Abstract:
A motor-generator includes a rotor that rotates about an axis of rotation, and a stator that is stationary and magnetically interacts with the rotor. The rotor is constructed of two spaced apart rotor portions having magnetic poles that drive magnetic flux across an armature airgap formed therebetween. An armature, located in the armature airgap, has a substantially nonmagnetic and low electrical conductivity form onto which wire windings are wound. The form has a free end that extends inside the rotor, and a support end that attaches to the stationary portion of the motor-generator. The form is constructed with a thin backing portion and thicker raised portions extending from the backing portion in the direction of the magnetic flux. The wire windings have multiple individually insulated conductor wire. The conductors of a single wire are electrically connected together in parallel and electrically insulated between each other along their length inside the armature airgap. The wire windings are wound on to the form by inserting the wire to lie between the raised portions; the form provides position location and support for the wire windings during the winding process, and subsequently reacts the electromagnetically induced torque on the windings to the stationary portion of the motor-generator through the support end of the form and prevents the windings from contacting the rotor portions during rotation of the rotor.

Description:
[0001]    This invention pertains to electrical machines for converting between electrical and mechanical energy, and more particularly to a low cost, high efficiency motor-generator having a special low cost armature winding construction that very high efficiency and is easily and rapidly wound and assembled. 
       BACKGROUND OF THE INVENTION  
       [0002]    Electric motors consume more than half of all electrical energy used in most countries. Currently, the majority of all motors used are induction motors. Induction motors provide simple and reliable operation and have relatively low costs. Unfortunately, induction motors do not provide the highest efficiency. 
         [0003]    Some applications of motors and generators run continuously or near continuously. In such applications, the annual electricity consumption costs can be several times greater than the acquisition cost of the motor. Increasing the efficiency of motors used for these applications could provide significant economic benefits. 
         [0004]    Another type of motor that can provide higher efficiency than induction motors is the brushless permanent magnet motor. Because the field flux in permanent magnet motors is produced by magnets instead of electrically excited windings as in induction motors, they can operate more efficiently. Unfortunately, conventional brushless permanent magnet motors are much more expensive than induction motors. They also do not always provide a great enough increase in efficiency and energy savings to justify the increased cost and make them an economically viable alternative, except where the functional benefits of brushless permanent magnet motor necessitate their use. 
         [0005]    One way to increase the efficiency of brushless permanent magnet motors is to eliminate eddy current and hysteresis losses occurring in laminations by eliminating the use of electrical laminations in construction. In such motors, the armature comprises windings with an air core instead of being wound into slots in the laminations. This type of construction also provides the benefits of reduced winding inductance for higher speed operation. As such, these types of motors are small and typically used in cameras, dental drills, flywheel energy storage systems and specialized application servomotors. Despite their potential for increased efficiency, air core permanent magnet brushless motors utilize more magnet material and require an armature that is more difficult, time consuming and costly to construct. In addition, some constructions can also result in increased armature losses, reducing the potential energy efficiency gains. For these reasons, such motors have not been widely used in commercial applications for replacement of induction motors. 
         [0006]    Thus, a new construction motor-generator is needed with high efficiency and lower costs for widespread commercial applications. Such a motor-generator could provide a higher efficiency and commercially attractive replacement for induction motors as well as other types, and would reduce electricity consumption operating costs. 
       SUMMARY OF THE INVENTION  
       [0007]    The invention provides a lower cost, high efficiency motor-generator. The motor-generator achieves both high efficiency and lower costs by utilizing an air core topology with a special low cost armature winding construction that is more easily and rapidly wound and assembled. 
         [0008]    The motor-generator has a rotor that rotates about an axis of rotation, and a stator that is stationary and magnetically interacts with the rotor. The rotor is comprised of two spaced apart rotor portions that define therebetween an armature airgap in which a stationary aircore armature is located. Magnetic poles on the rotor portions drive magnetic flux across the armature airgap and through the armature. The rotor portions provide a low reluctance, high efficiency magnetic flux path for linking the magnetic flux traversing back and forth across the air gap. The rotor portions are preferably constructed of ferromagnetic material, such as steel, which reduces the circumferential reluctance of the flux path and increases the total flux density, power capability, and efficiency. 
         [0009]    It should be noted that in the past, some motor-generators for unique applications have been construction using a single rotor surface acting on a single side of an air core armature. Such designs provide the benefits of simplified assembly and can provide acceptable performance for some applications, especially high speed ones where only a low magnetic flux density is needed for production of high power due to the high operating speed. However, for maximized power capability and efficiency per cost in accordance with this invention, a split rotor is used, enclosing an air core armature on both sides. This is especially applicable for low speed and very cost sensitive applications typical of induction motors. 
         [0010]    The stator of the motor-generator according to this invention has an air core armature, located within the magnetic air gap. The air core armature has windings that are wound onto a winding form designed to afford simplified winding of the armature, and a simple and speedy winding process for applying the windings onto the form. The form is made of a substantially nonmagnetic and low electrical conductivity material, and has a free end that extends inside the rotor and a support end that attaches to the stationary portion of the motor-generator. The form has a thin backing portion and thicker raised portions in the direction of the magnetic flux through the magnetic airgap. Wire windings are wound onto the form by inserting wire to lie between the raised portions, wherein the form provides position location and support for the wire windings during the winding process, and subsequent transmission or reaction of the electromagnetically induced torque on the windings to the stationary portion of the motor-generator through the support end of the form, and prevents the windings from contacting the rotor portions during rotation of the rotor. Use of raised portions on the form in the active length portions can circumferentially hold the wires. 
         [0011]    One consideration in the selection of the material properties of the form material is to preclude development of significant eddy current losses in the form from the rotation of the magnetic poles and currents in the windings to avoid wasted power and generation of waste heat internally in the motor. In motors used for high efficiency, the losses arising from the form are preferably less than 1% and more preferably less than 0.1%. Low electrical conductivity materials for the form have a resistivity of greater than 1×10 −6  ohm-m and more preferably greater than 0.001 ohm-m. A good class of materials are plastics having sufficient strength and temperature capability to carry the magnetically induced forces acting on the windings. The addition of a very thin film, layer or foil of conducting material on the form surface for heat reflection or shielding would not change the resistivity of the majority of the form material in the airgap or develop significant eddy current losses. As such, this conducting film would constitute another aspect of the invention. 
         [0012]    An additional preferred property of the form material is that it be substantially nonmagnetic. The goal is to prevent loss of performance from significant circumferential direction flux leaking through the form. Preferably the form material has a relative magnetic permeability of less than 100 and more preferably of less than 3. Again, plastics are one such class of good materials. 
         [0013]    Unlike many conventional air core motor-generators that are made by winding the armature coils separately, then assembling them together around the circumference of the armature and compressing the windings to minimum thickness for potting, the armature according to this invention is constructed by manufacturing the form first and then winding the windings directly onto the winding form, simplifying construction. The form then allows easy mounting of the armature and windings directly into the motor-generator. 
         [0014]    Use of the winding form in accordance with the invention has several significant drawbacks that generally make it considered an unattractive approach in the art for armature construction For a given amount of expensive magnet material in a motor-generator, the power capability and efficiency of air core motor-generators is directly related to the winding density of the armature. For this reason designers have typically sought to maximize the winding density of armatures and minimize the required airgap thickness. Smaller magnetic airgaps allow significantly greater magnetic flux density and hence the greater efficiency and power capability. Winding forms for armature windings occupy substantial space in the magnetic airgap that could other wise be filled with additional windings if manufactured by conventional techniques. The space occupied by the backing of the form increases the armature thickness in the direction of the magnetic flux and the raised portions reduces the number of windings circumferentially around the armature compared to non-form constructions. Compounding the lower winding density from a single armature form is that large armatures in accordance with the invention may utilize multiple forms that are assembled or stacked together in the magnetic airgap. The use of stacked armature form construction can have each phase wound in a single layer, and results in larger winding end turn lengths, and greater resistive losses. 
