Abstract:
A permanent magnet (PM) machine has a fault condition mechanism disposed within a back iron of the stator portion, the mechanism operational to automatically reduce fault currents associated with the PM machine during a fault condition. The fault condition mechanism disposed within the stator back iron is reconfigurable to automatically reduce internal heat associated with the PM machine during a fault condition. A method of reconfiguring the PM machine upon detecting a fault condition includes the steps of 1) selecting the reconfigurable fault condition mechanism from a) a plurality of magnetically anisotropic rotatable cylinders, b) a plurality of ferrofluid-filled cavities, and c) a dual-phase material selectively embedded within the stator core; and 2) reconfiguring the fault condition mechanism to automatically reduce fault currents or internal heat associated with the PM machine upon detection of a fault condition.

Description:
BACKGROUND 
   The present invention is directed to permanent magnet machines, and more particularly to a method of making a permanent magnet machine more fault-tolerant. 
   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. 
   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. 
   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. 
   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. 
   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. 
   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. 
   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. 
   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 
   The present invention is directed to a permanent magnet (PM) machine having a fault condition mechanism disposed within a back iron of the stator portion, the mechanism operational to automatically reduce fault currents associated with the PM machine during a fault condition. 
   The fault condition mechanism disposed within the stator back iron is reconfigurable to automatically reduce internal heat associated with the PM machine during a fault condition. 
   A method of reconfiguring the PM machine upon detecting a fault condition includes the steps of 1) selecting the reconfigurable fault condition mechanism from a) a plurality of magnetically anisotropic rotatable cylinders, b) a plurality of ferrofluid-filled cavities, and c) a dual-phase material selectively embedded within the stator core; and 2) reconfiguring the fault condition mechanism to automatically reduce fault currents or internal heat associated with the PM machine upon detection of a fault condition. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  illustrates a portion of a permanent magnet (PM) machine depicting rotatable laminated magnetic cylinders in the PM machine back iron (yoke) under normal operating conditions according to one embodiment of the present invention; 
       FIG. 2  illustrates a portion of a permanent magnet (PM) machine depicting rotatable laminated magnetic cylinders in the PM machine back iron (yoke) under a fault condition according to one embodiment of the present invention; 
       FIGS. 3   a  and  3   b  illustrate an actuator or gear assembly for rotating the rotatable cylinders shown in  FIGS. 1 and 2 . 
       FIG. 4  illustrates a portion of a permanent magnet (PM) machine depicting a stator core that is selectively or fully made of a dual-phase magnetic material according to one embodiment of the present invention; 
       FIG. 5  illustrates a portion of a permanent magnet (PM) machine depicting cylindrical tubes in the PM machine back iron (yoke) filled with ferrofluid under normal operating conditions according to one embodiment of the present invention; 
       FIG. 6  illustrates the portion of a permanent magnet (PM) machine shown in  FIG. 5  depicting the cylindrical tubes in the PM machine back iron (yoke) in which the cylindrical tubes are passively drained of the ferrofluid under fault conditions according to one embodiment of the present invention; 
       FIG. 7  illustrates in more detail, a back iron (yoke portion) of a permanent magnet (PM) machine depicting a set of axially-oriented chambers, each chamber having a static ferrofluid sealed therein, in which the chambers are passively drained of the ferrofluid under fault conditions according to one embodiment of the present invention; 
       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 
       FIG. 9  illustrates a conventional permanent magnet machine architecture that is known in the prior art. 
   

   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 
   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. 
   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. 
   The principles described herein are not limited to PM machines with fractional-slot concentrated windings. They can also be applied to any PM machine with any winding configuration to achieve the desired results. 
   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. 1-6  herein below. 
   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 . 
