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EOOW Oral Prep > Electrics > Flashcards

Electrics Flashcards

1
Q

What is the purpose of an earthing device on a propeller shaft

A

To ground the static electricity to the hull caused by propeller rotation

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2
Q

What is megger testing? What range does it work in (Ohms)

A

Used to measure insulation resistance of cables or electrical equipment. Works in Millions of Ohms

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3
Q

What is the difference between earthed and short circuit?

A

Earthed is a fault in the circuit where a conductor has came into contact with the ships hull or a metal enclosure.

Short circuit is where a low resistance connection occurs between two points in a circuit, causing large current flow

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4
Q

What are switchboard safety trips/devices

A

Preferential Trips
Overload Trip
Under voltage trip
Over voltage trip
Reverse power relay
Reverse current trip
Low frequency trip
Earth lamps
Synchroscope

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5
Q

What meters are fitted on a main switchboard

A

Ammeter
Voltmeter
Wattmeter
Frequency meter
Ohm meter
Synchroscope

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6
Q

What could affect the reactive power?

A

Starting large equipment such as bow thrusters

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7
Q

How would you manually parallel generators?

A

1) Start the new generator and observe its condition for normal running.
2) Switch on the synchronising switch of the generator on the synchro panel, where the bus frequency and upcoming frequency will be shown.
3) Make frequencies the same by operating the governor switch for the incoming generator. (Voltage will be taken care of by AVR.)
4) Look at the dial indicator on the synchro panel, which should be rotating slowly clockwise.
5) When the pointer is at 11’oclock position press the circuit breaker operating switch, connecting the new generator to the switchboard.
6) Operate the governor switch to increase load slowly until the load has equalised between the two generators. If changing generators over reduce fully the load of one generator and disconnect.
7) Switch off the synchroscope.

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8
Q

Why is it important to turn off the synchroscope after paralleling a generator?

A

It is rated equipment and will be damaged otherwise.

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9
Q

What would you check before going onto shore power?

A

1) Connection hatch is free and unobstructed.
2) Receiving terminals are in good order.
3) Indication lamps.
4) Ships batteries are fully charged.
5) Shore power cable connection.
6) Carry out a megger test on shore power cable.

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10
Q

How would you go onto shore power?

A

1) Connect shore power cables.
2) Ensure the earthing cable is arranged to shore earthing to the ship’s hull.
3) Note down and take pictures of watt readings from ship and shore.
4) Switch on shoreside power. Confirm the SHORE ALIVE indicator lamp is illuminated.
5) Ensure voltage and frequency are rated and phase sequence is correct.
6) Isolate the emergency generator to ensure that it does not start.
7) Isolate all non-essential services and reduce load to a minimum.
8) Set generators to manual so they do not connect to the board.
9) Open the generator ACB’s. Vessel blacks out.
10) Close the breaker for shore power.

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11
Q

How would you go from shore power to generator?

A

1) Reduce load to a minimum.
2) Set all generators to manual, including the emergency generator.
3) Run up a generator on local control.
4) Open the shore power breaker. Blackout.
5) Close the ACB for the generator which connects to the board.
6) Check voltage and adjust to 60Hz.
7) Set emergency generator to normal auto start and the same with the generators.
8) Isolate shore power supply and remove cables.
9) Note down and take pictures of the shore and ship watt meter readings.

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12
Q

What safety devices are fitted to a typical switchboard?

A

1) Overload trip
2) Preferential trip
3) Under voltage trip
4) Over voltage trip
5) Reverse power relay
6) Reverse current trip
7) Air circuit breaker
8) Earth lamps
9) Low frequency trip
10) Meters
11) Synchroscope
12) Emergency synchronising lamp
12) Ebonite handrail and rubber footstep

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13
Q

How does an overload relay work?

A

If the current rises above a certain limit over a period of time, the overload relay will trip which operates an auxiliary contact interrupting the control circuit and de-energising the contactor.

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14
Q

How does an under-voltage relay work and what are the negatives of under-voltage?

A

Operates when voltage falls below a set-point.

Lower voltage forces equipment to draw more current leading to over heating and damage.

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15
Q

What are the issued with over-voltage?

A

Premature failure of electronic components due to overheating.

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16
Q

What is reverse power and what are reverse power relays used for?

A

When the prime mover cannot supply sufficient torque to keep the alternator rotating at the same frequency of the bus and therefore starts motoring.

Detective a negative power flow, usually -10% of maximum load.

