Abstract:
The invention provides a method and apparatus for controlling a machine in which the magnetic flux emanating from a rotor is selectively diverted to a second path in the stator which bypasses the winding or windings so as to magnetically de-couple the winding or windings from the rotor.

Description:
CROSS-REFERENCE TO RELATED U.S. APPLICATION  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/461,356 filed Jun. 16, 2003, which prior application is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates to an electric machine and, in particular, to the control of such machines.  
       BACKGROUND OF THE ART  
       [0003]     Controlling electric machines, such as motors and generators, in fault conditions is important to providing a safe and controllable machine. Some types of machines, by reason of their architecture, are inherently more difficult to control. For example, a permanent magnet alternator which is continuously driven while suffering an internal short-circuit in its windings can present a particular concern, especially in high power applications. Consequently, the prior art presents many solutions to the control of electric machines in short-circuit situations.  
         [0004]     The prior art shows both electrical/electronic and mechanical means for responding to or preventing electric machine internal faults. One example of a mechanical solution in presented in U.S. Pat. No. 4,641,080 to Glennon et al., in which the stator of a permanent magnet generator is divided into two semi circular halves which, in the event of a detected fault, are opened by an actuator motor to increase the air gap between the stator and the rotor. Increasing the gap reduces the magnetic flux in the stator and thereby prevents burn out in the windings. The mechanisms involved, however are complex, expensive and present reliability issues to the designer. An improved solution is therefore desired, and it is an object of this invention to provide improved solutions to the problem of electric machine control.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention may be used advantageously to control an electric machine upon the occurrence of an internal short circuit in the machine or its associated control circuitry. The invention may also be used to control the machine in other circumstances, such as a loss of coolant, over-temperature, or other fault or non-fault situations.  
         [0006]     In one aspect the invention provides a method of operating an electricity generating machine, the machine having a rotor, stator and at least one winding disposed in at least one slot in the stator, the method comprising the steps of circulating magnetic flux along a first magnetic path through the stator to thereby induce an output voltage and current in the at least one winding, interposing a member across the at least one slot to provide a second magnetic path through the stator, the second path bypassing the at least one winding such that magnetic flux circulating along the second magnetic path induces substantially no voltage in the at least one winding, and diverting said magnetic flux from the first path to the second path.  
         [0007]     In another aspect, the present invention provides a method of operating an electric machine, the machine having a rotor, a stator and at least one winding disposed in the stator, the method comprising the steps of circulating magnetic flux along a first path in the stator, the first path at least partially encircling the at least one winding such that, when magnetic flux is circulated along the first path, the at least one winding is magnetically coupled to the rotor and selectively diverting the magnetic flux to a second path in the stator, the second path bypassing the at least one winding such that, when magnetic flux is circulated along the second path, the at least one winding become magnetically de-coupled from the rotor.  
         [0008]     In another aspect, the present invention provides a method of operating an electric machine, the machine having a rotor adjacent a stator assembly, the stator assembly having at least one slot and at least one winding in the slot, the slot defining a slot gap, the method comprising the steps of moving the rotor to generate electricity in the at least one winding, and moving at least a piece of the stator assembly to substantially close the slot gap and thereby provide a low reluctance path for guiding rotor magnetic flux away from the at least one winding and thereby substantially deactivating the at least one winding  
         [0009]     In another aspect, the present invention provides a method of controllably generating electricity, the method comprising the steps of guiding magnetic flux along a first path around an electrical conductor to induce a voltage in the winding and then selectively guiding said magnetic flux along a second path to substantially reduce said voltage induced in the electrical conductor.  
         [0010]     In another aspect, the present invention provides an electric machine comprising a magnetic rotor, a stator having a face adjacent the rotor, the stator including at least one slot defined therein and at least one winding disposed in the at least one slot, the at least one slot defining a slot gap across the at least one slot, wherein the at least one slot at least partially defines a primary magnetic circuit in the stator, the primary magnetic circuit having a reluctance and extending around the at least one slot, and a member moveable between a first and second position, the member substantially bridging the slot gap when in the second position to substantially close the slot gap and thereby close a second magnetic circuit in the stator, the second magnetic circuit bypassing a portion of the at least one slot containing the at least one winding, the second magnetic circuit having a reluctance not greater than the primary magnetic circuit reluctance.  
