Diesel Electric Locomotives

About 80% of the locomotives in Sri Lanka are Diesel Electric locomotives. Class M locomotives & Class S DMUs imported after S8 have this transmission method. The significance of this locomotive type is the ease in maintenance.

The main parts in a diesel electric locomotive is the fuel tank, diesel engine which is also known as the prime mover, radiator, air compressor, alternator or the generator, rectifier, inverter & traction motors. Let’s take these parts one by one & check what that part does.

First the diesel in the fuel tank flows to the diesel engine where the combustion takes place. The shaft connected to the diesel engine starts to rotate & the other end of the shaft is connected to an Alternator or the Generator. It acts as a turbine & converts the kinetic energy to generate AC (Alternative Current) electricity which is needed to move the train. This AC electric output travel through a rectifier. The process done here is converting AC input to DC (Direct Current) electricity.

Traction motors in most of the diesel electric locomotives are operated by DC electricity. Therefore the output from the rectifier can be used directly to power up traction motors. But most of the new locomotives have AC traction motors. For these motors to work, DC output needs to be converted to 3 phase alternative current. Therefore an Inverter is used to fulfil this task. Traction motors are fixed to the bogies of the locomotive. The axil of the traction motors are connected to axles of the bogies. Then the driver can drive the locomotive forward or backward at a required speed by changing the direction of the current & adjusting the amount of electricity. An air blower is included in this locomotives to cool down the traction motors when they get heated. This also helps to cool down the alternator.

When using electro dynamic brakes traction motors act as dynamos to generate power which needs to slow down the train. There are some advantages of this transmission type. Some of them are minimum wear & tear, easy to maintain, low maintain cost, less lubrication, smoothness in transmission & ability to run from overhead electricity if available. Apart from these there are some disadvantages when comparing to other transmission methods. The acceleration of DE locomotives are less than DH type locomotives & the power loss is higher than diesel hydraulics.

Based on a discussion in Yakada Yaka – Sri Lanka Railways facebook group. Special thanks to Nipuna Dodanthenna, Subhash Dhanasekara, Lahiru Kannangara, Mike Collins & Thurya Owitipana