Coil with twisted wires and stator assembly of a rotary electric machine

A rotary electric machine includes a stator having an open slot configuration and a plurality of stator poles with a coil positioned about each stator pole. Each coil has a plurality of electrically conductive wires defining a group of wires and the group of wires is wrapped generally around a stator pole to define a plurality of turns. At least a portion of the group of wires is twisted, and the portion of the group of wires has between approximately 1 and 5 twists per turn. A method of fabricating a stator assembly is also disclosed.

TECHNICAL FIELD

This disclosure relates generally to a rotary electric machine and, more particularly, to a coil and stator assembly of a rotary electric machine and a method of fabricating a coil and stator assembly.

BACKGROUND

Work machines may be powered by electrical propulsion systems. The electrical propulsion systems sometimes include electric drive traction systems that provide driving forces to traction devices of the work machines. In some electric drive fraction systems, switched reluctance motors are used to provide the driving force.

Switched reluctance motors may have various motor topologies (e.g., the number of stator poles, the number of coils, and the number of rotor poles). In addition, a switched reluctance motor may be configured with a plurality of phases (e.g., 2 phases, 3 phases, 4 phases, or more). A switched reluctance motor may have a plurality of stator poles, each with a winding of electrically conductive wires or coil positioned therearound. The number of wires and the configuration of the coil is one factor that affects the efficiency of the operation of the switched reluctance motor.

Many switched reluctance motors are designed to optimize operation under certain operating conditions. However, it is desirable for switched reluctance motors used to power certain work machines to operate efficiently at both low speeds with high current and at higher speeds with lower current. The coils of some motors perform well electrically but lack the ability to carry significant amounts of current without excessive coil heating. Other coils have increased current carrying capacity but do not perform efficiently as operating frequencies increase, which also limits their ability to power work machines.

U.S. Pat. No. 7,201,244 discloses a work machined powered by a switched reluctance motor. The coils used in the switched reluctance motor are formed with wires having a square cross-section to increase the density of the conductor or fill within the stator slot between each stator pole. However, as the operating frequencies of the motor increase, the electrical characteristics of the coil will limit the efficiency of operation of the motor.

The foregoing background discussion is intended solely to aid the reader. It is not intended to limit the innovations described herein, nor to limit or expand the prior art discussed. Thus, the foregoing discussion should not be taken to indicate that any particular element of a prior system is unsuitable for use with the innovations described herein, nor is it intended to indicate that any element is essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims.

SUMMARY

In one aspect, a rotary electric machine includes a stator having an open slot configuration and a rotor positioned within the stator. The stator has a plurality of stator poles and a plurality of stator slots. Each stator slot is positioned between a pair of the stator poles. The rotor has a plurality of rotor poles. A coil is positioned about each stator pole. Each coil has a plurality of electrically conductive wires defining a group of wires and the group of wires is wrapped generally around its respective stator pole to define a plurality of turns of the group of wires. At least a portion of the group of wires is twisted, and the portion of the group of wires has between approximately one and five twists per turn.

In another aspect, a rotary electric machine includes a stator having a plurality of stator poles and a plurality of stator slots. Each stator pole has first and second oppositely facing side surfaces and each stator pole is positioned between the first and second stator slots. Each stator slot has an outer surface, an inner boundary, and a centerline extending generally along a midpoint between opposed side surfaces of adjacent stator poles. A rotor is positioned within the stator and has a plurality of rotor poles. A coil is positioned about each stator pole. Each coil has a plurality of electrically conductive wires defining a group of wires with the group of wires being wrapped generally around its respective stator pole to define a plurality of turns of the group of wires about the stator pole. At least a portion of the group of wires is twisted with the portion of the group of wires has between approximately one and five twists per turn. Each coil has a first portion and a second portion with the first portion and the second portion being positioned within adjacent stator slots separated by the respective stator pole. Each first portion extends generally between the first side surface of the respective stator pole and the centerline of the first stator slot along a path generally from the outer surface of the first stator slot towards the inner boundary of the stator slot, and each second portion extends generally between the second side surface of the respective stator pole and the centerline of the second stator slot along a path generally from the outer surface of the second stator slot towards the inner boundary of the stator slot.

