 |
|
| Home> traction resistors> |
Cressall resistors have been used in transportation applications for more than sixty years and remain at the forefront of resistor technology.
Our customer demands are for ever-higher braking powers to be safely dissipated from resistors which have to be compact, lightweight, easily-maintained and with lifetimes to match the locomotives in which they are installed. Using our sophisticated 3D modeling and thermal simulation software Cressall’s engineers can rapidly offer optimized designs.
To prove our resistors once built, we have within our Group the best-equipped test laboratory of any resistor manufacturer in the world, equipped with a 2 ton capacity 3-axis vibration table, a controlled power source of 1.5MW, and monitoring equipment that allows a fully realistic test of the complete run-cycle of any resistor.
We make traction resistors for:
Dynamic Braking
Crowbar Circuits
Capacitor Charging/Discharging
Starting and Speed Control
Snubbers in solid state chopper controls
The energies involved in stopping high speed trains are so great that disc brakes alone are unsuitable because of their very high wear rates and consequent maintenance costs. Whenever possible regenerative braking is used. In this case the drive motors convert the kinetic energy of the train into electricity, which is fed back into the power supply and used elsewhere on the network. Alternatively the same regenerated electricity may be dissipated as heat in on-board or trackside resistive (or rheostatic) brakes.
This is an effective and easy to control braking method. Rheostatic brakes are non-wearing and unlike regenerative braking systems are totally independent of the external network conditions.
 |
| 3kV crowbar resistor and mounting frame |
The two types of crowbar resistor, hard and soft, are both used in traction power supply circuits to deal with the effects of transient or longer lasting over-voltage conditions. The soft crowbar is pulsed to dissipate transient over-voltages; if these persist or worsen then the main breakers are opened and the system is short-circuited through the hard crowbar to absorb the stored energy.
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.
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.
 |
| Snubber resistor on Eurostar |
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
Print Page
Cressall Resistors Ltd, Evington Valley Road, Leicester, LE5 5LZ U.K. Tel: (+44) (0)116 273 3633