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
Slow start resistor
This resistor sits in series with the motor and resists initial starting current. A centrifugal or time switch bypasses it completely so that the resistance is removed and full current is applied at the appropriate time.
Series Wound Motors
- How they deal with heavy loads
Field loops around magnet in series with the motor. With heavy loads the motor speed decreases, reducing back EMF and increasing current through the series field, thus providing the higher torque required.
Series Wound Motors
- Use cases
Motor speed VARIES so can’t be used if speed needs to be constant.
Do deliver high starting torque though so good for starter motors and flaps.
Shunt Wound Motors
- How they deal with heavy loads
AKA parallel wound
Field loops in parallel with motor so receive constant voltage from power source. If load on the motor increases it slows down, back EMF reduces allowing current to increase, delivering torque to drive the load.
Shunt Wound Motors
- Use cases
Speed variation from no load to full load is only 10%, so they are effectively CONSTANT SPEED motors.
Disadvantage is LOW TORQUE.
Used for fans, centrifugal pumps and motor generator units.
Compound wound motors
Have field coils both in series and in parallel (shunt) with the armature (i.e. motor) so can deliver a range of torques.
Starter-generator systems
Used to start turbine engines then generate power from them. Only suitable for smaller turbine engines as motor power is limited, but saves a lot of weight to use one item for 2 purposes.
Split Field Series Actuator
Reversible motor to give 2 directional control over something (e.g. opening and closing). Two separate field coils which are selected with a control switch and coiled in opposite directions.
Rotary actuators
Reversible DC motors (series for torque) acting through 350 degrees. Hit limit switches at end of travel to remove power and change field direction ready to reverse.
Used for fuel cocks and butterfly valves.
Linear actuators
Series wound DC for torque.
Motor drives a screw jack creating up/down motion.
Have limit switches at end of travel and can have 3 way controls (up/down/off) to allow “inching”, e.g. for moving flaps up and down.
Electromagnetic indicator
E.g. doll’s eye and prism indicators.
These are replacements for the filament lamp indicator, will display words (e.g. “OPEN”, “CLOSED”) or a hash pattern if there is no power.
Solenoid/electro-magnet causes the ball (dolls eye) to rotate, showing a different side to the indicator.
Dipole vs single pole system
Generator +ve connected to Bus Bar.
Single pole (unipole or earth return) connects -ve side of generator(s) to metal airframe and all components also connect to airframe.
Dipole (2 wire) requires each component to be wired to the bus bar (+ve) and the -ve generator terminal.
Generator vs Alternator
Alternator requires a rectifier to achieve DC output.
Full power of generator not achieved until engine operating at half of full RPM, alternator much earlier. Generator needs to be geared to about 3 x engine speed.
Generator cut-out
- description
- difference for alternators
Aka reverse current relay
Sprung switch connects the generator to the bus bar which is closed by the magnet only when voltage is greater than battery voltage. This prevents reverse current from the battery when generator voltage is low.
Not required in alternator as the rectifier achieves the same role
Static inverter
Solid state device which converts DC to AC.
Typically 18-30v DC converted to 115v AC at 400hz.
Rotary inverter
Converts DC power to AC by using a constant speed DC motor to drive an alternator (thus constant frequency AC).
Generator Differential Relay
Fitted to multi-engine aircraft to prevent the second generator coming on line until it has output voltage 2% above output of the generator which is already online (i.e. the bus bar voltage).
Generator/alternator warning light
Activated when the voltage of generator/alternator has fallen below battery voltage.
Ammeter & voltmeter
- what they detect
- what they indicate
BOTH detect CURRENT, but ammeter indicates current and voltmeter indicates voltage.
Ammeter & voltmeter in the circuit
Voltmeters are placed in parallel between the two points over which potential difference is to be measured. Will have a multiplier (high value resistor) in series with it to extend indication range.
Ammeter in series with the flow it is measuring, may have a shunt (low value resistor) in parallel around it to extend indication range.
