1. V A KUPPUSAMY, M.E., MISTE., Sr. Lecturer / EEE K.S.R. POLYTECHNIC COLLEGE TIRUCHENGODE – 637 215 Prepared by Electric Traction Unit - III DISTRIBUTION AND UTILIZATION K.S.R. POLYTECHNIC COLLEGE TIRUCHENGODE 637 215 Electric Traction1

2. Unit - III Electric Traction2

3. Electric traction means a locomotion in which the driving force is obtained from electric motors. There are many advantages of electric traction over other forms of locomotion. In present days the traction is an index of development of any country. In fact it is an essential requirement of a man in modern days to travel and transport goods in a fastest, safest and economical way. INTRODUCTION Electric Traction3

4. Requirements of Ideal Traction System The unit must be compact and self contained and able to run on any route. The unit should develop high starting torque. The system should withstand the sudden temporary over load. Efficiency should be high. The initial and running cost of the system should be low. The wearing of the track should be minimum. The system should have easy speed control. The system should not interface with the existing telephone system. The braking system should not cause excessive wear and tear on brake shoes. Electric Traction4

5. In this type of drive the locomotive is equipped with d.c. series motor or a.c series motor or three phase induction motor. The electric energy is directly fed into motor and torque developed is used for propelling the train. Electric Traction5

6. Advantages Absence if smoke and dust it is very clean. Maintenance and repair cost are about 50%. Time required for Maintenance and repair is low. It can be put into service immediately. This type of drive is most suitable for high traffic density route. Traction motors can withstand high over loads. Traction motor develop a continuous torque which results in smooth acceleration with out any jerks. Electric braking is superior to mechanical braking as it avoids wear on wheels, brake shoes and track. Electrical energy required for lights and fans of the train can be drawn directly from the lines. Therefore no generator is needed. Electric traction helps in saving of high grade coal which is limited in quantity in our country. Electric Traction6 Disadvantages Capital cost is very high. A failure in power supply may affect the whole system. Electric traction system is tried up to only electrified routes. Where a.c supply is used for traction, it causes interference to the telephone lines. In cold climates the steam locomotive can use its steam for heating the compartment of train cheaply. The electric train has to do it at extra cost.

7. The various components of an ac locomotive running on single-phase 25 KV, 50 HZ ac supply. OH contact wire Pantograph Circuit breakers On-load tap- changers Transformer Rectifier Smoothing Choke. DC traction motors. Block Diagram of an AC Locomotive Electric Traction7

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9. SYSTEM OF TRACK ELECTRIFICATION There are two methods of supplying power to electric traction. They are. Conductor rail system. Overhead system. Methods of supplying power Electric Traction9

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12. Conductor rail system (or) Rail connected system This method is usually employed in railways operating at low voltage upto 600 V. The current is supplied through one rail conductor or two rail conductors. In case of one conductor the track rail is employed as the return path of current. Whereas in case of two conductors the track rail is not being used as the return path. The rails are mounted on insulators. For collecting the current a shoe. The shoe presses on to the rail with a force of about 15kg with a current collecting capacity of 300 to 500 Amps per shoe. Electric Traction12

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14. Current Collection Gear for OHE The current in the overhead system is collected with the help of sliding contact collectors mounted on the roof of the vehicle. The current collecting system should have a sufficient flexibility to adjust itself to the variation of the level of the conductor at high speeds. Following types of collectors are used for this current collection purpose. Electric Traction14

15. Bow collector Bow collector with car roof. It consists of a light metal strip fixed on one end of two poles mounted on the car roof. This metal strip is pressed against the conductor and it collects the current from the over head conductor. Springs are used to obtain necessary upward pressure. The bow collector has to be reversed for the reverse direction of the train movement. Main advantage of bow collector is that it can be used for high speed trains. Main disadvantages are current collecting capacity is low and irreversible operation. Electric Traction15

16. Pantograph Collector Where the vehicles run at high speeds and current to be collected is 2000 to 3000A. It has a copper strip which act as contact surface. This strip can be changed as and when it wears. The copper strip is mounted on pressed steel pan. Arrangement is supported with the help of springs on a frame work of pentagonal shape. Operate in either direction of motion. Risk of jumping off the collector is minimum. Erection of overhead network is simple. Its height can be increased or decreased by simple operations from the driver’s cabin. Advantage Electric Traction16

