Patent Publication Number: US-2022231553-A1

Title: Rotor magnet retainer

Description:
RELATED APPLICATION 
     This patent application claims the benefit of U.S. Provisional Patent Application No. 63/139,989 filed Jan. 21, 2021, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     This disclosure relates to a brushless motor assembly for a rotary tool, and particularly to a rotor magnet retainer for rotor of a brushless motor. 
     BACKGROUND 
     Use of brushless direct-current (BLDC) motors in power tools is well known. BLDC motors typically provide higher power and higher efficiency than comparable brushed universal or permanent-magnet DC motors. A BLDC motor typically includes a stator that includes a series of coils that are electronically commutated in sequence, and a rotor that includes a rotor core and a series of permanent magnets that magnetically interact with the stator coils to cause rotation of the rotor. The rotor may be an inner rotor with the magnets located inside the stator or an outer rotor with the permanent magnets surrounding the stator. The permanent magnets may be surface-mounted on the surface of the rotor core or embedded within magnet pockets provided in the rotor core. 
     Permanent magnets embedded within the rotor core may be supported within a series of longitudinally-extending pockets of the rotor core. Normally, end caps are mounted on one or two ends of the rotor core to retain the permanent magnets within the pockets. A problem that arises is that, due to stack-up tolerances associated with manufacturing inefficiencies and inaccuracies, the length of the permanent magnets may not always match the length of the rotor core, leaving room for the permanent magnets to wobble within the pockets. This discloses attempts to solve this problem. 
     SUMMARY 
     According to an embodiment, a motor assembly is provided including a stator assembly; and a rotor assembly rotatably disposed relative to the stator assembly. The rotor assembly includes a rotor core having magnet pockets formed therethrough along a longitudinal direction, permanent magnets received within the magnet pockets, and a spring structure disposed in contact with the end of the rotor core. In an embodiment, the spring structure includes spring elements configured to apply biasing forces to the permanent magnets along the longitudinal direction of the magnet pockets. 
     In an embodiment, the spring structure includes a planar body having a center opening and slots formed within the planar body along directions normal to the center opening. In an embodiment, the spring elements are wave springs extending from the planar body into the slots along approximately the plane of the planar body. 
     In an embodiment, at least one of the wave springs includes humped portions projecting relative to one surface of the planar body and penetrating a corresponding magnet pocket of the rotor core along the longitudinal direction of the magnet pockets. 
     In an embodiment, at least one of the wave springs includes a first humped portion projecting relative to a first surface of the planar body and a second humped portion projecting relative to a second surface of the planar body. 
     In an embodiment, the motor further includes a magnet retention cap configured to axially support the spring structure at the end of the rotor core. 
     In an embodiment, the magnet retention cap is provided separately from the spring structure but holds the spring structure against the end of the rotor core. 
     In an embodiment, the spring structure includes an outer diameter that is approximately equal to an outer diameter of the rotor core. 
     In an embodiment, the magnet retention cap is configured unitarily include and support the spring structure. 
     In an embodiment, the spring structure includes an outer diameter that is smaller than an outer diameter of the rotor core, and the magnet retention cap includes an annular rim portion formed around an outer periphery of the spring structure. 
     In an embodiment, the rotor core and the magnet retention cap are securely mounted on a rotor shaft. 
     According to an embodiment, a motor assembly is provided including a stator assembly; and a rotor assembly rotatably disposed relative to the stator assembly. The rotor assembly includes a rotor core having magnet pockets formed therethrough along a longitudinal direction, permanent magnets received within the magnet pockets, and a magnet retention cap mounted at an end of the rotor core to axially stop the movement of the permanent magnets out of the magnet pockets. In an embodiment, the magnet retention cap includes a spring structure disposed in contact with the end of the rotor core and configured to apply a biasing force to the permanent magnets in a direction away from the magnet retention cap. 
     In an embodiment, the spring structure includes a planar body having a center opening and slots formed within the planar body along directions normal to the center opening. In an embodiment, the spring elements are wave springs extending from the planar body into the slots. 
     In an embodiment, the planar body includes an outer diameter that is smaller than an outer diameter of the rotor core, and the magnet retention cap includes an annular rim portion formed around an outer periphery of the planar body. 
