Patent Application
20120275942
1. Field of the Invention
The invention relates generally to the construction of electric motors, and more particularly to systems and methods for constructing electric motors in which a stator core includes inner and outer portions that enable slots for magnet windings to be open during construction and closed in the completed motor.
2. Related Art
A typical electric motor has two primary components: a rotor; and a stator. The stator is a stationary component, while the rotor is a movable component which rotates within the stator. Typically, in a DC motor, or in a permanent magnet motor, one or the other of these components has a permanent magnet, while the other uses coils of electrical wire to generate changing magnetic fields. In an AC induction motor, a magnetic field is induced into the rotor. The interaction of the magnetic fields created by the stator and the rotor cause the rotor to rotate within the stator.
The motor incorporates electromagnets that generate changing magnetic fields when current supplied to the electromagnets is varied. These electromagnets are normally formed by positioning coils (windings) of insulated wire around ferromagnetic cores. In an AC induction motor, the ferromagnetic cores are formed between “slots” in the stator core. When electric current is passed through the wire, magnetic fields are generated around the wire and consequently in the ferromagnetic cores. Changing the magnitude and direction of the current changes the magnitude and polarity of the magnetic fields generated by the electromagnets.
Electric motors that are designed for downhole applications (such as driving an electric submersible pump) are typically AC induction motors. These motors, generally speaking, are long and skinny. Usually, downhole motors are less than 10 inches in diameter, and they may be tens of meters long. This extremely elongated shape drives many aspects of a downhole motor's design. For example, although an open-slot stator design is generally better at inducing magnetic fields in the rotor, the length of the motor makes it very difficult to keep the magnet wires properly positioned with the slots, whereas the wires are confined in a closed-slot design. Further, in oil-filled motors in which the rotor is a close fit within the bore of the stator, open-slots can cause shearing and turbulence within the oil that reduces the efficiency of the motor. Still further, after the magnet wires are positioned in the slots, the slots are often at least partially filled with epoxy to maintain the positions of the wires and to provide additional electrical insulation around the wires, so a closed-slot design better contains the epoxy (the epoxy cannot get into the bore of the stator) and reduces the epoxy clean-up that is necessary. For these reasons, a closed-slot design is usually preferred for purposes of improved manufacturability.
Closed-slot designs, however, are not without their own drawbacks. For instance, because the slots of the stator are closed, the windings of magnet wire must be threaded through the individual slots. Each winding may have tens of turns (or loops) of wire, so the wire may be threaded through each slot tens of times. This presents a considerable risk of damage to the wire. Further, because the wire is being threaded through slots that may be tens of meters long, it is difficult, if not impossible, to control the positions of the individual turns of wire within the slots. The motor may therefore be referred to as a random-wound motor. As a result of the random winding, the turns with the highest voltage may be positioned adjacent to turns with the lowest voltage, thereby leading to high voltage-stresses that can damage the insulation around the wire.
It would therefore be desirable to provide systems and methods to improve the manufacturability of downhole motors that employ closed-slot stator designs.
The present invention includes systems and methods for the construction of electric motors, where the motor has a stator core that includes inner and outer portions that enable slots for magnet windings to be open during construction and closed in the completed motor.
One embodiment comprises a method. In this method, an inner stator core is provided. The inner stator core includes a plurality of slots which are open radially outward from an axis of the inner stator core and which are configured to accommodate turns of magnet wire. Turns of magnet wire are positioned in each of the open slots of the inner stator core, and then an outer stator core is positioned around the inner stator core, thereby enclosing the slots of the inner stator core. The inner and outer stator cores are then positioned within a housing. Each of the inner and outer stator cores may be formed by stacking a plurality of laminations together, where each of the plurality of laminations has a shape which is a cross section of the respective inner/outer stator core. The turns of magnet wire may be wound on a separate form, or on the inner stator core itself. Because the slots are open, the positioning of the magnet wires within the slots can be controlled, and shaped wire can be used.
Another embodiment comprises a stator for a downhole electric motor. The stator includes an inner core and an outer core. The inner core forms a plurality of slots that are open radially outward from an axis of the stator. Magnet wires are positioned within the slots of the inner core. The outer core is positioned around and coaxially with the inner core so that the outer core encloses the slots of the inner core. A rotor may be positioned coaxially within a central bore in the stator to form a motor. The motor may be used for such purposes as driving the pump of an electric submersible pump system. The motor may have an outer diameter of less than approximately 10 inches to enable it to be used in a downhole environment. The inner and outer cores may be formed by stacking a plurality of individual laminations together. Mating dimples may be provided in the laminations to retain the laminations in the stacked position and to thereby facilitate placement of the magnet wires in the slots. The inner and outer cores (and inner and outer laminations) may have interlocking shapes that prevent rotation of the inner core with respect to the outer core when the outer core is positioned around the inner core. Additionally, the outer and/or inner cores may be shaped to interlock with the stator housing.
Numerous other embodiments are also possible.
Other objects and advantages of the invention may become apparent upon reading the following detailed description and upon reference to the accompanying drawings.
While the invention is subject to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and the accompanying detailed description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular embodiment which is described. This disclosure is instead intended to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims.
One or more embodiments of the invention are described below. It should be noted that these and any other embodiments described below are exemplary and are intended to be illustrative of the invention rather than limiting.
As described herein, various embodiments of the invention comprise systems and methods for construction of electric motors in which a stator core includes inner and outer portions that enable slots for magnet windings to be open during construction and closed in the completed motor.
In one embodiment, a motor for a system such as an electric submersible pump (ESP) is constructed using a stator having a multi-part core. In this embodiment, the stator core includes two separate parts—an inner core and an outer core. Each of the inner and outer core is itself constructed using a set of individual laminations that are stacked together to form the respective part. The inner core is formed using identical laminations that have a plurality of teeth. Between the teeth are slots that are open radially outward from the center of the lamination (the axis of the stator). The outer laminations are designed to be positioned around the inner laminations to enclose the slots. Each of the outer laminations is identical.
The inner and outer core are formed by stacking the appropriate laminations. Because the inner stator core has slots that open radially outward from the axis of the inner core, the slots are easily accessible, facilitating positioning of magnet wires within the slots. After the magnet wires are positioned in the slots (e.g., by winding the wires on the inner core), the outer stator core is positioned around the inner core. This encloses the slots, resulting in a stator core that has an overall structure similar to that of a conventional closed-slot stator core. The wound stator core is then inserted into a housing and construction of the stator, and subsequently the motor, proceeds in a conventional manner.
Referring to
Rotor 130 is caused to move within stator 120 by changing magnetic fields. In an AC induction motor, varying electric currents in the windings of stator 120 create magnetic fields. These magnetic fields induce an electromotive force in rotor 130, thereby causing the rotor to generate its own magnetic fields. The interaction of the magnetic fields of stator 120 and rotor 130 causes the rotor to rotate within the stator.
Referring to
The slots (e.g., 231-232) extend entirely through the stator core so that wires can be threaded through them. A wire is threaded through one slot and back through a different slot to form a turn of wire. The wire is threaded through these same slots multiple times to form a coil. The walls between the slots, commonly referred to as “teeth”, serve as ferromagnetic cores, so that when a wire is wrapped around one or more of them, and current is passed through the wire, an electromagnet is formed. Although a wire could be threaded through adjacent slots in the stator core, this typically is not the case with induction motors. Thus, for example, a wire may be threaded upward through slot 231, and then back through slot 232, as shown by arrow 250. The other arrows in the figure show how wires may be threaded through the other slots to form the remaining wire coils. The particular winding pattern shown in the example of
The wires that are threaded through the passageways in the stator core are typically copper wires that have an insulating coating. This insulating coating is intended to electrically insulate each turn of wire from the others so that current will pass through each of the turns, rather than bypassing one or more turns of wire if a short-circuit is created by electrical contact between the wire of two or more turns. As noted above, each time one of the wires is threaded through one of the slots, the layer of insulation around the wire may be damaged.
Because of the difficulty of threading the magnet wires through the closed slots, and the potential for damaging the wires, the systems and methods of the present disclosure utilize a multi-part stator core to facilitate installation of the wires and minimize damage to the wires. The stator core consists of an inner core and an outer core (which itself may include multiple components, as will be described in more detail below). The inner core contains the slots in which the magnet wires will be positioned, but the slots are open to the exterior of the inner core. That is, each of the slots has an opening that faces away from the axis of the inner core. The outer core fits around the inner core and encloses each of the slots. Because the stator core consists of these two components (inner and outer cores), the magnet wires can be positioned in the slots while they are open, and then the slots can be closed by positioning the outer core around the inner core. This design therefore provides the advantages of a closed-slot design while eliminating the disadvantages associated with having to thread the magnet wires through the closed slots.
Referring to
In the present embodiment, a similar lamination-based construction is used, but each layer uses a two-piece lamination rather than a single piece. As shown in
Outer lamination 320 is sized to fit around Inner lamination 310. Outer lamination 320 contacts Inner lamination 310 at the outer edge of each tooth of the inner lamination. The contact between inner lamination 310 and outer lamination 320 allows the magnetic fields generated by the magnet wires to be channeled from the teeth to the outer lamination. Outer lamination 320 serves as what is sometimes referred to as the “back iron” of the electromagnets formed in the stator. Inner lamination 310 and outer lamination 320 have interlocking shapes to prevent the inner lamination from rotating within the outer lamination. In this embodiment, inner lamination 310 has small protrusions (e.g., 313) which extend radially outward from the teeth (e.g., 311) and fit into corresponding notches (e.g., 323) in outer lamination 320.
It should be noted that both inner lamination 310 and outer lamination 320 include small dimples (e.g., 314, 324). The dimples form small indentations on one side of the laminations and small protrusions on the opposite side. When the laminations are stacked, the protruding side of the dimples in one lamination fit into the indented side of the dimples of an adjacent lamination. This mechanism helps to hold the laminations together after they have been stacked. This is useful because it is necessary to hold the laminations together while the magnet wires are positioned in the slots. This function is conventionally performed by the stator housing, but in this embodiment the housing would prevent access to the open slots, as will be explained in more detail below.
Although the stator housing may not be necessary in this embodiment to hold the laminations together, it is contemplated that the assembled inner and outer stator cores formed by the laminations will be inserted into a housing. The housing serves as an additional means to hold the laminations in place and also protects the stator cores. In embodiments such as illustrated in
Referring to
The windings of the stator core are then placed in the open slots of the inner stator core (420). The windings may be pre-formed, or wrapped on a form of the appropriate shape (i.e., “form-wound”) and then positioned in the slots, or the magnet wire may be wrapped around the inner stator core. Because the slots are open to the exterior of the inner stator core, the entirety of the slots is accessible, and the magnet wire can be positioned exactly as desired, without the risk of damage that is normally posed by threading the wire through closed slots. The open slots also allow the wire to be wound by a machine, which minimizes the variability that is associated with the hand-winding that is conventionally required.
After the windings of magnet wire have been positioned in the open slots of the inner stator core, the outer stator core is positioned around the inner core (430). The outer stator core is then in contact with the inner stator core and encloses the slots, thereby protecting the magnet wires and providing the advantages of a conventional closed-slot stator. In one embodiment, the outer stator core is constructed by stacking laminations of the appropriate shape (see, e.g., outer lamination 320 of
The assembled stator core (including the inner core, the magnet windings and the outer core) is then inserted into a stator housing (440). The stator core may, for example, be retained in the housing conventional means such as a pair of locking rings. The rings may be modified if necessary to retain the two-part (inner/outer) laminations if necessary. The stator housing provides some protection for the stator core and also helps hold the components together, as in a conventional design. Once the stator is assembled, a rotor can be inserted into the stator, and the construction of the motor can proceed using conventional techniques. The motor can be used in downhole applications, such as to drive electric submersible pumps.
Because the slots of the inner stator core are initially open during the construction of the stator, there is a great deal of flexibility in the type and installation of the magnet wires. For instance, conventional round wire may be used (see
Referring to
Referring to
The two-part stator core design shown in
Referring to
In the embodiment of
Referring to
Stator segment 1300 may be formed by stacking laminations in the same manner as described above. Dimples (e.g., 1305) may be used to hold the stacked laminations together. Magnet wire (e.g., 1350) is wound around the tooth (e.g., 1340) of each segment (e.g., 1300), and then the segments are joined to form the stator core. As shown in the figure, each stator segment (e.g., 1300) has key/keyway features (e.g., 1307, 1308) which interlock to hold adjacent segments together. The two segments that are adjacent to segment 1300 are shown in the figure by dotted lines. The joined segments are then inserted into a stator housing, and subsequent construction of the motor can proceed in a conventional manner.
The benefits and advantages which may be provided by the present invention have been described above with regard to specific embodiments. These benefits and advantages, and any elements or limitations that may cause them to occur or to become more pronounced are not to be construed as critical, required, or essential features of any or all of the claims. As used herein, the terms “comprises,” “comprising,” or any other variations thereof, are intended to be interpreted as non-exclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements, and may include other elements not expressly listed or inherent to the claimed embodiment.
While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed within the following claims.
