Three dimensional magnetic field manipulation in electromagnetic devices

Electromagnetic devices and near field plates for three-dimensional magnetic field manipulation are disclosed. In one embodiment, an electromagnetic device includes a rotor, a stator, and a magnetic field focusing device. The rotor may include a rotor body and a plurality of radially extending rotor poles. The stator may include a plurality of stator poles radially extending inwardly from a stator body toward the rotor body. Each stator pole may have a magnetic flux generating device and a stator pole tip, wherein an air gap may be located between each stator pole tip and each corresponding rotor pole. The magnetic field focusing device is coupled to at least one stator pole tip and produces a magnetic field profile having at least one concentrated magnetic flux region proximate the stator pole tip. The magnetic field focusing device twists the magnetic field profile by an angle α.

TECHNICAL FIELD

The present disclosure generally relates to magnetic field manipulation and, more particular, to three-dimensional magnetic field manipulation in electromagnetic devices, such as switched reluctance motors, to reduce torque ripple.

BACKGROUND

Electromagnetic devices, such as switched reluctance motors, have numerous applications. Desirable properties may include high average torque and low torque ripple, which is the amount of torque measured by subtracting the minimum torque during one revolution from the maximum torque from the same motor revolution. High torque ripple may negatively affect the efficiency and operation of the electromagnetic device.

Accordingly, alternative electromagnetic devices and approaches and devices that increase average torque and reduce torque ripple are desired.

SUMMARY

Embodiments of the present disclosure are directed to electromagnetic devices (e.g., switched reluctance motors) including one or more magnetic field focusing devices, such as a near field plate or a shaped magnetic element operable to manipulate a magnetic field distribution in three dimensions at an air gap between stator and rotor poles. Optimal magnetic field distribution may be designed using a gradient based optimization approach. The magnetic field focusing device may be designed to give any desired magnetic field distribution.

Previous applications of near field plates have included high frequency devices, in particular devices for which diffraction limiting is a serious problem. Near field plates have not previously been used in relatively low frequency magnetic devices, such as motors. The operating frequency of the motor may be on the order of kilohertz, for example in the range 100 Hz-10 kHz. A magnetic field focusing device such as a near field plate may be attached at the tip of each of the stator poles, so as to focus the magnetic field produced by the stator coil at specific desired locations.

A near field plate may be a thin grating-like device used to manipulate the electromagnetic field distribution. In some cases, it may be thought of as an impedance sheet having a modulated surface reactance. When a magnetic field passes from one side of the plate to the other side, the field distribution is modified to a desired configuration. The near field plate may be configured to manipulate the magnetic field distribution in three dimensions.

In one embodiment, an electromagnetic device includes a rotor, a stator, and a magnetic field focusing device. The rotor may include a rotor body and a plurality of radially extending rotor poles. The stator may include a plurality of stator poles radially extending inwardly from a stator body toward the rotor body. Each stator pole may have a magnetic flux generating device and a stator pole tip, wherein an air gap may be located between each stator pole tip and each corresponding rotor pole. The rotor is configured to rotate within the stator upon sequential energization of the magnetic flux generating device. The magnetic field focusing device is coupled to an end face of at least one stator pole tip and produces a magnetic field profile having at least one concentrated magnetic flux region proximate the stator pole tip within the air gap when an associated magnetic flux generating device is energized. The magnetic field focusing device is configured to twist the magnetic field profile with respect to a motor axis of the electromagnetic device by an angle α.

In another embodiment, an electromagnetic device includes a rotor, a stator, and a near field plate. The rotor may include a rotor body and a plurality of radially extending rotor poles. The stator may include a plurality of stator poles radially extending inwardly from a stator body toward the rotor body. Each stator pole may have a magnetic flux generating device and a stator pole tip, wherein an air gap may be located between each stator pole tip and each corresponding rotor pole. The rotor is configured to rotate within the stator upon sequential energization of the magnetic flux generating device. The near field plate includes a spatially modulated surface reactance, and is coupled to an end face of at least one stator pole tip. The near field plate is configured to produce a magnetic field profile having at least one concentrated magnetic flux region proximate the stator pole tip within the air gap when an associated magnetic flux generating device is energized. The near field plate is configured to twist the magnetic field profile with respect to a motor axis by an angle α such that each rotor pole experiences at least a portion of the at least one concentrated magnetic flux region during 360 degree rotation of the rotor.