         [0015]    Despite the disadvantages resulting from reduced armature winding density, motor-generators in accordance with the invention have surprisingly been found to be a very attractive construction and manufacturing technique. Although reduced winding density does require some increased amount of magnet material and magnet costs to achieve the same high power level and efficiency as other motor-generators, this increased magnet cost has been found to be much less than the manufacturing cost savings achieved from the simplified armature manufacturing and reduced total manufacturing time. Any efficiency reduction has also been found to be made up for by the other features for the windings, as will be explained, that are offered by the form winding process. 
         [0016]    The form on which the windings are wound is preferably made of a substantially nonmagnetic and low electrical conductivity material, such as nylon, Noryl, Ultem, ABS or a fiber-reinforced plastic. It may also be made of thermally conducting polymers, ceramics such as Macor or other suitable materials that are easily and inexpensively produced, for example by machining, pressing, molding or forming, and afford adequate strength to support the forces acting on the windings in the particular size of motor or generator. Thermally conductive polymers are considered to be those with a thermal conductivity of greater than 1 W/mK. These materials can increase the thermal energy transport from the windings through the forms and to the convection cooling in the armature airgap. A preferred manufacturing technique for the form is injection molding because it is rapid and low cost and allows formation of complex channel construction easily. 
         [0017]    In one embodiment, the form has channels on the surface for placement of the wire. The channels provide structural support for the windings and preferably hold the windings in place on the form while winding. Particularly for large size, or high speed or high pole count electrical motor-generators, eddy current losses can become significant in the actual winding conductors from the rotating rotor flux passing through them. To reduce these losses to a low level, the windings are preferably wound using Litz wire, or wire comprised of multiple individually insulated strands. The stranded wire facilitates easy winding of large wires due to the reduced bending stiffness. However, because Litz wire windings are up to 100 times or more less stiff and rigid as solid wire windings, they do not stay in the desired place unless constrained in some fashion. For this reason, winding air core armatures with Litz wire by previous methods would be difficult. The wound coils or windings would tend to spring out of the desired winding pattern, which would make construction difficult. However, winding Litz wire windings onto the form in accordance with the invention is easily accomplished because, after the wires are pressed into the channels, the channels hold the wires in place. In a preferred embodiment of the invention, the channels clamp the wire and hold it tight. Other configurations of loss mitigating windings can be constructed from parallel-connected square or ribbon wires that are electrically insulated between each other along their length in the active length region. Again, the high flexibility of these windings takes substantial advantage of the forms clamping during the winding process. The efficiency gains from the windings can overcome the efficiency losses resulting from lower winding density as a result of the form. 
         [0018]    Multiple wires may be laid into a single wind channel, or each wire may be laid in an individual channel. Individual channels provide greater support since a group of wires in a single channel may bow outward in the center of the channel and come loose, but individual channels may further reduce the winding density depending on their construction and location. Individual channels are formed when the space between adjacent raised portions on the low electrical conductivity form has a width approximately equal to the width of one wire. For easiest winding and manufacturing, the armature is constructed preferably with only one wire per channel. When the wire is inserted in to the channel, it is squeezed across its&#39; diametral cross-section to hold it in place. For Litz wire windings, the wire can become slightly compressed as it is held in place. 
         [0019]    The channels or surface features of the form allow for rapid and reliable armature winding. Unlike other manufacturing methods, the winding wires need not be threaded through multiple holes or openings. Instead, the windings are simply pressed or snapped into the correct channels on the form surface. The winding process may be automated or simply and reliably done by hand by putting the windings in place in the channels with a hand roller. The forms may be machined with the channels or surface features or, for high volume production, are injection molded. 
         [0020]    A further benefit of the invention is that the armatures can be wound with high precision. Unlike hand winding with other constructions, the forms militate against winding mistakes and error by having surface features that facilitate winding by providing positive position location for wires during the winding process. Incorrect positioning of coils in a multiple phase motor-generator, as can occur with wound, arranged, assembled and potted construction, is precluded with the inventive winding process. No phase voltage or angle imbalance occurs, further improving performance. 
         [0021]    The motor-generator can also facilitate easy replacement of windings when required. Unlike conventional slot wound armature windings that are epoxied into the motor laminations and housing, the armature, or even a part of the armature, in accordance with the invention can unbolted and removed and replaced, if needed. 
         [0022]    The channels or surface wire holding features of the forms may be constructed by several means depending on the desired winding pattern. The windings may comprise coils or more preferably are constructed in a serpentine path around the circumference. The channels can provide a full channel that receives the wire completely over the entire winding surface of the form. This method provides the greatest structural support and also provides insulation between adjacent turns of the windings. Non-sheathed Litz wire, which is more compressible, can be utilized if desired. The use of a complete channel however can reduce the possible winding density, especially for axial gap motor-generators. To prevent further reduction in the potential winding density and to reduce costs for machined forms, the channel-defining raised portions can extend from the form backing only in selected portions, such as in the active length region, a portion of the active length region or only near the ends of the active lengths. An incomplete channel over the form surface can slightly increase the difficulty of winding, but this increase is generally small compared to the benefits gained in winding density. 
         [0023]    Many motor-generators utilize a three-phase construction. In accordance with the invention, all three phases may be wound onto a single form if end turn channels are omitted. Alternatively, in a preferred embodiment, each phase is wound identically into separate forms. The forms are then axially stacked together, shifted in angular orientation to form a multi-phase axial gap motor-generator. For radial gap motor-generators, the form or forms would be in the form of a thin walled cylinder with radially extending channels. In most cases, axial gap construction is the easiest and most cost effective and therefore is the most preferred when allowable. An adhesive such as epoxy can be applied to the wound forms to increase the structural integrity of the armature in the stacked configuration. Vacuum resin impregnation can also be performed on the assembled armature, if desired for greater structural integrity, however this may increase cost and manufacturing time and may be unnecessary in many cases. 
         [0024]    To allow for stacking or assembly of multiple armature forms, the backing portion of the form is preferably omitted in regions where overlapping of winding wires on a single form occurs. Such overlapping is typically the result of the input and output connection wires to the armature. Multiple forms preferably omit the backing from the same regions so that the overlapping does not prevent orderly stacking. 
         [0025]    In many cases, the armature construction uses multiple wires per phase and hence multiple wires electrically in series but mechanically in parallel serpentines around the diameter. Multiple wires may be run at one time with one pass around the diameter to form the windings. The wires are then connected together appropriately such that successive turns are electrically in series. In a preferred embodiment, the windings are preferably wound by winding one wire, or less than the total number of wires, several turns around the diameter. This method eliminates the multiple intra-winding connections or soldier joints and speeds manufacturing. This type of winding technique is made possible with the use of individual winding channels so that the turns of each pass around the diameter are positioned and held in place in the appropriate locations. 
         [0026]    Although the motor-generator can be utilized for many applications using motors and generators, it is particularly well suited for applications with continuous or near continuous operation. One such preferred application is for fans or blowers used for providing airflow. A particular application is in clean rooms. In these and similar applications, the motor-generator may provide substantial electricity cost saving compared to standard induction motors, currently employed. The low cost of the motor-generator compared to other brushless permanent magnet motor-generators enables it to be a commercially viable and cost effective alternative. The benefits of the motor-generator, which also include substantially reduced size, weight and noise along with efficiency, provide benefits for other motor-generator applications as well. 
         [0027]    In a further aspect of the invention, the winding density can be increased and assembly made easier by precompressing the Litz wire prior to winding the armature. When the channels in the form are made to have a rectangular cross-section, the Litz wire can be precompressed to a substantially matching rectangular or near-rectangular cross-section. The precompressing of the wire can be done in the wire manufacturing process through the use of set gap rollers. The rollers provide a high instantaneous pressure on the wire passing through and hence greatly compact the wire bundle. The rectangular wire can then be wound and pressed into the rectangular form channels. Little or no further compaction pressure on the whole armature may be needed. Care should be taken during the winding process to orient the wire properly into the channels and the wire spool is preferably supported for rotation to prevent twisting of the wire in the winding process. 