2 types of aircraft ammeter
Left zero: Measures load going out of the alternator (zero means power coming from the battery).
Centre zero: Measures charge/discharge of the battery, so +ve value means battery being charged.
Ammeter & voltmeter setup in multi generator aircraft
Have one ammeter for each generator and a single voltmeter which can be switched between the battery and each of the generators
Categorisation of electrical load items
- vital
- essential
- non-essential
Vital: Required in an emergency after wheels up landing (e.g. emergency lighting, crash switch fire extinguishers). Connected directly to battery.
Essential: Required for safe flight during in-flight emergency. Connected to DC & AC bus bars to be supplied by generator or batteries.
Non-essential: Can be isolated during an in-flight emergency for load shedding, connected to DC & AC bus bars and supplied from generator.
Parallel bus bar system
Used when there are 2 generators.
Battery (or hot) bus is connected to battery and supplies vital consumers. This is connected to centre bus bar via battery switch, which powers essential consumers (DC directly, AC via inverter).
Each generator has a bus bar which powers non-essential consumers (DC directly, AC via inverters) and also links to the centre bus bar to power it and thus the battery when working.
Bonding
Linking of metal components of an aircraft so the static can be shared and shedded (also part of earth return circuit).
Without this there is a risk of arcing of static creating fire risk.
Screening
Metal sheathing required around certain components to suppress interference which would affect communications (e.g. ignition systems, DC generator and motors, slip ring machines over 200 RPM and circuits which open and close at > 10 Hz).
Static wicks
- purpose
Used to be cotton, thickness of cigarettes. Now metal wicks or barbed antenna.
Disperse static to prevent it impacting radio communications.
AC advantages over DC
- Better power to weight ratio
- Generators simpler & more robust
- Easy voltage conversion with transformers
- DC voltage can be obtained easily with rectifier
- 3 phase AC motors are simpler, more efficient than DC motors
- Less maintenance (no brushes)
- Produce voltage at lower RPM
Calculating frequency of AC generator
(# of poles / 2) x (RPM / 60 seconds)
e.g. 8 pole generator has 4 magnets so need to do 4 x RPM / 60 seconds
Root Mean Squared (RMS) value
This is the effective value of AC used to compare it to DC.
Average value of AC is at 45 degrees of phase, so related to sin(45).
RMS value = 0.707 x peak value
Phase difference/Phase angle
This is the phase difference between AC sine curves for current and voltage.
If it is zero, the circuit is said to be “resistive”, however in most cases there is a difference due to inductance and capacitance.
Induction
When AC power in a primary coil creates constantly moving magnetic flux fields, which generate current in a nearby secondary coil.
Inductance
- description
- units
When a coil causes induction across itself, which according to Lenz’s law has to act in opposition to the voltage that creates it, i.e. a back EMF.
Measured in henrys (one henry means 1 amp per second creates 1 volt of back EMF).
Inductance effect on phase difference
Inductance causes current to lag voltage. If inductance is the only effect present, current will lag voltage by 90 degrees.
Factors to increase amount of inductance
- Increase # turns of the coil
- Addition of a soft iron core
- INCREASE in AC frequency
Inductive reactance
- description
- relationship with frequency
The opposition to current flow (i.e. resistance) in a circuit with inductance.
Positive correlation with frequency.
Inductance in series & parallel
Opposite to capacitance
Series: Ltot = L1 + L2 + …
Parallel: 1/Ltot = 1/L1 + 1/L2 + …
Mnemonic for adding inductors and capacitors in series/parallel
ISA - Inductors in series ADD
CSI - Capacitors in series INVERSE
Capacitance
- Symbol
- Units
- Description
Capacitance has symbol C and units are the farad.
1 farad means 1 ampere flowing for 1 second creates a potential difference of 1 volt between the plates.
Capacitance equation with charge & voltage
Q = VC
Charge = Voltage x Capacitance
Capacitance equation for capacitor factors
C = Dielectric K x Plate area / Distance
Capacitance effect on AC phase difference
As capacitor constantly charges and discharges back to the source in the opposite direction, it causes current to lead voltage.
