Abstract:
A permanent magnet (PM) machine has a reconfigurable fault condition mechanism disposed solely within a stator core portion, wherein the mechanism is automatically reconfigurable to reduce fault currents associated with the PM machine during a fault condition. The reconfigurable fault condition mechanism is automatically reconfigurable to also reduce internal heat associated with the PM machine during a fault condition. A method of reconfiguring the fault condition mechanism upon detection of a fault condition includes the steps of 1) selecting the reconfigurable fault condition mechanism from a) a plurality of rotatable magnetically anisotropic cylinders disposed within stator core slots, b) a plurality of ferrofluid-fillable cavities associated with stator core slots, and c) a sliding shield within the stator core; and 2) reconfiguring the fault condition mechanism 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 having a reconfigurable fault condition mechanism disposed solely within the stator core portion, wherein the mechanism is automatically reconfigurable to reduce fault currents associated with the PM machine during a fault condition. 
         [0011]    The reconfigurable fault condition mechanism is automatically reconfigurable to also reduce internal heat associated with the PM machine during a fault condition. 
         [0012]    A method of reconfiguring the fault condition mechanism upon detection of a fault condition includes the steps of 1) selecting the reconfigurable fault condition mechanism from a) a plurality of rotatable magnetically anisotropic cylinders disposed within stator core slots, b) a plurality of ferrofluid-fillable cavities associated with stator core slots, and c) a sliding shield within the stator core; and 2) reconfiguring the fault condition mechanism 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 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 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 a portion of a permanent magnet (PM) machine depicting hollow tubes in the PM machine stator core slot openings that are empty under normal operating conditions according to one embodiment of the present invention; 
           [0018]      FIG. 5  illustrates the hollow tubes depicted in Figure filled with a ferrofluid under fault conditions; 
           [0019]      FIG. 6  illustrates the portion of a permanent magnet (PM) machine depicting a sliding shield having magnetic and non-magnetic sections in the PM machine stator side during normal operating conditions according to one embodiment of the present invention; 
           [0020]      FIG. 7  illustrates the sliding shield depicted in  FIG. 6  under a fault condition; 
           [0021]      FIG. 8  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 
           [0022]      FIG. 9  illustrates a conventional permanent magnet machine architecture that is known in the prior art. 
       
    
    
       [0023]    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 
       [0024]    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. 
         [0025]    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. 
         [0026]    The fault-tolerance techniques described herein are not limited to PM machines with fractional-slot concentrated windings. The can also be applied to any PM machine with any winding configuration to achieve the desired results. 
         [0027]    A conventional PM machine that is known in the art is shown in  FIG. 9  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. 1-8  herein below. 
         [0028]    As can be seen in  FIG. 9 , 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 . 
         [0029]    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 in permanent magnet machine stator core slot openings  12  of the stator core  14  according to one embodiment of the present invention. The magnetically anisotropic material can be constructed by forming the cylinders using magnetically anisotropic materials or a plurality of magnetic (and non-magnetic laminations), in which the orientation of the laminations either impede or conduct a flux path through the slot openings  12 . 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 rotatable magnetically anisotropic (laminated magnetic and non-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 shunt the flux in the stator slot openings  12 , diverting more flux away from the normal magnetic flux path  16  in the stator back iron  11 . 
         [0030]      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 magnetically anisotropic cylinders  10  are engaged to divert more flux away from the normal flux path (orthogonal to the flux path)  16 . In this manner, the rotatable magnetically anisotropic cylinders  10  in the stator core slots  12  are rotated 90° under fault conditions to reduce the magnetic flux coupling the stator windings and limit the fault current. 
         [0031]      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 anisotropic cylinders  10  are engaged to provide a normal flux path  16  through the back iron  11  such as depicted in  FIG. 1 . During a fault condition, the rotatable anisotropic cylinders  10  are engaged by the actuator or gear assembly  20  as seen in  FIG. 3   b , to rotate the rotatable anisotropic cylinders  10  by approximately 90° to divert more flux away from 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. 
         [0032]      FIG. 4  illustrates a portion of a permanent magnet (PM) machine depicting hollow chambers or cavities  40  in the stator core slot openings  12  of the PM machine according to one embodiment of the present invention. Under normal operating conditions, the hollow chambers  40  are completely empty, thus impeding a flux path through the stator slot openings  12 . 
         [0033]      FIG. 5  illustrates the hollow chambers  40  shown in  FIG. 4  under fault conditions. The hollow chambers  40  are now filled with a magnetic ferrofluid  42 , shunting more flux through the magnetic flux path  18  that is further away from the permanent magnet machine windings (not shown), thus reducing winding fault currents. 
         [0034]      FIG. 6  illustrates a sliding shield  45  in the stator side of a permanent magnet (PM) machine stator core  14 . Sliding shield  45  has magnetic sections  52  and nonmagnetic sections  54 . The magnetic sections can be constructed of an anisotropic material or can optionally be constructed, for example, of a solid magnetic material or magnetic laminations. The sliding shield  45  can also be made of a dual-phase magnetic material wherein the nonmagnetic sections are heat-treated. During normal operation, 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 allowing flux to flow through the normal flux path  16  through the stator back iron  11 . 
         [0035]      FIG. 7  illustrates the sliding shield depicted in  FIG. 6  under a fault condition. 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 such as described further herein below with reference to  FIG. 8 . The thermal overload detector will then activate movement of the sliding shield  45  such that the magnetic sections  52  now allow a flux path  18  across the stator core slot openings  12  to divert more flux through the low-reluctance flux path  18  through the stator core slot openings  12 , thus diverting more flux away from the flux path through the stator back iron  11  and thus reducing the magnetic flux coupling the stator windings and limiting the fault current. 
         [0036]      FIG. 8  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. 
         [0037]    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  shown in  FIGS. 1 and 2  as described herein before. It will be appreciated that the protective mechanism actuator  20  can also be employed to activate a pumping mechanism (not shown), such that a ferrofluid  42  will be injected into the hollow chambers  40  discussed herein before with reference to  FIGS. 4 and 5 . 
         [0038]    With continued reference now to  FIG. 8 , 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  shown in  FIGS. 1 and 2  as described herein before, or in the alternative, to pump ferrofluid(s)  42  into the hollow chambers  40  described herein before with reference to  FIGS. 4 and 5 . 
         [0039]    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 magnetically anisotropic cylinders  10  in the stator core slot openings  12  to divert more stator flux away from the flux path through the stator back iron  11  under fault conditions, 2) hollow chambers (tubes)  40  in the stator core slot openings  12  that each are filled with a magnetic ferrofluid under fault conditions in order to reduce stator fault currents, 3) a sliding shield in the stator core side that operates to shunt more flux away from the normal flux path through the stator back iron  11  under fault conditions, and 4) combining desired features described above as necessary to achieve desired system performance, reliability, cost, size, specifications/requirements, and so on. 
         [0040]    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. 
         [0041]    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.