         [0028]    The low electrical conductivity form with channels can be fabricated several different way, including machining from a plastic blank, forming, or by molding, such as matched die molding or injection molding. Use of injection molding affords low unit costs for high volume production. In a preferred embodiment, the form is constructed by injection molding prior to winding the armature. To facilitate improved heat transfer, the form can be made of a thermally conductive polymer, if desired. Regardless of the form thermal conductivity, the mounting of the form to the stationary motor-generator structure can be providing with means to accommodate thermal expansion of the armature with respect to the stationary housing. Such accommodations can include oversized mounting holes, radial slots or an elastic or flexible connection. Such mounting precludes development of potentially deleterious stresses in the form from operation and relative thermal expansion of the form. 
         [0029]    The multi-phase motor-generator can be wound with one phase on each form, with the forms superimposed and angularly off-set to provide the desire number of phases, or can be wound with multiple phases on a single form. Each method has desirable properties for different applications and designs. For instance, in a radial gap motor-generator, using a single form can have advantages of simplicity by avoiding the nesting together of several individual cylinders with different radii for the different phases. When more than one phase is wound onto a single form, end turn overlapping can be used to allow the active length region to be thinner in the direction of the magnetic airgap than the end turns, for increased performance. 
         [0030]    Several different winding patterns can be constructed, again depending on the design of the machine, operating parameters and cost. In some cases, it is possible to increase the efficiency of the motor-generator by the spacing of the windings. In an additional embodiment of the invention, the windings of a phase are spaced more closely together circumferentially in the active length region than if circumferentially uniformly distributed about the pole pitch. This configuration places more windings near the center of a pole for higher voltage inducement and less windings located in the region of inter-magnet leakage. 
         [0031]    For radial gap motor-generators, the form can comprise a tube with radially raised channels and a thin backing portion. To facilitate easier assembly of the rotor and armature, the end turns of both ends of the armature preferably extend radially in opposite directions. This allows the armature to be inserted into the rotor and the magnets to be installed. For simplicity, the backing portion of the tube form can be made to allow the end turns of the windings to extend radially inward at the free end of the air core armature. One way to do this is to omit the backing form of the form at the free end end-turns. The rotor and stator can be assembled by attaching the magnets to the outer rotor tube, sliding the armature axially into the rotor, and then axially sliding the magnets onto the inner rotor portion. Alternatively, the magnets can be attached to the inner rotor portion before attaching it to the rotor, and then, after the armature has been inserted into the outer rotor tube, the magnet-loaded inner rotor portion can be inserted inside the armature and attached to the rotor. 
         [0032]    Another aspect of radial gap motor-generators is a armature winding compaction method. The windings can be wound on the channels on the outer surface of the form. A tension wound wrapping, such as a filament, tape, etc., can be wound circumferentially around the form. The wrapping compacts the windings into the channels for an accurate dimensioned and high-density air core armature. 
         [0033]    In yet a further embodiment of the invention, axial gap motor-generators can be constructed to provide easy winding, high dielectric strength, high density, and easy assembly. The windings are wound with only one phase per form and the forms are axially stacked together to construct a multiple phase armature. To allow flat stacking and prevent overlapping of the windings, the forms preferably have axial holes for exiting of the winding leads from the forms. In a preferred version, the windings are wound as a serpentine path and a multiple wire serpentine is formed by winding multiple times circumferentially around the form. In this construction, the coil of a single phase has only two wires and only one end needs to use an axial hole in the forms for exiting to prevent overlapping. Holes for both ends can also be used for simplicity to allow all the wires to exit the armature from one axial side. A benefit of this construction is that the windings can be fully supported throughout the whole armature. Additionally, the channels can dielectrically isolate all the windings. This can allow the use of un-served Litz wire, which is more flexible and more easily compacted. 
         [0034]    Features of the invention can be used in the both small and large motors and generators. For very large motor-generators, such as large turbine generators, the armature form can be constructed from multiple circumferential sections that are easier to manufacture. The sections can then be assembled and electrically connected together to form a large armature that would otherwise not be practical or economical to construct as a single piece. Applications for the motor-generators include stationary and transportation, wherever the advantages of high efficiency and high performance at reduced cost are desirable, including industrial process motors, manufacturing, hybrid electric vehicles and ship propulsion. 
     
    
     
       DESCRIPTION OF THE DRAWINGS  
         [0035]    The invention and its many attendant features and benefits will become better understood upon reading the following detailed description of the preferred embodiments in conjunction with the drawings, wherein: 
           [0036]      FIG. 1A  is a schematic partial elevation of a brushless permanent magnet motor-generator with single sided rotor. 
           [0037]      FIG. 1B  is a schematic partial elevation of a brushless permanent magnet motor-generator with single sided rotor and ferromagnetic back iron. 
           [0038]      FIG. 1C  is a schematic partial elevation of a brushless permanent magnet motor-generator with double sided rotor and ferromagnetic back irons. 
           [0039]      FIG. 1D  is a schematic partial elevation of a brushless permanent magnet motor-generator with double sided rotor with magnets on both sides and ferromagnetic back irons. 
           [0040]      FIG. 2  is a graph comparing horsepower capability of the configurations  1 A- 1 D. 
           [0041]      FIG. 3A  is a schematic elevation of an axial gap brushless permanent magnet motor-generator in accordance with the invention. 
           [0042]      FIG. 3B  is a schematic plan view of one rotor half of the brushless motor-generator in  FIG. 3A  in accordance with the invention. 
           [0043]      FIG. 4  is a schematic partial elevation of a radial view of a circumferential section of a brushless permanent magnet motor-generator in accordance with the invention. 
           [0044]      FIG. 5  is a schematic partial radial elevation along a circumferential section of another configuration brushless permanent magnet motor-generator in accordance with the invention. 
           [0045]      FIG. 6  is a schematic partial elevation, looking in the circumferential direction at a radial section, of a brushless permanent magnet motor-generator in accordance with the invention. 
           [0046]      FIG. 7  is a schematic partial elevation of a circumferential view of a radial section of an alternate configuration brushless permanent magnet motor-generator in accordance with the invention. 
           [0047]      FIG. 8  is a schematic partial elevation of a circumferential view of a radial section of a second alternate configuration brushless permanent magnet motor-generator in accordance with the invention. 
           [0048]      FIG. 9  is a schematic partial elevation of a radial view of a circumferential section of a second alternate configuration brushless permanent magnet motor-generator in accordance with the invention. 
           [0049]      FIG. 10A  is a schematic drawing of the phase stacking of the armature of brushless motor-generator in  FIG. 9 . 
           [0050]      FIG. 10B  is a schematic drawing of the phase stacking of the armature of brushless motor-generator in  FIG. 9 . 
           [0051]      FIG. 11  is a schematic plan view illustrating a process to manufacture the armature of brushless motor-generators in accordance with the invention. 
           [0052]      FIG. 12  is a graph comparing manufacturing costs for separately wound motor and a brushless motor-generator in accordance with the invention. 
           [0053]      FIG. 13  is a schematic plan view of an armature winding for a brushless motor-generator in accordance with the invention. 
           [0054]      FIG. 14  is a schematic plan view of an alternate configuration armature winding for a brushless motor-generator in accordance with the invention. 
           [0055]      FIG. 15  is a schematic partial plan view of a second alternate configuration armature winding for a brushless motor-generator in accordance with the invention. 
           [0056]      FIG. 16  is a schematic drawing of a fan with brushless motor-generator in accordance with the invention. 
           [0057]      FIG. 17  is a schematic drawing of an alternate configuration fan with brushless motor-generator in accordance with the invention. 
           [0058]      FIG. 18  is a graph illustrating the electricity cost savings from use of a motor in accordance with invention compared to standard induction motors. 
