Abstract:
The present invention relates to a a unipolar current electrical machine operable as a switched reluctance motor, comprising a motor structure and a control unit. The motor structure comprises a plurality of disk-shaped annular stator elements stacked and spaced equidistantly from each other and mounted on a frame; a plurality of disk-shaped rotor elements mounted on a rotational axis and spaced equidistantly from each other such that successive rotor elements are positioned between successive stator elements; a plurality of electrical windings on each of the stator elements, which when energized with a current flow, produce a magnetic field in a direction substantially parallel to the axis; and a return path for completing a magnetic flux path in a second direction perpendicular to the first direction on successive stator elements and through the rotor elements for generating a rotational force in the rotor elements; and a rotor position sensor. A rotor position sensor provides a signal to the control unit. The control unit utilizes an input signal in combination with the rotor position signal to switch current to each of the windings of the stator elements in a controlled manner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 11/150,777, filed Jun. 10, 2005, now pending, which is continuation-in-part of application Ser. No. 10/933,711, filed Sep. 3, 2004, now abandoned, which is a continuation of application Ser. No. 10/459,358, filed Jun. 11, 2003, now U.S. Pat. No. 6,803,847 B2, which is a divisional application of application Ser. No. 10/077,278, filed Feb. 15, 2002, now U.S. Pat. No. 6,713,982 B2, which claims benefit of U.S. Provisional application 60/270,032, filed Feb. 20, 2001 which are all incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a unipolar current electrical switched reluctance machine which uses a segmented coil construction to maximize active surface area of magnetic flux between rotor and stator elements, resulting in higher efficiency of flux utilization, to a method of operating such a machine in a fault tolerant manner and to a method of manufacture of such a machine. In contrast with induction motors, which utilize alternating current, a switched reluctance machine operates with a unipolar current flowing through its coils.  
       BACKGROUND OF THE INVENTION  
       [0003]     Electric machines such as motors and generators, are generally used because they are extremely rugged, reliable, easy to control, and in particular have a high torque capacity and high power density ratings. Switched reluctance machines operate on the principle that current traveling in stationary coils or windings of a stator produces a rotating magnetic field which in turn interacts with a rotor occupying the space where the rotating magnetic field exists. The magnetic teeth of the rotor react with the rotating magnetic field to produce a rotational force.  
         [0004]     Heretofore, it was believed that there was a fundamental limit to torque density in such machines. Although flux density is limited by material considerations, while current density is limited by (1) heating, (2) machine reactance, and (3) the fact that too much current density produces localized magnetic saturation, the present invention optimizes the configuration of the rotor and stator elements so that the machine output can be increased without substantially increasing the volume of the machine. Conventional belief in the design of electric machines is that power density is limited and the only way to increase power output is to increase the volume of the machine.  
         [0005]     Switched reluctance motors operate on the principle of unipolar current, i.e., the current flows only in one direction in the windings regardless of whether positive or negative torque is required. This principle requires only one switch to be in series with each winding in each stator element. The turning on or off of this switch regulates the flow of current in the winding. It should be noted that in the motor literature an individual winding of a stator element is sometimes generally referred to as a “phase”. In the context of a unipolar current electrical machine the term “phase” is somewhat analogous to a phase of a multiphase alternating current motor.  