In still another aspect, a method of fabricating a stator assembly of a rotary electric machine includes providing a stator having a plurality of stator poles. A plurality of coils are formed by supplying a plurality of electrically conductive wires to define a group of wire, twisting the group of wires at a predetermined rate, and wrapping the group of wires a predetermined number of turns to form a coil. The step of twisting the group of wires includes twisting the group of wires between approximately one and five twists per turn of the coil. The group of wires is configured so that each of the electrically conductive wires of each turn is laterally movable relative to others of the electrically conductive wires of the turn along at least a portion thereof. After forming, each coil is mounted on a stator pole.

DETAILED DESCRIPTION

Referring toFIG. 1, a machine10is schematically depicted including a chassis12with a front axle13and a rear axle14. A traction device15(e.g., wheels, tracks, etc) may be mounted on each end of each axle and may be driven by a switched reluctance traction system16. A power source17provides electrical power to the switched reluctance traction systems16. Power source17may use a prime mover (not shown) such as an internal combustion engine coupled with a generator (not shown) to supply electrical power to the switched reluctance fraction systems16. In another embodiment, power source17may be a fuel cell generator (not shown) configured to supply directly electrical power to the switched reluctance traction systems16. Still further, power source17may include a hybrid system including two or more different types of devices for converting an energy supply to electrical energy or for directly supplying electrical energy.

A controller21may be used to control operation of the switched reluctance traction systems16as well as the power source17and other components and systems of the machine10. Controller21may be a component of a control system shown generally at20inFIG. 1to indicate association with machine10. Control system20may include one or more sensors to provide data and other input signals representative of various operating parameters of the machine10.

The controller21may be an electronic controller that operates in a logical fashion to perform operations, execute control algorithms, store and retrieve data and other desired operations. The controller21may include or access memory, secondary storage devices, processors, and any other components for running an application. The memory and secondary storage devices may be in the form of read-only memory (ROM) or random access memory (RAM) or integrated circuitry that is accessible by the controller. Various other circuits may be associated with the controller such as power supply circuitry, signal conditioning circuitry, driver circuitry, and other types of circuitry.

The controller21may be a single controller or may include more than one controller disposed to control various functions and/or features of the machine10. The term “controller” is meant to be used in its broadest sense to include one or more controllers and/or microprocessors that may be associated with the machine10and that may cooperate in controlling various functions and operations of the machine. The functionality of the controller21may be implemented in hardware and/or software without regard to the functionality. The controller21may control the switched reluctance traction systems16and other functions of the machine10.

FIG. 2depicts a schematic view of a rotary electric machine such as a switched reluctance motor25that may be associated with each switched reluctance traction system16. As depicted inFIG. 2, an 8/4 2-phase switched reluctance motor25(i.e., eight stator poles28, four rotor poles29, and 2-phase conduction) may include a stator26and a rotor27rotatable relative to the stator. The number of phases as well as the number of stator poles28and rotor poles29is exemplary only and not intended to be limiting. In other words, the switched reluctance motor25may have a first plurality of stator poles28and a second plurality of rotor poles29.

As depicted, stator26includes eight radially inwardly projecting stator poles28and rotor27includes four radially outwardly projecting rotor poles29. Each stator pole28projects radially inward and has an inward end face31and a pair of oppositely facing side surfaces32. The distance, indicated at33, between the oppositely facing side surfaces32(i.e., a first side surface and a second side surface of the stator pole28) is generally constant so that each pole has a generally constant width in a circumferential direction around the stator26.

Stator26further includes a plurality of stator slots35with each stator slot being angularly positioned between a pair of the stator poles28and thus each stator pole28is angularly positioned between a pair of adjacent stator slots35. Accordingly, the number of stator slots35is equal in number to the number of stator poles28. Each stator slot35opens towards rotor27and has an edge or outer surface36, opposed side edges37defined by the oppositely facing side surfaces32of adjacent stator poles28, and an inner boundary38extending generally along or across the opening39of the stator slot. As described below, a retention structure55may span the opposed side edges37across the opening39to retain coils45positioned on each stator pole28within the stator slots35. Accordingly, the inner boundary38of each stator slot35may be spaced from the inward end face31of each stator pole28as best seen inFIG. 6.