           [0059]      FIG. 19A  is a schematic side elevation of a radial gap brushless permanent magnet motor-generator in accordance with the invention. 
           [0060]      FIG. 19B  is a schematic end elevation of the brushless motor-generator in  FIG. 19A . 
           [0061]      FIG. 20  is a schematic plan view of a third alternate configuration armature winding for an axial gap brushless motor generator in accordance with the invention. 
           [0062]      FIG. 21  is a developed schematic elevation of the phase stacking of the armature winding of  FIG. 20  to produce an armature for a three-phase motor-generator in accordance with the invention. 
           [0063]      FIG. 22  is a schematic plan view of a fourth alternate configuration armature winding for a brushless motor generator in accordance with the invention. 
           [0064]      FIG. 23  is a schematic drawing of the phase stacking of the armature winding of  FIG. 21  to produce an armature for a three-phase motor-generator in accordance with the invention. 
           [0065]      FIG. 24  is a schematic cross-sectional side elevation of a second alternate configuration brushless permanent magnet motor-generator in accordance with the invention. 
           [0066]      FIG. 25  is a schematic developed plan view of the armature form of the brushless motor generator of  FIG. 24 . 
           [0067]      FIG. 26  is a schematic drawing of a portion of the assembled armature of the brushless motor-generator of  FIG. 24 . 
           [0068]      FIG. 27  is a schematic drawing of a large armature form assembled from multiple circumferential section forms in accordance with the invention. 
           [0069]      FIG. 28  is block flow diagram illustrating the steps of a process for construction of an air core armature for use in a brushless motor-generator in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0070]    Turning to the drawings, wherein like reference characters designate identical or corresponding parts, four different magnetic configurations for air core motor-generators are shown in  FIGS. 1A-1D  for comparison.  FIG. 1A  shows a small portion of a motor-generator  30  with rotor  31  and stator  32 . The stator  32  is comprised of electrical armature windings  33  in close proximity to the rotor  31  for conversion of energy. The rotor  41  comprises alternating polarity magnets  34  attached to a lightweight non-magnetic rotor portion  35 . The rotor rotates relative to the stationary stator so, in operation, the magnets  34  pass laterally across the windings  33  in a direction perpendicular to the length of the wire and parallel to the plane in which the wire lies. The magnets  34  drive flux in a loop  36  through the windings  33  and back again around the loop  37  through the rotor portion. 
         [0071]    A motor-generator  40  shown in  FIG. 1B  has a rotor  41  rotating relative to a stationary stator  42 . The stator  42  includes electrical armature windings  43  in close proximity to the rotor  41  for conversion of energy. The rotor  41  comprises alternating polarity magnets  44  attached to a ferromagnetic rotor portion  45 . As the rotor rotates and carries the magnets across the armature windings  43 , the magnets  44  drive flux in a loop  46  through the windings  43  and back again through the loop  47  through the low reluctance steel rotor portion  45 . 
         [0072]      FIG. 1C  shows a motor-generator  50  with rotor  51  and stator  52 . The stator  52  is comprised of electrical armature-windings  53  in close proximity to the rotor  51  for conversion of energy. The rotor  51  comprises alternating polarity magnets  54  attached to a ferromagnetic rotor portion  55  and a spaced apart ferromagnetic rotor portion  56  acting as a back iron. The rotating array of magnets  54  drive flux in a loop  57  through the windings  53  and low reluctance steel portion  56 , and through the loop  58  through the low reluctance steel rotor portion  55 . 
         [0073]      FIG. 1D  shows a motor-generator  60  with rotor  61  and stator  62 . The stator  62  is comprised of electrical armature windings  63  in close proximity to the rotor  61  for conversion of energy. The rotor  61  comprises alternating polarity magnets  64  and  65  attached to ferromagnetic rotor portions  66 ,  67 , which rotate together. The rotating array of magnets  64 ,  65  drive flux in a loops  68 ,  69  through the windings  63  and the low reluctance steel portions  66 ,  67 . 
         [0074]    A comparison of the power capacity of the different magnetic configurations  1 A- 1 D is shown in  FIG. 2 . Each design uses an equivalent amount of magnet material and the power ratings are calculated based on achieving 98% efficiency. Although configurations  1 A and  1 B allow simple assembly by having the rotor on only one side of the armature, the power capability is lower at 7.75 Hp and 11.75 Hp. This power per cost is acceptable for some applications but not for competition with low cost induction motors. The configuration of  FIG. 1C  provides steel back irons for efficient circumferential flux paths on both sides of the armature. This provides a substantial improvement, more than doubling the power capability. Use of ferromagnetic rotor portions on both sides of the armature is therefore preferred for use with electrical machines in accordance with the invention.  FIG. 2  shows that the configuration of  FIG. 1D  can afford yet more improvement by placing half of the magnet on each ferromagnetic rotor. This configuration causes more flux to jump across the magnetic airgap and through the armature instead of leaking circumferentially. As a result, the power capability increases from 23.25 Hp to 40 Hp, providing the highest power rotor magnetic design configuration. In low inertia requirement applications, it is also contemplated that a Halbach motor magnet array with non-magnetic rotor portions, could alternatively be used. However, this configuration results in significantly higher costs that are typically not compatible for most commercial applications. 
         [0075]    Turning now to  FIGS. 3A and 3B , a brushless motor-generator  80  includes a rotor  81  mounted for rotation relative to a stator  82 . The rotor  81  has two steel rotor portions  83  and  84  that are connected together with a high reluctance connection tube  94 . The tube  94  and rotor portions  83 ,  84  may include fan air moving features to provide air flow in the motor-generator  80  for cooling, if desired. As shown in  FIG. 3B , multiple circumferentially alternating permanent magnets  85 ,  86  are attached to each rotor portion  83 ,  84  to drive magnetic flux across a magnetic airgap  87  defined between axially facing surfaces of the two rotor portions  83 ,  84 . Located in the magnetic airgap  87  is a special air core armature  88  that has multiple windings for conversion of power. Several configurations of the armature  88  are described in more detail below. The rotor portions  84 ,  83  have shafts  89  and  90  that are journalled in bearings  91 ,  92 . The bearings  91 ,  92  are supported by housing end plates  95 ,  96  that are held in spaced-apart position by an outer tube  97 . Bolts  98  hold the end plates  95 ,  96  together and also support the air core armature  88  within the magnetic airgap  87 . The axial plan view of half of the rotor  81 , shown in  FIG. 3B , shows the rotor portion  84  with multiple magnets  85  that drive flux back and forth across the magnetic airgap  87  and circumferentially through flux paths in the rotor portions  83 ,  84 , as noted in  FIG. 1D . 
         [0076]    Prior air core motor-generator designs have attempted to maximize the winding density of the air core armature. In motor-generators of similar design, increasing the volume of windings per magnetic airgap thickness increases the efficiency and power capability per unit amount of magnet material. Although air core armatures made in accordance with this invention achieve a reasonably high winding density, another goal is make the armature manufacturing several times faster, easier and more cost effective. 
         [0077]    A radial view of a circumferential section of a brushless motor-generator  110  of the type shown in  FIG. 3  is shown in  FIG. 4 . The motor-generator  110  includes a rotor  111  mounted for rotation about a vertical axis extending behind the plane of the figure, and an armature  117 , in the position of the armature  88  in  FIG. 3A . The rotor  111 , like the rotor  81  in  FIG. 3A , has spaced apart ferromagnetic rotor portions  112 ,  113  onto which are attached multiple alternating polarity permanent magnets  114 ,  115 . An axial airgap  116  is defined between opposing faces of the magnets  114 ,  115 . The magnets drive flux  120  across the airgap  116  and through the armature  117 , which is fixed in the airgap  116 . The magnetic flux passes circumferentially through loops  121 ,  122  in the ferromagnetic rotor portions  112 ,  113  to complete the flux loop. The armature  117  is constructed of a form  118  made of a substantially nonmagnetic and low electrical conductivity material, such as nylon or other plastic or ceramic, selected to provide the strength and temperature capability needed for the particular size and application of motor-generator. It may preferably be thermal conducting to facilitate heat transfer out of the armature  117 . The form  118  has a thin backing  124  and thicker raised portions  125 . The spaces between the raised portions  125  form channels  126 . 
