Patent Description:
The present disclosure relates generally to synchronous motors for use in submersible pump systems that use rotor-mounted permanent magnets, and more particularly to a rotor configuration that provides improved magnet protection and shaft deflection resistance against submersible pump environmental conditions.

Electric submersible pumps (ESP) (also referred to as deep well submersible (DWS) pumps) are especially useful in extracting valuable resources from deep well geological formations. In one example, an ESP can be used to retrieve crude oil or natural gas from significant subterranean depths. In another widely-used example, an ESP provides the motive power to large quantities of water, such as those used in municipal waterworks. ESPs conventionally include a centrifugal pump section and a motor section that are axially aligned with one another and oriented vertically in the well. More particularly, the motor section may be configured to drive one or more pump section stages. An example of a rotor for a submersible motor is shown in <CIT>.

Because ESPs are relatively inaccessible (often completely submerged at distances between about <NUM> and <NUM> meters beneath the earth's surface), they must be able to run for extended periods without requiring maintenance. Such extended operating times means that the economic feasibility of a system utilizing such pumps requires dependable, robust componentry. This can frequently be at odds with the need to design more efficient pumping systems as a way to alleviate increasing energy costs and more stringent environmental policies.

One particular motor configuration that has shown promise for high-efficiency operation is based on the use of a permanent magnet rotor, and is known as a permanent magnet synchronous motor (PMSM). Unfortunately, these are more expensive to build than conventional (asynchronous) induction motors, thereby offsetting the gains made possible by the inherent efficiency improvements. More significantly, the peculiar environment associated with submersible motors can hamper the long-term robustness that is needed in order to justify the higher cost of the PMSM; such environmental concerns include (a) requiring the use of a long, thin rotor to fit in limited diameter boreholes or pipes as a way to maximize the power output at a given outer motor diameter, (b) preventing the corrosive effects on the magnets of the motor lubricant or working fluid, and (c) the lengthy meantime between service of ESPs. More particularly, the first concern takes into consideration that the motors (and their respective shafts) used in an ESP define a long, thin profile where the magnetic and rotordynamic forces would tend to cause shaft deflection. Such deflection can undesirably lead to rotor contact with the stator. Likewise, the second concern is not ameliorated by the use of coatings on the magnets, as the prolonged exposure of the magnets to fluids (which are often at elevated temperature) will invariably lead to material degradation. Regarding this third concern, such exposure (and related magnet and motor damage) is inconsistent with the desired long times between service that are needed to make submersible pumps economically viable.

What is needed is a high-efficiency PMSM design for use in a submersible pump system that provides improved protection of the motor's rotor-mounted permanent magnets, as well as enhanced durability for the rotor.

A method of constructing a rotor for an electric submersible pump. The method requires the assembly of a yoke, a stepped collar, and a cladding tube of the rotor to form a magnet channel able to have rare earth permanent magnets positioned in the channel. The method further requires coupling a shaft to rotate the pump impellers, the shaft having a concentric yoke integrally-formed to the shaft, and disposing the magnets on the circumference of the yoke, having the magnets axially align with the yoke, and positioning a stepped collar on the fore end and the aft end of the yoke. The method is completed with the placement of a cladding tube on the circumference of the magnets, the cladding tube securing the magnets to the shaft. In embodiments of the invention, the yoke interface with the one or more permanent magnets comprises an outer dimension form closure on the circumference of the yoke and form-closed torque transmission is present between the one or more permanent magnets and the yoke. The form closure is defined by a faceted spot for each respective permanent magnet around the circumference of the yoke.

The following detailed description of the preferred embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which the various components of the drawings are not necessarily illustrated to scale:.

The embodiments set forth in the drawings are illustrative in nature and are not intended to be limiting of the embodiments defined by the claims. Moreover, individual aspects of the drawings and the embodiments will be more fully apparent and understood in view of the detailed description that follows.

Referring first to <FIG>, the ESP <NUM> is placed within well piping <NUM> and includes a motor section <NUM>, a pump section <NUM>, a fluid inlet section <NUM> to accept a flow of incoming extracted fluid <NUM>, and a fluid outlet section <NUM> that can be used to discharge the fluid <NUM> to a riser, pipestack or related fluid-conveying tubing. The motor section <NUM> and the pump section <NUM> are substantially vertically aligned in order to fit inside of the well piping <NUM>. Conserving axial space is a primary concern in the use of ESPs <NUM>. As shown, both the motor section <NUM> and the pump section <NUM> may be made of modular subsections. Thus, within pump section <NUM>, there may be numerous serially-arranged subsections in the form of pump bowls <NUM> that each house respective centrifugal impellers. Within the motor section <NUM>, there may be one or more motor assemblies or separate subsections for various components within the motor section <NUM>. The fluid inlet section <NUM> is situated axially between the motor and pump sections <NUM>, <NUM>, and may include a mesh or related screen to keep large-scale particulate out in order to avoid or minimize particulate contact with the rotating components in the pump section <NUM>. Because the operating temperature of the motor section <NUM> is higher than that of the extracted fluid <NUM>, any heat exchange between the fluid <NUM> and the outer surfaces of motor section <NUM> tends to cool the motor section <NUM> and the various components within it.

