Patent Publication Number: US-2020304002-A1

Title: Permanent Magnet Motor For Electrical Submersible Pump

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application Ser. No. 62/821,528, filed Mar. 21, 2019. 
    
    
     FIELD OF DISCLOSURE 
     This disclosure relates to electrical submersible well pumps (ESP), and in particular to a permanent magnet ESP motor having orthogonal magnets mounted between the pole magnets. 
     BACKGROUND 
     ESP&#39;s are often used to pump well fluid from hydrocarbon wells. One common type of motor for an ESP is an induction electric motor having stator windings encircling a rotor mounted to a drive shaft. The rotor has a stack of steel laminations with copper rods extending through them. Three-phase power applied to the stator windings induces rotation of the rotor. 
     Another type uses permanent magnets in the rotor, each providing one pole of the motor, which may have two, four or a different number of poles. Each permanent magnet is an arcuate member having an inner side bonded or otherwise attached to a sleeve mounted to the shaft for rotation and an outer side facing and spaced from the stator by a gap. Each pole magnet has two ends circumferentially spaced apart from each other, such as about 70 degrees in a four-pole motor. Each pole magnet has a north pole on either its inner arcuate side or its outer arcuate side and a south pole on the opposite side. In a four-pole motor, the north poles of two of the pole magnets 180 mechanical degrees from each other are on the outer sides. The south poles of the other two pole magnets are on the inner sides. Non-magnetic spacer bars may be positioned between the juxtaposed ends of adjacent pole magnets. 
     While permanent magnet ESP motors work well, improvements are desired. For example, some of the improvements could be to reduce thermal stresses in the pole magnets and improve the power factor and output torque. 
     SUMMARY 
     An electrical submersible pump motor comprises a stator having a stack of laminations with windings extending through slots in the laminations, the stator having a bore with a longitudinal axis. A shaft extends through the bore. Rotor sections mounted along a length of the shaft cause rotation of the shaft. Each of the rotor sections comprises a tubular core mounted to the shaft for rotation in unison. Pole magnets are spaced apart from each other and mount to an outer side of the core. Each of the pole magnets has an inner side, an outer side and two pole magnet ends joining the inner and outer sides. Each of the pole magnets is polarized with a north pole on one of the inner and outer sides and a south pole on the other of the inner and outer sides. Orthogonal magnets are also mounted to an outer side of the core. Each of the orthogonal magnets locates between two of the pole magnets. Each of the orthogonal magnets has an inner side, an outer side, and two orthogonal magnet ends joining the inner and outer sides of each of the orthogonal magnets. Each of the orthogonal magnets is polarized with a north pole on one of the orthogonal magnet ends and a south pole on the other of the orthogonal magnet ends. 
     Each of the orthogonal magnets of a first group has its north pole facing clockwise and its south pole facing counterclockwise. Each of the orthogonal magnets of a second group has its north pole facing counterclockwise and its south pole facing clockwise. The orthogonal magnets of the first group alternate with the orthogonal magnets of the second group. 
     In one embodiment, each of the orthogonal magnets has magnetic flux lines that are curved in a circumferential direction. In another embodiment, each of the orthogonal magnets has magnetic flux lines that are straight and tangent to the outer side of each of the orthogonal magnets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an schematic sectional view of an ESP in accordance with this disclosure. 
         FIG. 2  is a schematic, partial axial sectional view of a portion of the motor. 
         FIG. 3  is a transverse sectional view of the motor, taken along the line  3 - 3  of  FIG. 2  and with the housing removed. 
         FIG. 4  is a top view of the permanent magnets shown in  FIG. 3 , but with the stator and shaft removed. 
         FIG. 5  is a an sectional view of the rotor assembly of the motor of  FIG. 2 , with the shaft removed. 
         FIG. 6  is a sectional view of the rotor assembly of  FIG. 5 , taken along the line  6 - 6  of  FIG. 5 . 
         FIG. 7  is a top view of an alternate embodiment of one of the orthogonal magnets shown of  FIG. 4 . 
     
    
    
