Patent Publication Number: US-2023155466-A1

Title: Multi-rotor electric machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application No. 62/723,515 filed on Aug. 28, 2018, the entire contents of which are hereby incorporated by reference. 
    
    
     FIELD 
     This relates generally to electric machines, and more particularly to multiple-rotor electric machines such as motors and generators. 
     BACKGROUND 
     Electric machines with multiple rotors are known and may provide enhanced power over conventional electric machines. However, multiple-rotor electric machines may be heavier and less powerful than necessary or desirable, and may not be as efficient as desirable. 
     Accordingly, there is a need for lighter and more powerful electric machines which make more efficient use of materials. 
     SUMMARY 
     According to an aspect, there is provided a multi-rotor electric machine comprising: a stator; a first rotor magnetically coupled to the stator and rotatably mounted relative to the stator; a second rotor magnetically coupled to the stator and rotatably mounted relative to the stator; and a gear drivingly coupled to both the first and second rotors, the first rotor being drivingly coupled to a first face of the gear and the second rotor being drivingly coupled to a second face of the gear. 
     According to another aspect, there is provided an electric machine comprising: a plurality of stators circumferentially distributed about a central axis of the electric machine; a plurality of rotors arranged in triplets, each triplet of rotors sharing a common magnetic circuit with a respective one of the stators, each triplet of rotors comprising an adjacent pair of radially-outer rotors and a radially-inner rotor relative to the central axis; and a common gear rotatable about the central axis and drivingly coupled to the plurality of magnetic rotors, the radially-outer rotors being drivingly coupled to a radially-outer face of the common gear and the radially-inner rotors being drivingly coupled to a radially-inner face of the common gear. 
     According to another aspect, there is provided a method of operating an electric machine having a stator, a first rotor magnetically coupled to the stator and a second rotor magnetically coupled to the stator, the method comprising: driving a gear via a first face of the gear using the first rotor; and while the gear is driven using the first rotor, driving the gear via a second face of the gear using the second rotor. 
     Other features will become apparent from the drawings in conjunction with the following description. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the figures which illustrate example embodiments, 
         FIG.  1    is a schematic perspective view of portions of an embodiment of a multi-rotor electric machine; 
         FIG.  2    is a schematic front cut-away view of portions of the electric machine of  FIG.  1   ; 
         FIG.  3    is a schematic side view of a cross-section of a portion of an embodiment of a multi-rotor electric machine; 
         FIG.  4    is a schematic front partial cut-away view of a portion of the electric machine of  FIG.  1   ; 
         FIG.  5    is a schematic front partial cut-away view of the portion of the electric machine of  FIG.  4    showing representations of a magnetic circuit associated with the electric machine; 
         FIG.  6    is a schematic front partial cut-away view of a portion of an electric machine showing rotor orientations in accordance with an embodiment; and 
         FIGS.  7 A to  7 E  are schematic front partial cut-away views of the portion of the electric machine depicted in  FIG.  6    throughout a half-cycle of operation. 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects of preferred embodiments of electric machines according to the disclosure are described herein with reference to the drawings. 
     Electric machines may have more than one rotor. An example of a multi-rotor electric machine is provided in U.S. Pat. No. 8,232,700 B2, the contents of which are incorporated by reference in their entirety. 
       FIG.  1    is a schematic perspective view of an embodiment of an electric machine. As depicted, electric machine  100  includes central shaft  104  connected to common main gear  150 . For example, central shaft  104  may be drivingly coupled to main gear  150  via one or more radially-extending arms or a circumferentially-continuous surface.  FIG.  1    shows a single radially-extending arm for clarity to provide visibility to internal components of electric machine  100 . 
     Main gear  150  engages with multiple (e.g., pinion) gears  118 ,  119  arranged so as to be in meshing engagement with teeth on an outer face of main (e.g., ring) gear  150  and an inner face of main gear  150 . Each gear  118 ,  119  is connected to a respective rotors  102 ,  103  via a rotor shaft  116 . One or more windings  108  (see  FIG.  2   ) are wrapped around stator  122  to induce a magnetic field (when current is applied to winding(s)  108 ) or to have a magnetic field induced therein (when main gear  150  rotates, thereby causing rotors  102 ,  103  and consequently gears  118 ,  119  to rotate). 
       FIG.  2    is a schematic front view of the example electric machine depicted in  FIG.  1   . As illustrated, machine  100  comprises a plurality of outer magnetic rotors  102 , a plurality of inner magnetic rotors  103 , windings  108 , stators  122 , main gear  150 , and shaft  104 . As depicted in  FIGS.  2  and  3   , machine  100  includes a plurality of outer magnetic rotors  102 , each configured to rotate with a separate rotor shaft  116 , and a plurality of inner magnetic rotors  103 , each configured to rotate with a separate rotor shaft  116 . In some embodiments, the number of inner rotors  103  may be half the number of outer rotors  102  in machine  100 . 
     As depicted in  FIGS.  2  and  3   , each rotor shaft  116  is configured to drive shaft  104  by means of outer (e.g., pinion) gears  118  and inner (e.g., pinion) gears  119  interacting with central main (e.g., ring) gear  150 . The rotor shafts  116  are caused to rotate by respective magnetic rotors  102 ,  103 . In some embodiments, the machine  100  is operable as a motor and current is applied to windings  108  to cause the gears  118 ,  119  to drive the main gear  150 . In some embodiments, the machine  100  is operable as a generator so that when a torque is applied to shaft  104 , main gear  150  causes rotors  102 ,  103  to rotate via gears  118 ,  119  and thus causes a flow of electrical current in windings  108 . 
     Please note that in some figures, gears are shown without teeth for the sake of clarity. As described herein, gears may be provided in any suitable form, including in the form of toothless wheels engaged by friction, as well as gears with teeth which engage with other gears. 
       FIG.  3    is a schematic side view of a cross-section of a portion of an embodiment of an electric machine  100 . As depicted, each rotor shaft  116  is rotatably supported by front plate  134  and back plate  136 , with suitable bearings. Rotor shafts  116  may be formed integrally with or otherwise be connected to or coupled to a gear (e.g. outer gear  118  or inner gear  119 ). Outer gears  118  are configured to engage an outer face of main gear  150 . Inner gear  119  is configured to engage an inner face of main gear  150 . 
     Main gear  150  is connected to shaft  104 , such that rotation of one or more outer rotors  102  and/or inner rotor  103  causes outer gears  118  and/or inner gear  119  to drive main gear  150 , and therefore shaft  104 , into rotation—or vice versa, depending upon the mode of operation (motor vs. generator). During a motor mode of operation, outer rotors  102  and inner rotors  103  can be used simultaneously to drive main gear  150 . Alternatively, during a generator mode of operation, rotation of main gear  150  can drive outer rotors  102  and inner rotors  103  when generating electricity in windings  108 . 
     In some embodiments, outer rotors  102  and inner rotor  103  are configured to operate in electromagnetically independent triplets  160 . That is, the rotors  102 ,  103  can be separated magnetically into triplets  160 , such that there is no provision of magnetic material linking any two triplets  160  of rotors  102 ,  103  together, and the only linkages between separate triplets  160  are mechanical (e.g. support structure, gears  118 ,  119  or other mechanical couplings). 
     In some embodiments, each triplet  160  includes two radially-outer rotors  102  and one radially-inner rotor  103 . The outer rotors  102  and inner rotors  103  of a given triplet  160  can benefit from the provision of common magnetic circuit components, such as stators  122  and/or windings  108 , as shown, for example, in  FIGS.  4  and  5   . Such a shared configuration can significantly reduce the amount of magnetic material required for operation of the rotors, with corresponding cost and weight savings. Such arrangement can also promote an efficient use of space and power-density. Relative to other multi-rotor electric machine configurations, the use of both outer and inner rotors  102 ,  103  with shared magnetic components to engage main gear  150  on both inner and outer faces may increase the power output by as much as 50% without significant addition of weight to machine  100 . Thus, the power-to-weight ratio of machine  100  may be substantially increased relative to electric machines which do not incorporate the disclosed configuration of outer rotors  102  combined with an inner rotor  103 . 
     For example, since the magnetic circuit for the two outer rotors  102  and inner rotor  103  in triplet  160  is provided in common (see, e.g.,  FIG.  5    which illustrates the magnetic flux path within the magnetic circuit for a given triplet  160  of rotors), the source of magnetic energy (winding  108 ) may also be common to the three rotors in the triplet  160 , and as such shared by the three rotors  102   a,    102   b,    103  in triplet  160 . This means that the three rotors  102   a,    102   b,    103  in a triplet  160  can be energized by a single winding  108 , if desired, which results in a substantial weight savings in the weight of the overall machine. Although the figures depict a configuration in which there is one winding  108  per stator  122 , it is contemplated that other embodiments may include more than one winding  108  per stator  122  for each triplet  160  of rotors  102 ,  103 . 
     Referring to  FIG.  4   , each rotor  102   a,    102   b,    103  comprises one or more magnets  128  mounted on a shaft  116  and retained, particularly when rotating, by a containment sheath  126 . Magnets  128  comprise north and south poles (denoted “N” and “S”, respectively). In some embodiments, rotors  102 ,  103  comprise single pairs of north and south poles, and may be referred to as bi-pole rotors  102 ,  103 . Moreover, rotors are provided in triplets  160 , each triplet  160  comprising a pair of outer rotors  102  (denoted as first outer rotor  102   a  and second outer rotor  102   b ) and an inner rotor  103 . The rotors are indexed such that magnets  128  are mounted, and rotate, (a) as individual rotors in a desired phase with respect to their triplet-mates ( 102   a,    102   b,    103 ), and (b) by triplet  160 , in a desired triplet phase with respect to other triplets  160   a  and windings  108   a.    
     When using triplet  160  sets of bi-pole rotors indexed as described herein, particular advantage may be gained by phasing rotors within each triplet  160 . Such a configuration may make efficient use of the flux paths  132  (denoted in  FIG.  5   ) around the rotors  102 ,  103  and therefore provide better efficiency of interactions between rotors  102 ,  103  and winding  108 , resulting in greater power being developed from electric machine  100 . 
     It should be noted that during operation, the direction of rotation of inner rotor  103  is opposite to that of the outer rotors  102   a,    102   b.  This is illustrated In  FIG.  4   , which shows the first and second outer rotors  102   a,    102   b  rotating in direction A, while inner rotor  103  rotates in direction B (which is the same as the direction of rotation of the main gear  150 ). Although  FIG.  4    shows outer rotors  102   a,    102   b  rotating in a counter-clockwise direction and inner rotor  103  and main gear  150  rotating in a clockwise direction, it should be noted that the converse is also possible (i.e. outer rotors  102   a,    102   b  can rotate clockwise and inner rotor  103  and main gear  150  can rotate counter-clockwise). 
     It should also be noted that in some embodiments, during operation of machine  100 , the speed of rotation of each rotor  102   a,    102   b,  and  103  in each triplet  160  is the same magnitude. That is, rotors  102   a  and  102   b  rotate at a given speed in a given direction of rotation, and inner rotor  103  rotates at substantially the same speed as outer rotors  102   a,    102   b,  but in the opposite direction. As noted above, the rotation of rotors  102   a,    102   b,  and  103  causes the rotation of outer gears  118  and inner gear  119 , which in turn causes rotation of main gear  150 . It is understood that the circumference of the main gear  150  at its outer face  151  is greater than the circumference of the main gear at its inner face  152 . As such, the linear (tangential) speed of the outer face  151  will be greater in magnitude than the linear (tangential) speed of the inner face  152  for a given angular velocity. Accordingly, it is understood that radially-outer gears  118 , radially-inner gear  119  and faces  151 ,  152  of main gear  150  may be sized and configured to accommodate the common rotation speed of radially-outer gears  118  and radially-inner gear  119  that are meshed with common main gear  150 . For example, the diameter of the inner gear  119  may be selected so as to be shorter than the diameter of the outer gears  118   a,    118   b  in one rotor triplet  160 . 
     It has been found that an increase in magnetic path utility due to such (e.g., triangular) triplet configurations can allow for significant savings in weight and bulk, as compared to electric machines which assign one magnetic circuit to each rotor. For example, the addition of a third rotor (e.g. inner rotor  103 ) may cause an increase in power output by up to 50% relative to a configuration including only rotor pairs sharing a magnetic circuit without the third rotor. As will be understood by those skilled in the relevant arts, an increase in the diameter of an individual rotor magnet  128 , and the corresponding strength of that magnet  128 ′s surface area and the corresponding strength of the magnet&#39;s electromagnetic interaction with its corresponding winding  108  can be utilized to increase the power provided by machine  100 . However, to optimize the power provided, the cross-section of the corresponding stator  122  may be increased, in order to maintain the desired flux density. By grouping magnets  128  in rotor triplets  160  and employing a shared stator  122 , flux density may be maintained with a minimal weight penalty, which may be particularly important in weight-critical applications such as aerospace and transportation activities. 
     Windings  108  may be provided in any configuration suitable for use in accomplishing the purposes described herein. A wide variety of such configurations are known which may maximize the efficiency of machine  100  for a given application. For example, single Litz or multiple strand windings  108  may be used in configuring either machine  100 , individual rotors  102 ,  103 , rotor triplets  160 , or other sets of rotors. The use of multiple windings  108  in machine  100  may be employed, for example in conjunction with a suitable mechanical indexing of the rotors  102 ,  103  to fully or partially provide desired phasings in torque applied by rotors  102 ,  103  to main gear  150  and shaft  104 . In some embodiments, three-phase windings may be employed. 
     As depicted in  FIG.  5   , in some embodiments the machine  100  includes one winding  108  per stator  122 . That is, one winding  108  is provided for each triplet  160  of rotors  102   a,    102   b,    103 . The use of a single winding  108  per rotor triplet  160  may provide improved efficiency for the machine  100 , as compared to machines in which multiple windings are used. 
     As noted previously, the efficiency of machine  100  can be increased through the suitable phasing (i.e., indexing) of rotors  102 ,  103  with respect to each other and with respect to winding  108 . In particular, the operation of machine  100  can be controlled by phasing outer rotors  102   a,    102   b  and inner rotor  103  with respect to each other and to winding  108  in triplets. This may be accomplished, for example, by suitable gearing of outer rotors  102   a,    102   b  and inner rotor  103  with respect to each other and to motor shaft  104 . 
     In the example embodiment shown in  FIGS.  3  and  4   , each gear  118  driven by outer rotors  102  engages the outer face  151  of main gear  150 , and each gear  119  driven by inner rotors  103  engages the inner face  152  of main gear  150 , so that total torque applied to main gear  150  is the sum of the torques applied by the inner and outer gears  118 ,  119 . If windings  108  are configured substantially circumferentially about axis  200  of shaft  104  and therefore machine  100 , an index angle  112  may be defined between equators  202  (a theoretical line dividing a magnet into a north and south half) of individual magnets  128  and radii  204  extending from axis  200  to a corresponding rotor  102 ,  103 . By suitable arrangement of rotors  102 ,  103  and/or gears  118 ,  119 , index angles  112  may be set at desired values for individual rotors, and triplet sets thereof, with the result that phased torque output applied by each of the rotor triplets  160  can be applied to provide smooth, continuous, and powerful torque to shaft  104  via main gear  150 , in the case of motor operation. In the case of generator operation, smooth and continuous current output may be obtained from winding(s)  108  by applying a torque to shaft  104 . 
       FIG.  6    is a schematic front partial cut-away view of a portion of an electric machine showing rotor orientations in accordance with an embodiment. Radius  204   a  extends from axis  200  through the centre of outer rotor  102 a. Radius  204 b extends through the centre of outer rotor  102   b.  Radius  204 c extends through the centre of inner rotor  103 . As depicted, equator  202   a  of outer rotor  102   a  is perpendicular to radius  204   a.  Equator  202   b  of outer rotor  102   b  is perpendicular to radius  204   b,  with the polarity of the magnet in outer rotor  102   b  being reversed relative to the magnet in outer rotor  102   a.  Equator  202   c  of inner rotor  103  is parallel to radius  204   c.  It will be appreciated by a person skilled in the art that  FIG.  6    is an example depiction of inner rotors  103  and outer rotors  102   a,    102   b  at a moment in time, and that the rotors will be rotating throughout operation. The difference between the index angles of each rotor  102   a,    102   b,    103  in a triplet may be substantially maintained throughout operation, although there may be variation by several degrees depending on the driving current, the loading on the machine  100 , and the phase advance. 
       FIGS.  7 A to  7 E  are schematic front partial cut-away views of the portion of the electric machine depicted in  FIG.  6    throughout a half-cycle of operation (that is, throughout 180 degrees of rotation).  FIG.  7 A  illustrates the same initial configuration as  FIG.  6   , as well as arrows indicating the outer rotors  102   a,    102   b  moving in direction A, and the inner rotor moving in direction B, and the flux lines associated with the magnetic circuit in stator  122 . 
       FIG.  7 B  depicts the electric machine of  FIG.  7 A  after 45 degrees of rotation. As can be seen, each of outer rotors  102   a,    102   b  has rotated substantially 45 degrees in direction A (in this example, direction A is counter-clockwise), and inner rotor has rotated substantially 45 degrees in direction B (in this example, direction B is clockwise). The flux path is also depicted. 
       FIG.  7 C  depicts the electric machine of  FIG.  7 A  after 90 degrees of rotation relative to the initial configuration in  FIG.  7 A . As can be seen, each of outer rotors  102   a,    102   b  has rotated substantially 90 degrees in direction A, and inner rotor  103  has rotated substantially  90  degrees in direction B. It can be seen that at this moment in time, the flux path is temporarily broken between the three rotors  102   a,    102   b,    103 . 
       FIG.  7 D  depicts the electric machine of  FIG.  7 A  after 135 degrees of rotation relative to the initial configuration in  FIG.  7 A . As can be seen, each of outer rotors  102   a,    102   b  has rotated substantially 135 degrees in direction A, and inner rotor  102  has rotated substantially 135 degrees in direction B. The flux path is also depicted and once again travels through each of the rotors in the triplet, although the flux now follows a clockwise path relative to the initial counter-clockwise flux path in  FIG.  7 A . 
       FIG.  7 E  depicts the electric machine of  FIG.  7 A  after 180 degrees of rotation relative to the initial configuration of  FIG.  7 A . As can be seen, each of outer rotors  102   a,    102   b  has rotated substantially  180  degrees, and inner rotor  103  has rotated substantially  180  degrees. As such, each of rotors  102   a,    102   b  and  103  is now in the opposite polarity relative to the initial configuration of  FIG.  7 A , and the flux path depicted is substantially opposite in direction to the flux path depicted in  FIG.  7 A . 
     It will be appreciated that in  FIGS.  4  and  5   , the inner rotor angle is different than the embodiments shown in  FIGS.  6  and  7 A- 7 E . It should be appreciated that the indexing of the inner rotor  103  relative to outer rotors  102   a,    102   b  may be different than what is depicted in  FIG.  7    by several degrees, as the configuration may vary depending on whether a machine is optimized for torque, speed, or the like. 
     In the embodiment shown in  FIGS.  1  and  2   , an 18-rotor (12 outer, 6 inner), 6-phase system is shown. As will be understood by those skilled in the art, the disclosure is also applicable to a 9-rotor (6 outer, 3 inner), 3-phase system, a 36-rotor (24 outer, 12 inner), 12-phase system, and other combinations. 
     In the case of the 18-rotor, 6-phase system depicted in  FIG.  2   , each of the 12 outer rotors and 6 inner rotors may be grouped into six triplets  160 , each triplet having 2 outer rotors  102   a,    102   b,  and one inner rotor  103 . 
     Further, each of the 6 rotor triplets  160   a  may be phased relative to its adjacent two rotor triplets  160   b,    160   c.  For example, equators  202  of the 1 st  and 4 th  triplets  160   a,    160   d  may be aligned with their respective radii  204  from axis  200  (though the outer rotors  102   a  and  102   b  may be  180  degrees out of phase with one another), while equators  202  of the 2 nd  and 5 th  triplets  160   b,    160   e  are indexed by 60 degrees with respect to the 1 st  and 4 th  triplets  160   a,    160   d,  and equators  202  of 3 rd  and 6 th  triplets  160   c,    160   f  may be indexed by 60 degrees with respect to 2 nd  and 5 th  triplets  160   b,    160   e,  and by  120  degrees with respect to  1   st  and  4   th  triplets  160   a,    160   d.    
     In a 9-rotor (6 outer, 3 inner), 3-phase system, each adjacent rotor triplet  160  may be indexed by 120 degrees with respect to its neighbour triplets (this may be implemented as a single channel, 3 phase system). In a 36-rotor (24 outer, 12 inner) system, each adjacent rotor triplet  160  can be indexed by 30 degrees relative to its neighbour triplets (this may be implemented as a dual channel 6-phase system, a 3-channel 4-phase system, or a 4-channel 3-phase system). In an 18-rotor (12 outer, 6 inner) system, each adjacent rotor triplet  160  can be indexed by 60 degrees relative to its neighbour triplets (this may be implemented as a single channel, 6 phase system, or a dual channel, 3-phase system). 
     As will be readily apparent to those skilled in the relevant arts, a wide variety of combinations and geometries of indexing and phasing may be chosen, depending on the desired input and output characteristics, and geometry, of the machine  100 . For example, adjacent rotor triplets  160  can be indexed relative to each other such that when a current is passed through one or more windings magnetically coupled to the respective stators, the rotor triplets  160  provide phased rotary power to the common main gear  150 . 
     As will be further apparent to those skilled in the relevant arts, the desired indexing of adjacent rotor triplets can be accomplished mechanically, electrically, or in any suitable or desired combination thereof. 
     As previously noted, in various embodiments this disclosure provides electric machines having a plurality of flux paths (i.e., magnetic circuits) defined between triplets of rotors, each triplet of rotors being associated with a shared stator  122 . Respective triplets of rotors may further be associated with a single winding  108 , shared by the triplet. 
     Any materials suitable for use in accomplishing the purposes described herein may be used in fabricating the various components of machine  100 , including, for example, those used in fabricating analogous components of known electric machines. The selection of suitable materials would be within the knowledge of those skilled in the art. 
     As has already been noted, machine  100  may be operated as a motor by applying a suitable AC or commutated DC voltage across windings  108 , or as a generator by applying mechanical torque to shaft  104  and tapping current from leads suitably connected to windings  108 . 
     Electric machines in accordance with the disclosure can be operated, with appropriate rectifiers, solid state switches, capacitors, and other electronic components, using either direct- or alternating-current input, to provide either direct or alternating-current output, depending upon whether electrical or mechanical input is applied to the windings  108  or shaft  104 , respectively. 
     The above descriptions are meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the described subject matter. Still other modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art, and such modifications are intended to fall within the scope of the appended claims. 
     Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention is intended to encompass all such modification within its scope, as defined by the claims.