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Essay on Electric Traction

Essay Contents:

  1. Essay on the Meaning of Electric Traction
  2. Essay on the Characteristics of Electric Traction
  3. Essay on the Different System of Electric Traction
  4. Essay on the Types of Motors Used for Electric Traction
  5. Essay on the Advantages of Electric Traction
  6. Essay on the Disadvantages of Electric Traction

Essay # 1. Meaning of Electric Traction:


In this system of traction, driving force is obtained from electric motors. If electrical energy could be generated cheaply and the heavy initial cost can be borne, then system of Electric Traction is definitely better then all over systems of traction.

Electric trains were working in some Metropolitan cities viz. Bombay and Madras even before independence. In some cities tram cars were also working. These systems are working on D.C. supply of 1.5 KV and 600 volt for trains and tram cars respectively. Recently the whole of track between Calcutta and Delhi has been electrified at 25 KV, 50 Hz a.c. single phase supply.

The system of traction involving the use of electricity is called electric traction system. There are various systems of traction in our country such as steam engine drive, diesel electric drive etc. The electric system of traction consist vehicles which receive electric power from a distributing network fed at suitable points from the substation.

Essay # 2. Characteristics of Electric Traction:


An ideal traction system should have the following characteristics:

(i) It should possess high starting torque so that rapid acceleration can be achieved.

(ii) It should be self-contained and compact unit.

(iii) The traction equipment should be capable of withstanding temporary overloads.


(iv) Re-generative braking should be incorporated in the system so that energy can be returned to the supply during braking period and wear and tear can be reduced on the brake blocks.

(v) It should not cause interference to the communication lines.

(vi) The speed control of the system should be easy and convenient.

(vii) The traction system should cause minimum wear on the track.


(viii) It should not be too costly. However, no single system is ideal system.

Track Electrification:

(1) A.C System

(2) D.C System


1. The A.C. System:

A.C. Single phase system is used for overhead distribution system and D.C. series motors are used for necessary drive. The essential conversion is achieved by rectifiers. The locomotive carries transformer and mercury arc or metal rectifiers.

The former steps down H.V.A.C. to desired lower value and the later converts A.C. into D.C. The voltage employed is 25 KV A.C. at 50 Hz supply. This system is used for main lines in all the areas of high traffic density. In our country this system of electrification is being used for Howrah to Delhi and Jagatpuri to Nandgaon.

2. The D.C. System:

This system is used for local trains, trolley buses and tram cars. For trolley buses, the leads as well as return conductors are overhead and the potential difference is 600 volts D.C. 3 phase A.C. supply is converted by rectifiers into 600 volts D.C. supply.

For sub-urban area, the sub-stations are placed 2 to 3 Km apart. For tram cars 600 volt D.C. is used but the rail acts as return path. The rail track is so designed that it offers good electric continuity and conductivity so that leakage current is minimum. The positive terminal of D.C. supply is the overhead conductor and -ve terminal is connected to rail track.

The potential difference between any two points on the track should not exceed 7 volts, so that cross- sectional area of the rail will have to be increased. As, it is not economical to increase the cross-sectional area of rails, so negative boosters are used.

Advantages and Disadvantages of A.C. and D.C. Systems:

Single phase A.C. system is preferred for main line railway service due to the following advantages:

1. As the distribution voltage is high, the substations can be placed much apart as compared to D.C. traction. So the number of sub­stations required is less.

2. As higher voltages of distribution can be used, so that cost of over­head conductor will be reduced, hence smaller section of conductor is required.

3. Maintenance and running costs of A.C. substations are less as the substation equipment required for A.C. substations is less, cheap, and efficient.

The D.C. system is used for suburban service due to the following reasons:

1. For exerting the same torque current required by D.C. system is less than that of A.C. system.

2. Under similar conditions the energy consumption in D.C. system is less than in A.C. traction system.

3. The D.C. locomotive and motor coach equipment is lighter in weight, cheap in initial cost as well as in maintenance cost. It is also more efficient.

4. No interference in communication lines.

Types of AC System of Electric Traction:

(1) Single Phase Low Frequency System:

Single phase 15 KV, 16⅔ HZ system is used in West Germany and Australia for main line service. In U.S.A. 11 KV at 25 Hz is also used in some urban areas. High voltage A.C. reduces the current flowing through the overhead line resulting in lesser voltage drops which keeps the sub­station at larger distances say 50 Km to 80 Km.

Low frequency used reduces the importance of the line which further reduces current, reduces the voltage drop and justifies the location of sub-stations at longer distances. The high voltage is fed to overhead line from the sub-stations and is tapped by the locomotive through pantograph and the voltage stepped down to 300 V to 400 V. A.C. Series motors are used at low frequency which improves its commutation properties and also increase P.F. and efficiency of the motor.

