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INTRODUCTION TO DIESEL SHED RATLAM Diesel Shed Ratlam(M.P.) is established on 1 may 1967. It is awarded with ISO 9001-2000 and ISO 14001-1996 certificate firstly in western railway. It is located on main line between Delhi and Mumbai. Shed is a place where maintenance and repair of locomotive is done. As on today ,the shed has holding of 140 diesel locos. The entire diesel shed is divided into mainly two sections :(1) Mechanical Section(2) Electrical Section
LOCOMOTIVE AND ITS TYPE Locomotive is an automatic device which kept on steel frame having ability to run on rails and has power to pull passenger as well as goods trains. Locomotives are of three types :(1) Steam Locomotive(2) Diesel Electric Locomotive(3) Electric Locomotive
(1) STEAM LOCOMOTIVE Steam locomotive played a key role during development and golden age of railroading. Steam locomotive is a self contained power unit consisting of a steam engine , a boiler with fuel and water supplies. Superheated steam is admitted to the cyllinders by a suitable valve arrangement and the pressure on the pistons being transmitted through main rod to the driving wheels. The superheated steam is controlled by a throttle.
Steam locomotive has been replaced in developed nations by electric locomotives and diesel- electric locomotives because of following disadvantages of steam locomotive :(a) It has strictly limited overload capacity.(b) It has very low thermal efficiency of about 6-8% because installation of a condenser on locomotive is very difficult.(c) It is available for hauling work for about 60% of its working days, remaining 40% being spent in preparing for service, in maintence and overhaul.
(2) DIESEL ELECTRIC LOCOMOTIVE Diesel electric locomotives were introduced firstly in united states in 1924 and have become the most widely used type of locomotive. It was introduced for first time in India in 1958. Diesel electric locomotive has electric drive in form of traction motors driving the axles and controlled with electronic controls. It differs from electric locomotives principally in that it has its own generating station instead of being connected to a remote generating station through overhead wires. The generating station consists of a large diesel engine coupled to dc generator that provides power to traction motors. These motors drive the driving wheels.
Advantages: (a) It provides high starting acceleration in comparison to steam locomotive.(b) It is more efficient than a steam locomotive.Disadvantages: (a) It is costlier than either steam or electric locomotive for same power.(b) Life of diesel engine is shorter comparatively.(c) Regenerative braking cannot be employed though rheostatic can be.(d) Overload capacity is limited because diesel engine is a constant output prime mover.
(3) ELECTRIC LOCOMOTIVE Electric locomotives generally have two or more dc or ac motors. In these locomotives , power is collected from an electric trolley which is running on an overhead wire. The overhead wire can carry both types of supply ac as well as dc. Indian railway also uses both types of supply systems :(a) AC system- 25 kv single phase 50 hz ac supply(b) DC system- 1500 V dc supply Only western zone and central zone of railway uses dc system.
Advantages: (a) It does not produce any smoke and flue gases, so it is most suited for undergrounds trains.(b) The maintenance cost of electric locomotive is lower than steam locomotive.(c) Traction motors used in it have very high starting torqe.(d) Regenerative braking is used for ac systems.(e) An electric locomotive can be started in a moment but a steam locomotive takes about 2 hours to heat.
Disadvantages: (a) Electric traction has high initial cost of laying out overhead electric supply system.(b) Power failure for few minutes can cause traffic dislocation for hours.(c) Communication lines which usually run parallel to the power supply lines suffer from electrical interference.(d) Electric locomotive can be use only on those routes which have been electrified.
DC SYSTEM OF RAILWAY ELECTRIFICATION In dc system with overhead catenary, dc traction motors are supplied 1500 V DC by catenary. In overhead electrification systems, electricity is supplied through an overhead system of suspended cable which is known as catenary. Indian Railway uses catenaries of constant tension type. At one end of each section of catenary the cable connects to a pulley by going over this ,is terminated by hanging a weight. Catenary wires are usually made of copper alloys such as cadmium- copper which has high tensile strength of 63 kg per sequare mm.
For transmission of 1500 V DC by overheadwire, substations are made and they are located 40km apart with each other. These substationsreceive power from 132 kv ,3-phase network. Atthese substations, this high-voltage 3-phase supplyis converted into low-voltage 1-phase supply withhelp of Scott-connected 3-phase transformers. Nextthis low voltage ac is converted into 1500 V DC byusing suitable rectifiers. The dc supply so obtainedis fed to dc traction motors via suitable contact.