In yet another embodiment, a near field plate for a switched reluctance motor application includes a perimeter region having a first edge region opposite from a second edge region and a third edge region opposite from a fourth edge region, and a plurality of conductive loops extending between the third edge region and the fourth edge region. The conductive loops are angled at an angle α with respect to the first edge region and the second edge region, such that the conductive loops provide a spatially modulated surface reactance and modify magnetic flux passing therethrough to produce a magnetic flux profile having at least one concentrated magnetic flux region. The near field plate twists the magnetic flux profile by the angle α with respect to the first edge region and the second edge region. The near field plate is configured to generate the magnetic flux profile from magnetic flux resulting from an alternating signal at a frequency that is less than 100 kHz.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to electromagnetic devices and near field plates wherein a magnetic field is manipulated in three dimensions to reduce torque ripple. By angling the magnetic field distribution, and thus manipulating the three dimension magnetic field, it is hypothesized that the motor torque ripple can be reduced by creating a reasonably sustained higher torque field throughout a full 360 degree rotor rotation angle.

Embodiments of the present disclosure are described in the context of electromagnetic devices, such as motors, and in particular, switched reluctance motors having one or more magnetic field focusing devices. The magnetic field focusing device may be operable to manipulate the magnetic field distribution at an air gap between stator and rotor poles of the electromagnetic device. In one embodiment, the magnetic field focusing devices are configured as near field plates positioned at the tip of each stator pole, so as to manipulate the magnetic field produced by a stator coil. In another embodiment, the magnetic field focusing devices are configured as shaped magnetic elements. Various embodiments of electromagnetic devices and near field plates are described in detail below.

FIG. 1schematically depicts an electromagnetic device configured as a switched reluctance motor10comprising a stator12including an annular stator body14and stator poles16extending inwardly from the stator body14. Each stator pole16has a stator pole tip20, and a magnetic flux generating device, such as a stator coil18, which may be wound around each stator pole16. A near field plate, such as near field plate22, may be supported on one or more stator pole tips20. The annular stator body14need not have a circular outer cross-section, but may be configured in any suitable arrangement for supporting the inwardly directed stator poles.

The motor also comprises a rotor24having a central rotor body26supporting a plurality of outwardly extending rotor poles28. The rotor24is operable to rotate within the stator12, for example on appropriate energization of the stator poles16by the stator coils18. For illustrative clarity,FIG. 1does not depict other possible components, such as a driveshaft turned by the rotor, for example.

In the illustrated embodiment, an end face of at least one stator pole tip20supports a near field plate22, the near field plate22being configured to modify the magnetic flux distribution passing through the near field plate, so as to increase the torque and/or reduce the torque ripple of the switched reluctance motor10. The near field plate22may be configured to provide one or more regions of increased flux density within a rotor pole, as the rotor pole passes proximate the stator pole. A near field plate22may effectively focus the magnetic field at one or more locations within a proximate rotor pole.

As the rotor24rotates within the stator12, in some configurations a stator pole16and rotor pole28are proximate to one another and separated by an air gap. Magnetic flux generated by the stator coil(s) passes through the stator pole, and out of the stator pole tip20through the near field plate22. The near field plate22focuses the magnetic field distribution so as to create regions of focused or concentrated flux density. The near field plate therefore may reduce the uniformity of the magnetic field at the rotor pole.

It is noted that although embodiments of the electromagnetic devices and the near field plates are described herein in the context of switch reluctance motors, embodiments are not limited thereto. The near field plates described herein may be utilized in any electromagnetic and/or electromechanical application in which magnetic field focusing is desirable. For example, near field plates may be used to focus magnetic fields for increasing the magnetic force on a moveable component (e.g., the plunger of an actuator, the rotor of an inductance motor, etc.). Electromagnetic devices or systems that embodiments of the near field plates described herein may be utilized include, but are not limited to, magnetic motors (e.g., a switched reluctance motor as may be used in devices such as air-conditioners, washing machines, etc.), spectrometers (e.g., a magnetic resonance spectrometer or imaging device), metal detectors, relays, loudspeakers, magnetic levitation devices, transformers, inverters, and transducers. Other applications in which embodiments of the near field plates may be used include power transmission systems, such as systems operating at 13.56 MHz or other operational frequencies.

As described below, the use of one or more magnetic field focusing devices that manipulate the magnetic flux distribution in three dimensions, such as near field plates or shaped elements, may allow for improvements in average torque and/or torque ripple of a motor in which they are implemented.

FIG. 2schematically illustrates a geometry used for optimization of the magnetic field, and hence design of the near field plates22. The simulated structure includes stator40, stator pole42, permanent magnet44, rotor46, and geometrical design domain48. The dotted lines around the stator pole tip regions represent geometrical design domains which include a stator pole tip having a shaped element. Accordingly, the simulated structure ofFIG. 2may correspond closely to that of the apparatus to be optimized (such as the switched reluctance motor10depicted inFIG. 1), using a shaped element to represent the effect of the near field plates.

By optimizing the shaped elements on the stator pole tips, an improved field distribution may be determined. A near field plate may then be tailored to give any desired field distribution. As described below, near field plates that twist the magnetic field profile or distribution may be designed to further reduce torque ripple. The use of permanent magnets to replace the stator coils may allow geometry effects of the shaped elements to be removed from the simulation, thereby more closely representing the effect of the near field plates.

FIGS. 3A-3Cschematically illustrate the magnetic field focusing effect of an exemplary near field plate.FIG. 3Adepicts a stator pole60having a stator pole tip supporting a near field plate62. The near field plate may be thin compared to the geometrical extent of the stator pole60. Field lines, indicated as dotted lines66, are focused after passing through the near field plate62and are concentrated within a central region of the rotor portion64. This illustration is generally true for any magnetic device, and may also represent an improved actuator component where element60represents the electromagnet of an actuator, and element64corresponds to the movable plunger of the actuator. However, for conciseness, the present examples are in the context of motors. Accordingly, magnetic focusing devices as described herein may be utilized in applications other than switched reluctance motors.

FIG. 3Bis a similar configuration, in which the field lines are generated by an electromagnet in the form of a coil68disposed around element60. The flux lines, shown as dotted lines66, can be seen to concentrate within a central portion of the rotor element64.

FIG. 3Cfurther illustrates the novel design approach that may be used to obtain optimized field distributions. The electromagnet68inFIG. 3Bis replaced by a permanent magnet70, indicated by the letters S and N corresponding to south and north magnetic poles. In this example, the near field plate is replaced by shaped element72. The flux lines propagate through the pole and are focused by shaped element72into the central portion of rotor portion64. Optimization of the shaped element using a gradient-based approach allows the optimal field distribution to be determined. A near field plate or similarly shaped magnetic element can then be used to provide such a magnetic field distribution in an improved electromagnetic device.

In representative examples, a near field plate is configured to provide magnetic field focusing similar to that obtained using the shaped elements in the optimization process. Optimization of the shaped element allows the optimal field to be determined, and a near field plate can then be designed to provide that optimal field. In other examples, a magnetic field focusing device having a similar form to the optimized shaped element may be used in an improved switched reluctance motor. Additional information regarding optimization methods is described in U.S. patent application Ser. No. 13/028,712, filed on Feb. 16, 2011, which is hereby incorporated by reference in its entirety.

FIGS. 4A and 4Billustrate results of a design approach for a switched reluctance motor. In this example, similar to that shown inFIG. 2, the stator40includes a permanent magnet44corresponding to the stator coil within that particular stator pole.FIG. 4Bshows an enlarged view of the shaped element100, showing protruding portions102and104. A near field plate may then be designed to give a field distribution similar to that represented by the optimized shaped element.

FIG. 4Cdepicts an exemplary field distribution obtained without a near field plate110(no focusing) and with a near field plate112(field focusing) that manipulates the magnetic field distribution in two dimensions. An air gap15, which is exaggerated for illustration purposes, is located between the rotor pole28and a near field plate22coupled to a stator pole16. The rotor rotates relative to the fixed stator as indicated by arrow A. As shown inFIG. 4C, the two dimensional magnetic field distribution does not vary as a function of out-of-plane distance. As described below, manipulating the magnetic field distribution or profile in three dimensions may further reduce torque ripple.

The near field plate may be a patterned, grating-like plate having sub-wavelength features. In some embodiments, the near field plate may comprise capacitive elements, a corrugated surface, or other configuration. Near field plates may focus electromagnetic radiation to spots or lines of arbitrarily small sub-wavelength dimensions.

Embodiments of the present disclosure contemplate and use near field plates to achieve focusing of low frequency (for example kilohertz) electromagnetic signals used in electromagnetic devices such as reluctance motors, rather than in high frequency signal applications where the diffraction limit is a problem to overcome. A near field plate is used to modify the magnetic field distribution at a movable rotor to achieve a greater magnetic torque on the rotor.

