Patent Publication Number: US-11377325-B2

Title: Linear propulsion system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 15/100,740 filed Jun. 1, 2016, which is a U.S. national stage of Patent Application no. PCT/US2013/073303 filed Dec. 5, 2013, all of which are incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to linear propulsion system, and, in particular, relates to self-propelled elevator systems. 
     BACKGROUND OF THE DISCLOSURE 
     Self-propelled elevator systems, also referred to as ropeless elevator systems are envisioned as useful in various applications (i.e., high rise buildings) where the mass of the ropes for a roped system is awkward and there is a desire for multiple elevator cars in a single hoistway. There exist self-propelled elevator systems in which a first hoistway is designated for upward traveling elevator cars and a second hoistway is designated for downward traveling elevator cars. A transfer station at each end of the hoistway is used to move cars horizontally between the first hoistway and the second hoistway. 
     A cost effective elevator system is desired. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the disclosure, a linear propulsion machine is disclosed. The linear propulsion machine may comprise a first stator and a first mover. The stator may include a plurality of teeth. The first mover may be adjacent to the first stator and moveable in a linear direction along the first stator. The mover may include a plurality of spaced apart ferromagnetic strata, a plurality of slots, each of the slots adjacent to at least one of the strata, a plurality of wire coils, and a plurality of magnet layers. Each magnet layer may be sandwiched between two of the strata and disposed inside one of the plurality of coils. Each coil may be disposed in at least one slot, and the coils may have an activated state and a deactivated state. Each coil may be disposed substantially perpendicularly to the direction of magnetic flux of the permanent magnet layer around which the coil is wound. The teeth or the permanent magnet layer may be disposed at an angle from a plane substantially perpendicular to the direction of thrust on the first mover generated by the interaction of the first mover with the first stator. 
     In a refinement, the plurality of slots may be a multiple of a number of phases of the linear propulsion machine. 
     In another refinement, each of the plurality of teeth may include a skewed side surface. 
     In yet another refinement, each of the plurality of teeth has a tooth width and a distance across the slot between two strata is a slot width, wherein the slot width is about the same as the tooth width. 
     In another refinement, each of the strata may include a skewed strata side surface. 
     In another refinement, the linear propulsion machine may further include a second stator. The first mover may be disposed between the first and second stators. 
     In another refinement, the linear propulsion machine may further include a second mover moveable in a linear direction along the first stator. The first stator may be disposed between the first and second movers. In a further refinement, the magnet layers of each of the first and second movers may be angled in relation to the first stator. In another refinement, the linear propulsion machine may further include a third and fourth mover moveable in a linear direction along the first stator. The first stator may be disposed between the third and forth movers and the third and fourth movers may be offset from the first and second movers. 
     In accordance with another aspect of the disclosure, another linear propulsion machine is disclosed. The linear propulsion machine may comprise a first stator, and a first mover. The stator may include a plurality of teeth, each of the teeth having a magnetic pole. The first mover may be adjacent to the first stator and moveable in a linear direction along the first stator. The teeth of the first stator and the first mover may define a gap. The mover may include a plurality of spaced apart ferromagnetic strata, a plurality of slots, a plurality of wire coils, and a plurality of permanent magnet layers. 
     Each permanent magnet layer may be sandwiched between two of the strata and disposed inside one of the plurality of coils. Each of the slots may be adjacent to at least one of the strata. Each coil may be disposed in at least one slot. The coils may have an activated state and a deactivated state. The plurality of permanent magnet layers may be mounted on the mover to have reversed polarities with consecutive permanent magnet layers in a longitudinal direction along the mover. Each coil may be disposed substantially perpendicularly to the direction of magnetic flux of the permanent magnet layer around which the coil is wound. The teeth or the permanent magnet layer may be disposed at an angle in the range of about −60° to about 60° from a plane substantially perpendicular to the direction of thrust on the first mover generated by the interaction of the first mover with the first stator. 
     In a refinement, each permanent magnet layer may have a trapezoidal shape. 
     In another refinement, each permanent magnet layer may be comprised of first and second permanent magnets. The first permanent magnet may be disposed between the stator and the second magnet. The first permanent magnet may be a bonded magnet. 
