Patent Application
20120169171
Embodiments of the invention relate generally to permanent magnet electrical machines. Other embodiments relate generally to permanent magnet generators for wind turbines.
Permanent magnet (PM) electrical machines are used both as motors and as generators and may be constructed for alternating-current (AC) or direct-current (DC) operation. Such machines typically employ magnets that are attached to a rotor. The magnets may be attached in a variety of ways including through the use of a “pole cap” rotor topology in which permanent magnets are sandwiched between a rotor rim and a pole cap. Pole caps are typically formed from a ferromagnetic material such as laminated electrical steel sheets, but may also be manufactured from low or medium carbon steel sheets and soft magnetic composites. For simplicity of manufacture, and for dynamic balancing of the PM machine, conventional pole caps are formed with identical structural profiles and are attached to a rotor so as to be symmetrical about the circumferential direction of the rotor.
Such machines, however, are often susceptible to a form of electromagnetic feedback called “torque ripple.” In particular, torque ripple occurs when currents induced in a stator form magnetic fields that induce opposing currents, magnetic fields, and torque in a rotor. Torque ripple varies depending at least on relative rotational position of the rotor and the stator and on the number of rotor pole pieces and stator windings. As will be appreciated, torque ripple is undesirable, as it causes noise and vibration, reduces the life of gears and bearings, and performs no useful work. Accordingly, various means have been proposed to reduce torque ripple, as discussed below.
One way of potentially reducing torque ripple is to skew the stator windings in the machine. Skewing stator windings adds complexity and cost to the windings, however, and reduces the overall performance of the electrical machine. Stator skewing also increases the amount of copper required, increases the stator copper losses, and reduces the torque per amp performance of the machine, in addition to significantly complicating the design and formation of the stator coils and core, and the insertion of the stator windings into the stator core.
Skewing the rotor may also reduce torque ripple. Rotor skewing is typically done by twisting the entire rotor (magnet and rotor core) at discrete axial locations. Skewing a rotor is challenging to accomplish correctly, however, mainly due to the rigid/solid/nonconforming structure of the permanent magnet material. In particular, permanent magnets are not amenable to being “bent” at a skew angle, unlike copper coils of the stator windings.
Other approaches to rotor skewing that do not involve twisting the rotor are also challenging to accomplish. For example, rotor skewing may be attained by mounting selected permanent magnets at aperiodic locations about a rotor circumference, or by mounting unmagnetized permanent magnets at offset locations on a rotor, and then magnetizing the entire rotor using a special wound fixture. These approaches require considerable care in meeting tolerances for relative positioning of components, and, accordingly, both are time consuming and expensive.
In view of the above, a need remains for a simple and inexpensive yet effective method to reduce torque ripple in PM electrical machines.
In one embodiment of the invention, an apparatus includes a rotor with at least a first magnet disposed on the rotor, and producing a magnetic field. A first pole cap is proximate to the first magnet, and a second pole cap is proximate to the first magnet or to a second magnet also disposed on the rotor. At least one of the first pole cap and the second pole cap magnetically couples to at least one of the first magnet and/or the second magnet, and displaces at least a portion of the magnetic field produced by the first and/or second magnet.
In another embodiment of the invention, an electrical machine includes a rotor rotatably mounted within the electrical machine, and a plurality of magnets disposed on the rotor, each magnet producing a magnetic field. The electrical machine also includes a first plurality of pole caps proximate to the plurality of magnets disposed on the rotor, and magnetically coupling with a first group of the plurality of magnets to produce a first plurality of pole cap magnetic fields displaced from the magnetic fields of the magnets. The electrical machine further includes a second plurality of pole caps proximate to the plurality of magnets disposed on the rotor, and magnetically coupling with a second group of the plurality of magnets to produce a second plurality of pole cap magnetic fields displaced from the magnetic fields of the magnets. The displacement of the first and second pluralities of pole cap magnetic fields from the first and second plurality of magnet magnetic fields reduces torque ripple induced by rotation of the rotor within the electrical machine.
In yet another embodiment of the invention, circumferential locations of first and second pluralities of pole pieces producing pole piece magnetic fields are established. The first plurality of pole piece magnetic fields then are displaced by a first offset. Displacing the first plurality of pole piece magnetic fields may include disposing proximate to selected pole pieces a first plurality of pole caps having a first asymmetric magnetic profile.
In a further embodiment of the invention, an apparatus includes a rotor having a magnet disposed on the rotor, and producing a magnetic field. The apparatus also includes a pole cap proximate to the magnet disposed on the rotor. The pole cap is magnetically coupled to the magnet and has a laterally asymmetric profile.
The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings; wherein below:
Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals used throughout the drawings refer to the same or like parts.
Embodiments of the present invention reduce torque ripple in electrical machines. Such electrical machines may include, but are not limited to, PM motors, brakes, and synchros. In particular, such electrical machines include PM generators that may be employed in wind turbines.
