Abstract:
An electric machine including a magnetic component, forming part of its rotor or stator that loses its magnetic characteristics above a certain chosen temperature is disclosed. This magnetic material forms part of a magnetic circuit that guides flux about the stator. As a result, any magnetic flux emanating with the rotor stops circulating about the stator above this temperature, and the machine stops acting as generator. The component is thermally coupled to windings carrying current from the machine&#39;s stator. The material forming the component is selected so that the chosen temperature is lower than the temperature at which the machine would be thermally damaged. This, in turn, limits the operating temperature of the windings, and thus prevents overheating of the machine during operation, typically caused by a fault. Preferably this magnetic material is formed from a ferrite material, such as a Manganese Zinc ferrite material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electric machines, and more particularly to electric generators that are thermally protected from damage resulting from high currents in such machines. 
     BACKGROUND OF THE INVENTION 
     Permanent magnet electric motors and generators are well known and understood. Typically, such permanent magnet machines include a rotor formed, at least in part, from a magnetic material such as Samarium-Cobalt. Electric windings on a stator about the rotor are used to carry current that either generates a magnetic field or is the result of a magnetic field about the rotor. As a motor, current through the windings induces the rotating magnetic field, which in turn applies a torque to the magnetic portion of the rotor causing it to act as motor. Similarly, as a generator, torque applied to the rotor, results in a rotating magnetic field that induces a current in the windings. 
     Such electric machines provide significant benefits over synchronous machines, squirrel cage motors and other types of electric machines. Significantly, permanent magnet machines do not require brushes; are relatively light; use conventional and developed electronics to generate any required rotating magnetic field; and can act as both motors and generators. 
     In view of these benefits, such machines appear well suited for aircraft applications. Particularly, such machines would appear to lend themselves for use as starters and generators within a turbine engine. 
     Conveniently, such machines can be connected directly to the engine shaft. When required, generated electricity can be rectified and filtered using conventional lightweight electronics. When DC currents are required, as in traditional aircraft applications, the speed of rotation and frequency of generator output does not need be controlled. Heavy gearing is therefore not required. Operating as motors, such machines can act as starters. 
     Disadvantageously, however, machines coupled to such engines can potentially generate extreme power limited only by the power of the turbine engine driving the rotor of the machine. Unabated, generation of such electric power can result in extreme heat, particularly in the stator windings, that may cause the motor to melt and potentially burn. This is clearly undesirable. Obviously, current provided by the machine to interconnected electrical equipment may be limited by fusing the interconnected equipment or even the electronics used to rectify or regulate AC currents. However, such fusing will not react to short circuits internal to the machine. While unlikely, such short circuits might, for example, occur in the stator windings. Should this happen, a permanent magnet machine will invariably overload and overheat causing damage to the machine, and perhaps even to the associated engine. In the extreme case, this may cause the main engine to fail as a result of the high temperature of the engine shaft coupled to the motor. Similar problems may be manifested in other types of electric machines. 
     Accordingly, an improved electric machine that is thermally protected is desirable. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an electric machine includes a ferrite portion, forming part of its rotor or stator that loses its magnetic characteristics above a certain chosen temperature. As a result, any magnetic flux circulating between the rotor and stator is significantly reduced above this temperature, and the machine stops acting as generator. The component is thermally coupled to windings carrying current from the machine&#39;s stator. The material forming the component is selected so that the certain temperature is lower than the temperature at which the machine would be thermally damaged. This, in turn, limits the operating temperature of the windings, and thus preventing overheating of the machine during operation. 
     In accordance with an aspect of the present invention an electric machine includes a permanent magnet motor and a stator mounted about the rotor, at least partially forming a magnetic circuit guiding a magnetic flux emanating from the permanent magnet. At least one winding extends about the stator for picking up a current induced by the magnetic flux. At least a portion of the magnetic circuit is thermally coupled to the winding and is formed from magnetic material having a Curie temperature below a temperature at which the machine is damaged. This limits the magnetic flux about the magnetic circuit above the Curie temperature, and thus limits the operating temperature of the windings, and prevents overheating of the machine during operation. 
     In accordance with another aspect of the invention, an electric generator includes a rotor assembly including a permanent magnet; and a stator formed of a ferrite material mounted about the rotor, at least partially forming a magnetic circuit guiding a magnetic field emanating from the permanent magnet. At least one winding extends about the stator for picking up a current induced by the magnetic field. Preferably, the ferrite material is a Manganese-Zinc ferrite material. 
     Other aspects and features of the present invention will become apparent to those of ordinary skill in the art, upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In figures which illustrate, by way of example only, preferred embodiments of the invention, 
     FIG. 1 is an exploded view of a permanent magnet machine, exemplary of an embodiment of the present invention; 
     FIG. 2 is a cross sectional view of the machine of FIG. 1; 
     FIG. 3 is an exploded view of a partial stator assembly that may form part of the machine of FIG. 1; 
     FIG. 4 is a side perspective view of an exemplary stator assembly forming part of the machine of FIG. 1; 
     FIG. 5 is a back end view of FIG. 4; 
     FIG. 6 is a front end view of FIG. 4; 
     FIG. 7 schematically illustrates the flow of current about the stator assembly of FIG. 4; and 
     FIG. 8 is a top view of a portion of a further stator that may be used with a machine exemplary of a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 illustrate a permanent magnet electric machine  10 , exemplary of an embodiment of the present invention. As illustrated, electric machine  10  includes a stator assembly  12  and rotor assembly  14 , preferably mounted within a housing  16 . Rotor assembly  14  is mounted for free rotation about its central axis within housing  16  by bearings  20  and  22 . 
     Housing  16  includes an outer cylindrical shell  24 , and generally disc shaped front and rear end plates  26  and  28 . End plates  26  and  28  are fixed to shell  24 , and thereby retain stator assembly  12 , rotor assembly  14 , and bearings  20  and  22  within housing  16 . Annular walls  30  and  32  extend inwardly from the interior of end plates  26  and  28  and retain bearings  20  and  22  at defined axial positions within housing  16 , about rotor assembly  14 . A further retaining washer  23  assists to retain bearings  20  and  22 . Housing  16  is preferably formed of high-grade stainless steel. 
     Example rotor assembly  14  includes a generally cylindrical core section  38 . Two smaller diameter cylindrical shafts  34  and  36  extend axially outward from core section  38 , toward the front and rear of housing  16 , respectively. Spacing ledges  42 ,  44  and  46 ,  48  separate shafts  34  and  36 , respectively, from core section  38 . Ledges  42  and  46  abut with bearings  20  and  22 . A further smaller diameter concentric drive shaft  40  extends axially outward from shaft  34  and the front of housing  16 . As will be appreciated, core section  38 ; shafts  34 ,  36  and  40  are preferably machined from a single piece of relatively low strength magnetic steel, such as maraging steel. A thin shell  18  formed of non-magnetic material, such as a Nickle alloy, at least partially encapsulates core section  38  and contains the relatively low strength magnetic steel. Shell  18  is preferably formed of AMS 5662 or AMS 5663 Nickel Alloy and may be shrink fitted to the core portion  38  and then ground to achieve a desired overall thickness of shell  18 . 
     Stator assembly  12  is further illustrated in FIGS. 3-7. As illustrated, stator assembly  12  includes a magnetic circuit defined by an exemplary hollow cylinder  50 . Cylinder  50  includes a plurality of lengthwise extending, evenly spaced slots  52   a ,  52   b  and  52   c  (collectively slots  52 ) extending on its interior. In the preferred embodiment, a total of eighteen such slots extend along the cylinder&#39;s length. Conveniently, the eighteen slots  52   a ,  52   b  and  52   c  may be grouped into three groups, with all slots  52   a  belonging to one group, all slots  52   b  and  52   c  to another. Each third slot belongs in one of the groups. As best illustrated in FIG. 3, a set of six rectangular conductors  54   a  that are complementary in shape to slots  52   a , occupy the entire length of these slots. Each of these conductors is formed of a material such as copper, and is insulated by a thin plastic coating. Each of conductors  54   a  is identical in length, and extends slightly beyond the ends of cylinder  50 . Adjacent conductors within the group of conductors  54   a  are interconnected by arced conductors  56   a  extending radially about the central axis of cylinder  50 , and exterior to cylinder  50 . Alternating pairs of conductors  54   a  are connected at opposite ends of cylinder  50 . Thus, two arced conductors  56   a  are at one end of cylinder  50  and three are at the opposite. Conductors  54   a  and  56   a  thus form an electric circuit (referred to as circuit  58   a ) traversing the length of cylinder  50  six times, at intervals spaced sixty degrees about a central axis of cylinder  50 . Diametrically opposed rectangular conductors (ie. spaced by one-hundred and eighty degrees) have currents running in opposite direction along the length of cylinder  50  and thus form current loops or windings about the central axis of machine  10 . As illustrated in FIGS. 4-6, conductors  54   b ,  56   b  and  54   c ,  56   c  are similarly arranged to occupy the remaining slots  52   b  and  52   c , and thus form circuits  58   b  and  58   c . Resulting circuits  58   a ,  58   b  and  58   c  (collectively circuits  58 ) thus form nine current loops or windings about central axis of machine  10 . As illustrated in FIG. 6, conductors  54   b  and  54   c  have the same length as conductors  54   a  and are arranged at axial positions so that conductors  54   a ,  54   b ,  54   c  (collectively conductors  54 ) and  56   a ,  56   b  and  56   c  (collectively conductors  56 ) are not in contact with each other. Moreover, these conductors are preferably insulated so that they are not electrically connected with cylinder  50 , and are thermally coupled to cylinder  50 . The conductors may be coupled to cylinder  50  by way of a known thermal conductive varnish or epoxy. Cylinder  50  and conductors  54  may be encapsulated using this varnish or epoxy. Contact points for each circuit  58   a ,  58   b  and  58   c  extend from the rear end of cylinder  50 , as illustrated in FIG.  5 . Current flow in circuits  58   a ,  58   b  and  58   c  as viewed at the rear of machine  10 , resulting from a potential difference across the contact points is schematically illustrated in FIG.  7 . 
     As illustrated, stator assembly  12  and cylinder  50  are coaxial with core section  38 . A small air gap separates core section  38  from cylinder  50 . 
     A conventional three phase circuit (not shown) may be used to drive circuits  58   a ,  58   b  and  58   c  to cause machine  10  to act as a motor. Specifically, driving circuits  58  results in a rotating magnetic field generated by the nine windings or current loops, travelling circumferentially within cylinder  50 . This field is guided by cylinder  50  acting as part of a magnetic circuit about the center axis of this cylinder  50 , and in turn the core section  38  of rotor assembly  14 . As will be appreciated by those of ordinary skill in the art, the rotating magnetic field exerts a torque on the magnetic portion of rotor assembly  14 , causing it to rotate. 
     Cylinder  50  is preferably formed of a ferrite material. As is understood by those of ordinary skill in the art, ferrite materials exhibit magnetic properties and have high relative permeability resulting in low magnetic reluctance, allowing such materials to guide magnetic flux. Ferrites typically have cubic crystalline structure with the chemical formula MO.Fe 2 O 3 , where MO is typically a combination of two or more divalent metals, such as zinc, nickel, manganese or copper. Ferrites are typically classified as “hard” or “soft”. “Soft” ferrite materials only exhibit significant magnetic characteristics in the presence of a magnetic field, while “hard” ferrite materials tend to permanently retain their magnetic characteristics. As is further, understood, the nature of most magnetic materials is typically temperature dependent. Most magnetic materials lose their magnetic properties above a critical temperature, referred to as the Curie temperature of the material. For many materials, and for most ferrites, once the temperature of the material drops below the critical temperature, their magnetic properties return. Iron, for example, has a Curie temperature of about 770° C. In fact, most magnetic materials used in electric machines have Curie temperature far exceeding the operating temperature of the machine. In machine  10 , however, cylinder  50  and hence the magnetic circuit defined by cylinder  50  is formed of a material (preferably a ferrite) having a Curie temperature above conventional operating temperatures, but below a critical temperature at which damage might be caused to the circuits  58  or the remainder of machine  10 . For reasons that will become apparent, this Curie temperature may be considered to be the desired shut-down temperature of machine  10 . Preferably, cylinder  50  is formed of a “soft” ferrite having a Curie temperature of approximately 200° C. A ferrite having such property is, for example, a ManganeseZinc available from Phillips under material type 3C85, having a Curie temperature of 215° C. Of course, other materials may be suitable, and will be easily identified by those of ordinary skill in the art. Preferably the material will have a Curie temperature between 95° C. and 300° C. depending on the desired shut-down temperature. Of course, some machine designs may require lower or higher shut-down temperatures. 
     In operation then, circuits  58  may be driven by a three-phase power source, as describe above, causing machine  10  to act as a motor. Instead of using an alternating current three-phase power source, each of circuits  58   a ,  58   b  and  58   c  may be driven by a square wave source, with each square way source out of phase with another square wave source by 120°. As will be appreciated, this has the same effect of using a poly-phase AC source, driving rotor assembly  14 . 
     More significantly, however, machine  10  may be operated as a generator by driving shaft  40  using a rotational source of mechanical power. For example, shaft  40  may be interconnected with the power shaft of a gas turbine engine, and driven at very high speeds (potentially in excess of 100,000 rpm). As will be appreciated, rotating rotor assembly  14 , and more particularly magnetic shell  18  will generate a rotating magnetic field about the central axis of rotor assembly  14 . This, in turn, establishes an alternating magnetic flux in the magnetic circuit defined by cylinder  50 . This flux, in turn, induces an electric current in the windings defined by circuits  58   a ,  58   b  and  58   c . As will be appreciated, the current so generated will be three-phase current, having a frequency proportional to the speed of rotation of rotor assembly  14 , with current through circuits  58   a ,  58   b  and  58   c  being out of phase with each other by 120°. If desired, this current may be rectified using a conventional rectification circuit (also not shown). 
     Now, in the event machine  10  is subject to an internal fault, such as for example, caused by a short across conductors  54  or  56 , current in the conductors will increase, resulting in increased heat in the conductors. Moreover, as conductors  54 , and  56  are preferably in physical contact with, and thermally coupled to cylinder  50 , increase in temperature of conductors  54  or  56  will be transferred to cylinder  50 . As the temperature of cylinder  50  approaches the Curie temperature of the material forming cylinder  50 , cylinder  50  loses its magnetic properties, thereby severely limiting the flux through cylinder  50  and the current induced in the windings formed by circuits  58 , and effectively shutting down machine  10  acting as a generator. Clearly, as the current is reduced, the temperature of the conductors is reduced until the temperature of cylinder  50  again drops below the Curie temperature of the material and its magnetic properties return. As will be apparent, in steady state and in the presence of a fault, machine  10  will operate with cylinder  50  at or near the selected shut-down or Curie temperature. Clearly, for a properly chosen Curie temperature, cylinder  50  acts as temperature activated fuse, limiting the operating temperature of machine  10 , and thereby any damage to its components. 
     Additionally, the use of ferrite material in the formation of stator assembly  12  advantageously reduces Hysteresis and Eddy current losses within stator assembly  12 . This becomes particularly beneficial at high speeds. 
     In yet another embodiment, rotor assembly  14  may include a material having the desired shut-down Curie temperature. Preferably, a ferrite material is placed radially outward of magnets forming part of rotor assembly  14 , effectively as part of the magnetic circuit formed coupling the flux from rotor assembly  14  to stator assembly  12 . Cylinder  50  may be formed of a material having a much higher Curie temperature. The ferrite material on rotor assembly  14  may then be thermally coupled to the conductors forming circuits  58 . These conductors, could for example, be coupled to rotor assembly  14  by radiation or convection. In the event that the temperature of these conductors increases, the increase in temperature is conducted to the ferrite portion of the rotor assembly  14 , thereby causing the ferrite material to lose its magnetic properties near the Curie temperature. This results in a portion of the magnetic circuit about the magnets of rotor assembly  14  having a very low permeability, thereby reducing the magnetic flux emanating with rotor assembly and coupled to cylinder  50 ; the resulting flux in cylinder  50 ; and the resulting current in circuits  58 . Again, at steady state this second embodiment will operate with the temperature of the windings and rotor at or near the selected shut-down or Curie temperature. 
     In a further embodiment, a cylinder  50 ′ illustrated in FIG. 8 may form part of a machine that is otherwise identical to machine  10 , may be formed of more than one material. A portion  62  of the cylinder  50 , is preferably formed of ferrite material having the desired shut-down Curie temperature, and the remaining portion  64  of the cylinder formed of a material having a different Curie temperature. For example the toothed portion (ie. the lengthwise extending teeth or ridges) of cylinder  50 ′ may be formed of laminated iron, while the remainder of cylinder  50 ′ may be formed of Manganese-Zinc having a Curie temperature of about 200° C. Individual iron teeth or ridges may be epoxied to a Manganese-Zinc portion. Above the Curie temperature, the resulting magnetic circuit would have a very high reluctance, severely limiting the magnetic flux guided about rotor assembly  14 , and therefore the current through windings about the cylinder  50 ′, again causing cylinder  50 ′ to operate at or near the chosen Curie temperature. Of course, other configurations of cylinder  50 ′ having other portions formed of a magnetic material having the desired Curie temperature will be readily apparent to those of ordinary skill in the art. 
     Clearly, the above embodiments may be modified in many ways while still embodying the invention. For example, the shape of cylinder  50  could be modified—a toroid or other shape could take its place; the arrangements of conductors and windings could be changed in any number of known ways; the permanent magnet of rotor assembly  14  can be formed in numerous ways; and the size of the machine can be scaled (increased or decreased) as required; other magnetic materials having suitable Curie temperature may be used. Thus it is apparent that the described invention may be embodied in many ways. As further examples, the invention could be embodied in a salient pole DC machine; or in a synchronous machine. 
     The above described embodiments, are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention, are susceptible to many modifications of form, size, arrangement of parts, and details of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.