In both diesel as well as electrical engine, the main goal is to supply power to the traction motor (which could be either an Induction Motor with 3 phases or DC Motor). These traction motors are linked to wheels that make the train move. The first step of each electric engine is the similar: Pantograph
Pantograph is composed of collector bars that are placed on overhead lines to take in electrical energy. It then flows over the bus bars on the roof of the locomotive, and then through a surge arrester as well as a vacuum circuit breakers (These are devices that protect locomotives from short circuitsand overload currents/effects of lightning, etc.)
The circuit breaker’s output is fed to the transformer
From there, based on whether your locomotive is equipped with DC motors (and tap changers) or an Induction motors (and VVVF control) the procedure will differ.
Locomotives with a Tap changer
In locomotives with a tap changers there are two transformers that are wound around the same core. The first has an automatic transformer that has 31 taps. Depending on where your taps are, voltage of the transformer changes and this variation of taps happens within the locomotive’s cab. The output of the auto-transformer is then fed into another transformer that is a step-down transformer that has a fixed ratio. It lowers the voltage to an appropriate, operating level that is suitable for semiconductor devices to which the output of the transformer is intended to be fed.
From here , the output of transformers are sent to rectify (that converts to AC into DC) because we are using Dc motors. The output from these rectifier block is fed to filters, where the ripples of rectifier output are smoothed out and then fed to DC switchgear as well as the combination control of motors for traction (as there are many motors that traction) as well as to motors for traction, which are controlled by DC.
The control of torque and speed is done by tapping changer that changes the voltage (and the consequent the current) that is fed to the system.
Locomotives equipped with VVVF control
The output of the transformer is fed into the power converter stage which converts AC into DC (convertor is a different name used to describe rectifier). These converters employ the GTO (old technologies) as well as IGBT (current technology) to achieve this. The output is then filtered by another circuit, which provides an almost smooth Dc output. There is also an amplification stage that can stifle the harmonics in the system as well as stages to increase the efficiency for the entire system.
The output is fed into an inverter stage, which transforms the DC into a three-phase Ac. Then, it is fed to the motors that traction.
This control is made possible by the use of VVVF controlsthat can alter the frequency and voltage. Control of Induction motors is very difficult, however the advent of power electronics have made controlling frequency quite simple
Final stage
The output of traction motors are not directly connected to wheels. We have reduction gears on top of them that reduce speeds (while increasing the torque). The different ratios of reduction gears allows us to utilize the same motors to traction passenger and freight locomotives.