MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION 1 ...

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous)

(ISO/IEC - 27001 - 2005 Certified)

Subject Code :12233 .

SUMMER – 12 EXAMINATION Model Answer

Q.1a. i)

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As primary side of transformer is H. V. therefore low current in primary.

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Because of low current large number of tapping can be taken for on load tap changing arrangement.

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Therefore smoother acceleration and speed control. (1pt 1 mark, 2pts 2 marks, 3 pts – 4 marks)

ii)

1) Smoothing reactor Smoothen out ripples in the dc output of rectifiers 2) Reverser To control the direction of current in the field of traction motor (dc series motor) and hence its direction of rotation. 3) Tap changer – To obtain variable ac voltage for precise speed control of traction motor. 4) Pantograph – To maintain a link between the overhead system (OHE) and power circuit of the locomotive at varying speed in different climatic and wind condition on varying stiffness OHE. (1 mark for each component’s function)

iii) Sources of overvoltage in electric traction – 1) Direct lighting strokes or electrostatic induction (Indirect strokes). Eg. i) Direct lighting of strokes on overhead line. ii) Electro-statically induced charges on conductors due to passage of charged clouds over overhead lines 2) External switching surges from the transmission line and grid system. Eg. Closing and opening of feeder and transformer circuit breaker under the conditions when earthing is not proper for the switchgears provided in the system. 3) Internal switching surges within the locomotive eg when circuits get interrupted through tripping of the system under fault or no power conditions. (List of 3 sources of over voltage 1 mark, Eg. of each type 1 mark for each) 4) Faults which require overload protection – •

Shorting of segment of transformer winding

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Failure of diodes of rectifiers. 1/12



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Flasher of traction motors

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Shorting of power circuit due to the cable insulation breakdown. (1 mark for each type of fault)

b) i) Power circuit of electric locomotive – Refer fig. 1

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The diagram should cover following power circuit equipment i) contact wire ii) pantograph iii) circuit breaker, iv) tap changer v) traction transformer, vi) rectifier, vii) smoothing reactor viii) traction motors. (Unlabeled diagram: 2 marks, partially labeled: 3 marks, fully labeled: 6 marks) ii) Three types of maintenance activities :-

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1) Running repairs :When equipment has actually failed. (method of repair technically unsound). 2) Trouble shooting :

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Rectification of defects by detection of faults and making sometimes temporary repairs (if immediate repairs not possible) or patch repairs, so that locomotive may get going on (only temporary relief) 3) Preventive maintenance :-

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Advance actions taken to avoid future failures, systematic inspection of various components and timely repairs. (trouble free service up to next inspection)

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Q2a) i) Arno converter : Working : •

Main function is to convert single phase supply to three phase supply.

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Arno converter is basically induction motor.

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For starting 1φ supply given to 2 leads of Induction Motor directly while 3rd lead connected to supply through starting resistance and starting contactor.

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The motor starts as split phase Induction Motor, when its speed reaches 90% of synchronous speed, starting resistance circuit is operated out by means of relay.

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The machine then pulls into synchronism and gives 3φ output.

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Stator winding carries single phase motoring current and 3φ generating current.

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Currents in the 3 windings are not equal. This gives rise to voltage unbalance in the 3φ supply obtained from Arno converter. The degree of unbalance varies with load.

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Motoring phases carry both motoring and generating currents, therefore their resultant current is small where as generating phase carries only generating current, therefore its magnitude is higher than motoring phases.

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Catenary voltage varies between 17.5 kV to 27.1 kV, 3φ output voltage obtained from Arno convertor fluctuates from 290 V to 460 V. Also Arno converter has 5% voltage unbalance.

