electric traction

INTRODUCTION
Driving forward of vehicle is called Traction and the
system, which employs this type of mechanism is called
Traction system.
Electric trains are those trains which use electric
traction or in general “electric power”.
UTKARSH(srmscet,unnao)

Main types
There are mainly two types of electric trains, Which are
described as below-
(a.)trains which use electric power to drive
electric motors for the traction.as all indian
electric trains.
(b.)trains which use electric power to generate a
magnetic field for traction.as the most
advanced bullet trains.

WHY ELECTRIC TRACTION?
Traction System Efficiency
Steam locomotive 5-7%
Gas turbine electric
locomotive
10%
Diesel electric locomotive 26-30%
Electric locomotive with
thermal power plant
34-36%
Electrical locomotive with
Hydroelectric power plant
40-42%

The use of electric locomotives
primarily came in order to reduce
problems of smoke pollution. Though
electric locomotives are more efficient
than steam and diesel locomotives,
electrification of railway lines is
expensive. For this reason, electric
locomotives are used mainly for busy
passenger lines .

Systems of track electrification
 THERE ARE BASICALLY TWO TYPES OF
ELECTRIFICATION-
 A.) Dc system.
 B.) Single phase ac system.

Existing Tractions System
Existing tractions system uses D.C. motors.
a) The 25 KV over head voltage is step down to 2000 V
with the help of step down transformer.
b) Rectifier rectifies this A.C. voltage to D.C. voltage.
c) This rectified D.C. voltage is used to operate the D.C.
motors in existing system engine.
Causes favoring the DC motors
a) D.C. series motors are less costly, however for some H.P
more efficient and requires less maintenance than A.C.
series motor.
b) Rail conductor system of track electrifications which is
less costly with D.C. system than with A.C. System

 There are two kinds of power transmission that are used
for electric locomotives. These include alternate current
and direct current. Alternate current, as the name
suggests, flows in more than one direction where as DC
is restricted to one direction only .

 Alternating current has an advantage over
DC current such that it can be used over large
distances even through a wire that has a smaller
radius. Direct current locomotives typically require
wires with greater diameter and in some cases a
third rail. At present, alternating current
locomotives haul trains over large distances or
main line areas where as DC locomotives are
restricted to shorter distances. The voltage
difference is also great as AC is varied between
15KV to 50KV while DC is confined to below 3KV


 There are different methods in which the current
is transferred. One way is through a third rail and the
other is through overhead wires. The third rail is most
popular with DC electric systems. Examples of trains
that use the third rail are usually metro rail systems but
they can be found in main-line routes in southern
England to provide speeds of up 100 miles per hour at
750V DC.

THIRD RAIL
There are certain disadvantages of the third rail. The first
one is that there is a safety hazard. If pedestrians were to
trample on the rail, they are likely to have a shock hazard.
Secondly, weather conditions such as snow can cause the
system to not function

 Third Rail systems also have rail gaps in order to cope
with excess voltage supply. For each gap, there is what
is called a “substation”. These substations ensure
continuity and provide voltage in the direction of
propagation and are off support if one substation were
to fail.
 Substations also give an indication for the train to
continue on its route or to halt for any reason. In cases
where there may be faults along the electric lines, there
are switches to stop the flow of current which means
that the train must stop before entering that section

SCHEMATICS OF WASHINGTON METRO

CURRENT COLLECTOR
PANTOGRAPH

 AC electric systems are more likely to be used
overhead wires otherwise known as a catenary.
 The design of the overhead wires are created such that
it is held in tension horizontally and also subjected to
lateral pulling such that it can accommodate for
curves.

GENERAL LAYOUT OF TRAMSMISSION

 In order to makeup for interference along the catenary,
there are booster transformers connected at frequent
intervals throughout the route. The main purpose of
this is to reduce any inequality of voltages that are
induced by other electric lines that run parallel to the
catenary. A return wire is connected to the track such
that the current returns to the transformer. Without
this, there can also be a safety hazard.

POWER CIRCULATION
 The NEXT circuits show a circuit of an electric railway
system used by the Indian Railways. The current
IC flows through the catenary and then enters the
locomotive through the pantograph. The departing
current IR returns to the main power supply through
the tracks. The second circuit diagram is an example
of booster transformers used in Indian
Railways. Notice that the there are insulated rail
joints to be sure that the current flows at the correct
sections .

CIRCUIT DIAGRAM REPRESENTATION

Circuits of electric locomotives

 , the locomotive receives its power through the
pantograph from the catenary. The voltage received is
decreased by a step-down transformer.
 The amount of current is controlled by a device known
as the “tap changer”. The “tap changer” is basically a
camshaft set of operated switches and connects more
sections of the transformer. The next device is the AC-
DC rectifier.
 The kind of rectifier used in this circuit is the bridge
rectifier. It is basically an arrangement of diodes that
enable the current to flow in one direction only. The
capacitor is used to decrease the fluctuation of voltage.
The drop of voltage from the rectifier is eliminated by
the voltage drop in the capacitor

RECTIFIER USED IN TRAINS
 Instead of using a bridge rectifier, it is possible to use a
thyrister. A thyrister is a kind of diode such that it
allows current to flow when it receives a command
through a third terminal .

 When the thyristor is used to rectify from AC to DC,
the sinusoidal wave display on the voltage and time
graph is confined to above the positive axis. In order to
create a steady supply of voltage and smoothen the
delivery, a smoothening circuit is connected. The
smoothening circuit may include inductors and
capacitors .

 It is also possible to have AC electric locomotives
with AC motors. As seen in the circuit below, there
is both an AC-DC rectifier and a DC-AC inverter.
The connection between the rectifier and the
inverter is known as a DC link.
 The DC-AC inverter is necessary in order to
provide input to the 3 phase traction motors. The
speed of the motor also depends on the frequency
submitted towards it.
 There may be more inverters connected to the DC
link in other to serve other parts of the locomotive
such as the compressor and cooling fans

MODERN AC ELECTRIC LOCOMOTIVES

Braking:
Generally braking is classified into two types in the
traction.
 Electrical braking.
 Plugging
 Rheostatic or dynamic braking.
 Regenerative braking.
 Mechanical braking.
 Compressed air brakes
 Vacuum brakes
 Hydraulic brakes

PLUGGING
 Plugging is a method of braking used in induction
motors. Plugging involves interchanging the supply to
two of the stator phase windings. This method is used
in applications which require immediate stop
applications. When the phase supply is reversed,
torque is produced in the opposite direction. This
leads to braking of the electric motor.
 Motors which are operated this way have a plugging
switch. This switch operates when the stop command
is given to the motor circuit. The operation of this
switch applies reverses the supply to two of the
windings. When the motor comes to a complete halt,
this reversed supply is disconnected.

DYNAMIC BRAKING
 Another method of reversing the direction of torque and
braking the motor is dynamic braking. In this method of
braking the motor which is at a running condition is
disconnected from the source and connected across
a resistance. When the motor is disconnected from the
source, the rotor keeps rotating due to inertia and it works
as a self –excited generator. When the motor works as a
generator the flow of the electric current and torque
reverses. During braking to maintain the steady torque
sectional resistances are cut out one by one.

DYNAMIC BRAKING

Regenerative braking
 Regenerative braking takes place whenever the speed of the
motor exceeds the synchronous speed. This baking method is
called regenerative baking because here the motor works as
generator and supply itself is given power from the load, i.e.
motors. The main criteria for regenerative braking is that the
rotor has to rotate at a speed higher than synchronous speed,
only then the motor will act as a generator and the direction
of electric current flow through the circuit and direction of the
torque reverses and braking takes place. The only disadvantage
of this type of braking is that the motor has to run at super
synchronous speed which may damage the motor mechanically
and electrically, but regenerative braking can be done at sub
synchronous speed if the variable frequency source is available.

Loco Transformer
This is a main transformer of
locomotive.
The 25 KV single phase AC
power supply of OHE is fed
to the winding of regulating
transformer through main
bushing. The winding is
equally divided into 32 taps.
These taps are connected to
tap changer.

LOCO TRANSFORMER
There are two traction transformer connected in parallel of
same rating for the purpose of reliability the rating of
transformer are as follows
Capacity : 25 KV.
Frequency : 50 Hz
taps on HT : 32 taps
cooling :forced oil & air

ARNO-CONVERTER
1. Arno converter is a device
which convert single phase AC
in to three phase AC.
2. The three phase supply needed
for the three phase induction
motors which used in blowers,
exhausters an oil pumps. To
supply three phase power to
three phase induction motors
arno converter is used.
3. Arno converter is rotating
device.

DC LINK
 This is essentially a bank of capacitor and inductor
or active filter circuitry to further smooth.
 Also to trap harmonics generated by drive converter
and traction motors.

REFERENCES
 1.) INDIAN RAILWAYS INSTITUTE OF ELECTRICAL
ENGINEERING handbook.
 2.)WIKIPEDIA.ORG
 3.)ELECTRICAL4U.COM
 4.) http://irfca.org
 5.) http://www.railway-technical.com

electric traction

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