Patent Application
20140021809
This disclosure relates to electric machines and more specifically to electric motors that utilize magnetic reluctance.
Conventional switched reluctance motors (SRMs) have a cylindrical stator that surrounds a rotor within the stator. The stator typically includes a fully cylindrical outer body, also known as the “yoke” or “back-iron,” and a plurality of stator poles that project radially inwardly from the outer body. The rotor includes outwardly projecting rotor poles that differ in number from the plurality of stator poles. Such SRMs typically include independently controlled electrical windings positioned around each of the inwardly projecting stator poles. The different windings are variably energized to create variable magnetic flux paths to drive the rotation of the rotor. Typically, each of the flux paths travel circumferentially through the same cylindrical outer body of the stator between varying sets of energized stator poles. For example, in a conventional 12-8 SRM, when four stator poles are activated, each flux paths travels 90° through the outer body of the stator between activated poles, such that flux is observed around the entire 360° of the outer body of the stator at the same time.
Described herein are exemplary embodiments of switched reluctance motors (SRMs), including embodiments of Multiple Isolated Flux Path (MIFP) SRMs. Also disclosed are winding techniques, structural designs, and control concepts, which can provide improved performance, specific power, power density, and/or other features. Various coil winding configurations and stator support features are disclosed. MIFP SRMs can facilitate various novel electrical control techniques since the torque overlap between phases can be considerable. Such control techniques can provide torque ripple reduction, acoustic noise reduction, and/or other advantages.
Some exemplary reluctance motors disclosed herein comprise a central rotor having a plurality of radially outwardly projecting rotor poles and a plurality of stator units positioned circumferentially around the rotor. The stator units are spaced circumferentially apart and magnetically isolated from adjacent stator units. Stator units can comprise a circumferentially extending yoke and two stator poles extending radially inwardly from the yoke, such that the stator poles are positioned adjacent to the rotor poles. The motor further comprises a plurality of coils of electrical conductors, wherein at least one of the coils is coiled around one of the yokes of the stator units.
In some embodiments, the stator units comprise a generally U-shaped lamination stack and the stator units are magnetically isolated from one another.
In some embodiments, the coils comprise an outer portion and an inner portion, the outer portion being located along a radially outer side of the respective yoke and the inner portion being located along a radially inner side of the respective yoke between the two stator poles. The outer portion of the coil can have a radial thickness that is less than a radial thickness of the inner portion of the coil and the outer portion of the coil can have a circumferential width that is greater than a circumferential width of the inner portion of the coil. The outer portion of the coil and the inner portion of the coil can have about the same cross-sectional area perpendicular to current flow through the coil.
In some embodiments, each stator unit is associated with only one coil. In some embodiments, each stator pole comprises a circumferentially lateral side that faces away from an opposing stator pole of the same stator unit and a circumferentially medial side that faces the opposing stator pole of the same stator unit, and the circumferentially lateral sides of the stator poles are free of the coils.
In some embodiments, the motor further comprises an annular body, such as a cooling jacket, positioned along radially outer surfaces of the outer portions of the coils, that is configured to remove heat from the outer portions of the coils.
In some embodiments, the stator units further comprise first and second ridges projecting radially outwardly from the yoke along circumferentially lateral sides of the outer portions of the coils.
In some embodiments, the motor comprises a plurality of non-magnetic stator supports positioned between the stator units and configured to engage circumferential sides of the stator units to hold the stator units in alignment with one another and the rotor. The stator supports can be generally wedge shaped and/or taper in circumferential width moving radially inward. The stator units can comprise first and second circumferentially extending support projections that engage with corresponding support recesses in the adjacent stator supports. The motor can further comprise first and second axial end supports, or plates, positioned on opposing axial sides of the plurality of stator supports, wherein the axial end supports retain the plurality of stator supports in a fixed alignment relative to one another and relative to the rotor, thereby retaining the plurality of stator units in a fixed alignment relative to one another and relative to the rotor.
Disclosed embodiments can provide many advantages over conventional SRMs. For example, disclosed embodiments can provide increased power density, reduced noise, reduced torque ripple, reduced overall size and weight, simplified and lower cost manufacturability, improved heat transfer, improved ease of winding coils around stator units, improved ease of removing and inserting individual stator units, increased available space between stator units for placement of other components, and/or reduced of flux leakage between stator components. The foregoing and other objects, features, and advantages of this technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
Using the 12-10 embodiment of
Some or all of the stator units 102A-102F can be magnetically isolated from one another. For example, non-magnetic material and/or air-filled space can be positioned between the stator units 102. This prevents a direct flux path between adjacent stator units. Instead, flux paths are localized to a single stator unit 102 and travel around the U-shaped path of the individual stator unit. An exemplary isolated flux path 120 is shown in
In MIFP SRM embodiments described herein, the rotor and the stator units can comprise stacks of several thin layers, or laminations, of magnetic material that are built up in the axial direction to provide a desired axial depth. See the rotor 304 in
Three significant energy loss factors in SRMs are hysteresis, eddy currents, and “copper” losses. For a given excitation level and frequency, hysteresis and eddy current losses are generally proportional to the length of the flux path. Therefore, a MIFP SRM can have reduced hysteresis and eddy current losses relative to conventional SRMs since the flux path lengths in a MIFP SRM are shorter. Furthermore, hysteresis loss is generated from molecular friction when magnetic particles in the metal are subject to a reversal of magnetic field. For some MIFP SRM configurations, such as 12-10 embodiments, there can be essentially no flux reversal in the stator units, and therefore minimal hysteresis losses. This can be a significant advantage when compared with conventional flux reversal frequencies in conventional SRMs, which can be three or more times the electrical frequency, since all phases share the same cylindrical outer stator body. Additionally, disclosed MIFP SRMs can have a smaller amount of wasted copper at the end-turns compared to conventional motors since the phase windings do not overlap each other and thus can have lower copper (I2R) losses than what is typical for permanent magnet or induction machines, as well as reduced weight, volume, and cost from reduced copper amounts. Other benefits of disclosed MIFP SRMs include low material and manufacturing costs, high durability, and torque versus speed performance similar to permanent magnet machines.
In some embodiments of a MIFP SRM, one or more of the stator units includes two continuous wire coils, one positioned around each of the two stator poles 106. For example, in a 12-10 MIFP SRM, each of the twelve stator poles can have an individual coil wrapped around it. The two coils on the two stator poles of a stator unit can be electrically coupled in series, and such pairs of coils can be electrically coupled in series or in parallel with other pairs of coils in the same phase.
In other embodiments of a MIFP SRM, one or more of the stator unit comprises a continuous wire coil positioned around the yoke portion of the stator unit. For example,
As shown in
As shown in
MIFP SRM embodiments having coils positioned around the yokes of the stator units can provide various advantages relative to embodiments having coils positioned around the stator poles. These advantages can include improved heat transfer. Copper has a thermal conductivity that is approximately 40 times higher than steel. Thus, by having the outer portions 132 of the coils located radially outwardly of the yokes 108, the outer portions 132 of the coils can readily interface with a cooling structure, such as a heat exchanger, located radially outward of the stator units. By contrast, in motors wherein each stator pole includes an individual coil wrapped around it, the heat generated by the coils has to travel through the material of the stator before reaching the cooling structure located radially outwardly of the stator. In some embodiments, the outer portion 132 of the yoke coils 130 can be in direct contact with, or adjacent to, a cooling structure positioned radially outwardly of the stator units. Such improvement of heat transfer can allow for increased power density and specific power for the motor since higher output power can be obtained with an equivalently sized motor.
Another advantage of yoke-wound coils is manufacturability. Coils 130 can be wound around the yokes 108 of the individual “U” shaped stator units 102 before the stator units are installed in the support structure of the motor. This can make the winding process quicker, easier and less expensive. Also, the coils 130 can be machine wound around the yokes 108 by rotating the individual stator units 102 (about the axis of the yoke) while a feed of the coil wire is caused to become wrapped around the yoke. In motors having coils wrapped around the stator poles, by contrast, the coils are typically hand-wound around the stator poles or pre-wound away from the stators and subsequently slid over the stator poles. Winding a coil wire directly around the stator poles is more difficult because the opposing stator pole interferes with the winding path and can therefore require sophisticated equipment and/or tedious manual labor. Additionally, the pole-wound approach can require the interconnection of the two separate coils on each stator unit, whereas this can be avoided with the yoke-wound approach.
Furthermore, individual stator units 102 can be readily removed from and inserted into the motor without having to remove or insert other stator units. For example, an individual stator unit can be removed from the motor to replace or fix a damaged portion and then the individual stator unit can be reinserted into the motor without having to remove and reinsert other stator units. Further, each stator unit can be individually wound with a coil separate from the rest of the motor and inserted into the motor one at a time. By contrast, individual removal or manipulation of stator units is not possible with motors having a one-piece stator unit with a fully circumferential back-iron.
Another advantage of having coils 130 positioned around the yokes 108 of the stator units 102 instead of around the stator poles 106 is that the stator poles can be wider since the coils are not located on the outer-lateral side of the stator poles between adjacent stator units. Wider stator poles can allow for the torque production from each phase to be broader, increasing the overlap of torque production among phases. This can also increase the overall torque, and facilitates the reduction of torque ripple and acoustic noise reduction. Wider stator poles 106 can also allow for wider rotor poles 114, which can be more mechanically substantial, resulting in a reduction in vibration and acoustic noise.
Another advantage of having coils 130 positioned around the yokes 108 of the stator units 102 instead of around the stator poles 106 is that more loops of a single coil can be located in the middle of the stator units 102 between the poles 106 since two different coils do not share this same volume. Because individual stator units 102 include a single yoke-wound coil 130, the issues of winding two coils in place through the same volume or installing two pre-wound coils onto the stator poles can be avoided. Using two pole-wound coils can result in compromises on fill factor. Additionally, some applications using pole-wound coils may require the two coils to be separated from each other with an insulation material to keep the coils electrically isolated and/or mechanically protected from vibration. The increased fill factor provided by using a single yoke-wound coil instead of two pole-wound coils that share the same space can allow for a corresponding increase in power density and/or specific power for the motor.
Another advantage of having coils 130 positioned around the yokes 108 of the stator units 102 instead of around the stator poles 106 is that it can facilitate acoustic noise damping techniques. Since the spaces between the adjacent stator units 102, indicated as 140 in
In some embodiments, the spaces 140 between the stator units 102 can be used in other ways. For example, the spaces 140 can be used to locate coolant passageways or other cooling mechanisms, additional separate coils, magnets configured to counteract magnetic leakage, electronics and controllers, and/or other features.
The motor 300 can further comprise stator supports positioned circumferentially around the motor in the regions between the stator units 302. Such stator supports can be generally wedge shaped to conform to the shape of open regions between the stator units. The supports can be configured to structurally support one or both adjacent stator units 302. For example, as shown in
Because one or more of the stator units 302 can be secured in place via the ridges 330 engaging with the supports 350, these stator units can be free of axial bolt apertures (such as the apertures 190 in
In some embodiments, the ridges 330 on the stator units 302 and/or the slots 352 in the supports 350 can be replaced by engagement features other than axially extending ridges, such as any engagement features that restrict the motion of the stator units in the radial and circumferential directions. For example, the ridges 330 can be replaced with prongs, tabs, or other non-axially extending projections and the slots 352 can be replace with corresponding recesses. In other embodiments, the stator units 302 can comprise recesses and the supports 350 can comprise projections, or a combination of both. Desirably, the engagement between the stator units 302 and the supports 350 is such that the circumferential spacing of the stator units can be maintained and such that the radial spacing of the stator poles 312 from the rotor poles 306 can be maintained. In some embodiments, bolts, screws, latches, or other mechanisms can be included to secure the engagement between the stator units 302 and the stator supports 350.
Similarly, the engagement between the supports 350 and the end supports 360, 370 can comprise an interface that is sufficient to restrict the motion of the supports 350 and stator units 302 in both the radial and circumferential directions, as well as in the axial direction.
As shown in
The MIFP SRMs disclosed herein can be controlled using novel electrical control techniques due to the considerable torque overlap between phases and other novel characteristics. Such control techniques can provide torque ripple reduction, acoustic noise reduction, and/or other advantages. An exemplary system can comprise at least one MIFP SRM as described herein that is electrically coupled to at least one controller and an electrical power source. The controller can comprise computing hardware, such as a processor, memory, and programmed control logic in the form of software and/or other computer readable instructions stored in the controller or a storage device associated with the controller.
In some embodiments, control algorithms can be used to optimize control waveforms as a function of speed and torque of the motor. This can allow for near-zero torque ripple at least for low and moderate torque levels, such as up to about 150 Nm in some embodiments, and greater torque levels in other embodiments, and can allow for reduced torque ripple at all torque levels.
Dynamic testing of an embodiment similar to the embodiment 300 shown in
For purposes of this description, certain aspects, advantages, and novel features of the embodiments of this disclosure are described herein. The disclosed methods, apparatuses, and systems should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The methods, apparatuses, and systems are not limited to any specific aspect or feature or combination thereof, nor do the disclosed embodiments require that any one or more specific advantages be present or problems be solved.
Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed methods can be used in conjunction with other methods.
As used herein, the terms “a”, “an” and “at least one” encompass one or more of the specified element. That is, if two of a particular element are present, one of these elements is also present and thus “an” element is present. As used herein, the term “and/or” used between the last two of a list of elements means any one or more of the listed elements. For example, the phrase “A, B, and/or C” means “A,” “B,” “C,” “A and B,” “A and C,” “B and C” or “A, B and C.” As used herein, the term “coupled” generally means physically or electrically coupled or linked and does not exclude the presence of intermediate elements between the coupled or associated items absent specific contrary language.
Unless otherwise indicated, all numbers expressing properties, sizes, percentages, measurements, distances, ratios, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods. When directly and explicitly distinguishing embodiments from discussed prior art, numbers are not approximations unless the word “about” is recited.
In view of the many possible embodiments to which the disclosed principles may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the disclosure. Rather, the scope of the disclosure is at least as broad as the scope of the following claims. We therefore claim all that comes within the scope of these claims.
This application claims the benefit of U.S. Provisional Patent Application No. 61/672,824, filed on Jul. 18, 2012, which is incorporated by reference herein.
This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61672824 | Jul 2012 | US |