(1 mark for every 2 points, max. 4 marks) ii) Uses of compressor in electric locomotive : About 1000 liters per min at 9 kg/cm2 compressed air pressure is needed for the 1)operation of brakes, 2)wind screen wipers, 2)horn, 3)sanding equipment, 4)tap changer drive, 5)air blast circuit breaker, 6)pantograph contactors and 7)other pneumatic controls. ( 1 use 1 mark, max 4marks) b) i)

Measures for reducing idle period of locomotive for maintenance: 1) Proper inspection schedule – (More frequent and detailed inspection, longer would be the outage of locomotive and lower is the availability. Improper inspection increases the failure rate of locomotive. (Hence frequency and duration of inspection is important) 2) Coordination between maintenance and operating staff of locomotive. 3) Coordination of work in shed. 4) Availability of standby and quality spares. ( 2 mark for each point, max 4 marks)

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ii) Factors through which optimum maintenance can be obtained – 1) Proper inspection. 2) Proper method of trouble shooting and repairs 3) Technical investigations. 4) Suitable repairs facilities in the sheds 5) Test and trials. 6) Design for minimum maintenance skills and tools. 7) Use of standard parts and components. ( 1 point – 1mark, max 4 marks) c) Moving primary fixed secondary, double side LIM

(diagram 3 marks)

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Secondary Al reaction rail faces a primary on either side. (1mark)

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Primaries are so connected that opposite poles are formed in primary members along the length, on either side of secondary member. (1mark)

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Because of travelling field produced propulsive forces are produced in the same direction. (1mark)

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No Need of backing of iron along the track as laminated core is used in each primary. (1mark)

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Problem of maintaining uniform clearance between secondary Al plate and two primaries all along the track, particularly at points and crossing. (1mark)

Q. 3 a) Need for temperature monitoring : •

Prevention of fires. To prevent fires due to excessive temperatures.

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Increase life of equipment.

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Temperature sensing device can trigger an alarm which attracts attention of crew.

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Common practice is to monitor transformer oil temperature where fire potential is 4/12



maximum.

(4 marks for any 2 points)

b) Use of static converter instead of Arno converter •

Unbalanced current in the 3 phase of stator winding

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Undue heat produced in the winding.

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Voltage fluctuation and voltage unbalance

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When output voltage of Arno converter rises to 460 V , because of excessive magnetizing current, poor p.f.

(1 marks for each factor) c) Functions of equipments : i)

Blowers : Forced air supplied by blowers used for coding of semiconductor rectifiers and traction motors.

ii)

Oil pumps : To maintain continuous oil circulations in the transformer tank through a radiator for cooling of transformer oil.

iii)

Battery – Before pantograph in raised and circuit for auxiliaries is energized, batteries are required to raise the pantograph, to operate the air blast C.B. and provide lighting in the drivers cabin.

iv)

Vacuum Exhauster: - To produce vacuum for operators of vacuum brakes.

(1 mark for each equipment function) d) Need for maintenance : •

To correct the effects of Wearing out.

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To correct the effects of deterioration of locomotive component during normal use.

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In the long run (aging) performance gets affected and finally beyond safe limit components may fail altogether.

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Increase the life of equipment.

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Maintain the proper quality of output of equipment.

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Maintain the efficiency of the equipment.

(1 mark for each point, any four; max 4 marks) e) Strengths of LIM propelled traction system 1) Pull produced for propelling the system is independent of adhesion between the wheels of motorized coach and rail. This feature permits negotiation of steeper gradients. 2) Two wheels fixed to an axle can rotate independent of each other and hence curve negotiating property improves. 3) System does not require gears therefore reduction in the associated noise level and cost of maintenance. 5/12