     In accordance with yet another aspect of the disclosure, an elevator system is disclosed. The elevator system may comprise a hoistway, a car disposed within the hoistway, and a linear motor. The linear motor may include a first stator disposed in the hoistway and including a plurality of teeth, and a first mover mounted on the car and adjacent to the stator. Each of the teeth of the first stator may be a magnetic pole. The first stator may be made of laminated ferromagnetic material. The first mover may be moveable in a linear direction along the first stator. The mover may include a plurality of spaced apart strata made of laminated ferromagnetic material, a plurality of slots, a plurality of permanent magnet layers, and a plurality of wire coils. Each of the slots may be adjacent to at least one of the strata. Each permanent magnet layer may be sandwiched between two of the strata. Each permanent magnet layer may be comprised of a first magnet and a second magnet. The first magnet may be disposed between the second magnet and the stator. The first magnet may be a bonded magnet. The second magnet may be a sintered magnet. The coils may have an activated state and a deactivated state. Each coil may be disposed in at least one slot and wound around one of the permanent magnets. Each coil may be disposed substantially perpendicularly to the direction of magnetic flux of the permanent magnet around which the coil is wound. The linear motor may be a polyphase motor. A first group of the plurality of wire coils may carry alternating current with a different phase than a second group of the plurality of wire coils. 
     In a refinement, the teeth may be disposed at an angle in the range of about −60° to about 60° from a plane perpendicular to the direction of thrust on the first mover generated by the interaction of the first mover with the first stator. 
     In another refinement, the permanent magnet layers may be disposed at an angle in the range of about −60° to about 60° from a plane perpendicular to the direction of thrust on the first mover generated by the interaction of the first mover with the first stator. 
     In another refinement, the plurality of teeth may be one greater than the plurality of slots. 
     In another refinement, the elevator system may further include a second stator. The first mover may be disposed between the first and second stators. 
     In another refinement, the elevator system may further include a second mover mounted to the car and moveable in a linear direction along the first stator. The first stator may be disposed between the first and second movers. In a further refinement, the teeth of the first and second movers may be disposed at an angle in the range of about −60° to about 60° from a plane perpendicular to the direction of thrust on the first mover generated by the interaction of the first mover with the first stator. 
     In another refinement, the elevator system may further include a third and fourth mover mounted to the car and moveable in a linear direction along the first stator. The first stator may be disposed between the third and forth movers and the third and fourth movers may be offset from the first and second movers. The offset may be a distance in the range of more than zero to about one per unit stator channel pitch. 
    
    
     
       These and other aspects of this disclosure will become more readily apparent upon reading the following detailed description when taken in conjunction with the accompanying drawings. 
         FIG. 1  is an embodiment of an exemplary elevator system; 
         FIG. 2  is an another embodiment of an exemplary elevator system; 
         FIG. 3  is a perspective view of one exemplary embodiment of a linear motor for an elevator system constructed in accordance with the teachings of this disclosure; 
         FIG. 4  is a perspective view of another exemplary embodiment of a linear motor; 
         FIG. 5  is a schematic front view of the linear motor of  FIG. 4  with the lines of magnetic flux illustrated; 
         FIG. 6  is a perspective view of another exemplary embodiment of a linear motor; 
         FIG. 7  is a schematic front view of the linear motor of  FIG. 6  with the lines of magnetic flux illustrated; 
         FIG. 8  is an alternative embodiment with two linear motors mounted on an exemplary car; 
         FIG. 9  is a graph of the average thrust force and percentage thrust force ripple as a function of offset between the two linear motors of  FIG. 8 ; 
         FIG. 10  is a graph of the thrust force as a function of linear motor offset for the two linear motors of  FIG. 8 ; 
         FIG. 11  is a front view of the exemplary linear motor of  FIG. 3  with the stator teeth angled; 
         FIG. 12  is a front view of the exemplary linear motor of  FIG. 3  with the magnet layers of the mover angled; 
         FIG. 13  is an enlarged schematic of an exemplary magnet layer of a mover; 
         FIG. 14  is a schematic of another exemplary embodiment of a magnet layer; 
         FIG. 15  is a schematic showing an enlarged view of a portion of an exemplary mover; and 
         FIG. 16  is a schematic showing an enlarged view of a portion of an exemplary stator and tooth. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to be limited to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the present disclosure. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The linear propulsion system  10  disclosed herein may be utilized in applications that require movement of a vehicle along a track. For example, the linear propulsion system may be utilized for elevators, trains, roller coasters, or the like. 