     While the disclosure will be described in connection with one embodiment, it will be understood that it is not intended to limit the disclosure to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the scope of the claims. 
     DETAILED DESCRIPTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term “about” includes plus or minus 5% of the cited magnitude. In an embodiment, usage of the term “substantially” includes plus or minus 5% of the cited magnitude. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
     Referring to  FIG. 1 , a well  11  has casing  13  that is perforated or has other openings to admit well fluid. An electrical submersible pump assembly or ESP  15  is illustrated as being supported on production tubing  17  extending into well  11 . Alternately, ESP  15  could be supported by other structure, such as coiled tubing. Although shown installed vertically, ESP  15  could be located within an inclined or horizontal section of well  11 . The terms “upper”, “lower” and the like as used herein only for convenience, because ESP  15  can be operated in inclined or horizontal sections of a well. ESP  15  has several modules, including a motor  19 , normally a three-phase electrical motor. A motor protector or seal section  21  connects to motor  19  and has components, such as a bladder, for reducing a pressure differential between lubricant in motor  19  and the hydrostatic pressure of well fluid. Seal section  21  may be mounted to an upper end of motor  19  or alternately to a lower end. An optional gas separator  23  connects to the upper end of seal section  21  in this example. 
     A pump  25  connects to gas separator  23  if one is employed; if a gas separator is not used, pump  25  may connect to seal section  21 , as shown, or to motor  19 . Pump  25  has a well fluid intake  27  that will be in gas separator  23  if one is used, and if not, at a base of pump  25 . Pump  25  is normally a rotary pump, such as a centrifugal or progressing cavity pump, but it could be a reciprocating pump. The connections between the modules of ESP  15  are shown as bolted flanges, but they could be threaded connections. 
     A power cable  29  extends from a wellhead (not shown) alongside tubing  17  for supplying power to motor  19 . Spaced apart clamps  31  (only one shown) secure power cable  29  to production tubing  17 . A motor lead  33 , which may be considered to be a lower part of power cable  29 , connects to a lower end of power cable  29  by a splice  35  in this example. Motor lead  33  extends alongside ESP  15  and has an electrical connector  37  on its lower end that secures to a receptacle at the upper end of motor  19 . Splice  35  is illustrated at the upper end of pump  25 , but it could be a considerable distance above pump  25 . 
     Referring to  FIG. 2 , motor  19  has a housing  39  containing a non-rotating stator  41 . Stator  41  is conventional, having a stack of thin steel discs or laminations  43 . Windings  45  (shown in one of the slots in  FIG. 3 ) extend through slots in laminations  43 . Stator  41  has a cylindrical central bore  47  with a longitudinal axis  49 . A rotatable drive shaft  50  extends through bore  47  on axis  49  for driving pump  25  ( FIG. 1 ). 
     Rotor sections  51  are mounted to shaft  50  by a key arrangement  52  for causing shaft  50  to rotate. Rotor sections  51  are positioned along the length of shaft  50  and spaced apart from each other a short distance. A radial bearing  53  locates between adjacent ends of the rotor sections  51 . Bearings  53  frictionally engage the inner diameter of stator  41  to prevent their rotation. 
     Each rotor section  51  has a number of permanent pole magnets  55  mounted circumferentially around shaft  50 . Pole magnets  55  are indicated by dotted lines in  FIG. 2 , and may be located in segments, each segment having an array of pole magnets  55  spaced around shaft  50 . In this example, there are four segments  56   a,    56   b,    56   c  and  56   d,  each segment containing pole magnets  55  encircling shaft  50 . The array of four segments  56   a,    56   b,    56   c  and  56   d  extends approximately a full length of each rotor section. The lower ends of the pole magnets  55  in each segment  56   a,    56   b  and  56   c  may abut the upper ends of the pole magnets  55  in the next lower segment. There are at least two pole magnets  55  in each segment  56   a,    56   b,    56   c  and  56   d,  and they are separated from adjacent pole magnets  55  by orthogonal permanent magnets  57 . The orthogonal magnets  57  in each segment  56   a,    56   b,    56   c  and  56   d  have the same axial dimension as the pole magnets  55  in the same segment. 
     Referring to  FIG. 3 , in this example, there are four pole magnets  55  in each segment  56   a,    56   b,    56   c  and  56   d,  but other numbers are feasible, such as two, six or other numbers. There are also four orthogonal magnets  57  in each segment  56   a,    56   b,    56   c  and  56   d,  each located between two adjacent pole magnets  55 . In this example, pole magnets  55  and orthogonal magnets  57  are mounted to the outer surface of an inner sleeve or tubular core  59  that is keyed or affixed to shaft  50  for rotating shaft  50 . Pole magnets  55  and orthogonal magnets  57  may attach to inner sleeve  59  in various manners, such as by epoxy or an adhesive. Optionally, a protective outer sleeve  61  encloses the array of magnets  55 ,  57  and rotates with each rotor section  51 . Shaft  50  and inner sleeve  59  are normally of a magnetically permeable material, such as a steel. Outer sleeve  61  is non-magnetic and may be of different materials. An annular gap exists between outer sleeve  61  and the inner diameter of stator  41 . 
