Why no neutral wire electric traction and why AC powered trains are more common than DC powered ones

Grounding Mechanism

As you might have seen in our video on electric trains, we are using the tracks as a ground for the train. Considering that this is a huge waste of energy, why are we doing that? Can’t we use a neutral wire along with the tracks or another neutral OHL wire?

Fig:1 - Electric train with OHL wire and grounding system
Fig:1 - Electric train with OHL wire and grounding system

Using a neural wire to provide the return path for the current is a complex, impractical, and unreliable solution. To feed the return current into the neutral wire, we need a sliding mechanism that can slide over the neutral wire either below or above the train. Not only would this be complicated to implement, but it would be very costly. You may be wondering whether the tracks can be used as a neutral wire. Althogh we are using it as a path to the ground. Leaving them on the ground is comparable to leaving live wire on the ground, which is obviously not recommended. In this case, the tracks will be connected to the power grid through poles at regular intervals. Although this decreases the possibility of getting shocked, it is still impractical and generally inadvisable.

Fig:2 - Basic model of single-phase motor-wheel pair
Fig:2 - Basic model of single-phase motor-wheel pair

Drive Train System

In our video, the simplest electric train we demonstrated was with a single-phase induction motor. We could have used a DC motor instead, because in our video, we converted AC power into DC power via a rectifier and then DC power into variable frequency AC power through an inverter. However, after converting AC into DC, can’t we directly use DC power to run the train with the help of DC motor. There are many DC motor operated trains in use, but most of the electric locomotives work using AC motors.

Characteristics of DC motors

In our video, we used induction motors in electric trains. But are DC motors more reliable and advantageous than AC motors? The DC motor is somewhat easy to control, which is why it is more often used for frequent start-and-stops. It also has a very high starting torque, which makes it a good choice for traction purposes. On the other hand, AC three-phase induction motors have a low starting torque but highly efficient for the long run.

Fig: 3a- Torque curve for AC motor
Fig: 3a- Torque curve for AC motor
Fig: 3b-Torque curve for DC motor
Fig: 3b-Torque curve for DC motor

In the electric train, the type of DC motor used is a combination of DC series and a shunt motor. Initially, the DC motor operates on series and then gets converted into a shunt motor. While a DC series motor has very good starting torque, regenerative braking cannot be applied to it because as we reduce the voltage applied to the motor, the back EMF becomes more than the applied voltage due to rotor rotation. The current gets reversed in the rotor as well as in the stator. The net torque remains positive and the motor continues to run in the same direction. That’s why a convertible motor is used in DC traction.

Fig:4 – Third Rail System for DC traction
Fig:4 – Third Rail System for DC traction

For the DC traction system, two types of transmission systems are used: overhead wire and third rail. The third rail option is very prevalent nowadays in many countries because it is very reliable and economical for short run. But it is not safe because of security reasons and DC traction operates on low voltage and as the transmission losses depend on the current so losses are very high that’s why it requires substation at very frequent intervals.

We hope you enjoyed this Lesics members-only article. Thank you for being part of our team!

 144