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17
Q

What is an air circuit breaker?

A

Used for protecting low voltage circuits, mainly for energising and cutting off high currents.

An Electronic Trip Relay (ETR) trigger outputs a signal to energise the trip coil when the value exceeds a set-point.

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18
Q

What is the earth lamp?

A

Set of lamps that show a presence of an earth fault in the system.

Will show dim light due to low potential, and the location of the fault can be traced by switching off individual branches and checking the condition of the earth lamp.

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19
Q

What is the effect of under frequency?

A

Generally caused by a system event and will try to extract excessive current from the stator and drag the rotor speed down.

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20
Q

Emergency generator safeties?

A

All of that on a normal switchboard but only alarms, will only trip on overspeed.

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21
Q

What would be examples of a preferential trip sequence?

A

PT1 - AC, refrigeration, galley, accommodation
PT2 - Cargo pumps, ballast pumps, air compressors
PT3 - Navigation, machinery, control stations, steering gear

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22
Q

Why would a bow thruster be high voltage?

A

For the same power the motor could be made to be smaller. Also power loss can re reduced due to low currents.

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23
Q

How would you parallel a generator to the bus bar?

A

1) Start up the new generator and allow it to come to temperature and proper running parameters.
2) On the MSB select the new generator for paralleling which should display its frequency and voltage.
3) Ensure the voltage on the incoming matches that of the bus-bar.
4) Using the governor control switch increase or decrease the frequency of the incoming generator until it matches that of the bus, or is just slightly faster.
5) Start the synchroscope and observe for a clockwise lamp direction, proving the incoming is slightly faster.
6) When the lamp comes to just before the top position close the breaker.
7) The generator should now be connected to the board with no load.
8) Balance the load sharing and then stop the synchroscope.

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24
Q

What checks must be made prior to closing the breaker on a generator?

A

1) Voltage is the same.
2) Frequency of the incoming is slightly faster.
3) Phase angle is the same.

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25
Q

What should happen if you parallel a generator at different frequency to supply?

A

Circuit protection should trip and separate the connection instantly.

Voltages will not match generating a large current between the 2 generators and causing one to surge to catch up, massively damaging generator windings and rotor.

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26
Q

What can cause the frequency to drop?

A

1) Generator tripping.
2) Heavy machinery starting.
3) Fuel blockage or restriction.
4) No functioning AVR.

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27
Q

What methods of starting the emergency generator are there?

A

1) Batteries
2) Hydraulic
3) Compressed air
4) Manual cranking
5) Inertia starters

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28
Q

How would you battery test the emergency generator?

A

1) Switch to test mode. Generator will start but not come onto load.
2) Check voltage and frequency.
3) Keep running for 10-15 minutes checking the exhaust temperature and other parameters.
4) Check sump level.
5) Put switch in manual, stop generator and put into emergency mode.

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29
Q

How would you hydraulic test the emergency generator?

A

1) Change to manual and check oil pressure.
2) Open valve from accumulator to generator.
3) Push spring loaded valve and generator should start.
4) Check voltage and frequency.
5) Keep running for 10-15 mins and check exhaust gas temperature and other parameters.
6) Check sump level.
7) Stop generator.
8) Pressurise hydraulic accumulator.
9) Close accumulator valve.

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30
Q

How would you test the automatic start of the emergency generator?

A

1) Open the breaker from the tie line which should start the emergency generator with batteries.
2) Close breaker and due to interlock should come of load.
3) Stop generator and put to emergency status.

31
Q

What regulation dictates the start up methods for the emergency generator?

A

SOLAS Chapter II-1 Regulation 44:

Starting Arrangements for Emergency Generating Sets

32
Q

What readings would you expect to find on the switchboard?

A

1) Ammeter (current)
2) Voltmeter
3) Wattmeter (power)
4) Frequency meter
5) Ohm meter (resistance)
6) Synchroscope

33
Q

What is an earth fault?

A

An Open Circuit fault where the live wire breaks and gets into contact with earth directly or through a conductor.

34
Q

How can an earth fault be caused?

A

Insulation damage during installation, impact damage to the cable sheath or improper installation.

35
Q

How would you determine the location of the earth fault?

A

1) Isolate distribution panels one by one until the earth indicator goes back to normal.
2) Once the distribution panel has been identified, use the same process for their individual consumers.
3) Once identified make repair.

36
Q

Describe an earth fault indication system.

