Abstract:
An air core motor-generator has a rotor that is journalled to rotate about an axis of rotation, and a stator that is stationary and magnetically applies torque to the rotor. The rotor has magnetic poles that drive magnetic flux across an armature airgap, and the stator has an air core armature located in the armature airgap. Windings on the armature cause AC voltage to be induced in the windings as the rotor rotates. The windings include active length portions that are located in the armature airgap to receive the magnetic flux and induce the AC voltage, and end turn portions that traverse circumferentially and connect together the active length portions. The magnetic poles have a circumferential pole pitch, Y, and the active length portions of the windings having an active length circumferential width of a single phase, X, such that 0.5 Y&lt;X&lt;Y.

Description:
[0001]     This invention pertains to brushless motor-generators and more particularly to air core motor-generators that employ a new armature with a special windings configuration that increases the efficiency and power capability while also facilitating easy manufacturing.  
       BACKGROUND OF THE INVENTION  
       [0002]     Air core motor-generators have the potential to provide higher efficiency and performance than conventional type electrical machines. They achieve these advantages by eliminating slot wound armature windings wherein the windings are wound in slots in a steel stator, and instead locate the windings within the magnetic airgap. Air core motor-generators can utilize single rotating or double rotating construction. Single rotating construction utilizes a loss mitigating ferromagnetic stator on one side of the airgap. Double rotating air core motor-generators eliminate the need to pass a circumferentially varying flux through a ferromagnetic stator by bounding both sides of the magnetic airgap by rotating surfaces of the rotor.  
         [0003]     Various different methods for constructing air core armatures have been utilized along with different winding pattern configurations. Unfortunately, existing air core motor-generators do not achieve their maximum possible potential for efficiency and performance. A new type of air core armature for motor-generators is therefore needed.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention provides a brushless air core motor-generator having an armature with special windings configuration that increases efficiency and power capability with easy manufacturing. The motor-generator is comprised of a rotor that is journalled to rotate about an axis of rotation and a stator that is stationary and magnetically applies torque to the rotor. The rotor comprises magnetic poles that drive magnetic flux across an armature airgap and the stator comprises an air core armature located in the armature airgap and comprising windings such that AC voltage is induced in the windings as the rotor rotates. The windings comprise active length portions that are located in the armature airgap, receive the magnetic flux and induce the AC voltage, and end turn portions that traverse circumferentially and connect together the active length portions. The magnetic poles have a circumferential pole pitch, Y, and the active length portions of the windings have an active length circumferential width of a single phase, X, such that 0.5 Y&lt;X&lt;Y. More preferably, 0.55 Y&lt;X&lt;0.90 Y. Unlike trapezoidal windings wherein X=Y/3 or full phase layer windings wherein X=Y, the invention provides a unique and unexpected reduction of the armature resistive losses and an increase of the efficiency and power capability of the motor-generator. The result is particularly surprising because the armature has a lower winding density, yet it achieves higher performance. This result is contrary to the design principles that are well known in the art of air core armatures.  
         [0005]     The functioning of the motor-generator of this invention can be understood by studying the circumferential field flux distribution and its interaction with the windings for generation of the back emf and in the resistive loss contributions of different wires in an air core armature. As will be shown, the field flux density at the circumferential ends of the magnetic poles of the rotor suffers from fringing and leakage. Because of the much larger magnetic airgap used in air core motors and generators, the leakage portion between adjacent poles is much larger. As a result, the circumferential flux density distribution in the armature airgap suffers from significant circumferential areas near the interfaces between adjacent poles where the flux density is greatly reduced. It has been found that reducing the number of windings and particularly, the circumferential width of the active length portion of a phase to be less than the pole pitch but greater that one half of the pole pitch, the resistive losses can be reduced while the back emf produced is not as appreciably affected. The end windings of a phase approaching wherein the active length width is equal to the pole pitch do not significantly participate in the voltage generation due to the circumferential armature airgap flux density distribution, yet they significantly add to the armature resistance. Eliminating these end windings by reducing the active length width as specified actually increases the motor-generator performance despite the fact that the armature has a lower windings density.  
