7 Commonly Used Systems of Traction | Electrical Engineering

The various traction systems commonly used are described as follows: 1. Direct Steam Engine Drive 2. Direct Internal Combustion Engine Drive 3. Steam Electric Drive 4. Internal Combustion Engine Electric Drive 5. Petrol Electric Traction 6. Battery Electric Drive 7. Electric Drive.

System # 1. Direct Steam Engine Drive:

The steam engine drive used to be widely employed for railway work. In this type of drive, the reciprocating steam engine is invariably used for getting the necessary motive power because of its inherent simplicity, operational dependability, and simplified maintenance, the simplicity of connections between the cylinders and the driving wheels and easy speed control. The locomotive and train unit is self-contained therefore it is not tied to a route.

It causes no interference to the communication lines running along the track. The capacity is very high in comparison to direct internal combustion engine drive, and battery electric drive etc. Initial investment required is low in comparison to that of electric drive as no overhead structure, distribution system and power generation is required. It is cheap for low density traffic areas and in initial stages of communication by rail.

Steam engine drive is losing ground because of the following drawbacks:

i. Because of difficulty of installing a condenser on a locomotive, the steam engines employed are of non- condensing type, therefore, efficiency is very poor (6 to 8 per cent). Further fires have to be banked even when locomotive is idle.

ii. Adequate supplies of feed water at regular intervals is required.

iii. The overload capacity of steam locomotive is limited.

iv. Steam locomotive has to carry sufficient quantity of coal and water which correspondingly reduces the payload that can be hauled.

v. Limited availability and cruising range which are influenced not only due to quantity of water and coal to be carried in the locomotive but also due to periodic stops necessary to clean the ash pan. For standard passenger locomotives this cruising range is about 160 km.

vi. Performance of steam locomotive is governed by the rate of firing of the coal. Single fireman can maintain a firing rate of about 2 tonne/hour for short periods and with two firemen it may be about 2.7 tonne/hour.

vii. More number of crew (one driver and two firemen) are required, therefore, wage bill is increased.

viii. Extensive and costly ancillary equipment, such as coaling cranes, ash and fire cleaning pits, turntable, water supply plants, is required. The maintenance of these facilities also costs money.

ix. Steam locomotive requires more repair and maintenance. The repair costs of steam locomotive are heavy especially for the boiler. Steam locomotive requires large number of unskilled and semi-skilled staff for maintenance.

x. Larger size of running sheds and workshops are required.

xi. Owing to coal dust, cinder and ash the steam engine drive is not clean drive.

xii. Steam locomotive cannot be put into service at any moment as time is required for steam raising.

xiii. Since driving wheels are very close, more concentrated adhesive weight is required.

xiv. Steam locomotive causes considerable wear on the track owing to unbalanced reciprocating parts and corrosion of steel structures by the smoke emitted.

xv. Because of unbalanced reciprocating masses of steam engine riding qualities are not good.

xvi. Because of unbalanced forces produced by reciprocating masses, coefficient of adhesion of steam locomotive is low (0.25 as compared with that of electric ranging from 0.3 to 0.45).

xvii. Due to low coefficient of adhesion power-weight ratio of steam locomotive is low.

xviii. Owing to high centre of gravity of steam locomotive, speed is limited.

xix. Owing to smoke, steam engine drive cannot be employed for underground railways.

xx. Steam locomotive is only available for hauling for about 60 per cent of its working day, the remainder of the time is spent in coaling, fire cleaning, watering and servicing.

xxi. Because of low coefficient of adhesion it is not possible to have acceleration and retardation with steam engine drive as high as with electric drive. Steam engine drive is, therefore, not suitable for urban or suburban services where distance between stations is small. It is not also suitable for hauling heavy loads on steep gradients.

A few steam turbine locomotives have been built having a condenser and reduction gearing between the turbine and driving wheels for experimental purposes. Though the efficiency of such locomotives is between 10 and 15 per cent but due to their high initial and maintenance costs they could not be put into general use.

Gas-turbine locomotives have also been built on an experimental basis.

System # 2. Direct Internal Combustion Engine Drive:

Direct internal combustion engine drive is widely employed for road transport (buses, trucks, cars etc.). The efficiency of internal combustion engine at its normal speed is about 25 per cent. It is self-contained unit and therefore, it is not tied to any route. Initial investment required, being only the cost of vehicle and garage, is very low. Speed control and braking system employed is very simple. It is cheap drive for the outer suburbs and country districts.

The disadvantages of this type of drive are:

i. Its overload capacity is limited on account of the speed- torque characteristics of the engine and even 10% of overloading may result in stalling of the engine.

ii. Its operation at any but the normal speed is very uneconomical.

iii. As torque exerted by an internal combustion engine is approximately constant, hence an engine capable of giving high starting torque required is not used to its full capacity when running at full speed unless some form of gear is employed.

iv. Speed control is possible only by employing a gearbox.

v. It is not suitable for heavy railway work as the arrangement possible for speed control and utilizing the torque of engine to full capacity when running at full speed is not altogether satisfactory for it.

vi. The life of propulsive equipment is much shorter than that of electrical equipment of a tramcar or a trolley bus.

vii. The maintenance and running costs are higher.

viii. Since the fuel oil is to be imported in our country, therefore, there is drain on foreign exchange which is vital for expanding economy of the country. In wartime this system may get completely paralysed.

ix. The oil engine bus accommodate slightly less passengers than the trolley bus.

x. It has no starting torque and must be started by some auxiliary means such as compressed air or electrically.

System # 3. Steam Electric Drive:

A few locomotives employing steam turbine for driving a generator used for supplying current to electric motors have been built for experimental purposes. Such locomotives have not been put into general use because of some mechanical difficulties and complications.

System # 4. Internal Combustion Engine Electric Drive:

In this drive the reduction gear and gearbox are eliminated as the diesel engine is to drive the dc generator coupled to it at a constant speed. This type of drive has found considerable favour for railway work and locomotives of this type are becoming widely used.

The advantages and disadvantages of this drive are given below:

Advantages:

i. No modification of existing track is required for conversion from steam traction to diesel engine electric traction.

ii. Initial investment required is low as no overhead structure distribution system and equipment is required.

iii. The diesel engine electric vehicle can accommodate more passengers than steam engine vehicle and because of its higher acceleration and braking retardation the schedule speed over a given route will be higher than that of the latter vehicle, so that fewer diesel-electric locos will be required for operating a given service.

iv. Power loss in speed control is very low because it can be carried out by the field control of generator.

v. The locomotive and train is a self-contained unit and, therefore, it is not tied to any route.

vi Though the maintenance cost is fairly heavy but the time spent in maintenance and overhaul is very less (only 5 to 10 per cent of its working day). Hence such a locomotive is available for hauling for almost 6½ days in a week for 24 hours duty.

vii. It can be put into service at any moment.

viii. Overall efficiency is greater than that of steam locomotives (about 25 per cent in comparison of 8 per cent or so in case of steam locomotives).

Disadvantages:

i. Its overload capacity is limited as diesel engine cannot be overloaded.

ii. Life of the diesel engine is comparatively shorter.

iii. Special cooling system is required for cooling the diesel engine in addition to motor generator set.

iv. Running and maintenance costs are high.

v. The dead weight of such locomotives is more. It requires more axles (six for diesel electric locomotive in comparison with four for electric locomotive).

vi. Since the fuel oil is to be imported in our country, therefore, there is drain on foreign exchange which is vital for expanding economy of the country. In wartime this system may be completely paralysed.

vii. For the same power output diesel electric locomotive is costlier than steam or electric locomotive because it carries diesel engine, main generator and traction motors, all capable of handling full locomotive power. This also makes diesel electric locomotive considerably heavier than plain electric locomotive.

viii. Regenerative braking cannot be used with such drives but there is no bar in making use of rheostatic braking.

System # 5. Petrol Electric Traction:

This system has been used in heavy lorries and buses. Due to electric conversion it provides a very fine and continuous control which makes the vehicle capable of moving slowly at an imperceptible speed and creeping up the steepest slope without throttling the engine.

System # 6. Battery Electric Drive:

In this drive the locomotive carries the secondary batteries which supply power to dc motors employed for driving the vehicle. Such a drive is well suited for frequently operated service such as for local delivery of goods in large towns with maximum daily run of 50 to 60 km, shunting and traction in industrial works and mines. Battery driven vehicle is easy to control and very convenient to use. Other advantages are low maintenance cost and absence of fumes.

The major limitation of this type of drive is the small capacity of the batteries and the necessity for frequent charging, speed range is also limited. Battery vehicles are started by series-parallel grouping of batteries—in parallel for starting and running at the speed up to half maximum speed and in series for running at full maximum speed.

System # 7. Electric Drive:

The drive of this type is most widely used. In this system of traction the vehicle draws electrical energy from the distribution system (from a contact wire suspended above the track or from an additional rail laid alongside it) fed at suitable points from either a central power station or substations.

The advantages and disadvantages of electric drive are as follows:

Advantages:

i. It is the cleanest of all other types of systems of traction. Due to this only it is ideally suitable for the underground and tube railways.

ii. No water and coaling depots are required in electric drive.

iii. Ancillary equipment, such as coaling cranes, ash and fire cleaning pits, water supply plants, is not required. Size of running sheds and workshops is comparatively smaller.

iv. The electrical energy required for lights and fans of the train can be drawn from the lines directly and, therefore, there is no need of providing Rosenberg generators.

v. Owing to better speed control in electric drive locomotion on steep gradients at slow speed is possible.

vi. It has got another advantage of rapid acceleration and braking retardation. An electric locomotive has an acceleration of 1.5 to 3.5 kmphps for urban service. Trolley buses have acceleration of 4 to 6 kmphps. The steam locomotive has an acceleration of only 0.8 kmphps. Hence the schedule speed may be from 50 to 100 per cent higher than that corresponding to steam operation over short runs and with frequent stops.

vii. Because of larger passenger carrying capacity and higher schedule speed the electric railway can handle the traffic of any amount—up to double the amount possible with steam railway.

viii. The electric train can be divided and run in sections during the periods of light traffic, thereby enabling a frequent service to be maintained.

ix. Less terminal space is required on account of speedy movement and lesser number of units required for operating a given service.

x. An electric locomotive requires much less time for maintenance and repairs than a steam locomotive and such vehicles can, if desired, be kept in service for 95 per cent or more of the working day.

xi. Maintenance and repair cost is about 50 per cent of that of steam locomotive.

xii. The electric locomotive can be put into service immediately whereas steam locomotive takes about two hours to get up steam.

xiii. There is no damage to the plant, equipment and building due to corrosive smoke fumes because of absence of unwanted gases and fire hazards.

xiv. This system is healthier from the hygienic point of view owing to complete absence of smoke and fumes.

xv. Electric traction is most economical in areas of high traffic density particularly if electrical energy is cheap.

xvi. Centre of gravity of electric locomotive is lower than that of steam locomotive and, therefore, electric locomotive is able to negotiate curves at comparatively higher speeds.

xvii. The vibrations in electrically operated vehicles are less as the torque exerted by the electric motors is continuous.

xviii. In electric traction it is possible to install power units in two or more power cars in the train and to control them from one point whereas in steam traction each locomotive is to be manned by its own crew.

xix. The coefficient of adhesion is better in electric traction than that in steam traction. This is due to continuous torque of electric motors and distribution of driving wheels along the length of the train. Higher coefficients of adhesion not only reduces the ratio of weight to output kW of the locomotive but also results in improvement in riding qualities and reduction in wear and tear of the track.

xx. Electric equipment can withstand large temporary overloads and can draw relatively large power from the distributing system.

xxi. Electric traction helps in saving of high grade coal which is limited in quantity in our country.

xxii. Power requirement for railway electrification has been of the order of 50 kW/track km. This is a sizeable load which tends to increase power development schemes. Traction load has high load factor of the order of 60 to 70%. Electric traction, therefore, provides a most important base load. This, therefore, enables the use of large generator units having high thermal efficiency possible in thermal stations. High base load also affords economic development of hydroelectric potential.

xxiii. Railway electrification encourages rural electrification as no special transmission lines have to be run for this purpose.

xxiv. Electric braking is superior to mechanical braking because of less wear on brake shoes, wheels and track. In case of regenerative braking a part of energy is returned to the supply instead of being wasted as heat in the brake shoes, thus saving in overall consumption is affected. Goods traffic on gradients is more safe and speedy.

xxv. In case of steam traction brake blocks and wheels are likely to overheat on continuous grades and sometimes trains have to be stopped for allowing these brake blocks and wheels to cool down but in case of electric traction it is not so. In electric traction dynamic braking is applied to hold the trains under control and brake blocks are applied for stopping the train and thus wheels and brake blocks are always cool whenever required for stopping the train.

Disadvantages:

i. The greatest disadvantage of electric traction is that it involves a heavy initial expenditure for the power supply system and, in the case of tramways, for the track rails and electrical return system, in addition to the cost of vehicles, depots etc.

ii. Number of somewhat severe statutory regulations, concerning leakage currents from overhead conductors and the voltage drop in the track rails, have to be complied with.

iii. Failure of power supply for a few minutes may paralyses the whole system. Such an occurrence is, however, rare nowadays due to increased reliability of power supply.

iv. Steam locomotives can use their steam for heating the compartments in cold weather, very cheaply whereas the electric locomotive have to do it at extra cost.

v. Electric traction system is tied up to only electrified routes.

vi. In case of ac traction the communication lines running along the track experience considerable interference from the power lines. The communication lines therefore must, either be removed away from the track, or replaced by special expensive cables. The cost incurred for doing this may be as high as 15% of the capital outlay.

vii. Additional equipment is required for regeneration. In case of dc series motors regeneration is not simple process.

viii. In cold countries a service locomotive is required to run up and down the line in order to prevent the formation of a layer of ice on the conductor rails.

The main consideration in the electrification of railways or other traction services is one of the economics. If operation by electric traction proves to be cheaper than any other method (i.e., the electrical energy is available at cheaper rates and the density of the traffic is high) and necessary capital can be raised then no other factor or disadvantage would prevent the conversion of steam railway to electric railway. There is no technical difficulty in adoption of electric traction.

So far as Indian Railways are concerned the route kilometrage on which electrification is justified is quite considerable.

The reasons are:

1. About 72% of route kilometrage comprises of single line section and average traffic density even at sixties level of traffic is very much higher than that necessary for justification of electrification of single line sections.

2. The traffic densities for justification of electrification of railways in case of graded sections are much less due to relative increase in fuel energy charges and savings in the energy affected by employing regenerative braking and in our country nearly 30% of railway track is having gradients steeper than 0.33%.

3. In our country electric energy is available at cheaper rates comparatively because of availability of substantial sources of hydropower generation and nuclear power generation. By electrification of railways high grade coal, which is limited in quantity in our country, will be saved which can be used for some other useful purposes. Efficiencies of different types of locomotives are given below in tabular form.