A

1) 3 lamps are connected to 3 phases.
2) If there is no earth fault all lamps will shine at equal brightness.
3) If there is an earth fault in one of the lines, one of the lamps will be dimmer or be off as the potential difference between the line and the earth would be zero.

37
Q

What are the effects of an earth fault?

A

1) Creates a fault current that can harm equipment.
2) Interrupts continuity of supply which may shock the user.

38
Q

How would you check the insulation resistance of a motor?

A

1) Remove power cables and start/delta sequence as the high voltage can damage electronics.
2) Connect the black lead into the tester and the other to the body of the motor, usually the mounting bolts.
3) Remove paint on connection as will insulate.
4) Plug the red lead into the tester and then onto U1.
5) Select suitable DC voltage (typically 1/2 the norm).
6) Press the test button and apply for a minimum of 1 minute.
7) Test between the 3 other windings, U1V1, U1W1, V1W1

39
Q

What is the difference between insulated and earthed distribution systems?

A

In an insulated system the star point is insulated from earth whereas an earthed distribution system sees it directly connected to the ships hull.

40
Q

Why is an insulated system usually used?

A

Requires 2 earth faults on 2 different lines for a fault current to flow, ensuring continuity of supply to equipment.

41
Q

Why is earthed distribution used for high voltage?

A

It trips after one short circuit which is essential as there are many hazards associated with high voltage such as arcing, arc blast and arc flash.

42
Q

What is a multi-meter?

A

Measuring instrument that can measure voltage, resistance and current.

43
Q

What is a proving unit?

A

An electronic voltage source that can verify the operation of electrical testing tools.

44
Q

What is an insulation resistance tester?

A

Measures the total resistance between two points separated by electrical insulation.

45
Q

How would you isolate a motor to ensure it was safe to work on?

A

1) Conduct a risk assessment.
2) Create a PTW.
3) Isolate power to the motor locally and lock-out tag out.
4) If possible isolate on the switchboard also and lock-out tag-out.
5) Place keys in separate lock box which only you have access to. Keep key on person at all times.
6) Open up junction box using electricians tools and gloves.
7) Using a proving unit check the multimeter is working.
8) Put clips on each phase together and then the body.
9) Once satisfied dead test the multimeter in proving unit again.
10) Tape electrical connections so if power does come through it will short circuit.

46
Q

How does an induction motor work?

A

1) Stator has 3 sets of windings spaced 120 degrees apart to create 3-phase.
2) Uses a squirrel cage design (conductive copper bars that are shorted at each ring inside laminated sheets).
3) Stator produces a rotating magnetic field.
4) This field cuts the bars of the rotor cage, inducing and EMF.
5) Current flows in the rotor cage which generates a magnetic field.
6) Rotor field is dragged behind the stator field which generates torque.

47
Q

How does a synchronous motor work?

A

1) Uses a wire wound rotor design.
2) DC excitation current fed via slip rings into windings on the rotor.
3) Current flows.
4) Magnetic field generated.
5) Locks together with the rotating stator field.
6) Rotor field is pulled by stator field at synchronous speed.
7) Torque is produced by rotor.

48
Q

Why is the induction motor more common?

A

Less moving parts means its simpler, cheaper and has greater longevity. Highly efficient.

49
Q

Where would synchronous motors be used more often?

A

Large industrial appliances where exact speed is required.

50
Q

What is a DOL?

A

The start button energises the coil that pulls the contactors to close the circuit.

51
Q

Advantages and disadvantages of DOL?

A

Advantages:
1) Simple & cost-effective design.
2) Easy to understand and operate.
3) Provides high starting torque due to high starting current.

Disadvantages:
1) High inrush current can damage the windings.
2) High inrush current causes voltage to dip in the power supply.
3) Not suitable for heavy motors.
4) Can decrease the lifespan of a motor.

52
Q

What is an autotransformer starter?

A

A step-down transformer to reduce the voltage applied to the stator during the starting phase.

53
Q

Advantages and disadvantages of an autotransformer?

A

Advantages:
1) Provides a better starting torque.
2) Used for starting large motors with a significant load.
3) Offers manual speed control.
4) Offers flexibility in starting characteristics.

Disadvantages:
1) Takes up a lot of space.
2) Complex and expensive starter.

54
Q

What is a star-delta starter?

A

1) Connected in star to start to reduce the phase voltage by 1/root3 and reduces the starting current by 1/3.
2) As the motor accelerates a timer relay switches to star connection.
3) This allows full voltage across the windings.

