The constant increase of speeds for passenger trains has imposed the improvement of their braking systems in order to ensure the safety of traffic and shorter braking distances. Apart from the automated pneumatic brake with direct acting, the basic braking system of any train, it was necessary to introduce additional braking systems.
As we all know, braking is used either to reduce the traffic speed of train up to an inferior limit (in the case of restrictions or speed limits), or to stop the train. When traffic speeds are high, the braking work is very high which means that a lot of energy is dissipated. To ensure the shortest possible braking distances, the energy of the train has to be dissipated as fast as possible, which means that the braking force has to be higher. Braking forces are, however, limited by the possibility of the system to dissipate heat, by imprinted decelerations (for the comfort of passengers), and in the case of basic railway braking systems, braking forces are limited by the adhesion of the wheels and rails – in order to avoid the blockage of wheels, the braking force has to be smaller or at least equal with the adhesion force. The wheel-rail adhesion coefficient decreases with the increase of speed and reaches small values in high-speed traffic. At high speeds, this limits the braking forces of the basic braking systems.
Due to the higher values of the energy that has to be dissipated when braking at high traffic speeds, a very serious problem is the wear of brake linings and discs, elements which participate in the development of braking forces. That is why it is recommended to also use the dynamic brakes: electric brake, dynamic and recuperative braking, eddy-current brakes, hydro-dynamic brake, etc. These additional braking systems don’t rely on the development of a direct friction force.
The eddy-current brake releases the braking force by inducing in the rail (linear eddy-current brake – see fig. 1) or into a pivoting conductor (pivoting eddy-current brake – see fig. 2) of the eddy currents, the electromagnetic field thus generated leading to the generation of a force with a direction opposite to the conveyance direction of the vehicle. The linear eddy-current brake has a great advantage compared to all the above-mentioned braking systems, namely that it is independent of the wheel-rail adhesion. Consequently, high braking forces can occur in the field of high-speed which leads to shorter braking distances. Apart from the above-mentioned advantage, there is also the reduced maintenance due to the lack of mechanical contact between the metallic mass and rail (such is the case of electromagnetic brake) and implicitly the lack of wear, braking is noiseless, controllable and different steps of braking can be achieved and it can also be used in combination with other braking systems etc.
The electromagnets are thus supplied so as to ensure the pole N and pole S successively to the metallic mass. When standing, the field lines look like in fig. 3a. When running, there is a relative movement between the electromagnets and the metallic mass and the field lines change their arrangement generating a force which opposes conveyance and thus reducing speed.
The implementation of this braking system was developed for the first time in 2002 by German company Knorr Bremse on the ICE 3 train manufactured by Siemens AG for Deutsche Bahn AG. Apart from the electromagnetic brake formed of yoke, pole core and coils, the braking system includes the following elements: bearings, crowbars for the transmission of braking forces, integral beams, connection bars and air gaps .
In normal exploitation conditions, the EWB 154 R system receives energy from the catenary through the traction converter, 4QS converter, of the intermediate continuous voltage circuit and of the dedicated chopper. In case the current in the overhead lien is interrupted, the traction engines operating as generators supply the coils of the brake with eddy-currents, the system being redundant.
In traction, the train and its 16 electric engines develop a total haulage power of 8 MW, in dynamic brake 7,2 MW (Velaro S 103 for Spain) , while the eddy-current brake can develop a maximum power of 13 MW [3].
Bibliography :
[1] Cătălin Cruceanu – “Brakes for Railway Vehicles”, Matrix Rom Publishing House, Bucharest, 2007
[2] Knorr Bremse – “Linear Eddy Current Brake EWB 154 R” – Presentation brochure
[3] Dr. Martin Schöpf – Knorr-Bremse – „Eddy Current Brake – An innovative wear-free braking system independent from wheel-rail adhesion”, 6th World Congress on High Speed Rail, Amsterdam 2008
[4] Cătălin Cruceanu (2012). Train Braking, Reliability and Safety in Railway, Dr. Xavier Perpinya (Ed.)
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