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AGK Electrics (Batteries (DC only
EX: Must last 30mins after AC…
AGK Electrics
Batteries
- DC only
- EX: Must last 30mins after AC electrical fail
- Powers starter for small AC, APU for large + Emergency Power
- A chemically driven electron pump.
- Chemical redox, (reduction/oxidation) reaction occurs at 2 electrodes separated by conductive electrolyte
- REDUCTION @ CATHODE, +ve electrode.
- OXIDATION @ ANODE, -ve electrode
- Oxidation (loss of electrons) occurs at the ANODE (-ve electrode) producing a negative charge. Electrons flow to +ve cathode (reduction combines them with the cathode).
- Anode eventually depleted = dead battery
- 1° cell (non-rechargeable): Used mainly for emergency lighting. Eg carbon zinc cells
- 2° cell(Rechargeable): Used as main batteries in AC i.e. Lead-Acid or Nickel-Cadmium
- Lead Acid Battery
- Typical Light AC
- Uses 2.2V cells IN SERIES to give Nominal voltage 12V or 24V
- Electrolyte = Sulphuric Acid diluted with distilled water
- Each cell fully chrgd Voltage = 2.2V, SG = 1.27
- Each cell discharged Voltage = 1.8V , SG = 1.1
- Disadv: Voltage ⬇️as discharge time ⬆️
- Alkaline Battery (Ni-Cd)
- NiCd cell voltage < Lead-Acid ∴ more cells reqd ∴ batteries bigger
- Electrolyte = Potassium Hydroxide
- Each cell fully chrgd Voltage = 1.3V, SG = 1.26 (const)
- Each cell discharged Voltage = 1.1V , SG = 1.26(const)
- Adv: relatively constant Voltage over discharge time + charge quicker
- DisAdv: Prone to thermal runaway (soln = thermal switch)
& Memory effect = growth of crystals ⬇️charging capacity & voltage when repeatedly not fully discharged
- Lithium Ion Batteries:
- Good energy density, no memory effect and holds charge well with time.
Disadv: prone to thermal runaway & fires
- Cross Cell Vent used to waft corrosive gases from chem. reactions
- Non-spill vent blocks vent if inverted ∴no spill
- EX: Electrolyte Spillage
- Acid Batteries: Dilute Sodium Bicarbonate neutralises
- Alkaline Batteries: Boric acid neutralises
- Sealed Battery
- Adv: will not emit corrosive gasses or spill.
- But need specific chargers cos wet-battery chrgr will cause irreversible damage
- Charging
- Assess state of chrg of a batt by c/f 'on-load' & 'off-load' voltages
- Current in opposite directn from which it discharged to restore chemical composition of electrolyte
- Thermal Runaway
- As temp ⬆️while charging, resistance ⬇️ so charging current ⬆️more, then temp more, etc....unless stopped
- Soln = Pulse Charging via THERMAL SWITCH = disconnect, allow to cool, reconnect....charge some more...disconnect...etc..
- Capacity
- Measure total energy it contains. Depends on size & nbr of plates
- Unit = Ampere hours (Ah) e.g. 40Ah battery = 40A for 1h OR 1A for 40h or any combo
- AC Batts capacity checked 3month intervals - replaced if below 80%
- Batts. in SERIES
- Batteries in series ⬆️the voltage (i.e. PD), Current same!!!
- Batts in PARALLEL
- ⬆️capacity i.e. the surface area of plates available, BUT Voltage (PD) same!!!
Wiring
- Dipole (2 wire) Electrical system (aka Insulated Return Cable system???)
- Less susceptible to short cct cos spanner has to touch both supply and rtn
- Open cct can be in supply or rtn lines
- Single Pole/Unipole/Earth Return System
- Use airframe as rtn circuit
- Resistance negligible due to massive X-section of airframe c/f wiring
- Less wiring = less weight = less complex
- More susceptible to short cct (cos wire just has to touch airframe)
- Easier to trace fault (cos half amt of wires)
- Open cct can only be in supply line
- Busbars
- Low resistance distributn pts for loads
- Loads and battery connected in parallel to the Gen(s)
- Hot Battery Busbar: connected directly to battery, cannot be isolated from it. Provide electrical power for emergency eqmnt, fire extinguishers, etc.
- Essential Busbar: provides power for things essential for safe flight i.e radios, nav eqmnt, etc
- Non-Essential Busbar: provides power for non-essential loads i.e. galley, etc..
7/8 DC Generators
- Electromag Induction = induction of EMF (Voltage) by cutting through lines of mag flux with a conductor
- Right Hand Rule for Generators - MFI (Thumb,1st 2nd)
- Genny converts mechanical energy 2 electrical
- DC Generator (Dynamo)
- DC Gen (Dynamo) = rotating armature of conductor (Rotor) in a stationary mag fld (Stator)
- Output of Dynamo = AC, uses SPLIT RING COMMUTATOR to convert AC->DC (only takes +phase of AC). Multiple splits of commutator and windings = multiphase voltage = smoother DC rather than lumpy
- AC Generator (Alternator)
- Rotating mag fld(Rotor) in stationary conductors (Stator)
- Output = AC, may be rectified to become DC alternator
- Voltage Regulator(Carbon Pile): ~variable resistor in series with field coil. Electromag coil senses ⬆️voltage so pulls carbon pile apart =⬆️resistance = current limited = voltage limited
- Circuit Protection - Over-voltage protection unit
- EXAM: Over voltage can only be reset on the ground - it’s permanent until you land and reset!!
- Series Wound Genny: Field windings connected in series with load. As load ⬆️ output voltage ⬆️
- Shunt Wound Genny: Field windings connected in parallel w/ load. As load ⬆️voltage⬇️slightly
- If Nominal battery voltage 12V, genny output 14V, if 24 then genny 28V
- Higher cos ensures battery isn't used to provide power when genny running. Also helps charge battery. ∴output voltage generally 112% of battery
- Genny cut-out prevents battery current feeding back to genny when genny voltage drops below battery
- DC Genny usually excited by residual magnetism. If this mag is lost "flashing the field" can restore resid. mag by breifly passing large current thru fld coil in normal direction of flow
- Parallel DC Gens: Load sharing
- A twin with a genny on each eng has them linked in parallel with a voltage regulator for each genny
- If output differs, current flows from hi output genny to low
- For Gens to loadshare output voltage must =. Done via EQUALISING CCT. This detects out of balance V corrects field current through "equalising coils" in voltage regulators.
- Only voltages equalise NOT currents!
- Loadshedding: shed unnecessary loads if 1 genny down
-Gen. fail shown by AMMETER = 0 or WARNING LIGHT
- A LOADMETER will show total Current from GEn to loads incl. battery charging
- CENTRE ZERO AMMETER fitted btwn battery & busbar shows directn of current flow in/out of battery
- Rectifier converts AC to DC
- Inverter converts DC to AC
Intro 1
- Conventional flow = +ve to -ve (FYI Electron flow = -ve to +ve)
- Coulomb(C) = qty of charge
- Ampere (A) = rate of electron flow (Coulombs passing per sec)
- Voltage(V) = EMF = Potential Diff
- Ammeter in Series, Voltmeter in parallel
- Resistance (R) = opposition to flow
- Specific R = resistance of a cube of material at 0deg
- +ve Temp Coeff (MOST COMMON) = resistance ⬆️ with an ⬆️ in temp
- -ve Temp Coeff = resistance ⬇️ with an ⬆️ in temp
- R determined by Specific R (⍴), Length, Area
- Resistors in series: R = R1 + R2 +R3
- Resistors in parallel: 1/R = 1/R1 + 1/R2 + 1/R3
- Ohms Law
- Kirchoffs Law
- 1st Law: ∑currents in a junction = ∑ currents out
- 2d Law: ∑Voltage drops = supply voltage OR Voltage produced = Voltage consumed
Power
- P = VI ∴ P = I²R or P = V²/R
- Capacitors - Farads
- Stores charge as an electrostatic field
- Capacitance determined by 3 factors:
- 1 = Area of the plates; 2 = distance btwn the plates; 3 = nature of the dielectric
Circuit Protection
- Protect from excessive current
- Rated by Amperes i.e. max current it can carry
- Normally pick slightly higher amp fuse than cct current
- Fuses
- Fuses/Circuit Breakers should NEVER be used as a switch or means of controlling the current in a circuit
- Fuse: cheap, but need to carry spares
- EX: EU-OPS must carry at least 10% fuses per rating or 3 which ever is greater
-> ONLY REPLACE A FUSE ONCE!!!
- Current Limiters (~heavy duty fuses)
- Large capacity 100-350Amps
- Circuit Breakers
- White band = tripped
- Tripped CB only reset unless deemed necessary
- ONLY 1 RESET should be done!!
- No spares needed
- CBs should NOT be used as Switch unless specified e.g. in an emergency procedure!
- Themal CB
- Bimetalic strip, expands if current too hi cos of heat. Trips latch which breaks cct
- Used to protect circuits in the event of PROLONGED Over-Current
- Mag CB
- Faster than Thermal. Electomag coil uses. Over current = EMF = Move latch = break cct
- Used to protect circuit that are too sensitive to tolerate prolonged over-current
- Trip-free CB:Stops returning cct if re-set button held in and fault still exists
- Load shedding: shed load by pulling CBs
Switches
- Rocker: Standard Off/On (like a light switch)
- Push: Click on/off. May have illuminated message (ON, FAULT etc)
- Rotary: Progressive ctrl i.e. temp range OR function selector
- Inductive Proximity – Uses magnetism to detect when desired object is close (e.g. U/C gear)
- Toggle – May have several positions. May be lift to move (+ve Action Swtch), or may be guarded (Guarded Swtch)
Static Electricity
- Occurs due to friction (DRY Air is poor conductor)
- Caused by friction on skin / props, lightning, electrical circuitry or equipment
- St.Elmos Fire: Luminous plasmas by SE discharge
- Static Discharge Wicks: Located on trailing edges. Provide a discharge point for static electricity and lightning. Sacrificial in the case of a lightning strike.
- Bonding Strips: Provide a continuous conductor around the AC for earth return, safe transmission of lightning. It also equalises static charges and reduces interference from static discharges (which normally affects radio reception)
- Screening: To prevent radio interference from electrical components, conductors should be screened with suitable insulating material
- Grnd Earthing: Earthing cable to prevent shocks to personnel, and to prevent sparking (esp when fuelling)
- AC can be grnded to designated pts on the ground. Ensures uniform Potential Difference (PD)
-
DC Motor
- A motor converts electrical energy to mechanical
- Left Hand Rule for Motors - MFI (Thumb,1st 2nd)
- Back EMF
- V is induced in moving conductor in a mag fld in opposite directn to supply V
- Series Wound Motor
- Field connected in SERIES with ARMATURE.
- Hi starting torque = used for starter motors & actuators
- Shunt wound motor
- Field connected in parallel with armature.
- They are constant speed (within 100%) and are used for pumps and fans
- Actuators
- Rotary: used for fuel shut off, air control & hydraulic shut-off valves
- Linear: driven via reduction gearbox. Used for trim tabs
- Brake solenoids/ElectroMag Brake:
- are energised to release actuator brake and de-energised to apply the brake. Fitted in series with fld coil
- Limit Switches
- Control the normal range of travel by changing the directn of the mag fld