     In an embodiment, the wave springs is arranged to penetrate the magnet pocket of the rotor core to engage the permanent magnet. 
     In an embodiment, the spring structure includes resiliently-deformable bosses. 
     According to an embodiment, a motor assembly is provided including a stator assembly; and a rotor assembly rotatably disposed relative to the stator assembly. The rotor assembly includes a rotor core having magnet pockets formed therethrough along a longitudinal direction, permanent magnets received within the magnet pockets, and a magnet retention cap mounted at an end of the rotor core to axially stop the movement of the permanent magnets out of the magnet pockets. In an embodiment, the magnet retention cap includes resiliently-deformable members arranged to at least partially penetrate the magnet pockets and to apply a biasing force to the plurality of permanent magnets in a direction away from the magnet retention cap. 
     In an embodiment, the resiliently-deformable members are made of rubber bosses extending along axis normal to a center opening of the magnet retention cap. 
     In an embodiment, the magnet retention cap further includes a spacer formed at least partially around the resiliently-deformable members and in contact with the end of the rotor core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a side view of a power tool with a housing half removed, according to an embodiment. 
         FIG. 2  depicts a perspective view of a brushless DC (BLDC) motor of the power tool, according to an embodiment. 
         FIG. 3  depicts an exploded view of the motor including stator and rotor assemblies, according to an embodiment. 
         FIG. 4A  is a side cross-sectional view of the rotor assembly provided with a conventional prior art rotor end cap, according to an embodiment. 
         FIG. 4B  depicts a zoomed-in view of the rotor assembly, particularly showing the interface between the rotor end cap and permanent magnets of the rotor assembly, according to an embodiment. 
         FIG. 5A  depicts a side cross-sectional view of the rotor assembly provided with a magnet retention cap and a spring structure for engagement and retention of the permanent magnets in place of a conventional rotor end cap, according to an embodiment. 
         FIG. 5B  depicts a perspective view of the spring structure alone mounted on the rotor core, according to an embodiment. 
         FIG. 5C  depicts a side exploded view of the rotor assembly with the spring structure mounted and the magnet retention cap provided at a distance, according to an embodiment. 
         FIG. 5D  depicts a side view of the spring structure, according to an embodiment. 
         FIG. 5E  depicts a perspective view of the spring structure, according to an embodiment. 
         FIG. 6A  depicts a partial side cross-sectional view of a spring structure, according to an alternative embodiment. 
         FIG. 6B  depicts a perspective view of the spring structure, according to an embodiment. 
         FIG. 7A  depicts a side cross-sectional view of rotor assembly provided with an integrated end cap assembly for retention of the permanent magnets, according to an alternative embodiment. 
         FIG. 7B  depicts a perspective view of the integrated end cap assembly including a spring structure integrally supported by a magnet retention cap, according to an embodiment. 
         FIG. 7C  depicts a side cross-sectional view of the integrated end cap assembly, according to an embodiment. 
         FIG. 7D  depicts an exploded view of the integrated end cap assembly relative to the rotor assembly, according to an embodiment. 
         FIG. 7E  depicts another exploded view of the integrated end cap assembly relative to the rotor assembly, according to an embodiment. 
         FIG. 8A  depicts a side cross-sectional view of rotor assembly provided with a magnet retention cap integrally including resiliently-deformable bosses for retention of the permanent magnets, according to an alternative embodiment. 
         FIG. 8B  depicts a perspective view of the magnet retention cap including the resiliently-deformable bosses, according to an embodiment. 
         FIG. 8C  depicts a partial perspective view of the rotor assembly with resiliently-deformable bosses alone shown in engagement with rotor magnets, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description illustrates the claimed invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the disclosure, describes several embodiments, adaptations, variations, alternatives, and uses of the disclosure, including what is presently believed to be the best mode of carrying out the claimed invention. Additionally, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     With reference to the  FIG. 1 , a power tool  100  constructed in accordance with the teachings of the present disclosure is illustrated in a longitudinal cross-section view. The power tool  100  in the particular example provided may be an impact wrench, but it will be appreciated that the teachings of this disclosure is merely exemplary and the power tool of this invention could be a drill, impact driver, hammer, grinder, circular saw, reciprocating saw, or any similar portable power tool constructed in accordance with the teachings of this disclosure. Moreover, the output of the power tool driven (at least partly) by a transmission constructed in accordance with the teachings of this disclosure need not be in a rotary direction. 
