Helix windings for linear propulsion systems

Maglev propulsion systems commonly employ either synchronous fields with a serpentine winding or a linear induction motor winding. Another alternative is a simpler heliical winding, the current for which is injected via a sliding contact. Long stator machines are forced to excite a lot more track than is required at any time and to place expensive switch gear along the track. Short stator induction machines are forced to perform much of the power handling on the vehicle and to deal with entry drag effects. A brush on the vehicle excites a helix winding on the track and eliminates both problems and uses the same magnetic field employed to get lift and guidance to supply propulsion. Because only a small section of the track is excited at a time, the efficiency is very high.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT AND DRAWINGS Motors with a source of magnetomotive force (MMF) on both sides of the air gap get better the bigger they are. They are typically preferred over motors that have a source of MMF on one side of the air gap only, for example the LRM. This invention uses this principle while attempting to circumvent the shortcomings of the conventional LSM and LIM. In the preferred embodiment of the present invention, a simple winding is used on the stator into which current is to be injected. This current interacts with a magnetic field on the vehicle to generate thrust. The current is injected through brushes, which slide over the helix winding in front of and behind the electromagnets. The electromagnets also serve the purpose of providing lift and guidance. The lift is active. To achieve this active lift, the air gap between the vehicle, specifically the electromagnets, and the track is monitored by a sensor. When the sensor detects a change in the size of the monitored air gap, the current in the electromagnets is adjusted to maintain a lifted position of the vehicle. The guidance in the system, however, is passive. Any lateral displacement of the vehicle from the alignment results in magnetic force directed at the maglev vehicle and acting to realign the guideway steel with the electromagnets. The guideway preferably consists of steel laminations around which the helix winding is wound. Shown in FIG. 1 is one preferred embodiment of the maglev system design. The active electromagnet exerts an upward force on the vehicle. The electromagnetic field from the electromagnet 9 is driven into the guideway steel, along the track travel direction, and then back into a paired electromagnet. The vehicle body 1 is attached to the bogie 2 through an air spring 3 . A landing skid 4 is set to catch the vehicle if the electromagnet support should fail. The guideway laminations 5 orient along the travel direction and are affixed to the structural concrete support 6 through bolts 7 . An aluminum end plate 8 fits over the wire helix (not shown). The electromagnet 9 is attached to the bogie and supplies the magnetic field to generate lift, guidance, and thrust, itself being excited by a control winding 10 . Current is injected into the helix via a sliding contact of brush 11 in the same manner that a dc motor is excited. To augment guidance, a separate lateral electromagnet 12 may be employed. Shown in FIG. 2 is the helix winding 13 wound around the guideway lamination structure. Current is driven into the winding through a brush 11 affixed to the vehicle. As the vehicle drives past the guideway, the brush 11 maintains a slidable contact with the helix winding, thereby exciting it. An end plate 8 preferably of aluminum is notched so that the wire of the helix winding fits into the notches, as shown in the end-plate blow up of FIG. 2 . Bolts 7 affix the guideway 5 with laminations oriented in the travel direction to the structural concrete support 6 . In this configuration the magnetic field for lift and thrust is the same. The lift field in the system is precisely controlled, and is proportional to the B field squared. The thrust is proportional to the B field. In practice, performing both tasks results in an electromagnet with a weaker B field, covering more distance to allow the thrust to build. Consequently it is preferable to utilize a maglev system having two paired electromagnets flanking the guideway, where vertically corresponding poles of these paired electromagnets have opposite polarity. This preferred geometry is shown in FIG. 3 . In this embodiment, when the paired electromagnets 9 a and 9 b are activated by the control winding 10 , they drive flux into the same point of the helix winding and then down the travel direction. Such a geometry allows the B field to be driven up considerably. The brush 11 is preferably attached to the side of the bogie 2 for inserting current into the helix winding 13 of the guideway 5 . An inductive sensor 14 is used to monitor the air gap 20 . When the air gap 20 is increased or reduced to fall outside its allowed size, the inductive sensor 14 adjusts the current in the control winding 10 , thereby changing the intensity of the magnetic field and the lift force generated by the paired electromagnets 9 a and 9 b. This adjustment continues until the original air gap is restored. Although not necessary, it is convenient in practice to separate the electromagnet functions. Shown in FIG. 4 is a side view of the layout of electromagnets on the bogie. In this embodiment, the two leading electromagnets 16 preferably only perform the function of lift. They are followed by paired electromagnets 17 which drive magnetic flux into the track from above and below, and use a higher magnetic field. The current in the leading electromagnets 16 is controlled by the air gap sensor (not shown), and directed to maintain a fixed air gap usually in the neighborhood of 10 mm. The trailing electromagnets 17 are driven at a higher magnetic flux level. When the guideway is centered between electromagnets, i.e. when the air gap is stabilized at its desired size, the trailing pair of electromagnets will not generate any lift regardless of the field strength. This high B field is then used to generate thrust with a nominal current. Since thrust is equal to BLi, where L is the working conductor length, B is the magnetic field density, and i is the current flowing in the helix winding, a high thrust can be achieved with a modest current. The brushes 11 are excited to realize the polarity indicated by the (±) signs. In this preferred embodiment, the current injected into these paired electromagnets 17 is also controlled. The control logic can be configured to maintain a zero vertical force, or to augment the lift force from magnets 16 slightly. Either is possible. In both cases the magnetic field density should be near the material saturation strength. Having only electromagnets on the lower surface is generally insufficient to produce adequate guidance. The stronger magnets placed above and below greatly enhance guidance forces even when the lift forces cancel completely. In order to suppress arcing, conventional dc motors employ interpoles. These small magnetic poles may be utilized with the present invention by inserting them between the primary magnet poles to suppress any voltage induced in the helix winding during commutation. In addition, to maintain an even voltage distribution over the helix, and allow operations at higher voltages, a compensation winding may also be employed with the present invention to counteract armature reaction. Both are shown in FIG. 5 , as they might be excited in a more appropriate electromagnet lamination. The interpole winding 18 drives flux into the helix winding 13 of the guideway 5 so as to lower the induced self voltage. The compensation winding 19 lies on the surface of the electromagnet and offsets the self field from the helix current. Having described this invention with regard to specific embodiments, it is to be understood that the description is not meant as a limitation since further variations or modifications may be apparent or may suggest themselves to those skilled in the art. It is intended that the present application cover such variations and modifications as fall within the scope of the appended claims.