Electrical

Question 1

What is supplied by UPS?

Answer 1

Bridge control consol
Engine control consol
Nav lights
Emergency lighting

Question 2

What is supplied by Emergency Switchboard?

Answer 2

Steering gear
Radar motor
Emergency fire pump
Air compressor
Economiser pump
Emergency bilge pump
Navigation Lights
Emergency lights
Water mist
Radio Equipment
Public announcement speaker
Light on embarkation deck
Water tight doors

Question 3

Transformer regulations?

Answer 3

Must be double wound
Must have a back up for continuity of power
Must have means of isolation on either side
With stand a short circuit for 2 seconds
Short Circuit on primary and Overload on Secondary, Temperature monitoring, Earth

Question 4

What are the protection devices on the switchboard?

Answer 4

Overcurrent - Protects from high currents thus high temperature and breakdown of insulation

Reverse Power - Protects the generator from becoming a motor and thus leading to catastrophic mechanical damage

Preferential Trip - Protects the generator from overloading by removing non essential loads such as galley or Air conditioning while maintaining essential services.

Synchronizer - prevents the generator from connecting to the switchboard in the phase or phase angle which would cause high torque and mechanical failure

Question 5

What is dynamic braking?

Answer 5

Dynamic braking removes the AC from the stator windings and applies a DC current. This current creates a static magnetic field and thus has a rotational speed of 0. The magnetic field in the rotor slows down as it aligns itself with the static field.

Question 6

What is Plugging?

Answer 6

Plugging is a form of electrical breaking where two phases are swapped, this cause the magnetic field to operate in the opposite way and therefore slow the motor down. As the motor closes in on 0 RPM the phases are switched back to prevent the motor running in reverse.

Question 7

Why is Braking required?

Answer 7

Braking is required as a safety feature, in order to either slow down or control the rate of descent or operation.

Question 8

What makes it preferable to mechanical braking?

Answer 8

Electrical braking is much easier to achieve through an electrical circuit rather than a complex mechanical brake. Electrical braking also reduces the maintenance as there is no need for pad or shoe replacement. It also removes the mechanical aspect freeing up space.

Question 9

What is ICCP? How does it work?

Answer 9

AC current is stepped down and rectified into a DC current. Lead or Titanium anodes are fitted across the ships hull. The DC current are fed to the ships hull and therefore the hull becomes a cathode. The Flow of ions means that the anodes corrode instead of the cathode. The DC current is controlled by reference anodes, the speed, temperature, thickness of paint and salinity.

Question 10

What tests are carried out on Equipment in hazardous areas?

Answer 10

Insulation resistance test
Earth loop resistance test
Earth continuity test

What safety precautions?
Instruments and tools will be made from a material which doesnt cause a spark when dropped
No equipment is to be worked on till its been made dead
Competent person
Use of a gas tester to ensure the environment is safe

Question 11

What is Increased safety?

Answer 11

Components are designed to a high standard to prevent faults developing and creepage distances are used to increased the distance needed to arc. As well as an enclosure to contain open sparking.

Question 12

What is intrinsically safe?

Answer 12

Intrinsically equipment is designed to limit the current and voltage to ensure no sparks or thermal effects occur

Question 13

What is Flame Proof Enclosure?

Answer 13

Enclosure must be strong enough to with stand internal explosion without suffering damage
prevent flame and hot gases escaping
Limit the external surface temperature

Question 14

What is the maintenance on ICCP?

Answer 14

Check the brushes on the shaft to ensure a complete circuit

When the ship is stationary, portable electrodes can measure the voltage to the hull.

When dry docked the earthing on the fins and rudder can be checked.

Question 15

Lithium ion - thermal runaway

Answer 15

Thermal runaway is when a cell enters a self heating state, the heat created by the cell is more than the heat dissipated, thus an uncontrolled rise in temperature which leads the a hazardous electrical fire or explosion. This can be cause by overcharging or physical damage.

Question 16

Lithium ion - cell drift

Answer 16

Cell Drift is when two or more cells within a lithium ion battery are in an energy imbalance. This can lead to over charging or over discharging which causes the cells to be defective and pose a safety risk.

Question 17

Lithium ion advantages and disadvantages

Answer 17

+ves

High energy density
Low self discharge

-ves
thermal runaway
Expensive

Question 18

Why will prolonged low voltage affect the circuit breaker?

Answer 18

There many not be sufficient voltage to operate the breaker as the break will contain a solenoid.

Question 19

What is a wound rotor?

Answer 19

A wound rotor uses wingdings on the core of the rotor connected to a variable resistance resistor bank via slip rings and brushes. When the motor is starting the resistance is at a maximum, this reduces the starting in rush current and increases the torque, then as the RPM increases the resistance can be reduced thus reducing the current.

Question 20

What is a double squirrel cage?

Answer 20

A double cage uses an inner and outer squirrel cage improve the starting characteristics of the motor. The outer cage is made from a smaller cross sectional area thus having a larger resistance. When the Ac voltage is applied to the stator then the initial magnetic field penetrates the outer cage, the larger resistance enusres the current is lower but the torque is higher. Then as the RPM increase the magnetic field penetrates the inner cage, this has a larger cross section thus a larger magnetic field is produced.

Question 21

What does an AVR do?

Answer 21

The AVR Receives feedback from the output of the generator and uses thyristors to to control a DC current for excitation in the rotor which then increases the magnetic field thus inducing more voltage.

Question 22

What types of excitation are there?

Answer 22

Self excitation where the voltage produced is used to create a excitation current
Static uses transformers and slip rings to induce a current to the rotor