         [0006]     The primary object of this invention is to provide a switched reluctance electrical machine having a high torque capacity for a given machine volume. A second object of this invention is to provide such a switched reluctance electrical machine in which the commonly accepted limit to torque density in electrical machines is overcome by utilizing the same magnetic flux among one or more parallel air gaps. A third object of this invention is to provide a switched reluctance electrical machine in which the magnetic flux is passed through multiple air gaps, interacting with a rotor element at each air gap, thereby increasing the torque density for a give volume of the machine. A fourth object of this invention to provide such an electrical machine in which force density is increased substantially by the number of air gaps present in the machine but the overall machine transverse dimension is increased only by a smaller factor because the magnetic return path remains nearly constant. A fifth object of this invention is to provide a fault tolerant switched reluctance electrical machine which can continue to operate even when one or more winding faults have been detected. A sixth object of this invention is to provide a method of manufacture for such a switched reluctance electrical machine.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention is a unipolar current electrical machine having a housing, a stator mounted in the housing, and a rotor, the rotor having a shaft with an axis therethrough and being supported by bearings for rotation about the axis in the housing, comprising:  
         [0008]     the stator having a plurality of stator elements each in the form of an annular disk, spaced apart from each other, each stator element comprising a plurality of magnetically isolated magnetic teeth;  
         [0009]     a plurality of electrical windings on each of the stator elements, each winding being associated with a group of magnetic teeth of the stator element, each group of magnetic teeth being arranged at a predetermined angular position with respect to an adjacent group of magnetic teeth, each of the windings being arranged such that, when energized with a current flowing in the windings, a magnetic flux is created in a first direction;  
         [0010]     the rotor having a plurality of rotor elements, the rotor elements being spaced from each other and interstitially disposed with the stator elements in an interdigitated manner;  
         [0011]     each rotor element being in the form of a circular disk mounted on the shaft, each rotor element comprising a plurality of magnetically isolated magnetic teeth arranged in an annular portion of the circular disk;  
         [0012]     means for completing a magnetic flux path in a second direction through the magnetic teeth of the rotor elements and through corresponding groups of teeth on successive stator elements.  
         [0013]     The present invention further comprises a modular control unit arranged to individually control electrical energy applied to each winding of each stator element, the control unit comprising:  
         [0014]     (a) a microprocessor controller, a load sensing means, a rotor angle position sensor, and a plurality of stator control modules, each control module comprising an electrical switching device connected to a winding of the respective stator element; the microprocessor controller being responsive to the load sensing means and to the rotor angle position sensor to generate control signals to the control modules, each control module being responsive to the control signals to control the flow of current to the connected winding of the stator element in a pulse-width control manner,  
         [0015]     the flow of current to each winding being turned on at a first predetermined rotor angle position and turned off at a second predetermined rotor angle position by the control unit in response to control signals from the controller, thus causing the rotor to rotate at a speed responsive to the control signals with a power output proportional to the load.  
         [0016]     In the present invention each control module may further comprise current sensing means to sense current in the windings of the associated stator element and means to generate a corresponding signal to the microprocessor controller, the controller being responsive to the signal to compensate for the sensed current and to generate control signals to the control modules to equalize the current in each phase of the stator windings.  
         [0017]     In the control unit of the present invention, the microprocessor controller compares each current signal to a predetermined fault threshold to detect a winding fault and then causes that control module to deenergize one or more windings of the stator element in response to the detected fault, permitting the motor to continue to operate. Alternatively, the controller may cause the control modules to deenergize all the windings in a stator element is response to a detected fault.  
         [0018]     In the present invention the load sensing means may comprise a motor speed sensor or a torque sensor.  
         [0019]     The present invention also comprises a method of operating a unipolar electrical machine, as described above, in a fault tolerant manner in response to a controller,  
         [0020]     the controller being responsive to the load sensing means, the rotor angle position sensing means, and the signal representing the sensed winding current, to generate control signals to the control modules;  
         [0021]     the controller comparing each current signal to a predetermined fault threshold to detect a fault in a winding in a stator element and causing the corresponding control module to deenergize one or more windings in the stator element in response to the detected fault,  
         [0000]     thereby permitting the motor to continue to operate in the presence of a winding fault. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
       [0022]      FIG. 1  shows an arrangement of components for a motor of the present invention;  
         [0023]      FIG. 2  shows a representative assembly overview of shaft, rotor disks, stators and housing of the present invention;  
         [0024]      FIG. 3  shows a segmented stator construction of the present invention, showing only a portion of the stator windings, the windings surrounding individual magnetic teeth;  
         [0025]     FIGS.  4  shows an exemplary arrangement of a stator element construction of the present invention with the windings surrounding groups of magnetic teeth;  
         [0026]      FIG. 5A , B and C show an exemplary arrangement of a rotor element construction of the present invention;  
         [0027]      FIG. 6  shows a view with the magnetic teeth of the stator elements and the magnetic teeth of the rotor elements misaligned;  
         [0028]      FIG. 7  shows a view with the magnetic teeth of the stator elements and the magnetic teeth of the rotor elements aligned; and  
         [0029]      FIG. 8  shows a plot of winding current versus rotor position angle for a four-phase switched reluctance motor.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention is exemplified by the Figures. As may be seen in  FIG. 1 , the electrical machine  100  the present invention comprises a motor unit  110  and a modular control unit  160 . The motor unit  110  (best seen in  FIG. 2 ) comprises a housing  120 , a stator  130  mounted in the housing, and a rotor  140 , the rotor  140  having an axis  140 A therethrough and being supported in the housing  120  by bearings  122  for rotation about the axis  140 A. The housing  120  oprionall may have air vents  124  or heat radiating ribs  126 . The stator  130  has a plurality of stator elements  132  in the form of annular disks  132 D (best seen in  FIGS. 3 and 4 ), spaced apart from each other, each stator element  132  comprising a plurality of magnetically isolated magnetic teeth  136  ( FIGS. 3 and 4 ).  
