Electrical oral questions in MMD Kolkata,MEO class IVB 1. Dynamic Breaking: It is a form of regenerative breaking in which the breaking energy is dissipated in a auxiliary resistor instead of economically absorbed. 2. Regenerative Breaking: The thyristors are operated in such a way that the reverse power flow occurs from the motor to the supply. This allows quick reduction in the rpm of the motor and breaking operation is taken place. 3. Plugging Breaking: for plugging breaking, normally in the dc motors the direction of current is reversed hence the motor flux get reversed and tend to rotate the motor in acw direction and hence the motor rpm will reduce quickly. IN AC motors, the direction of ac can not be reversed hence the phase sequence is reversed by the inverter thyristors and hence reduce the rpm quickly. 4. PWM convertor ( ac-dc-ac) diode bridge rectifier(not thyristor) then IGBT(insulated gate bipolar transistor). Transistor is used hence only medium range voltage and current can be supplied. Used as VSD variable speed drive for induction motor. Since thyristor are not used hence signal can not be reversed and hence cannot be used in regenerative braking operation. 5. synchronous converter or static frequency convertor FREQUENCY OUTPUT(0.5-120Hz) (Contains rectifier and inverter thyristor bridges. Ac-dc-ac. Amount of Current is controlled by the rectifier thyristors and the frequency is controlled by the invertor thyristor bridge. This convertor is for synchronous motor variable speed drivesVSD . Switching Inverter thyristor bridge is also acting as Static commutator because it supply the ac to the armature of the motor from the dc input . 6. Cycloconvertor FREQUENCY OUTPUT (MAX 20Hz) less frequency output is due to the wave form produced by this convertor. This is used as VSD in synchronous motors. 7. What is azipods? Very large propulsion motors are being fitted within the rotating pods mounted outside to the ships hull. These ate generally refer to as the azipods. They can be rotated in 360 degree to apply thrust in any horizontal direction 8. What precautions you will take before entering battery room. a.Inform the bridge & the engine control room b.Check the oxygen content. c. Should be ventilated properly. d. All PPE to be donned properly & rubber hand gloves to be used. e. No Naked lamps to be taken inside 9. What is meaning of power factor. What is best value. What is normal value onboard. How do you improve? Power Factor :- The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power and is a dimensionless number between 0 and 1 The best value is 0.85. The normal value onboard varies in between 0.8 to 0.9. Power factor can be improved by connecting a Capacitor in parallel 10. Main switchboard safeties. a. Ebonite Rod. b. Rubber pad infront of switchboard. c. Dead-front type switchboard. d. 0.6 meter gap behind the switchboard. e. Fuse f. Circuit Breakers. g. Earth Fault indicators. h. Panel doors are earthed. i. No water, steam or oil pipelines to pass in its vicinity. j. Undervoltage Relay k. Reverse Power trip. l. Preferential trip. m. Overcurrent trip n. Arc Chute o. Short-circuit trip. p. CT and other instruments are earthed q. interlock provided so that when open the door the supply will automatically disconnected r. Fused isolators are provided.
11. Maintenance on Alternator? > Stop and lock prime mover and alternator. > Lock in tag out For help :
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Electrical oral questions in MMD Kolkata,MEO class IVB > Open up the terminal box and check for the tightness of the terminals and integrity of the terminal box. a. Ventilation passages & air filters to clean. b. Insulation resistance to check for Stator, rotor winding. If low, then it should be regain and varnishing to be done. Check the earthing wire is properly connected b/w the stator and the bedplate. Before connecting megger, the electronic instrument and circuit should be separated. c. Air gap to check using plastic tapered long feeler gauge. d. Bearing oil to be renewed. e. Use vaccum cleaner to remove dirt/ dust. h. AVR components, diodes to keep free from oil, moisture or any dirt. i. Heaters to be checked. j. Commutator and brushes condition to be checked. Brush pressure etc. 12. An example of each. - Where does magnetic field cuts the conductor?alternator -where does conductor cuts the magnetic field ?motor
13. How to test a fuse? -apart from multimeter 14. Which is the largest induction motor used onboard? Alternator
15.What is the supply given to shipboard general alarm. 24 Volt DC. Abhishek Sharma 6228
