18 December 2016

ORAL QUESTIONS AND ANSWERS FOR ETO - PART - 2

Oral on Marine Electro Technology  Part - 2


1. What is induction motor ?
It is a motor which run at a speed below that of the rotating magnetic field.
Types are squirrel cage induction motor and slip ring induction motor.

2. Squirrel cage induction motor
  • Most widely used of all types of AC motors, due to simplicity, strength of construction and ease of maintenance
  • Made up of two main parts, rotor and stator and no direct electrical connection between them.
  • No wire winding or slip rings.
  • Rotor has a series of plain bars (copper or Al) running in slots the length of the iron core
  • Each end of the bar is brazed into 2 heavy copper rings, one at each end. Those bars form a cage, that looks like squirrel cage
  • Stator has 3 separate windings supplied from a 3 phase AC supply
  • Phase difference 120 and number of poles 2, 4, 6 or more, depending on speed required


3. Difference between Synchronous Motor and Induction Motor ?
  • Synchronous motor is almost exactly the same as an alternator
  • Induction motor cannot run normally at synchronous speed. It has slip.


4. What are AC motor overloads protections ?
  • Magnetic overload relay
  • Thermal overload relay
  • Built-in protective device


5. Causes of motor overheat ?
  • Motor running at overload
  • One phase failure (single phasing)
  • Defective cooling fan blades
  • Bearing failure
  • Poor ventilation
  • Misalignment


6. When phase failure occur, A.C motor run or not ?
  • When motor is running, the motor keep on running but over load and over heat will occur.
  • When motor is starting, the motor cannot run.


7. What is moving iron instruments ?
It is an instrument suitable for measuring A.C voltage or current as well as D.C voltage or current.

8. What is potentiometer ?
It is an instrument which is used to measure or adjust the potential difference in the circuit.

9.  What is moving coil instruments ?
It is an instrument used for D.C only and as voltmeter or ammeter.

10.  What is Static Electricity ?
  • Electricity at rest instead of in motion
  • Electric charges may be induced by friction or atmosphere effect


11. How to prevent Static Electricity hazards in oil tanker ?
Earthing device, earth bond across flanges on pipeline
Inert gas

12. What are the causes and effects of Single phasing in motors ?
  • Single phasing occurs when one of the 3 phase circuits is opened, hence the remaining circuits carry excess current.
  • One phase of the circuits becomes open, due to blown fuse, faulty contactor, or broken wire.
  • It prevents a motor from starting, but a running motor may continue to run with this fault.
  • For a running motor, it can be detected by overloaded device in supply line, or overheating.
  •  For an idle motor, it cannot be started.
  • Due to single phasing, overheating in a stalled or running motor will cause, burnt out overloaded coil.


13. What is residual magnetism ?
  • Magnetism remaining in the fields of a generator, after exciting current is cut off.
  • Residual magnetism is essential for initial generation of current, necessary for further build up of shunt field strength.
  • Generator may fail to excite, if there is loss or reversal of residual magnetism of the pole.


14. What action, when generator fails to excite ?
  • Pass a current through shunt field coil in correct direction
  • Correct direction means the current will re-magnetise the iron core in the right way
  • Current for restoration can be obtained from another DC generator or from a battery.
  • If battery is used fro re-magnetising: A 12 V battery is connected [exclusively] across shunt field coil with the machine stopped
  • Current flow in right direction, for a few seconds, only will establish the field.
  • During this time faulty generator must be in stopped position.


15. What are windlass safety devices ?
  • Emergency stop
  • Overload trip
  • Over speed trip
  • Mechanical break
  • Slipping clutch for over load, to prevent any undesirable damage such as hull damage due to anchor and rope brake out.


16. What is Slipping Clutch in windlass ?
  • Slipping clutch is commonly fitted between prime mover and gearing
  • It is incorporated with motor, magnetic brake and drive shaft
  • Set to slip at approximately 133% of full load torque
  • Letting go or dropping speed is controlled by friction brake
  • Hauling speed is 0.15 m/sec


17. Why Slipping Clutch is fitted in windlass ?
  • In windlass, undue stresses must not be applied to chain cable and machinery
  • Without slipping cultch, excessive stresses could be applied to cable, by armature momentum, by sudden obstruction when heaving, or when bringing the anchor into hawsepipe.
  • Fitted also to avoid inertia of prime mover being transmitted to windlass machinery in the event of shock loading on cable, when anchor is being housed
  • When ship is riding at anchor, bow stopper prevents the strain for windlass