     Referring to  FIG. 4 , each pole magnet  55  has an arcuate inner side  63  and an arcuate outer side  65 , and each side  63 ,  65  has a radius with a center point on axis  49 . Each pole magnet has circumferential ends  67 ,  69  that join inner and outer sides  63 ,  65 . In this embodiment, each end  67 ,  69  is flat and located on a radial plane of axis  50 . A circumferential length between ends  67 ,  69  is the same for each of the pole magnets  55 . This circumferential length may vary and is illustrated to be equivalent to about 70 degrees. Each pole magnet  55  has a south pole on one of its inner and outer sides  63 ,  65  and a north pole on the opposite side. The south and north poles are indicated by the letters “S” and “N” in  FIG. 4 . The four-pole design alternates the north and south poles between adjacent pole magnets  55 . Two of the pole magnets  55  have the south pole on the inner side  63  and two on the outer side  65 . The two pole magnets  55  with the south pole on the inner sides  63  are 180 mechanical degrees from each other. Similarly, the two pole magnets  55  with the south pole on the outer sides  65  are 180 degrees from each other. 
     Orthogonal magnets  57  are also curved, having arcuate inner sides  73  and arcuate outer sides  75 . Flat circumferential ends  77 ,  79  are located in radial planes of axis  49  and join inner and outer sides  73 ,  75 . Each orthogonal magnet  57  has the same radial width between inner and outer sides  73 ,  75  as pole magnets  55 , resulting in a constant outer diameter for the array of magnets  55 ,  57 . The circumferential length of each orthogonal magnet  57  is less than the circumferential lengths of pole magnets  55 . In this embodiment, the circumferential length of each orthogonal magnet  57  is equivalent to about 20 degrees. 
     In the embodiment shown in  FIG. 4 , each orthogonal magnet  57  is polarized in an orthogonal manner with a south pole on one end  77 ,  79  and a north pole on the opposite end. The flux lines emanated from each north and south pole are normal to the flat ends of faces  77 ,  79 . This polarization creates a magnetic flux  81  that curves in a circumferential direction, from one end  77 ,  79  through the opposite end  77 ,  79 . 
     Each orthogonal magnet  57  has a polarity opposite to the orthogonal magnet  57  closest to it. For example, the orthogonal magnets  57  at about 30 and 210 degrees in the drawing have magnet flux  81  directed in a counterclockwise rotational direction, and the orthogonal magnets  57  at 120 and 300 degrees have magnetic flux  81  directed in a clockwise rotational direction. 
     Each orthogonal magnet  57  completely fills the space between adjacent ones of the pole magnets  55 . Each orthogonal magnet end  77 ,  79  is flush with and abuts one of the pole magnet ends  67 ,  79 , creating a continuous annular shape for the magnetic array. 
     Orthogonal magnets  57  may be of the same material as pole magnets  55 , typically a rare earth magnetic material. The same or similar material results in orthogonal magnets  57  having the same coefficient of thermal expansion as pole magnets  55 , avoiding thermal stresses that occur as motor  19  heats. 
       FIG. 5  illustrates rotor section  51  in an axial section with shaft  50  ( FIG. 3 ) removed. End rings  80  attach to the upper and lower ends of inner sleeve  59  to secure orthogonal magnets  57  and pole magnets  55  ( FIG. 4 ) to the outer side of inner sleeve  59 . Inner sleeve  59  may have a number of apertures  82  through its side wall to facilitate in assembling magnets  55  and  57 . 
       FIG. 6  is an assembly view similar to  FIG. 4 , but showing that magnets  55 ,  57  are located between inner sleeve  59  and outer sleeve  61 .  FIG. 6  also shows rotor section  51  with shaft  50  removed. 
     During operation, three phase AC power will be supplied to stator windings  45 . A variable speed drive at the surface of the well may vary the frequency of the power for startup and other reasons. The current in windings  45  results in magnetic flux being created that revolves around stator  41 . The revolving electromagnetic field interacts with the magnetic flux  71  of pole magnets  55 , causing rotor sections  51  and shaft  50  to rotate. Orthogonal magnets  57  increase the overall magnetic flux  71  linked with windings  45  by redirecting or adjusting the magnetic flux  71  near pole magnet ends  67 ,  69 . The redirection of magnetic flux  71  results in improved torque capacity and a higher power factor for motor  19 . 
     In the alternate embodiment shown in  FIG. 7 , orthogonal magnet  83  may be machined from a rectangular magnetized block of magnetic material. Orthogonal magnets  83  also have flux lines  85  that emanate in rotational directions. However, flux lines  85  are generally straight, rather than curved circumferentially like flux line  81  of orthogonal magnets  57  ( FIG. 4 ). Flux lines  85  emanate generally tangential to the curvature of outer side  87  or inner side  89  at the center point between the flat circumferential ends  91 . Flux lines  85  do not emanate normal to flat ends  87 . Rather, flux lines  85  emanate at an angle other than 90 degrees relative to flat ends  87 . Otherwise, orthogonal magnets  83  are the same as orthogonal magnets  57 . 
     The present disclosure described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While only two embodiments of the disclosure have been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the scope of the appended claims.