17. Different system of track electrification There are five system of track electrification. D.C system. Single phase low frequency A.C system. Single phase high frequency A.C system. Three phase A.C system. Composite system. Electric Traction17

18. Single phase low frequency A.C system In this system the operating voltage is between 300 V to 400 V at frequency 25Hz or 16 2/3 Hz. The energy is obtained at high voltage in the range of 15KV to 25KV at standard frequency. This high voltage is stepped down to the operating low voltage by step down transformers and he frequency is also converted by a motor – alternator set. A.C series motors are used for this system. Advantages Advantages High efficiency. Power factor is improved. Required less number of sub stations. Disadvantages Disadvantages Frequency convertor equipment is necessary. Step down transformer is needed. Electric Traction18

19. Booster Transformer In A.C traction system, return current which flows from the locomotive to the track soon leaks to the ground within short distance and returns to the substation through earth. These ground currents cause heavy interference with the communication lines. To minimise the interference due to ground currents booster transformer is used. It consists of two windings of 1:1 ratio. The primary winding is connected in series with contact wire. Any amount of current flowing through primary requires to be balanced by equal current in the secondary and therefore tendency of currents flowing through stray path is reduced. Necessity of Booster Transformer Electric Traction19

20. Methods of connecting Booster Transformer Rail connected booster system. Booster transformer with return feeder. Electric Traction20

21. Rail connected Booster transformer Primary is connected in series with the contact line and secondary is connected in series with rails. Induced voltage in the secondary constrains the return current to flow through rails. At all locations of booster transformer, we have to provide insulated rails joint with small neutral section in the OHE. Insulation puncture between rails of insulated joint may cause short circuiting the secondary and make the booster transformer ineffective. If voltage raise of above 200V between rails and above 100V between rail and earth may cause danger life. It requires closer spacing of booster transformer. Electric Traction21

22. Incoming Supply Feeder Station Current Collection Red Bonds Mid Point Connection Along – track Conductor Booster transformer Electric Traction22

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24. Booster transformer with return feeder Connected midway between booster transformers. Rails are relieved of return current which now flows through the return feeder back to substation. Running the return feeder very close to the contact wire reduces the tendency of magnetic coupling between power line and telecommunication line. This method of connecting booster transformer is more effective and therefore provide at large space intervals. The turns ratio of the booster transformer should be 1:1. This enables the secondary to suck out the current from rails equal in magnitude to that flowing in the primary. Electric Traction24

25. Neutral Sectioning In A.C traction system, return current which flows from the locomotive to the track soon leaks to the ground within short distance and returns to the substation through earth. These ground currents cause heavy interference with the communication lines. To minimise the interference due to ground currents booster transformer is used. It consists of two windings of 1:1 ratio. The primary winding is connected in series with contact wire. Any amount of current flowing through primary requires to be balanced by equal current in the secondary and therefore tendency of currents flowing through stray path is reduced. Electric Traction25

26. Traction Mechanics Units of speed and distance etc in the mechanics of train are altogether different from those used in solving the problems of ordinary applied mechanics. Various such units used and their notations are given as follows. T = Time in seconds. S = Distance in kilometres V = Speed in kilometre per hour (kmph) α and β = Acceleration and retardation in kilometres per hour per second (kmphps) W = Weight in tonnes. F = Tractive effort or force in newtons. T = Torque in newton metres. Units and Notations used in Traction Mechanics Electric Traction26

27. Speed Time Curve The speed time curve gives complete information of the motion of the train. This curve gives the speed at various times after the start and run directly. The distance travelled by the train during a given interval of time can be obtained by determining the area between the curve and the time axis corresponding to this interval. A typical speed time curve for main line service. A B C D E 0 t1t1 t2t2 t3t3 t4t4 SPEED TIME NOTCHING ACCELE RATION FREE RUNNING COASTING BRAKING Electric Traction27

28. Speed Time Curve Notching up period (0 to t 1 ). Acceleration period (t 1 to t 2 ). Free Running period (t 2 to t 3 ). Coasting period (t 3 to t 4 ). Braking or retardation period(t 4 to t 5 ) A B C D E 0 t1t1 t2t2 t3t3 t4t4 SPEED TIME NOTCHING ACCELE RATION FREE RUNNING COASTING BRAKING t5t5 Electric Traction28

