Main forms of electrical power found on aircraft

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Main forms of electrical power found on aircraft

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    • 28V DC
    • Direct current
    • Maximum current limited to 400 A due to practical DC generator and contracts arcing limits
    • Hence, max power limited to 12 KVA per channel
    • Widely used to power DC motor
      • Suitable for intermittent and reverse operations (easy to turn on and off)
      • E.g. Fuel system control valves, flap and slat lead screw liner actuators
    • Used for low power aircraft lighting (e.g. Cabin), and low power heating and cooling applications
    • Used to charge battery
    • Main forms of electrical power found on aircraft

      • 115 V AC
      • Advantage of AC over DC: 
        • smaller losses, as there is a lower current,
        • Easy to change from low to high voltage
        • And from high to low current
      • Alternating current, 400 Hz
      • Max power up to 100 K VA per channel - a lot more power
      • 3 phase supply
      • Used to power induction motors
        • Suitable for continuous single direction operation - disadvantage ,only goes at one speed in one direction, e.g. Air conditioning
      • Used for high power heating
        • E.g. Anti-ice and de-icing, gallery services, air-con
      • Used to power many avionics systems
        • E.g. Weather radar, cockpit displays, in flight entertainment
      • 26 V AC used for high power external lights
      • Power generation

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          • DC
            • Voltage induced by rotating electromagnets
            • Known as a DC generator
            • AC converted to DC by means of a commutator to achieve half rectification
            • Output smoothed using capacitors leading to a DC voltage with some ripples
          • AC
            • Voltage induced by rotating permanent magnet or electromagnet
            • Known as an alternator
            • Slip rings used instead of commutator - mechanically simpler
            • Older aircraft alternators run at a constant frequency of 400 Hz and constant voltage (CFCV)
            • Requires a constant speed shaft input, and hence complicated continuously variable transmission between prime mover and alternator
            • New systems use variable frequency constant voltage alternators
            • Mechanically much simpler and hence more reliable than CF designs, however requires additional control hardware on AC motors 
            • Primary and secondary electrical power generation systems

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                • 1. Primary power generating system
                  • Power input provided by engine
                • 2. Secondary(stand-by) power generating systems; i.e. If one fails there is another power source so there is a constant voltage
                  • Battery system
                  • Auxiliary power unit (APU)
                  • Ram-air turbine (RAT)
                • Most modern civil aircraft have three independent channels of electrical power generation - 2 primary systems, and 1 secondary system (usually APU based)
                • Critical design case is failure during an automatic landing (no avionics ... No landing) 
                • Industry perspective: aluminium wiring on the A380

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                    • Some 300 of the 500 kilometres of wiring on the A380 uses aluminium instead of the conventional copper as the current conductor, giving a 20% weight saving - wires over long distances, where weighting is important - Al is used
                    • Aluminium wiring technology has been used on all types of Airbus aircraft for wiring or cable where the cross-sectional

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