Aircraft - Electrics & Electronics Flashcards
Difference between insulators and conductors
Conductors have free electrons, insulating materials have very few free electrons.
Current flow in reality
The flow of (negatively charged) electrons across a material from a negative terminal to a positive terminal.
Current flow by convention
Flow from positive to negative terminals, which was the assumed way of things before electrons were discovered.
Electromotive Force (EMF)
The force making electrons flow, measuring in units of Voltage.
Aka potential difference.
Symbol V or E [or U]
Ohms Law
V = IR
Power
- Description
- Formula
Power is the rate at which work is done.
W (watts) = V x I
Factors affecting resistance of a wire
Double the length to double resistance.
Decreasing width increases resistance.
Resistance and temperature
Most metals have positive temperature co-efficient (resistance proportional to temperature).
Insulators often negative temperature co-efficient (resistance negatively proportional to temperature).
Resistors connected in:
- Series
- Parallel
Series: Add resistance together
Parallel: 1/R(T) = 1/R(1) + 1/R(2) + …
More power loss with high current or high voltage
More power lost with high current, thus power lines at very high voltages
Unit of electric charge
Coulomb
Voltage and current around a circuit
[Model for doing calculations at various parts of the circuit]
Voltage (drops) add up around the circuit, so the total supplied voltage is split around all the series sections of the circuit.
Current on the other hand flows at a constant level around the circuit. It only gets “split” by parallel section, which can be figured out with V=IR on each component.
Momentary action vs alternate action switch lights
Momentary action: Press and hold to activate (release to deactivate)
Alternate action: Press and release to activate, press and release again to deactivate
Microswitch
Detect movements by allowing some physical item to move a contact away from a terminal.
Bimetallic switch
Activate when temperature at a certain level is detected, via two metal strips with different heat properties being fixed to each other.
Guarded switch colour codes
Red guard means once the switch is activated it can’t be undone.
Black guard means the switch can be put back.
Do fuses and circuit breakers protect from current or voltage?
Protect from high current, they will be rated in terms of an amount of current.
Minimum # spare fuses
10% of number of each rating in aircraft, minimum of 3 of each
Fuses vs circuit breakers
Fuses normally open circuits before full current is released, circuit breakers afterwards. So to use circuit breakers need to make sure components can handle high current for short time.
Circuit breakers can be reset and can be used as circuit isolation switches.
Dummy fuses
Circuits not in use have a dummy fuse with red streamer attached to it, square cross section with corrugated sides to identify.
Can also have tripped circuit breakers with warning flags or plates for the same purpose.
Operation of circuit breakers
Single button can just be pushed back in to reset. White band shows when the button is out.
Higher rated version have reset button which can be pushed to reset, and a trip button.
Non-trip free vs Trip free circuit breakers
Non-trip free circuit breakers can be “tripped” by holding them in under fault conditions and the circuit will be closed.
NOT allowed in aircraft.
Circuit Breaker colours
- Red
- Yellow/white
Red: May need to be reset in flight
Yellow/white: Can be pulled to isolate a service
Thermal vs magnetic circuit breakers
Thermal (bi-metallic) bend with temperature to move a latch. The heating takes time though so protect against PROLONGED over current.
Magnetic provide QUICK TRIP RESPONSE, can also sense reverse flow so can be used as reverse flow breakers.
General timing requirement of current protection devices
“Short term overcurrent is normal” (for example on startup) so protective devices shouldn’t trip on inrush currents that are normal for the device.
Capacitor roles (3)
- Stores electrical charge (by creating electric field between 2 plates)
- Acts as if passing AC flow
- Blocks DC flow
Construction of capacitor
2 metal plates separated by an insulator called a “dielectric”
Units of capacitance
Farad
Generally use microfarad (millionth), nanofarad nF (thousand millionth), picofarad pF (millionth millionth)
A 1 farad capacitor with 1V applied to it will store 1 coulomb of energy.
Factors affecting capacitance
Area of the plates (large plates => large capacitance)
Distance between plates (small gap => large capacitance)
Dielectric material (different materials have different dielectric constant k)
Capacitor working voltage
Maximum DC voltage or peak AC voltage the capacitor can handle, or dielectric will break down
Capacitor in a DC Circuit
Each plate will build up charge from the voltage it is connected to (+ve on side connected to +ve terminal), eventually building up to the amount of the voltage applied. Current flows through the circuit up to the fully charged point, but no current flows through the dielectric.
The capacitor maintains this charge if the circuit is opened and will discharge it if connected to an external circuit.
Capacitor in an AC Circuit
Charge builds up on each plate from the AC supply allowing the AC current to flow through the circuit. No current flows through the dielectric.
Capacitance of capacitors in series and parallel
Opposite of resistors
Series: 1/C(T) = 1/C(1) + 1/C(2) + …
Parallel: C(T) = C(1) + C(2) + …
Purpose of aircraft battery
Provides emergency power and power to start engine.
Primary cell
Two electrodes in an electrolyte which encourages electron transfer, building up a potential difference (c. 1.5v) between the two electrodes.
When connected to a circuit electrons flow from the -VE to the +VE and negative electrode is gradually eaten away.
Can’t be recharged.
Secondary cell
- Description
- Unit of capacity
Can be recharged by passing a reversed charging current through them.
Capacity measured in Ampere hours (Ah), i.e. amps x hours (called “rated load”)
Usually measured at the 1 hour rate, in reality capacity is lower at higher discharge rates.
Calculation for battery cells in series or parallel
Series: Add voltages together
Parallel: Add currents (amp hours) together
In aircraft we want the same voltage but more capacity so wire in parallel.
Lead Acid Battery
- component materials
- chemical changes
+ve plate: Lead peroxide
-ve plate: Lead
electrolyte: Water & sulphuric acid
Hydrogen gas produced when working (vented through vent cap). Lead sulphate forms at both plates and acid becomes weaker.
Top up with distilled water.
Lead Acid Battery
- Detecting charge level (numbers)
- Recharging issues
Charge level can be detected through the specific gravity of the electrolyte - 1.27 fully charged, 1.17 for discharged.
Recharging too fast causes gassing, evaporation and boiling the battery.
It will cause the lead sulphate to dissolve.
Lead Acid Battery Voltage levels
- On load (full)
- Off load (full)
Off load (full): 2.2v
On load (full): 2v
Nickel Cadmium (NiCad) battery
- Description
- Component materials
Aka alkaline battery, more common in larger aircraft as more stable voltage.
+ve plate: nickel oxide
-ve plate: nickel cadmium
electrolyte: potassium hydroxide
NiCad vs lead acid battery
NiCad is lighter, stronger, has longer life, wide temperature range, no acid spills, fast charge rate, constant voltage and can be stored discharged.
However more expensive.
Nickel Cadmium (NiCad) battery
- Voltages
Off-load: 1.3v
On-load: Steady @ 1.2v
Thermal runaway
Caused by too fast recharging speed, which increases temperature.
Increase in temperature lowers thermal resistance, increasing discharge and further heat, leading to “thermal runaway”. Need in built thermal switches to prevent this.
Possible with lead acid, but more of a problem with NiCad and even worse in Lithium.
Effect of temperature on battery performance
Batteries perform better at high temperature due to decreased internal resistance, but working life will be reduced.
Battery capacity requirements to remain in service
Capacity tests carried out every 3 months and need at least 80% efficiency (i.e. current capacity / rated load).
This is to ensure that essential loads can be provided for a period of 30 minutes following generator failure.
On-load check
Apply the rated voltage to the battery for a period of time (10-20 secs) and check that voltage doesn’t reduce too much, which would indicate low state of charge.
Voltage should recover when load is removed.
Storage condition of lead acid vs nickel cadmium batteries (charging condition)
Lead acid must be stored fully charged, nickel cadmium can be stored in partial charged state.
Charging systems for lead acid and nickel cadmium batteries
Constant Voltage charging for lead acid, at 112% of battery voltage.
For alkaline to avoid thermal runaway use constant current charging. May use pulse charging that pulses once charge reaches 85%.
Total voltage of secondary cell determined by
Number of plates
Lithium battery
- advantages & disadvantages
Many types, lithium ion and lithium polymer are popular.
High energy density and wide temperature range (although need cooling as life reduces above 25C), but expensive and prone to thermal runaway (more so than NiCad). Resulting heat affects neighbouring cells and is hard to extinguish due to flammable medium.
Dent to a lithium battery case can be a big problem.
Purpose of metal box around lithium battery
To contain thermal runaway.
Direction of lines of flux
From N to S
Lines of flux when magnets are close
Lines of flux never cross, magnetic fields of two close magnets will cancel each other out if opposed or intensify a shared field if in the same direction.
Temporary vs permanent magnets
Temporary magnets made from soft iron, which can be magnetised but readily loses magnetic properties.
Permanent magnets made of hard alloy steels, hard to magnetise but retain magnetism well.
Permeability
The property of a piece of soft iron allowing lines of flux from nearby magnet to flow through it so that the soft iron itself becomes magnetised.
Reducing magnetism of a material (3)
- Heating it
- Hammering it
- Placing it inside a solenoid supplied with AC current
Molecular structure of magnetised soft iron
Being placed in magnetic fields causes molecules to line up N/S along with lines of flux.
Once all cells are lined up in N/S direction the soft iron magnet is said to be “saturated”.
Corkscrew rule
For magnetic field created around a conductor which is carrying current.
Magnetic field will be in concentric circles around the wire (imagined on a piece of paper the wire travels through).
Direction of the magnetic flux lines based on direction corkscrew turns, if corkscrew moves in direction of conventional current (i.e. +ve to -ve).
Solenoid
General term for an electromagnet (also alternative tool to relay).
Series of insulated coils of wire through which a current is passed.
Solenoid
- Factors increasing strength
More coils
Increase in current
Add a soft iron core
Right hand rule for solenoid
For solenoid field direction, hold the coils of wire in RH with fingers pointing in direction of conventional current (+ve to -ve), thumb indicates north pole of the electromagnet.
Solenoid vs relay
Solenoid moves a soft iron core which is connected to a contact that is opened or closed. Used to operate very low torque valves or switches.
Relay is an electromagnet used to switch another electrical circuit. Used to switch low to medium current circuits.
Fleming’s right hand rule
For induced current due to magnetism (geneRIGHTor).
ThuMb - points in direction of motion
First finger - direction of Field (N to S)
SeCond finger - direction of current (conventional, +ve to -ve)
How to remember left hand vs right hand rule
Gene-RIGHT-or
RH for generators
LH for motors
Ways to increase magnetically induced voltage
Increase speed
Increase strength of magnetic field (flux density)
Increase number of coils
Simple Generator
Rotating loop (armature) within magnetic field from permanent magnet, connected via brushes and slip rings (separate one for each end of coil).
Results in AC flow due to current changing direction as each side of armatures moves back and forth through the field.
Simple DC Generator
Uses a “split ring commutator”, slip ring divided in 2 with the halves insulated. Both ends of coil connect to the same split ring commutator so that one side of the ring becomes +ve and one -ve.
Resultant voltage output is absolute sine wave (still bounces between 0 and max value).
Field coils in generators
Wire coils around the permanent magnets to strengthen magnetic field.
Series wound DC generator
- description
- nature of field loops
Armature, field coils and external circuit are all wired in series, so drawing more current as load increases magnetic force and increases voltage. Eventually there is a point where magnet is saturated and voltage and current are limited.
Not suitable for aircraft which need a constant voltage.
Series wound => FEW, THICK loops
Commutator ripple
The fluctuation of voltage output of a DC generator (for simple DC generator bounces between 0 and max voltage which is problematic).
Multiple coil armatures can combat this as they will be out of phase.
Shunt wound DC generator
Field wiring is in parallel to the load circuit and a variable resistor added, which controls current through the field coils and thus magnetic strength and the terminal voltage of the generator.
Rises to terminal voltage quickly because the circuit of armature and field coils is complete even when there is no load.
As more load is added parallel to the field coil, less current goes through field coil and output voltage starts to fall.
Compound wound DC generator
Includes a series field and a shunt field.
The series field combats the drop in terminal voltage experienced in shunt wound generators as load is increased.
Produces more steady voltage than series or shunt.
Self-excited vs externally excited generators
Self-excited have residual magnetism in the permanent magnets which automatically generates a field.
Externally excited require a current in field loops before current can be generated.
“Flashing the field”
Practice (perhaps by pressing a button) which passes current through a field to excite the generator magnet and allow power to be generated.
Alternator
Uses a rotating magnet rather than rotating armature and a rectifier (diode) to get DC current, rather than the split ring commutator.
This also eliminates arcing and sparking at the split ring, which is an issue with DC generators.
Used commonly for this reason in light aircraft.
Carbon pile voltage regulator
Carbon pile is a type of variable resistor which reduces resistance when compressed.
Regulator is wired in SERIES with the shunt field coil and compressed by a parallel “voltage” coil.
At high rpm/low load, voltage increases and carbon pile is de-compressed, increasing resistance and reducing current through the shunt coil.
Vibrating contact voltage regulator diagram
Vibrating contact voltage regulator description
Fixed resistor in series with the shunt field, which is bypassed by a non-resistor path with a closed contact switch.
The voltage coil in parallel opens the contact switch at high voltage, causing higher resistance in the shunt field circuit and reducing voltage.
Points open and close 50 to 200 times a second to maintain voltage.
Not suitable for high output as points would fuse.
Load sharing circuits
- Description
Two generators working in parallel to supply aircraft. Each need the same VOLTAGE otherwise current flows from stronger to weaker generator (recirculating current) which becomes a motor!
Reverse current relays prevent this.
Load sharing circuits
- How this is achieved
Have carbon pile regulators in both generators. The bars within the voltage coils are also wrapped with equalising coils that join the two circuits.
If one generator experiences higher load, current will flow through the equalising coils to balance it and shunt coil current will be adjusted to balance voltage.
Failure of a load sharing generator
Shut down the failed generator.
Load-shedding: reduce load items to reduce demand on remaining generator.
Voltage level in generator (relative to battery)
Generator voltage for recharging will be higher than battery voltage (112%) to ensure the battery remains charged.
e.g. 12v battery/circuit => 14v generator
Fleming’s left hand rule
Opposite of right hand rule, governs directions for a motor (the opposite of when acting as a generator).
Complete motor
This is a DC motor with multiple armatures and many turns of wire (around the magnet) so that there are always wires at the maximum torque position.
Which armature position gives strongest torque in DC motor?
When armature is parallel to the magnetic field
Back EMF
Back EMF is the electromotive force induced in a motor armature due to its movement through the magnetic field. It opposes the supply voltage and is proportional to motor speed, but never as high as the supply input voltage.
The difference between applied EMF and back EMF will always allow current to flow in the conductor and produce motion.
Impact of DC motor speed on current and torque
Due to back EMF, increase in DC motor speed will reduce current and therefore torque