     The power tool shown in  FIG. 1  may include a tool housing  102  that houses a motor assembly  200  and a control module  106 , an input unit (e.g., a variable speed trigger)  110 , and a transmission assembly  114  having a gear case (not shown). The motor assembly  200  may be coupled through the gear case to an output spindle (not shown), which is rotatably coupled to a square wrench  107 . The tool housing  102  additionally includes handle  112  that, in an embodiment, houses the control module  106 . 
     According to an embodiment, motor  200  is disposed in housing  102  above the handle  112 . Motor  200  may be powered by an appropriate power source (electricity, pneumatic power, hydraulic power). In embodiments of the invention, the motor is a brushless DC electric motor and is powered by a battery pack (not shown) through a battery receptacle  111 , though it must be understood that power tool  100  may alternatively include a power cord to receive AC power from, for example, a generator or the AC grid, and may include the appropriate circuitry (e.g., a full-wave or half-wave bridge rectifier) to provide positive current to the motor  200 . 
     In an embodiment, input unit  110  may be a variable speed trigger switch, although other input means such as a touch-sensor, a capacitive-sensor, a speed dial, etc. may also be utilized. In an embodiment, variable speed trigger switch may integrate the ON/OFF, Forward/Reverse, and variable-speed functionalities into a single unit coupled and partially mounted within control unit  106  and provide respective inputs of these functions to the control unit  106 . Control unit  106 , which receives variable-speed, on/off, and/or forward/reverse signal from the input unit  110 , supplies the drive signals to the motor  200 . In the exemplary embodiment of the invention, the control unit  106  is provided in the handle  112 . It must be understood that while input unit  100  is a variable-speed unit, embodiments of the invention disclosed herein similarly apply to fixed-speed power tools (i.e., tools without a speed dial or speed trigger, having constant speed at no load). 
     In an embodiment, brushless motor  200  depicted in  FIG. 1  is commutated electronically by control unit  106 . Control unit  106  may include, for example, a programmable micro-controller, micro-process, digital signal processor, or other programmable module configured to control supply of DC power to the motor  200  and accordingly commutate of the motor  200 . Alternatively, control unit  106  may include an application-specific integrated circuit (ASIC) configured to execute commutation of the motor  200 . Using the variable-speed input, forward/reverse input, on/off input, etc., from the input unit  110 , control unit  106  controls the amount of power supplied to the motor  200 . In an exemplary embodiment, control unit  106  controls the pulse width modulation (PWM) duty cycle of the DC power supplied to the motor  200 . For example, control unit  106  may include (or be coupled to) a series of power switches (e.g., FETs or IGBTs) disposed in a three-phase inverter circuit between the power source and the motor  200 . Control unit  106  may control a switching operation of the switches to regulate a supply of power to the motor  200 , via motor wires  109 . 
     Commutation details of the brushless motor  200  or the control unit  106  are beyond the scope of this disclosure, and such details can be found in co-pending International Patent Publication No. WO 3081/1596212 by the same assignee as this application, which is incorporated herein by reference in its entirety. An example of an integrated switch and control module embodying an input unit  110  and a control unit  106  described herein may be found in application Ser. No. 14/6210,617 filed Mar. 30, 3085 by the same assignee as this application, also incorporated herein by reference in its entirety. 
       FIG. 2  depicts a perspective view of a brushless DC (BLDC) motor  200 , according to an embodiment of the invention.  FIG. 3  depicts an exploded view of the same motor  200 , according to an embodiment. 
     As shown in these figures, the exemplary motor  200  is a three-phase BLDC motor having a rotor assembly  210  rotatably received within a stator assembly  230 . Various aspects of motor  200  are described herein. It must be noted that while motor  200  is illustratively shown in  FIG. 1  as a part of an impact driver, motor  200  may be alternatively used in any other device or power tool. Further, while motor  200  is a three-phase motor having six windings, any other number of phases or winding configurations may be alternatively utilized. 
