rail traction resistors

Resistors for rail traction

London Underground trains with Cressall brake 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.


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. 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.


Crow bar 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.