MEO Class 4 Exam : Function 5 Electrical Questions and Answers part 2

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Function 5 – EECEOL Most frequently asked questions in MMDs

Function 5: EECEOL(Electrical, Electronics, and Control Engineering at Operational Level)  generally called “Electrical” is one of the oral functions in class 4 exams. This is the second article of Function 5 Questions and Answer in which a few more most frequently asked questions by surveyors are being posted with answers.

Common faults in DC Generators and Motors:

  • Sparking at brushes.
  • Overheating
  • Failure to excite.

Sparking at Brushes:

  • Wrong brush position.
  • Dirty commutators.
  • Brushes not properly bedded.
  • Incorrect spring pressure on brushes.
  • Wrong grade of the brush.
  • Overloading.

Overheating:

  • Overloading
  • Blocking up of ventilation passages with dirt.


Why Megger Reading is taken?

  • To verify insulation resistance.
  • To detect insulation fault.

How Insulation Resistance of a motor is tested?

  • Switch-off at Main Switchboard.
  • Disconnect the connector from the starter.
  • Test with Megger.
  • Insulation Resistance is tested while in hot conditions because it is minimum at that time.

Test for Over Current Trip:

  • By increasing current injectors; (e.g. by using Welding Machine.)

Test for Reverse Power Trip:

  • By reducing Frequency.

Excitation Loss:

  • Energize with Battery.
  • Tap with a hammer to field coil core of Excitation Motor.

Rotary Converter:

  • Convert AC to DC.
  • Synchronous motor and generator combined unit.
  • Field and armature coils are similar to DC generators, except that slip rings are located on the end of the shaft opposite to the commutator.
  • AC turns the converter (as a synchronous motor) and DC is taken from the commutator brush.

Carbon Pile Regulator: (AVR)

  • Resistance from a carbon pile (stack), which is varied by pressure changes, controls the current flow through the exciter shunt field.
  • Pressure is applied by springs and relieved by the magnetic field strength of the electromagnetic coil.
  • Current for the electromagnetic coil is supplied from Alternator output to switchboard.
  • AVR is designed such that variations in Alternator Voltage, due to load change will affect the strength of the 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 the exciter shunt field and then increase the output voltage.
  • When Alternator voltage is high, the electromagnetic coil relieves pressure on carbon pile and resistance becomes high. Less current flows through the exciter shunt field and decreases the voltage.
  • (Strength of Electromagnetic coil relieves spring pressure on carbon pile.)

Fuse, to order:

  • Amperage of the circuit. (AC/DC)
  • Type of fuse wire (Tin or Lead wire).
  • Standard Wire Gauge (SWG)
  • 12 (0.104″dia.),14,16,18 or 20.

Battery, to order:

  • Voltage
  • Ampere/hour
  • Size
  • Type ( Lead Acid or Alkaline )

Static Electricity in an oil tank, Prevention;

  • Earthing device, earth bond across flanges on the pipeline.
  • Inert gas

Ward Leonard System:

  • Used for fine control of shunt motor speed from zero to full in either direction.
  • Able to give the motor a robust torque characteristic.
  • Can be used for motors of electric steering gears of ships with DC power.
  • Used today on ships with AC power for deck machinery such as windlass, mooring winch, etc.
  • Working motor, which powers the steering gear, windlass, or other equipment is a DC machine because speed control of these is easy.

Method:

  • A DC generator is driven by an AC squirrel-cage induction motor (AC powered ship).
  • The output voltage of the DC generator is applied as a power supply to an armature of the working motor.
  • Speed and direction of the working motor vary with the magnitude and direction of the applied voltage.
  • Output voltage of the DC generator is increased or decreased by Potentiometer, as the magnetic field strength is altered by changing the field current to field windings of the generator.
  • As the output voltage of the generator varies, the speed of the working motor also varies.
  • Change of current flow direction, also by Potentiometer, through the field poles of the generator, will cause the change in direction of the generated current, supplied to the working motor and thereby also the running direction of the motor.

How to change the speed?

Through Potentiometer: 

By changing the strength of field current to field winding of DC generator

  • Output voltage of DC generator changed 
  • Working motor speed is also changed.

How to change direction?

Through control handle movement of Potentiometer: 

by changing the direction of field current through field poles of DC generator

  • Direction of generated current is changed
  • Running direction of the working motor is changed.

Surveyor approved IR (mΩ)

  •  1 mΩ 10⁶ Ω

Switchboard Survey, what documents to give to Surveyor?

  • Voltmeter, Wattmeter, Ammeter calibration test result.
  • ACB test result (Survey result).
  • Control Circuit Safeguard test results (Safety trips and Alarm test results).
  • Maintenance reports.
  • IR test results of each terminal: all 440V and 220V main circuits.
  • Data to be given to Surveyor for Electrical Survey:
  • Voltmeter, Wattmeter, Ammeter calibration test result.
  • ACB test result (Survey result).
  • Control Circuit Safeguard test results (Safety trips and Alarm test results).
  • Maintenance reports.
  • IR test results of each terminal: all 440V and 220V main circuits.
  • IR test results of all motors & their safety factors:
    • Test date, time, place
    • Voyage No.
    • Ambient temperature
    • Weather condition, hot or humid
    • Machine is hot or cold.

Navigation lights:

  • Fore Mast (No. 1&2 or Up, Down).
  • Main or Aft Mast (No. 1&2 or Up, Down).
  • Stern light (No. 1&2 or Up, Down).
  • Port light (No. 1&2 or Up, Down).
  • Starboard light (No. 1&2 or Up, Down).

Emergency Lighting:

  • Engine room lighting
  • Bridge lighting
  • Passageway lighting
  • Embarkation light

How will you check the frequency of shore power supply?

  • Shore Power Supply Connection Box shows Phase Sequence of shore power generator with bright and dark lights.
  • Frequency can be checked at Main Switchboard after shore supply is ‘on’ through link switch or circuit breaker.

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