     In an embodiment, rotor assembly  210  includes a rotor shaft  212 , a rotor core (or rotor lamination stack)  214  mounted on and rotatably attached to the rotor shaft  212 , and rear and front bearings  220 ,  222  arranged to secure the rotor shaft  212 . In an embodiment, rear and front bearings  220  and  222  provides radial and/or axial support for the rotor shaft  212  to securely position the rotor assembly  210  within the stator assembly  230 . 
     In various implementations, the rotor core  214  may be a lamination stack including a series of flat laminations attached together via, for example, an interlock mechanical, an adhesive, an overmold, etc., that house or hold two or more permanent magnets (PMs) therein. The permanent magnets may be surface mounted on the outer surface of the rotor core  214  or embedded therein. The permanent magnets may be, for example, a set of four PMs that magnetically engage with the stator assembly  210  during operation. Adjacent PMs have opposite polarities such that the four PMs have, for example, an N—S—N—S polar arrangement. The rotor shaft  210  is securely fixed inside the rotor core  214 . While rotor core  214  may be made of a lamination stack, it should be understood that a solid-piece rotor core may be alternatingly utilized. 
     In an embodiment, rotor assembly  210  also includes a sense magnet  216  attached to an end of the rotor core  214 . Sense magnet  216  includes a similar magnetic arrangement as the rotor permanent magnets and may be made of, for example, four magnet segments arranged in an N—S—N—S polar arrangement that align with the rotor permanent magnets. The sense magnet  216  is disposed in close proximity to and is sensed via a series of positional sensors (such as Hall sensors), which provide positioning signals related to the rotational position of the rotor assembly  210  to control module  106 . In an embodiment, sense magnet  216  additionally axially limits the movement of the magnets on one end of the rotor core  214 . 
     In an embodiment, on the other end of the rotor core  214 , a rotor end cap  226  is disposed, which also axially limits the movement of the magnets, described later in detail in this disclosure. Various embodiments and improvements to rotor end cap  226  are described later in this disclosure. 
     In an embodiment, a fan  218  is mounted on and rotatably attached to a distal end of the rotor shaft  212 . Fan  218  rotates with the rotor shaft  212  to cool the motor  200 , particularly the stator assembly  230 . In an embodiment, a pinion  205  may be disposed on the other distal end of the shaft  212  for driving engagement with the transmission assembly  114 . 
     According to an embodiment, stator assembly  230  includes a generally cylindrical lamination stack  232  having a center bore configured to receive the rotor assembly  210 . Stator lamination stack  232  includes a plurality of stator teeth extending inwardly from the cylindrical body of the lamination stack  232  towards the center bore. The stator teeth define a plurality of slots therebetween. A plurality of stator windings  234  are wound around the stator teeth. The stator windings  234  may be coupled and configured in a variety of configurations, e.g., series-delta, series-wye, parallel-delta, or parallel-wye. The stator windings  234  are electrically coupled to motor terminals  238 . Motor terminals  238  are in turn coupled to the power switch inverter circuit provided in (or driven by) control module  106 . Control module  106  energizes the coil windings  234  via the power switch inverter circuit using a desired commutation scheme. In an embodiment, three motor terminals  238  are provided to electrically power the three phases of the motor  200 . 
     In an embodiment, front and end insulators  236  and  237  may be provided on the end surfaces of the stator lamination stack  232  to insulate the lamination stack  232  from the stator windings  234 . The end insulators  236  and  237  may be shaped to be received at the two ends of the stator lamination stack  232 . In an embodiment, each insulator  236  and  237  includes a radial plane that mates with the end surfaces of the stator lamination stack  232 . The radial plane includes teeth and slots corresponding to the stator teeth and stator slots. The radial plane further includes axial walls that penetrate inside the stator slots. The end insulators  236  and  237  thus cover and insulates the ends of the stator teeth from the stator windings  234 . 
     According to an embodiment, motor  200  is additionally provided with two bearing support members  250  and  270  formed as motor caps disposed at and secured to the two ends of the stator assembly  230 . In an embodiment, both bearing support members  250  and  270  are made of insulating (e.g., plastic) material molded in the structural form described herein. In an embodiment, first and second bearing support members  250  and  270  are provided with axial post inserts  280  and  290  shaped to be received within the slots of the stator lamination stack  232  between respective adjacent stator windings  234 . In this manner, the bearing support members  250  and  270  are supported and piloted relative to the stator assembly  230 , thus structurally supporting the rotor assembly  210  relative to the stator assembly  230 . 
       FIG. 4A  is a side cross-sectional view of rotor assembly  210  provided with a conventional prior art rotor end cap  226 , according to an embodiment.  FIG. 4B  depicts a zoomed-in view of the rotor assembly  210 , particularly showing the interface between the rotor end cap  226  and rotor magnets  215 , according to an embodiment. 
     In these figures, the rotor core  214  includes a series of axially-oriented magnet pockets  213  within which four discrete permanent magnets  215  are embedded. One end of the permanent magnets  215  engage sense magnet  216 , which axially stops and retains the permanent magnets  215  within the magnet pockets  213 . The other end of the permanent magnets  215  is similarly retained by the rotor end cap  226 . In this embodiment, rotor end cap  226  includes a planar body secured to the end of the rotor core  214  that the permanent magnets  215  from moving out of the end of the rotor core  214 . It should be understood that a second rotor end cap may be used in place of the sense magnet  216 . 
     Due to stack-up tolerances and other inefficiencies, the length of the permanent magnets  215  and the rotor core  214  may include slight variations, causing a gap  228  to form between the rotor end cap  226  and one or more of the permanent magnets  215 . This gap  228  provides room for the one or more permanent magnets  215  room to axially wobble within the magnet pockets  213 , causing an imbalance in the rotor assembly  210 . It has been found by the inventors that this imbalance can attribute to high noise and vibration in the motor. 
     Embodiments of the invention described herein provide solutions for this problem. It has been found that these solutions can help reduce motor noise and vibration by up to approximately 80%. 
       FIG. 5A  depicts a side cross-sectional view of the rotor assembly  210  provided with a magnet retention cap  300  and a spring structure  310  for engagement and retention of the permanent magnets  215  in place of a conventional rotor end cap  226 , according to an embodiment. In this embodiment, the spring structure  310  is provided between the magnet retention cap  300  and the axial end of the rotor core  214 . The spring structure  310 , as discussed below, includes features that apply a biasing force to the permanent magnets  215  along the axial direction to counteract the stack-up tolerances and eliminate gaps at the ends of the permanent magnets  215  within the magnet pockets  213 , but reducing or substantially eliminating magnet wobble. 
       FIG. 5B  depicts a perspective view of the spring structure  310  alone mounted on the rotor core  214 , according to an embodiment.  FIG. 5C  depicts a side exploded view of the rotor assembly  210  with the spring structure  310  mounted and the magnet retention cap  300  provided at a distance, according to an embodiment.  FIG. 5D  depicts a side view of the spring structure  310 , according to an embodiment.  FIG. 5E  depicts a perspective view of the spring structure  310 , according to an embodiment. 
     As shown in these figures, in an embodiment, the spring structure  310  includes a planar disc-shaped body  312  having a diameter that is approximately equivalent to the diameter of the rotor core  214  and a center opening  320  through which a rotor shaft  212  extends. 
     In an embodiment, the spring structure  310  further includes a series of rectangular-shaped slots  316  provided equidistantly around the center opening  320  at a normal orientation relative to the center opening  320 . In an embodiment, each slot  316  is aligned with and is approximately the same size as a corresponding magnet pocket  213  of the rotor core  214 . In an embodiment, the spring structure  310  further includes a series of linear wave springs  314  extending from the planar body  312  into the series of slots  316 . There are the same number of slots  316 , and therefore the same number of springs  314  (four in this example), as there are permanent magnets  215 . 