The PM generator, in turn, includes a rotor with a plurality of pole assemblies. Each pole assembly has several permanent magnets or pole pieces disposed on an outer surface of a rotor body to provide a pole field. The permanent magnets are covered by pole caps to protect them from mechanical damage.
By placing one of the ferromagnetic pole caps 60A, 60B over one or more permanent magnets 78, variations in magnetic reluctance across the pole cap profile 62 can produce a pole cap magnetic field with an effective magnetic-axis e (the “e-axis”) that is shifted across the pole cap profile 62 relative to the magnetic axis d (the “d-axis”) of the magnetic field produced by the permanent magnet(s) 78 beneath the pole cap. For example,
Turning now to
As shown in
Alternatively, the pole caps 60A, 60B and the underlying permanent magnets 78 can be mounted onto an outer surface 92 of a rotor body 94 via a combination of clamping bars 96 and mounting bolts 98, as shown in
As shown in
Another aspect of the invention relates to establishing circumferential locations within an electrical machine of first and second pluralities of pole pieces producing pole piece magnetic fields, and displacing the first plurality of pole piece magnetic fields by a first offset. The first plurality of pole piece magnetic fields may be displaced by disposing a first plurality of pole caps adjacent to the first plurality of pole pieces. Additionally, the second plurality of pole piece magnetic fields may be displaced by a second offset different from the first offset. The first and second pluralities of displaced magnetic fields may form an axial stair step pattern around the rotor. The first and second pluralities of displaced magnetic fields may be axially continuous and skewed from the rotor axis. The first and second pluralities of displaced magnetic fields may be symmetrically axially skewed.
In use, an embodiment of the invention may include a rotor with at least a first magnet disposed on the rotor to produce a magnetic field, a first pole cap operatively connecting at least the first magnet to the rotor, and a second pole cap operatively connecting the first magnet, and/or at least a second magnet, to the rotor. At least one of the first pole cap and the second pole cap magnetically couples to the first magnet and/or the second magnet to displace at least a portion of the magnetic field produced by the magnet or magnets. The first and second pole caps may have respective first and second magnetic profiles that are asymmetric, and may be magnetically coupled to selected magnets to displace the magnetic fields of the selected magnets. The selected magnets may be the same or different magnets. The first and second pole caps each may circumferentially displace the magnetic field of at least one selected magnet, such that the displaced magnetic fields form a substantially continuous axially skewed pattern or an axial stair step pattern. The respective first and second magnetic profiles may be mirror-symmetric to each other. The first and second pole caps may be members of respective first and second pluralities of pole caps that are mounted in a repeating pattern. Each of the second plurality of pole caps may have an inverse axial profile to an adjacent one of the first plurality of pole caps. A rim of the rotor may include a flattened region, and the magnet may have a flattened surface complementary to the flattened region. The first pole cap may be mounted to the rotor by a mounting bar that engages an axially extending indentation of the first pole cap and that also engages an axially extending indentation of the second pole cap to align the first and second pole caps. The mounting bar may engage with a mounting structure formed on a bracket secured to the rotor adjacent to the flattened region formed at the rim of the rotor. The bracket and the mounting bar may mutually align the first and second pole caps with each other and with the magnet for locating the displaced magnetic field of the magnet relative to the rotor.
In other embodiments, the inventive apparatus may also include a rotor rotatably mounted within an electrical machine, a first plurality of pole caps operatively connecting a first plurality of magnets to the rotor, each of the magnets in the first plurality having a magnetic field, and magnetically coupling with the first plurality of magnets to produce a first plurality of pole cap magnetic fields circumferentially displaced from the magnetic fields of the magnets, and a second plurality of pole caps operatively connecting a second plurality of magnets to the rotor, each of the magnets in the second plurality haying a magnetic field, and magnetically coupling with the second plurality of magnets to produce a second plurality of pole cap magnetic fields displaced from the magnetic fields of the magnets. The mutual displacement of the first and second pluralities of pole cap magnetic fields from the first and second plurality of magnet magnetic fields may reduce the magnitude of torque ripple induced by rotation of the rotor within the electrical machine. The first plurality of pole cap magnetic fields and the second plurality of pole cap magnetic fields may form an axial stair step pattern. Alternatively, the first plurality of pole cap magnetic fields and the second plurality of pole cap magnetic fields form substantially continuous helices. The first plurality of pole cap magnetic fields and the second plurality of pole cap magnetic fields may be mirror-symmetrically axially skewed.
One of ordinary skill in the art will understand that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions and types of materials described herein are intended to define the parameters of the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of ordinary skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second.” “third,” “upper,” “lower.” “bottom.” “top,” etc. are used merely as labels, and are not intended to impose numerical or positional requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.
This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable any person of ordinary skill in the art to practice the embodiments of invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including.” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property.
Since certain changes may be made in the above-described embodiments, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.