         [0006]     In another embodiment, the circumferential width of a section of the air core armature comprising one set of active lengths of each phase is substantially greater than the circumferential pole pitch, and the circumferential width of the active length portion of a single phase is less than the circumferential pole pitch.  
         [0007]     In an additional embodiment, the air core armature has two sides that are perpendicular to the magnetic flux and has a first winding layer that is closest to one side and a second winding layer that is closest to the second side. The active lengths of one phase winding lie only in the first winding layer, active lengths of a second phase winding lie only in the second winding layer and active lengths of a third phase winding lie in more than one winding layer.  
         [0008]     The air core armature can be used with both radial and axial gap motor-generators. When the armature airgap is axial, the pole pitch and the active length circumferential width are herein defined by their values at the location of the inner diameter of the magnetic poles.  
         [0009]     In yet a further embodiment, the armature can utilize the teachings of having the active length circumferential width lying in the specified range but can also choose a specified width to increase the armature winding density and further increase performance. In this construction, the active length circumferential width is approximately equal to ⅔ of the circumferential pole pitch and the circumferential space between adjacent active length portions of a given phase is approximately equal to ½ of the active length circumferential width. By this means, the air core armature can be compressed into a thinner structure, as the windings will readily allow for nesting of the phases. In one case, the windings are wound with three phases and compressed into an even number of layers in the active length region. The windings active length width can also be made less than the circumferential pole width in instances when pole width is made less than the pole pitch.  
         [0010]     One preferred method for construction of the air core armatures is through the use of a substantially nonmagnetic form wherein the windings are wound onto the form. The form can provide for both location placement and structural support, which is particularly useful when the windings are wound with flexible Litz wire. For axial gap motor-generators one or multiple forms may be stacked together. For radial gap motor-generators it is possible to use only a single form having radial channels for the wires.  
         [0011]     The air core armatures may be effectively utilized in both single and double rotating air core motor-generators. In an additional embodiment, the armatures are used in double rotating electrical machines, providing the benefits of higher efficiency and performance and eliminating the need for laminations. In this case, the magnetic airgap is bounded on both sides by rotating surfaces of the rotor.  
         [0012]     Although in most cases the air core motor-generator is permanent magnet excited, particularly by attaching a circumferential array of alternating polarity permanent magnets to the rotor for driving the magnetic flux, it is also applicable for use in electrically excited versions of air core motor-generators. These electrical machines employ a field coil to produce the flux in the armature airgap are used in some applications such as flywheel energy storage systems. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic sectional elevation of a brushless air core radial gap motor-generator in accordance with the invention.  
         [0014]      FIG. 2  is a schematic drawing of a prior art winding pattern for air core armature.  
         [0015]      FIG. 3  is a schematic drawing of an alternate prior art winding pattern for air core armature.  
         [0016]      FIG. 4  is a graph showing airgap flux distribution for an air core motor-generator in accordance with the invention.  
         [0017]      FIG. 5  is a schematic drawing of a winding pattern for air core armature in accordance with the invention.  
         [0018]      FIG. 6  is a schematic drawing of an alternate configuration winding pattern for air core armature in accordance with the invention.  
         [0019]      FIG. 7  is a schematic drawing of a second alternate configuration winding pattern for air core armature in accordance with the invention.  
         [0020]      FIG. 8  is a schematic drawing of a third alternate configuration winding pattern for air core armature in accordance with the invention.  
         [0021]      FIG. 9  is a schematic sectional elevation of a brushless air core radial gap motor-generator in accordance with the invention.  
         [0022]      FIG. 10  is a schematic sectional elevation of a brushless air core axial gap motor-generator in accordance with the invention.  
         [0023]      FIG. 11  is a schematic drawing of an air core armature for an axial gap motor-generator such as the one shown in  FIG. 10 .  