16. What do u mean by rotary convertor?what does it do!! ..... Rotary convertor is used to convert AC to DC. It is a mechanical convertor whose construction is almost same as the DC generator but its rotor consist of both commutator and slip rings. During normal operation, AC is injected through slip ring and the convertor is rotated like a synchronous motor, the part of AC current supply through slip ring is directly passed to the commutator and the current generated in rotor due to the rotation is also pass through the commutator so DC is the output. It can also change DC to AC by supplying DC through the commutator and AC output from the slip ring but in that case it is called reverse rotary convertor. AC and DC both can also be produced by acting it as a generator. It is used in the railway engines. 17. what do u mean by slip..why there is no slip in synchronous motor. 18. what is short ckt,how will u find which ckt is short cktd. Short circuit can be tested by comparing the continuity readings taken for all phases . If anyone reading is found to be smaller than other, means that winding is short-circuited. 19. what is earth fault...what is the a criteria to find a earth fault in a particular machinery 20. fuses hw many types, every type functioning 21. ampere rating of a wire Max current that a wire can carry continuous without increment in the rise in temp by 35 degree from 45 degree ambient temperature. i.e. the conductor temp should not rise above 80 if ambient temp is 45 degree. And if ambient temp rise the rating should decrease considerably. 22. hw to chk tht the cell is completely charged By checking the specific gravity(measure and add 0.007 per 10 degree rise in temp above 15 degree and subtract 0.007 at each 10 drop in temp below 15 degree celcius) and voltage(during on-load operation of battery. As V=E-Ir, at no load terminal voltage will be higher and false indication of voltage will be measured) 23. Synchronous motor: Bow thruster motor, governer motor(also can be dc motor because of high torque), radar, gyro, clear glass, propulsion motor, synchronous compensator. 24. ACB drawn out procedure : During switchboard maintenance or during the safety trips , the ACB is to be drawn out to test position or disconnect position. The procedure is simple and as follows: Ensure the acb is open and disconnected from the main busbar. Remove the locking nut and pull the release lever on both sides.
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Electrical oral questions in MMD Kolkata,MEO class IVB -
Push the trip button so that the cover on the drawn out handle insertion hole will be open automatically. Infact this is the safety provided in the ACB that is until the acb is not in trip position, the drawn out handle insertion hole will be in close position and you can not put the draw out handle to draw the acb out. Insert the drawn out handle in the insertion hole and rotate it acw until the acb is drawn out to the TEST POSITION. ACB will be locked in this position and will not draw out further. If you want to draw out the acb further to the DISCONNECTED position, again pull out the release lever and turn the drawn out handle acw. Under voltage trip: Sets at the 30-70% of the rated voltage. When voltage goes down to this value , it will trip the alternator acb. To test the UV Trip :It can only be checked by the voltage injection directly to the UV relay. Draw out the ACB and note down the voltage up to which the UVT is de-energised make a slow closing operation and check the breaker does not close. And at the voltage when UVT energised, check that the breaker can be normally closed. The time for activation of uv trip can be note down and compare it with the time characteristics supplied by the manufacturer. To test the Overcurrent relay setting : Only a proper current injection test will prove the correct setting. Draw out the ACB to test position and a set of calibrated currents from a current injection set( which provide the currents equal to the trip current with lower voltage like welding transformer) are fed directly to the closed circuit breaker. The tripping current and time delay can now be compared with the manufacturer’s I/t characteristics. Normal set point 150%FLC at 20 sec time delay. TESTING OF THE REVERSE POWER TRIP: If the output of a generator is reduced below 0 kW during the load shift operation, the reverse power relay (RP setting (Normally): 10% of rated output, Time setting:10 seconds) will activate and trip the generator ACB and the possibility of “Black-out” to be ensued if the remaining generator power is affected. During load shifting-Observe the transferring of load and the frequency of both generators to be maintained at 60Hz. When D/G1 output is reduced to about 5% of its rated output, operate the ACB switch for D/G1 to “Open” and D/G1 will be removed from the bus. All loads were transferred to D/G2 Also Testing relay is provided to test the reverse power trip. Programmable Logic Controller (PLCs): also called sequencer : it is electrical PID controller. All programmable controllers are composed of a memory, processing part, input interface and output interface. The sequencer requires input data and output devices. The input data are push button switches for start/stop, change over switches for AUTO/MAN, Limit switches for level high/low, magnetic switches, thermal relay signal, etc. The output devices are contactors, indicator lamps, solenoid valves, etc. Mitra’s oral questions electrical: 26. What is the composition of the insulation materials? 27. How does raise lower knob of the governer of the alternator is connected to the governer ?(dc motor) why dc motor? 