     To facilitate the understanding of this disclosure, the linear propulsion system will be described as utilized in a linear motor propelled elevator system. It is to be understood that the linear propulsion system is not intended to be limited to elevator applications. The elevator application described herein is an exemplary embodiment described in order to facilitate understanding of the disclosed propulsion system. 
     Referring now to  FIG. 1 , a propulsion system  10  is shown in schematic fashion. The propulsion system is an exemplary elevator system that utilizes one or more linear motors. As shown in  FIG. 1 , the elevator system  10  comprises a hoistway  18  that includes a first hoistway portion  12  and a second hoistway portion  16 . The first and second hoistway portions  12 ,  16  may each be disposed vertically within a multi-story building. The first and second hoistway portions  12 ,  16  may be dedicated to directional travel. In some embodiments, the first and second hoistway portions  12 ,  16  may be part of a single open hoistway  18 . In other embodiments, the first and second hoistway portions  12 ,  16  may be part of a divided hoistway  18  that has a wall or other divider between the first and second hoistway portions  12 ,  16 . The hoistway  18  is not limited to two hoistway portions. In some embodiments, the hoistway  18  may include more than two hoistway portions disposed vertically within a multi-story building. 
     In the embodiment illustrated in  FIG. 1 , elevator cars  14  may travel upward in the first hoistway portion  12 . Elevator cars  14  may travel downward in the second hoistway portion  16 . Elevator system  10  transports elevator cars  14  from a first floor to a top floor in the first hoistway  12  and transports elevator cars  14  from the top floor to the first floor in the second hoistway  16 . Above the top floor is an upper transfer station  20  where elevator cars  14  from the first hoistway  12  are moved to the second hoistway  16 . It is understood that the upper transfer station  20  may be located at the top floor, rather than above the top floor. Below the first floor is a lower transfer station  22  where elevator cars  14  from the second hoistway  16  are moved to the first hoistway  12 . It is understood that lower transfer station  22  may be located at the first floor, rather than below the first floor. Although not shown in  FIG. 1 , elevator cars  14  may stop at intermediate floors to allow ingress to and egress from an elevator car  14 . 
       FIG. 2  depicts another exemplary embodiment of the elevator system  10 . In this embodiment, the elevator system  10  includes an intermediate transfer station  24  located between the first floor and the top floor where the elevator car  14  may be moved from the first hoistway portion  12  to the second hoistway portion  16  and vice versa. Although a single intermediate transfer station  24  is shown, it is understood that more than one intermediate transfer station  24  may be used. Such an intermediate transfer may be utilized to accommodate elevator calls. For example, one or more passengers may be waiting for a downward traveling car  14  at a landing on a floor. If no cars  14  are available, an elevator car  14  may be moved from the first hoistway portion  12  to the second hoistway portion  16  at intermediate transfer station  24  and then moved to the appropriate floor to allow the passenger(s) to board. It is noted that elevator cars may be empty prior to transferring from one hoistway to another at any of the upper transfer station  20 , lower transfer station  22 , or intermediate transfer station  24 . The elevator system  10  includes one or more stators  30  disposed within each hoistway portion  12 ,  16 . The stator  30  generally extends the length of the hoistway portion  12 ,  16  and may be mounted on a support frame, a wall of the hoistway  18 , or the like. The elevator system  10  further includes one or more movers mounted to each car  14 . 
     Turning now to  FIG. 3 , therein is shown one exemplary arrangement of a first stator  30  disposed within a hoistway portion  12 ,  16  and a first mover  32  mounted to a car  14 . The stator  30  may include a plurality of teeth  34  and is made of ferromagnetic material. For example, in one embodiment, the stator  30  may be made of silicone steel, or the like. In some embodiments, the stator  30  may also be a laminated material. In between each of the teeth  34  is a channel  35 . Typically, the quantity of channels  35  is an even number. 