         [0024]      FIG. 12  is a schematic drawing of an air core armature (section view) for motor-generator in accordance with the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     Turning to the drawings, wherein like reference characters designate identical or corresponding parts,  FIG. 1  shows a brushless air core radial gap motor-generator  30  constructed of a rotor  31  mounted for rotation relative to a stationary stator  32 . The rotor  31  is comprised of two spaced apart steel tubes  33 ,  34  to which circumferential arrays of alternating polarity magnets  35 ,  36  are attached. The magnets  35 ,  36  drive magnetic flux across the armature magnetic airgap  37  formed within the rotor  31 . Located in the magnetic airgap  37  is an air core armature  38  that is comprised of windings having active length portions  39  and end turn portions  40 ,  41 . The active length portions  39  are located in the magnet airgap  37  such that AC voltage is induced in the windings as the rotor  31  rotates. The end turn portions  40 ,  41  traverse circumferentially and connect together the active length portions  39 . The rotor  31  is connected to a shaft  42  that is journalled by bearings  43 ,  44 . The outer housing  45  supports the bearings  43 ,  44  and air core armature  38 . The winding leads  46  connect to an electrical junction box  41  for external connection.  
         [0026]     A prior art winding pattern for air core armature is shown in  FIG. 2 . The armature  50  utilizes a trapezoidal type winding. The rotor  59  has alternating poles  51 ,  52  and a pole pitch  57 . The armature  53  has three phase windings  54 ,  55 ,  56  such that the active lengths of all three phases  54 ,  55 ,  56  has a combined width  58  that is substantially equal to the pole pitch. Accordingly, the single phase active length width  59  of any particular phase is less than ½ of the pole pitch  57  and approximately equal to ⅓ of the pole pitch for a three phase motor-generator with this type of armature construction. This type of winding, which can be fabricated by winding individual coils, nested stacking them, and pressing the active lengths into a single layer, or by winding together is complicated by the end turn overlapping that is inherent. The end turn portions make winding and fabrication difficult. In addition, only two layers of winding can be used by this method because of the end turn overlapping which must be offset in opposite directions to achieve a compact armature. Performance from this winding construction is limited.  
         [0027]     An alternate prior art winding pattern for air core armature is shown in  FIG. 3 . The armature  60  utilizes a full phase layer type winding. The rotor  70  has alternating poles  61 ,  62  and a pole pitch  67 . The armature  63  comprises three phase windings  64 ,  65 ,  66  wherein each phase lies in a different layer. The windings  63  have an active length width  68  made up of same direction traversing winding wires  69 . The active length width  68  is substantially equivalent to the pole pitch  67 . By this winding method, the armature  63  can be constructed of unlimited thickness and numbers of layers. Since the winding end turns lie in the same planes as the active lengths, end turn overlapping and stacking from the different phases does not occur. Additionally, the armature  63  also achieves maximum windings density for high power and efficiency. As a result, this winding method would seem to be very good. However, it has surprisingly been found to be less than optimal for use in air core motor-generators and especially in ones employing double rotating topology that has an even larger magnetic airgap.  
         [0028]     The cause of less than optimal performance for a full phase layer winding construction can be understood by looking at the armature airgap flux density distribution for an air core motor-generator, as illustrated in  FIG. 4 . Air core motor-generators have a much larger magnetic airgap because the windings are placed directly in the airgap instead of in slots in a steel stator. The magnetic air gap can be ten times larger or more. Because of the very large magnetic airgap, substantial inter-pole magnetic flux leakage occurs whereby magnet end flux jumps between adjacent magnets on one part of the rotor instead of jumping the magnetic airgap to provide torque. The circumferential airgap flux distribution  83  has high flux regions  80  that occur in the central regions of the magnetic poles. The circumferential airgap flux distribution  83  also has a reduced flux region  82  resulting from the leakage and fringing. The reduced flux region  82  is typically less than the pole pitch  81 . Because of the significant reduced flux region between the poles, the end conductors of the active length width in the armature are not exposed to significant flux density and hence provide little torque. However, the end conductors do contribute substantially to the armature resistance.  