4) Floor height above the rail level is substantially reduced. This results in reduction in the height of bridges, diameter of tunnels along the route. Weakness : 1) The system requires looping of Al reaction rail all along the route. This considerably adds to the cost of track. 2) Since Al is relative expensive material, hence chances of theft of more, hence disruption of service. 3) Due to more air gap in LIM than RIM, p.f. is poor and increased motor losses. 4) Due to discontinuity in the magnetic and electric fields at entry and exit ends of motor, therefore poor motor efficiency. (1/2 mark for each strength, max 2 marks) (1/2 mark for each weakness, max 2 marks) Q.4 a) i) Modifications required for adopting LIM propulsion in existing system. 1) Track Modifications Case A – If track is already electrified and can be made available for LIM propelled. System modification required- Reaction rail consisting of Al sheet backed by ms sheet should be laid between the existing steel rails. Case B – If track is not electrified. Modification required – i) track should be electrified. ii) Modification similar to case A 2) Vehicle modifications : i) Axles to be redesigned with wheels of reduced diameter, such that two wheels of same axle, rotate independently. ii) Primary member to be maintained between axles of each bogie. iii) Matching modifications in existing mechanical braking. (2 marks for each modification). ii) Energy saving strategy that can be implemented with the help of computers. 1) Accelerations to the “booked speed” promptly. 2) Running at “booked speed” to maintain the time table. 3) Coasting (without power) before application of brakes at stoppages or speed restriction locations. (2 marks for each strategy, any two, max 4 marks). iii) Power circuit equipments : Pantograph circuit breaker, tap changer, traction transformer, rectifier, smoothing choke, 6/12



traction motor. Auxiliary circuit equipments : Batteries, Arno converter, Blowers, Exhausters, Compressor, Head light, Flasher light, Horn etc. (2 marks for power circuit. Equipments , 2 marks for Auxiliary circuit equipments) iv) R-C networks used for projection of switching surges

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RC networks reduce steepness of the voltage surge wave-front and help to absorb the energy in the resistance.

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Precaution to be observed in connecting R.C. networks is to avoid use of unduly long leads.

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The inductance of the lead should be negligible as compared to the capacitance. (2 marks for fig., 2 marks for description)

b) i) Test that are carried out on different electrical equipments: i) Insulation test : •

The test is carried out on transformers, rotating machines (traction motor) , circuit breaker, contactors etc.

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Insulation resistance test is carried out with megger of suitable voltage rating.

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Voltage rating of merger depends upon the voltage of the installation under test.

Type of installations

Insulators resistance to each at

1) Circuit breakers and For 400 V rating

400C 2MΩ 7/12



transformer winding For 11 kV rating

2) Traction motors

50 M Ω

For 25 kV rating

200 M Ω

For 132 kV rating

2000 M Ω

Up-to 750 V rating

1 M Ω at 75 0C

Above 750 v rating

3 M Ω at 75 0C

3) Passer equipment in general

1 to 2 M Ω

2) Induced voltage test : * To find inter turn and inter lager defects in transformer Twice the normal voltage is applied on test voltage winding with other winding left open circuited. This will produce high voltage in every turn. 3) Crackle test : * Aim of the test is to determine presence of water or miosture in the oil. Transformer oil under test is taken in tube and heated on burner if water or moisture is present n oil, cracking sound is heard. (2 marks for each test) b) ii)

1) Reasons of proneness of fire a) Presence of large no. of joints in electrical circuits, arcing contactors, possibility of inter turn shorts in machines and transformers. b) Presence of oil in the transformer and tap changer and leakage of oil. c) Absence of overload protection for auxiliaries. (1 mark for each reason, total 3 marks) 2) measures to prevent fire: a)Making proper and tight connections. No connection should be left loose. b)Monitoring of transformer oil temperature and preventing leakage of oil. c) Providing smoke detectors. d) Providing at least 2 to 4 fire extinguishers in loco engine. (1 mark for each measure, any 3 measures, total 3 marks)

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Q.5 a) Equipments of electric locomotive that require constant monitoring Need – To provide uninterruptable service of electric locomotive, failure prone equipments are constantly monitored, to minimize the need for frequent preventive checkups and dismantling. Avoid major breakdown. To avoid serious consequences arising from the parts which an subjected to stresses and susceptible to fatigue failures, eg axle, to avoid risk of five. ( 4marks for need) Following equipments need constant monitoring silicon diode rectifier, transformer oil cooling blower, bearings of rotating machines, axles, Bogie frames and couplers, smoothing reactors, insulting in traction motors and transformer, Armature banding etc. (4 marks for list of equipments) b) Prevention from accidental contact. •

All equipments at voltage exceeding 400 V are enclosed by expanded metal frames which are fitted with locks and doors at susceptible locations.

