Abstract:
A permanent magnet (PM) machine includes a plurality of reconfigurable fault condition mechanisms disposed within a stator core portion, the plurality of reconfigurable fault condition mechanisms together automatically reconfigurable to reduce fault currents and internal heat associated with the PM machine during a fault condition. The plurality of reconfigurable fault condition mechanisms are disposed solely within the stator core portion according to one embodiment to automatically reduce stator winding fault currents and internal heat associated with the PM machine during a fault condition. A method of reconfiguring the fault condition mechanisms upon detecting a fault condition includes the steps of 1) selecting the plurality of reconfigurable fault condition mechanisms from a) a plurality of rotatable magnetically anisotropic cylinders disposed both within a stator back iron and stator slot openings, and b) a plurality of rotatable magnetically anisotropic cylinders disposed within a stator back iron and a sliding shield disposed with a stator slot opening portion of the stator core, and 2) reconfiguring the plurality of fault condition mechanisms together to automatically reduce fault currents associated with the PM machine upon detection of a fault condition.

Description:
BACKGROUND 
       [0001]    The present invention is directed to permanent magnet machines, and more particularly to a method of making a permanent magnet machine more fault-tolerant. 
         [0002]    Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly. 
         [0003]    Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than rely solely on the HP engine spool to drive the electrical generators. The low-pressure (LP) engine spool provides an alternate source of power transfer. 
         [0004]    PM machines (or generators) are a possible means for extracting electric power from the LP spool. However, aviation applications require fault tolerance, and as discussed below, PM machines can experience faults under certain circumstances and existing techniques for fault tolerant PM generators suffer from drawbacks, such as increased size and weight. 
         [0005]    Permanent magnet (PM) machines have high power and torque density. Using PM machines in applications wherein minimizing the weight is a critical factor is therefore advantageous. These applications are wide ranging and include aerospace applications. 
         [0006]    One of the key concerns with using PM machines is fault-tolerance since the magnets cannot be “turned off” in case of a fault. Traditionally, the use of PM machines has been avoided in applications where fault-tolerance is a key factor. When PM machines have been used in such applications, fault-tolerance has been achieved by paying a penalty in the form of oversized machines and/or converter designs, or using a higher number of phases which complicates the control process and adds to the overall system weight and cost. 
         [0007]    As is known to those skilled in the art, electrical generators may utilize permanent magnets (PM) as a primary mechanism to generate magnetic fields of high magnitudes. Such machines, also termed PM machines, are formed from other electrical and mechanical components, such as wiring or windings, shafts, bearings and so forth, enabling the conversion of electrical energy from mechanical energy, where in the case of electrical motors the converse is true. Unlike electromagnets which can be controlled, e.g., turned on and off, by electrical energy, PMs always remain on, that is, magnetic fields produced by the PM persists due to their inherent ferromagnetic properties. Consequently, should an electrical device having a PM experience a fault, it may not be possible to expediently stop the device because of the persistent magnetic field of the PM causing the device to keep operating. Such faults may be in the form of fault currents produced due to defects in the stator windings or mechanical faults arising from defective or worn-out mechanical components disposed within the device. Hence, the inability to control the PM during the above mentioned or other related faults may damage the PM machine and/or devices coupled thereto. 
         [0008]    Further, fault-tolerant systems currently used in PM machines substantially increase the size and weight of these devices limiting the scope of applications in which such PM machines can be employed. Moreover, such fault tolerant systems require cumbersome designs of complicated control systems, substantially increasing the cost of the PM machine. 
         [0009]    In view of the foregoing, it would be advantageous and beneficial to provide a method for limiting winding currents for all types of faults, especially a turn-to-turn fault associated with a PM machine to significantly improve the fault-tolerance capability of the PM machine without substantially increasing the size, weight and/or complexity of the PM machine. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    The present invention is directed to a permanent magnet (PM) machine comprising a plurality of reconfigurable fault condition mechanisms disposed within a stator core portion, the plurality of reconfigurable fault condition mechanisms together automatically reconfigurable to reduce fault currents and internal heat associated with the PM machine during a fault condition. 
         [0011]    The plurality of reconfigurable fault condition mechanisms are disposed solely within the stator core portion according to one embodiment to automatically reduce stator winding fault currents and internal heat associated with the PM machine during a fault condition. 
