AGK - Electricity Flashcards
What is current?
Charge over time
How do electric field lines flow?
Out from positive
In toward negative
What is coulomb’s law?
The strength of the electric force/field can be increased by
increasing the amount of charge
decreasing the distance between charges
What is Kirchoff’s 1st law?
Total current flowing into a point must be the current flowing out of that point.
What is Kirchoff’s 2nd law?
The sum of the voltage drops of a circuit must equal the emf.
Which factors affect resistance?
Length - Proportional
Cross-sectional area - Inversely proportional
Material
Temperature - Proportional
Resistors in series?
Add resistances together
Resistors in paralell?
1/Rt = 1/R1 + 1/R2 + 1/R3 …
What is a pole?
The connector
What is throw?
Moving from 1 position to another
Another name for push switches?
Light switches because they have lights in indicating condition.
What is momentary and alternative action on a switch?
Momentary - only while holding
Alternate - Toggle
What does a thermo switch do?
Make or break circuit when it reaches certain temps
What is a unipole system?
Old aircraft using ground as airframe.
What is a dipole system?
Composite aircraft have a return wire to ground.
What is a non-trip free circuit breaker?
Old type of breaker where the button can be pushed in again to complete the circuit even if the fault still exists.
What is a trip free circuit breaker?
Trip free circuit breaker prevents current from returning if the button is held in and the fault is still present through the use of secondary contacts.
What are thermal circuit breakers used for?
Protected circuits in event of prolonged over current.
Through use of two bitmetallic strips with two different coefficients of expansions
What is a magnetic circuit breaker and what is it used for?
A magnetic circuit breaker uses current instead of heat to generate a magnetic field to pull the breaker at a very specific point.
They are used in sensitive circuits where there is no delay and strict current limits.
How many spare fuses must an aircraft carry?
10% of number of fuses which use that specific rating
3 of each specific rating of fuse
The higher of these 2 values.
What are current limiters? (4 facts)
Allow short term overloads
High current fuses
Cannot be changed in flight
Normally found in series with busbars
How is static electricity generated?
Rubbing and friction exchanging electrons
What is a static discharge wick?
Static electricity likes sharp discharge points, a wick is a sharp discharge point which can be used to dissipate static electricity back into the atmosphere from the airframe.
Where do we find out how many static discharge wicks we can lose?
The Minimum equipment list (MEL)
What is bonding?
Bond each part of the airframe together to allow a low resistance path between all parts of the airframe and the static wicks.
Allows a uniform potential across the airframe.
What is grounding?
Airframe can still have charge after landing.
Ground to prevent arcing from fuel nozzle and prevent fire.
Why are cables covered in protection screening?
Wires carrying current have magnetic field.
This can interfere with other circuits.
Shield prevents this.
What is in a cell?
Anode (-)
Cathode (+)
Electrolyte separating them
How is a primary cell different to a secondary cell?
You can’t recharge a primary cell
Why is a secondary cell different to a primary cell?
You can recharge it
What are the two types of secondary cell?
Lead
Alkaline (NiCad)
The ELECTROLYTE determines the type!
What is the electrolyte in a lead acid battery?
Dilute sulphuric acid
What is in a lead acid battery
Anode - Lead peroxide
Cathode - Spongy lead
Electrolyte - Dilute sulphuric acid
What is the terminal voltage of a lead acid cell?
2.2 V
What is the nominal voltage of a lead acid cell?
2 V
What is terminal voltage?
Voltage of battery/cell off load
What is nominal voltage?
Voltage of battery/cell on load
What is the terminal voltage of a lead acid battery?
13.2 V (off load)
What is the nominal voltage of a lead acid battery?
12 V (On load)
How many cells in a lead acid battery?
6 Cells (2.2 V Term, 2 V nom)
What does the voltage of a battery/cell depend on?
Material and not size
Characteristics of lead acid discharge?
Voltage drops off gradually with discharge
Think of dimming torch
What is Sulphation?
Lead acid battery suffers from sulphites blocking pours in the cathode (spongy lead) preventing charging of the battery.
Occurs when left discharged for a long period.
How do we prevent sulphation of lead acid batteries?
Store them fully charged
What is specific gravity?
Similar to density
What is the specific gravity of a charged lead acid battery?
1.27 SG
What is the specific gravity of a discharged lead acid battery?
1.17 SG
How many cells in NiCad battery?
10
NiCad terminal cell voltage?
1.3V
NiCad nomimal cell voltage?
