J.SOMLAL, M.Tech.,(Ph.D), MISTE, MIETE,
K L University,
vWhat is Traction?
ØThe act of drawing or pulling, as by an elastic or
ØTraction refers to the maximum frictional force that
can be produced between surfaces without slipping.
ØTRACTION is resulting from a specific
FRICTION coefficient (i.e. friction between rubber
and ground) combined with area of ground covered
by the tire – FOOTPRINT combined with vehicle
WEIGHT pressing a tire onto the ground.
ØTRACTION is a resistance between tire and
Why do we need traction?
vWell, that question comes up every once in a while.
Its a good question?
Ø If our feet do not find a surface with good grip
(traction) our legs and feet could not move us
Ø Same story for a car - if the ground would not
provide enough resistance (traction) the force
generated in the engine would not be able to move
the car forward.
Ø The scenario is quite simple. The more traction can
be made available the more torque can be
generated to move more weight.
Requirements of an IDEAL Traction System
The following are some of the important requirements
of the driving equipment used for traction purposes:
Ø The coefficient of adhesion(sticking) should be high
so that high tractive effort at start is possible and
rapid acceleration of the train can be obtained.
Ø It should be possible to overload the equipment for
Ø The wear(consume by use) caused on the shoes,
wheel tires and the track should be minimum.
Ø It should be possible to use regenerative braking so
that on descents it should be possible to generate
energy and feed back to the supply system.
Ø It should be pollution free.
v 1. Non-Electric Traction System: Does not use
electricity at any stage. Ex: Steam Engine Drive,
Internal Combustion Drive.
v 2. Electric Traction System: If electric supply is
used for driving a locomotive, the system is
known as electric traction.
ØInvolves use of electricity at some stage or other.
Ex: Battery Electric Drive, Diesel Electric Drive,
ØElectric traction is the most efficient of all other
systems and is going to be the future system to be
adopted by almost all countries of the world.
Types of Electric Traction
v Vehicles which receive electric power from a
distribution network fed at suitable points from either
a central power station or substations suitably spaced.
It is further subdivided into
Ø 1) Systems operating with d.c. such as trolley buses,
tramways and railways.
Ø 2) Systems operating with a.c. such as railways.
v The group consists of self contained locomotives i.e.
they generate their own power.
It is further subdivided into
Ø 1) Diesel electric trains and ships
Ø 2) Petrol electric trucks and lorries
Ø 3) Battery driven road vehicles
DIESEL ELECTRIC TRACTION
v The diesel electric locomotive uses a diesel engine to drive an electric
generator, which then supplies the current to traction motors, which are
geared directly to the locomotive's wheels.
Ø In India, the diesel locomotives were introduced in 1945 for shunting
purposes on broad gauge section and in 1956 for main line services on
medium gauge section. The diesel electric locomotives employed in
practice are of the following types:
Ø 1. Main line diesel electric locomotive having engines of output not
exceeding 1500 kW and speeds of 160 kmph.
Ø 2. Shunting diesel electric locomotive having an engine of 225 to 375
kW output and speed between 25 to 50 kmph.
Ø 3. Diesel electric multiple units stock of which each motor has an engine
of 135 to 150 kW output and train is capable of having speeds between
80 to 110 kmph.
Ø 4. Diesel electric rail car having an engine of 75 to 450 kW output which
may operate as a single car or car with one or more trailer coaches.
ü In diesel electric system used for traction, electric motors are used for
driving the locomotive which are fed by a d.c. generator driven by diesel
engine mounted on the same locomotive as shown by the schematic
diagram in fig.1
Ø The initial investment required is low Ø Its overload capacity is limited.
as compared to direct electric traction
since there is no need of overhead Ø The life of the diesel engine is
structure distribution system and
equipments. comparatively shorter.
Ø Due to its higher acceleration and Ø In addition to motor generator set,
retardation, the schedule speed over a special cooling system is
given route will be higher.
Ø It can be put into service at any required for cooling the diesel
moment since hardly any time is engine also.
required to start up the engine and put
it on duty. Ø Its running and maintenance
Ø The power loss in speed control is very costs are high.
low because it can be carried out by
field control of generator. Ø For the same power output,
Ø Its overall efficiency is higher than diesel electric locomotive is
that of steam locomotive about 25%. costlier than steam or electric
Ø There is no interference with the
adjoining communication lines. locomotive.
Ø Since a diesel electric locomotive is a Ø Regenerative braking cannot be
self contained unit and does not
requires any overhead structure hence used with such types of drives.
it can be used on any route.
Ø It is the most widely used traction system in which the vehicle draws
electrical energy from a distribution system fed at suitable points from a
central power station or substation.
Ø In India both AC and DC type of electrified train systems operate today.
Ø 1500 V DC based train system is mostly operating in Mumbai area. It is
being converted to 25 kV AC system. Rest of the India where routes are
electrified mostly operates under 25 kV AC overhead wire.
Ø The electric locomotives are of two types
ü A.C. locomotive
ü D.C. locomotive
Ø In case of d.c. locomotive d.c. motors are used for traction. The fig.1
shows a schematic block diagram of d.c. locomotive.
Ø It basically consists of a step down transformer, a full wave rectifier with
filters and d.c.motors. The fig.2 shows an a.c. type electric locomotive.
Ø The following are the advantages and disadvantages of electric system:
Ø Since electric motors are used as the drives , the system is clean and
pollution free. Starting torque is high, so high acceleration is possible.
Speed control is very simple. Braking is simple and efficient. Electric
braking is used in this case which is superior to mechanical braking used
by steam and diesel locomotives. It is possible to apply regenerative
braking which has the following advantages
ü Above 80% of energy spent during ascent(upward movement) is
pumped back during descent.
ü Less maintenance of brake shoes, wheels, tyres and rails on account of
less wear and tear.
Ø An electric locomotive requires much less time for maintenance and
repairs than a steam locomotive; and hence can be kept in service for
95% or more of the working day if desired.
Ø Its maintenance and repair cost is about 50 % of that of steam
Ø The electric locomotive can be put into service immediately whereas
steam locomotive requires about two hours getting up steam and be
ready for service.
Ø The centre of gravity of electric locomotive is lower than that of steam
locomotive due to which it is able to negotiate curves at comparatively
Ø The most important factor against electric traction is
high capital outlay on overhead supply system.
Therefore, unless heavy traffic is to be handled electric
traction becomes uneconomical.
Ø Power failure for few minutes can cause disruption of
traffic for hours.
Ø The electric traction system is tied to electric routes only.
Hence it cannot be used on any of the routes.
Ø In case of A.C. traction the communication lines
running along the track experience considerable
interference from power lines. The communication
lines therefore must either be removed away from the
track or replaced by special expensive cables (this
increases the capital cost outlay by 15%)
What are the voltages used for electric traction in India?
Ø Voltages used are 1.5kV DC and 25kV AC for mainline trains.
Ø Calcutta had an overhead 3kV DC system until the '60s.
Ø The 1.5kV DC overhead system (negative earth, positive catenary) is used
around Bombay (This includes Mumbai CST - Kalyan, Kalyan - Pune,
Kalyan - Igatpuri, Mumbai CST - Belapur - Panvel, and Churchgate -
Ø Conversion to 25kV AC has already been done on the Titwala-Kasara
section; next to be converted are Khapoli-Vangani, Vangani-Thane, and
Ø The Madras suburban routes (Madras-Tambaram in the '60s, extended
later to Villupuram) used to be 1.5kV DC until about 1967, when it was
converted to 25kV AC (all overhead catenary supply).
Ø The 25kV AC system with overhead supply from a catenary is used
throughout the rest of the country.
Ø The Calcutta Metro uses 750V DC traction with a third-rail mechanism
for delivering the electricity to the EMUs.
Ø The Calcutta trams use 550V DC with an overhead catenary system with
underground return conductors. The catenary is at a negative potential.
Ø The Delhi Metro uses 25kV AC overhead traction with a catenary system
on the ground-level and elevated routes.
System of Track Electrification
Presently, following four types of track electrification systems are available:
v Direct Current System: DC at 600-750 V is universally employed for tramways in urban
areas and for many suburban railways while 1500-3000 V dc is used for main line railways.
Low voltage dc system is undoubtedly superior to single phase ac system for heavy
v (1) Single Phase AC System: In this system ac series motors are used for getting the
necessary motive power. The voltage employed for distribution network is 15 to 25 Kv at
162/3 or 25 Hz, which is stepped down on the locomotive to a low voltage (300 to 400 V)
suitable for supplying to single phase ac series motors.
v (2) Three Phase AC System: In this system 3-phase induction motors operating at 3300 to
3600 V systems consist of two overhead wires and track rail for the third phase and receives
power either directly from the generating station or through the transformer substations.
v (3) The Composite Systems: Such systems incorporate good points of two systems while
ignoring their bad points. Two such composite systems presently in use are:
Ø A. Single Phase To Three Phase System or Kando System: In this system single phase
hv ac system is employed for distribution purposes and 3-phase induction motors for getting
the necessary driving power in order to have the advantage of low cost of single phase
overhead distribution system together with the desirable characteristics of 3-phase induction
motors (at low frequency 3 phase induction motor develops high starting torque without
excessive current). Speed control is also conveniently achieved by varying the supply
frequency. This system is likely to be developed in future.
Ø B. Single Phase To Direct Current System : This system
combines the advantages of hv ac distribution system and dc series
motors for traction. The voltage used for overhead distribution
system is 25 kv at normal supply frequency of 50 Hz. The
locomotive carries transformer and converting machinery to step-
down the voltage and convert into dc. This system of track
electrification using 25 kv, 50 Hz, single phase ac supply has been
adopted for all future track electrification in India. The advantages
of such a system are light overhead catenaries owing to lower
currents, less number of substations (usually spaced at 50-80 km
distances), flexibility in the location of substations, simplicity of
substation design, lower cost of fixed installations, higher adhesion
coefficient and higher starting efficiency. The draw-backs of this
system are unbalancing effect on the supply and interference to
telecommunication circuits. Fortunately both of these undesirable
effects can be minimized.
Typical Railway Services
S.No Parameter of Urban or City Service Sub-Urban Main Line
Comparison Service Service
1 Acceleration 1.5-4Kmphps 1.5-4Kmphps 6-8Kmphps
2 Retardation 3-4Kmphps 3-4Kmphps 1.5Kmphps
3 Max. Speed 120Kmph 120Kmph 160Kmph
4 Distance 1Km 1-8Km >10Km
5 Special remarks Free running period is Free running running and
if any absent and coasting period period is absent coasting period s.
is small. and coasting Acceleration and
period is long. braking periods
TRAIN MOVEMENT-SPEED TIME CURVES
Ø A train is to be run most optimally as for as possible.
Ø for this it is necessary to know the speed time curves so that it
is possible to find out what energy must be supplied to the train
to perform a particular job.
Ø The slope at any point of the curve gives the acceleration or
retardation of the train at that instant of time.
Ø The area between the curve and the time axis up to a certain
time gives the distance covered by the train till that instant of
Ø Positive slope-Acceleration; Negative slope-Retardation.
Ø (i) Acceleration while notching up or constant Acceleration:
During this period of run (0 to t1), Rst is gradually cut off so
that the motor current is limited to a certain value and the
voltage across the motor is gradually increased.
Ø (ii) Speed curve running: From t1 to t2, the current starts
decreasing with the increase in speed.
Ø (iii) Free running(t2-t3):This period occurs when
the power output from the driving axels balances
the rate at which energy is expended against the
resistance to motion.
Ø (iv) Coasting period(t3-t4): At the end of free
running period , supply to the motors is cut off and
train is allowed to run under its own momentum.
Ø (v) Braking period(t4-t5):At the end of coasting
period, brakes are applied to bring the train to stop.
Crest Speed, Average Speed And Schedule
Ø Crest speed is the maximum speed (Vm) attained by a
train during the run.
Ø Average speed is the mean speed from start to stop i.e.
the distance covered between two stops divided by the
actual time of run is called the average speed.
Ø Schedule speed is the ratio of distance covered
between two stops and total time of run including the
time of stop.
Ø The schedule speed of a given train when running on
a given service (i.e. with a given distance between
stations) is affected by (i) acceleration and braking
retardation (ii) maximum or crest speed and (iii)
duration of stop.
Direct Current Motor Control
v DC Motors:
ØFor a DC motor, Torque
T=0.1592Φ[Z Ia /A]P N-m =KΦIa
Ø For shunt motor: Φ is constant for a constant
supply voltage, so, the torque is directly
proportional to the current drawn by the motor.
ØFor series motor: Φ α Ia, therefore T α I2.
Ø Eb=V-Ia r= ΦZNP/60A
Ø N=(60A/ΦZP)(V-Ia r)
• THE SIEMENS ELECTRIC RAILWAY laid down at
the Berlin Exhibition of 1879. The short line
was about 600 yards long, and current was
drawn from a third rail between the track
which acted as the return to the dynamo. The
locomotive hauled a maximum of some thirty
passengers at a speed of about four miles an
hour. This historic illustration shows one of
the earliest trainloads of passengers, seated
upon three carriages.