29. Types of Services There are three types of electric traction services. Main line Service.Sub - Urban Service.Urban Service. Electric Traction29

30. SPEED – TIME CURVE FOR DIFFERENT SERVICES The distance between two successive station in main line service is considerably more (more than 10Km). In this service free run is of longer duration. The duration of acceleration and retardation is small. 1. SPEED – TIME CURVE FOR MAIN LINE SERVICE SPEED in kmph TIME FREE RUNNING COASTING BRAKING ACCELERATION Electric Traction30

31. The distance between two successive station in the range of 1.5km to 8 km. Represents speed – time curve for sub – urban service. Acceleration and braking retardation required are high. Free running period is not possible and coasting period will be comparatively longer than urban service. 2. SPEED – TIME CURVE FOR SUB – URBAN SERVICE SPEED in kmph TIME ACCELERATION COASTING BRAKING Electric Traction31

32. In city service the distance between the two station is very short i.e., between 0.75km and 1km. The required for this run between the adjacent station is very small. Therefore acceleration and retardation should be sufficiently high. It will be seen that there will be no free running period. The coasting period is also small. 3. SPEED – TIME CURVE FOR URBAN OR CITY SERVICE SPEED in kmph TIME ACCELERATION COASTING BRAKING Electric Traction32

33. Simplified speed – time curve Such a curve has simple geometric shape so that simple mathematics can be used to find the relation between acceleration, retardation, average speed and distance etc. The simple curve would be fairly accurate provided it. Retains the same acceleration and retardation. Retains the same acceleration and retardation. The same area as the actual speed/time curve. The same area as the actual speed/time curve. The simplified speed/time curve can have either of the two shapes. Trapezoidal Speed time Curve. Trapezoidal Speed time Curve. Quadrilateral Curve. Quadrilateral Curve. Electric Traction33

34. SPEED in kmph TIME A1A1 A2A2 B2B2 B1B1 AB Curve OA 1 B 1 C is the equivalent trapezoidal curve. In this curve, the acceleration period and coasting period of actual speed – time curve are replaced by constant speed periods. Trapezoidal speed time curve gives closer approximation of the condition of main service. Hence this curve is suitable for main line service and long running service calculation. Trapezoidal Speed time Curve O C Electric Traction34

35. SPEED in kmph TIME A1A1 A2A2 B2B2 B1B1 AB Curve OA 2 B 2 C is the equivalent quadrilateral curve. In this curve, the acceleration period and coasting period of actual are extended. This curve is suitable for urban and suburban service calculation. Quadrilateral Speed time Curve O Electric Traction35 C

36. SPEED in kmph TIME O C tan -1 α tan -1 β BA VmVm D E T t1t1 t2t2 t3t3 Let S = Distance between stops in km. T = Actual time of run between stops in second. α = Acceleration in km per hour per second (kmphps) β = retardation in km per hour per second (kmphps) V m = Maximum Speed in km per hour (kmph) t 1 = Time for acceleration in seconds =V m / α t 2 = Time for free running in seconds = T - (t 1 + t 3 ) = T – (V m /α + V m /β) t 3 = Time for retardation in seconds = V m /β Derivation of Maximum Speed from Trapezoidal Speed – Time Curve Electric Traction36

37. area of Rectangle ABED area of Triangle BCE Area of Triangle OAD + + Electric Traction37

38. Electric Traction38

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40. a = K b = T c = 3600 S Electric Traction40

41. The maximum speed attained by the train during the run is known as crest speed. Crest Speed The average speed is defined as the ratio of the distance covered between two stops to the actual time of run. Average speed = = = Average Speed It is the ratio of the distance between the stops and the total time taken including time for stops. Schedule speed = Schedule Speed S Electric Traction41

42. The effective force, necessary to propel the train at the wheels of the vehicle is called the tractive effort. It is tangential to the driving wheels and measured in newtons. The tractive efforts required to run a train on tack are Tractive effort needed to provide acceleration (F a ) Tractive effort needed to overcome the train resistance (F 1 ) Tractive effort needed to overcome gradients (F g ) Tractive Effort Electric Traction42