         [0029]     A winding pattern for air core armature in accordance with the invention that provides increased power capability and efficiency is shown in  FIG. 5 . The winding pattern  90  is hereinafter denoted as an optimal phase layer winding pattern. The rotor  100  comprises alternating magnetic poles  91 ,  92  with a pole pitch  97 . The armature  93  is comprised of three phase windings  94 ,  95 ,  96  that are wound in layers. A different number of phases could also be used instead. Each of the phase windings  94 ,  95 ,  96  comprises active length conductors  98  in a single direction that have a total active length width  99 . To achieve increased performance, the active length width is made less than the pole pitch but also greater than ½ the pole pitch. In this way, the end conductors of a full phase layer winding are omitted, actually reducing the windings density of the air core armature. According to accepted principles, the performance should therefore be reduced. However, the elimination of the end conductors reduces the armature resistance to a much greater extent that it reduces the back emf due to the reduced flux region resulting from the very large magnetic airgap in the air core motor-generator. The efficiency and power capability of the motor-generator have been found to be appreciably increased. An additional benefit of this construction is that it reduces the need for tighter bend radii of the windings in full phase layer construction and has no end turn overlapping winding difficulties as with trapezoidal type windings, making it easier than both as well.  
         [0030]     Another winding pattern  110  for an air core armature  113  affording yet further increased efficiency and performance is shown in  FIG. 6 . The winding pattern  110  hereinafter is denoted as an optimal integer winding pattern. Armature  113  is in an airgap bounded on at least ones side, preferably both sides, by a rotor  124  having magnetic poles  111 ,  112  and a pole pitch  117 . The armature  113  comprises three phase windings  114 ,  115 ,  116 . Each of the phases  114 ,  115 ,  116  has active length conductors  118  that together form a circumferential active length width  119 . Again, the active length width is set to be between ½ the pole pitch and the pole pitch to achieve high performance. However, in this winding construction the active length circumferential width is approximately equal to ⅔ of the pole pitch and the circumferential inter-active length width  124  is approximately equal to ½ of the active length width  119 . Because of this construction, the windings  110  can be compressed into a thinner armature  120  with higher winding density for a reduced airgap thickness and increased efficiency and performance.  
         [0031]     Another air core armature  120 , shown juxtaposed to the other side of the rotor  124  for convenience (although both armatures would not be used in the same motor at the same time) has two sides that are perpendicular to the magnetic flux (shown as the hollow arrow  128 ) and has a first winding layer  125  that is closest to one side and a second winding layer  126  that is closest to the second side. The active lengths of one phase winding  121  lie only in the first winding layer  125 , active lengths of a second phase winding  122  lie only in the second winding layer  126  and active lengths of a third phase winding  123  lie in both winding layers  125 ,  126 .  
         [0032]     One desirable method for armature construction is to wind the wires onto a substantially nonmagnetic form. The windings preferably utilize Litz type wire to reduce winding eddy current losses. When utilizing the optimal integer winding patter in a form with individual slots the width of the wires and three phase construction, the number of slots around the diameter preferably is equal to the number of conductors per active length width times 3/2 times the number of poles. Additionally, the number of conductors per active length circumferential width is an integer multiple of 4.  
         [0033]     Another winding pattern  130  for air core armature  133 , shown in  FIG. 7 , is an optimal integer winding pattern with eight conductors per active length width. The armature  133  is in an airgap of a motor-generator having a rotor  144  with poles  131 ,  132  and a pole pitch  137 . Spaces may also be included between poles to reduce magnet costs in which case the pole width becomes less than the pole pitch. The armature  133  is comprised of multiple phase windings  134 ,  135 ,  136  that each comprises active length conductors  138  in a single direction forming the circumferential active length width  139 . The active length width is equal to ⅔ of the pole pitch  137  and the windings are compressed into a compacted armature  140 , shown on the opposite side of the rotor  144  for convenience of illustration. The armature  140  has the windings  141 ,  142 ,  143  that are nested together in the active length region as shown. The end turns, not shown, will be thicker but will not require an increased magnetic airgap thickness by locating them outside of the armature airgap in the motor-generator.  