28. Why alternator is not generate the voltage? Residual magnetism. Every thing? Not generating voltage but still running, the only cause is seems to be the flux is not generating, hence problem is with the exciter or the residual magnetism is lost. Avr is faulty or the voltmeter is faulty. 29. what is reverse power trip? how it is tested ? what are the settings(2-3% TURBINE and 5-15% diesel time delay of 0.53sec) ? how is the setting adjusted? why we use reverse power trip and not reverese current trip..? TESTING OF THE REVERSE POWER TRIP: If the output of a generator is reduced below 0 kW during the load shift operation, the reverse power relay (RP setting (Normally): 10% of rated output, Time setting:10 seconds) will activate and trip the generator ACB and the possibility of “Black-out” to be ensued if the remaining generator power is affected. During load shifting-Observe the transferring of load and the frequency of both generators to be maintained at 60Hz. When D/G1 output is reduced to about 5% of its rated output, operate the ACB switch for D/G1 to “Open” and D/G1 will be removed from the bus. All loads were transferred to D/G2 Also Testing relay is provided to test the reverse power trip. 30. megger, what are the safeties on megger, why insulation resistance measured every month and not just once, what is steady state temperature and hot spot temperatur of a insulation class..? MEGGER SAFTIES : Use meggers on high-resistance measurements only (such as insulation measurements or tocheck two separate conductors on a cable). Never touch the test leads while the handle is being cranked. Deenergize and discharge the circuit completely before connecting a megger. Disconnect the item being checked from other circuitry, if possible, before using a megger Disconnect the diode and semiconductors before megger testing.
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Electrical oral questions in MMD Kolkata,MEO class IVB CURRENT LIMITING RESISTANCE is provided in the megger in the deflecting coil so that if the two terminals touch each other , the deflecting coil will not damage due to shortcircuit’s high current. Also, in the rotating handle type megger, the gear train are provided inside the megge so that still rotating the handle at normal speedm high rpm can ve generated. THIS gear train will generate particular voltage, if handle will be turned faster, the gear will slip and the rpm remains constant. HOT SPOT TEMP : the temperature at the hottest point that is important, and this is referred to as “Hot spot” temperature. In field coil, for instance, the hot spot is somewhere near the center of winding and there is temperature gradient from there to the surface, so that temperature is not uniform throughout the coil. The only means available in practice therefore is to determine the temperature either by the change in resistance of the winding, or by measuring the surface temperature by thermometer. STEADY STATE TEMP: steady temperature at which the heat in windings and magnetized cores and the heat arising from frictional losses will be dissipated at the same rate as they are generated. Eg : Class A insulation -55’C steady state surface temp and 105’C as the hot spot temp in the centre of the coil is generally accepted. 31. thyristor, draw a simple circuit usking thyristor, applications of thryistor, i said cycloconvertor, so he asked what is cycloconvertor, and how does it work..? 32. what are space heaters, why they are there, what is the other name for space heaters(Strip Heater), where al u hav seen it., suppose if earth fault alarm came in ecr and it was due to space heater in emergency generator, how would u go about detection and clearance of earth faut..? 33. shore suppy connection box, where is it, what all is there ( dont forget to mention dataplate), how it is earthed, what if phase sequence is wrong, how to correct it..?How a PSI works(other than the 2 lamp type, i.e. the pointer type) Shore connection box is having an earthing terminal along with other terminals to earth ships hull to the shore earth. P.S.Indicator can be of pointer type which is rotating in cws direction when the phase sequence is correct. It is a small 3phase induction motor. If phase sequence is not correct this motor and hence the pointer will move in acw direction. Electrical 34. What is Max. Continues Current rating of motor? (give ans please ) 35. What is the function of commutator r k das and mukherjee 36.what is residuel magnetisim.give atleast 4 reason for loss of rm. Reasons of loss of residual magnetism: Non-operation for long time cause lost of residual magnetism. Improper operation AVR terminals to the exciter winding gets open then during starting RM will over and volt will become zero. In series winding, if reverse current flow the reversal of residual magnetism will occur. Recovery of RM : procedure for field flashing a generator: stop the engine, disconnect exciter field leads from the voltage regulator (note the polarity of the leads), and turn the circuit breaker off. Then briefly apply voltage from an external battery or another DC source in series with a 10-20 Ohm 25W limiting resistor or a bulb to the field coil while observing polarity 37. working of dc motor.why used in deck machinery. 38. arc protection in ckt Arc chutes provided to cool down the arcs . They are made up of insulating and arc resisting alloy like silver-tungsten. 39. what is phase sequence indicator. how it works. Two lamps are connected. One lamp b/w R phase and B phase and another b/w Y phase and B .A capacitor is provided in the b phase will shift the phase towards R and the voltage b/w phases will change and hence one lamp will light up more than other due to voltage difference.During reverse phase, another lamp will glow more( in AK Mitra) 40. working of dc generator. 41 .definition of non essential load. Non-essential loads are the loads which are not required in safe operation of the ship and safety of the personnels like Air conditioning, provisional refer plants, refer cargo containers etc. 42. what is mutual inductance. 43. single phasing cause and effect : Single phasing cause: Terminal contact become loose Open circuit across one phase For help :
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Electrical oral questions in MMD Kolkata,MEO class IVB One of the contact in the contractor is not making proper contact One fuse blown up Contact points coated with oxidation products and not making proper contact. Single phasing effects : Large current drawn up in the remaining phases causes overheating of the windings. Motor can only be operated at less load. Winding insulation may fail due to overheating Once stopped, motor can not be start again. Probable overloading of alternator Due to unbalance currents in the windings, fluctuation of torque Fluctuation in RPM Motor will vibrate Humming noise 44. d,c motor, zener diode working, purpose, use, and its break down voltage, back emf............... Breaking voltage governs by the material and the amount of impurity added and thickness of layer. It has a specific reverse breakdown voltage. It can be varied from 1 to several hundred volts.
45. maintenance of alternators, where to take air gap(6months routine for insulation testing and air gap) ,overhaul of induction motor,how to take alternator on load in dead busbar condition,causes of no voltage in stator while primemover running at its rated speed(voltmeter is faulty, voltage regulator knob is at zero setting, residual magnetism lost, avr terminal open circuit, avr faulty) and remedial action,specification of alternator,checking of emcy generator auto onload operation 46. how to synchronize without sync and lamp test?. if sync rotating anti clckwise then what thats mean? That means the incoming generator frequency is less than the busbar frequency. If now acb is close, the incoming generator will take power from bus bar to gain its speed and hence motoring will occur and the generator will trip on reverse power trip. 48. how to synchr and put on parallel? 49. what happened when we increased the governor knob on main swtc board? 50. what governor will do to alternator? 51. how frequency will increase? 52. what is semiconductors? 53. how many types are their? Two - Intrinsic and extrinsic semiconductors. 54. tell me any 4 loads are by DC operators? Electronic circuits, battery charging and its supply( emergency lights, general alarms circuit, fire alarm circuits, ships whistle, day light signalling lamp), wardleonard control of dc motor used in deck machine and in electroplating. 55. can we start the motor in single phase? 56. what will be the effects? 57. where can we use semiconductors? Mukopadhyay 58. how alternator cooling is done, how will u come to know any abnormality in same? 59. what does the pointer in synchronoscope indicate(phase difference is zero at 12o clock, frequency is more or less can be seen by the cws or acw direction of the needle),lampdark system and its connections and disadvantages,what happens synchroscope is moving anticlockwise and u close the circuit breaker(Alternator will start taking power from the busbar and motoring of alternator will cause the reverse power trip to operate)? 60. why and how insulation testing is done how much should be its value for motors? if a motor shows a little lees value what should be done?minimum insulation value for motors is 1MOhms. (output voltage(volts) x 3 / output power(KW)+1000) 61. types of fuses ? Re-wirable, semi-enclosed, HRC fuses, 62.Humming noise of transformers(dadiyal’s question) Electric hum around transformers is caused by stray magnetic fields causing the enclosure and accessories or loose core to vibrate if the enclosure etc are not properly tightened up and loose. Hence a vibration and humming noise is heared.
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Electrical oral questions in MMD Kolkata,MEO class IVB
electrical... 63. what all indication lamps are provided on the emergency switchboard? Source lamp MSB Bustie acb – open/close Emergency generator acb – open/close e.gen – run E/G Auto St-by lamp Voltage indicator Frequency indicator Rpm indicator EG WILL BE CUTOUT AUTOMATICALLY WHEN THE MSB POWER IS RESTORED. If the MSB is restored to the normal voltage while E/G is in operation, the E/G ACB trip signal will be automatically sent to E/G ACB to trip. The bus-tie breaker on the E/G panel will be closed in instant, resulting in feeding from the MSB to ESB.
64. what is over current trip. 150%full load (20sec timer) for generator and 105%full load(not less then 2hr timer) for motors. 65. what is preferential trip To cutout the non-essential load in case of overloading to prevent full power cutout. 110% full load(instantaneous trip) rkd/mukherjee66. e/r crane safeties and maintenance E/R crane motor is a induction motor with speed control from thyristor control method. Safety features in e/r crane : - Electromagnetic fail safe brake (which do not allow the crane to fall with the load even when there is a failure) - Emergency stop - Distance limit switches in both transverse and longitudinal direction. - Mechanical stoppers are also provided in both transverse and longitudinal direction. - Automatic stopper for up-down travel of the hook to prevent the motor overloading. - Thermistor trip for motor winding. Overload and short-circuit trip . No voltage trip. - load limit switch- trip motor Maintenance: Motor must be isolated from the supply. Insulation check of the motor. Cable should be checked for any damage. If damage found then replace it. Grease up the cables and the sprockets. Check the operation of the limit switches and the functioning of other trips and safeties. Check the operation of the electro magnetic fail safe break during on load test. 68. which motor onboard does not have overload trip then what does it have... Steering gear motor does not have overload trip. It generally provided with shortcircuit trip only and overload and single phasing alarm are provided. If overcurrent trip is provided, it should not be less than 200% of the full load current. Also the under voltage trip and no voltage protection is also not provided. If the ship is having earthed neutral system then the steering gear motor is supplied through a star-delta transformer and the motor is connected in the delta side to prevent tripping of the motor supply during earth fault. 69. What are the safeties in windlass? mechanical brake, slipping clutch ,if hyd driven l.o.low pr trip.l.o.high temp trip.....if elect driven short circuit, over load protection, single phasing.
70. what is diff. between switch and starter? Starter controls the starting voltage and the current and hence reduce the voltage drop. It is provided with the safety trips. starter s only for starting purpose , after came up to rotor speed then only line voltage s applied to the stator....the only purpose is to reduce the starting voltage...... switch s continuously supply the line voltage if it s come into operation....that is make or break the circuit.. both r used fr d same purpose..but the only diff z dat a starter z equiped wid safety devices like fuses or relay or thermal switches etc.. but a switch z not...dnt confuse wid D.O.L starting system..it z also same like switch but hv safety devices..
71. how to test emergency generator operation? 72. paralleling of generator?
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Electrical oral questions in MMD Kolkata,MEO class IVB 15)exi,exd ,exn Ex nnon-incendive or non-sparking protection. Ex i – low power ( spark is not capable for igniting flammable gas or mixture) Made invulnerable to the external energies. Exi(a) safety is maintain up to two faults. Exi(b) safety is maintain up to one fault. 30V, 50mA Capacitance and inductance in the circuit are kept to minimum to prevent spark Zener diodes are provided to prevent excessive electrical energies from reaching the hazardous area. Ex d -
Used at the places where the sparks and arcing occur during normal operation. Flame proof (flame path made long enough) Explosion proof Gas tight No gap in the flanges should be greater than specified maximum (may be 0.15mm) Example: Electric motors of Ex d protection have the labyrinth design to increase the flame path. Explosion test : test by the spark-plug ignition of an explosive mixture. Further static or dynamic test at 1.5 times the explosion pressure or 3.5 bar. Flame proof test : Repeat the explosion test at least 5 times with the machine in a chamber which is filled with the same explosive mixture. If the mixture in the chamber is not ignited, then the second set of tests is considered to have been passed.
Rkd/ mukopadhyay 73. other than msb where else earth fault can be detected Local terminal, by insulation resistance meter. 74. uv trip working? Its function is to trip the breaker when a severe voltage dip occur (around 50 %). It is achieved by lifting the mechanical latch (which keeps the contacts closed) to allow the trip spring to function, this opens the breaking contacts. Its function is also to prevent closing the contacts while generator voltage is very low or absent. Also provide backup protection to short circuit protection.
75. use of all p+i+d Where the rate of change of error is slow ( like in jcw control, sensor is in middle of tank and steam coil is in the bottom, o whatever the sensor is reading(say 62 C) will increase slowly after, as the actual temp at the bottom near steam coil is 72 o C. Rate of change of error is very slow, here the derivate came in action. C(t) = Kp.E(t) + Kp/Ti . int(E(t). dt + Kp.Td dE/dt
Func 4: r k das and mukherjee 75. During paralleling two generators hw do u adjust speed from ecr to fuel injector explain wt s happening ??? mechanism?? There is synchronous motor/dc motor mounted on governor .from ecr we adjust current signal to increase its speed. Synchronous motor pull the governor shaft against spring force.
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