Design

DESIGN CONSIDERATIONS – THE MEGAPOINT 750 VDC SWITCH ASSEMBLIES 12A, 50A AND 80A VERSIONS

Rail Heating
Using (DC) Power from the Third Rail

General considerations

Heating elements for rails are usually in the form of tapes or cables clipped to the rail. There are a number of different ways of taking and controlling power from the third rail to heat a section of rail or a the rails of a switch point. There are two basic possibilities. Heating elements can be rated at the live to running rail potential, or they can be rated at some other potential.

Low voltage elements

If heating elements designed for any voltage other than the live-rail-to-running-rail potential some form of switch mode converter must be operated directly from the rails. It may be that low voltage elements are cheaper than live-to-running rated elements, but an inverter that is reliable under all conditions and does not pass any electrical interference back to the rail system will be expensive.

Live-rail-to-running-rail rated elements .

Essentially all that is needed is a switch controlled by a temperature sensor on the heated rail. As usual it is not quite so simple as that, but it is much simpler than the use of a switching inverter.

Choice of basic system:

Any reasonably efficient device which can convert power at a direct pressure of 750 volts to, say, 110 volts has to use a fast switching process. The obvious configuration uses a semiconductor switch to connect an inductor between the source and the load until a certain current is reached. The switch then disconnects and a flywheel diode allows the inductor to continue to force current into the load until its magnetic field collapses. After a delay the process starts again. Output voltage (and therefore heat delivery) is varied by controlling the delay.

Whatever the configuration, the switching device is only separated from the power source by whatever filtering is necessary to control the flow of unwanted energy in either direction. Switching device rating, depending on the type of converter, will need to be at least double the load current or at least double the worst case source voltage.

On the other hand, using live-rail-to-running-rail rated elements the switching device can be separated from the power source by the heating element itself. Not only does this limit any possible surge current into the switch, it provides part of the suppression system for unwanted energy in both directions. Device rating needs to be worst case load current and worst case source voltage.

Suppression of unwanted energy is much easier. Switching events happen at times measured in seconds rather than microseconds. There is therefore far less energy to absorb, and it can be absorbed easily without disrupting system action.

Since live-rail-to-running-rail rated elements are readily available without significant cost penalty, the choice of basic system has to be the straightforward switch.

Switching Devices

Since heating elements rated at the live rail to running rail potential are readily available the system choice is to use them with a temperature controlled ON/OFF switch.

Historically speaking, many devices have been used to switch DC power. Currently the realistic choice of device to perform the actual switching operation rests between an IGBT (Isolated Gate Bipolar Transistor) and a contactor.

Superficially, a contactor is easier to use. It needs less protection from incoming noise and surges. However, frequent cycling, breaking 50A DC (note that there is no zero crossing to extinguish the arc) with a source voltage of up to 1KV suggests serious life problems or much expense.

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A suitably rated IGBT has no problems with repeated cycling. It must be driven correctly, and it must be protected from incoming interference.

In practice the devices needed to give a contactor some sort of life expectancy and those needed to protect an IGBT are very similar. The devices needed to stop outgoing (to the rails) interference are probably also very similar, although the contactor’s arcing must be expected to get worse as its contacts burn and distort.

One further consideration is that the temperature sensing and switch control electronics need some power. This low voltage power has its tedious side. For example, a 30mA at 24 volt supply can be derived using a 24 volt zener and a 22K ohm resistor. The problem with the resistor is that it has to specified to dissipate 37 watts continuously.

In practice, some current is needed for ancillary devices such as telephone diallers. Control package current is therefore at a premium. The IGBT control circuits take almost zero average current.

Specifications for relays capable of repeatedly switching 50A with a source pressure of up to 1KV DC are not available but the solenoid current requirements will certainly not be insignificant.

The conclusion is clear. Provided, of course, that incoming noise can be controlled the only realistic choice is an IGBT.