43. Let M is the dead (or)stationary mass of train in tonnes. Dead mass of train=M tonnes =1000 M kg [1 tonne = 1000kg] Acceleration =α km/hr/sec =α x 1000/3600 m/sec 2. =α x m/sec 2 When a train is accelerated in a linear direction, its rotating parts like the wheels, and armature of motors have to be accelerated in and angular direction. Therefore the accelerating mass of the train is greater than the dead mass of the train. Generally the effective or accelerating mass is 10% more than the dead mass. Tractive Effort needed to provide acceleration (F a ) 1000 3600 Electric Traction43

44. i.e., M e = 1.1 M. Let the effective mass of train=M e tonnes =1000 M e kg [1 tonne = 1000kg] Therefore Force required for acceleration = Mass x acceleration i.e., F a = M e x a = 1000 M e x α = 277.8 M e α Newtons Tractive Effort needed to provide acceleration (F a ) 1000 3600 Electric Traction44

45. While moving, the train has to overcome the opposing force due to the surface friction and wind resistance. The train resistance depends upon various factors such as shape, size, condition of track etc., Tractive effort required to overcome the train resistance. F r = M x r Netwons Where M = Mass of train in tonne. r = Train resistance in Newtons/tonne. Tractive Effort needed to overcome the train resistance(F r ) Electric Traction45

46. While train is moving upwards on a slope as shown in the fig. Dead mass of the train along the slope will tend to bring it downward. To overcome this effect of gravity, tractive efforts are required in opposite direction. Tractive effort to overcome the effect of gravity = F g. Tractive Effort required to overcome gradients (F g ) MgMg M g cos θ M g sin θ Train Track FgFg C B A θ In railway practice the gradient is expressed in terms of rise or fall in every 100 meters of track and it is denoted by G%. θ Electric Traction46

47. Total tractive Effort (F t ) Electric Traction47 MgMg M g cos θ M g sin θ Train Track FgFg C B A θ θ

48. Power required for a given speed Electric Traction48

49. Specific Energy Output Electric Traction49

50. Specific Energy Output: - Specific Energy Consumption: - Electric Traction50

51. The various factors affecting specific energy consumptions. Distance between stops. Greater the distance between stops, less will be the specific energy consumption Acceleration and retardation. If the value of acceleration and retardation are high, the period of coasting is more, hence power consumed is less. Therefore specific energy consumption is less. Gradient. Steep gradients will naturally involve more energy consumption even though regenerative braking is employed. Train resistance. If the train resistance is more, greater will be the specific energy consumption Types of train equipment. If the overall efficiency of the train equipment is greater, then the energy consumption will be less. Factors Affecting Specific energy consumption: - Electric Traction51

52. High starting torque. Speed should fail with the increase in load. Speed control should be simple. Possible to employ dynamic or regenerative braking. Withstand which sudden voltage fluctuations. High efficiency. Suitable for series parallel control. Traction Motors and Control Desirable Characteristics of Traction Motors (i) Electrical Characteristics Electric Traction52

53. Motor must be robust and capable to withstand continuous vibrations. Small in overall dimensions. Minimum possible weight. Totally enclosed type to provide protection against different types of weather and dirt, dust, water, mud etc. (ii) Mechanical Characteristics The following motors are used for electrictration. D.C series motor. A.C series motor. Repulsion motor. 3 – phase induction motor. Linear induction motor. Motors used for Traction Purpose Electric Traction53

54. Plain rheostatic starting (Notching). Series – parallel starting. Metadyne control. Methods of Starting and speed control of D.C Traction motors Voltage across the armature is increased gradually from zero to full voltage. Its help to limit the starting current and suitable starting of the D.C motors. Plain rheostatic starting Electric Traction54 ON, the back emf is zero. Voltage drop in the I a and I se is negligible, drop in external resistance. As the motor accelerates and speed increases, motor current will go on reducing with the development of back emf in the motor armature. Certain value of minimum motor I reached, a portion of external resistance is cutout. Motor current again reaches the maximum value.

55. Energy saving with Plain Rheostatic Control Electric Traction55

56. Energy saving with Plain Rheostatic Control Electric Traction56 Con…

57. Electric Traction57 Accordingly two traction motor are connected in series and supply to them is given through the starting resistance in series. Series Parallel Control This resistance is progressively cut out until only two motor remain in series. At least two motors are being used. Connected in series at the starting for low speed and are in parallel for full speed running. When the motors are left running in this position, voltage across each will be nearly half of the supply voltage. Due to this, the motors will be running nearly at half the full speed.

