response which can have properties different from each individual induction motor, such as  34 A and  34 B. The resultant third motor response of the concatenated motor assembly  15 , for instance, can achieve the effect of additional poles for the concatenated pump system  10 , thus allowing the concatenated motor assembly  15  to achieve the equivalent of a larger number of poles than is physically present in the individual induction motors, such as  34 A and  34 B. 
   The operation of the concatenated pump system  10  will be described with reference to  FIGS. 6 through 8 . As described above, a concatenated pump system  10  is formed when the shafts of two or more motors are connected in series to form the continuous rotatable shaft  49  as shown in FIG.  6 . In the present concatenated motor assembly  15 , the variable speed results from the unique use of slips  72  and resultant change in frequency applied to the second motor module  28 . This change results in the system taking on different performance characteristics than any of the individual motor modules of the originally designed unit. The resultant speed of the concatenated motor assembly  15  is inversely proportional to the sum or difference of the number of poles in the concatenated motors. If the synchronous speed of a two pole motor is 3600 rpm on 60 hertz (Hz) power, then the synchronous speed of a four pole motor is 1800 rpm. The speed of an eight pole motor is 900 rpm, and the speed of a twelve pole motor is 600 rpm. If the concatenated motor system has two motors and one has four poles and the other has eight poles, the resultant “third motor” or concatenated motor assembly  15  could run at 1800 rpm (4+0) or 900 rpm (8+0) or 600 rpm (8+4). It could also run in the reverse direction at a speed of 1800 rpm (8−4). 
   This effectively allows different pole configurations and different windings to be combined in the same concatenated pump system  10  by using the resultant slip of the slips  72  in the motor coupling unit  26 . This is preferable because the effective speeds and resultant torque that can be obtained using concatenated motor assembly  15  are sufficient to power a progressive cavity pump, at the higher horsepower and torques required, absent the use of a gearbox. 
   The progressive cavity (PC) pump  17 , as shown in  FIG. 1 , is connected to the second motor module  28  via the seal assembly  18 . The second motor module  28  is in turn connected to the first motor module  24  via the motor coupling unit  26 . The first motor module  24  and the motor coupling unit  26  collectively form the control concatenation unit  20 . The output response of the first motor module  24  coupled to the second motor module  28  via the motor coupling unit  26  work together to produce the resultant equivalent “third motor” discussed above. If the concatenated motor assembly  15  for the PC pump  17  has two motors with a synchronous speed of 3600 rpm on 60 hertz (Hz), one motor with twelve poles and the other with eight poles, then the resultant equivalent “third motor” could run at a slow speed of 360 rpm (8+12 poles) with high torque or at a faster speed with low torque such as 600 rpm (12+0 poles) or 900 rpm (8+0 poles). These are speeds within the range of those shown in  FIG. 7 , which are those for a typical PC pump, such as PC pump  17 . 
     FIG. 7  shows a curve  90  of torque as a percentage of full load versus motor speed for a typical progressive cavity (PC) pump. On the y-axis  92  is plotted the torque as a percentage of full load and on the x-axis  94  is plotted the corresponding motor speed (rpm). These speeds typically range from 100 to 800 rpm for the PC pump and require high initial torques. At start-up the torque can be nearly 145% of the running torque as shown at  96 . In contrast, the torque at 100 rpm is typically much lower as shown at  98 . The concatenated motor assembly  15  can operate at these speeds and torques with a particular combination of motors in the control concatenation unit  20  and the secondary concatenation unit  22  as described above. 
   The operation of the concatenated pump system  10  can be further understood with reference to  FIG. 8 , which is a flow chart of the steps necessary to pump fluids using the concatenated pump system  10 . It should be noted that, in general, this is a submersible system but it could be used as a surface system, or a combination of both. It should also be noted that the diagrams imply a vertically disposed well-bore but in most circumstances the well-bore will have an incline. As shown in  FIG. 1 , the motor module assembly  28  of the secondary concatenation unit  22  rotates the combined continuous rotatable shaft  49  that rotates the pump  17 , such as a progressive cavity pump, and moves produced fluids  100 , such as oil and gas, from the producing formation via the pump  17  to the surface. The rotational speed of the continuous rotatable shaft  49  is influenced by the control concatenation unit  20  through the motor coupling unit  26 . 
   Referring to  FIG. 8 , starting with fluids  100  in the well-bore ready to be pumped to the surface, as shown by step  200 , the wound rotor induction motor  34 A of the control concatenation unit  20  is energized by the power cable  14  through power connection  30 A (step  202 ), the wound rotor induction motor  34 A rotates the continuous rotatable shaft  49  of the motor coupling unit  26  (step  204 ). At step  206 , the direct mechanical linkage from the first wound rotor induction motor  34 A drives the inner slips (rotor)  76 , thereby creating an electromagnetic coupling to the outer slips (stator)  74 . As shown in step  208 , the output from the slips  72  provides the power input to the stator  36 B of the second induction motor  34 B; thus the two induction motors  34 A and  34 B exhibit properties of a resultant equivalent third motor as described above, thus powering the pump  17  at the appropriate speed to operate without additional controls. At step  210 , fluids  100  enter the pump  17 . At step  212 , the pump  17  energizes the fluids  100 ; and at step  214 , the fluids  100  are pumped to the surface. 
   The present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, numerous changes may be made which will readily suggest themselves to one skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.