AGK Electrics 5

Aircraft General Knowledge - electrics chapter 5

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  • Created by: Gul
  • Created on: 05-06-09 09:17

Faraday's Law

Generator: Mechanical energy to electrical energy

Faraday's law

EMF produced around closed path proportional to rate of change of mag flux thru any surface bounded by that path.

(elec current will be induced in any closed circuit when mag flux through a surface bounded by conductor changes - whether field itself changes in strength or conductor is moved through it.)


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Lenz's law

Lenz's law

Direction of any induced EMF is such that it opposes change that created the EMF (electromotive force)

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Commutator

  • Slip rings replaced by commutator to make output DC.
  • Mechanical rectifier mounted on armature shaft comprising cylindrical arrangement of copper bars with brushes.
  • Brushes ride on bars and carry current from armature.
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Diodes

  • Semi-conductor device with high resistance in one direction and low resistance in the other.
  • A bridge rectified comprising of a no. of diodes can provide DC power from AC source.
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Transformer Rectifier Unit (TRUs)

  • The speed of a frequency wild generator varies with engine speed
  • On an aircraft utilising AC as primary power supplies, batteries are charged in flight from a TRU.
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Inverter

  • DC ---> AC power
  • Usually feeds instruments and flight system equipment (Avionics, instruments, FMC)
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DC Generators - types

  • In order that DC generators will achieve equal load sharing when operating in parallel, necessary for voltages to be equal.
  • 3 types: shunt, series, compound


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Excitation

  • Generator armature rotated by engine but initially by battery power.
  • In constant speed DC gen providing constant output voltage, if electrical load increases, voltage reg will increase the intensity of excitation current.
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Series + Compound Wound Gens

Series:

  • In series with load so as load increases, curent flowing through field coil increases + field excitation increases.
  • Thick copper wire (can take more current)
  • Good for more load

Compound wound

  • Combo of series and parallel
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Shunt Gens

Shunt:

  • field coil parallel with armature.
  • conditions to be met activiate a shunt gen are:

1)presence of perm magnet

2)min rotation speed

  • Good for fast speed
  • To reverse direction of rotation of shunt-type DC elec motor, it is necessary to reverse polarity of either stator or rotar.
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Voltage Regulator

  • Adjusts field current
  • Regulated to higher voltage than battery to ensure battery is charged
  • Voltage reg of DC gen connected in series with shunt field coil.
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Alternators

Essential diff between aircraft AC alternators & DC generators (dynamos) is that induced (output) windings of alternators are fixed (stator) & dynamos have a fixed inductor field coil.



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