         [0078]    The armature is fabricated by winding the windings  119  directly into the spaces or channels  126  in the form  117 . In a preferred embodiment, the width of the channels  126  is made to tightly fit the windings  119  such that the winding process is completed by simply pushing the windings  119  into the channels  126 . 
         [0079]    As illustrated, the armature construction does not maximize the winding density, as is also the case with previous methods. The form backing  124  and raised portions  125  occupy some space that might otherwise be able to hold more windings in the absence of the form  117 . However, the armature manufacturing process is made much simpler, faster and more reliable. No wires need be threaded and pulled through multiple openings during winding, no specialized holding and potting equipment is required and potting resins can be eliminated or minimized, although potting resins may be sometimes be used to hold the windings in place after winding. Unlike armature fabrication, wherein coils are separately wound and later assembled and potted together, armatures in accordance with this invention can be manufactured in a reduced number of simpler steps. Likewise, the use of the form  117  and similar such forms disclosed herein allows use of Litz wire. Litz wire is wire comprised of multiple individually insulated strands  123 , illustrated schematically in  FIG. 4A . The insulation between strands precludes development of significant eddy current losses in the windings. Use of Litz wire becomes significantly important for higher power motor-generators where the wire size and armature size become large. It is especially advantageous for use in motor-generators designed to replace large induction motors. Winding with Litz wire provides the additional benefit of easier wire bending in the wire process. However, unlike solid wire conductors, the Litz wire does not hold its shape to which it is bent. Therefore, winding air core armatures with Litz wire by previous separately wound methods is difficult. The windings or coils do not maintain shape after winding and while being assembled. The process disclosed herein overcomes this deficiency because the channels hold the wires, and the process is completed quickly and simply. 
         [0080]    Another configuration brushless motor-generator  130 , shown in  FIG. 5 , is similar to the configuration shown in  FIG. 4  in that it includes a fixed stator, including an armature  137 , and a rotor  131  having two spaced apart co-rotating steel rotor portions  132 ,  133  with attached magnets  134 ,  135 . The armature  137  is fixed in an axial airgap  136  defined between opposing faces of magnets  134 ,  135 . The magnets  134 ,  135  drive magnetic flux  143  across the armature airgap  136  and through the armature  137  in the airgap  136 , and through circumferential paths  132 ,  133  in the rotor  131 , as in  FIGS. 3A and 4 . 
         [0081]    The armature  137  includes a substantially nonmagnetic and low electrical conductivity form  138  with a thin backing  139  and channels  141  between raised portions  140 . In this configuration, the winding is further facilitated and the windings  142  are held more securely in the channels  141  which are of a width approximately equal to the width of the wire of the windings  142  such that each wire is placed in a separate channel. When multiple wires are run in a single channel, as shown in  FIG. 4 , the group of wires can tend to bow upward and loose shape and support from the channel, and hence come out of the channel, although this tendency can be counteracted by covering the wire-filled channels with a bonded on cover plate or another form, as shown in  FIGS. 9 and 21 . Otherwise, uncovered wires in a multi-wire channel can make winding more difficult, especially when many wires are required for the armature winding. The configuration of  FIG. 5  eliminates that problem and also provides other benefits to the winding pattern such as facilitating the winding of multiple wires electrically in series, as will be described in more detail later. When one wire occupies a single channel, winding can become easier and the wires have a greater tendency to stay in place, whether interference fit with the channel or loose. The wires  142  are illustrated as having a diameter equal to the width of the channels  142 , but they can also be sized with a diameter greater than the width of the channel  142  so that they must be pressed into the channel, as described below in connection with  FIG. 11 , and fill the channel completely, as shown in  FIG. 21 . Alternatively, the wires can be preformed with a square or rectangular cross-section so they fit snuggly into the channel. Also, for manufacturing convenience, the wires can be preformed with a slightly tapering profile so they can be inserted easily into the channel and then fill the channel completely when pressed therein. 
         [0082]    As shown in  FIG. 6 , one side of a brushless motor-generator  150  has a rotor  151  mounted for rotation about a vertical axis, and a stationary stator  152 . The rotor  151  comprises two spaced apart co-rotating steel portions  153 ,  154  with attached magnets  155 ,  156  that drive magnetic flux  160  across an armature magnetic airgap  157 , as illustrated in  FIGS. 3A-5 . The stator  152  is comprised of a substantially nonmagnetic and low electrical conductivity form  158  that has a thin backing  160  and thicker raised portions  161  that form channels on the surface, as illustrated in  FIG. 5 . Windings  159  are wound directly onto the form  158  in the channels between the raised portions  161 . As shown in this configuration, the windings  159  are completely contained within the channels between the raised portions  161  and are supported for all portions of the windings  159 , including the end turns, as further illustrated in  FIG. 22 . This provides high structural support and ease of winding, but may slightly increase the cost of the form  158 . 
         [0083]    In some applications of motors in accordance with this invention, the form need not support the windings at all portions of the winding on the form. It may be desirable to reduce the form cost or simplify manufacturing by supporting the windings only in selected places, although it is preferable to provide enough support to hold the windings in place and ensure easy winding. Leaving a portion of the winding on the form unsupported by channels can also allow for cooling of the windings, or potting after winding. Potting with high thermal conductivity material may be beneficial for a specific motor-generator application. A brushless motor-generator  170  having a winding form  177  with channels supporting the windings only in the active lengths is shown in  FIG. 7 . “Active lengths” as used herein means the lengths of the armature windings that interact with flux in the airgap to produce torque in a motor, or electrical power in a generator. The motor-generator  170  has a rotor  171  supported for rotation about a vertical axis, as shown on the right side of the drawing in  FIGS. 6-8 , and a stator  172 . The rotor  171  comprises two spaced apart co-rotating steel portions  173 ,  174  with magnets  175 ,  176  that drive magnetic flux  180  across the armature magnetic airgap  178 , as illustrated in  FIGS. 3A-5 . The stator  172  includes a substantially nonmagnetic and low electrical conductivity form  177  that has a thin backing  181  and thicker raised portions  182  that form channels on the surface. Windings  183  are wound directly onto the form  177  in the channels. In this configuration, the raised portions  182  bounding the sides of the channels hold only the active length portion of the windings. The end turns of the windings are free and can be exposed to air-cooling if desired. Without end turn channels, windings of multiple phases, angularly displaced by several channels per phase can also be wound on to a single form. This uses a number of wires divisible by three and is a more complicated winding and than using individual forms, but can reduce the required airgap thickness for multiple phases. 
         [0084]    Another configuration of a brushless motor-generator, shown in  FIG. 8 , likewise has a rotor  191 , mounted for rotation about a vertical axis, and a stator  192 . The rotor  191  comprises two spaced apart co-rotating steel portions  195 ,  196  with magnets  193 ,  194  that drive magnetic flux  199  across an armature magnetic airgap  198 , as illustrated in  FIGS. 3A-5 . The stator  192  includes a substantially nonmagnetic and low electrical conductivity form  197  that has a thin backing  201  and thicker raised portions  202  that form channels on the surface. Armature windings  203  are wound directly into the channels on the form  197 . As shown in this configuration, the channels  202  only hold the wires  203  near the ends of the active lengths or magnetic airgap  198 . This configuration provides less structural support but still facilitates winding by holding the windings in place during the winding process. Such a construction could be utilized when potting of the wires with epoxy, if desired. Thermally conductive epoxy can be useful to minimize heat build-up in the armature. 
