Other Generator Controls and Monitoring Devices

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    Constant Speed Drive

      • The purpose of the Constant Speed Drive (CSD) is to take rotational power from the engine and, no matter the engine speed, turn the generator at a constant speed
      • This is necessary because the generator output must be 400Hz
      • CSD Operation
        • The engine turns the CSD which uses a differential assembly and hydraulic pumps to turn the generator

      Integrated Drive Generator

      • Another method of regulating the generator speed is with the use of an Integrated Drive Generator (IDG)
      • An IDG is simply a CSD and generator combined into one unit
      • There are two ways to mount the IDG:
        • Co-axially
        • Side-by-side

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    Engine Driven AC Generators

      • Each of the engines on an aircraft drives an AC generator
      • The power produced by these generators is used in normal flight to supply the entire aircraft with power

      APU Power

      • Most often the APUs power is used while the aircraft is on the ground during maintenance or for engine starting
      • However, most aircraft can use the APU while in flight as a backup power source
        • One exception to this is the B272, which only allows APU operation in the ground

      External Power

      • External power may only be used with the aircraft on the ground
      • This system utilizes a Ground Power Unit (GPU) to provide AC power through an external plug on the nose of the aircraft
      • GPUs may be either portable or stationary units

      Ram Air Turbine

      • Some aircraft are equipped with Ram Air Turbines, or “RATs”
      • These may be used, in the case of a generator or APU failure, as an emergency power source
      • When necessary, the RAT may be deployed to be used as an AC power source

      Aircraft Batteries

      • The aircraft’s nickel cadmium battery is final source of backup power
      • The battery provides 28 VDC
      • It is also possible to change the 28 VDC into 115 VAC 400Hz with the use of a static inverter
      • When using the battery, power usage is limited by the short life of the battery

      Electrical Power System Components

      • AC Generator
      • Constant Speed Drive
      • Integrated Drive Generator
      • Transformer Rectifier Unit
      • Generator Control Unit

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      Aircraft Electrical Systems

        • The function of the aircraft electrical system is to generate, regulate and distribute electrical power throughout the aircraft
        • New-generation aircraft rely heavily on electrical power because of the wide use of electronic flight instrument systems

        Electrical Power Uses

        • Aircraft electrical power is used to operate:
        • Aircraft Flight Instruments
        • Essential Systems
        • Passenger Services

        Electrical Power Uses (cont.)

        • Essential power is power that the aircraft needs to be able to continue safe operation
        • Passenger services power is the power that used for:
          • Cabin lighting
          • Operation of entertainment systems
          • Preparation of food

        Power Used

        • Aircraft electrical components operate on many different voltages both AC and DC
        • However, most of the systems use:
          • 115 VAC @ 400 Hz
          • 28 VDC
        • 26 VAC is also used in some aircraft for lighting

        Power Sources

        • There are sever different power sources on large aircraft to be able to handle excessive loads, for redundancy, and for emergency situations.
        • These power sources include:
          • Engine driven AC generators
          • Auxiliary Power Units
          • External power
          • Ram Air Turbines

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        Ideal Diode Model.

          • The ideal small signal diode conducts current in one direction (forward-conducting) and blocks current in the other direction (reverse-blocking).
          • Signal Diodes are used in a wide variety of applications such as a switch in rectifiers, limiters, wave-shaping circuits.

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          Practical Diode Model (model 1, 2).

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          • The practical model 1 take into consideration the actual voltage drop across the diode that makes it ON. However, the model still neglects the diode internal resistance that is considered zero.
          • Signal Diodes are used in a wide variety of applications such as a switch in rectifiers, limiters, wave-shaping circuits.
          • The practical model 2 take into consideration the actual voltage drop across the diode that makes it ON.
          • Also, it takes into consideration the diode internal resistance.

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          Junction Diode Symbol and Static I-V Characteristics

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          • Is : is the saturation current.
          • VT : is a constant votage called the thermal voltage.
          • n : is a constant has a value between 1 and 2 depending on the material and physical structure of the diode.
          • If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
          • By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased.

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          There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:

          1. Zero Bias – No external voltage potential is applied to the PN-junction.
          2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN-junction width.
          3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN-junction width.

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            Semiconductor Basics .

              • Semiconductors have intermediate energy gape. In their pure (intrinsic) state, semiconductors are neither good conductors nor good insulators.
              • Semiconductors do not used in their pure state because at room temperature very few electrons can jump the energy gap to the conduction band and become free electrons that causing conduction current.
              • To effectively increase the conductivity of semiconductors we have to add impurities to intrinsic semiconductors to increase the free electron in conduction band.

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              There are two types of semiconductors; N-type and P-type.

              • In N-type semiconductors, penta-valent impurity atoms (atoms with five electrons in the valence band) are added to intrinsic semiconductors. Four electrons of these five are bonding with different four atoms of semiconductors substance leaving one free electron.
              • While for P-type semiconductors, tri-valent impurity atoms (atoms with three electrons in the valence band) are added to intrinsic semiconductor. These three electrons are bonding with three semiconductor’s atoms. The fourth semiconductor adjacent atom find no valence electron to share but instead imaginary hole (same but positively charge as electron).
              • When the N and P-type semiconductor materials are first joined together a very large density gradient exists between both sides of the junction so some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions.
              • However, because the electrons have moved across the junction from the N-type silicon to the P-type silicon, they leave behind positively charged donor ions (ND) on the negative side and now the holes from the acceptor impurity migrate across the junction in the opposite direction into the region were there are large numbers of free electrons.

               

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              • As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions (NA), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the junction is known as diffusion.
              • The significance of this built-in potential across the junction, is that it opposes both the flow of holes and electrons across the junction and is why it is called the potential barrier.
              • In practice, a PN junction is formed within a single crystal of material rather than just simply joining or fusing together two separate pieces.
              • Electrical contacts are also fused onto either side of the crystal to enable an electrical connection to be made to an external circuit.
              • Then the resulting device that has been made is called a PN junction Diode or Signal Diode.

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              • If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
              • By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased.

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              Semiconductor Basics

                • Electrons that are in orbits further from the nucleus have higher energy & are less tightly bound to the atom.
                • The force attraction between the positive charged nucleus & the negatively charged electron decreases with increasing distance from the nucleus.
                • This outermost shell is known as the valence shell & electrons in this shell are called valence electronics.

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                • When a valence electron absorb energy from a heat or light source it can actually escape from the outer shell and the resulting is a positive charged (more protons than electrons) atom is called positive ion i.e. H+. The escaped valence electron is called a free electron.
                • When a free electron loses energy and falls into the outer shell of an atom, the atom becomes negatively charged (more electrons than protons) and is called a negative ion i.e. H– .
                • Materials can be categorized into conductors, semiconductors or insulators by their ability to conduct electricity.
                • Conductors: Metals conduct electricity easily because there is no band gap since the conduction overlaps the valence band.
                • Semiconductors: The band gap is small enough that electron that absorb thermal energy can bridge the gap to the conduction band.
                • Insulators: Very large band gap between the valence & conduction bands makes it hard for electrons to bridge the gap.

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                • Two types of semi conductive materials are silicon and germanium. Both have four valence electrons.
                • The valence electrons in germanium are in 4th shell while the ones in silicon are in 3rd shell, closer to the nucleus.
                • This means that the germanium valence electrons are at higher energy levels than those in silicon. Thus require a smaller amount of additional energy to escape from the atom.
                • This property makes germanium more unstable than silicon at high temperatures, which is the main reason silicon, is the most widely used semi conductive material.

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