Muqadsa Iftikhar (03)
Zunaib Ali (09)
Tayyaba Saleem (24)
Zenab Fazal (34)
Ayaz Khan (43)
Driving forward of vehicle is called Traction and the system,
which employs this type of mechanism is called Traction
classified broadly into two groups namely:
(1)non-Electric traction systems
(2)Electric traction systems
The system which use electrical power for traction system i.e.
for railways, trams, trolleys, etc. is called electrical traction.
Electric traction means a locomotion in which the driving
force is obtained from electric motors.
Electric traction will continue to play a dominant role in the
Railway since it is an environmentally clean technology
compared to diesel traction and also its running cost is low.
There are many advantages of electric traction over other
forms of locomotion.
Ideal Traction System
(i) Maximum tractive effort should be exerted at
starting in order to have rapid acceleration.
(ii) Equipment should be capable of overloads for short
(iii) The wear caused on the track should be minimum
(iv) Braking should be possible without excessive wear
on the brush
(v) Capability of withstanding voltage fluctuations and
temporary interruption of supply.
(vi) Parallel running usually more than one motor (two or
four motors ) should be possible.
Advantages of Electric Traction
Cheapness. It is cheapest method of all other methods of traction.
Cleanliness. It is free from smoke and flue gasses
Maintenance cost. Maintenance and repair cost is about 50% of
steam traction system.
Starting time. It can be started without loss of time.
High starting torque. This system uses of d.c. & a.c. series
motors, which has a very high starting torque.
Braking. In electric traction , regenerative breaking is used which
feeds back 40%of the energy.
Saving in high grade coal. No coal is required for electric traction.
Disadvantages of Electric Traction
(i) High initial expenditure
(ii) Failure of supply is a problem
(iii) Electrically operated vehicles have to move
only on electrified track
(iv) For braking & control, additional equipments
(v) Interference with telegraphs and telephone
System Of Track Electrification
(i) D.C. Systems
(ii) A.C. Systems
(a) Three phase A.C. system
(b) Single phase standard frequency system
(c) Single phase low frequency system
(d) Single phase to three phase system [Kondo
e.g 600V dc is given to tramways and trolley buses.
Track System Classification
In general following electric traction systems exist :
• AC 3 phase 3.7 kV system
• AC single phase 15/16 kV -161/25 Hz
• AC single phase 20/25 kV - 50/60 Hz
• DC 600 V
• DC 1200 V
• DC 1.5 kV
• DC 3 kV.
Advantages & Disadvantages of A.C. System Over
1. Light overhead catenary
2. Saving in sub-stations
3. More flexibility
4. Lesser equipment used & easy to maintain
5. Starting efficiency
6. Cheap as far as cost for fixed installation
1. Single-phase A.C. system imposes Unbalancing effect
2. Produces interference with telecommunication lines
Catenary - This is the overhead wire of special cross-section.
Pantograph - This is a device used for collecting current from the
catenary i.e. the over head lines.
Auxiliary Equipment Cont…
Circuit Breakers - The function of circuit breaker is to disconnect the
engine from the line side in case of some fault
On Load Tap Changer - On load tap changer is used for varying the
output voltage for controlling the speed of motor
Transformer - A tap changing transformer is installed in the locomotive
for stepping down the voltage according to the requirement of
Rectifiers - Semiconductor rectifiers are used for conversion of A.C
power into D.C power
Smoothing Reactors - Any ripples found on the output side of the
rectifier are eliminated by the smoothing reactor fitted after the
Independent Control- SEPEX
Short form of SEParate EXcitement of traction motors where
the armature and field coils of an electric motor are fed
with independently controlled current.
This has been made much more useful since the introduction
of thyristor control where motor control can be much
Schematic of AC Electric Locomotive Power System with
Thyristor Control and Separately Excited DC Motors
Thyristor Control Circuit for DC Supply to DC Motors
DC Resistance Control
Simplified DC Traction Motor Power Control Circuit
Parallel Motors - DC Resistance Control
Schematic of Simple DC Traction Motor Power Control Circuit
DC Traction Motor
It consists of two parts, a rotating armature and a fixed field.
• The fixed field consists of tightly wound coils of wire fitted
inside the motor case
• The armature is another set of coils wound round a central
• The armature is connected to the field through "brushes"
which are spring loaded contacts pressing against an extension
of the armature called the commutator.
The DC Traction Motor: How it Drives the
How a DC motor drives the axle through a pinion and
AC Motor Traction
Schematic of Single Phase AC Supply Powering 3-Phase AC
AC Locomotives with AC Drive
AC Motor Traction
Schematic of AC locomotive with Tap Changer Control of
AC Locomotives with DC Drives
If any electric motor is to be stopped, electric supply to it is
In this way, the motor shall take long time to stop, as the
motor shall continue to rotate due to inertia.
Some braking system must be used so that it stops within the
pre-determined short time.
A good braking system must posses the following features
(i) It should be fast and reliable.
(ii) The braking force must be capable of being controlled
(iii) Kinetic energy of the rotating parts of the motor and its
driven machine must be suitably dissipated & suitable
means must be provided.
Advantages & Disadvantages of Electric Braking
(i) It is quite fast
(ii) It is quite cheap as far as maintenance part
(iii) Higher speed can be maintained
(iv) Heat produced during braking is not harmful
(v) It is free from fires and is smoother
(i) It can stop the motor but it cannot hold it
(ii) It cannot be applied to all motors
(iii) Its initial cost is very high.
Types Of Electric Braking
Dynamic Braking: where the motors become generators
and feed the resulting current back.
It classified into three categories:
2. Rheostatic Braking
3. Regenerative Braking
PLUGGING: Generally plugging is applied by changing the
phase sequence of the three phase induction and
• The motors become generators and feed the resulting
current into an on-board resistance.
• When the driver calls for brake, the power circuit
connections to the motors are changed from their power
configuration to a brake configuration and the resistors
inserted into the motor circuit. As the motor generated
energy is dispersed in the resistors and the train speed
• The motors become generators and feed the resulting
current back into the supply system
• A train could use its motors to act as generators and that
this would provide some braking effect if a suitable way
could be found to dispose of the energy.
• Trains were designed therefore, which could return
current, generated during braking, to the supply system
for use by other trains.
•No single motor is ideal for traction purposes. But; DC
Series Motors are most suitable for this work
•Power Electronics have been resorted to in a major way as in
case of thyristor control.
•Different types of braking for the traction system was