Resistors for rail traction 2
Capacitor charge/discharge resistors
Modern solid state traction systems use capacitor/inductor filter circuits to decouple the traction power supply from the drives. These filters prevent surges and unwanted harmonics entering the supply. Resistors are used in these circuits to limit inrush currents to the capacitors during charging and also to discharge them safely when required.
Motor starting and speed control resistors
This is the traditional application for power resistors in traction systems. Resistors are used where simplicity, reliability and robust construction are key considerations – for example, in mining locomotives.
Chopper and GTO thyristor drives can generate very high back EMFs across the various solid state components. This is a result of the high frequency switching of the power to the traction motors, typically several hundred Hertz. These potentially damaging voltages are dissipated in parallel resistor/capacitor snubber circuits, for which a key requirement is low inductance. Expanded mesh resistors are particularly suited to this requirement.
Construction: The basic construction will consist of banks of appropriately rated elements, supplied either for mounting within equipment cases, or as self-contained purpose-designed enclosures. All Cressall resistor elements are constructed from high grade stainless steel alloys, with mica and ceramic insulators, suitable for use in the most severe environments, and for roof or undercar mounting. Enclosures where supplied are in galvanised mild steel or stainless steel.
Thermal shock: All Cressall traction designs will meet the thermal shock rain test requirements of IEC60322. Expanded mesh resistors have the advantage that although the heated part of the element and the metal supporting it are formed from a single sheet, heat is kept out of the mountings, terminations and supporting insulators to a much greater extent than with other designs.
Cooling: The design of expanded mesh ensures exceptionally effective heat transfer. Compact designs for both natural and forced air cooling are available.
Locomotive testing: Load banks for diesel-electric rail generators
Download: Cressall Transit Catalogue