Due to the circular cross-section of the stator26and the generally constant width of each stator pole28in an arcuate or circumferential direction, each stator slot35has a width that tapers or narrows generally uniformly or linearly from the outer surface36towards the opening39. With such a structure, the stator26may be referred to as having an open slot configuration. A centerline40of each stator slot35extends between adjacent pairs of stator poles to extend generally along the midpoint between the opposed side edges37of the stator slot.

The stator poles28may be grouped in two or more stator pole28sets that correspond to the number of phases (e.g., 2) of the switched reluctance motor25. In the depicted example, the eight stator poles28are grouped in two phase sets with four stator poles (depicted as A+and A−) grouped into one phase set and four stator poles (depicted as B+and B−) grouped into the other phase set. The rotor poles29may be grouped in diametrically aligned pairs.

Each stator pole28has a conductive winding or coil45wrapped therearound. The coils45positioned about the stator poles28of each group of a phase set (A+, A−and B+, B−) are electrically connected and may be configured as part of an electrical circuit, either in parallel or in series.FIG. 3depicts the coils of phase A connected in parallel. The coils of phase B may be arranged in a manner similar to the coils ofFIG. 3.

Switched reluctance motor25has a rotor27with no windings or magnets. The rotor27may be formed of a stack of vertically laminated iron, one-piece continuous annular members (not shown). Rotors27having other structures and configurations are contemplated. In addition, while the motor ofFIG. 2is depicted as a switched reluctance motor, the concepts disclosed herein are applicable to other rotary electric machines such as a switched reluctance generator. The concepts are further applicable to other rotary electric machines, for example, one in which the rotor27has permanent magnets or some other structure or configuration.

In operation, rotation of the rotor27of the switched reluctance motor25is achieved by the sequential excitation or energization of adjacent sets of stator poles28by supplying DC current to the coils45of the stator poles28. Energization of the stator poles28creates magnetic flux towards which the rotor poles29are attracted which tends to align the rotor poles29with the energized stator poles28. As the rotor poles29become aligned with the energized stator poles28, the DC current to the energized poles is terminated and subsequently supplied to the next sequential stator poles28. The rotor poles29are then attracted to the next set of sequential poles, which causes continued rotation of the rotor27. This process is continued during operation of the switched reluctance motor25. Torque is generated by the tendency of rotor poles29to align with energized stator poles28. Continuous torque may be generated by synchronizing excitation of consecutive stator poles28with the instantaneous position of rotor poles29.

Referring toFIG. 4, a portion of the stator26is depicted with two coils45positioned about adjacent stator poles28. Each coil45may be generally identical and is formed of a plurality of electrically conductive wires46that define a group of wires that are wrapped in a generally oval manner around the central opening47a predetermined number of times or turns. The central opening47generally corresponds in size to the cross section of the stator poles28so that the coil45may be slid onto the stator pole28during the fabrication of the switched reluctance motor25. Each coil45has a pair of long or major sides48and a pair of short or minor sides49interconnecting the pair of major sides. Accordingly, each coil45has a first portion51that extends through a first stator slot35and a second portion52that extends through a second, stator slot35. The second stator slot35is adjacent the first stator slot with the first and second stator slots separated by a stator pole28.

The coil may be formed of a plurality of electrically conductive wires46, each having a generally circular cross-section. The electrically conductive wires46may have a non-circular cross-section such as oval, square or rectangular in some configurations. The electrically conductive wires46may be formed of a highly conductive, flexible material, such as copper, and have a layer of insulation thereon. In one embodiment, magnet wires having a layer of enamel insulation may be used.

In one embodiment, a coil45may be fabricated with a pair of major sides48of approximately eight inches in length and a pair of minor sides49of approximately two inches in length. The group of wires may include seven electrically conductive wires46, each having a diameter of approximately 0.05 inches, and may be wrapped around the central opening47fifty-six times. Such a coil may also be referred to as having fifty-six turns. In other similar embodiments, the group of wires may include between approximately five and nine electrically conductive wires46. Other numbers of electrically conductive wires46may be used if desired. Examples using as few as two electrically conductive wires46and as many as thirty wires have been contemplated.

The electrically conductive wires46may also have other diameters. In another embodiment, the electrically conductive wires46may be approximately 15-18 gauge wire. Other numbers of electrically conductive wires46and those having other diameters may also be used. For example, increasing the number of wires while decreasing their diameter may result in comparable performance. However, in some circumstances, increasing the number of wires may undesirably increase the amount of insulation as a percent of the cross-section of the group of wires.

The number of turns or times that the group of wires is wrapped around the central opening47may be determined or set based upon the desired electrical performance of the switched reluctance motor25. Accordingly, the number of turns about the central opening47may be adjusted as desired.

The group of wires that is wrapped around the central core is formed of individual electrically conductive wires46that may also be twisted together. The twisting of the wires may be achieved in a variety of manners. In one example, the electrically conductive wires46may be generally continuously twisted generally about an axis through the group of wires. In another example, the electrically conductive wires46may be fed from a plurality of wire supplies (not shown) through a tensioner (not shown) and the tensioner (as well as the wire supplies, if desired) may be rotated to twist the wires. If desired, the electrically conductive wires46may be twisted as they are being fed and wrapped around a fixture (not shown) to form the coil45.

Regardless of the manner of twisting the electrically conductive wires46, the twist forms a relatively loose twist of the wires. Such a loose twist of wires may permit the individual electrically conductive wires46to move laterally relative to the other wires of the group as the coil45is being mounted on a stator pole28. In the example described above, the group of wires may be twisted approximately two times as they travel about the central opening47. As the electrically conductive wires46are twisted, the group of wires may take on a somewhat circular cross-section. However, the loose twist of the wires permits lateral movement of each electrically conductive wire46relative to other wires within the group of wires. Once the coil45is mounted on the stator pole28, the electrically conductive wires46may have moved laterally sufficiently so that each turn or wrap of the group of wires may not be readily discernible from other turns or wraps of the wires and the individual electrically conductive wires46may appear to be relatively randomly positioned or positioned in a non-uniform array within each stator slot35.

Although the group of wires in the depicted embodiment is twisted twice for each wrap around the central opening47, in other embodiments the electrically conductive wires46may be twisted between approximately one and five times as they are wrapped about the central opening47. In other applications and configurations, it may be possible to twist the group of wires at a slower or a faster rate if such twist permits the electrically conductive wires46to be moved laterally and the positioning of the electrically conductive wires satisfies the desired electrical performance.

FIG. 6depicts a cross-section through a stator slot35and a pair of adjacent stator poles28having coils45positioned therearound. The first portion51of one coil45and the second portion52of a second, adjacent coil45are each positioned within the stator slot35. A non-conductive spacer56may be positioned generally along the centerline40of each stator slot35to separate the first portion51of one coil45from the second portion52of the second, adjacent coil45. Because the electrically conductive wires46are loosely twisted when forming the group of wires, they are laterally movable relative to other of the electrically conductive wires within a turn at least along the major sides48of the coil45. Accordingly, as the coils45are slid onto the stator poles28, the individual electrically conductive wires46may move to fill relatively tightly the stator slot35. This movement may result in a significant number of openings or gaps between adjacent turns of the group of wires being filled due to the lateral movement of the electrically conductive wires46.

As stated above, because of the relatively loose twist of the electrically conductive wires46, the wires may move laterally as each coil45is mounted on its stator pole28. The electrically conductive wires46along the major sides48of the coil may move laterally within their stator slot35to reduce the number and size of the openings between wires. In some instances, the portion of the wires along the minor sides49of each coil45may move so as to relatively reduce or eliminate the twist of the wires along the minor side. As such, each turn of the group of wires is still twisted but the twisted portion may tend to be concentrated in a portion of the turn along the major side48of the coil45. As such, while the electrically conductive wires46may be relatively consistently twisted as the coil45is formed, the twist may not be consistent along the length of each turn.

