Linear current collection is similar to carbon brush collection except that in linear collection the collector is moving rather than the conductor. Current collectors have relative motion along a line instead of around a rotating cylinder; point or line contact is present, as opposed to the area contact of a brush. There is more frequent and more complete rupture of contact with the consequence of sparking and arcing. In addition, linear current collectors are more likely to operate outdoors, subject to the variations of environment and weather. In order to cope with these disadvantages, collector materials have to be much harder, denser and more abrasive than brush materials.
The main benefit of using carbon collectors is the skin development on the contact wire. This gives a substantial reduction in wire wear when compared with metal-to-metal surfaces. Since manufactured carbon has no molten state, this eliminates one of the major problems arising from metal-to-metal contacts under arcing conditions.
A pantograph carbon is a long extruded carbon fitted into an aluminium carrier that collects current from an overhead wire. The wire is at low current and high voltage.
At least half the world’s railway systems use carbon as a means of current collection and most new systems are specifying carbon. Every railway system has its own design of carbon and of carrier.
The carbon can be plain or metallised. Plain carbons are used on high voltage AC systems, for example 15kV or 25kV. Metallised carbons are used on low voltage DC systems of 75v to 3000v. The choice of material will depend on the current being taken through the carbon in static and /or moving condition.
Plain carbon is rated at 5Amm-1 of line contact, metallised at 12Amm-1 and up to 16Amm-1 for MY258. The figures refer to new carbon on a new wire. New wire has only 1mm of line contact, whereas once a wire is in regular service the flat can be up to 10mm, thus increasing the rating considerably.
Metallised material has a lower electrical resistance and greater strength than plain material, but metallised carbons are heavier and there is a current manufacturing limit of 450 mm.
The ideal situation is to have the right amount of carbon, strongly supported, without the pan head being too heavy. Theoretically, a larger carbon will last longer but, in practice, a large had mass and small contact-force will give short life.
The most popular methods of support are:
1) Tee bolts fitted to a sub-frame
T-bolts: uses short lengths of carbon mechanically crimped into tinned steel sheaths. The carbon can be straight or curved.
Each system has to be designed specifically to meet its own particular requirements.
Carbon has distinct advantages over other collector media, such as copper, sintered metals, or steel:
|i)||Longer collector life.|
|ii)||Longer wire life.|
|iii)||Lower maintenance costs.|
|iv)||Reduced running costs.|
|v)||Reduced pan head mass.|
|vi)||Ability to operate at high speed.|
|vii)||Elimination of Electrical interference.|
|viii)||Negligible audible noise.|
However, it is not possible to convert every system to carbon from metal strip. The design of the pan head, the pivot angle and wear patterns all have to be considered before any change can be undertaken.