55
Q

Advantages and disadvantages of a star-delta starter?

A

Advantages:
1) Simple and cheap design.
2) Does not require maintenance.
3) Low surge current.
4) Used for large induction motors.
5) Best for long acceleration time.

Disadvantages:
1) Works on delta connected motor.
2) More wire connections.
3) Low starting torque which cannot be maintained.
4) Limited flexibility in starting characteristics.
5) Mechanical jerk switching from star to delta.

56
Q

What is a variable frequency drive?

A

Ac from supply is converted into DC using rectifiers. This has adjustable frequency and voltage using pulse width modulation technique through a transistor.

Provides full control of the motor speed from 0 to rated.

57
Q

Advantages and disadvantages of VFD?

A

Advantages:
1) Provides better and smooth acceleration.
2) Offers full speed control.
3) Increases lifespan due to absence of stress.
4) Offers forwards and reverse operation.

Disadvantages:
1) Relatively expensive unless speed control is necessary.
2) Heat dissipation.
3) Creates harmonics in lines which can affect equipment and power factor.

58
Q

What is an earth fault?

A

The unintentional flow of current to the earths hull.

59
Q

What can cause earth faults?

A

1) Loose connections
2) Water ingress
3) Insulation breaking down
4) Overloaded motor melting insulation

60
Q

What are the 2 types of IR test?

A

1) Phase to phase
2) Phase to earth

61
Q

How would you test the emergency generator on load?

A

Open the interconnector feeder breaker.

Under voltage relay detects a dead bus and pulls contact in, connecting the ESB.

62
Q

How are batteries charged?

A

Trickle charging:

Low amperage, charging a fully-charged battery at a rate equal to its self-discharge rate.

63
Q

How does a bridge rectifier work?

A

Converts AC to DC using diodes made of a semiconductor.

64
Q

How does an alternator work?

A

1) Prime mover rotates rotor which is made of wound copper windings.
2) Passing DC through rotor windings generates magnetic field.
3) As rotor cuts stator windings EMF is induced.
4) 3 windings spaced 120degrees apart.
5) AVR monitors voltage output.
6) AVR controls level of excitation to rotor which alters the rotor EMF strength.
7) This controls the voltage produced.
8) Governor controls power production.

65
Q

What is excitation?

A

A means of sending DC field current to the rotor to create a magnetic field.

66
Q

What are the 2 methods of excitation?

A

1) Rotary - Brushless AC generator
2) Static - Slip rings

67
Q

What is the equation for the different voltage in a star system?

A

Vp = Vl / root(3)

Voltage reduced by 1/root(3) which reduces the starting current.

68
Q

How does the AVR work?

A

1) Generator sends current to voltage sensing unit via a voltage transformer to take it down to a proportional value to protect AVR.
2) Voltage sensing unit feeds it to a comparator which compares between the set point.
3) Comparator sends this error voltage to the amplifier.
4) Amplifier sends a voltage signal to the thyristor to increase or decrease the excitation to the DC field current.

69
Q

How do alternators work?

A

1) AVR controls generators voltage output.
2) Generator is excited with DC voltage to create a fixed magnetic field.
3) Exciter rotor windings rotate and cut through the fixed magnetic field which generates an AC voltage output.
4) AC voltage output gets rectified by the diode pack and fed to the generator rotor windings with DC.
5) The DC in the generator rotor windings creates a fixed magnetic field which rotates and cuts through the stator windings to generate and AC output.

70
Q

What to do when motor is flooded with seawater?

A

1) Disconnect stator in the rotor and wash using freshwater.
2) Dry by putting it somewhere hot or get heaters next to it.
3) Re-varnishing
4) Maybe change bearings

71
Q

What maintenance would you carry out on batteries?

A

1) Condition and tightness of cables.
2) Clean connections and terminals. Petroleum jelly.
3) Vent caps are clean.
4) Electrolyte level.
5) Diluted and topped up with distilled water.
6) Electrolyte level should never fall below the plates.

72
Q

What is specific gravity?

A

A measurement of defining electrolyte activity. Only on lead acid.

73
Q

How should a motor be well maintained?

A

1) Clean
2) Dry
3) Rotates smoothly
4) Tight motor connections

74
Q

What can cause motor bearing damage?

A

1) Over and under greasing
2) Wrong grease
3) Vibration in storage

EOOW Oral Prep (29 decks)

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