Synchronous Generator Basics
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Questions and Answers

What is the type of winding in a rotor of a synchronous generator?

  • Single-phase winding
  • DC excited winding (correct)
  • 3-phase winding
  • AC excited winding

    • What is the characteristic feature of a revolving-armature alternator?

  • A stationary magnetic field and a rotating armature
  • A rotating magnetic field
  • A rotating armature through a stationary magnetic field (correct)
  • A stationary armature

    • Why are revolving-armature alternators not generally used?

  • Because of their high power rating
  • Because of their low power rating
  • Because they require slip rings and brushes (correct)
  • Because they have a stationary magnetic field

    • What is the type of rotor used in a 2-pole alternator?

    <p>Cylindrical Rotor</p> Signup and view all the answers

    What is the purpose of the field excitation voltage in a revolving-field alternator?

    <p>To generate the rotating magnetic field</p> Signup and view all the answers

    What is the type of alternator found in small-scale power ratings?

    <p>Revolving-armature alternator</p> Signup and view all the answers

    What is the advantage of a revolving-field alternator?

    <p>It can connect the voltage to the load directly</p> Signup and view all the answers

    What type of rotor is used in a 4-pole alternator?

    <p>Salient Rotor</p> Signup and view all the answers

    What is the major advantage of having a stationary armature winding in an alternator?

    <p>It is easier to insulate and can withstand higher voltages</p> Signup and view all the answers

    What is the characteristic of the stator in a single-phase alternator?

    <p>It has two pole groups</p> Signup and view all the answers

    In a three-phase alternator, what is the phase displacement between the voltage induced in any one phase and the other two?

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

    What is the relationship between the line voltage and phase voltage in a star-connected three-phase alternator?

    <p>Line voltage is 1.73 times the phase voltage</p> Signup and view all the answers

    What is the advantage of a rotating DC field in an alternator?

    <p>High speed of rotation is possible</p> Signup and view all the answers

    In a single-phase alternator, how are the armature conductors connected?

    <p>Connected in series</p> Signup and view all the answers

    What is the characteristic of the rotor in a single-phase alternator?

    <p>It has two pole groups, adjacent poles being of opposite polarity</p> Signup and view all the answers

    What is the purpose of connecting one point from each winding to form a neutral in a three-phase alternator?

    <p>To create a wye connection</p> Signup and view all the answers

    What is the relationship between the line and phase currents in a delta connection?

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

    What is the advantage of using three-phase connections in alternators?

    <p>To provide two possible connections: wye and delta</p> Signup and view all the answers

    How does the number of poles affect the speed of the rotor?

    <p>A two-pole machine must rotate at twice the speed of a four-pole machine</p> Signup and view all the answers

    What is the relationship between the frequency of the AC voltage and the speed of the rotor?

    <p>The frequency increases as the speed of the rotor increases</p> Signup and view all the answers

    What is the angle of rotation required for one electrical cycle in a two-pole alternator?

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

    How many electrical cycles are produced by a four-pole alternator in one mechanical rotation?

    <p>Two electrical cycles</p> Signup and view all the answers

    What is the formula to calculate the speed of the rotor?

    <p>Ns = f * s / 2P</p> Signup and view all the answers

    What is the relationship between the rotor poles and the electrical cycles?

    <p>One electrical cycle for two rotor poles</p> Signup and view all the answers

    What is a phasor diagram typically used to represent in a synchronous generator?

    <p>Voltage and current within one phase</p> Signup and view all the answers

    What type of power factor results in a larger internal generated voltage EA?

    <p>Lagging power factor</p> Signup and view all the answers

    What is the formula to calculate the electrical line frequency produced by the alternator?

    <p>(nrp)/120</p> Signup and view all the answers

    What are the three quantities that must be determined to describe a synchronous generator model?

    <p>Field current, armature resistance, and synchronous reactance</p> Signup and view all the answers

    What is the purpose of dividing the speed of the rotor by 60 in the formula?

    <p>To change from revolutions per minute to revolutions per second</p> Signup and view all the answers

    What is the purpose of the open circuit test in a synchronous generator?

    <p>To measure the internal generated voltage EA</p> Signup and view all the answers

    What happens to the flux when the saturation point is reached in a synchronous generator?

    <p>It increases much slower</p> Signup and view all the answers

    What are the two types of voltage drops that occur in an alternator?