As may be seen inFIG. 6, interstices or uniform voids between adjacent turns of the group of wires along the cross-section through the stator slot35may be significantly reduced or even generally eliminated. In other words, because of the loose twist of the individual electrically conductive wires46of the group of wires, the wires may move laterally so that the individual wires appear to be generally randomly placed and the outline or boundary of each turn or wrap of the group of wires is substantially eliminated. For example, inFIG. 7, a first turn60of the group of wires is depicted with a somewhat circular cross-section or boundary. A second turn61of the group of wires is depicted with a somewhat rectangular cross-section. A third turn62of the group of wires and a fourth turn63of the group of wires each have a generally different cross-section. As such, even though the group of wires may have initially had a generally circular-cross section, upon positioning the coil45within the stator slot35, the wires within many or even all of the turns of the group of wires may be shifted to have non-circular cross-sections.

Referring back toFIG. 6, a retention structure55such as a generally non-conductive board-like member may span the opposed side edges37of the stator slots generally adjacent and across the opening39to retain the coils45positioned on each stator pole28within the stator slots35. (It should be noted that retention structure55is not depicted inFIG. 4for clarity but extends between the stator poles28and across each stator slot35as depicted inFIG. 6.)

With the disclosed structure, a relatively dense coil configuration (i.e., a reduced amount of air) may be created. The first portion51of each coil45generally fills one stator slot35between the side surface32of one stator pole28and the centerline40of the stator slot along a first path generally from the outer surface36of the stator slot35to the inner boundary38of the stator slot. Similarly, the second portion52of the same coil45generally fills the stator slot35on the opposite side of the stator pole28between the opposite side of the stator pole28and the centerline40of its stator slot35along a second path generally from the outer surface36towards the inner boundary38of the stator slot. Upon positioning a coil45around each stator pole28, each stator slot35will have a first portion51of one coil45and a second portion52of an adjacent coil positioned therein. The non-conductive spacer56may be positioned between the first portion51of one coil45and the second portion52of a second, adjacent coil. In addition, the first portion51and second portion52may be retained within a stator slot35by retention structure55.

To form the coil45, a plurality of individual wire supplies (not shown) may be positioned on a rotatable fixture or turntable (not shown) with the electrically conductive wires46defining a group of wires being fed through a tensioning structure (not shown). The tensioning structure may include a plurality of openings through which the electrically conductive wires46pass or a single common opening.

The electrically conductive wires46may be fed from the tensioning structure and wound about a collapsible bobbin or fixture (not shown) having an outer surface generally conforming in size and shape to the central opening47. As the group of wires is wound about the fixture, the turntable may be rotated at a predetermined rate so that the electrically conductive wires46are twisted as they are also wound about the fixture. In other words, the group of wires may be twisted by rotating the turntable as the group of wires is wound around the fixture to fabricate the coil45. In another embodiment, the group of wires may be twisted as part of a separate process prior to winding the wires around the fixture or provided by a supplier as a pre-twisted group of wires.

The speed at which the turntable is rotated to twist the electrically conductive wires46may be set based upon the rate at which the group of wires is wrapped around the fixture. In one embodiment, the turntable may be set to twist the electrically conductive wires46a predetermined number of times (e.g., between approximately one and five twists) per turn of the group of wires about the fixture. This relationship (approximately one to five twists per turn) may be maintained even with a relatively wide range in size of the coil45. Some coils are contemplated having a major side48as small as three inches in length and others are contemplated with a major side48as large as twenty-five inches in length. The disclosed structure may also be used with coils of other sizes outside of this range.

Still further, the disclosed concepts may be used by specifying the number of twists of the electrically conductive wires46that form the group of wires per unit length. In other words, rather than specifying the number of twists of the electrically conductive wires46per turn of the group of wires, it may be desirable to specify the number of twists per unit length (e.g., twists per inch) of the wires.

Once the electrically conductive wires46have been wound or wrapped about the fixture a desired number of times, the wires of the coil45may be secured together such as by tape57and the coil removed from the fixture. Coils45of this type are sometimes referred to as concentrated coils as the windings form a multi-turn coil having the same magnetic axis and are fixed around a single stator pole28.

The stator26may be formed by stacking a plurality of one-piece continuous annular iron members (not shown) together. A layer of insulative material (not shown) may be provided between each iron member. The coils45may be mounted on the stator26by moving the coils relative to the stator to slide a stator pole28through the central opening47of each coil45. While sliding the coils45onto each stator pole28, at least a portion of at least some of the electrically conductive wires46of each turn may move laterally relative to other electrically conductive wires46of the same turn. In one embodiment, the portions of the electrically conductive wires46being laterally moved are positioned along the major sides48of the coil45. This configuration generally eliminates interstices or uniform voids between adjacent turns of the group of wires along a cross-section of each coil45across each stator slot35. It should be noted that the coils45may be initially formed with a generally symmetrical cross-section and the lateral movement of at least some of the electrically conductive wires46of each turn while mounting the coils45on the stator poles28may modify the shape of the coil to form a generally asymmetrical cross-section across a portion thereof. The asymmetrical cross-section may extend across a portion of a pair of adjacent stator slots35that are separated by a stator pole28.

The coils45may be secured within the stator slots35such as by inserting a retention structure55that may span the opposed side edges37of the stator slots generally adjacent and across the opening39. Coils45mounted on opposite stator poles28may be electrically connected to form opposed coil pairs. The windings of such opposed coil pairs may be electrically connected in parallel or series as part of an electrical circuit as desired.

INDUSTRIAL APPLICABILITY

The industrial applicability of the rotary electric machine described herein will be readily appreciated from the foregoing discussion. The foregoing discussion is applicable to rotary electric machines such as switched reluctance motors25in which it is desirable to increase the electrical efficiency and performance of the rotary electric machine over a range of operating conditions.

Through the disclosed rotary electric machine configuration and the method of fabricating a stator assembly, improved electrical performance, efficiency and decreased copper losses may be achieved. For example, the switched reluctance motor25using the coil45depicted herein may significantly reduce copper losses in the motor. For example, losses due to skin effects of the conductors may be substantially reduced due to the relatively small diameter of the electrically conductive wires46as compared to the operating frequency of the switched reluctance motor25. In addition, the relatively small diameter of the electrically conductive wires46also reduces eddy currents within the conductors. Still further, the higher density of copper within the stator slot35(as depicted inFIG. 6) also may improve the thermal conductivity by reducing the air gaps between the individual electrically conductive wires46of the group of wires.

Proximity effects are also reduced through the disclosed structure by the relatively random positioning of the individual electrically conductive wires46within the group of wires. This is in part due to the relatively loose twist of the wires and the lateral movement of the wires as they are positioned within stator slot35. Inter-strand circulation currents are reduced by the disclosed structure due to the relatively random positioning of the individual electrically conductive wires46of the group of wires within the stator slot35. For example, electrically conductive wires46of the coil45that are closer to the rotor poles29may experience a difference of induced voltage as compared to wires that are farther away from the rotor pole29. By twisting the individual electrically conductive wires46as they are wound about the central opening47, the conductors change position within each turn of the group of wires which results in generally averaging of the distance of the individual electrically conductive wires46from the rotor poles29. As a result, the voltage induced in each of the electrically conductive wires46through the rotor poles29is generally averaged which reduces the inter-strand circulation caused by exposure of the individual wires to different induced voltages.

The relatively loose twisting of the electrically conductive wires46within the group of wires results in efficient volume utilization or packing of the wires within the stator slots35as best seen inFIG. 6. If the individual electrically conductive wires46of the group of wires are tightly twisted, the tightly twisted wires will form a relatively large, somewhat rigid generally circular structure. As coil45is slid onto its stator pole28, relatively large air gaps may exist between adjacent turns of the group of wires. By relatively loosely twisting the electrically conductive wires46, the individual wires are able to move laterally relative to each other as the coil45is slid onto its respective stator pole28and thus the individual wires may more efficiently fill the stator slots35. Through such a structure, a greater volume of the conductor (e.g., copper wire) may be positioned within the stator slots35. This higher density of the conductor results in a lower resistance through the coil45and, for a given voltage, a higher current carrying capacity and thus a more efficient operation of the switched reluctance motor25.