(2) Three Phase A.C. System:

3 phase slip-ring motors are used for obtaining the necessary propelling torque. The motor works at 3.3 KV and 16⅔ HZ. The speed control is obtained by a combination of pole changing and rotor resistance method.

The main advantage of this system that regenerative braking is obtained immediately as the speed exceeds the synchronous speed, without any change in connections. Two overhead conductors are used and the return conductor is obtained through the rails. This method is now almost out of use.

(3) Single Phase to Three Phase System (KANDO SYSTEM):

Single phase A.C. supply of 15 KV at 50 HZ is fed from the sub-station to the over-head system. Single phase power is then tapped by the locomotive through pantograph. The locomotive has a phase convertor installed in it which converts 1-phase A.C. to 3-phase A.C. frequency remaining the same. This three phase a.c is then fed to the 3-phase induction motors.

Essay # 3. Different System of Electric Traction:

Diesel-Electric Traction:

The diesel electric traction system is quite economical where the cost of diesel is low as in U.S.A. This system is also adopted in the areas where coal for steam traction is not available in plenty and water is sometimes scared as in Rajasthan.

In diesel electric traction system the electric motors employed for driving the locomotive are fed from the d.c. generators driven by the diesel engine put on the same locomotive. Thus electric locomotive is self-contained motive power unit and, therefore, heavy expenditure on track electrification and sub-stations etc. is eliminated.

The diesel engine employed for diesel electric locomotive is usually high speed type with three or four different running speeds with b.h.p. Diesel engine is made self-starting employing the supply from the battery. The voltage control of main generator becomes easier. The overload capacity of diesel engine is very small (not more than 10 percent).

Since in traction service the torque required is roughly inversely proportional to the speed, and therefore the constant-torque characteristic of a diesel engine has to be modified to suit the requirement of traction work. This is achieved by employing some form of torque converter which must be included in the transmission between the engine shaft and the driving axles.

Urban or City Service:

Types of Railway Services:

The distance between two stops is comparatively very short (of the order of 1 KM or so) and the time required for this run is very small i.e. few-minutes. The acceleration and retardation should be kept high to average speed and short time of run. Free run is not present in this run. The coasting period is also small.

Suburban Service:

In this run the distance between two stops is little longer than urban service but smaller than main line service (between 1 KM and 8 KM). Free run is not possible. Coasting is for a comparatively longer period. Acceleration and retardation required are as high as urban service.

Main Line Service:

The distance between two stops is about 20-40 Km. and free run is of longer duration. The duration of acceleration and retardation is a small fraction of total running time and so they do not effect the average and schedule speeds. Hence acceleration and retardation are of little importance in main line service.

Speed-Time Curves:

A curve drawn between speed and time is known as speed-time curve. The speed is taken (in Km/hour) along the Y-axis and time (in seconds or minutes) on X-axis. The curve given complete information of the motion of the train. A typical speed time curve is shown in figure 7.1.

Typical Speed-Time Curve

Speed-Time Curve mainly consists:

(i) Initial acceleration

(ii) Constant speed Run or Free Run Coasting

(iii) Retardation or Braking period.

(i) Initial Acceleration:

It consists of two parts. They are:

(a) Constant Acceleration or Acceleration during Notching Up:

During notching up period (0 to t1) the current is maintained approximately constant and voltage across the motor is gradually increased by cutting out the starting resistance. Hence, tractive effort is constant and acceleration remains constant during the period 0 to t1.

(b) Speed Curve Running or Acceleration on Speed Curve:

During period t1 to t2, the voltage across the motor remains constant and current starts decreasing with the increase in speed and finally the current taken by the motor becomes constant. The train accelerate, during this period but acceleration decreases with the increase in speed and finally becomes zero at the speed at which the tractive effort developed by the motion of the train.

(c) Constant Speed Run or Free Run:

At t2 i.e. the end of speed curve running the train attains the maximum speed. During this period the train runs with constant speed attained at t2 and constant power is drawn from the supply.

(iii) Coasting:

At t3, which is the end of free running period, the power supply is cut and the train is allowed to run under its own momentum. The speed of the train starts decreasing due to resistance to the motion of the train. The rate at which speed decrease during coasting period is called coasting retardation.

(iv) Retardation of Braking Period:

At t4 i.e., the end of coasting period the brakes are applied to bring the train to rest. During this period the speed decreases rapidly and finally reduces to zero.

Crest Speed, Average Speed and Schedule Speed:

1. Crest Speed:

The maximum speed attained by the vehicle during the run is known as crest speed.

2. Average Speed:

The mean of the speed from start to stop i.e. the distance covered between two stops divided by the actual time of run is known as average speed.

3. Schedule Speed:

The ratio of distance covered between two stops and total time of run including time of stop is known as schedule speed.

Trapezoidal Speed-Time Curve:

Figure 7.2 shows a trapezoidal speed time curve.


α = Accelration in k.m.p.h.s

β = Retardation in k.m.p.h.s.

Vm = Maximum or crest speed in k.m.p.h

T = Total time of run in seconds

Trapezoidal Speed-Time Curve

Total distance of run in Km, S = Distance travelled during acceleration + distance travelled during free run + distance travelled during braking

The +ve sign is not possible as the value obtained by using +ve sign will be high which is not possible in practice. Hence -ve sign will be used.

Factors affecting Schedule Speed:

The schedule speed of a given train when running on a given service (i.e. with a given distance between stations) is affected by the following factors:

(i) Acceleration and braking retardation.

(ii) Maximum or crest speed.

(iii) Duration of stop.

(i) Effect of Acceleration and Braking Retardation:

For a given run and fixed crest speed the increase in acceleration will result in increase in actual time of run and therefore increase in schedule speed. Similarly increase in braking retardation will affect the schedule speed. Variation in acceleration and retardation will have more effect on schedule speed in case of shorter distance run in comparison to longer distance run.

(ii) Effect of Maximum Speed:

For a constant distance run and with fixed acceleration and retardation the actual time of run will decrease and therefore schedule speed will increase in crest speed. The effect of variation in crest speed on schedule speed is considerable in case of long distance run.

(iii) Effect of Duration of Stop:

For a given average speed the schedule speed will increase by reducing the duration of stop. The variation in duration of stop will affect the schedule speed more in case of shorter distance run as compared to longer distance run.

Comparison between A.C. and D.C. Systems of Traction

Essay # 4. Types of Motors Used for Electric Traction:

1. D.C Series Motor:

As the d.c. series motor possess a high starting torque and variable speed characteristics, it is very much used for electric traction purpose. It is more robust and capable of withstanding very severe mechanical shock. It can be designed to take more overloads.

In d.c. series motors commutation is excellent, therefore replacement of brushes is not required frequently. The d.c series motors, when operated in parallel, to drive a vehicle by means of different driving axles, share load almost equally even there is unequal wear of different driving wheels.

2. A.C Series Motor:

D.C series motor rotates in the same direction regardless of the polarity of the supply. So the motor should be operated on a.c. The operating voltage is kept low i.e. 300 to 400 V in order to reduce the reactance. Some a.c series motors are built in large size for traction service and are design to operate at low frequency 25 Hz or 16⅔ Hz.

The single phase a.c series motors are provided with commutating or interpoles. Commutating poles increase the efficiency of the machine. The single phase a.c series motor has similar characteristic as d.c series motors. For a given h.p output the single phase, a.c series motor have 1.5 times weight than d.c motors. At start the power factor is low, therefore for a given current, starting torque is much lower than that of d.c motor. Due to poor starting torque, these motors are not well suited for suburban service where stops are frequent. However the single phase a.c series motor is extensively used for main line work where high acceleration is not important.

3. Three Phase Induction Motors:

The three-phase induction motors have the advantages of simple and robust construction; less maintenance cost, high voltage operation. However due to constant speed operation and complicated overhead feeding systems they are not suitable for electric traction work.

Essay # 5. Advantages of Electric Traction:

1. This system of traction is the cleanest of all other systems.

2. In electric traction, acceleration and braking are smooth and rapid.

3. In electric trains, the fans and lights can be connected directly to the supply lines and as such there is no need for providing Rosenburg generators.

4. Because of larger passenger carrying capacity and higher speed, the electric railways can handle double traffic as is not possible with steam railway.

5. Maintenance and repair cost is about 50% less.

6. Electrical energy is cheaper.

7. There is no damage to the plant, equipment and buildings due to corrosive smoke fumes and due to absence of unwanted gases.

8. Where electric traction is used, the auxiliary equipment such as coal, ash and fire cleaning fits water-supply plants etc.) is not required.

9. Electric locomotive require less maintenance than steam locomotives.

10. Speed control is possible.

11. Electric locomotive can be started immediately.

12. Fire hazards are less in electric traction.

13. High grade coal which is required for steam locomotive can be saved.

Essay # 6. Disadvantages of Electric Traction:

1. Power break-down of a very short duration causes dislocation of the service for long duration.

2. Additional equipment is required for regeneration.

3. The foremost disadvantage of electric traction is that it involves a heavy initial expenditure.

4. In A.C. traction the communication lines running along the track experience considerable interference from the power lines.

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