In a purely capacitive circuit current will lead voltage by 90 degrees.
Capacitive Reactance
- description
- relationship with frequency
The opposition to current flow in a circuit due to capacitance.
Increases with DECREASE in frequency.
Impedance
This is the combined resistance to current flow in an AC circuit, arising from:
- Resistance;
- Inductive reactance; and
- Capacitive reactance
They can’t simply be added together as they aren’t in phase with each other so don’t act together.
Resonant circuit
As frequency has opposite effect on capacitance and inductance, there is a frequency where they will balance each other. When the AC frequency causes inductive reactance and capacitive reactance to be equal, the circuit is said to be resonant.
Resonant circuits in series and parallel
If a capacitor and inductance component are in series with each other at resonant frequency, current flowing in the circuit will be at its maximum.
If they are in parallel, the current flowing in the circuit at resonant frequency will be at a minimum.
Mnemonic for voltage and current phase relationship for inductive and capacitive circuits
CIVIL
CIV:
In a capacitive circuit I leads V
VIL:
V leads I in an inductive (L) circuit
Power in AC circuits:
- Purely resistive
- Purely capacitive/inductive
Purely resistive means I and V in phase. Can multiply to get a power curve.
Purely capacitive or inductive means 90 degrees out of phase therefore real power = 0.
AC circuit power measurements:
- True/real/watfull power
- Apparent power
- Reactive power
True/real/wattfull power is V x I, however this is zero for purely capacitive/inductive so not useful.
Apparent power (KVA) = RMS V x RMS I
Reactive power (KVAR) is power needed to overcome that absorbed by inductive/capacitive elements.
Power factor (PF)
This is equal to True Power / Apparent Power = KW/KVA
It will be 1 for purely resistive, 0 for purely inductive.
Can be calculated as cosine of the phase angle.
Which type of power is used as a rating for an AC alternator?
Apparent power, measured in VA or kVA.
What is the effect of an alternator running at below normal frequency?
Low frequency reduces inductive reactance which increases current, therefore:
Inductive devices will overheat.
Industry standard AC power supply
- phase voltage
- line voltage
- frequency
115V (phase voltage)
200V (line voltage)
400hz
3 phase
Advantage to rotating field alternator (vs rotating armature)
Only a small current (DC to maintain polarity of magnet) is required to be passed through slip rings to the rotating component (magnet).
The high level of current generated is in the fixed component.
Star connected 3 phase alternator
All phases connected at one end to a single neutral point (usually aircraft earth), then to the other at their individual phase loads.
Star connected 3 phase alternator
- Diagram
Star connected 3 phase alternator
- Advantages
- Disadvantages
Advantage is that the star connection can cope with different loads on each bus bar, unlike the delta connection.
Disadvantage is that if there is an earth fault on one phase, the neutral will carry a very high load.
Line voltage and phase voltage (star connection)
Phase voltage is the voltage across a single phase’s coils. i.e. connection from the neutral point to the phase line (before loads).
Line voltage is between 2 phase lines.
Line voltage = 1.73 x Phase voltage
Line current and phase current (star connection)
Line current = phase current
Flow not measured between two points but through a single point, so there is no difference
Star connection - phase imbalance
Any imbalance between loads across phases will result in a flow through the return (neutral) connection. Ideally this should be low.
Any fault in one phase (e.g. grounding to the return) will affect the other phases.
Delta connected alternator
- Description
- Line/phase voltage
- Line/phase current
Each set of phase coils connected to the next one in a “loop”.
No neutral point so can only measure voltage between phases, so line voltage = phase voltage.
Line current however (current heading out to loads) is 1.73 x phase current (current across a set of coils).
Delta connected alternator
- Diagram
Practical use of star and delta alternators
Star are used predominantly.
Delta less useful as no neutral connection and can’t handle variation in loads, but some specific uses such as speed sensors and tacho generators.
Star vs delta connection, voltage & current
Star - generates high V at low I
Delta - generates low V at high I
Brushed alternator
Uses slip rings and brushes to provide current to the field coils around the rotating magnet.
DC current provided from 28V bus bar (rectified from AC by TRU) initially, then via a voltage regulator from the AC (with a regulator) to control voltage.
Brushed alternator
- diagram
Brushless alternator
Rectified AC current supplied via voltage regulator to generate N and S poles around a rotating core & coil which is on the alternator shaft. This generates AC in the coil which is in a circuit with the coil around the main alternator magnet. A rectifier (diode) in that circuit ensures the magnet receives DC current.
Initial excitation from permanent magnet.
Brushless alternator
- diagram
Frequency wild alternator
Alternator whose output frequency varies with engine speed. 2 such systems can’t be connected in parallel.
Can be used for specific systems where frequency isn’t important such as de-icing heating mats, or to be rectified to DC.
Converting frequency wild to constant frequency AC
Need a rectifier to convert to DC and an inverter to achieve the constant frequency AC.
Constant Speed Drive Unit (CSDU)
Obtain frequency within of 380 to 420Hz using an engine driven HYDRAULIC pump driving a hydraulic motor, which then drives an alternator.
Separate oil supply used for hydraulics, lubrication and cooling.
A “load controller” senses alternator output frequency, with drives the speed governor to adjust the pump swash plate.
How is the CSDU protected from alternator failure?
Quill drive, a weak link that breaks before damage is caused
Operating modes of CSDU
- overdrive
- straight through
- underdrive
Overdrive: generator speed > engine speed
Straight through drive: generator speed = engine speed
Underdrive: generator speed < engine speed
Integrated Drive Unit (IDU) or Integrated Drive Generator (IDG)
Combination CSDU and generator
CSDU fault indicators
- Low oil pressure
- High oil temperature
Drive disconnect unit
- description
- reconnection
Disconnects the engine input from the CSDU. Can be automated in some cases based on generator feedback but mostly pilot initiated.
Can be disconnected easily but only reconnected on the ground when engines are off.
Permanent Magnet Generator
Type of IDG with 3 generators on the same shaft:
- Permanent magnet generator (for initial power)
- Exciter generator (to control the field of the…)
- Main generator
Needs a generator control unit.
Variable Speed Constant Frequency (VSCF) systems
Alternative to the CSDU, frequency wild generator is converted electronically to 400 Hz 115/200V 3 phase using a generator converter control unit, to rectify to DC and then form the AC supply.
Running AC generators in parallel
As with DC generators they need to share the load. This requires monitoring of:
- Real Load; and
- Reactive Load (aka wattless load)
Controlling real load in parallel generators
Real load is the actual working load of the generator, measured in kW, it is controlled by managing the TORQUE supplied to the alternator driver via the CSDU.
Torque the same => real load the same.
Generator Control Unit or Bus Power Control Unit
- items monitored
Monitors:
- Over & under voltage
- Over & under frequency (speed)
- Over current
- Differential fault (internal short)
If activated by these (or fire handle) will pull GCB to disconnect bus bar and also excitation breaker. May be able to reconnect later if problem resolved.
What is a differential fault?
Difference between generator load and busbar, indicating a short
Controlling reactive load in parallel generators
This is the vector sum of inductive and capacitive currents & voltages expressed in kVAR.
It is controlled by controlling the voltage output of each generator via their EXCITER FIELD CURRENT.
Requirements before parallel AC generators connected
3 factors need to be equalised
- Voltage (real and reactive loads)
- Frequency
- Phase sequence (i.e. phase 1, 2, 3 waves appear on top of each other for the 2 generators)
Effect of connecting AC generators which aren’t frequency synched
Alternators are synchronous machines so they will seek to lock frequencies when connected. This causes damage as one generator tries to speed up and another to slow down.
Parallel 3 phase system layout
Each generator connected to 3 bus bars, one for each phase. Generator circuit breakers can cut off the generator from its bus bar.
