Suresh Kamble,Electrical Dept.GPA

Electric Traction

Block Diagram of A.C Locomotive

1. Overhead Contact wire

2. Pantograph

3. Circuit breaker4. On-load tap changer

5. Transformer7. Smoothing choke

8. D.C . Traction motor

Trolley Wires or Contact wire

Trolley wire or Contact wire

Trolley Wires or Contact wire

Contact wire

Trolley Wires

The trolley wire or contact wire must be suspended with the minimum of sag

so that it remains practically horizontal.

So that the contact between the wire & current collector may be maintained at high speeds.

This is achieved by which the trolley wire(contact wire) is supported by another wire known a the “catenary” or “Messenger”

The catenary construction are different types1. Single catenary construction.2. compund catenary construction

Single catenary construction

Fig.

A steel wire with seven or more strands supports the trolley wire by means of droppers clipped to catenary & trolley wire at equidistant horizontal intervals.

The span of catenary wire 45 to 90 metres with a sag of from 1 to 2 metres for straight track.

Fig. Single catenary construction

Contact wire

Catenary

Droppers

Single catenary construction contd…..

Single catenary construction contd………

The distance of droppers is between 3 to 5 metrescurved track the spans of shorter length are employed.

Such construction provides for speed upto 120 kmph.

Advantages

It is relatively cheaper. Less maintenance. Single catenary construction is considered suitable for most light Weight a.c. system. Suitable for secondary routes where traffic density is low & operating speeds are low.

Compound catenary construction

Employed for heavy current i.e where traffic density is high.

It consist of three wires.1.The upper wire is catenary wire, which is insulated from the supporting structure2. The another wire known as intermediate catenary, is supported by droppers clipped to both wires.3. Third wire i.e trolley wire.

Catenary or messenger wire

DropperIntermediatecatenary

Loop wire

Trolley wireFig. shows compund catenary construction

Compund catenary Contd…

Trolley wire is suspended from intermediate catenary by solid wire loop droppers which are to slide vertically on the intermediate wire & fixed the trolley wire.

The purpose of intermediate catenary, besides increasing the current carrying capacity, is to provide for more uniformity in Elasticity.

The compound catenary is used on lines where speed ranges is 190 to 224 kmph.

Current Collector

Current Collector

Current Collector contd…..

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 current collector is that it should keepContinuous contact with trolley wire at all speeds.

There are three types of current collector

1. Trolley collector2. Bow collector3. Pantograph collector.

Trolley Collector

This collector is used for Tramways & trolley buses & is mounted on the roof of the vehicle contact with overhead wire.

It is made either grooved wheel or a sliding shoe carried at the end of a light trolley pole attached to the top of the vehicle& held in contact with overhead wire by means of a spring.

Fig. Trolley wire section

1888 on a double wire conductor tramway.

Trolley Collector contd….

The trolley collector is suitable for low speeds upto 32 kmph

Beyond this speed there is risk of its jumping off the overhead contact wire particularly at points & crossing.

Experimental trolley bus in Berlin in 1882 1989 trolleybus design

Bow Collector

It consist of two roof mounted trolley poles at the ends of which is placed a light metal strip or bow, about 1 metre long for current collection

Fig. Bow collectorTrolley Bus

Bow Collector contd….

Collector strip is made of soft material like copper, aluminum or carbon.

It never wear instead of trolley wire as it is easy to replace collection strip than trolley wire.

The bow collector also operates in the trailing position.Hence, it requires provision of the either duplicate bows or an arrangement for reversing the bow for running in the reverse direction.

Bow collector is not suitable for railway work where speeds up to 120 kmph and currents upto 3000A

Pantograph Collector

The main function is to maintain link between overhead contact wire & power circuit of the locomotive at varying speeds in different climate & wind conditions on varying stiffness of overhead equipments.

Fig show a diamond type single pan pantograph Fig show a actual diamond type pantograph

Pantograph Collector contd…….

Fig show a pantograph collector

Spring

pan

A positive pressure has to be maintained at all Times to avoid loss of contact & sparking.

But pressure must be as low as possible in order to minimize wear of overhead contact wire.

It consists of a pantograph framework of steel tubing.

The contact portion consists of a pressed steel pan fitted with renewable copper wearing Strips which are forced against the overhead contact wire by the upward direction of pantograph springs.

The pantograph can be raised or lowered from cabin by air cylinders.

Desirable Characteristics of Traction motor

The main requirements of electric motors used for traction work

1. Electric characteristics:

i. High starting torqueii. Simple speed controliii. Possibility of electric brakingiv. Capacity of withstanding voltage fluctuationsv. Capability of withstanding temporary interruption of supplyvi. Overload capabilityvii. Parallel running

2. Mechanical features:

i. Robustness & ability to withstand continuous vibrations.

ii. The weight of the traction motor should be minimum in order to increase the pay load capacity of the vehicle.

iii. The traction motor must be small inn overall dimensions.

iv. The traction motor must be totally enclosed type, to provide protection against dirt, dust, water, mud etc.