    <p>Resistive and inductive drops</p> Signup and view all the answers

    What is measured in a short circuit test in a synchronous generator?

    <p>Armature current IA</p> Signup and view all the answers

    What is the term used to describe the change in voltage from no-load to full-load in an alternator?

    <p>Voltage regulation</p> Signup and view all the answers

    How is a constant output voltage maintained in an alternator?

    <p>By varying the field strength</p> Signup and view all the answers

    What is the plot of armature current vs. field current in a synchronous generator known as?

    <p>Short-circuit characteristic</p> Signup and view all the answers

    What is a key difference between a DC generator and a synchronous generator?

    <p>DC generators do not have a synchronous speed, while synchronous generators do</p> Signup and view all the answers

    What is the purpose of rotating the generator at the rated speed in both open circuit and short circuit tests?

    <p>To ensure consistent test results</p> Signup and view all the answers

    What is a structural difference between medium and large scale synchronous generators and DC generators?

    <p>Synchronous generators have the armature on the stator, while DC generators have the armature on the rotor shaft</p> Signup and view all the answers

    What is a key component that DC generators require, but synchronous generators do not?

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

    Study Notes

    Basic Parts of a Synchronous Generator

    • A synchronous generator consists of a rotor with a DC excited winding and a stator with a 3-phase winding in which the AC emf is generated.
    • There are two types of rotors: salient or projecting poles and cylindrical or round rotors.

    Types of Rotors

    • Salient pole rotors have a 2-pole or 4-pole design and are typically used in low-power applications.
    • Cylindrical rotors have a 2-pole design and are typically used in high-power applications.

    Principle of Operation

    • In a rotating armature alternator, the armature rotates through a stationary magnetic field, and the generated AC voltage is brought to the load through slip rings and brushes.
    • In a revolving-field alternator, the stationary armature has a rotating magnetic field, and the generated voltage can be connected directly to the load without using slip rings and brushes.
    • The majority of alternators use the revolving-field design.

    Advantages of Stationary Armature and Rotating Field

    • The stationary armature is easier to insulate, with voltages up to 30 kV or more.
    • It is easier to collect current from the stationary parts of the generator.
    • Slip ring voltages are lower.
    • The simple construction allows for high-speed rotating DC fields, resulting in high efficiency.

    Single-Phase Alternator

    • A single-phase alternator has all armature conductors connected in series.
    • The stator has two poles, and the winding is wound in two distinct pole groups, both poles being wound in the same direction around the stator frame.
    • The rotor also consists of two pole groups, adjacent poles being of opposite polarity.

    Two-Phase Alternator

    • A two-phase alternator has two single-phase windings spaced to produce a 90-degree phase shift between the two phases.

    Three-Phase Alternator

    • A three-phase alternator has three single-phase windings spaced to produce a 120-degree phase shift between the three phases.
    • The voltage waveforms generated across each phase are phase-displaced by 120 degrees from each other.

    Three-Phase Stator Connection

    • The three-phase stator can be connected in a star (wye) or delta configuration.
    • In a star connection, the line voltage is 1.73 times the phase voltage, and the line and phase currents are equal.
    • In a delta connection, the line voltage is equal to the phase voltage, but the line current is 1.73 times the phase current.

    Frequency of AC Voltage

    • The frequency of the AC voltage generated by an alternator depends on the number of poles and the speed of the rotor.
    • The frequency can be calculated using the formula: f = (nrp)/120, where nr is the speed of the rotor in rpm, p is the number of poles, and f is the electrical line frequency.

    Voltage Regulation

    • The output voltage of an alternator varies with the load due to resistive and inductive voltage drops.
    • The voltage regulation is the change in voltage from no-load to full-load.
    • A constant output voltage can be maintained by varying the field strength as required by the load.

    Comparison with DC Generator

    • DC generators do not have a synchronous speed and operate at whatever speed is needed to obtain the desired output voltage.
    • Synchronous generators rotate and generate EMF at the same rotor frequency, whereas DC generators generate DC voltage with no frequency.
    • Structurally and mechanically, synchronous generators are built with the field windings on the rotor shaft and the armature winding built on the stator, whereas DC generators are built with the armature on the rotor shaft connected to the load through a commutator and the field windings in the stator.

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    Description

    Learn about the basic components and types of rotors in synchronous generators, including salient and cylindrical rotors.

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