یادگیری JAES

It might be surprising to know that in electric trains, the power collected from the overheadlines ends up in the grounding cable of the track after flowing through the wheels. Three phase power conversion, regenerative braking and zig-zag overheadlines - all these make electric train technology quite unique. Let’s understand all the engineering secrets behind the electric trains starting from the simplest design possible.

The simplest version of an electric train is shown here. Here, a single sliding wire collects electric power from the overhead lines. This power is fed to a single-phase induction motor. The induction motor's rotor is connected with the wheels. To complete the circuit, the induction motor's other terminal is grounded. This grounding connection is possible because the wire is first connected to the wheels through an axle brush. The wheel is always in contact with the track, and the track is grounded, as shown. You can see how the current from the OHL flows to the ground. It first passes through the induction motor, then the axle brushes, then the wheels, and finally to the track and ground. When the current passes through, the induction motor's rotor spins, along with the wheels. However, a spinning wheel will move forward, which means the next grounding circuit should be available at a very short distance from the previous one.

Now, let’s look at some axle brush details. The axle brushes are fitted to the rotating wheel, and the current is transferred to the wheels through these carbon brushes, which slip over the disk attached to the wheel axle. Again, this is the simplest version of an electric train.

Now,let’s improve this design and make our train more functional. The voltage used in the overhead line is 25 Kilo Volt, which is a huge voltage. Motors require much less voltage to run. For this reason, we need to feed it to a step down transformer, which transforms the voltage to the desired level. The power supply from the Overhead Line initially passes through the transformer's primary, and here, the circuit is completed due to the grounding. Because of the transformer's action, the current will be induced in the secondary winding and reduced voltage power is fed to the motor.

For high traction like the train, the motor should supply a high torque. Moreover, the torque curve should be uniform, even if the motor speed varies. Three-phase induction motors are the perfect choice to achieve high, uniform torque requirements. Stretching three-phase wire to power this motor is not a good idea, as it’s highly uneconomical. That’s why a rectifier and inverter is used to convert the single-phase supply to the three-phase supply. A rectifier converts the single-phase AC power into DC power, and then the inverter converts the DC power into three-phase AC power. Now this engine is ready to run over the track.

We can increase the torque output further if we add a transmission system with some gear ratio between the motor shaft and the wheel axle. At this point, the engine we just made seems perfect.

However, if we make a complete train by connecting several coaches to this engine, it becomes quite clear that the single motor engine we developed doesn't have sufficient power to pull all these coaches. So, let’s add more motor-wheel pairs to make the engine more powerful. An engine bogie is constructed with three three-phase induction motors, obviously to drive three pairs of wheels. In an engine, generally two such engine bogies are used. In this complete engine you can see how the transformer and rectifiers are positioned.


So far in this video, we've shown how we transfer power to the train using a single hanging wire. However, this method is not practical. Sometimes it is not possible to keep the distance between the train and overhead line the same, which means for proper power collection, a height-varying mechanism should be used. Pantographs accomplish this task. A modern pantograph looks like this. You can see that based on the pressure of a pneumatic system, the pantograph can adjust its height. If you observe carefully, you can see that during this height adjustment, the current-collector remains horizontal. The current-collector has to be perfectly horizontal; otherwise, power transmission will be in trouble. We hope from this animation you can understand why the current collector always remains horizontal; for more details of it, please watch our detailed video about the pantograph. Now, let’s see the pantograph in action. If for some reason the pantograph loses its connection with the wire, the train will continue its free run for a few kilometres due to its high momentum.