         [0085]    One of the benefits of the use of armature forms in accordance with this invention is the ability to simply and rapidly construct air core armatures. Although it is possible to wind multiple phases on a single form in some cases, this makes winding more difficult, and it makes the holding of the windings in the channels more difficult as well. To overcome these deficiencies and to make motor-generator manufacturing easier, multiple phases can be wound using multiple forms, such that each phase is wound on a separate form, and the separately-wounded forms and phases are then stacked together to form a multiphase armature, as shown in a brushless motor-generator  210  in  FIG. 9 . In this way, only one configuration of winding is needed. Each form is identical and is rotationally offset  120  degrees when stacking a three-phase armature. The number of wires per phase can then be chosen independently to provide the optimum power capability and efficiency for the motor-generator. This construction has the disadvantage of requiring multiple forms and an lower armature winding density. Such a construction would not typically be considered desirable However, when the cost savings in the armature manufacturing are considered, it has been found to be an advantageous construction. 
         [0086]    The motor-generator  210  is comprised of a rotor  211 , mounted for rotation about a vertical axis (out of the plane of  FIG. 9 ), and a stationary stator  212 . The rotor  211  has two spaced apart co-rotating steel rotor portions  215 ,  216  with magnets  213 ,  214  that drive magnetic flux  224  across an armature magnetic airgap  227  and in circumferential paths  225 ,  226  through the rotor portions  215 ,  216 , as in  FIGS. 3A-5 . The stator  212  is comprised of a triple stack of armature forms  217 ,  218 ,  219 . Each form contains a thin backing  220  and raised portions  221  that form channels  222 . Windings  223  are wound into the channels  222 , as previously described for  FIGS. 4-8 , and then the forms  217 ,  218 ,  219  are stacked and attached together at the proper angular orientation to each other for correct phasing to form a complete armature for the stator  212 . 
         [0087]    The phase stacking of the armature of the brushless motor-generator in  FIG. 9  is shown in  FIGS. 10A and 10B . The armature windings stacking  229  is comprised of three phases  230 ,  231 ,  232  comprising windings  233 ,  236 ,  239 . Each winding  233 ,  236 ,  239  has active lengths  235 ,  238 ,  241  located in the magnetic airgap for power conversion, and end turns  234 ,  237 ,  240  (and also end turns at the other end of the active lengths) that traverse circumferentially. The windings  233 ,  236 ,  239  are each circumferentially offset 120 degrees for production of three-phase power. The offset angle would be adjusted accordingly for other numbers of phases. 
         [0088]    A process to manufacture the armature of brushless motor-generators in accordance with the invention is illustrated in simplified form in  FIG. 11 . The winding process of the armature forms could be automated if desired. However, one benefit of the invention is that the armature can be fabricated easily by hand as well. This allows armature manufacturing in relatively low volume to be cost effective, or manufacturing by hand in geographical regions where labor cost is low, so no significant capital equipment investment is required, a substantial contrast from other air core armatures. The process illustrated in  FIG. 11  for manufacturing an armature  251  comprises pushing Litz wire  253  into channels  255  on the low electrical conductivity form  252 , such as the form shown in more detail in  FIG. 20 . The wire  253  can be directly fed from its spool  254  and pressed into the channels in the form  252  with a roller  256 . The compression of the wire into the channel can be such that, after the wire has been pressed into the channel, it is squeezed or clamped by the sides of the channel, holding the wire firmly in place. Depending on the required windings for a given motor-generator, a single wire  253  can be run around the form  252  one, or more typically, multiple times. Alternatively, multiple spools can feed wire into multiple channels at the same time, running wires physically in parallel. Such a configuration minimizes winding time, however some addition time may be required for electrical connections between the multiple wires. 
         [0089]    After winding the forms, they may be assembled into the motors and attached in place within the rotor. Alternatively, if potting is desired, after winding they may be removed to a potting station where the potting operations are performed. Separation of the winding operation and the potting operations can be beneficial in manufacturing operations. Potting of multiple forms at once can be done by applying the potting compound to multiple wound forms, stacking multiple armature stacks together with a release film between the stacks, and enclosing the entire stack in a vacuum bag for evacuation of any air bubbles in the channels. Use of a heated chamber or autoclave can increase throughput and manufacturing consistency. The top form in a multi-form stack may be covered with a thin sheet, such as fiberglass or the like, to further ensure that the windings stay in place during operation of the motor-generator. 
         [0090]    As noted previously, air core motor-generator designers have typically sought to maximize armature winding density in order to maximize the power capability and efficiency for a given amount of expensive permanent magnet material, with manufacturing ease a secondary concern. This allowed the greatest cost effectiveness for the rotor magnets. Contrary to this conventional approach, a benefit of this invention is to enhance armature manufacturing speed, reliability, and economy. Although achieving a high winding density is desirable, improved manufacturing ease and economy have been found to be achievable, so that the total manufacturing cost is lower. When looking at the manufacturing cost breakdown for moderate volume, the cost savings afforded by manufacturing motors in accordance with the invention become apparent. The manufacturing cost breakdown for 40 Hp motors of equivalent efficiency are shown in  FIG. 12 . Other size motors, such as much smaller motors, and other manufacturing volumes, such a very high volume, would have a different comparison. The chart compares a conventional separately wound motor wherein the coils are separately wound and later assembled and potted, with a motor made in accordance with the invention utilizing a form wound armature with wire channels. What can be seen is that the new motor requires approximately 20% higher magnet costs due to a lower winding density from the inclusion of the form backing and space from the raised channel portions. Despite the increased magnet costs, the new motor provides a roughly 40% cost reduction for the total manufacturing cost due to the much easier and more rapid armature manufacturing. Another factor in the armature cost savings is the reduction of capital equipment costs for manufacture, which is particularly advantageous for manufacturing of new larger air core motor-generators such as those greater than several horsepower. The new motors are also particularly well suited to compete with low cost induction motors for widespread industrial applications. 
         [0091]    An armature winding for a brushless motor-generator, such as the one shown in  FIG. 3A , is shown schematically in  FIG. 13 . The armature  260  has a substantially nonmagnetic and low electrical conductivity form  261  with wire channels  262  in the surface. Wires  263  are wound into the channels  262 . The wires  263  and the channels  262  are shown using the same lines in  FIG. 13  for simplicity and clarity of illustration. In the configuration shown, the wires  263  are wound in a serpentine path around the circumference with all the wires in parallel. Such a configuration is applicable for use in high power motor-generators. In the parallel serpentine configuration, no winding overlapping of the wires  263  is required, providing a significant advantage. The winding of the wires  263  into the channels  262  is also very easy and can be completed rapidly, and there is no overlapping of wires. The wires  262  have active lengths  264  that traverse non-circumferentially (radially, as shown in  FIG. 13 ) across the magnetic airgap (shown as the annular zone between the dashed lines  268  and  269 ) and also inner and outer end turns  267  that traverse circumferentially. The end turns  267  are preferably located outside the magnetic airgap in order to maximize the power conversion per amount of magnet material. It is also possible to have some end turns located in the magnetic airgap, if desired, but with the result of less total active length conductor length in the magnetic airgap. The transitions between active lengths  264  and end turns  267  are shown as corners  266 . The corners  266  can be rather sharp or more preferably are rounded, as shown in  FIGS. 20 and 22 , to facilitate keeping the wires  263  in the channels. The end turns may also be completely rounded from one active length to another. 