         [0034]     Another configuration winding pattern for windings  150  of air core armature  153 , shown in  FIG. 8 , is a double layered version of an optimal integer winding with four conductors per active length width. Again the windings  150  can be wound as coils or alternatively as a serpentines around the diameter which can be easier and faster. The rotor  164  comprises poles  151 ,  152  with a pole pitch  160 . The armature  153  is wound with phase layers  154 ,  155 ,  156 ,  157 ,  158 ,  159  wherein layers  154  and  157 ,  155  and  158 , and  156  and  159  are each of the same phases. Each layer  154 ,  155 ,  156 ,  157 ,  158 ,  159  has active length wires  162  of a single direction forming the active length width  161 . The windings  150  can then be compressed into a compacted armature  163 , shown on the other side of the rotor  164  for convenience of illustration.  
         [0035]     The air core armature windings are applicable for use in both double rotating air core motor-generators as previously shown and single rotating versions. A single-sided brushless air core motor-generator  170 , shown in  FIG. 9 , has a rotor  171  and a stator  172 . The rotor  171  has a circumferential array of magnetic poles  173  that drive flux across an armature airgap  174 . Located in the magnetic airgap  174  is an air core armature  175  that rests against a loss mitigating ferromagnetic stator  176 , such as a steel lamination stack. The rotor  171  is connected to a shaft  177  that is journalled in bearings  178 ,  179 . The bearings  178 ,  179  and armature  175  are supported by the housing  180 . This type of air core motor-generator construction can have higher losses due to eddy current and hysteresis losses in the laminations  176 . However, the rotor  171  can have lower inertia, which may be beneficial in some applications.  
         [0036]     The disclosed air core armature is applicable for use in axial gap air core motor-generators as well as radial gap types shown. A brushless axial gap air core motor-generator  190 , shown in  FIG. 10 , is comprised of a rotor  191  and stator  192 . The rotor  191  is constructed with two steel discs  193 ,  194  that have circumferential arrays of magnetic poles  195 ,  196  that drive flux across a magnetic airgap  197  created within the rotor  191 , and then circumferentially through to discs  193 ,  193  to the circumferentially adjacent magnet  195 ,  196  to continue the flux loop. Located within the magnetic airgap  197  is a stationary air core armature  198 . The rotor  191  is coupled to a shaft  199  that is supported for rotation by bearings  200 ,  201 .  
         [0037]     An axial air core armature  210  for an axial gap motor-generator, such as the one shown in  FIG. 10 , is shown in  FIG. 11 . Although the windings can be assembled in accordance with the invention by several different means including individual winding and potting, a preferred method uses a nonmagnetic form wherein the windings are wound onto the form. For flexible Litz wire windings the form provides both windings location and structural support during the winding process and in operation. The armature  210  is comprised of a plastic form  211  and windings  212  that are wound into surface channels. The windings  212  have at least one start lead  213  and end lead  214 . A cut out section  215  can be provided in the form  211  to account for overlapping of the exit lead  214 . The armature  210  preferably is inserted in the motor-generator such that the magnetic poles have an inner pole diameter  217  and an outer pole diameter  216 . When using an axial gap motor-generator with the windings in accordance with the invention, the pole pitch and the active length circumferential width are defined by their values at the location of the inner diameter  217  of the magnetic poles. When using an optimal phase layer type winding, the channels for the wires  212  may be complete to support active lengths and end turns (as shown) or they can be incomplete, supporting only a portion of the winding pattern, for example, only the active lengths. When the optimal integer winding pattern is utilized, the channels for the wires  212  can not support the end turns and can only be located in the active length region.  
         [0038]     A three-phase air core armature  220  for an axial gap motor-generator in accordance with the invention is shown in  FIG. 12 . The armature  220  utilizes a triple stack construction for the three phases. The armature  220  is comprised of phases  221 ,  222 ,  223  that are axially stacked together. Each phase  221 ,  222 ,  223  comprises a plastic form  224  with windings  225  that are wound onto the form  224 . The form  224  has a thin backing portion  226  and raised channel walls  227  such that the windings  225  lie between the channel walls  227 .  
         [0039]     Obviously, numerous modifications and variations of the described preferred embodiment are possible and will occur to those skilled in the art in light of this disclosure of the invention.