Motor section <NUM> has a casing, outer wall, or related enclosure <NUM> that is preferably filled with a thermal transfer fluid <NUM> that in addition to providing lubrication to the motor bearings, possesses a high thermal coupling of the heat sources to the enclosure <NUM>. In certain embodiments, the thermal transfer fluid <NUM> may be water or an antifreeze (such as glycol with water). Thus, heat generated within the motor section <NUM> is efficiently carried by this internal filling to the enclosure <NUM>, where it can be exchanged with the fluid <NUM> being pumped that passes in the circumferential channel <NUM> that is formed between the outside of the enclosure <NUM> and the inside of well piping <NUM>.

Within the casing <NUM>, the motor section <NUM> defines a synchronous motor that includes a rotor <NUM> and a stator <NUM> that operate by electromagnetic principles well-known to those skilled in the art. The rotor <NUM> and stator <NUM> are magnetically coupled so that the rotor <NUM> is made to spin along its vertical axis when acted upon by the stator <NUM>, thereby operating the pump section <NUM> of the ESP <NUM>. As will be additionally understood by those skilled in the induction motor art, stator <NUM> may further include coil winding (not shown). Electric current is provided to stator <NUM> by a power cable <NUM> that typically extends along the outer surface defined by casing <NUM>. Power cable <NUM> is in turn electrically coupled to a source that may include computer-controlled variable-frequency drives. It is further contemplated that the motor section <NUM> may comprise more than one synchronous motor, where each motor may comprise a rotor <NUM> and a stator <NUM> to operate the pump section.

Referring next to <FIG>, the rotor <NUM> includes a shaft 114A that extends over the length of the motor section <NUM> (and optionally over the entire length of the ESP <NUM> through suitable coupling (not shown)). In one form, the shaft 114A extends out of the upper end of the motor section casing <NUM> into the pump section <NUM>, and is fluidly isolated between the motor and pump sections <NUM> and <NUM> by seals <NUM> that are disposed between them. The seals <NUM> prevent the extracted fluid <NUM> from entering the motor section <NUM> of the ESP <NUM>, which may cause corrosion of the motor components. The seals <NUM> in turn prevent the thermal transfer fluid <NUM> from exiting the motor section. Operation of motor section <NUM> causes the shaft 114A to turn, which in turn causes the serially-arranged centrifugal impellers <NUM> in the pump section <NUM> to rotate so that fluid <NUM> can be pressurized and conveyed through tubing and exit through the fluid outlet section <NUM>.

Details associated with providing magnet protection against the ambient deep well environment discussed above include having a concentric yoke 114B that axially coincides with the placement of a series of permanent magnets 114C; importantly, rather than having the yoke 114B be formed of numerous laminated iron plates, it is integrally-formed with the shaft 114A, which by virtue of its larger integrally-formed radius and larger polar and area moment of inertia increases the effective bending stiffness of rotor <NUM> by defining a larger radial profile. As shown in <FIG>, the yoke 114B is comprised of only one piece, without the addition of separate sections or parts. A stepped collar 114D, in one form a balancing ring, is placed fore and aft of the yoke 114B in order to provide axial load support and further increases in bending resistance, as well as sealing closure, of the magnets 114C. Regarding the increase in bending resistance due to the the stepped collar 114D, inadvertent grinding or related machining or wear on the magnets 114C is avoided. Importantly, the stepped collar 114D has an inherently low part count and ease of assembly.

In one form, the magnets 114C are surface mounted to the yoke 114B. In order to secure the magnets 114C to the yoke 114B, the magnets 114C may be glued thereon. By placing magnets 114C on the surface of the rotor yoke 114B, the pole-pitch factor can be maximized, which in turn promotes high energy densities. Such a construction is optimal for long thin rotors such as those used in submersible motors such as ESP <NUM>. As such, the magnets 114C are fixed to the shaft 114A through the yoke 114B using a cladding tube 114E in addition to an optional castable resin <NUM> to fill all gaps between tube 114E, magnets 114C and rotor yoke 114B. Through this fixation, very high centrifugal forces can be withstood; moreover, the magnets 114C may be hermetically sealed against the ambient environment. Thus, through the isolation against the motor filling liquid (not shown), the likelihood of corrosion to the magnets 114C is reduced or eliminated. The tube 114E may fully encompass the circumference of the magnets 114C joined to the yoke 114B, and therefore surround the circumference of the yoke 114B itself. The cladding tube 114E in one or more embodiments may be a strong, prefabricated cladding tube 114E, allowing for ease of assembly and improving the rigid structure and durability of the rotor <NUM>, being fully produced and finished before being assembled to the yoke 114B. In addition, the cladding tube 114E can provide a highly accurate surface with close dimensional tolerances, which is valuable in reducing friction losses in configurations where rotor <NUM> and stator <NUM> are closely-coupled. Furthermore, by using a non-conductive material, eddy current losses in the tube 114E can be avoided. In one form the cladding tube 114E is made from a wound carbon fiber impregnated with epoxy resin, while in another it is made of fiberglass.

