One form of electrical drive for transit vehicles is the linear induction motor. Linear induction motors offer the advantage of mechanical simplicity, with consequent reduced maintenance requirements and greater reliability. Such a linear induction motor has a primary and a secondary, one of which is carried by the vehicle, and the other of which is provided in elongate form along the track.
For transit systems, it is common to provide the linear induction motor primary, which is supplied with the drive current, on the vehicle itself. The secondary is then provided along the track. The secondary is, in this case, a passive component of the system, which provides a low reluctance path for the magnetic field and a low resistivity facing material which enables the necessary induction currents to be generated. This enables the required thrust between the primary and the secondary to be generated.
It is to be appreciated that, since the secondary extends along the full length of the track, it is important that the secondary should be as cheap and economical as possible. Any construction that is excessively elaborate, will have a significant effect on the overall cost of a transit system. At the same time, the magnetic and electrical properties of the secondary should be as optimum as possible, otherwise the overall efficiency of the system will be impaired.
A common configuration for the primary and secondary of the linear induction motor is the so-called single-sided linear induction motor. In this case, both the primary and the secondary have just a single planar side or surface facing each other. As compared to, for example, double-sided systems where the primary might run in a slotted secondary, this has the advantage of relative simplicity. For a transit system using railway vehicles, the secondary is formed as a reaction rail extending longitudinally between the two rails of the track. At switches and the like, small gaps can be provided in the ion rail. This has little effect on the overall efficiency on the system, whilst not affecting the layout of the tracks themselves.
A simple and cheap construction for such a reaction rail could utilize solid iron and solid aluminum. Thus, for example, one could have a continuously extruded aluminum top surface. Below this, a continuous length of iron, of appropriate magnetic properties, having the required cross-section would extend. The aluminum would provide the electrical path necessary for the currents induced by the magnetic field from the primary, whilst the iron would complete the magnetic circuit formed between the primary and the secondary. Such an arrangement has the disadvantage that unwanted eddy currents would be generated in the conductive iron impairing the effectiveness of the magnetic circuit. This factor results in a reduction in LIM performance.
As is known for rotary machines, the problem of eddy currents can be eliminated by the provision of laminated iron cores. In rotary machines, this is achieved by clamping together a large number of thin iron sheets, with thin layers of electrical insulation between. The direction of the laminations is such as to provide a relatively uninterrupted magnetic path in the required direction, whilst interrupting the paths along which eddy currents might arise. Such an arrangement can considerably reduce eddy currents.
However, for a reaction rail, which might be provided along many miles of track, such a construction is complex and costly. Theoretically, one could construct the iron core of a reaction rail from a large number of elongate strips, with insulation between, each strip being relatively thin and flexible. For a typical reaction rail, experience has shown that assembly of such an iron core requires care and special techniques. It is necessary to secure the laminations together by transverse bolts or the like.
Accordingly, it is desirable to provide a reaction rail construction, which has good electrical and magnetic properties, but which is relatively simple and economic to produce.