         [0011]     In another aspect, the present invention provides an electric machine comprising a magnetic rotor, and a stator defining at least part of a primary magnetic circuit and a secondary magnetic circuit therein, the primary magnetic circuit adapted to guide magnetic flux emanating from the rotor around at least one winding to induce a voltage in the winding, the secondary magnetic circuit adapted to guide said magnetic flux emanating from the rotor substantially along a stator surface adjacent the rotor to thereby bypass the at least one winding, wherein the secondary magnetic circuit is selectively closeable, and wherein the secondary magnetic circuit has a reluctance not greater than a reluctance of the primary magnetic circuit. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0012]     In order that the invention may be readily understood, several embodiments of the invention are illustrated by way of example in the accompanying drawings, in which:  
         [0013]      FIG. 1  is a cross-sectional view of a permanent magnet electric machine according to the prior art;  
         [0014]      FIG. 2  is an enlarged partial sectional schematic view of a machine rotor and stator incorporating the present invention, the invention shown in the ‘open’ position;  
         [0015]      FIG. 3  is an enlarged partial sectional schematic view of the device of  FIG. 2 , shown in the ‘closed’ position;  
         [0016]      FIG. 4  is a further enlarged cross-sectional view of another embodiment of the present invention, the invention shown in the ‘open’ position;  
         [0017]      FIG. 5  shows the device of  FIG. 4  in the ‘closed’ position;  
         [0018]      FIG. 6  is an axial sectional view of the device of  FIG. 4 , shown in the ‘open’ position;  
         [0019]      FIG. 7  is an axial sectional view of the device of  FIG. 4 , shown in the ‘closed’ position; and  
         [0020]      FIG. 8  shows an example application of an electric machine incorporating the present invention.  
         [0021]      FIG. 9  is similar to  FIG. 4 , showing an alternate configuration for the invention. 
     
    
       [0022]     Details of the invention and its advantages will be apparent from the detailed description included below.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]     A typical prior art machine configuration will first be examined, in order to permit a comparison with the present invention described later below.  FIG. 1  shows a typical permanent magnet (PM) machine  100  according to the prior art, which has a rotor  102 , with permanent magnets  104  mounted thereto by a retaining ring  106 , which is mounted on a rotatable shaft  108 . Rotor  102  is adjacent a stator  110  having a plurality of windings  112  interspersed between a plurality of teeth  114  mounted to a back iron  116 . An “inside rotor” configuration is shown in  FIG. 1 , but the positions of the rotor and stator may also be reversed. As is well understood, PM machine  100  may operate in a generator/alternator mode or a motor mode. When operated in a generator/alternator mode, an external torque source forces rotation of the shaft (and thus the rotor and the magnets), and the interaction of the magnets and the windings causes a magnetic flux to loop the windings in the slots. As the rotor rotates, the magnetic flux in the stator structure changes, and this changing flux results in generation of voltage in the windings, which results in an output current that can be used to power electrical devices, or be stored for later use.  
         [0024]     Referring to  FIGS. 2-8 , the present invention will now be described.  FIG. 2  shows an electric machine  10 , generally similar to machine  100 , is shown schematically with the rotor and stator ‘flattened’ for convenience, having a stator  12  and rotor  14 . Rotor  14  has a plurality of permanent magnets  16  and is separated from stator  12  by an rotor air gap  18 . Stator  12  has a rotor face surface  20  and includes a plurality of teeth  22  extending from a back iron portion  24  to thereby define a plurality of slots  26  for housing winding or windings  28 . As will be described in better detail below, stator  12  defines a primary magnetic flux path  30 , for guiding magnetic flux from rotor  12  through teeth  22  and back iron  24 , and around winding(s)  28 . Retaining means may be required for magnets  16 , depending on machine design, but for clarity, none are shown here.  
         [0025]     The present invention also includes a ‘gate’ member  40  configured to substantially close, and preferably intimately close, a stator slot gap  42  between adjacent teeth  22  in stator  12 . Gate  40  is moveable between at least a first or ‘open’ position ( FIG. 1 ) and a second or ‘closed’ position ( FIG. 2 ), as will be described in more detail below. In the ‘open’ position, a gate gap  44  separates faces  46  and  48 , which are preferably mating faces. In the ‘closed’ position, gate  40  and teeth  22  form a second path  32  which may be used to guide magnetic flux through stator  12 , as will be described in more detail below.  
         [0026]     The means by which gate  40  is moved is not shown in  FIGS. 2 and 3 , but is shown and depicted in more detail later in this specification. Also not shown is a control means for determining when the gate is to be actuated for movement between its open and closed positions. Such control means preferably includes a fault sensor, such as an appropriate temperature or current sensor, but may comprise any suitable means of determining when gate  40  is to move, and any suitable means for signaling, releasing or activating a movement mechanism for gate  40 .  
         [0027]     Machine  10  is preferably generally constructed of known materials. The gate member  40  is manufactured preferably from a high magnetic permeability material such as silicon iron or similar suitable materials common to the industry. The gate material is preferably of lower reluctance (or higher permeability) material than the material forming the primary magnetic path in the stator, thereby optimizing the functioning of the invention as described in more detail below.  
         [0028]     In use, under ‘normal’ machine operating conditions (i.e. rotor  14  id driven for the purpose of generating electricity using machine  10 ), the gate is positioned in its ‘open’ position (i.e.  FIG. 2 ) and, when there, preferably has a negligible effect on the operation of machine  10 . As rotor  12  passes adjacent stator  12 , magnetic flux from magnets  16  is guided down tooth/teeth  22 , through back iron  24  and back up tooth/teeth  22  to a successive magnet  14 , and thus magnetic flux circulates along primary magnetic circuit path  30 , around windings  28 , and thereby induces voltage in windings  28  which may be used to generate an output current from windings  28  and, as well, machine  10 .  
         [0029]     Referring to  FIG. 3 , upon the appropriate condition (e.g. upon reaching a threshold temperature, or upon receiving the appropriate command from a sensor sensing an internal fault condition, etc.), gate  40  is moved upward and into contact with the stator teeth  22 , and preferably contact is intimate along faces  46  and  48 , thereby substantially completely closing gate gap  44  and, thereby, slot gap  42 . Gate  40  thus forms a new, second magnetic circuit path  32  for guiding magnetic flux in the stator. Since the second flux path  32  preferably has a lower reluctance than the primary magnetic circuit path  30  (i.e. it is an ‘easier’ path for the magnetic flux to follow), the majority of the magnetic flux will bypass the generating path  30  and the windings  28  when the gate is in the ‘closed’ position. Therefore, when the gate  40  is closed, magnetic flux in the stator will be guided such there is preferably no or negligible voltage induced in windings  28 . The machine  10  may be in this manner ‘shut down’ or ‘turned down’ to control the machine  10  in specific circumstances, such as an internal fault or short circuit in a winding  28 , or another fault such as over-heating in the machine, etc. Preferably, gates  40  are individually closeable, to thereby permit a selective activation of the gate elements of the present invention. In this manner, specifically located internal faults in the machine may be individually isolated without substantially disrupting the normal operation of the remainder of the machine.  
         [0030]     The present invention provides for a shutdown capability which may be activated in a much faster way than the prior art, and need not be activated by temperature or the exceedance of a threshold current. It could be activated for any control purposes, and may even be manually (i.e. non-automatically) actuated, if desired. The gate is preferably a wedge-shaped component, as depicted, but need not necessarily be so. The faces  46  and  48  may be stepped, curvilinear or straight, and are preferably intimately mating to reduce the overall reluctance of the secondary path. The gate  40  is preferably located in the slot of the stator assembly when in its ‘open’ position but, again, but need not necessarily be so.  
         [0031]     A benefit of the present invention is that, when the gate is in the ‘closed’ position, the machine windings become encircled by a relatively high permeability material, which has the effect of multiplying the ‘self’ or ‘leakage’ inductance of the winding which, in turn, causes a significant increase in the impedance of the winding. This thereby beneficially significantly reduces the short circuit current value in the machine winding, adding yet another element of increased safety to the machine. The present invention has many applications and is particularly suited to, among many other things, use in an electric machine  10  used as an starter/generator in an aircraft prime mover gas turbine engine  70  (see  FIG. 8 ).  
         [0032]     Referring to  FIGS. 4-7 , an apparatus for automatically actuating the present invention is shown. Referring to  FIG. 4 , gate  40  is supported on winding  28 , in this case a single copper conductor, which thereby also forms a support  50  and pinned to support plate  52  by a plurality of pins  54 . Gate  40  is held to pins  54  in a slot  56  and brazed therein by a braze  58  selected and configured to melt above a chosen temperature limit to thereby permit automatic deployment (as described below) of gate  40  once the threshold temperature is exceeded. An example of suitable alloy for braze  58  is a gold eutectic alloy with a melting range of 600° F. to 625° F. Referring to  FIG. 6 , a slot  60  is provided between gate  40  and support  50  for retaining a leaf spring  62 , which is preferably slidably mounted to support  50  by a pin  64 . (The arrow I indicates the direction of current flow through the winding  28 ).  
         [0033]     Referring to  FIGS. 5 and 7 , in use, when the machine heats up (e.g. due to local or general short circuit current in the windings, or an unwanted over-temperature situation), braze joint  58  will melt thereby releasing pins  54  from slots  56 . Support  50  preferably also helps conduct heat in machine  10  to braze joint  58  so that the joint is in good thermal communication with at least the stator slot. Spring  62  thereby biases gate  40  towards its ‘closed’ position to close gate  40  as desired. Preferably, spring  62  will move gate  40  at least about 0.060″, to thereby permit the ‘open’ position of gate  40  to be sufficiently far away from teeth  22  (i.e. to provide a gate gap  44  large enough) to prevent unwanted circulation of magnetic flux through the secondary magnetic circuit  32  when the gate  40  is in the ‘open’ position. The leaf spring stiffness is chosen, among other things, to overcome the surface tension of the melted braze to ensure release and movement of gate  40 . Once release, gate  40  will also naturally be attracted to the teeth  22  of the stator  12  due to the action of the magnetic flux from the rotor  14 .  
         [0034]     It will be understood that it is preferable that the braze release mechanism of the present invention is actuated by the heating of winding  28 /support  50  before the risk of fire or significant damage to machine  10  occurs. The independent nature of this particular actuation means for gate  40  makes it possible for one phase conductor (i.e. the winding in one slot) to have actuated gate  40  while the remaining phases (i.e. slots) are not shutdown (i.e. some useful power is still produced).  
         [0035]     The motion of the gate can be achieved in a number of different ways. For example, support  50  itself may thermally expands sufficiently to close the gate gap  44 . Alternately, another type of spring or other biasing means may be provided. The motion required between the ‘open’ and ‘closed’ positions of the gate may be as small as about 0.060″, depending on the machine design, but may be smaller or larger, as will be understood by the skilled person in light of a review of this disclosure.  
         [0036]     The above description is meant to be exemplary only, and one skilled in the art will recognize that many changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the machine may be single or multi-phase, single or multi-channel. The windings may have single or multi turns per slot. A variety of winding types may be used (squirrel cage, lap, etc.), and the windings may be any conductor(s) (i.e. single conductor, more than one wire, insulated, laminated, etc.) or may be superconductors. The winding(s)  28  need not be integral with the mechanism support structure  50 , but may be separate features as shown in  FIG. 9 , with support  50  held appropriately within the slot preferably so as not to interfere with the operation of windings  28 . In multiphase machine, there may be zigzag, delta, or Y-connected windings in accordance with known techniques. The rotor can be electromagnetic (i.e. permanent magnet not necessary), and may be provided in an outside or inside configuration, or any other suitable configuration.  
         [0037]     Therefore, although the above description relates to specific preferred embodiments as presently contemplated by the inventors, it will be understood that the invention in its broad aspect includes mechanical and functional equivalents of the elements described herein.