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The locks are opened by keys which get released after operator closes main earthing switch to ‘Earth’ position. This becomes possible only after pantographs are lowered.

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Similar locking arrangement is provided for releasing the ladder which is required for climbing on the roof of locomotive. For doing this permit to work is needed from traction power controller.

(Any 2 points, 8 marks) c) i) Simulators for training drivers : Simulator is a combination of hardware and software for training drivers in good driving practices and route learning It consists of – Replica cab, Instructor console, Data base, Matching computer model with necessary degree of sophistication, A visual display system giving impression of driving a real train with images of track side signals and other visual events Need of simulator : Route learning using route films on videotapes, Time table adherence, Energy conservation measures, Traction and braking control. Passenger comfort and stopping accuracy Systematic and planned response to failure and accident. (2 marks for meaning of simulator, 3 marks for simulator requirements) ii) Factors on which drivers can be trained using simulator : Route learning using route films on videotapes, Time table adherence 9/12



Energy conservation measures Traction braking control Passenger comfort and stopping accuracy Systematic and planned response to failure and accidents (3 marks for 6 factors) Q.6 a) Basic skills required in artisans for maintenance of electric traction Electricians: with basic practice on working with high voltage/ high current systems. Mechanical fitters capable of analyzing fitting of parts etc. Electronic technicians with knowledge of basic trouble shooting & soldering practices, Switch gear technicians Apart from these armature winders and welders are also needed in the base sheds and workshops. (4 skills, 4 marks) b) Need of history cards •

It is useful when loco is transferred from one shed to another or sent to workshop for POH, as complete record of various defects in loco is maintained, in it.

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Maintenance staff can get idea about the defects which have been already noticed in past and which parts have been replaced.

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Repetition of particular faults/ malfunctions can be detected.

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Specification of parts which are used are readily available.

Use of computers in replacing cards /sheets. •

By eliminating clerical efforts, integrity of records is maintained.

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By using stored information, it helps in decision making for major interventions or preventive measures. (2 marks for need of history sheets, 2 marks for how computers can replace history cards )

c) Features useful in adopting LIM for Metro Rail system: i)

Motor efficiency does not significantly affect the total energy consumption used in Metro rail .

ii)

With the use of VVVF drives improved power factor and energy efficiency obtained.

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With LIM coach floor height is brought down, so wheel diameter is reduced, Therefore tunnel diameter in Metro rail reduces to approximately 75% which reduces tunneling cost to about 60% 10/12



iv)

With reduction in tunnel diameter, total volume of air required for air conditioning reduces, this reduces energy consumption for air conditioners

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Wear and tear of motor rotor and gears is eliminated as only moving part is the wheel fixed via bearings to the axle. This also reduces noise in tunnel.

vi)

As adhesion is not limiting feature in LIM, hence tracks can be laid with steeper gradients and improved curve negotiating property of vehicle.

(2 factors: 1mark, 3 factors – 2 marks, 4 or 5 factors: 3 marks, 6 factors – 4 marks) d) Use of computers in failure analysis. •

Through suitable software solution for repetitive causes can be obtained.

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Clues regarding developing trends due to unexpected wear /tear pattern or fatigue arising in service conditions can be obtained.

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(2 points, 4 marks).

e) 3φ VVVK ac supply from 1φ

From the 25 kv and power available from OHE, in locomotive alogn with transformer converter/Inverter arrangement is provided, to get 3φ VVVF ac supply. •

In this voltage as well as frequency are varied on demand during inversion.

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Input to the inverter is a fixed dc voltage supplied by controlled converter.

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To enable movement in either direction and regenerative braking, converter and inverter are both controlled and function as converter or inverter as required . This is achieved through four quadrant controller.

( 2 marks for fig. 2 marks for description.)

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