         [0031]     The rotor  140  has a plurality of rotor elements  142  ( FIGS. 5A, 5B ,  5 C) mounted on a shaft  144 , the rotor elements  142  spaced from each other and interstitially disposed with the stator elements  132  in an interdigitated manner ( FIG. 2 ).  
         [0032]     As may best be seen in  FIG. 3 , in a first arrangement, each of the stator elements  132  has a plurality of electrical windings  134 , each winding  134  being associated with a magnetic tooth  136  of the stator element  132 . As may best be seen in  FIG. 4 , in a second arrangement, each of the stator elements  132  has a plurality of electrical windings  134 , each winding  134  being associated with a group  136 G of magnetic teeth  136  of the stator element  132 , the windings  134  being arranged such that, when energized with a current  134 C flowing in the windings  134 , a magnetic flux M 1  is created in a first direction D 1 . It should be noted that some of the stator element components have been omitted so that the winding  134  may be better seen. Although the Figures illustrate an electrical machine having four windings  134 - a,    134 - b,    134 - c,    134 - d  in each stator element  132 , the machine of the present invention may have any desired number of windings  134  in each stator element  132 .  
         [0033]     The magnetic flux path is completed in a second direction D 2  through corresponding groups  136 G of teeth  134  on successive stator elements  132 , and through the rotor elements  142 , thereby resulting in a rotational force F being applied to the rotor elements  142  causing the rotor  140  to rotate.  
         [0034]     As seen in  FIG. 1 , the electrical machine  100  further comprises a modular control unit  160  arranged to individually control electrical energy applied to each winding  134  of each stator element  132 . The control unit  160  comprises a microprocessor controller  162 , a load sensing means  170 , a rotor position sensor  176  and a plurality of stator control modules  180 .  
         [0035]     Each control module  180  comprises a plurality of electrical switching devices  182  connected to the respective winding  134  of a stator element  132 . Suitable electrical switching devices  182  include isolated gate bias transistors (IGBT) and similar devices capable of switching the required current. The microprocessor controller  162  is responsive to the load sensing means  170  and the rotor position sensor  176  to generate control signals S to the control modules  180 . The load sensing means  170  may comprise a motor speed sensor  172  or a torque sensor  174 . Each control module  180  is responsive to the control signals S to control the flow of current  134 C to the connected winding  134  of stator element  132  in a pulse-width control manner by turning on and off the electrical switching devices  182 , thus causing the rotor  140  to rotate at a desired speed with a desired power output proportional to the load.  
         [0036]     The rotor angle position sensor  176  is typically implemented by an optical encoder. Rotor shaft position feedback is needed to synchronize the current flow in each winding  134  with rotor position in order to generate the desired motoring torque. The rotor angle position is also used by the controller  162  to compute actual rotor rotational speed, which is compared with the desired speed and used to control the rotor rotational speed.  
         [0037]     Each control module  180  further comprises current sensing means  190  to sense current  134 C in the windings  134  of the associated stator element  132  and current signaling means  192  to generate a corresponding signal  192 S to the microprocessor controller  162 , the controller being responsive to the signal  192 S to generate control signal S to the control modules  80 , which in turn generate signals  166 S 1 ,  166 S 2 ,  166 S 3 ,  166 S 4  to the switching devices  82  to equalize the current  134 C-a,  134 C-b,  134 C-c,  134 C-d in each phase of the stator windings  134 .  
         [0038]     The microprocessor controller  162  compares each current signal  192 S to a predetermined fault threshold  164 F (schematically represented as being supplied by potentiometer  164 ) to detect a winding fault. When a winding fault is detected the microprocessor controller  162  sends a control signal S to that control module  180  which in turn generate signals  166 S 1 ,  166 S 2 ,  166 S 3 ,  166 S 4  to the switching devices  82  to deenergize one or more windings  134  of stator element  132  in response to the detected fault, permitting the motor  100  to continue to operate.  
         [0039]     The power supplied to the control module  180  is direct current (i.e., is unipolar). The direct current may be supplied by any suitable direct current source, such as a DC power supply having a conventional rectifier, filter and voltage regulation means. The DC power supply may be fed from a single phase AC powerline source V 1  or from a multiphase AC source V M .  
         [0040]     The electrical machine of the present invention may have any desired number of windings  134  in each stator element  132 . An exemplary embodiment having four windings (i.e., four phases)  134 - a,    134 - b,    134 - c,    134 - d  is shown. When multiple windings  134  are employed the current sensing means  190  is arranged to sense current  134 C in each individual winding (phase)  134 - a,    134 - b,    134 - c,    134 - d  in each stator element  132 . Signal generating means  192 - a,    192 - b,    192 - c,    192 - d  generate corresponding sensing signals  192 S 1 ,  192 S 2 ,  192 S 3 ,  192 S 4  to the microprocessor controller  162 . The controller  162  is responsive to each sensing signal  192 S 1 ,  192 S 2 ,  192 S 3 ,  192 S 4  to generate control signal S (which may comprise components  166 S 1 ,  166 S 2 ,  166 S 3 ,  166 S 4 ) to the control modules  180  to equalize the currents  134 C-a,  134 C-b,  134 C-c,  134 C-d in each of the stator windings  134 - a,    134 - b,    134 - c,    134 - d.    
         [0041]     The microprocessor controller  162  compares each current sensing signal  192 S 1 ,  192 S 2 ,  192 S 3 ,  192 S 4  from each winding  134  in each stator element  132  to a predetermined fault threshold  164 F to detect a winding fault in a stator element  132 , and causes the control module  180  connected to that stator element  132  to deenergize one or more windings  134  of that stator element  132  in response to the detected fault, permitting the motor  100  to continue to operate.  
         [0042]     The microprocessor controller  162  may respond to the sensed current in the faulted winding  134  in a number of ways. The microprocessor controller  162  may increase the current in the other windings  134  of that stator element  132  to compensate for the fault. Alternatively, the microprocessor controller  162  may respond to the sensed current in the faulted winding  134  to increase the current in the windings  134  of other stator elements  132  not having the faulted winding  134  to compensate for the fault. In response to a fault at least two windings  134  in a stator element  132  may be deenergized, the deenergized windings  134  being symmetrically arranged on the stator element  132 , so that torque produced by that stator element fluctuates in a symmetrical manner. When one or more windings in a stator element  132  not containing the faulted winding  134  are deenergized, those deenergized windings are arranged on the unfaulted stator element rotationally symmetrical to the faulted winding  134  on the faulted stator element  132 , so that torque produced by the electrical machine remains substantially constant.  
         [0043]     A unipolar electrical machine operated as a switched reluctance motor is a singly-excited motor with salient poles  136 P on the stator  130  ( FIGS. 6 and 7 ) and salient poles  146 P on the rotor  140  ( FIGS. 6 and 7 ). Only the stator  130  carries windings  134  on the stator elements  132 . The rotor  140  has neither windings nor magnets and is built up from a stack of steel laminations. Each stator winding  134  (phase) consists of two series connected windings  134 X,  134 Y ( FIG. 3 ) on diametrically opposite magnetic teeth  136  to create magnetic poles. Alternatively, stator winding may consist of windings that are wound around groups  136 G of magnetic teeth  136  ( FIG. 4 ).  
         [0044]     In a switched reluctance motor torque is produced by the tendency of its moveable part (i.e., rotor  140 ) to move to a position where the inductance of the excited winding  134  of the stator element  132  is maximized. During motor operation, each winding  134  of the stator element  132  is excited (i.e., current is turned on) when its inductance is increasing, and unexcited (i.e., current is turned off) when its inductance is decreasing. The air gap  200  ( FIGS. 6 and 7 ) is at a minimum at the aligned position, the position where a pair of rotor poles is exactly aligned with a stator pole as seen in  FIG. 7 , and the magnetic reluctance of the flux flow is at its lowest. The magnetic reluctance will be highest at the unaligned position as seen in  FIG. 6 . Thus, when a winding  134  of the stator element  132  is energized a magnetic pole on the stator is created. If a magnetic tooth  146  on the rotor  140  is not aligned with that magnetic stator pole  136 P created by the current in the winding  134  around magnetic tooth  136 , the rotor  140  will start to move and attempt to align with the stator pole  136 P. If the rotor position is known (as sensed by rotor angle position sensor  176 ) the current to successive stator windings  134  may be switched on and off in a controlled manner, causing the rotor  140  to rotate in a desired direction and at a desired speed.  
         [0045]     The controller  162  implements conventional switched reluctance motor torque, speed and position control protocols (i.e., current or torque method control) with control of control modules  180  connected to individual windings  134  of the segmented stator elements  132 . Combined with current sensing inputs  192 S the control unit therefore implements a self-regulating variable power output device.  
         [0000]     Operation of a Four-Phase Switched Reluctance Motor  
         [0046]     Since switched reluctance motors operate on the principle of unipolar current, i.e., the current flows only in one direction in the windings  134  regardless of whether positive or negative torque is required. This principle requires only one switch  182  to be in series with each phase winding  134 . The turning on or off of this switch  182  regulates the flow of current  134 C in the phase winding  134 . The exemplary four-phase construction, where the phases are identified respectively as: a, b, c, d, when operating, has phase currents  134 C-a,  134 C-b,  134 C-c,  134 C-d plotted as shown in  FIG. 8  (lines i a , i b , i c , i d  respectively).  
         [0047]     As seen in  FIGS. 3, 4 ,  6  and  7 , there are a plurality of magnetically isolated magnetic teeth  136  on each of the stator elements  132 . There are a plurality of electrical windings  134  on each stator element  132 . Each of the windings  134  surrounds a different group  136 G of magnetic teeth  136  to create a magnetic pole  136 P. There are also return means including magnetic material for establishing a magnetic flux path axially in series through corresponding groups of teeth  136  on successive stator elements  132  and teeth  146  on the interstitial rotor elements  142 , and azimuthally in the return means.  
         [0048]     The number of teeth  146  on each rotor element  142  may be the same as or different in number from the number of teeth  136  on each stator element  132 . There are return means including magnetic material for establishing a low reluctance azimuthal flux path and a plurality of conductor paths surrounding the teeth on each of the rotors. In a preferred embodiment the stator elements  132  are each in the form of an annular disk  132 D. The magnetic teeth  136  of the stator elements  132  are imbedded in the annular disks  132 D, and the annular disks  132 D are of laminated construction.  
         [0049]      FIGS. 5A, 5B  and  5 C show a preferred embodiment of the rotor elements  142 , which are each in the form of a circular disk  142 D mounted on the shaft  144 . In a similar manner the magnetic teeth  146  of the rotor elements  142  are imbedded in the circular disks  142 D, and the circular disks  142 D are of laminated construction.  
         [0050]     In the unipolar current electrical machine of the present invention materials of construction known and used in conventional designs of motors, generators, transformers and the like may be used. In the present invention the various control means and electrical components for control may be assembled from electrical components known and used in conventional electrical control circuits. For example, the load sensing means  170  ( FIG. 1 ) may be a conventional load or torque sensor and the current sensing means  190  may be a plurality of control transformers (CT).  
         [0051]     In the present invention, the specific geometric relationship of the rotor and stator elements according to the present invention is unique. This unique segmented geometry not only provides advantages in the efficiencies of operation, but also results in a surprising ease of manufacturing and interchangeability electrical components.  
         [0052]     The present invention provides a method of manufacture of a unipolar current electrical machine having a housing, a stator mounted in the housing, and a rotor, the rotor having a shaft with an axis therethrough and being supported by bearings for rotation about the axis in the housing, the method comprising the steps of:  
         [0053]     (a) selecting from a plurality of individual stator elements the number of stator elements necessary to produce the desired horsepower or kilowatt rating for the electrical machine wherein stator elements are in the form of annular disks comprising a plurality of magnetically isolated magnetic teeth, each stator element having thereon a plurality of electrical windings on each of the stator elements, each winding being associated with a group of magnetic teeth of the stator element, the windings being arranged such that, when energized with a current flowing in the windings, a magnetic flux is created in a first direction;  
         [0054]     (b) selecting from a plurality of individual rotor elements necessary to produce the desired horsepower or kilowatt rating for the electrical machine, wherein the rotor elements are in the form of circular disks comprising a plurality of magnetically isolated magnetic teeth arranged in an annular portion of the circular disks;  
         [0055]     (c) mounting the selected stator elements in the housing such that the elements are spaced apart from each other;  
         [0056]     (d) mounting the selected rotor elements on the shaft such that the rotor elements are spaced from each other and interstitially disposed with the stator elements in an interdigitated manner, so that a magnetic flux path in a second direction is completed through corresponding groups of teeth on the rotor elements and through corresponding groups of teeth on successive stator elements.  
         [0057]     Stator and rotor element combinations according to the present invention may be produced in selected sizes, for example 2, 5, 10 or 20 horsepower. To build a 40 horsepower motor, for example, one could select and stack two 20 horsepower stator rotor combinations, four 10 horsepower combinations, eight 5 horsepower combinations, or twenty 2 horsepower combinations.  
         [0058]     This ability to combine combinations of simple elements or segments simplifies the manufacturing process. Motors may be produced in an assembly line operation rather than by custom manufacture. Repairs are also simplified. For example, a motor may be repaired by simply replacing a bad stator or rotor element with a good one. Field repairs may be possible in many situations. Parts inventories are simplified by the present invention in that simple stock parts can be manufactured and selected in combinations to produce the desired power output in a particular electrical machine.  
         [0059]     The present invention is a unipolar current electrical machine having a novel arrangement of rotor and stator to produce increased torque compared to a conventional machine design, yet in a machine that is smaller in size than a conventional machines. It is believed that this increase in output/decrease in size is accomplished by passing the same magnetic flux created by the stator through multiple air gaps and interacting the magnetic flux at each gap with an adjacent rotor element, so that the force density is multiplied by the number of gaps.  
         [0060]     The present invention is believed to offer the following advantages over conventional designs:  
         [0061]     (1) Improved efficiency: The power output is maximized by optimizing the cross-sectional area of magnetic flux interaction between the stator and the rotor. This results in an increase of magnetic energy transfer and improved efficiency compared to conventional machines. Due to the increased magnetic energy transfer, less electrical power will be required to produce the same amount of mechanical power compared to prior art machines. Unlike prior art designs, the present invention utilizes independent stator elements and a split motor housing design, allowing individual segments of the stator to be removed and replaced, facilitating easier maintenance.  
         [0062]     (2) Enhanced Control Capability: The motor design includes an integral microprocessor-based control unit that allows control of each individual winding of each stator element. This feature allows the motor to run at a constant speed with the optimal horsepower rating for a given load. Motor locked-rotor and in-rush currents are reduced compared to conventional designs. Due to the utilization of the individual stator element control, the motor can be started at a lower power rating, resulting in further reduction of locked-rotor and in-rush currents.  
         [0063]     (3) Reductions in Size, Weight and Cost: The machine of the present invention can be smaller than conventional motors of an equivalent horsepower rating. The segmented design permits the manufacture of standardized modules of stator elements and rotor elements. By varying the number of modules which are assembled, machines of various sizes and power ratings may be achieved. The manufacture of interchangeable modules can reduce costs.  
         [0064]     Those skilled in the art, having the benefits of the teachings of the present invention as hereinabove set forth, may effect numerous modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention, as defined by the appended claims.