         [0092]    Another wiring configuration for an armature  270  for a brushless motor-generator, such as the one shown in  FIG. 3A , is shown in  FIG. 14 . The armature  270  includes a substantially nonmagnetic and low electrical conductivity form  271  with multiple surface channels  272 . Wires  273  are wound onto the form  271  and into the channels  272 . The wires  273  and the channels  272  are shown using the same lines in  FIG. 14  for simplicity and clarity of illustration. The windings  273  have active lengths  274  in the magnetic airgap and have radially inner and outer end turns  281  that traverse predominately circumferentially, connecting active lengths together. In the winding configuration shown, the wire  273  is wound in a serpentine pattern in a single wire with multiple passes around the entire circumference of the form  271 , or it may be made with multiple wires with serial electrical connection between adjacent wires. The winding may be done by winding multiple wires  273  with one pass around the circumference and then making electrical connections  276  between adjacent wires  273 . Winding is rapid and easy by this process but more time is required for making the electrical connections and soldiering. Alternatively, if the form  271  has individual channels  272  for each wire, the entire winding process can be completely without the need for any electrical connections. The winding wire  273  is wound into the channels  272  and continues multiple times around the diameter of the form  271 . Each pass results in placing the wire  273  in the next adjacent channel  272 . Again, the winding corners  280  can be relatively sharp or alternatively rounded, as shown in  FIGS. 20 and 22 , for easier winding. 
         [0093]    When winding multiple wires  273  in series, some overlapping of the wires  273  occurs, and the thickness of the armature  270  in the direction of the magnetic flux can increase and could interfere with stacking or assembling of multiple forms. To overcome this potential problem, preferably no backing portion of the form  271  is provided at circumferential positions where overlapping  272  of windings  273  occur on a single form. The form  271  has a section  275  omitted to allow for the overlapping. Because the diameter of the wire  273  is typically greater than the form backing, the same portions of successively stacked forms may also be removed as well. Alternatively, the Litz wire sheath could be removed and the individual wires spread out in a shallow layer, with a dielectric film or tape between the wire layers to insulate. The armature has an input wire connection  279  and output wire connection  278  when completed. 
         [0094]    Although previously shown with serpentine pattern winding, the armature may also utilize a coil pattern winding. Coil pattern winding can allow for continuous serial winding around the circumference of the armature, without connections. However, coil winding has the drawback that it results in more positions of overlapping. An armature winding configuration for a brushless motor-generator using a coil winding pattern is shown in  FIG. 15 . The armature  290  includes a substantially nonmagnetic and low electrical conductivity form  291  with multiple channels  298  on the surface. Wires  293  are wound into the channels  298  in a pattern of multiple coils. The windings have active lengths  295  located in the magnetic airgap, and end turns  296  preferably located outside the magnetic airgap. Each coil  292  has a beginning end  293  and a terminal end  294 . The terminal end of one coil can be wound directly into the beginning end of the next adjacent coil to speed winding manufacturing. Whether the coils  292  are connected in series or in parallel, overlapping  297  will occur. To allow for stacking or assembly of multiple forms, a cut out portion  299  is preferably made in the form  291 . Multiple stacked forms would also have cut outs in the same locations when assembled to facilitate uniform stacking. 
         [0095]    There are many promising applications for the disclosed motor-generator, due in part to its high efficiency at low cost and ability to be readily constructed to large power levels. Besides applications normally utilizing brushless DC motors, such as servomotors, motors in accordance with the invention using a suitable motor controller discussed below, can complete with variable speed induction motor systems. The advantage of higher efficiency allows it to provide considerable electricity cost savings particularly in applications that run continuously or near continuously. One such application is in fans and blowers for air circulation. Fans used in clean rooms, for instance, run continuously and move large volumes of air. They also consume a large amount of electrical power and result in substantial annual electricity costs. Motor-generators in accordance with the invention can replace these induction motor driven fans and provide a significant energy savings without a significant acquisition cost penalty. The low initial cost allows them to be competitive and a commercially attractive solution. An example of such an application is a fan  300  shown in  FIG. 16  with a brushless motor in accordance with the invention. The fan  300  includes an air core permanent magnet motor generator  301  having a specialized armature construction as disclosed herein. The motor  301  drives a fan rotor  302  through connection to the shaft  303 . The motor  301  is mounted to the fan frame housing  304 . The armature wires  305  connect the motor  301  to a motor drive inverter  306 . The inverter provides synchronous AC to the motor-generator  301  to synchronously energize the armature windings to apply torque to the rotor. The inverter  306  can be a sensor feedback type or alternately a sensorless type for motor commutation as described in conjunction with  FIG. 24 . The inverter  306  is connected to supply power through a power connection  307 . 
         [0096]    When moving large volumes of air, larger slow speed fans are typically utilized. In this case, a belt drive is interposed between the motor and a larger fan rotor to drive the fan at a speed lower than the motor speed. Such a configuration of a fan with a brushless motor-generator and speed reducer is shown in  FIG. 17 . The fan  310  includes a brushless air core motor  311  having a specialized armature construction as disclosed herein. The motor  311  drives a large diameter fan rotor  312  mounted on an intermediate shaft  313  that is journalled by bearings  314 . The motor  311  drives the intermediate shaft  313  through pulleys  315 ,  316  and a v-belt  318 . The armature windings of the motor  311  have an electrical connection  320  to a variable speed motor drive inverter  321 . The inverter  321  provides variable frequency and synchronous AC power to the motor  311  to drive it at different speeds. The inverter  321  is powered by a connection to input line power  322 . 
         [0097]    The electricity cost savings from replacement of induction motors with the motor-generator invention can be substantial. An example of electricity cost savings from a motor in accordance with the invention over a standard induction motor is shown in  FIG. 18 . The calculation assumes continuous operation and the high efficiency EPACT rated induction motors for comparison with a new 98% efficient air core motor as disclosed herein. Two different motor sizes are shown, 5 Hp and 40 Hp. Each year, the new motor will save $400 for the 5 Hp size and $1,434 for the 40 Hp size. Over ten years, this translates to $4,000 and $14,340, respectively per each motor used. 
         [0098]    The motor-generator in accordance with this invention can be constructed with an axial magnetic airgap, as has been shown in  FIG. 3A , or radial gap construction can also be employed, using a corollary cylindrical structure and armature winding construction, as illustrated by a radial gap brushless motor-generator  330  shown in  FIGS. 19A and 19B . The motor-generator  330  includes a stationary stator  332  and a rotor  331  having shafts  339 ,  340  that are journalled for rotation in bearings  341 ,  342  to provide for rotation of the rotor  331  relative to the stator  332 . The bearings  341 ,  342  are supported by housing end plates  343 ,  344 , which are connected by an outer housing tube  345 . The rotor  331  has a central steel cylinder section  333  and an outer co-axial hollow steel tube  334  connected by a disc  348  at one end to the central section  333  and open at the other end. Together, the central section  333  and the outer steel tube form two radially spaced apart co-rotating ferromagnetic rotor portions. Radially magnetized magnets  335  of circumferentially alternating polarity are attached to the outer tube  334 , as shown in  FIG. 19B . The advantage of radial gap construction is a smaller diameter comparable with standard equivalent power rated induction motors. This diameter reduction does though come at the expense of a lower magnet tip speed and potentially increased magnet costs. Magnets may also be attached to the inner cylinder  333 , if desired, as shown in the embodiment of  FIG. 24 . The magnets  335  drive magnetic flux  336  across an armature magnetic airgap  337  defined between the inner surface of the magnets  336  and the outer surface of the central steel cylinder section  333 . The magnets  335  drive flux  336  through a flux path that includes a radial portion across the airgap  337 , and circumferential portions through the inner steel cylinder  333  and the outer co-axial hollow steel tube  334 . 
         [0099]    The stator  332  includes an air core armature  338  that is fastened to an end plate  334  facing the open end of the rotor  331 , and extends axially into the magnetic airgap  337 . The armature  338  is constructed with a substantially nonmagnetic and low electrical conductivity cylindrical form having channels and windings wound into the channels as described previously and as described in more detail in conjunction with  FIGS. 24-26 . For maximizing power density capability, liquid cooling can also be added to the armature. One method is to run liquid cooling lines through the form and adjacent to the windings. A synchronous variable speed motor drive inverter provides synchronous AC power to energize the armature windings. The synchronous AC power in the armature windings interacts with the alternating flux in the airgap  337  produced by the rotating array of magnets to exert a torque on the rotor  331  which is transmitted through a connection on the shaft  340  to drive a load. Air cooling holes  346 ,  347  provide for airflow through the rotor  331  and cooling of the armature  338 . 
