Chap 3
Electric Traction Systems
The system which use electrical power for traction system i.e. for railways, trams, trolleys, etc. is called
electrical traction. 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:
Direct Current (DC) electrification system
Alternating Current (AC) electrification system
Composite system.
Electrification System Types
1. 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.
DC Traction System
This type of DC supply is supplied to the vehicle through two different ways:
3rd and 4 the rail system operate at low voltages (600-1200V)
Overhead rail systems use high voltages (1500-3000V)
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 metro trains.
1500-3000V for suburban and mainline services like light metros and heavy metro trains.
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.
Advantages;
In case of heavy trains that require frequent and rapid accelerations, DC traction motors are better
choice as compared AC motors.
DC train consumes less energy compared to AC unit for operating same service conditions.
The equipment in DC traction system is less costly, lighter and more efficient than AC traction system.
It causes no electrical interference with nearby communication lines.
Disadvantages;
Expensive substations are required at frequent intervals.
The overhead wire or third rail must be relatively large and heavy.
Voltage goes on decreasing with increase in length.
2. 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.
AC Traction System
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.
Advantages;
Fewer substations are required.
Lighter overhead current supply wire can be used.
Reduced weight of support structure.
Reduced capital cost of electrification.
Disadvantages;
Significant cost of electrification.
Increased maintainance cost of lines.
Overhead wires further limit clearance in tunnels.
Upgrading needs additional cost especially in case there are brigdes and tunnels.
Railway traction needs immune power with no cuts.
3. Composite System
Composite System (or multi-system) trains are used to provide continuous journeys along routes that
are electrified using more than one system. One way to accomplish this is by changing locomotives at
the switching stations. These stations have overhead wires that can be switched from one voltage to
another. Another way is to use multi-system locomotives that can operate under several different
voltages and current types. In Europe, it is common to use four-system locomotives. (1.5 kV DC, 3 kV DC,
15 kV 16⅔ Hz AC, 25 kV 50 Hz AC).
Electric Traction Drives
Definition: The drive which uses the electric power for moving forward, such type of drive is called an
electric traction drive. One of the major application of an electric drive is to transport men and materials
from one place to another. The traction drives are mainly classified into two types, i.e., the single phase
AC traction drive and the DC traction drive.
Electric Traction Services
The electric traction services can be broadly classified as
1) Electric trains
• Main-Line Trains
• Suburban Trains
2) Electric buses, trams and trolleys.
3) Battery and solar powered vehicles
The electric traction services are explained below in details.
1. Electric Trains
The electric train run on fixed rails is further classified as main line train and suburban train.
Main-Line Trains – In such type of train the power supplies to the motor in two ways, i.e., either from an
overhead line in an electric locomotive or through diesel generator set in a diesel locomotive.
In electrical locomotive, the driving motor is placed in the locomotive. The overhead transmission line is
laid along or above the rail. The current collector is attached over the locomotive has conductor strip
which slides against the supply conductor. Thus, maintain the contact between the supply and the
locomotive.The supply conductor is normally known as contact wire. For ensuring the good contact
between the current collector and supply wire the catenary cable and dropper wires are used.
In the high-speed train, the pantograph collector is used. The collector has the shape of a pentagon and
hence it is called pantograph collector. The collector has the conducting strip, which is pressed against
the contact wire by springs.The collector strip is usually made of steel, and it maintains the constant
pressure between the collector strip and the contact wire for preventing vertical oscillations.
The single phase supply is laid all along the track, and the current enters into the locomotive through the
collector.The power enters through the primary of the step-down transformer and returns to supply
earth through locomotive wheels.The secondary of the power transformer feeds the power modulator
which in turns power the driving motor. The secondary of the transformer also supply power for a
lightning fan, air conditioning, etc.
Suburban Trains – The suburban trains are used for travelling at small distances. This train is also known
as the local train.The suburban train has a consecutive stop at a much smaller distance. This train
consists motorised coaches for increasing the ratio of the weight on driving wheels and a total weight of
the train due to which the acceleration and de-acceleration of the train increase.
The each motor coach is equipped with electric drive and pantograph collector. The usual pattern for
using the power and unmotorised coaches is in the ratio of 1:2. For high powered train, the ratio may be
increased from 1:1. The train which uses motorise coach and trailer coach is also known as electrical
multiple unit (EMU) train.The power supply for the suburban train is similar to that of the main train
except in an underground train.
In the underground train, the DC supply is used because the DC supply system required lesser clearance
from the supply conductor of the train body. Also, the power modulator becomes simpler and less
expensive. The underground trains do not use the overhead transmission line, and hence the power is
supplied from the running rails or on one side of the tunnel.
2. Electric Buses, Trams and Trolleys
Such type of drive usually consists single motor driven coach. It takes the supply from the low-voltage
DC overhead line which is running along the roadside. As the current is generally small, the collector
consists of a rod carrying at its end a grooved wheel or two rods bridged by a contact bow.The collector
system is provided with enough flexibility, and it also provided an additional conductor for the return of
current.
The trams are electric buses which run on rails, and it consists a single motor coach. Sometimes, two or
more unmotorised or trailer coaches are added. Their current collection system is similar to buses, and
its return can be through one of the rails. The trams run on rails, and their path through road is fixed.
Electric trolleys are used for transporting material in mines and factories. It is mostly run on rails. They
are similar to trams; only the shape is different.
Important Features of electric traction drives
The important features of the electric traction drives are explained below.
The traction drive required large torque during start and acceleration to accelerate the heavy mass.
Because of economic reason single phase supply is used in AC traction.
The supply has sharp voltage fluctuations, including discontinuity when the locomotive crosses from one
supply section to another.
The harmonics injected into the source, both in AC and DC traction can cause interference in telephone
lines and signals.
Traction drive mainly used dynamic braking. A mechanical brake is also used when the drive is
stationary.
Duty Cycle of an electricTraction Drives
The duty cycle of traction drive is explained below with the help of speed-time curve and power torque
time diagram. Consider the drive travel between two consecutive stations on a level track. The train is
accelerated at the maximum possible torque, and the power increases linearly with speed.
At time t1 the base speed and the maximum allowable power is reached. Further acceleration occurs at
constant power.The torque and acceleration decrease with speed. At time t2 the drive torque equal to
the load torque, and steady speed is reached. The acceleration 0 to t2 has two parts. From 0 to t1 the
acceleration has constant torque, and at t1 to t2 the acceleration has constant power.
From t2 and t3 train runs at a constant speed and constant drive power. This duration is known as free
running. At the suitable time t4, the brake is applied to stop the train at the next station.
