Permanent Magnetic Generators in Gas Turbine Engines
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Questions and Answers

What is a key advantage of permanent magnetic generators (PMGs) in comparison to conventional generators?

  • They are not suitable for high-altitude operations.
  • They need constant power supply to maintain field operation.
  • They are larger in size.
  • They require no brushes or slip rings. (correct)

    • How do brushless alternators differ from traditional alternators in terms of cooling?

  • Brushless alternators do not require cooling.
  • Traditional alternators are air cooled.
  • Brushless alternators are water cooled.
  • Brushless alternators are air cooled. (correct)

    • What is the main function of an exciter field in a brushless alternator?

  • To connect the generator to the power source.
  • To induce current into the field coil. (correct)
  • To generate output voltage directly.
  • To cool down the alternator.

    • Why are brushless alternators more efficient at high altitudes compared to traditional alternators?

    <p>Because brushless alternators have no brushes or slip rings that can arc at high altitudes.</p> Signup and view all the answers

    In a brushless alternator, what produces the magnetic flux to start the generator?

    <p>Permanent magnets</p> Signup and view all the answers

    What is the purpose of the main output field in a brushless alternator?

    <p>To generate electrical output.</p> Signup and view all the answers

    What determines the direction of current movement in an electrical circuit?

    <p>Variations in voltage terminal polarities</p> Signup and view all the answers

    What do variations in the negative alternation of a sine wave indicate?

    <p>Voltage terminal polarities have reversed</p> Signup and view all the answers

    What does the amplitude of a sine wave represent?

    <p>Magnitude of voltage</p> Signup and view all the answers

    In which waveform is the alternating current (AC) most commonly identified?

    <p>Sine wave</p> Signup and view all the answers

    How are both current and voltage represented in a sine wave?

    <p>Two vertical axes</p> Signup and view all the answers

    When is current considered to be flowing in a positive direction on a sine wave?

    <p>Above the time axis</p> Signup and view all the answers

    What is the phase difference between the voltages generated in the three windings of a three-phase alternator?

    <p>120°</p> Signup and view all the answers

    What is the relationship between the voltage at terminal C and the phase voltages A and B in a two-phase alternator?

    <p>The voltage at C is 1.414 times the phase voltages</p> Signup and view all the answers

    Why is the two-phase alternator seldom seen in actual use?

    <p>It has fewer advantages over the single-phase alternator</p> Signup and view all the answers

    What is the purpose of the small vector diagram mentioned in the text?

    <p>To show the phase relationship between the three windings</p> Signup and view all the answers

    What is the main advantage of the three-phase alternator over the single-phase type?

    <p>It is more widely used in aircraft</p> Signup and view all the answers

    Why is the schematic diagram of a three-phase stator complex and difficult to understand?

    <p>It shows all the individual coils of each phase</p> Signup and view all the answers

    What is the primary function of the exciter field winding in a brushless alternator?

    <p>To induce voltage into the exciter output winding</p> Signup and view all the answers

    How is the output of the main generator regulated in a brushless alternator?

    <p>By controlling the current to the exciter field winding</p> Signup and view all the answers

    Which component(s) rotate as a unit in a brushless alternator?

    <p>Permanent magnet, exciter output winding, six diodes, and output field winding</p> Signup and view all the answers

    Where are the main output stator windings located in a brushless alternator?

    <p>Wound in slots in the laminated frame of the alternator housing</p> Signup and view all the answers

    What is the purpose of the six silicon diodes in a brushless alternator?

    <p>To rectify the output from the exciter output winding</p> Signup and view all the answers

    Why is there no need to 'flash the field' in a brushless alternator?

    <p>Because the alternator utilizes a permanent magnet</p> Signup and view all the answers

    What is the primary advantage of using a permanent magnet in a generator?

    <p>It eliminates the need for brushes to carry current to the rotating assembly.</p> Signup and view all the answers

    What does a sinusoidal sine wave represent in the context of a generator?

    <p>The value of induced EMF at each instant during a 360° rotation of the coil.</p> Signup and view all the answers

    What distinguishes alternating current (AC) from direct current (DC)?

    <p>AC flows first in one direction and then in the opposite direction, while DC flows in one direction only.</p> Signup and view all the answers

    What is the most common waveform associated with alternating current (AC)?

    <p>Sine (or sinusoidal) wave</p> Signup and view all the answers

    When does a conductor produce a greater potential difference (or induced EMF) in a generator?

    <p>When it is cutting lines of flux quickly.</p> Signup and view all the answers

    What does each cycle of a sine wave represent in the context of a generator?

    <p>Two identically shaped variations in voltage, indicating current movement in opposite directions.</p> Signup and view all the answers

    What is the main difference in speed calculation between a three-phase motor with six poles and a single-phase motor with two poles?