         [0100]    As shown in  FIG. 20 , another armature  350  for an axial gap brushless motor-generator has serpentine windings  351  that are wound multiple times circumferentially into a serpentine slot in an armature form  356 . The windings  351  have a start  352  and an end  353 . To prevent overlapping  355  of the end  353  from interfering with flat stacking of multiple forms, as shown in  FIG. 21 , the armature  350  has a circumferential cut out section  354 . 
         [0101]    The stacking of a multiple phase armature is shown in  FIG. 21 . The stacked armature  360  is comprised of three phases  361 ,  362 ,  363  that are axially assembled together. Each phase  361 ,  362 ,  363  uses a substantially low electrical conductivity form  364  having a backing  367  and upwardly opening channels  366 . Litz wire windings  365  are placed in the channels  366 . As shown, the channels are rectangular and the wire has been precompressed to a rectangular cross section prior to winding. This facilitates a more easily compacted high-density construction without the need for high-pressure compaction after winding. The channels  366  are shown with radiused inner corners, however the corners can be made square to better match the rectangular compressed wire. Also shown with the stacked armature  360 , the windings of each phase are spaced more closely together circumferentially in the active length region (shown) than if circumferentially uniformly distributed about the width of that active length region. It has been found that this close packing can be done in some cases to increase the efficiency of the motor-generator by reducing the resistive losses and increasing the back emf for the given design. 
         [0102]    As shown in  FIGS. 22 and 23 , another configuration winding for a brushless motor generator armature  370  includes a substantially nonmagnetic and low electrical conductivity form  371  with Litz wire windings  372  that are wound into channels in the form  371 . The windings  372  have a start  373  and end  374 . A multi-phase armature  380 , shown in  FIG. 23 , is constructed by stacking together the different phases  381 ,  382 ,  383 , each wound as shown in  FIG. 22 . The forms  371  have axial holes  373 ,  374  for exiting of the windings  372  from the low electrical conductivity forms  371  to prevent overlapping of windings of a single phase on a form. Corresponding exit holes (not shown) are made in the middle and bottom forms for passing the leads of the top and middle phases  381 ,  382  to axially exit the stacked armature  380 . In this method, the windings can be dielectrically isolated from each other at all locations as well as be completely supported by form channels around the circumference of the armature. It is therefore possible to use un-served Litz wire without the need for the added dielectric breakdown strength of the outside wire sheath. This un-served Litz wire is more flexible and easier to wind as well as more compactable for higher winding density and motor-generator efficiency. Holes  375  are made around the inside and outside periphery of the form  371  to ensure proper alignment of the forms so that the phases are correctly positioned relative to each other, and to securely attach the forms together in the stacked assembly. The holes also receive fasteners for holding the armature in the desired position in the armature airgap. 
         [0103]    Another radial gap brushless motor-generator  390 , shown in  FIG. 24 , has a rotor  391  that includes a shaft  392  supported by bearings  393 ,  394 , and inner and outer co-rotating steel tubes  395  and  396  that are radially separated to form a radial armature airgap  406 . Radially magnetized magnets  397 , 398 , arranged in a circumferentially alternating polarity array as shown in  FIG. 19B , but in this embodiment on both sides of the airgap, drive magnetic flux radially back and forth across the armature airgap  406 . The magnetic flux flows circumferentially in the inner and outer steel tubes between circumferentially adjacent magnets to close the flux loops. A cylindrical air core armature  407  is located in the armature airgap  406  to covert between electrical and rotational energy. The armature  407  has an active length region  402  and axial end turns  400  and  401 . The end turns  400 ,  401  are thicker in the direction of the magnetic airgap than the active length region  402 . The end turns  400 ,  401  protrude radially in opposite directions to allow for easy assembly of the motor  390 . The winding leads  403  exit the armature  407  to an electric box  404  located external to the housing  405 . 
         [0104]    The box  404  can hold a motor controller, such as a synchronous variable speed motor drive inverter for providing synchronous AC power to energize the armature windings and electrically power the motor  390 . To eliminate the requirements for accurate motor commutation phasing and also the running of sensor wires from the motor to the drive that can pick up electrical noise impeding proper operation if long, the motor drive preferably utilizes a sensorless control. More preferably, to allow for accurate and robust motor control, the motor drive inverter employs sensorless flux vector control. This type of control provides for high efficiency with good power factor reflected to the power supply and also provides optimal torque and speed control capability with the double rotating air core motor construction. The high performance of sensorless flux vector control synergistically cooperates with the increased performance capability of the air core armature motor. This motor controller can also be used in the motors shown in  FIGS. 3A and 19A . 
         [0105]    The armature form for the radial gap motor-generator  390  is shown in  FIG. 25  in “unrolled” or flat view for clarity of illustration. The form  410  is a plastic tube having a backing portion  411  and radially protruding ridges  412  that define therebetween channels  413  that open radially outward, and into which the windings are wound. To allow for the end turns on the free end of the stator to be radially displaced inward, the backing portion  411  is omitted at the free end. The assembled air core armature of the motor-generator is shown in  FIG. 26 . The armature  407  is comprised of the low electrical conductivity form  410  that has a backing portion  421  and radially opening channels  413  in the active length region  402 . Litz wire windings  426  are wound onto the form with the end turns  401  and  400  located at the axial ends. A tension wrap  424  may be wound circumferentially around the form  411  to radially compress the windings  426  into the channels  413 . The wrap  424  may be a fiber band, tape or other means that is radially thin and has sufficient strength to provide the required compression. 
         [0106]    A large armature form assembled from multiple circumferential section forms is shown in  FIG. 27 . For larger motors or generators, it may be easier and more practical or cost effective to assemble the armature from multiple pieces. Machining or molding a single large form may not be economically feasible. One application of very large generators is in direct drive wind turbines that can have a diameter of 15 feet or more. The armature  430  is comprised of multiple circumferential form sections  431  that are attached to a supporting structure  432 , potentially through the use of bolts  433  or other fasteners. The windings, not illustrated, can be wound on to the form sections  431  prior to assembly together. Alternatively, it may be easier to do the winding after assembly of the form for reducing the required number of electrical connections. 
         [0107]    A process for construction of an air core armature for use in a brushless motor-generator in accordance with the invention is illustrated in  FIG. 28 . The armature form is made, preferably by molding or forming the form material into the desired configuration, as indicated in Step  441 . This contrasts with other air core armature construction techniques where the windings are wound first and then molded or encapsulated as the final step. After making the form in Step  441 , the windings are wound onto the forms in Step  442 . Multiple windings and or forms are assembled and/or compressed in Step  443 . The Litz wire ends are soldiered in Step  444  to ensure good electrical conduction to all individually insulated strands of the wires. The armature is then installed in Step  445  into the motor-generator. 
         [0108]    The motors made in accordance with this invention are low in cost and high in efficiency, and they can also function equally well as generators for power generation applications, and as motor-generators for flywheel storage systems. For example, a motor-generator in accordance with the invention can be use in flywheel energy storage systems such as those shown in application Ser. No. 09/977,678 entitled “Inductor Alternator Flywheel System” filed on Oct. 15, 2001. 
         [0109]    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. For example, although all disclosed embodiment shown herein use permanent magnets to produce magnetic flux that interacts with the armature in the airgap, it is contemplated that the flux could instead be produced by stationary field coils, conventional or superconducting. Field coil motor-generators are shown in application Ser. No. 10/319,190 entitled “Lightweight High Power Electrical Machine” filed on Dec. 13, 2002, and U.S. Pat. No. 6,750,588 entitled “High Performance Axial Gap Alternator Motor” issued on Jun. 15, 2004. Accordingly, we intend that these modifications and variations, and the equivalents thereof, be included within the spirit and scope of the invention as defined in the following claims, wherein we claim: