Abstract:
A system for propelling a vehicle in a desired direction using a linear induction motor situated proximate to, but off-board the vehicle. The linear induction motor has a suspension system adapted to suspend the motor a desired distance from a reaction rail, separate from any suspension system for the vehicle. The linear induction motor moves the vehicle using a generally rigid member extending between the linear induction motor and the vehicle. Application of a current to the stator of the motor communicates a desired force to the linear induction motor, the general rigid member, and the vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to U.S. App. Ser. No. 61/133,359, filed Jun. 27, 2008, titled Towing Linear Induction Motor for Transit Applications. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a propulsion system. In particular, the present invention is adapted for use with towed or driven vehicles, including in particular, a towing linear induction motor propulsion system for magnetic levitation (Maglev) vehicles suitable for use in transit systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Linear motors used in Maglev and other transit applications are generally either Linear Induction Motors (LIM) or Linear Synchronous Motors (LSM). LIMs are usually employed in the so-called “short stator” arrangement in which the motor primary or stator is short and vehicle mounted, while the motor secondary or reaction rail is passive and attached to or incorporated into the guideway along its full length. LSMs typically use a “Long Stator” arrangement, with a long motor primary is mounted on the guideway, while a short secondary is mounted on the vehicle. LIMs lead to a lower cost guideway, but a heavier and more expensive vehicle. LSMs lead to an expensive guideway, but offer a lighter, lower cost vehicle. It is generally accepted that LSMs are more efficient to operate. 
         [0004]    The weight of vehicle mounted LIMS has been identified as a performance and efficiency problem. In particular for Maglev applications, the levitation system for such vehicles must be strong enough to overcome the additional weight of the LIM stator or primary, its power supply, and any control systems. Operationally, this renders vehicle mounted LIMS less efficient to operate. Further, a vehicle mounted LIM can produce design challenges in inter-relation with the magnetic levitation system. For example, levitation systems typically are directed to producing larger air gaps, while the efficiency of LIMS is improved with a smaller air gap. 
         [0005]    Some approaches have employed LIMs installed within a track or guideway. As might be expected, however, such approaches erode the cost advantage for track or guideway construction adapted to support a Maglev vehicle having a vehicle mounted LIM. Accordingly, the main applications having LIMs installed within a track or guideway are cases that are functional with discrete propulsion over only a portion of the track, as with a roller coaster, for example. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, an aspect of the present invention is a system for propelling (i.e., towing or pushing) a vehicle in at least one desired direction. 
         [0007]    In one embodiment, the vehicle may be configured to levitate magnetically with respect to a levitation surface. The system involves a linear induction motor situated proximate to, but off-board, the vehicle. The linear induction motor has a stator that defines a channel adapted to receive a supported reaction rail, with “support” connoting the ability to receive a reaction force. The rail extends in the desired direction. The motor also includes a suspension system adapted to suspend the stator at a desired distance from the reaction rail. A motor bus subsystem is provided in conductive communication with the linear induction motor, a power supply, and a control system. A generally rigid member extends between the linear induction motor and the vehicle. This member has a motor attachment end affixed to the motor and a vehicle attachment end affixed to the vehicle. At least one pivot is within the member, such that the vehicle and the linear induction motor may be suspended independently of each other during operation without carrying the weight of the other. Application of a current to the stator of the linear induction motor via the motor bus subsystem creates a desired force along the at least one desired direction. The general rigidity of the member is such that the desired force along the at least one desired direction is communicated to the linear induction motor, the general rigid member, and the vehicle, such that they are propelled in the at least one desired direction. The motor suspension system may include rollers, wheels, a secondary magnetic levitation system, an air cushion system, at least a portion of the stator, etc. Optionally, the member further comprises a shock absorber. 
         [0008]    In some embodiments, attachment ends of the member comprise ball joints. Optionally, motor attachment end may be detachably affixed to the motor; optionally, the vehicle attachment end may be detachably affixed to the vehicle. In some cases, both attachment ends may be detachable. The power supply may be located off-board the vehicle, or on-board the vehicle. The control system may be located off-board the vehicle, or on-board the vehicle. 
         [0009]    The vehicle is not necessarily a Maglev vehicle. In one embodiment, the system involves a linear induction motor situated proximate to, but off-board, the vehicle. The linear induction motor has a stator that defines a channel adapted to receive a supported reaction rail, with “support” connoting the ability to receive a reaction force. The rail extends in the desired direction. The motor also includes a motor suspension system adapted to suspend the stator at a desired distance from the reaction rail. A motor bus subsystem is in conductive communication with the linear induction motor, a control system, and a power supply. A generally rigid member extends between the linear induction motor and the vehicle. This member has a motor attachment end affixed to the motor and a vehicle attachment end affixed to the vehicle. At least one pivot is within the member, such that the vehicle and the linear induction motor may be suspended independently of each other during operation without carrying the weight of the other. Application of a current to the stator of the linear induction motor via the motor bus subsystem creates a desired force along the at least one desired direction. The general rigidity of the member is such that the desired force along the at least one desired direction is communicated to the linear induction motor, the general rigid member, and the vehicle, such that they are propelled in the at least one desired direction. The motor suspension system may include rollers, wheels, a motor magnetic levitation system, an air cushion system, at least a portion of the stator, etc. Optionally, the member further comprises a shock absorber. 
         [0010]    In some embodiments, attachment ends of the member comprise ball joints. Optionally, motor attachment end may be detachably affixed to the motor; optionally, the vehicle attachment end may be detachably affixed to the vehicle. In some cases, both attachment ends may be detachable. The power supply may be located off-board the vehicle, or on-board the vehicle. The control system may be located off-board the vehicle, or on-board the vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will be better understood in relation to the attached drawings illustrating preferred embodiments, wherein: 
           [0012]      FIG. 1  is a front view of a prior art configuration of a Maglev vehicle with the guideway and LIM cross sections shown. 
           [0013]      FIG. 2  is a front view of an embodiment of the invention with guideway and LIM cross sections shown. 
           [0014]      FIG. 3  is a side view of an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An off-board LIM for a transportation system would enjoy some of the benefits of different approaches to propulsion. An aspect of this invention is that the LIM would be suspended (i.e., separately from vehicle suspension), either magnetically, by air cushion, or mechanically with respect to a dedicated, conductive passive reaction rail, which the LIM primary may wrap around. In some embodiments, a portion of the LIM stator may be used for suspension of the LIM. A wrap around design (sometimes referred to as tubular LIM) with motor suspension allows the LIM to have a very small air gap for increased efficiency. A separate, off-board LIM does not impose attractive or repulsive forces that could conflict with the forces of a primary suspension. The vehicle itself could be simpler than one carrying LIMs and the associated drive equipment. In one embodiment, the vehicle could be towed by a mechanical link attached to the LIM. Alternatively, the vehicle could be pushed by an off-board LIM. 
         [0016]    The uses of this invention may thus include magnetically levitated transportation systems, other transit vehicles including rail systems or highway vehicles, amusement rides, and a wide variety of other transport embodiments. Some of the advantages of this invention include that it is more efficient than conventional LIM approaches, and enables a lower cost, lighter vehicle. In addition, the present invention is less expensive than the LSM approach. Full advantage of the off-board LIM approach is manifested in Maglev transportation system, so primary description is to Maglev embodiments; however, the invention should not be construed as limited thereto. 
         [0017]    With reference to the drawings,  FIG. 1  is an illustration of an embodiment of a magnetic levitation vehicle  100  using a prior or conventional approach for LIM propulsion. The perspective is a front view of the prior art configuration with guideway  70  and LIM  30  cross sections shown. Bogie  50  supports vehicle  100  magnets  58  that inter-relate with magnets  78  supported by guideway  70 , forming a portion of the levitation system that enables the vehicle  100  to levitate magnetically above levitation surface  73  in a conventional manner. Guideway  70  also supports a passive, highly conductive reaction rail  75 , which engages electromagnetically with stator  35  of LIM  30 . Thus, in a conventional embodiment, vehicle  100  bears levitation power and control systems (not shown) as well as LIM  30  power and control systems (not shown). These can include elements such as direct current pickup from an energized rail, precharging resistors, inverters, etc. 
         [0018]      FIG. 2  is an illustration of an aspect of the present system for LIM propulsion of a magnetic levitation vehicle  100 , for comparison with  FIG. 1 . This figure is also a front view with guideway  70  and LIM  30  cross section shown, such that the desired direction of motion is out of the page. LIM  30  is situated near or proximate to, but off-board, the magnetic levitation vehicle  100 . The LIM  30  comprises a stator  35  defining a channel  31  adapted to receive a supported reaction rail  75 , which may be separate from the levitation surface  73 . The reaction rail  75  thus extends in the at least one desired direction. A motor suspension system  33  adapted to suspend the stator at a desired distance from the reaction rail  75 . Motor suspension system may take any of a variety of embodiments, such as rollers, wheels, balls, a separate or secondary maglev system, or air cushioning systems. 
         [0019]    A motor bus subsystem  37  in conductive communication with a power supply  77 . In this embodiment, drive inverter  60  and control system  65  are shown off-board vehicle  100  and LIM  30 . Elements of these components may be positioned on vehicle  100 , if desired for the application; however, such positioning may increase the levitation weight born by vehicle  100  during levitation and the weight to be propelled, decreasing efficiency for Maglev vehicle applications. Elements of these components may optionally be positioned on LIM  30 , which would avoid increasing the levitation weight, but still increasing weight to be propelled. In the embodiment shown, drive inverter  60  supplies three phase power to power supply  77 , with motor bus subsystem  37  shown as three phase conductive roller pickup as well. 
         [0020]    With respect to the side view of  FIG. 3  of a towing embodiment system with a single LIM  30 , a generally rigid member  20  extends between the LIM  30  and the vehicle  100 . The term “generally rigid” in reference to member  20  is such that a desired force generated by LIM  30  against reaction rail  75  (not shown) along the at least one desired direction is communicated to the LIM  30 , the generally rigid member  20 , and the vehicle  100 , such that they are all propelled in the at least one desired direction. Member  20  may take a variety of different forms, depending on the configuration of vehicle  100 , guideway  70 , reaction rail  75 , and relative positioning of LIM  30  with respect to vehicle  100 . 
         [0021]    Member  20  has a motor attachment end  21  and a vehicle attachment end  29 . At least one pivot  25  is provided such that the vehicle  100  and LIM  30  may be suspended or levitated independently of each other during operation without carrying the weight of the other. Pivot  25  may take a variety of embodiments, such as a ball pivot that permits relative motion in substantially all directions other than in the at least one desired direction. In another, simple embodiment, pivot  25  may be provided with member  20  in the form of a T-shaped bar with the transverse of the T-shape at the vehicle attachment end  29  riding in a vertical slot on vehicle  100  at a point corresponding to reaction rail  75 . Of course, simpler embodiments may lead to greater wear of member  20 , or a requirement for closer tolerances. 
         [0022]    Optionally, member  20  may include or incorporate other elements. For example, in some embodiments, motor bus subsystem  37  may be incorporated into member  20  if elements, such as drive inverter  60 , were to be positioned on vehicle  100 . In another example, member  20  may include a shock absorbing system (not shown). In another example, member  20  may include additional pivots or structure providing additional degrees of freedom, such as ball joints at motor attachment end  21  and vehicle attachment end  29 , as shown in  FIG. 3 . Optionally, member  20  may be detachably coupled to vehicle  100  at motor attachment end  21 , vehicle attachment end  29 , or some other desired location. Detachable coupling may permit separation for LIM  30  from vehicle  100 , for reasons such as maintenance, towing a different vehicle, or other desired use. 
         [0023]    Returning to  FIG. 2 , application of a current from power supply  77  to stator  35  via motor bus subsystem  37  creates a desired force along the at least one desired direction. LIM  30  may thus be used for acceleration, overcoming steady state resistance for operating a constant speed, or for braking. 
         [0024]    LIM  30  may take a variety of characteristics. For example,  FIG. 2  shows a circular reaction rail  75  with a LIM  30  having a wrap around, in facing, active stator  35  design, which can enable a very small gap for improved efficiency. 
         [0025]    Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.