MEO Orals on Marine Electro Technology Function 5- Part 7
Action when an electrical machine is flooded with seawater ?
- Machine is disconnected from power sources and dismantled
- All salt deposits washed out with freshwater
- If deposited with oily bilge water, wash out with Electro Cleaner
- Should be heated with lamp, and enclosed or covered up to retain the heat
- Moisture should be escaped, by lifting the cover continuously or periodically
- Hottest part of the machine shall not exceed 90°C, while heating
- IR readings and temperatures taken regularly, until constant value reach about 1 mega ohms
- Then spray the machine with insulation vanish
- Assemble and out in service with low load, if possible
Why drying out of electrical machine ?
- It is essential, when machine has been exposed to weather, or when accidentally immersed.
Short, open and grounded circuits ?
A low resistance path that actually shorten the intended path for the flow of current
The path for the flow of current is broken. A switch s one method of creating an “open” intentionally
A circuit that has come in contact with the earth, by coming in contact somewhere in itself, or by a conductor which is connected with the earth
What is shore connection on ships ?
- Shore connection box is provided at convenient position, to receive shore power supply, so that ship’s generators can be shut down, in port or during Dry Docking.
What are Lloyd’s requirement on shore connection ?
- Connection box contains a circuit breaker or isolating switch with fuses
- Provided full information of supply system, normal ship voltage and frequency for AC current
- Main Switch board must be provided with a link switch or a circuit breaker, voltmeter, ammeter, and an indicator to show that the cable is energized
- For 3-phase supply with earth neutral and earthing terminal must be provided, for connecting the hull to shore side.
- A phase sequence indicator is necessary to ensure correct connections
- Means shall be provided for checking polarity, and terminals should be labeled
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 check at Main Switchboard, after shore supply is ‘on’ through link switch or circuit breaker.
What are Common faults in DC Generator and Motors ?
- Sparking at brushes
- Failure to excite
Causes of Sparking at Brushes ?
- Wrong brush position
- Dirty commutators
- Brushes not properly bedded
- Incorrect spring pressure on brushes
- Wrong grade of brush
Causes of Overheating ?
- Blocking up of ventilation passages with dirt
Test for over current trip ?
By injecting current with injectors
Test for Reverse Power Trip ?
By reducing frequency
- 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 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 made easy
- A DC generator is driven by AC squirrel cage induction motor.
- Output voltage of DC generator is applied as power supply to armature of working motor.
- Speed and direction of working motor varies with magnitude and direction of applied voltage
- Output voltage of DC generator is increased or decreased by Potentiometer, as magnetic field strength is altered by changing the field current to field windings of the generator
- As output voltage of the generator varies, 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 generated current, supplied to the working motor and thereby also the running direction of the motor
How to control speed of AC motor ?
Adjusting number of poles by means of selector switch.
What are motor earthing symptoms ?
- 440V, low insulation alarm
- Earth lamp signal
What are the effects of motor earthing ?
- Over heat
- Burn out
- Get the smell
Specifications of fuses on ships ?
- Amperage of the circuit (AC/DC)
- Type of fuse wire (Tin or lead wire)
- Standard Wire Gauge (SWG)
Specifications of battery on ships ?
- Type (Lead Acid or Alkaline)
Oral Guide by – MIN ZAR TAR