18. Explain electric breaking system in marine machinery ?
  • When electric deck motor is used for lowering or lifting load, electric brake system is used. All brakes are failsafe types.
  • In the event of power failure, brake automatically applied, thus preventing the load running back. A number of brake pads [free to move] are located in carrier, which is keyed to motor shaft.
  • Armature plate applies pressure to brake pads, by means of a number of springs, to force against friction surface of back plate, so that it prevents the motor shaft from turning.
  • When energized, armature plate is attached to magnet, thus releasing the thrust pressure and allowing the shaft to rotate


19. What is synchroscope ?
  • An instrument fitted at the main switchboard which indicates when two electrical supplies are in synchronism and can be paralleled.
  • An instrument, which indicates that, voltages, frequencies and phase angle of two electrical supplies of running machine and incoming machine, are in synchronism and can be paralleled.
  • Synchroscope should not be left in circuit for more than 20 minutes as it cannot continuously rotate.
  • Synchroscope is a small motor with coils on the two poles connected across red and yellow phase of the incoming machine and the armature winding supplied from the red and yellow switchboard bus bars.


20. What is the synchronizing ?
The process of bring the voltage, frequency and phase angle of electrical supplies into line in order to be paralleled and share the loads.

21. Explain Synchronising methods ?
  • Synchroscope is the main method
  • Back-up methods are: Lamp dark method
  • Lamp bright method
  • Rotating lamp method or Sequence methods (preferable)

                                           
22. Explain Rotating lamp method or Sequence method ?
  • One of the lamps known as key lamp is connected in one phase
  • Other two lamps are cross-connected
  • If two frequencies differ, lamps will bright up in rotation. Clockwise indicates incoming machine is fast, and counter-clockwise indicates it is slow
  • Synchronising moment is when key lamp is dark and other two lamps equally bright
  • If phase rotation is wrong, all lamps will become bright and dark together.
  • Remedy is to interchange any two phase connections


23. How to parallel the two generator with the aid of synchroscope  on ships ?
  • All meters and indicators must have in good working order.
  • Start the incoming generator with the correct staring procedure.
  • Check the working condition by readings pressure gauges, thermometer and audible and visual.
  • Watch for a minutes until prime mover come to stable.
  • Check the voltage of existing and incoming generator on the switchboard, check trip if have reset.
  • Move the selector switch to incoming generator. At that time, synchroscope pointer will rotate clockwise or counter clockwise direction.
  • Check the frequency and voltage of existing and incoming.
  • Carry out the adjustment by means of speed adjuster of prime mover to obtain the condition such that synchroscope pointer rotate in the clockwise direction at a speed of about 4sec/rev.
  • Close the circuit breaker of incoming generator when the pointer reaches just before 12’ O clock position.
  • Made off the selector switch.
  • Made load shearing the two generators by the speed adjuster of generator.

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24. How do you parallel the two generators if synchroscope is out of order ?
  • If synchroscope is out of order, the two generators can be paralleled by emergency synchronizing lamps with sequence method.
  • The lamps are fitted as triangle forms, one on top and two on bottom.
  • To make parallel, reset if any trip
  • Move the selector switch to incoming generator. At that time, synchronizing lamps will give bright and dark sequence rotate clockwise or counter clockwise direction.
  • Check the frequency and voltage of existing and incoming.
  • Carry out the adjustment by means of speed adjuster of prime mover to obtain the required frequency.
  • Adjust the speed to meet the slow clockwise direction.
  • Incoming machine breaker is closed by hand when the top lamp (key lamp) dark and the both bottom lamps are equal brightness.
  • Off the selector switch and make load sharing the two generators by speed adjusting.


25. How do you monitor the correct instant for synchronizing without the aid of synchroscope or synchronizing lamps ?
  • It can be synchronized with 500V voltmeter as follow.
  • Connect a pair of 500V voltmeter probes across the one phase of the incoming machine circuit breaker and bus bar.
  • Adjust the generator speed until the voltmeter slowly fluctuates from zero to maximum.
  • Close the breaker when the voltmeter passes through zero.


26. Can you parallel the load of main G/E with emergency G/E and shore supply ?
It cannot paralleled, circuit breaker interlocks are arranged in this system.

27. What are the generator safety devices ?
  • Over speed trip
  • L.O low pressure trip and alarm
  • F.O low pressure alarm
  • Jacket water high temperature alarm
  • Jacket water high temperature trip
  • Thermometer
  • Pressure gauge
  • L.O high temperature alarm
  • Dip stick
  • Crankcase relief valve


28.  Methods of armature reaction compensation ?
  • By fitting inter poles between the main poles
  • By fitting carbon brush rocker
  • By fitting neutralizing winding


Reference:


Oral Guide by – MIN ZAR TAR

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ORAL QUESTIONS AND ANSWERS FOR ETO - PART - 2
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