7 January 2017

ORAL QUESTIONS AND ANSWERS FOR ETO - PART - 3

Oral on Marine Electro Technology  Part - 3


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1. Why AC is popular on board ship ?
  • Smaller, lighter and compact machine size, for a given kW
  • High power and high voltage AC generator can be easily manufactured
  • Voltage can be raised or lowered by transformer
  • AC can be easily converted to DC


2. Precautions while working on electrical machinery ?
  • Switch off the main switch
  • Out mechanical lock on
  • Take out fuse
  • Put a signboard “Man Working on Line”
  • Use LOTO (Lock out Take out)


3. What is AVR ?
  • A.V.R or Automatic Voltage Regulators are used in conjuction with generator for controlling the terminal voltage to give a steady voltage under varying load.
  • It senses and controls an A.C generator’s output voltage within (+ or -) 1 to 2%.


4. What are the types of A.V. R ?
  • Carbon pile regulator
  • Vibrating contact regulator
  • Static A.V. R.
  • Rotating sector
  • Multi contact
  • Magnetic amplifier
  • Electronic amplifier


5. Carbon Pile Regulator (AVR)
  • A resistance from a carbon pile (stack), which is varied by pressure changes, controls the current flow through exciter shunt field
  • Pressure is applied by springs and relieved by magnetic field strength of electromagnetic coil
  • Current for electromagnetic coil is supplied from alternator output to switchboard
  • AVR is designed such that variations in alternator voltage, due to load change will effect strength of electromagnetic coil and hence alters carbon pile resistance
  • When alternator voltage is low, spring exerts greater pressure and carbon pile resistance becomes low, so more currents flow through exciter shunt field and then increase the output voltage
  • When Alternator voltage is high electromagnetic coil relieves pressure on carbon pile and resistance becomes high. Less current flows through exciter shunt field and decreases the voltage (Strength of Electromagnetic coil relieves spring pressure on carbon pile)


6. What is rectifier ?
  • It is an electric device or circuit capable to allow an A.C current to be converted to D.C current.
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7.What is excitation ?
  • To supply and control the correct D.C current for the rotor pole winding to produce the required generator A.C output voltages.
  • Production of an electromagnetic field of a generator by supplying exciting current for magnetizing the field magnet
  • For excitation, DC is used because DC produces constant rate of magnetic flux
  • Continually regulated to maintain the generator output voltages as the load demand fluctuates.


8. Exciters 
  • The source, which generates the field current for excitation of field magnets.


9. Equalizing bar
  • Equaliser is a low resistance circuit, connected across armature ends of series coils of parallel compound generators, via a special bar in switchboard
  • Equaliser is fitted to stabilize parallel operation of compound Dc generators.


10. Rotary converter
  • A rotating diode to convert AC to DC current for alternator excitation
  • Synchronous motor and generator combined unit
  • Filed and armature coils are similar to DC generator, except that slip rings are located on the end of the shaft opposite to commutator
  • AC turns the converter (as synchronous motor) and DC is taken from commutator brush


11. Procedure for alternator overhaul ?
  • Shut down the generator prime mover and lock off the starting system.
  • Lock off the circuit breaker and isolated the alternator electric heater. Tag the label on the switchboard.
  • Inspect the tightness of terminal connection and insulation.
  • Check the bearing oil level and condition.
  • Clean the cooling air intake filter and exhaust opening.
  • Clean the rotor and stator windings by means of vacuum cleaner with rubber hose and nozzle.
  • If the oil is deposited on the surface of winding, open the drain plug and then removed by special degreasing liquids. (Electro cleaner)
  • Measure the air gap clearance between rotor and stator. (Carefully at lower part)
  • Baking the alternator with lamp. Keep a temp not more than 43°C
  • Disconnect the neutral point from the terminal box.
  • Measure insulation resistance between – rotor to earth, phase to earth, stator to earth, phase to phase
  • It is must at least one mega-ohm, if mega reading has reasonable value, the windings are to be covered with high quality air drying insulation varnish.

 After maintenance work
  • Reassemble all necessary parts.
  • Check no load running, the synchronizing and loading.
  • On load, practically check for excessive temperature rises and load sharing stability when running in parallel.