The bus bars then connected to a set of 3 synchronising bus bars, with bus tie breakers in between.
Ground power or APU connects to the synchronising bars.
Balancing real load in parallel 3 phase system
A single series circuit of 5 amps has coils around one of the phases from each generator. Error detectors in parallel with each of the coils detects current, which will be 5 amps if the generators are balanced.
If one generator has more real load, currents in each coil will be differenced (average out to 5A) and the error detector will feed back to the CSDUs to request change to speed/torque.
Balancing reactive load in parallel system
Similar to real load balancing, however the parallel circuit around each coil is phased by 90 degrees by a “mutual reactor”, before the error detector. Thus we are looking at the vector combined reactive load, not the real load.
The results feed back to the voltage regulator (excitation/field current) to control reactive load.
Alternator cooling
- IDGs, IDUs, wild frequency, CSDUs
Wild frequency generators and CSDUs use ram air cooling (or alternative air flow system on ground).
IDGs and IDUs use their oil to cool the stators, which requires an oil cooler.
Discriminatory circuits
These ensure that only the malfunctioning alternator gets disconnected from the system by the bus tie breakers or generator circuit breakers.
Synchronising Unit
Ensures alternators are synchronised before they can be connected.
Automatic control mostly (prevents bus connections until allowed).
Older systems use dark lamp (manual) method. Lights are on when the alternators aren’t synched, need to check they are off.
Exciter Control Relay
Aka Generator Control Relay or Generator Field Relay
Controls the exciter field current supply to the generator field, to protect against over excitation or overvoltage.
Alternative AC supply sources (4)
Ram Air Turbine (RAT) - Lowered into the airflow, drives hydraulics so powers flight controls and a hydraulic generator
Auxiliary Power Unit (APU) - Gas turbine engine mounted in tailcone run at constant speed (so no CSPU needed). Can’t be paralleled with other alternators.
Static Inverter - Can use battery or DC bus bar power to supply limited 115/200/400/3 AC power.
Hydraulic Drive Unit - Reduced power rating for essential AC. Constant speed hydraulic motors.
Protection checks required for ground power units (GPU)
A system to reject ground power if unsuitable.
- Own aircrafts alternators must be off
- Phase sequence of supply must be correct
- Overvoltage check
Can a 3 phase supply power single phase equipment?
Yes
Split Bus System
- Generator (AC) side
[including connection of generators, APU, external power and AC non-essentials]
2 generators are NOT paralleled. They are connected via a Bus Tie Breaker (BTB) which can close if one alternator fails, to allow the other to take over. The APU can be connected to either bus to take over from a failed alternator or both.
Each generator bus connects to a set of non-essential components and gen 2 bus connects to the external power.
Split Bus System
- DC Side
[including connection of DC essentials, DC non-essentials, vitals and AC essentials]
Gen 1 bus supplies a DC essentials bus (via a TRU), the other a DC non-essential bus. The two DC buses are connected but can be isolated by a DC isolation relay (to prioritise essentials in emergency). The battery bus powers the vital bus called the “HOT BUS” directly and can also connect to the DC essential bus via a battery relay in emergency.
Finally an AC essential bus can be switched via a change over relay between gen 1 AC bus or the DC essential bus (via an inverter).
Split Bus System
- Diagram
Situation when parallel supplies might need to be separated
When dual autopilot autolanding is carried out and two autopilots need separate power supplies.
Parallel Bus Bar system
Generators connect to load bus via GCBs, BTBs connect the load bars to synchronising bus bar.
Frequency, phase and voltage of the AC generators must be in line before they can be synchronised, which will happen automatically.
Parallel bus bar - 4 engine
Generator vs bus bar failure
Hard to distinguish initially, bus bar and generator disconnected. If generator is failed the bus bar can be powered from other sources. Bus bar failure (e.g. short in a component) is more serious as all loads on that bar are off until the problem can be isolated.
Oncoming Source Priority
When a new power source comes on, old one is kicked off.
e.g. initially ground power => APU => engine generators
close down in reverse
Step up or step down transformer
Refers to the voltage (i.e. step up is increase in voltage).
# coils proportional to voltage
Transformer for 3 phase power star or delta?
3 phase transformers will be delta wound
Autotransformer
A single coil with one circuit (primary or secondary) connected over the entire length of the coil, and the other connected half way down the same coil.
Thus one part of the coil carries both the primary and secondary current.
Less expensive than normal transformers as less wire, but lack of isolation between the two circuits means they’re only really used for lighting.
LOW CURRENT
UP AND DOWN
Half wave rectification
Stick a diode in the AC circuit, -ve half of the wave is cut out entirely so there is zero output half of the time.
Full wave rectification
Three phase rectification
Zener Diode
Stops reverse current up to a point, then allows it, but only at a constant voltage. Thus used as a voltage stabilising device.
Transformer Rectifier Unit (TRU)
Combined transformer and rectifier (usually 3 phase rectification) to achieve desired DC voltage.
Synchronous Motor
3 phase AC motor. Bar magnet rotates inside stator with 6 poles with magnetic field created by the 3 phases. Rotating magnetic field causes bar magnet to rotate based on frequency of AC supply.
Synchronous motor
- Rotation speed calculation
RPM = Frequency x 60 / # magnets
So 2 bar magnets rotate at half the speed of 1 bar magnet.
[# magnets = # poles / 2]
Synchronous motor
- Rotor magnetism
Rotor is energised by a DC power source. Will require some brushes and slip rings, but low power level so less problematic than a DC motor.
Synchronous motor
- Usage
Used for RPM measuring (e.g. tacho-generator) as rotation speed synch’s with engine frequency. Can use small 3 phase alternator on engine to create wild frequency source then synchronous motor in the instrument to match the engine RPM based on AC frequency.
Induction motor
Inside the 3 phase stator is a “squirrel cage” rotator (cylindrical laminated iron core centre, surrounded by lots of copper bars connected to end plates).
The rotating magnetic field creates a large current flow in the squirrel cage which creates its own magnetic field and thus rotates.
Squirrel cage is self-contained making this a simple solution.
Induction motor
- Slip speed
The induction motor rotor doesn’t match AC frequency, will typically “slip” by around 5%. Thus the motor is described as asynchronous.
Single phase induction motors
A single phase induction motor can work, but is not self starting like a 3 phase induction motor
Result of fault in 3 phase induction motor
If lightly loaded the motor will likely continue to run at half speed and might make a humming noise.
If it stops it won’t start again.
Changing direction of a 3 phase motor
Need to reverse two of the phases to achieve reverse direction
Flip flop
Semi-conductor which changes state between 1 and 0 when a pulse is fed to it.
A series of them (feeding pulses to the next in the chain) can create a binary counter.
AND
[Note no round terminal on exit]
OR
[Note no round terminal on exit]
[1 + 1 => 1]
NOT
[Round terminal on exit indicates NOT]
NAND
[Round terminal on exit indicates NOT]
NOR
[Round terminal on exit indicates NOT]
[Only 0 + 0 => 1]
XOR
Two most commonly used gate
AND and OR
Transistor
Low current devices (can be damaged by high current).
Made up of Emitter, Base and Collector terminals - current to the base allows flow from collector to emitter.
Can be used as a semi-conductor to act as an automatic switch or an amplifier.
Transistor - PNP vs NPN
PNP is normal version, flow into collector out of emitter.
NPN is backwards, flow goes into the emitter.
PNP - Pointing In (to emitter)
NPN - Not Pointing In
Bit, byte, word
A “bit” is a single 0/1 digit.
A series of bits make up a “word”.
A “byte” is the storage capacity taken to store a single character.
Impact of temperature on life expectancy of electrical components
Increase of 10C reduces lifespan by a half
Purpose of double pole trim switches
To prevent TRIM RUNAWAY.
Both switches required to action trim motors, thus having one switch runaway will not result in trim runaway.
