Most of the locomotives used in Sri Lanka are diesel electric locomotives. They move by rotating axles by traction motors which are powered by a generator connected to the diesel engine. There are some braking methods as vacuum brakes & air brakes. Apart from them there is another system called Dynamic Brakes & this is an additional brake system.
The functioning of this brake system can be described as follows. The current which goes to the traction motors is diverted through a large resister grid. Therefore the electric energy converts into heat by the resister grid. A fan is fixed near this resister grids to cool them down.
When dynamic brakes are turned on traction motors act as an electricity generator & axle act as the armature. Due to the connected resistor grid & provided heavy load the electromagnetic force induced in the traction motor reduces the speed of the armature. However a locomotive cannot stop by using only dynamic brakes. Dynamic brake system stops functioning when the locomotive reaches a certain low speed. After that train should be stopped using the primary brake system.
Dynamic brakes are functional in a given speed range only. If you apply dynamics when the train is traveling at a higher speed resister grids get destroyed due to overheating. When this happen it’s very difficult to control the locomotive & it may lead to an accident. It’s said that the accident faced by the Colombo Kandy Intercity Express train which hauled by Class M6 788 was happened due to this reason. Class M6 manufacturers have stated that the maximum speed the locomotive should maintain to apply dynamics is 20kmph.
The throttle should be at its maximum notch to apply full dynamic brakes & the recommended method for Class M6 locos is the throttle should be in idle position for about 10 seconds before applying dynamics.
The brake shoes get subjected to wearing quickly when using primary brakes. In the inclines of upcountry line brakes should apply frequently. Therefore dynamic brake is used to reduce wearing of the brake shoes.
Class M2, M5, M6, M9, M10 & W3 (Hydro-Dynamic) locomotives & Class S5, S12 DMUs have dynamic brake facility. Class M4 locomotives had this in early days but now it’s removed.