12. Inspection and Maintenance of Battery Room
  • Battery installation and its charging rectifier checked.
  • Battery room environment must be dry and well ventilated.
  • Battery tops shall be clean and dry, and terminal nuts must be tight and a smear of petroleum jelly applied to prevent corrosion.
  • Electrolyte at proper level, and shall have correct value of specific gravity by checking with a hydrometer.
  • Rubber gloves and goggles used when handling electrolyte.
  • Charging equipment checked for dirt, overheating, loose connection and correct functioning of indicators.
  • Ventilation arrangement for battery locker checked. Battery installation of both lead acid and alkaline needs good ventilation.
  • Since both type generates hydrogen gas during charging, no smoking and naked light allowed.
  • Steel works and decks adjacent to lead acid battery, should be painted with acid proof paint. [For Cad-Ni cell, alkaline resistance paints].
                                                            

13. Battery Room Safety Arrangement
Safety is provided by
  • Proper ventilation
  • Prevention of heat source for ignition

Ventilation
  • Independent exhaust fan provided
  • Inlet duct should be below battery level, and outlet at top of the compartment


Prevention of heat source for ignition
  • No naked light and no smoking
  • Uses of externally fitted light or flameproof light
  • Cables of adequate size and they are well connected
  • Never place Emergency Switchboard in this room
  • Use insulated spanner and plastic jug for distilled water, to prevent short circuit
  • Room temperature, maintained at 15 ~ 25 °C


14. Nickel Cadmium battery [alkaline battery]
  • + ve plate is Nickel hydroxide + graphite
  • –ve plate is Cadmium + Iron
  • Electrolyte is Potasssium hydroxide solution (strong alkaline)
  • Normal specific gravity [1.21] does not change with charging or recharging
  • But specific gravity of electrolyte gradually decrease, and electrolyte should be renewed when specific gravity becomes [1.160]
  • It is a sealed battery, thus no gassing during charging
  • Very low open circuit losses, but requires 67% more cells than Lead Acid battery [1.2 V per cell and when fully charged 1.7 V per cell]
  • Not harmful when overcharged
  • Left for long period, either fully charged or fully discharged, without deterioration
  • Better mechanical strength and durability than lead acid battery
  • High initial cost but longer life


15. Lead Acid Battery
  • + ve plate is Lead Peroxide [chocolate brown]
  • – ve plate is Spongy Lead [slate gray colour]
  • Electrolyte is H2SO4 , specific gravity 1.8 when fully charged, renew when specific gravity is 1.110.
  • 1.8 V per cell when fully charged 2 V per cell
  • When undercharge, + ve plates are pale brown or yellowish, instead of deep chocolate and very difficult to convert back to normal form
  • Efficiency (watt hour efficiency) is higher than Alkaline Cells


16. How do you know when the battery is full charge ?
  • It can be known by measuring the electrolyte specific gravity by using hydrometer.
  • If fully charge it has a specific gravity of about (1.27 – 1.285), hydrometer scale 1270 to 1285
  • If fully discharge which fall to a specific gravity of 1.1 (1100)


17. How can you know battery is over charge ?  Effect of overcharge ?
  • When battery is overcharged bubbles will form at the surface, the current flowing into the cell causes breakdown or electrolysis of water in the electrolyte.
  • Both hydrogen and oxygen are evolved and released through cell vent caps into the battery compartment.
  • There is an explosion risk if hydrogen is allowed to accumulate. (4% of H2 in air).


18. What happens when lead acid battery is overcharged ?
  • Overheating cause buckling of plates
  • Internal short circuit
  • Sludge formed at the bottom of cells (lead peroxide)
  • Battery may be ruined
  • Lower the capacity


19. What happens when lead acid battery is undercharged ?
  • Over discharging
  • + ve plates are pale brown or yellowish, instead of deep chocolate
  • - ve plates, almost white colour
  • Falling of voltage 1.8 V / cell, and specific gravity of H2SO4 1.15

Depolarisation
  • Liberation of hydrogen ions at – ve electrode (cathode) and that will decrease the current flow.
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Reference:

Oral Guide by – MIN ZAR TAR

                                                        

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ORAL QUESTIONS AND ANSWERS FOR ETO - PART - 3
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