     In an embodiment, each spring  314  includes a substantially rectangular profile when viewed along a longitudinal direction of the rotor shaft  212  and is provided in a floating manner within the corresponding slot  316 , connected only along one distal end to the planar body  312  and floating within the slot  316  on three sides. Further, each spring  314  is wave-shaped when viewed along a radial direction, including one or more humped portions  315  that project beyond the surface of the planar portion  312  facing the rotor core  214 . This arrangement provides a poka-yoke structure between the spring structure  310  and the rotor core  214 . When the spring structure  310  is mounted on the end of the rotor core  214 , the humped portions  315  of the linear wave springs  314  partially penetrate into the ends of the magnet pockets  213  of the rotor core  214  and engage the ends of the permanent magnets  215 . The linear wave springs  314  are resiliently moveable and capable of engaging permanent magnets  215  of various length variations. In this manner, linear wave springs  314  of the spring structure  310  apply biasing forces to the permanent magnets  215  in a direction away from the spring structure  310 , resiliently retaining the permanent magnets  215  within the magnet pockets  213  while providing enough flexibility to overcome slight manufacturing inconsistencies and stack-up tolerances. This arrangement significantly reduces or substantially eliminates magnet wobble within the magnet pockets  213  of the rotor assembly  210 . 
     In an embodiment, two fillets  318  are formed in the planar body  312  around the connection point of each spring  314 . The fillets  318  are recessed relative to the end of the spring  314  to protect against bending cracks. 
     In an embodiment, as shown in  FIGS. 5D and 5E , outer faces of the humped portions  315  of the linear wave springs  314  are substantially flat for approximately 1-3 mm to increase the surface contact area between the linear wave springs  314  and the permanent magnets  215 . 
     In an embodiment, a series of holes  322  are provided through the planar body  312 . In an embodiment, holes  322  are used to secure the spring structure  310  to the end of the rotor core  214  via, e.g., welding, soldering, etc. 
     In an embodiment, the spring structure  310  may be formed as a laminated steel using the same or similar die as the remaining steel laminations of the rotor core  214  lamination stack. The rectangular-shaped slots  316  may be stamped into the steel lamination and the wave springs  314  may be formed during the stamping process or at a later time via a machining process. The holes  322  may accordingly be used for interlocking the spring structure  310  to the remaining steel laminations using a single interlocking process that forms the rotor core  214 . 
     In an embodiment, the magnet retention cap  300  is mounted on the rotor shaft  212  to hold the spring structure  310  against the end of the rotor core  214 . In an embodiment, magnet retention cap  300  is mounted on the rotor shaft  212  via a bushing  302  press-fitted on the rotor shaft  212 . 
       FIG. 6A  depicts a partial side cross-sectional view of a spring structure  410 , according to an alternative embodiment.  FIG. 6B  depicts a perspective view of the spring structure  410 , according to an embodiment. 
     In this embodiment, similar to above, spring structure  410  includes a planar body  412 , a series of rectangular-shaped slots  416 , and linear wave springs  414 , among other features. Unlike the above structure, each spring  414  include humped portions  415  protruding from both surfaces of planar body  412 . Thus, either surface of the spring structure  410  provides a poka-yoke structure and can be mounted to the end of the rotor core  214  to retain the ends of the permanent magnets  215  within the magnet pockets  213 . 
       FIG. 7A  depicts a side cross-sectional view of rotor assembly  210  provided with an integrated end cap assembly  500  for retention of the permanent magnets  215 , according to an alternative embodiment.  FIG. 7B  depicts a perspective view of the integrated end cap assembly  500  including a spring structure  510  integrally supported by a magnet retention cap  502 , according to an embodiment.  FIG. 7C  depicts a side cross-sectional view of the integrated end cap assembly  500 , according to an embodiment.  FIGS. 7D and 7E  depict exploded views of the integrated end cap assembly  500  relative to the rotor assembly  210 , according to an embodiment. 
     In this embodiment, the spring structure  510  includes many of the same features as described above, including a planar body  512  including a center opening  520 , a series of slots  516  oriented along a normal direction to the center opening  520 , and a series of linear wave springs  514  extending into the slots  516 , among other features. In an embodiment, the spring structure  510  is designed to be securely mounted on the magnet retention cap  502  to form the integrated end cap assembly  500 . The integrated end cap assembly  500  is mounted as a unitary body on the end of the rotor core  214  with the linear wave springs  514  aligned with and received within the magnet pockets  213  of the rotor core  214  to axially retain the permanent magnets  215  therein. 
     As shown in these figures, in an embodiment, the spring structure  510  is mounted on and secured to a surface (front face) of the magnet retention cap  502  facing the rotor assembly  210  via a plurality of pins or fasteners (not shown) received through holes  522 . Alternatively, the magnet retention cap  502  may be formed via an overmolding or insert-molding process to capture the spring structure  510 . In yet another embodiment, magnet retention cap  502  may be included with one or more grooves into which the spring structure  510  is snapped and secured. 
     In an embodiment, the magnet retention cap  502  is mounted on the rotor shaft  212  via a bushing  504 . In an embodiment, the bushing  504  extend beyond the front face of the magnet retention cap  502 , allowing the bushing  504  to be received within the center opening  520  of the spring structure  510 . In an embodiment, the diameter of the center opening  520  is sized to be form-fittingly received around the outer circumference of the bushing  504 . In an embodiment, the spring structure  510  may be press-fit onto the bushing  504 . 
     In an embodiment, the magnet retention cap  502  includes a ring-shaped annular rim  506  projecting from its front face around the spring structure  510 . The outer diameter of the spring structure  510  sized to be form-fittingly received within the inner circumference of the ring-shaped portion  506 . In an embodiment, the outer diameter of the magnet retention cap  502  is approximately equal to the outer diameter of the rotor core  214 , and thus, the outer diameter of the spring structure  510  is smaller than the outer diameter of the rotor core  214 . 
       FIG. 8A  depicts a side cross-sectional view of rotor assembly  210  provided with a magnet retention cap  600  integrally including resiliently-deformable bosses  610  for retention of the permanent magnets  215 , according to an alternative embodiment.  FIG. 8B  depicts a perspective view of the magnet retention cap  600  including the resiliently-deformable bosses  610 , according to an embodiment.  FIG. 8C  depicts a partial perspective view of the rotor assembly  210  with resiliently-deformable bosses  610  alone shown in engagement with rotor magnets  215 , according to an embodiment. 
     In this embodiment, resiliently-deformable bosses  610  are provided in place of linear wave springs to retain the permanent magnets  215 . Each boss  610  is shaped to be received within a corresponding magnet pocket  213  of the rotor core  214  at a length of approximately 3-8 mm. Resiliently-deformable bosses  610  are made of elastic material such as rubber. In an embodiment, resiliently-deformable bosses  610  may be mounted on a face of magnet retention cap  600  directly. When received with the end of the magnet pockets  213  of the rotor core  214 , the bosses  610  partially penetrate into the ends of the magnet pockets  213  of the rotor core  214  and engage the ends of the permanent magnets  215 . The bosses  610  are resiliently deformable and thus capable of engaging permanent magnets  215  of various length variations. In this manner, bosses  610  apply biasing forces to the permanent magnets  215  in a direction away from the magnet retention cap  600 , resiliently retaining the permanent magnets  215  within the magnet pockets  213  while providing enough flexibility to overcome slight manufacturing inconsistencies and stack-up tolerances. This arrangement significantly reduces or substantially eliminates magnet wobble within the magnet pockets  213  of the rotor assembly  210 . 
     In an embodiment, a spacer  612  may also be mounted on the same face of magnet retention  600  as the bosses  610 . Spacer  612  may include slots  614  that correspond to magnet pockets  213  of the rotor core  214  and are formed around the bosses  610 . Spacer  612  has a smaller length than the bosses  610  and is designed to come into contact with the end of the rotor core  214  while the bosses  610  penetrate into the magnet pockets  213  of the rotor core  214 . 
     Example embodiments have been provided so that this disclosure will be thorough, and to fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Terms of degree such as “generally,” “substantially,” “approximately,” and “about” may be used herein when describing the relative positions, sizes, dimensions, or values of various elements, components, regions, layers and/or sections. These terms mean that such relative positions, sizes, dimensions, or values are within the defined range or comparison (e.g., equal or close to equal) with sufficient precision as would be understood by one of ordinary skill in the art in the context of the various elements, components, regions, layers and/or sections being described. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.