         [0012]    A method of reconfiguring the fault condition mechanisms upon detecting a fault condition comprises the steps of 1) selecting the plurality of reconfigurable fault condition mechanisms from a) a plurality of rotatable magnetically anisotropic cylinders disposed both within a stator back iron and stator slot openings, and b) a plurality of rotatable magnetically anisotropic cylinders disposed within a stator back iron and a sliding shield disposed with a stator slot opening portion of the stator core, and 2) reconfiguring the plurality of fault condition mechanisms together to automatically reduce fault currents associated with the PM machine upon detection of a fault condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
           [0014]      FIG. 1  illustrates a portion of a permanent magnet (PM) machine depicting rotatable anisotropic material cylinders in the PM machine stator core slots as well as the stator slot openings under normal operating conditions according to one embodiment of the present invention; 
           [0015]      FIG. 2  illustrates a portion of a permanent magnet (PM) machine depicting rotatable anisotropic material cylinders in the PM machine stator core slots as well as the stator slot openings under a fault condition according to one embodiment of the present invention; 
           [0016]      FIGS. 3   a  and  3   b  illustrate an actuator or gear assembly for rotating the rotatable cylinders shown in  FIGS. 1 and 2 . 
           [0017]      FIG. 4  illustrates portion of a permanent magnet (PM) machine depicting rotatable anisotropic material cylinders in the stator back iron and a sliding shield having magnetic and non-magnetic sections in the PM machine stator slot side during normal operating conditions according to one embodiment of the present invention; 
           [0018]      FIG. 5  illustrates portion of a permanent magnet (PM) machine depicting rotatable anisotropic material cylinders in the stator back iron and a sliding shield having magnetic and non-magnetic sections in the PM machine stator slot side during a fault condition according to one embodiment of the present invention; 
           [0019]      FIG. 6  is a block diagram illustrating a general provision for protection of a permanent magnet generator using active and/or passive detection of a thermal overload condition and triggering a protection mechanism actuator according to one embodiment of the present invention; and 
           [0020]      FIG. 7  illustrates a conventional permanent magnet machine architecture that is known in the prior art. 
       
    
    
       [0021]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0022]    Conventional PM synchronous electric machines employ permanent magnets as the magnetic poles of a rotor, around which a stator is disposed. The stator has a plurality of teeth that face the rotor. Alternatively, the machine may be designed so that the rotor surrounds the stator. For high-speed operation, a retaining sleeve is usually wrapped around the magnets as needed to keep the magnets in place. The retaining sleeve may be shrink fit upon the magnets to ensure a non-slip fit. Usually the retaining sleeve is made of one whole metallic piece for structural integrity. When the coils formed on the stator are energized, a magnetic flux is induced by the current through the coils, creating electromagnetic forces between the stator and the rotor. These electromagnetic forces contain tangential and/or circumferential forces that cause the rotor to rotate. 
         [0023]    In order to achieve inherent fault-tolerance in these PM machines, there has to be complete electromagnetic, thermal, and physical isolation between the coils of the various phases. This is achieved by using fractional-slot concentrated windings where each coil is wound around a single stator tooth and each stator slot is occupied by one side of a coil. Since slots formed between the teeth and the permanent magnets on the rotor are spaced from each other, the magnetic flux passing through a tooth will pass through the neighboring tooth in the next moment as the rotor rotates. 
         [0024]    The fault-tolerance techniques discussed herein are not limited to PM machines with fractional-slot concentrated windings. They can just as easily be applied to any PM machine with any winding configuration to achieve the desired results. 
         [0025]    A conventional PM machine that is known in the art is shown in  FIG. 7  to provide a background regarding PM machine architecture before describing several embodiments for implementing a synchronous permanent magnet machine that is fault-tolerant, and with particular focus on turn-to-turn faults, with reference to  FIGS. 1-6  herein below. 
         [0026]    As can be seen in  FIG. 7 , a PM machine  1  contains a plurality of magnets  2  provided in a radial arrangement upon a back iron  3  that is disposed around a shaft (not shown). The back iron  3  is also known as a yoke. The magnets  2  are surrounded by a retaining sleeve  4 . A stator  5  surrounds the retaining sleeve  4  and is separated from the magnets  2  by a gap  6 . The stator  5  has a plurality of radially disposed teeth  7  that form stator slots  8 . The teeth  7  are wound with coils  9  that substantially fill the stator slots  8 . 
