Stopping a train requires the dissipation of a lot of energy. Conventional disc brakes alone suffer from too much wear and maintenance. Dynamic braking is therefore often used as additional system. In dynamic train braking, the kinetic energy of the train is transformed into electric current at the traction motors of the locomotive.
Often this system is additional to a friction braking system. The generated current is dissipated under the locomotive carbody in large onboard banks of resistors. To protect the resistors, they are cooled by big air fans. Another protection system is thermal monitoring. Whenever the resistors become too hot, the train will switch to friction braking. Nowadays, most diesel trains are equipped with dynamic brakes, but in the past the system was used mainly in the mountains, where overheating on long descents caused overheating of the traditional brakes. When a train with dynamic brakes is passing by, one can hear the use of the brakes which make a humming sound.
Brake resistors have relatively low ohmic values and a high power rating, like Microhm LHR and NWP series. Therefore, the wirewound resistor is a popular solution. Often they have a ceramic core and are fully welded. They are usually encased in a frame to create a safe distance to other parts. To increase dissipation capability, the frames are often executed with cooling fins, fans or even water cooling.
