Alternator Types and Waveforms
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Questions and Answers

What is the main advantage of using a permanent magnetic generator (PMG) over a brushed generator?

  • PMGs can operate in both voltage and current modes.
  • PMGs require less maintenance due to the lack of brushes and slip rings. (correct)
  • PMGs are more efficient at high altitudes.
  • PMGs can provide power to ignition exciters and FADECs.
  • Why are brushless alternators more efficient at high altitudes compared to brushed alternators?

  • Brushless alternators have a more robust design that is better suited for high altitude conditions.
  • Brushless alternators have a more efficient cooling system.
  • Brushless alternators have a higher power output.
  • Brushless alternators do not experience brush arcing, which is a common problem at high altitudes. (correct)
  • What is the purpose of the exciter field in a brushless alternator?

  • To control the output voltage of the generator.
  • To regulate the current flow to the main output field.
  • To provide the initial magnetic flux to start the generator.
  • To induce voltage into the main output field. (correct)
  • What is the role of the permanent magnets in a brushless alternator?

    <p>They provide the initial magnetic flux to start the generator.</p> Signup and view all the answers

    Which of the following is NOT a key component of a brushless alternator?

    <p>Brushes and slip rings</p> Signup and view all the answers

    What is the primary purpose of a brushless alternator in a gas turbine engine?

    <p>To provide power to the ignition exciter.</p> Signup and view all the answers

    What is the key difference between a revolving armature type alternator and a revolving field type alternator?

    <p>The rotor is the field in the revolving armature type, while the rotor is the armature in the revolving field type.</p> Signup and view all the answers

    Why is the rotating armature configuration generally not used to supply electric power in large quantities?

    <p>The rotating armature configuration requires slip rings and brushes to connect the armature output voltage and current to the load.</p> Signup and view all the answers

    What is the key similarity between a DC generator and a revolving-armature alternator?

    <p>Both have a rotating armature and a stationary magnetic field.</p> Signup and view all the answers

    What is the main advantage of the revolving field type alternator over the revolving armature type?

    <p>The revolving field type does not require slip rings and brushes.</p> Signup and view all the answers

    Which type of alternator is generally used to supply electric power in large quantities?

    <p>Revolving field type</p> Signup and view all the answers

    What is the purpose of the slip rings in a revolving-armature alternator?

    <p>To connect the armature output voltage and current to the load.</p> Signup and view all the answers

    What happens when the rotor poles are opposite the windings of Phase B in the alternator?

    <p>Maximum voltage is induced in Phase B</p> Signup and view all the answers

    How much rotor rotation corresponds to one quarter of a cycle in the alternator?

    <p>90°</p> Signup and view all the answers

    What is the relationship between the voltages induced in Phase A and Phase B in the two-phase alternator?

    <p>They are 90° out of phase</p> Signup and view all the answers

    What is the purpose of the B1-A2 connection made internally in the two-phase three-wire alternator?

    <p>To provide a new output voltage</p> Signup and view all the answers

    What effect does connecting one end of B1 to one end of A2 have on the alternator's output?

    <p>Results in a new output voltage, C, larger than A or B</p> Signup and view all the answers

    What is unique about the output connections in a two-phase, three-wire alternator compared to a standard alternator?

    <p>Only three connections are brought out from the stator</p> Signup and view all the answers

    When the waveform is below the time axis, what is said to be flowing in a negative direction?

    <p>Current</p> Signup and view all the answers

    What is the total number of degrees in a complete cycle?

    <p>360</p> Signup and view all the answers

    At the initial position of 0, what is happening to the armature conductors?

    <p>They are moving parallel to the magnetic field</p> Signup and view all the answers

    As the armature rotates from 90 to 180, what happens to the induced voltage?

    <p>It decreases from a maximum positive value to zero</p> Signup and view all the answers

    What is the direction of the induced voltage as the armature rotates from 180 to 270?

    <p>Negative</p> Signup and view all the answers

    What type of alternator produces a single, continuously alternating voltage?

    <p>Single-phase alternator</p> Signup and view all the answers

    What is the relationship between the line voltage and phase voltage in a wye-connected three-phase system?

    <p>The line voltage is 1.73 times the phase voltage.</p> Signup and view all the answers

    What is the relationship between the line current and phase current in a wye-connected three-phase system?

    <p>The line current is equal to the phase current.</p> Signup and view all the answers

    How is the neutral connection used in a wye-connected three-phase system?

    <p>The neutral connection is used to supply single-phase voltage.</p> Signup and view all the answers

    What is the significance of the 'wye' connection in a three-phase system?

    <p>The windings appear as the letter 'Y' without the neutral connection.</p> Signup and view all the answers

    What is the purpose of the $ ext{sqrt}{3}$ factor in the relationship between line and phase voltages in a wye-connected three-phase system?

    <p>To convert from phase to line voltage.</p> Signup and view all the answers

    What is the main advantage of a wye-connected three-phase system over a delta-connected system?

    <p>Ability to supply single-phase loads.</p> Signup and view all the answers

    Study Notes

    Permanent Magnetic Generators (PMGs)

    • No requirement for power supply to the field, eliminating the need for brushes or slip rings
    • Can be designed to operate in voltage mode (open circuit) or current mode (closed circuit)
    • Crucial components for providing power to ignition exciter, FADECs, or other accessories on gas turbine engines

    Brushless Alternators

    • Used in large jet-powered aircraft
    • No current flow between brushes or slip rings, making them efficient at high altitudes
    • Consist of three separate fields: permanent magnetic field, exciter field, and main output field
    • Permanent magnets induce voltage into the armature, carrying current to a generator control unit (GCU)

    Alternator Types

    • Revolving armature type: rotor is the armature, stator is the field
    • Revolving field type: rotor is the field, stator is the armature (used almost exclusively in aircraft)

    Revolving-Armature Alternator

    • Similar to DC generator construction
    • Armature rotates in a stationary magnetic field
    • AC output is brought to the load unchanged via slip rings
    • Found only in low-power alternators, not used to supply electric power in large quantities
    • Disadvantages: requires strong construction, slip rings, and brushes

    Two-Phase Three-Wire Alternator

    • Three connections brought out from the stator
    • Electrically equivalent to a larger diagram
    • Two-phase alternator connected in this manner is called a two-phase, three-wire alternator

    Alternator Phase Types

    • Single-Phase Alternators: produce a single, continuously alternating voltage
    • Three-Phase Alternators: three-phase AC voltage is produced by three separate windings
    • 3-Ø Star and Delta Connections: windings connected together to form a star (wye) connection or delta connection

    Star (Wye) Connection

    • Three-phase alternator leads connected together to form a star connection
    • Neutral connection brought out to a terminal for single-phase load supply
    • Single-phase voltage available from neutral to each phase (A, B, C)
    • Line voltage is the vector sum of individual phase voltages
    • Line and phase currents are the same (equal)

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    AC Generators PDF

    Description

    Learn about the different types of alternators - revolving armature type and revolving field type, and understand the waveforms produced by each. This quiz covers the rotation and waveform production of three-phase alternators.

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