1.2V
NiCad battery terminal voltage?
13V
NiCad battery nominal voltage?
12V
Discharge characteristics of NiCad?
Alkaline batteries - Bright torch
Voltage drops slightly with charge, remains constant then drops off suddenly.
What happens to NiCad specific gravity with charge?
Doesn’t change.
How is a battery connected with a generator to charge it?
In paralell.
Why does generator produce higher voltage then battery?
So when generator is running, forces current into the battery charging it.
What is a NiCad thermal runaway?
In NiCads resistance decreases with temperature
Temp increases -> Resistance decreases -> Current increases -> Temp increases more
How do we stop NiCad thermal runaway?
Monitor the temperature of the battery and adjust current accordingly.
What do we clear up lead acid electrolyte spillage with?
Acid electrolyte
Alkaline - Bicarbonate of soda
What do we clear up NiCad spillages with?
Alkaline electrolyte
Acid - Boric Acid
What is the electrolyte in a NiCad?
Potassium hydroxide
What do we measure battery capacity in?
Amp-hours
How are battery capacities tested?
At 1 hour rate.
I.e current discharging battery in an hour
So 25 Ah battery would be 25 Amps for an hour.
How does battery capacity change in series?
It doesn’t
Voltage changes but capacity same
How does battery capacity in paralell change?
Add together the capacities of the batteries
What happens to capacity of a battery in lower temperatures?
It goes down
What is the minimum capacity of a battery?
80%
How long must main batteries last (CS25)
30 Minutes
How long must emergency light batteries last? (CS 25)
10 Minutes
What demagnetises a permenant magnet (2 things)?
Heat
Vibration
How do we find field direction in a coil electromagnet?
Right hand rule
Fingers are field direction
Thumb is current direction
What 3 things strengthen electromagnets?
Current
Number of coils
Soft iron core
What is a relay?
Low voltage control circuit operates electromaget switch to high voltage operating circuit.
Fixed core single contact EM switch allowing remote control of heavy loads
What is a solenoid?
Have a moving core with twin contacts
Heavy duty loads
Why is a relay different to a solenoid?
Relay light duty device
Solenoid heavy duty device
Relay has a fixed core
What is a contactor and how is it different to a solenoid?
The contactor latches on/off and stays in position unlike a solenoid.
Will not move until powered up again
What is Lenz’s law?
The current generated in a conductor produces it’s own magnetic field which opposes the field creating the current.
When do we use the right hand rule (FBI)?
Generators
When do we use the left hand rule (FBI)?
Motors
How does a DC generator create DC from the AC output in the armature?
It uses a split ring commutator
What is an armature?
Where we induce an emf (drive current)
Why is it bad to rotate the armature?
The armature is where we generate electricity and to extract this we will need to use brushes with rings to extract it, high currents will pass through these brushes causing wear.
What is the difference between a DC generator and alternator?
The alternator has a rotating field and a fixed armature.
How do we convert AC in the armature to DC in a DC alternator?
Bridge rectifier
What happens to the series wound field in a DC generator?
As load and current increases, current increases and so does field
What happens to the shunt wound paralell coil in a DC generator with load?
Emf stays constant even with load
But output goes down with load as current varies
What is a compound wound field?
Use both effects of series and shunt coils to smooth out voltage and excitation field drops in DC generator.
What is the purpose of a voltage regulator on a DC generator?
We place it in series with the shunt coil, it is a variable resistor which senses the voltage of the output and controls current through the shunt coil/field.
What do you monitor on your DC electric system?
Monitor Voltage and Current
Two types of voltage regulator?
Carbon pile, vibrating contact
What does a current regulator in a DC generator do?
Opens contact to field coil to prevent overload
How does DC generator overvoltage work and why is it needeD?
Electromagnet sense overvoltage cutting contacts to field coil and disconnects the entire generator. Now just rotating armature.
It is required because overvolting can cause electrical components on the aircraft to be damaged.
What is the reverse current cut out relay used for on a DC generator?
Protects you in the event the generator voltage drops lower than the battery, this stops the battery driving the generator.
In a system with 2 DC generators, what does the Differential cut out prevent?
If there is a larger than 2 voltage difference between the two generators, it removes one of them to prevent it doing all the work and going past it’s rating.
In a system with 2 DC generators, what does the equalising circuit/load sharing circuit do?
It fine tunes generator voltage regulators to balance field coil and generator voltages to get the same voltage out of both generators.