         [0027]    Looking now at  FIGS. 1 and 2 , there is shown, a portion of a permanent magnet machine depicting rotatable cylinders  10 . The rotatable cylinders  10  are constructed of a magnetically anisotropic material. Each magnetically anisotropic cylinder can be implemented by forming the cylinder from, for example, a magnetically anisotropic material or from a plurality of magnetic laminations. These laminations can be, for example, any grade of silicon-steel laminations (e.g., M19, M23, . . . , etc.) or any grade of iron-cobalt laminations. The magnetically anisotropic rotatable cylinders  10  are located both in permanent magnet machine stator core slot openings  12  of the stator core  14  as well as a stator back iron  11  according to one embodiment of the present invention. The orientation of the magnetically anisotropic material or magnetic laminations then either impedes or allows a flux path through the slot openings  12  or through the stator back iron (yoke)  11 . The rotatable magnetically anisotropic (laminated magnetic) cylinders  10  can be seen in  FIG. 1  to be oriented in a direction to conduct a normal magnetic flux path  16  through the stator core back iron (yoke)  11  under normal operating conditions. Under fault conditions, all rotatable magnetically anisotropic cylinders  10  are rotated to simultaneously impede or interrupt the normal magnetic flux path  16  in the stator back iron  11  and allow a flux path through the slot openings  12 . 
         [0028]      FIG. 2  depicts the new flux path  18  under a fault condition and shows the new flux path  18  does not pass through the back iron  11  of the permanent magnet machine. The rotatable anisotropic cylinders  10  in the stator back iron  11  are disengaged to block the normal flux path (orthogonal to the flux path)  16 . In this manner, the rotatable anisotropic cylinders  10  in the stator core slots  12  are rotated 90° under fault conditions to allow a flux path through the slot openings and thus reduce the magnetic flux coupling the stator windings and limit the fault current. 
         [0029]      FIGS. 3   a  and  3   b  illustrate actuation of the rotatable anisotropic cylinders  10  depicted in  FIGS. 1 and 2 . Rotation of the rotatable anisotropic cylinders  10  is implemented via an actuator or gear assembly  20 . The actuator or gear assembly  20  is affixed on permanent magnet machine end plates (not shown) in one embodiment. Many types of actuators and gear assemblies suitable for implementing this structure are easily constructed by those skilled in mechanical engineering; and so actuators and gear assemblies are not discussed in any detail herein to preserve brevity and provide clarity in describing the particular embodiments herein. Under normal operation, the rotatable magnetically anisotropic cylinders  10  in the stator back iron  11  are engaged, while the rotatable magnetically anisotropic cylinders  10  in the stator slot openings  12  are disengaged to provide a normal flux path  16  through the back iron  11  such as depicted in  FIG. 1 . 
         [0030]    During a fault condition, the rotatable magnetically anisotropic cylinders  10  in the stator back iron  11  are disengaged; while the rotatable magnetically anisotropic cylinders  10  in the stator slot openings  12  are engaged by the actuator or gear assembly  20  as seen in  FIGS. 3   a  and  3   b , to rotate the rotatable anisotropic cylinders  10  by approximately 90° to impede or block the normal flux path  16 , thereby shunting the magnetic flux away from the windings via a new flux path  18  as shown in  FIG. 2 , and reducing the fault currents. 
         [0031]      FIGS. 4 and 5  illustrate a sliding shield  45  in the stator slot opening side of a permanent magnet (PM) machine stator core  14 . Sliding shield  45  has magnetic sections  52  and nonmagnetic sections  54 . In one embodiment, a plurality of axial-laminated portions are inserted, with solid pieces of nonmagnetic material inserted between each laminated portion. The laminated portions can be constructed, for example, using the same, but not limited to, materials used for the rotatable magnetically anisotropic cylinders. The sliding shield  45 , according to one embodiment, can be made of a dual-phase magnetic material where the nonmagnetic sections are heat treated. The magnetic sections can also be constructed, for example, of a magnetically anisotropic material or can optionally be constructed of magnetic laminations. During normal operation as shown in  FIG. 4 , the sliding shield  45  is in its conventional operating mode in which the nonmagnetic sections  54  are aligned to impede a flux path through the stator core slot openings  12  and thus allow flux to pass through the normal flux path  16  through the stator back iron  11 . 
         [0032]    With continued reference to  FIGS. 4 and 5 , stator core  14  can be seen to also have a plurality of rotatable cylinders such as discussed herein before with reference to  FIGS. 1 and 2 , disposed within the back iron  11 . As shown in  FIG. 4 , the sliding shield  45  is positioned such that the nonmagnetic sections  54  are aligned with the slot openings  12  during normal fault-free operation to impede a flux path through the slot openings  12 ; while the rotatable cylinders  10  are rotated to conduct a flux path through the back iron  11  during fault-free operation. 
         [0033]      FIG. 5  illustrates the sliding shield  45  and the rotatable cylinders  10  during a fault condition in which the sliding shield  45  is positioned such that the magnetic material sections  52  are aligned with and provide a flux path through the slot openings  12 , while the rotatable cylinders  10  in the back iron  11  are rotated to impede the flux path through the back iron  11 . The magnetically anisotropic material may optionally be replaced with laminated magnetic portions, as stated herein before. 
         [0034]    If a localized electrical fault occurs in the stator core  14  of the permanent magnet machine, excitation provided by the permanent magnet rotor  21  can cause significant overload current to flow, as described herein before. Localized heating will occur in this case. When the foregoing localized heating occurs, the heat generated at the internal stator core  14  fault will be detected via an active or passive thermal overload detector mechanism such as described further herein below with reference to  FIG. 6 . The thermal overload detector mechanism will then activate movement of the sliding shield  45  such that the magnetic sections  52  now create a shunt across the stator core slot openings  12  to divert more flux through flux path  18  through the stator core slot openings  12 , and less flux through the normal flux path through the stator back iron  11  thus reduce the magnetic flux coupling with the stator windings and limit the fault current. In similar fashion, the thermal overload detector mechanism will activate rotation of the cylinders  10  in the back iron  11  to provide a flux path during normal fault-free operation. The thermal overload detector mechanism will then reorient the rotatable cylinders  10  during a fault condition to impede a flux path through the back iron  11 . 
         [0035]      FIG. 6  is a block diagram illustrating a permanent magnet machine (i.e. generator)  50  using active and/or passive detection of a thermal overload condition, and triggering a protection mechanism actuator  20  according to one embodiment of the present invention. The permanent magnet machine  50  is controlled in response to commands from a generator controller  53  that senses one or more loads  55  supplied by the machine  50 . The generator controller  53  is also in communication with an active thermal overload detection system  56  that operates to sense operating point conditions that are conducive to machine  50  overloading. Many types of active thermal overload detection methods and systems suitable for implementing the requisite active thermal overload detection system  56  are known in the art, and so further details of thermal overload detection systems will not be discussed herein. 
         [0036]    When the active thermal overload detection system  56  detects an operating condition that exceeds one or more desired or predetermined operating condition set points, the active thermal overload detection system  56  sends one or more command signals to the protective mechanism actuator  20 . The protective mechanism actuator  20  then operates in response to the command signal(s) to operate the rotatable cylinders  10  and the sliding shield  45  shown in  FIGS. 1-2  and  4 - 5  respectively as described herein before. 
         [0037]    With continued reference now to  FIG. 6 , a passive thermal overload detection system (sensor)  60  is configured to directly sense thermal conditions of the permanent magnet machine (generator)  50 . When the passive thermal overload detection system  60  is subjected to an operating condition that exceeds one or more desired or predetermined operating condition set points, the passive thermal overload detection system  60  physical state is altered. This changed physical state is detected by the protective mechanism actuator  20 . The protective mechanism actuator  20  then operates in response to the altered physical state to operate the rotatable cylinders  10  and the sliding shield  45  shown in  FIGS. 1-2  and  4 - 5  respectively as described herein before. 
         [0038]    In summary explanation, methods for improving the fault-tolerance of PM machines have been described to include various electrical, mechanical, hydraulic or thermal solutions that provide flexibility in choosing the optimal PM machine architecture from a system point of view. These solutions include, but are not limited to 1) rotatable anisotropic or laminated magnetic cylinders  10  in the stator core slot openings  12  to interrupt the stator flux through the stator back iron  11  under fault conditions, 2) a sliding shield in the stator core slot opening side that operates to impede a flux path through the stator back iron  11  under fault conditions, and 3) combining desired features described above as necessary to achieve desired system performance, reliability, cost, size, specifications/requirements, and so on. 
         [0039]    A key feature of the embodiments described herein before include the provision of a fault tolerant permanent magnet machine that is more robust than permanent magnet machines known in the art that employ more conventional types of fault sensing mechanisms, actuators, controllers, and so on. 
         [0040]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.