Rail Traction

Our resistors have been used in all kinds of transportation applications for more than sixty years and remain at the forefront of resistor technology.

Now we have integrated Cutler-Hammer's design and manufacturing expertise with our own, to form Cressall Resistors Transit, you can choose from the world's widest range of units with reliability and cost effectiveness that cannot be matched anywhere else. Dynamic Braking Snubbers in solid state chopper controls Crowbar Circuits Capacitor Charging/Discharging Starting and Speed Control

Rail Traction

Dynamic braking

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. Electrical braking is therefore used, although conventional disc brakes are still required for slow-speed braking.

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.

Snubber resistors

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.

Crowbar resistors

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.

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.

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.