     Each of the plurality of teeth  34  has a tooth width W. Further, each of the teeth  34  of the stator  30 , when adjacent to the mover  32 , may be a magnetic pole M. For example, in  FIG. 3  the stator  30  has eight (8) poles Mover the length of the first mover  32 . The number of poles M of the stator is different than the number of slots  42  of the mover (slots  42  are discussed in more detail later) over the length of the mover. For example, the number of poles M may be one (1) greater or less than the number of slots  42 , two (2) greater or less than the number of slots  42 , three (3) greater or less than the number of slots  42 , etc. In general, the closer the number of slots  42  of the mover is to the number of poles M of the stator (over the length of the mover), the better the performance of the linear motor. 
     The mover  32  is adjacent to the stator  30  and is moveable in a linear direction along the stator  30 . The teeth  34  of the stator  30  and the mover  32  define a gap  38 . In some embodiments, the gap  38  may be an air gap. The mover  32  may include a plurality of spaced apart strata  40 , a plurality of slots  42 , a plurality of wire coils  44 , and a plurality of permanent magnet layers  46 . 
     Each strata  40  may each be made of ferromagnetic material. In some embodiments, each strata  40  may be made of laminated ferromagnetic material. In some embodiments, each strata  40  may be generally L-shaped such that two consecutive strata may form a U-shape pair. The strata  40  are not limited to this shape and may be other shapes as well. 
     Each slot  42  is adjacent to at least one of the strata  40 . In linear embodiments, the slots  42  may be grouped as internal full slots  42   a  and external half slots  42   b . The internal slots  42   a  are disposed between two strata  40 . The external half slots  42   b  are adjacent to one strata  40 . Typically the external half slots  42   b  are the first and the last slots  42  on a mover  32 . The distance across a slot  42  between two strata  40  is a slot width Ws. In one embodiment, the slot width Ws may be about the same as the tooth width W. In other embodiments, the slot width Ws may be different than the tooth width W. Two of the external half slots  42   b  are equivalent to one full slot  42   a . The mover slot pitch Sp is the distance between the midpoint of a first slot  42  and the midpoint of the next adjacent slot  42 . 
     In one embodiment, the plurality of slots  42  is a multiple of the number of phases of the linear propulsion machine or linear motor  48  that comprises at least one mover  32  and at least one stator  30 . More specifically, the quantity of slots  42  is a multiple of the number of phases P of the linear motor  48  in order to achieve a balanced winding and may be defined by the equation: quantity of slots=k*P where k is an integer. For example, in the embodiment of  FIG. 3 , the linear motor is a three-phase machine. The value of P is three (3), the value of k is three (3) and the resulting number of slots is equivalent to nine (9) full slots (eight full slots  42   a  plus two half-slots  42   b ). 
     Each wire coil  44  is disposed in at least one slot  42 . Each coil  44  is wound around, or encircles, the combination of magnet layer  46  and the at least two strata  40  that sandwich the magnet layer  46 . As can be seen in  FIG. 3 , two separate coils  40  are wound through, or disposed in, each internal slot  42   a.    
     The coils  42  may be operably connected to a source of electrical current (not shown). The source may provide multi-phase current as is known in the art. For example, the linear motor illustrated in  FIG. 3  is a three-phase machine that can receive the three alternating currents A, B, C of a three-phase electrical source. In such a three-phase system, three groups of coils  42  (A, B, C) each carry one of the three alternating currents of the same frequency which reach their peak values at one third of a cycle from each other. As illustrated in  FIG. 3 , the coils  42 A and  42 A′ carry the A phase, the coils  42 B and  42 B′ carry the B phase, and the coils  42 C and  42 C′ carry the C phase. Current direction into the page is indicated by A′, B′ and C′. Current direction out of the page is indicated by A, B, C. 
     Each coil  44  may be made of a conductive material such as copper, aluminum, a combination of the two, or the like. Each coil  44  has an activated state and a deactivated state. When activated, current is flowing in the coil  44 . Each coil  44  disposed, in relation to the magnet layer  46 , perpendicular to the direction of magnetic flux of the magnet layer  46  around which the coil  44  is wound. This orientation ensures that the current in the coil  44  is also perpendicular to the magnetic flux. 
     Each magnet layer  46  is sandwiched between two of the strata  40  and disposed inside one of the plurality of coils  44 . Each magnet layer  46  may be a permanent magnet or an electromagnet. The plurality of magnet layers  46  are mounted on the mover  32  to have reversed polarities with consecutive magnet layers  46  in a longitudinal direction along the mover  32 . 
     In operation, the interaction of the activated coils  44  of the mover  32  with the stator  30  produces a thrust on the mover  32  attached to the car  14  and propels the car  14  along the stator  30 . While, the combination of stator  30  with the mover  32  is described in conjunction with use as a motor  48 , it may also be used as a generator during regeneration. 
     Turning now to  FIG. 4 , therein is illustrated another embodiment of a motor  48 . Elements of  FIG. 4  that correspond to elements in  FIG. 3  are labeled with the same reference numerals where practicable. In the embodiment of  FIG. 4  the elevator system  10  includes a first stator  30   a , a second stator  30   b  and a mover  32  disposed between the first and second stators  30   a ,  30   b . The stators  30   a ,  30   b  are similar to that discussed with reference to the embodiment of  FIG. 3  except that, in order to facilitate flow of flux between the mover  32  and each of the stators  30   a ,  30   b , each of the strata  40  are not L-shaped as they were in the embodiment illustrated in  FIG. 3 . The teeth  34   a  of the first stator  30   a  and the mover  32  define a first gap  38   a , and the teeth  34   b  of the second stator  30   b  and the mover  32  define another gap  38   b . As in the previous embodiment illustrated in  FIG. 3 , each gap  38   a ,  38   b  may be an air gap. Further, each of the teeth  34   a ,  34   b  of the each stator  30   a ,  30   b  may be a magnetic pole Ma, Mb. To achieve better performance, the teeth  34   a  of the first stator  30   a  may be offset (vertically) from the teeth  34   b  of the second stator  30   b . In one embodiment, the offset may be in the range of greater than zero to about one stator channel pitch Cp. The channel pitch Cp is defined as the distance from the midpoint of a first channel  35  to the mid-point of a second adjacent channel  35 . The offset is beneficial to the production of useful force/torque. Maximum thrust force may be generated when each of the teeth of one stator is offset from the teeth of the other opposing stator by half of a stator channel pitch Cp. 
     Turning to  FIG. 5 , therein is schematically illustrated the lines of magnetic flux of the exemplary embodiment illustrated in  FIG. 4  when the coils are excited by a current load. As can been seen the lines of magnetic flux  50  flow from one of the poles of the magnet layer  46 , in this embodiment a permanent magnetic layer, to the stator teeth  34  and back to the magnet layer  46 . In  FIG. 5 , the coils have been removed from the illustration so as not to obscure the magnetic lines of flux  50  in the illustration. 
     Turning now to  FIG. 6 , therein is illustrated an embodiment of the linear motor  48  for use in the elevator system  10 . Elements of  FIG. 6  that correspond to elements in  FIG. 3  are labeled with the same reference numerals where practicable. In the embodiment of  FIG. 6 , the linear motor  48  of the elevator system  10  includes a stator  30  disposed between two movers  32 , namely a first mover  32   a  and a second mover  32   b.    
     Both of the movers  32   a ,  32   b  are mounted to a car  14 . The first mover  32   a  and the teeth  34  of the stator  30  define a first gap  38   a  as does the second mover  32   b  and the teeth  34  of the stator  30 . This arrangement includes double the magnets of the embodiment shown in  FIG. 3  and thus provides a higher power density and force on the movers  32   a ,  32   b  relative to the other embodiment. 
     Turning to  FIG. 7 , therein is schematically illustrated the lines of magnetic flux of the exemplary embodiment illustrated in  FIG. 6  when the coils are excited by a current load. As can been seen the lines of magnetic flux  50  that flow from the poles of the magnet layers  46 , in this embodiment a permanent magnetic layer, to the stator teeth  34  and back to the magnet layers  46  are combined for the two movers  32   a ,  32   b , which generates a greater thrust or force on the movers  32   a ,  32   b  than in embodiments having only one mover  32 . In  FIG. 7 , the coils have been removed from the illustration so as not to obscure the magnetic lines of flux  50  in the illustration. 
