L Theory Flashcards
What holds the electron in orbit around the nucleus
Force of attraction
What is the outer obit of an atom called
Valance shell
What is the polarity of an electron
Negative
If an electronic is removed from the atom, what is the electron called?
Free electron
If an electron is removed from a balanced atom, what is the stomach now know as
Positive ion (positively charged)
Electrical flow is always ….
Negative to positive(-ve to +ve)
If current was to flow from positive to negative(+ve to -ve) what is it called
Conventional current flow
What is the symbol for current
I
What is current measured in
Amps
What is the electron called that is in the outer orbit of the atom
Valance electron
Potential differnce will exist between two points in a circuit will have a …..
Difference in charge
What is potential difference is measured in?
Volts
What is the force that drives current round a circuit
Voltage
Name 6 methods that can produce voltage
Induction heat
Friction pressure
Light chemical
To produce voltage in a chemically, what do the different plates need to be made of?
Different metals
Give an example of creating voltage using friction
Rubbing socks on a carpet
Name 3 requirements for induction voltage
Relative movement
Magnetic field
Electrical conductor
What determins the polarity of induced voltage
The movement of the conductor through the magnetic field
What determines the voltage in a thermocouple
The differnce in temperature and the matrial used
What is used in solar panels
Semiconductor
A material with many free electrons will pass current freely is called a…
Conductor
List at least 4 examples of conductors
Copper
Gold
aluminium
Steel
Carbon
A material with few free electrons and can only pass with difficulty is called an….
Insulator
Give at last 4 examples of insulators
Plastic
Fabric
Paper
Rubber
Glass
List 4 examples of what could cause resistance in a circuit
Heat
Cross face area
Length
Type of material
State ohms law
States that the current flowing through a conductor is directly proportional to the voltage applied across it. Providing the temperature remains constant, and is inversely proportional to the resistance
What is resistance measure in
Ohms
What is the formula for total resistance in a series circuit
Rt= R1+R2+R3
What is the equation for total resistance in a parallel circuit
RT=1/R1+1/R2+1/R3
Define electrical power
The rate of doing work
Power=(force×distance)÷time
What is the unit for power
Watts
How many Watts makes 1HP
746W
What’s the danger of passing constant high current through a conductor
Can overheat and cause a electrical fire
Name two types of fuses
High Rupturing Capacity(HRC)
Cartridge Fuse
What does the fuse protect
The power source
What happens to a HRC fuse when it blows
The powder covers and solidifies over the broken contacts and this prevents it from completing the circuit
What is the electrical potential of the sea
0volts
3 things that can cause a earth fault
Dampness
Accidental damage
Insulation becoming brittle with age or heat
Why is equipment earthed to the ships hull
Because the Hull is at earth potential thus 0 volts meaning someone standing on earth cannot be shocked from touching casing
What is the name of the natural magnetic ore
Magnetite
What direction do lines of magnetic flux travil
North to South
Negative to positive
List at least 3 materials mixed with steel, will produce stronger magnetics
Copper
Iron
Nickle
Cobalt
Aluminium
What is produced around a conductor passing current
Magnetic field
Heat
Permanent and electro-magnet are two ways of producing magnets what’s the third
Temporary magnet
List 2 advantages of AC over DC
Can transformed to get a range of voltage via a transformer
Equipment is usually lighter
At what speed is ac changing direction called(measured in)
Frequency
The level to which the voltage raises to in an ac wavelength graph
Peak value
How are AC cycles expressed(unit of measurement)
Hertz
What are the values of the ships electrical supplies draw
450V
60Hz
3 phase
Draw the frequency equation
(Seconds)
Time= 1
Frequency
What is the unit of inductance and what are the more common values
Henry
Milli Henry
Micro Henry
Label all parts of ac waveform
One cycle
+peak value
-Peak value
Amplitude
Time axis
Will a transformer work with dc current
No
Why won’t dc work with a transformer
They only work with AC as it is always changing which gives us our relative movement
Name two types of transformers
Shell
Core
What is the purpose of a transformer
To either increase or decrease the voltage
What is the purpose of a laminated soft iron core
It passes the magnetic field and voltage from one coil with another
State the output voltage equation
Output. No. of secondary coils
= ×input volts
Volts. No. Of primary coils
Purpose of a centre tapped transformer
Has an earth centre in the secondary coil, which half’s the voltage
This reduces the shock effect by half
Identify the input and output cables on hm ships
Input. Output
Red- L1. A
Yellow- L2. B
Blue. - L3. C
What are the new colour codes
Red is now brown
Yellow is now black
Blue is now gray
What the most common type of generator today
Brushless generator
2 things EMF output depends on
1)Amount of lines of force cutting across the conductor as they rotate
2) the speed the generator is rotating
How is the HF generator turned
Is belt driving, its output is 115volts
What current is fed to the exciter stator
Direct current
What is the purpose of the rectifier assembly
Converts ac output to DC for the main generator rotors
If the magnetic field around the around the ac generator is stable(made from DC supply) how does the generator produce ac output
The last generator produces an AC output as the rotor rotates round 3 AC stator, producing 3 Phase AC current
What is the output from generators on hm ships and how does the AVR know what’s being produced
450V
60 hertz
3 phase
It is connected to the 3 phase by a sensing line
Lists 4 advantages of AC motors
Less maintenance
More efficient than smaller dc motors
Cheaper than DC
More compact
What is the main disadvantage of an AC motor
Needs specialist equipment to vary speed
List the 2 main components of a AC motor
The stator
The rotor
What causes the movement in a 3-phase induction motor
Rotating magnetic field
What is the most common type of AC motor used on hm ships
Squirrel cage induction motor
List the components of a squirrel motor
(4)
Copper end ring
Drive end bearing
No drive end bearing
Copper bars
How does the rotating magnetic field from the stator cause rotor to rotate
It has the opposite polarity of the stator magnetic field, and the interaction of the two fields causes the rotor to turn to follow the rotating field
Why are there less maintenance on a AC motor
There is no electrical connection(carbon contacts) meaning you don’t need brushes and slip rings, simplifying construction and less parts to wear
What would happen to a running 3 phase motor if it lost 1 phase
It would start vibrating and loose power and start overheating
If a shutdown 3 phase motor looses 1 of its phases what affect would this have
It wouldn’t start
What is the DC motor generally made from
Coils of wire
What does maxwell right and grip indicates
Direction of the magnetic field around the conductor
What will lines of magnetic flux never do
Cross
Using flaming left hand rule.
What does each digit indicate
Thumb= motion
First finger= field
Second finger= current
What is back EMF
Is the emf created by the generator that flows in a differnt direction to the EMF.
What is The amount of torque produced dependant on(2)
Strength of the magnetic field
Magnitude of the armature current
List 3 classifications of DC motors(type)
Series wound
Shunt wound
Compound wound
Why must the carbon brushes fixed in the same position as and why
The must be fixed directly opposite along the magnetic neutral axes and this is to prevent any back EMF
Describe armature reaction
A magnetic field is created around the armature. The armature magnetic field will distort the winding magnetic field
What are inter-poles and where are they positioned and why
Inter-poles are similar poles just smaller.
They are placed between the main poles and wound in series with the armature.
They neutralise the effect of armature reaction