     FIGS. 1 and 2  illustrate a portion of a permanent magnet machine depicting rotatable magnetically anisotropic cylinders  10  in the permanent magnet machine back iron (yoke portion)  12  of the stator core  14  according to one embodiment of the present invention. The magnetically anisotropic rotatable cylinders  10  can be constructed by, for example, but not limited to, forming the cylinders using a magnetically anisotropic material or using a plurality of magnetic laminations. These laminations can be any grade of silicon-steel laminations (e.g., M 19 , M 23  . . . , etc.) or any grade of iron-cobalt laminations. The magnetic cylinders  10  can be seen in  FIG. 1  to be seen to include a plurality of magnetic laminations oriented in a direction to provide a normal magnetic flux path  16  under normal operating conditions. Under fault conditions, all rotatable laminated magnetic cylinders  10  are rotated to simultaneously interrupt the normal magnetic flux path  16  in the stator back iron  12 . 
     FIG. 2  depicts a new flux path  18  under a fault condition and shows the new flux path  18  does not pass through the stator back iron  12  of the permanent magnet machine. The rotatable laminated magnetic cylinders  10  are disengaged to block the normal flux path (orthogonal to the flux path)  16 . In this manner, the rotatable laminated magnetic cylinders  10  in the stator back iron  12  are rotated 90° under fault conditions to impede and thus reduce the magnetic flux coupling with the stator windings and limit the fault current. 
     FIGS. 3   a  and  3   b  illustrate actuation of the rotatable laminated magnetic cylinders  10  depicted in  FIGS. 1 and 2 . Rotation of the rotatable laminated magnetic 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 laminated magnetic cylinders  10  are engaged to provide a normal flux path  16  such as depicted in  FIG. 1 . During a fault condition, the rotatable laminated magnetic cylinders  10  are disengaged by the actuator or gear assembly  20  as seen in  FIG. 3   b , to rotate the rotatable laminated magnetic cylinders  10  by approximately 90° to 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. 
     FIG. 4  illustrates a portion of a permanent magnet (PM) machine depicting a stator core  14  selectively or fully constructed of a dual-phase magnetic material  30 . The present inventors found one suitable dual-phase magnetic material to consist of a Fe-17.5Cr-0.5C alloy (also known as YEP-FA1), available from Hitachi Metals, Ltd. This dual-phase magnetic material was found by the present inventors to possess magnetic properties superior to the ferrite materials generally known in the art. Use of known ferrite materials results, for example, in very large, bulky, PM machines to achieve the desired results because of the relatively poor magnetic properties of such ferrite materials. Further, the YEP-FA1 dual-phase magnetic material has transitive properties changing from magnetic to nonmagnetic beginning at a much higher temperature than the known ferrite materials that are generally bi-state in nature. The YEP-FA1 dual-phase magnetic material also exhibits a hysteresis-type effect in which it transitions to its original magnetic state upon cooling. Known ferrite materials generally do not return to their original magnetic state subsequent to cooling below their state-change temperature. 
   In one embodiment, only the back iron (yoke)  12  is constructed of the dual-phase magnetic material  30 . In another embodiment, the entire stator core  14  is constructed of the dual-phase magnetic material  30 . Under fault conditions, the high fault currents heat up the PM machine coils (not shown) causing stator core  14  heating. Beyond a certain temperature, the dual-phase magnetic material  30  becomes nonmagnetic, changing the normal flux path and resulting in a reduction of the stator flux and the fault current. It will be appreciated that any stator core  14  cooling could optionally be deactivated to increase the speed-up of stator core heating. 
     FIGS. 5 and 6  illustrate a portion of a permanent magnet (PM) machine depicting cylindrical chambers  40  in the stator core back iron  12  of the PM machine according to one embodiment of the present invention. Under normal operating conditions, the cylindrical chambers  40  are completely full with a magnetic ferrofluid  42  to create a magnetic flux path  44  as depicted in  FIG. 5 . Under fault conditions, the magnetic ferrofluid  42  is passively drained to empty the cylindrical chambers  40 , greatly impeding the main flux path and diverting more flux through the high-reluctance flux path  46  that is further away from the permanent magnet machine windings (not shown), thus reducing winding fault currents, as seen in  FIG. 6 . 