     Turning now to  FIG. 8 , therein is illustrated an alternative embodiment in which the elevator system  10  includes two of the linear motors of  FIG. 6 . More specifically, the elevator system  10  of  FIG. 8  includes first, second, third and fourth movers  32   a ,  32   b ,  32   c ,  32   d  mounted to a car  14 . Similar to the embodiment of  FIG. 6 , the stator  30  is disposed between the first and second movers  32   a ,  32   b . In  FIG. 8 , the stator  30  is also disposed between the third and fourth movers  32   c ,  32   d . Elements of  FIG. 8  that correspond to elements in  FIG. 3  are labeled with the same reference numerals where practicable. 
     Each of the third and fourth movers  32   c ,  32   d  is offset a distance D from the first and second movers  32   a ,  32   b . This offset reduces the thrust force ripple to provide a better quality ride in the car  14  for passengers. Thrust force ripple may occur due to variation in stator  30  permeance experienced by the movers  32   a ,  32   b ,  32   c ,  32   d  as they traverse the stator  30 . Reduction in thrust force ripple can be achieved by adjusting the position of the movers  32   a ,  32   b  of the first linear motor  48   a  relative to the movers  32   c ,  32   d  of the second linear motor  48   b  so that the instantaneous thrust force ripple generated by each is cancelled while the average thrust force is maintained constant. This reduces vibrations and improves ride quality. The distance D may be in the range of greater than zero to about one per unit stator channel pitch Cp. 
       FIG. 9  illustrates the average thrust force  52  and percentage thrust force ripple  54  as a function of offset between the two linear motors  48   a ,  48   b  of  FIG. 8 . The offset has been unitized in terms of stator channel pitch. It can be seen that there are three different offset distances, in this exemplary embodiment, that result in thrust force ripple being reduced from about 48% to about 12%. This is achieved by generating an instantaneous thrust force of linear motor  48   b  ( FIG. 8 ) that is out of phase to that of linear motor  48   a . In the exemplary embodiment illustrated in  FIG. 9 , the ranges of per unit stator channel pitch Cp that provided lower thrust force ripple were as follows: about 0.14 to about 0.17 per unit stator channel pitch Cp; about 0.47 to about 0.53 per unit stator channel pitch Cp; and about 0.80 to about 0.86 per unit stator channel pitch Cp. 
       FIG. 10  illustrates the thrust force as a function of linear motor offset. Line  56  illustrates the machine force per unit for linear machine  48   a , line  58  illustrates the machine force per unit for linear machine  48   b , line  60  illustrates the total force generated by the two linear motors  48   a ,  48   b  without offset, and line  62  illustrates the total force generated by the two linear motors with offset. 
       FIG. 11  illustrates a variation in which the teeth  34  of the stator  30  may be angled. Although shown with regard to the embodiment of  FIG. 3 , this variation is applicable to each of the aforementioned embodiments in  FIGS. 3-4, 6 and 8 . The term “angled” as used herein means rotated an angle from a plane H perpendicular to the direction of thrust on the mover  32  generated by the interaction of the mover  32  with the stator  30 . The angle for the stator  30  teeth  34  is the angle a and may be in the range of about −60° to about 60° from the plane H perpendicular to the direction of thrust on the mover  32  generated by the interaction of the mover  32  with the stator  30 . 
       FIG. 12  illustrates a variation in which the magnet layers  46  of the mover  32  may be angled. Although shown with regard to the embodiment of  FIG. 3 , this variation is applicable to each of the aforementioned embodiments in  FIGS. 3-4, 6 and 8 . The angle for the mover  32  magnet layer  46  is the angle  8  and may be in the range of about −60° to about 60° from the plane H perpendicular to the direction of thrust on the mover  32  generated by the interaction of the mover  32  with the stator  30 . 
       FIG. 13  illustrates in an enlarged schematic a variation of the exemplary magnet layer  46 . This variation is applicable to each of the aforementioned embodiments in  FIGS. 3-4, 6, 8, 11 and 12  and aids in reducing magnet losses and achieving higher operating efficiencies. In  FIG. 13 , each magnet layer  46  may be comprised of one or more magnets. In the example shown in  FIG. 13  the magnet layer is comprised of three magnets, a first magnet  66   a , a second magnet  66   b  and a third magnet  66   c . The magnet  66   a  disposed closest to the stator teeth  34  may be made of a bonded magnet with high resistivity properties. The second and the third magnets  66   b ,  66   c  may be sintered magnets. In another embodiment, each magnet layer  46  may be comprised of a plurality of magnets including a group of sintered magnets and a group of bonded magnets. Each magnet in the sintered group is a sintered magnet, and each magnet in the bonded group is disposed between the sintered group and the stator teeth  34  and is a bonded magnet with high resistivity properties. 
       FIG. 14  illustrates, in an enlarged schematic, a variation of the exemplary stator. This variation is applicable to each of the aforementioned embodiments in  FIGS. 3-4, 6, 8, 11, 12 and 13 . In  FIG. 14 , each magnet layer  46  may be trapezoidal shaped. The trapezoidal shape helps reduce magnetic saturation and allows the linear propulsion machine to operate at higher force densities. 
       FIGS. 15-16  illustrate variations in which a plurality of surfaces  70  of the strata  40  (of the mover  32 ) and/or a plurality of surfaces  72  of the stator  30  may be skewed. The term “skewed” as used herein means that the applicable surfaces  70 ,  72  taper from a plane perpendicular to the direction of thrust on the mover  32  generated by the interaction of the mover  32  with the stator  30 . The variations illustrated in  FIGS. 15-16  may be applicable to each of the aforementioned embodiments in  FIGS. 3-4, 6, 8, and 11-14 , and with each other. 
       FIG. 15  is an enlarged view showing a portion of an exemplary mover  32 .  FIG. 15  illustrates a variation in which one or more surfaces  70  of each of the strata  40  of the mover  32  may be skewed. In  FIG. 15 , the wire coils  44  and permanent magnet layers  46  have been removed in order to better show the skewing of the strata  40 . In the exemplary embodiment of  FIG. 15 , each strata  40  includes an outer side surface  70   a , an inner side surface  70   b ; a left side surface  70   c  and right side surfaces  70   d ; the outer side surface  70   a  of each strata  40  (that is adjacent to the permanent magnet layer  46 ) is skewed and the inner side surface  70   b  (that is adjacent to the wire coils  44 ) is skewed. In this particular embodiment, left and right sides surfaces  70   c ,  70   d  are not skewed. In an embodiment in which the strata  40  are skewed as described above, the permanent magnet layers  46  and wire coils  44  of the mover  32  may also be skewed in order to accommodate the strata  40  geometry. 
       FIG. 16  is an enlarged view showing a portion of an exemplary stator  30 .  FIG. 16  illustrates a variation of the stator  30  in which one or more surfaces  72  of the stator may be skewed. In the exemplary embodiment of  FIG. 16 , each tooth  34  has an upper and lower side surface  72   a ,  72   b . It can be seen that the lower side surface  72   b  of each tooth  34  is skewed. Although not visible in the illustration, the opposing upper side surface  72   a  of each tooth is also skewed in this embodiment. 
     INDUSTRIAL APPLICABILITY 
     In light of the foregoing, it can be seen that the present disclosure sets forth a linear propulsion system. In one exemplary embodiment the linear propulsion system is an elevator system utilizing one or more linear motors per car. Such elevator systems may be most appropriate for propulsion of non-counterweighted or ropeless elevator cars. 
     In the embodiments disclosed herein, the stator is free of active elements and is mechanically strong, rigid and simplified. The active elements, the magnetic layers and the coils of wire, are disposed on the mover instead of the stationary stator positioned in the hoistway. Because the magnets and coils of wire do not line the entire stator track, fewer are utilized overall. This results in a more cost efficient system without sacrificing thrust force. In addition, the angling of either the teeth of the stator or the magnet layer results in improved efficiency and skewing results in reduced thrust force ripple. 
     Furthermore, in embodiments in which a plurality of linear motors are utilized for each car, the linear motors may be so positioned on the car to reduce thrust force ripple. 
     While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure.