electric traction

INTRODUCTIONDriving 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 typesThere 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 Vwith 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.Pmore efficient and requires less maintenance than A.C.series motor.

b) Rail conductor system of track electrifications which isless 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

MAST


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.


http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/

http://www.electrical4u.com/electric-current-and-theory-of-electricity/

http://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/

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.


http://www.electrical4u.com/electric-current-and-theory-of-electricity/

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 devicewhich convert single phase ACin to three phase AC.

2. The three phase supply neededfor the three phase inductionmotors which used in blowers,exhausters an oil pumps. Tosupply three phase power tothree phase induction motorsarno converter is used.

3. Arno converter is rotatingdevice.

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


http://irfca.org/

electric traction

Engineering

electric traction

electric motors

types of electric trains

indian electric trains

dc electric systems

general electric power

dc system

dc locomotives