58. Electric Traction58 Series Parallel Control For full speed, motors are disconnected and then reconnected in parallel, again supplied through starting resistance. This resistance is then progressively cut out, leaving motors in parallel running condition.

59. Electric Traction59 Methods of Series – Parallel Connection Series – parallel method of starting is not only more efficient but also it gives two economical running speeds without any resistance circuit. i.e about half and full speeds. To obtain a satisfactory transition from series to parallel without affecting the torque, the following two methods are used. They are Shunt transition Bridge transition.

60. Electric Traction60 Shunt Transition In this method, first the motors are run up and brought to series with full resistance. First the motors are run up and brought to series with full resistance. Series resistance is cut out completely in step by step and the two motors are connected in full series. Then one of the motor is short circuited with some resistance ( First Transition ).

61. Electric Traction61 Shunt Transition In this method, first the motors are run up and brought to series with full resistance. Then one end of the short circuited motor is opened (Second Transition) and finally connected across in such a way that motors are placed in first parallel.. Then the external resistance is gradually reduced to zero and the motors are connected in (full parallel). In this method jerks will be high at the time of short circuiting and disconnecting the motor during starting. Hence it is used in tramways, industrial locomotives and main line locomotives.

62. Electric Traction62 Bridge Transition In this method, the starting resistance is split up into two equal parts. Motor and starting resistances are connected in series. Starting resistances are gradually cut- out and the motors come in full series.. During transition the free ends 4 the rheostats and the motors form the wheat stone bridge.

63. Electric Traction63 Bridge Transition Then the bridge link is removed and the motors are placed in first parallel. Now the external resistance is gradually reduced to zero and the motors come in full parallel. The advantage of this method is in both of the motor circuits are closed during transition. This is employed for suburban services. There is no jerk felt in this method.

64. Electric Traction64 Multiple Unit Control The coaches where electric motors are installed are known as motor coaches. For city and suburban services, it is usual to use motor coaches. Multiple unit trains are better suited for high speed running than locomotive hauled trains. This main advantage flexibility of operation. Each unit consists of a motor coach and number of trailing coaches for passengers. Depending upon the traffic requirements, a suitable number of motor and trailer coaches are used. Number of unit vary from time to time according to traffic density. Light traffic the train can be split up. Each motor coach may have two or four motors. Possible to control all motors in single point.

65. Electric Traction65 RECENT TRENTS IN ELECTRIC TRACTION In levitation system, ordinary electromagnets are used. These magnets produce an attractive force and levitate the vehicle i.e., to rise and float the vehicle in the air with no physical support. The electromagnets are attached to the car. These magnets are placed such that they are facing the underside of the rail. They produce an attractive force and levitate the car. Force controlled by a gap sensor. This measures the distance between the rails and electromagnets. Control continuously regulates the gap and fixed distance of about 8mm. Gap increases beyond 8mm, the current to the electromagnet is increased. Magnetic Levitation (MEGLEV)

66. Electric Traction66 Multiple Unit Control The primary side coils of the motor are attached to the car body. The secondary side reaction plates are installed along the guide way of the rail. The secondary plate is made of aluminium or copper plate. If the gap become less than 8mm, the current is decreased, to create less attraction (ie., Now the gap is increased). The levitation magnets are ‘U’ shaped and the rails are inverted ‘U’ shaped. In this system the electromagnetic attractive force levitate and guide the car. In this system linear induction motor is used. The linear induction motor is just like ordinary induction motor, but it has been split open and flattened. Cont…

67. Electric Traction67 Suspension systems In suspension arrangement the locomotive (train engine) is carried by two bogies. The bogies are coupled by two or more axles. As the locomotive moves on the track, shocks are delivered to the wheels and axles due to irregular tracks. The purpose of suspension arrangement is to minimise the transmission of these shocks to the locomotive body with the help of springs and dampers. The set of springs and dampers are interposed in between axles and bogie. This arrangement is called primary suspension. Generally upto 80 Kmph speed, railway vehicles do not produce any special vibration problem. At higher speeds vibration becomes more. Primary and Secondary suspension system Therefore for high speed vehicles, a second set of spring of low stiffness is provided in between the bogie and the body. Arrangement is called Secondary Suspension.

68. Electric Traction68