A Theoretical Guide to Electric Locomotive Systems

Owing to their wide variety of advantages compared to diesel and steam locomotive systems, electric locomotive systems have become the most popular and widely used systems for traction systems.

With the advent of power electronic devices, modern electric traction systems are using multilevel inverters for better traction performance like high accuracy, quick responsiveness, and higher reliability.

The evaluation of electric motor design and electrification technologies have not only led to the design of high-speed locomotives (Metros and suburban railways), but also have raised the overall energy efficiency.

What is an Electric Traction or Locomotive?

A driving force that causes propulsion of a vehicle is referred to as a traction system. The traction system is of two different types: non electric traction system and electric traction system.

Non-Electric Traction System

The traction system that doesn’t use electricity at any stage of a vehicle movement is referred to as a non-electric traction system. Such a traction system is used in steam locomotives, IC engines, and in the maglev trains (high -speed trains).

Electric Traction System

The traction system that uses electricity in all stages or some stages of a vehicle movement is referred to as an electric traction system.

In an electric traction system the driving force to pull a train is generated by the traction motors. The electric traction system can be broadly divided into two groups: one is self-powered and the other one is third-rail system.

The self-powered systems include diesel electric drives and battery electric drives that can generate their own power to pull the train; whereas, the third-rail or overhead-wire systems use the power from an external distribution network or grids, and the examples include tramways, trolley buses and locomotives driven from overhead electric lines.

Types of Track Electrification Systems

The track electrification refers to the type of source supply system that is used while powering the electric locomotive systems. It can be AC or DC or a composite supply.

Selecting the type of electrification depends on several factors like availability of supply, type of an application area, or on the services like urban, suburban and main line services, etc.

The three main types of electric traction systems that exist are as follows:

1. Direct Current (DC) electrification system
2. Alternating Current (AC) electrification system
3. Composite system.

Direct Current (DC) electrification system

The choice of selecting DC electrification system encompasses many advantages, such as space and weight considerations, rapid acceleration and braking of DC electric motors, less cost compared to AC systems, less energy consumption and so on.

In this type of system, three-phase power received from the power grids is de-escalated to low voltage and converted into DC by the rectifiers and power-electronic converters.

This type of DC supply is supplied to the vehicle through two different ways: the first way is through the 3rd rail system (side running and under running electrified track and providing return path through running rails), and the second way is through the overhead line DC system. This DC is fed to the traction motor like the DC series or compound motors to drive the locomotive, as shown in the above figure.

The supply systems of DC electrification include 300-500V supply for the special systems like battery systems (600-1200V) for urban railways like tramways and light metros, and the 1500-3000V for suburban and mainline services like light metros and heavy metro trains. The 3rd (conductor rail) and 4th rail systems operate at low voltages (600-1200V) and high currents, whereas the overhead rail systems use high voltages (1500-3000V) and low currents.

Due to high starting torque and moderate speed control, the DC series motors are extensively employed in the DC traction systems. They provide high torque at low speeds and low torque at high speeds.

An electric motor speed controller is used by varying the voltage applied to it. The Special drive systems that are used to control these electric motors include tap changer, thyristor control, chopper control and micro processor control drives.

The disadvantages of this system include difficulty in interruption of currents at high voltages when fault condition is raised, and the need for locating DC substations between short distances.

Alternating Current (AC) electrification system

An AC traction system has become very popular nowadays, and it is more often used in most of the traction systems due to several advantages, such as quick availability and generation of AC that can be easily stepped up or down, easy controlling of AC motors, less number of substations requirement, and the presence of light overhead catenaries that transfer low currents at high voltages, and so on.

The supply systems of AC electrification include single, three phase, and composite systems. The Single phase systems consist of 11 to 15 KV supply at 16.7Hz, and 25Hz to facilitate variable speed to AC commutation motors.
It uses step down transformer and frequency converters to convert from the high voltages and fixed industrial frequency.

The Single phase 25KV at 50Hz is the most commonly used configuration for AC electrification. It is used for heavy haul systems and main line services since it doesn’t require frequency conversion. This is one of the widely used types of composite systems wherein the supply is converted to DC to drive DC traction motors.

Three phase system uses three phase induction motor to drive the locomotive, and it is rated at 3.3.KV, 16.7Hz. The high-voltage distribution system at 50 Hz supply is converted to this electric motor rating by transformers and frequency converters. This system employs two overhead lines, and the track rail forms another phase, but this raises many problems at crossings and junctions.

The above figure shows AC electric locomotive operation wherein the catenary system receives single-phase power from the overhead system. The supply is stepped up by the transformer, and then converted to DC by a rectifier. A smoothening reactor or a DC link, filters and smoothens DC to reduce the ripples, and then the DC is converted to AC by an inverter that varies frequency to get variable speed of the traction motor (similar to VFD).

Composite system

This system incorporates the advantages of both DC and AC systems. These systems are of mainly two types: a single phase to three phases or Kando system, and the other single phase to DC system.

In a Kando system, a single overhead line carries the single-phase supply of 16KV, 50Hz. This high voltage is stepped down and converted to three-phase supply of same frequency in the locomotive itself through the transformer and converters.

This three-phase supply is further supplied to the three-phase induction motor that drives the locomotive. Since the two-overhead line system of the three-phase system is replaced by a single overhead line by this system, it is economical.

As we have already discussed in the AC electrification that a single-phase to DC system is highly popular, it is the most economical way of single overhead line and has wide variety of DC series motor characteristics.
In this particular system, a single-phase 25KV, 50Hz supply of overhead line system is stepped down by transformer inside the locomotive, and then converted to DC by rectifiers. The DC is fed to the DC-drive system to drive the series motor and to control its speed and braking systems.

This is all about the electric locomotive systems. And, we hope that we have given you ample and relevant information about the various supply systems used in the traction systems.

We encourage you to write your suggestions, comments, and feedback about this article or project ideas in the comment section given below, and also expect your suggestions to reduce the short circuits accidents in the traction systems.

Photo Credits

• Electric locomotive systems by railway-technology
• Electric Vs Non electric traction by static.flickr
• 3rd rail system by conductix
• DC electrification system byrailway-technical
• AC electrification system by hesston
• Single phase to three phase or Kando system by railway-technical