Desirable Characteristics of Traction motor contd…….

Traction motor

None of the motors can have all the desirable characteristics.

Some of the motors which finds application in traction are:

1.D.C series motors.

2.Single phase A.C series motors

3.Repulsion motors.

4.Three-phase Induction motors

5.Linear Induction motors

Suitability of D.C series motor for Traction

1. D.C series motor develops high torque at low speeds & low torque at high speeds. This is the exactly the requirement of traction units.

2. The field flux varies as the armature current, as in the case of series motor, torque corresponding to the given armature current. Therefore, is independent of line voltage & thus unaffected by the variations in the line voltages.

3. Commutating properties of series motor are also very good as increases in the armature current as a result of heavy load torque results in the decrease of armature speed. This reduces the magnitude of e.m.f induced in the coils under going commutation which helps in achieving sparkless current collections

Suitability of D.C series motor for Traction contd....

5. In case of dc series motor , upto the point of magnetic saturation, torque developed in proportional to the square of the current. Therefore, dc series motor requires comparatively less increased power input with the increase in load torque. Thus the series motor are capable of withstanding excessive loads.

6. Speed of dc series motor can be controlled by various methods.

7. Because of low time constant of field in case of series motor, the field flux dies away in very short time the result back emf of the motor cease with temporary interruption of supply. Therefore, there is inrush of initial current on temporary interruption of supply

8. The dc series motor is simple & robust in construction.

9. The dc series motor, owing to its characteristics, is most suitable for urban & suburban services where high rate of acceleration is essential.

Suitability of D.C series motor for Traction contd....

Draw back

The main drawback of dc series motor are commutation Which restricts speed, current & voltage. Therefore is a risk of flash over & the brush gear requires considerable maintenance.

Suitability of single phase a.c series motor for Traction

1. For a given KW rating ac series motor is 1.5 to 2 times in size & weight of the corresponding dc series motor.

2. The construction cost of an ac series motor is much more than of a dc series motor.

3. The starting torque of a.c single phase motor is lower than that of dc series motor due to poor power factor at the start.

4. A.C single phase motor are not suitable for urban & suburban services, it requires high acceleration. However, single phase motors are extensively used for main lines services.

Suitability of single phase a.c series motor for Traction contd…

5. The speed of an ac series motor may be controlled efficiently by taps on a transformer. Which is not possible in case of a dc series motor.

6. However, single phase ac series motor have better performance on reduced supply frequency says 25 Hz. The higher frequency results in higher leakage reactances & hence a relatively poor power factor.

7. The maximum operating voltage for these motors is limited to 400 volts

Suitability of Three phase Induction motor for Traction

It has simple & robust construction. Trouble free operation. Less maintenance. High voltage operation consequently requiring reduced amount of current. Automatic regeneration are the main advantages of 3-ph. Induction motor for traction.

But due to their flat speed- torque characteristics, constant speed operation, developing low starting torque, drawing high starting current, complicated speed control systems they are not suitable for electric traction work.

Suitability of Three phase Induction motor. contd…

With the development of thyristorised inverter circuits, it has now been possible to invert the supply and obtain a variable frequency supply which could be used for the 3- phase Induction motor & a very smooth speed control can be obtained.

Power supply Arrangement

Since Indian Railways have decided to adopt 25kv, 50 Hz supply system

Following arrangements of power supply to various substations exist on Indian Railways

1. Power is purchased from supply authorities who are responsible for the operation & maintenance of 132/110 kv transmission lines & grid substations upto 25 kv out - going terminals of substation. However 25 kv feeder circuit breaker is controlled from remote control centre by traction power controller. This arrangement enables railway authorities to pay full attention to operate & maintain traction service which is their prime responsibility.

2. All the 132 kv & 25 kv equipments of substation is owned, installed operated & maintained by the supply authorities. However 25 kv circuit breaker are owned, installed, operated & maintained by the railway authorities.

Power supply Arrangement contd…..

To have better control & expeditions execution of electrificationProgramme, it was considered necessary that the railways should own , install, operate &maintain all the 132/110 kv equipments at the substation. Supply authorities will give only voltage supply of 132/110 kv at the substation

Constituents or Parts of Power supply system

Various elements of power supply system are:

1.Sub-station.

2.Feeding posts.

3.Sectioning & paralleling posts.

4.Sub-sectioning & paralleling posts.

5.Sub-sectioning posts.

6.Elementary sections.

Substation

Fig. layout of a typical traction sub-station.

Feeding post