From the manufacturing point of view, the described construction of rotor <NUM> provides the robustness typically associated with traditional buried magnets while also being much simpler to assemble than that of known surface mounted magnet approaches. As such, the present use of the yoke 114B, stepped collar 114D (which in one configuration may operate as the aforementioned balancing ring) and the cladding tube 114E define a magnet channel 114I sized to accept the comparably sized and shaped magnets 114C that can be easily inserted therein. Significantly, ancillary manufacturing operations, such as drilling, milling or stacking of punched metal sheets to produce such a channel, are not necessary, thereby simplifying the rotor <NUM> assembly process.

As mentioned above, in one preferred form, the castable resin <NUM> may be in the form of a filler material placed between radially-adjacent magnets 114C to not only fill gaps that otherwise would permit unacceptably high levels of magnet 114C shifting, but also inhibit direct contact between the magnets 114C and the ambient environment that may include (among others) lubricant, coolant or working fluid. In a preferred assembly step, the castable resin <NUM> is injected into the channel of the cladding tube 114E before the magnets 114C are moved in; this helps reduce the likelihood of air bubble formation without having to use a vacuum process. As such, the tube 114E, along with resin <NUM> helps protect the magnets 114C from lubricants, working fluid and their associated thermal, chemical or frictional interaction.

Referring with particularity to <FIG>, two variants of the yoke 114B outer profile of are also contemplated, both of which are within the scope of the present disclosure. In one variant, numerous faceted spots are formed as chord-like spot facing around the yoke 114B periphery, while in another, the circumference defines a substantially constant axial profile <NUM>. Regarding the first, these faceted features define an outer dimension form closure 114F. Although not shown all around the yoke 114B circumference, were it to do so, form closure 114F would impart a polygonal shape to the outer periphery of yoke 114B. In particular, the faceted shape of the form closures 114F promote improved torque transferral to the magnets 114C during rotor <NUM> rotation. In one form, the magnets 114C may be shaped with planar lower surfaces such that they can be glued very securely to the shaft 114A. As discussed above, the balancing ring stepped collar 114D may also be used to help provide both axial confinement of the magnets 114C as well as additional shaft 114A stiffening and magnet 114C hermetic sealing.

While not necessary for operation of the pump <NUM>, the magnets 114C in one preferred form may be made from high-capacity rare-earth materials (such as NdFeB), while in others they may also include Dy or Tb. While such materials tend to have superior electromagnetic performance, they also have a greater tendency to corrode than with more conventional materials. In the event that such rare earths are used, the hermetically-sealed construction made possible by one or both of the cladding tube 114E and resin <NUM> ensures ample environmental resistance of the magnets 114C without significantly more complex rotor <NUM> construction.

It is noted that recitations herein of a component of an embodiment being "configured" in a particular way or to embody a particular property, or function in a particular manner, are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is "configured" denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component.

It is noted that terms like "generally," "commonly," and "typically," when utilized herein, are not utilized to limit the scope of the claimed embodiments or to imply that certain features are critical, essential, or even important to the structure or function of the claimed embodiments. Rather, these terms are merely intended to identify particular aspects of an embodiment or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment. Likewise, for the purposes of describing and defining embodiments herein it is noted that the terms "substantially", "significantly", "about" and "approximately" that may be utilized herein represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement or other representation. Such terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

Claim 1:
A method of constructing a rotor (<NUM>) for an electric submersible pump, the method comprising the assembly of a yoke (114B), a stepped collar (114D), and a cladding tube (114E) to define a magnet channel sized to accept one or more rare earth permanent magnets (114C), the method further comprising:
placing a shaft in rigid communication with and configured to rotate one or more pump impellers (<NUM>), said shaft comprising a concentric yoke (114B) integrally-formed therewith, the yoke (114B) comprising a fore end and an aft end;
disposing the one or more rare earth permanent magnets (114C) about the circumference of the yoke (114B), further positioning the one or more permanent magnets (114C) to axially coincide with the yoke (114B);
positioning a stepped collar (114D) on each of the fore end and the aft end of the yoke (114B); and
disposing a cladding tube (114E) around the circumference of the one or more rare earth permanent magnets (114C), the cladding tube (114E) securing the one or more permanent magnets (114C) to the shaft through the yoke (114B);
and wherein:
the yoke (114B) interface with the one or more permanent magnets (114C) comprises an outer dimension form closure (114F) on the circumference of the yoke (114B); wherein form-closed torque transmission is present between the one or more permanent magnets (114C) and the yoke (114B); and
the form closure (114F) is defined by a faceted spot for each respective permanent magnet around the circumference of the yoke (114B).