AIM: To study different types of Electric systems and collectors used in electric

traction.

THEORY:

Electric Traction

Traction is defined as a physical process in which a tangential force is transmitted

across an interface between two bodies through dry friction or an intervening fluid

film resulting in motion, stoppage or the transmission of power. There is a wide

variety of electric traction systems around the world, which have been built according

to the type of railway, its location and the technology available at the time of the

installation.

Electric systems

Two types of vehicles are used for electric traction. The first type of vehicles receives

power from a distribution network while the second type of vehicles generates their

own power. In the second category comes, diesel engine electric drives. The former

type of vehicles is used on a.c. or d.c. power from the overhead line. Accordingly

there are two main systems of electric traction.

D.C. Systems

The d.c. traction in India exists only in Bombay region (Western Railway) and some

parts of Madras. In this system the electric motors used are d.c. series motors. The

operating voltage is generally about 600 volts for suburban railways and tram cars.

For main line railways the operating voltage is from 1500 volts to 3000 volts. The

motors receive power from an overhead line with the help of a a pantograph and the

railway steel track is the return conductor. The overhead wire is fed from the various

substations. These sub-stations receive power from three phase 11kv or 3 phase, 33kv

or 3 phase, 66kv transmission lines. Then this a.c. power is converted into d.c. power

by using mercury arc rectifiers or rotory converters. The modern trend is to use

semiconductor rectifiers. The distance between sub-stations depends upon the factors

like voltage drop and density of traffic. For sub-urban services the distance between

the sub-stations is 3 to 5 km whereas for main line service it is about 40 to 50 km.

A.C.Systems

The a.c. system in India is being employed from Howrah to Tundla [Recently

extended upto New Delhi] and from Madras to Tambram. The modern development

in electric traction is to use single phase a.c. supply. The a.c. system used in practice

is further of four types;

(a) Three Phase A.C. System. This system employs 3 phase slip ring induction

motors, speed control being obtained by a combination of pole changing and rotor

resistance method. The main advantage of this system is that regenerative braking is

obtained immediately as the speed exceeds the synchronous speed, without any

change in conditions. The voltage and frequency at which the motor is made operate

are about 3600V and 16(2/3) cycles/second.

The major disadvantage is the use of two overhead conductors. (Third being the rail

itself) and hence the system is almost out of use.

(b) Single phase standard frequency system. This system is also known as

composite system of traction. This system is employed in India on South-Eastern and

Eastern railway. The system has a single overhead wire supplied at 25kV 50c/s which

is the standard industrial frequency. A transformer is mounted on the locomotive and

it steps down the voltage which is further rectified and supplied to the traction motors.

The sub-stations are supplied at a high voltage of upto 132kV which is stepped down

to 25kV by transformer installed at each substation.

With the developments in semiconductor rectifiers, the system has become very

popular and is being increasingly adopted throughout the world. All modern electric

traction work is being done by this system. As compared to mercury arc rectifiers,

semiconductor rectifiers are more reliable, more efficient and simple to maintain. The

driving force is obtained from D.C. series motors.

(c) Single phase low frequency systems. Single phase 15kV, 16(2/3) cycles/sec

system is used in West Germany, Swedon and Australia for the main line service. In

some urban areas of U.S.A.11 kV at 25 cycles/sec is also employed. A step down

transformer is carried in the traction unit which steps down the voltage to about 400V

for the use of traction motors.

Each substation is supplied at a high voltage at the standard frequency. The voltage at

the substation is stepped down and frequency is also converted by a motor-alternator

set. Series motor is employed for traction. Due to the commutating difficulties at

normal frequency on such motors, a low frequency supply is essential. The main

disadvantages of this system are that a special low frequency power distribution

network is required.

(d) Single phase to three phase system [Kondo System]: In this system single phase

high voltage a.c. system is employed for distribution network. The locomotives carry

a phase convertor which converts single phase a.c. into three phase. The three phase

supply is connected to three phase induction motor for getting the necessary driving

force. The system has the advantage of low cost distribution and cheap and robust

construction of induction motor. The voltage used for distribution network, is 16,000

V at 50 cycles/second. This system is adopted in Hungary.

Current collectors used in electric traction

The current in the overhead system is collected with the help of sliding contact

collector mounted on the roof of the vehicle. The main requirement of a current

collector is that it should not leave the contact of overhead equipment in any

circumstances.

The following types of current collection systems are employed in electric traction

system

1. Trolley collector

2. Bow collector

3. Pantograph collector

4. Pole collector

5. Cable collector

1. Trolley collector:

The trolley collector is universally employed with tramways and trolley buses. This

consists of a grooved gun metal wheel or grooved slider shoe with carbon insert

carried at the end of a long pole. The other end of this pole is hinged to a swivelling

base fixed to the roof of the vehicle. Necessary upward pressure for the pole and

current collector is achieved by means of springs. As two trolley wires are required

for a trolley bus a separate trolley collector is provided for each wire, the bases being

mounted side by side. The pressure for wheel is approximately 10kg and for a carbon

insert slider is approximately 17kg. The main drawback of trolley collector is that it

has to be rotated through 180° for reversing the direction of motion of the vehicle.

Another drawback is that there is poor contact between the wheel and trolley wire

which gives rise to high current density. The trolley collector is suitable for

comparative low speed (say 24 to 32 kmph) beyond this speed there is possibility of

its jumping of the trolley wire.

2. Bow Collector:

A bow collector is one of the three main devices used on tramcars to transfer electric

current from the wires above to the tram below. While once very common in

continental Europe, it has now been largely replaced by the pantograph.

Construction

The bow collector is one of the simplest and most reliable methods of current

collection used on tramways. The very earliest versions were simply very heavy-

gauge wire or steel bars bent into a rectangular shape and mounted long-side-down on

the tramcar roof. The height of the collector was such that its top edge would scrape

along the wire above. The top section is made of a 1 inch broad (or thereabouts) steel

rod, machined to have a bow-shaped cross section, hence the name. This bow shaped

rod is referred to as the 'collector plate', and in later models may be up to several

inches wide. Unlike many trolley poles, the bow collector does not normally have a

revolving base (one exception was in Rome, where the entire assembly could be

revolved), but is rather fixed centrally to the tramcar roof.)The changes of design are

most noticeable on systems where both double- and single-deck cars were used on the

same system. Single deck trams usually have tall and lightly constructed collectors

with complicated frames to support the heavy cast-steel collector plate, while double

deck cars usually have heavier collectors with less complicated frames. To maintain

good electrical contact, the bow collector must exert quite strong pressure on the wire

above, and so complicated systems of springs or weights were put into use to ensure

good electrical contact, and hence efficient operation was maintained. The steel rails

on the tracks act as the electrical return.

Operation

Properly the bow collector should be mounted in such a way so that the top edge of

the collector plate would rise several inches above the wire when the collector frame

is standing straight up. Thus the collector usually leans opposite to the direction of

travel; when the time comes to travel in the opposite direction, the collector must be

swung over. To allow this to happen, the overhead wire must be raised by several

inches at places where the bows are swung over, such as terminals and turn-outs. This

operation is usually achieved by ropes and pulleys. The collector is folded down to a

horizontal position when the car is not in use. Some early cars had no means to swing

the bows over. It was thought that this would happen automatically when the tramcar

started travelling the other way, but collectors such as these were a failure.

Advantages and Modern Usage

The bow collector has fewer moving parts than the trolley pole, but is heavier and

sometimes more complicated to construct. The construction of overhead wires for

bow collectors is simpler than trolley pole wiring. As bow collectors do not have

revolving mountings, the collector cannot jump off the wire or follow the wrong one

at intersections, as trolley poles sometimes do. Thus overhead 'frogs' and guides for

trolley poles are not necessary with bow collectors. Bow collectors are, however,

much noisier than trolley poles. The overhead wires for bow collectors are stretched

tighter than for trolley poles, and straight sections are 'staggered', that is, the wire does

not run completely straight down the centre line of the track, but rather zig-zags

slightly across a small distance. This distributes wear across the bow collector's

collector plate, and extends the collector's life.

3. Pantograph Collector:

A pantograph is a device that collects electric current from overhead lines for electric

trains or trams. The term stems from the resemblance to pantograph devices for

copying writing and drawings. The most common type of pantograph today is the so

called half-pantograph (sometimes 'Z'-shaped), which has evolved to provide a more

compact and responsive single-arm design at high speeds as trains get faster. The half-

pantograph can be seen in use on everything from very fast trains (such as the TGV)

to low-speed urban tram systems. The design operates with equal efficiency in either

direction of motion.

The electric transmission system for modern electric rail systems consists of an upper

weight carrying wire (known as a catenary) from which is suspended a contact wire.

The pantograph is spring loaded and pushes a contact shoe up against the contact wire

to draw the electricity needed to run the train. The steel rails on the tracks act as the

electrical return. As the train moves, the contact shoe slides along the wire and can set

up acoustical standing waves in the wires which break the contact and degrade current

collection. This means that on some systems adjacent pantographs are not permitted.

Pantographs are the successor technology to trolley poles, which were widely used on

early streetcar systems. Trolley pole are still used by trolleybuses, whose freedom of

movement and need for a two-wire circuit makes pantographs impractical, and some

streetcar networks, such as the Toronto Streetcar System, which have frequent turns

sharp enough to require additional freedom of movement in their current collection to

ensure unbroken contact.

Pantographs with overhead wires are now the dominant form of current collection for

modern electric trains because, although more expensive and fragile than a third-rail

system, they allow the use of higher voltages.

Pantographs are typically operated by compressed air from the vehicle's braking

system, either to raise the unit and hold it against the conductor or, when springs are

used to effect the extension, to lower it. As a precaution against loss of pressure in the

second case, the arm is held in the down position by a catch. For high-voltage

systems, the same air supply is used to "blow out" the electric arc when roof-mounted

circuit breakers are used.

Pantographs may have either a single or a double arm. Double arm pantographs are

usually heavier, requiring more power to raise and lower, but may also be more fault

tolerant.

4. Pole Collector:

For tramways grooved gun metal wheel trolley collector or grooved slider shoe with

carbon insert, attached to the end of a long pole provided on the top of the car is used.

Main drawback of trolley collector is that it has to be rotated by 180° before tram car

can have motion in reverse direction. It is suitable upto a speed of 22 to 30 kmph. The

other drawback is the poor contact between wheel and trolley wire, giving rise to high

current density, which in turn results in heavy arching. Its use is prohibited in gassy

mines.

5. Cable Collector:

It is used at places where range of operation is small and where it is hazardous to have

current collection from overhead wire by other means, such as in gathering service of

gassy mines. It consists of a long cable which pays out or winds upon a power driven

wheel/reel carried on the locomotive when it travels into or out of working place.

CONCLUSION:

Hence, different types of collectors used in electric traction are studied.