     FIG. 7  illustrates in more detail, a back iron (yoke portion)  12  of a permanent magnet (PM) machine stator core  14  depicting a set of axially-oriented chambers  40 , each chamber having a static ferrofluid  42  sealed therein, in which the chambers  40  are passively drained of the ferrofluid  42  under fault conditions according to one embodiment of the present invention. The set of axially-oriented chambers  40  are provided within the stator laminations  47  of the back iron  12  portion of a permanent magnet machine (generator). These chambers  40  are normally completely sealed, and filled with a static ferrofluid  42 . The ferrofluid  42  conducts the stator magnetic flux during normal machine operation. The ferrofluid chambers  40  are sealed by drain plugs  48  that are made of a material that will melt at an appropriate temperature. Suitable sealing materials are easily determined by one skilled in the thermal protection art, and so will not be discussed in further detail herein to preserve brevity and enhance clarity. 
   If a localized electrical fault occurs in the stator core  14  of the permanent magnet machine, excitation provided by the permanent magnet rotor 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 conducted to the ferrofluid chambers  40  and the drain plugs  48 . If the fault is severe enough, the plugs  48  will melt at that location and allow the ferrofluid to drain out of the stator laminations  47 . When the ferrofluid  42  drains, the magnetic reluctance in the nearby stator flux paths will be increased, reducing the effectiveness of the rotor excitation in that area. This will ultimately reduce the local fault current to acceptable levels and prevent the condition from causing further damage to the stator core  14 . The foregoing permanent magnet machine using passive thermal protection is advantageous in that it provides a more reliable, smaller, and cost competitive solution over similar protection mechanisms known in the art that require active actuation devices to pump ferrofluid out of the chambers  40  when a fault occurs. 
     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. 
   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 laminated magnetic 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 open the drain plugs  48  described herein before with reference to  FIG. 7 , such that the ferrofluid  42  will be released from the chambers  40  containing the ferrofluid  42 . 
   Further, the protective mechanism actuator  20  could just as easily be employed to shut down or deactivate a cooling mechanism or system to provide faster heating of a stator core  14  that is constructed of a dual-phase magnetic material such as discussed herein before with reference to  FIG. 4 . In this manner, fault currents are more quickly reduced to acceptable levels during a permanent magnet machine fault condition. 
   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  detects an operating condition that exceeds one or more desired or predetermined operating condition set points, the passive thermal overload detection system  60  alters its physical state that is sensed by the protective mechanism actuator  20 . The protective mechanism actuator  20  then operates in response to the physically altered state of the passive thermal overload detection system or device  60  to operate the rotatable laminated magnetic cylinders  10  shown in  FIGS. 1 and 2  as described herein before, or in the alternative, to open the drain plugs  48  described herein before with reference to  FIG. 7 , such that the ferrofluid  42  will be released from the chambers  40  containing the ferrofluid  42 . The passive thermal overload detection system  60  in one embodiment comprises one or more thermally sensitive drain plugs  48  that automatically rupture upon reaching a desired or predetermined temperature, such that the ferrofluid  42  will be released from the chambers  40  containing the ferrofluid  42 , such as discussed herein before. 
   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 back iron  12  to interrupt the stator flux under fault conditions, 2) hollow chambers (tubes)  40  in the stator back iron  12  that each contains a magnetic ferrofluid that is drained under fault conditions in order to reduce stator fault currents, 3) a stator core  14  that is fully or selectively constructed from a dual-phase magnetic material, such that under fault conditions, the windings heat up the core  14  and the core  14  then becomes nonmagnetic, thus reducing the stator flux and the fault currents, 4) using an external heat source and/or shutting down the stator cooling in order to speed up the heating of the dual-phase magnetic material  30  under fault conditions, and 5) combining desired features described above as necessary to achieve desired system performance, reliability, cost, size, specifications/requirements, and so on. 
   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. 
   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.