    <p>The calculated speed for the three-phase motor is three times too small.</p> Signup and view all the answers

    What is the primary characteristic of synchronous motors in terms of speed?

    <p>They operate almost exactly at the synchronous speed.</p> Signup and view all the answers

    What is the percentage difference between the full-load speed and the synchronous speed in asynchronous motors called?

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

    What are the two practical methods mentioned in the text to change the rotation speed of AC induction motors?

    <p>Use a frequency converter or use a motor with separate windings for different speeds.</p> Signup and view all the answers

    What type of device are induction motors considered to be?

    <p>Practically fixed-speed devices.</p> Signup and view all the answers

    For applications requiring accurate speed control, what is recommended?

    <p>Using a frequency converter.</p> Signup and view all the answers

    What is a disadvantage of a synchronous motor in terms of starting?

    <p>It cannot be started by applying three-phase AC power to the stator.</p> Signup and view all the answers

    What is the purpose of adding a squirrel-cage winding to the rotor of a synchronous motor?

    <p>To help the rotor start by interacting with the stator's magnetic field.</p> Signup and view all the answers

    What does the heavy copper bars shorted together by copper rings in a squirrel cage winding resemble?

    <p>A squirrel's wheel</p> Signup and view all the answers

    Why does a relatively large current flow in the squirrel cage of a synchronous motor?

    <p>Due to the short circuit in the squirrel cage winding.</p> Signup and view all the answers

    What occurs to the rotor in a synchronous motor when AC is applied to the stator?

    <p>The rotor is repelled first in one direction and then in the other.</p> Signup and view all the answers

    What is the main function of a squirrel-cage winding in a synchronous motor?

    <p>To interact with the rotating field of the stator for starting.</p> Signup and view all the answers

    What is the main reason an induction motor cannot run at synchronous speed?

    <p>There would be no induced voltage in the rotor if the speeds were the same.</p> Signup and view all the answers

    What is the name given to the difference between the speed of the rotating stator field and the rotor speed in an induction motor?

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

    What is the purpose of the small air gap between the rotor and stator in an induction motor?

    <p>To increase the interaction between the stator and rotor fields.</p> Signup and view all the answers

    What is the main force that causes the rotor of an induction motor to move in the same direction as the rotating stator field?

    <p>The induced EMF in the rotor.</p> Signup and view all the answers

    How does increasing the resistance in an induction motor affect its speed?

    <p>It decreases the speed of the motor.</p> Signup and view all the answers

    What is the purpose of using a star-delta arrangement for starting large 3-phase induction motors?

    <p>To limit the starting current and achieve full voltage at speed</p> Signup and view all the answers

    What is the relationship between the torque angle and the load on a synchronous motor?

    <p>The torque angle increases as the load increases</p> Signup and view all the answers

    What is the maximum value of torque that a synchronous motor can develop without losing synchronism?

    <p>Pull-out torque</p> Signup and view all the answers

    What happens to a synchronous motor if the mechanical load is increased beyond the pull-out torque?

    <p>The motor will stop</p> Signup and view all the answers

    In a star-wound stator winding, what is the effective voltage across each winding compared to a delta-wound winding?

    <p>$\frac{1}{\sqrt{3}}$ times the voltage</p> Signup and view all the answers

    What is the purpose of using a star-delta arrangement for starting large 3-phase induction motors?

    <p>To limit the starting current and achieve full voltage at speed</p> Signup and view all the answers

    What is the primary reason for split-phase motors being limited to small sizes?

    <p>The auxiliary winding is only a light winding, limiting the starting torque.</p> Signup and view all the answers

    What is the primary advantage of permanent-split capacitor motors?

    <p>Their running performance and speed can be tailored by capacitor selection.</p> Signup and view all the answers

    How does a resistance-start motor differ from a capacitor-start motor?

    <p>The phase shift is obtained by making the winding impedances unequal.</p> Signup and view all the answers

    What is a common application for permanent-split capacitor motors?

    <p>Fans and blowers with low-inertia loads.</p> Signup and view all the answers

    Why are split-phase motors used in applications where high starting torque is not required?

    <p>They have desirable characteristics despite the low starting torque.</p> Signup and view all the answers

    How can you identify a capacitor start motor?

    <p>By the bulbous protrusion on the frame where the starting capacitor is located.</p> Signup and view all the answers

    Study Notes

    Permanent Magnetic Generators (PMGs)

    • PMGs offer higher efficiency and reduced maintenance compared to conventional generators due to the absence of excitation losses.

    Brushless Alternators

    • Brushless alternators cool differently; they use airflow over their components instead of brushes which require maintenance and wear over time.
    • The exciter field in a brushless alternator generates the magnetic field necessary for operation.
    • Brushless alternators perform more efficiently at high altitudes due to minimized losses from air density changes affecting their cooling systems.

    Electromagnetic Principles

    • In a brushless alternator, magnetism is initially produced by a rotating permanent magnet or by the exciter.
    • The main output field in a brushless alternator converts electrical energy to mechanical energy, maintaining the operational frequency.

    Electrical Circuit Dynamics

    • Current direction in a circuit is determined by the voltage polarity applied to the circuit elements.
    • Negative alternation variations in a sine wave indicate changes in electrical activity and signal transmission.
    • The amplitude of a sine wave represents the peak voltage or current, determining the power capacity of the circuit.

    Alternator Waveforms

    • Alternating current (AC) is most commonly identified by the sine wave.
    • Sine waves visually depict both current and voltage fluctuations over time.
    • Current is considered positive during the upward slope of the sine wave.

    Three-Phase and Two-Phase Alternators

    • In a three-phase alternator, the voltages generated in the windings are phase-shifted by 120 degrees.
    • In a two-phase alternator, terminal C voltage results from the vector sum of phase voltages A and B, but it is rarely used in practice due to complexity and inefficiency.
    • A small vector diagram helps visualize phasing and voltage relationships within the alternator.

    Advantages of Alternator Designs

    • The three-phase alternator surpasses single-phase ones in efficiency and power output, allowing for smoother operation.
    • The complexity of the schematic diagram in a three-phase stator makes it challenging to understand due to multiple windings and phase interactions.

    Exciters and Regulation

    • The exciter field winding's primary function is to supply the necessary voltage to the main alternator.
    • Output regulation in a brushless alternator occurs through active monitoring and adjustments by the control systems.

    Component Functions in Brushless Alternators

    • In brushless alternators, the rotor and exciter typically rotate as a unit.
    • Main output stator windings are stationary and generate electrical output.
    • The six silicon diodes serve to convert AC to DC, allowing usage for various applications.
    • No need to 'flash the field' exists as the permanent magnets maintain the existing magnetic field.

    Generator and Current Characteristics

    • A permanent magnet enhances generator performance by eliminating the need for an external power supply for excitation.
    • A sinusoidal wave relates to alternating current generators and depicts its cyclical nature.
    • AC is characterized by its bidirectionality, contrasting with the unidirectional flow of DC.

    Induction Motor Dynamics

    • A conductor produces greater potential difference when moving within a magnetic field.
    • Each sine wave cycle represents a single period of voltage and current fluctuation in a generator.
    • A three-phase motor with six poles has a different speed calculation compared to a two-pole single-phase motor, affecting overall performance.
    • Synchronous motors maintain constant speed as they operate at synchronous speed, with slight variations divided by the full-load speed and synchronous speed termed slip.

    Motor Control and Characteristics

    • AC induction motors are considered rotating electrical devices.
    • For precise speed control, variable frequency drives (VFDs) are recommended.
    • Synchronous motors face challenges in starting due to their strict operational requirements.
    • Squirrel-cage windings provide torque in synchronous motors by inducing currents via the stator's rotating magnetic field.
    • High current in the squirrel cage results from induced EMF when the stator is energized.

    Induction Motor Operation

    • The rotor experiences movement through the interaction with the moving magnetic field generated by AC supply.
    • The air gap between the rotor and stator allows for effective magnetic coupling and torque generation.

    Torque and Load Relationships

    • The torque angle in synchronous motors is directly related to the mechanical load; higher loads increase the angle.
    • Maximum torque without losing synchronism is known as the pull-out torque, exceeding this leads to loss of synchronization under heavy loads.

    Winding Configurations

    • In star-wound stator configurations, voltage across each winding is less than in delta-wound configurations, thus influencing application in specific systems.
    • Star-delta arrangements are used to manage starting currents and gradually increase voltage in large three-phase induction motors.

    Split-Phase and Capacitor Motors

    • Split-phase motors are limited to small sizes due to design inefficiencies.
    • Permanent-split capacitor motors have advantages in maintaining efficiency and simplicity.
    • Resistance-start motors differ by using resistors to help initiate rotation, compared to capacitor-start motors that utilize capacitors for starting torque.
    • Permanent-split capacitor motors are common in single-phase applications.
    • Split-phase motors are suitable for low-torque applications while capacitor-start motors are identifiable by their additional starting capacitor.

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    Description

    Learn about permanent magnetic generators (PMGs) which do not require power to be supplied to the field, eliminating the need for brushes or slip rings. Discover how PMGs can be designed to operate in either voltage mode or current mode, and their importance in providing power to various components on gas turbine engines.

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