Advantages of dc system over 1-φ ac system:(a) DC system does not cause electrical interference with overhead communication lines.(b) DC motors are better suited for frequent and rapid acceleration of heavy trains than ac motors.(c) DC train equipment is lighter, less costly and more efficient than similar ac equipment.(d) When operating under similar conditions, dc trains consumes less energy than 1-φ ac train.Disadvantage: Only one disadvantage is the necessity of locating ac/dc conversion substations at short distance apart.
TRACTION MOTOR(1) Description: The traction motor is a four pole DC series motor in which field winding is connected in series with armature. It is a forced ventilated machine arranged for axle mounting on sleeve bearing. Transverse movement is limited by the flanges of axle suspension bearing. An electric locomotive as well as diesel-electric locomotive in indian railway contain six dc traction motors at once.
(2) Construction: The armature core is made from high permeability silicon steel stampings and these stampings are separated by thin coating of varnish as insulation with each other. The armature is lap wounded with 100% equalization. The commutator is built up with hard drawn silver bearing copper segment which are insulated with micanite segment. After the commutator is statically and dynamically seasoned to insure stability ,the complete armature is dynamically balanced.
The high permeability cast steel magnetframe is machined to insure alignment of the endshields, pole bores & axle way bores. The mainpoles are built from steel laminations. There are four brush holder per motor, eachcarrying 3 split carbon brushes. Each brush holderis carried on two insulated support pins. The armature is supported on greaselubricated roller bearing. Bearing assemblies aresealed type, so necessity of lubrication in about 2or 3 years.
(3) Rating: A traction motor has following ratings: Voltage - 285 volts Current - 980 ampere Speed - 360 rpm Power - 248 kw
(4) Principle: When a current carrying conductor is placed in a magnetic field, a force is exerted on it and direction of force is determined by fleming’s left hand rule. In a dc motor, dc supply is provided to field winding. As a result, unidirectional magnetic field is produced and magnetic field lines cut the rotor conductors which carry current equal to field current. Since the conductors are on circumference of rotor, force acts in tangential direction to the rotor. Thus a torque is developed on the rotor and it starts to rotate.
(5) Operation: To understand the operation of a traction motor, three transition panels are studied. Transition panels perform transition events. These events correspond to field weaking and changing the connections of traction motor. Three transition panels are performed at three different speeds – First transition at 30 km/hr speed Second transition at 50 km/hr speed Third transition at 80 km/hr speed
When 1500 V dc supply is provided to circuitof traction motors, a large current flows througharmature of motor as it is connected in series withfield. As a result armature, armature begins torotate and a back emf is generated which opposesthe main supply voltage. ( back emf Eb =PφNZ/60A Eb α N ) As speed increases, back emf alsoincreases and it offers resistance to flow ofgenerated current to traction motors. Hence forincreasing the speed , the supply voltage mustincrease but it is not
possible to increase supply above 1500 V.Therefore, when speed is reached at 30 km/hr, firsttransition panel is switched on. Now a parallelcombination of two series connected tractionmotors is under operation and resistance isconnected in parallel with the field of motors torefuse back emf. Now speed increases beyond 40 km/hrand back emf again starts increasing rapidly. Sosecond transition panel is switched on at speed of50km/hr. After second transition, all six motors runin parallel
without resistance parallel to field of motors. No longer back emf is controlled bysecond transition panel as speed increases beyond70 km/hr, so third transition panel is switched on atspeed of 80 km/hr. By this transition, resistancesare connected parallel with field of motors in circuitof second transition to reduce back emf.
(6) Speed-Torque characteristic:The speed Vs torque characteristic of dc series traction motor with a constant votltagesupply is shown on right side.As the speed decreases, torquefor motor increases sharply. Asload is removed from motor,speed increases sharply. Henceit must have a load connected.
(7) Braking: Dynamic or rheostatic braking is employed for electric locomotives. During the time of braking, traction motors are disconnected from supply and is connected to a dynamic resistance. Now traction motors are acted as traction generator because kinetic energy is converted into electrical energy. The direction of current is reversed during this period as before braking. Field current also reverses as field winding is connected in series with armature. Hence connections of field winding should be changed such that current flows in it in should
be changed such that current flows in it in samedirection as before braking. The output of generatoris given to grid resistance and it dissipates poweras heat. Due to friction between wheel and track, aopposite torque is developed. As a result,locomotive stops. If connections of field winding is notrevesed, no braking will occure. Dynamic resistance must be less thancritical resistance otherwise generator will not beself exciting.