A near field plate may comprise patterned conducting elements (such as wires, loops, corrugated sheets, capacitive elements, inductive elements, and/or other conducting elements) formed on or otherwise supported by a dielectric substrate. In one embodiment, a near field plate has a surface impedance with sub-wavelength structure. The surface impedance structure, and hence pattern of conducting elements, may be determined using back-propagation methods from the desired field focusing properties, for example as described by U.S. Pub. No. 2009/0303154 to Grbic et al. A near field plate may include grating-like sub-wavelength structures. Other structures may include a circular corrugated surface, such as a grooved surface with a radial profile in the form of a Bessel function. The near field plate may be generally planar, or in other embodiments, may be curved to conform to a surface (e.g. a curved or other non-planar end surface of an electromagnet, e.g., the tip of a stator pole).

Referring now toFIG. 5A, a near field plate22according to one embodiment is illustrated. The near field plate22generally comprises a sheet of electrically conductive material having a perimeter27with a first edge portion27a, a second edge portion27b, a third edge portion27cand a fourth edge portion27d, and a plurality of conductive loops23positioned therebetween. A gap25is located between each of the conductive loops23. The actual configuration of the near field plate22may result from the optimization methods described above, and may be different from that depicted inFIG. 5A. The configuration of the near field plate22therefore provides a spatially modulated surface reactance to manipulate the magnetic flux distribution in two dimensions.

FIG. 5Billustrates a magnetic field distribution130resulting from magnetic flux passing through the near field plate22depicted inFIG. 5A. The magnetic field distribution130has a plurality of concentrated magnetic flux regions133that are separated by valley regions135passing relatively little magnetic flux. It should be understood that the near field plate22may provide for more or fewer concentrated magnetic flux regions133depending on the application. As shown inFIG. 5B, the near field plate22only manipulates the magnetic field distribution in two dimensions (the x- and y-axes). The magnetic field distribution130does not vary along the motor axis (i.e., the z-axis).

Referring now toFIG. 6A, a near field plate22′ configured to modify the magnetic field in three dimensions is illustrated. In this embodiment, the near field plate22′ comprises a sheet of electrically conductive material having a perimeter27′ with a first edge portion27a′, a second edge portion27b′, a third edge portion27c′ and a fourth edge portion27d′, and a plurality of angled conductive loops23′ positioned therebetween. The first and second edge portions27a′,27b′ are parallel with respect to the z-axis, which, when the near field plate22′ is installed on a stator pole tip, is parallel to the motor axis of the electromagnetic device. As shown inFIG. 6B, the conductive loops23′ are angled with respect to the z-axis by an angle α.

By angling the conductive loops,23′, the magnetic field is twisted or tilted with respect to the motor axis. The angle α may be dependent on the desired amount of twisting of the magnetic field.FIGS. 6B and 6Cdepict a top perspective view and a front view of a magnetic field distribution140, respectively, resulting from magnetic flux passing through the near field plate22′ depicted inFIG. 6A. Similar to the magnetic field distribution130depicted inFIG. 5B, the magnetic field distribution140has a plurality of concentrated magnetic flux regions143that are separated by valley regions145passing relatively little magnetic flux. It should be understood that the near field plate22′ may provide for more or fewer concentrated magnetic flux regions143depending on the application. As shown inFIGS. 6B and 6C, the near field plate22′ with angled conductive loops23′ provides for three dimensional magnetic field manipulation as the magnetic field distribution140varies along the z-axis, as indicated by angle α near region147inFIG. 6B. In this manner, the magnetic field distribution140is twisted or tilted such that the rotor poles experience at least a portion of the concentrated magnetic flux regions143during 360 degree rotation of the rotor. This may provide a sustained higher average torque field throughout the motor rotor rotation angle.

FIG. 7shows the torque versus rotor angle curve for the non-field focusing200, two dimensional field focusing202, and three dimensional field focusing204approaches, showing that the torque ripple may be reduced using a near field plate (or other magnetic field focusing device) to obtain field focusing in two or three dimensions. It is noted that the curve for three dimensional field focusing204is based on hypothetical results. The reduced torque ripple provided by three dimensional magnetic field focusing may be due to higher sustained field magnitude over the distance/angle of travel of the rotor.

It should now be understood that the embodiments described herein may reduce torque ripple in an electromagnetic device, such as a switched reluctance motor, by manipulating a magnetic field in three dimensions. In one embodiment, a near field plate may comprise angled conductive loops or regions that are operable to twist the magnetic field along an axis. The near field plates may be coupled to stator pole tips to produce regions of concentrated magnetic flux that are twisted or tilted with respect to a motor axis of the machine. The three dimensional, manipulated magnetic field distribution may provide a sustained higher average torque field throughout the motor rotor rotation angle.