Electrical Engineering: Synchronous Generators
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Questions and Answers

What is the reason for ignoring the effect of salient-pole shape on the operation of synchronous machines?

  • All machines are assumed to have cylindrical rotors (correct)
  • It is not a significant factor in synchronous generator operation
  • To simplify the analysis of the machine
  • Because it is negligible in most practical cases

    • What is the terminal voltage in the synchronous generator electrical equivalent circuit?

  • EO
  • E
  • V (correct)
  • XS

    • What is the synchronous reactance (XS) composed of?

  • Armature resistance and reactance
  • Internal generated voltage and reactance
  • Armature leakage reactance and reactance of armature reaction (correct)
  • Terminal voltage and induced e.m.f

    • What type of load is represented in the phasor diagram of a synchronous generator at UPF?

    <p>Purely resistive load</p> Signup and view all the answers

    What is the significance of the phasor diagram of a synchronous generator at lagging power factor?

    <p>It represents an inductive load</p> Signup and view all the answers

    What is the internal generated voltage produced in one phase represented by?

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

    What is the effect of armature reaction allowed in the induced e.m.f?

    <p>It modifies the induced e.m.f.</p> Signup and view all the answers

    What type of rotor shape is assumed in the analysis of synchronous machines?

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

    What happens to the rotor velocity of a synchronous generator to maintain frequency for a stable system?

    <p>It remains constant</p> Signup and view all the answers

    What is the difference between the input mechanical power and the output power of a synchronous generator?

    <p>Losses of the machine</p> Signup and view all the answers

    What is the assumption made to simplify the phasor diagram?

    <p>The armature resistance is negligible</p> Signup and view all the answers

    What is the term for the maximum power indicated by the equation?

    <p>Static stability limit</p> Signup and view all the answers

    What is the relationship between the input mechanical power and the output power of a synchronous generator?

    <p>Input power is always greater than output power</p> Signup and view all the answers

    What are the losses in a synchronous generator composed of?

    <p>Copper losses, core losses, and stray losses</p> Signup and view all the answers

    What type of load is connected to the synchronous generator, as per the assumption?

    <p>Lagging load</p> Signup and view all the answers

    What is the expression for the input mechanical power of a synchronous generator?

    <p>Pm = τs ωm</p> Signup and view all the answers

    What is the primary condition for connecting an alternator to the infinite bus-bars?

    <p>The r.m.s line voltages of the incoming alternator and the infinite bus-bar must be equal.</p> Signup and view all the answers

    How can the frequency of the incoming alternator be controlled?

    <p>By controlling the speed of the prime mover driving the alternator.</p> Signup and view all the answers

    What is the purpose of a synchroscope?

    <p>To measure and display the frequency difference and phase shift angle between two power systems.</p> Signup and view all the answers

    What happens when two unsynchronized AC power systems are connected together?

    <p>High currents flow, which can severely damage equipment.</p> Signup and view all the answers

    What is the purpose of the three lamps method?

    <p>To check the phase sequence of the incoming alternator and the bus-bar.</p> Signup and view all the answers

    What happens if the lamps brighten in succession in the three lamps method?

    <p>The systems have opposite phase sequence.</p> Signup and view all the answers

    How is the process of paralleling a new generator to the line done in large generators?

    <p>Automatically by a computer.</p> Signup and view all the answers

    What is the purpose of automatic voltage regulation (AVR)?

    <p>To adjust the generated voltage of the incoming alternator.</p> Signup and view all the answers

    What happens to the armature current when the load on a synchronous generator is increased at a lagging power factor?

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

    What is the characteristic of an infinite bus in a power system?

    <p>Its voltage and frequency do not vary regardless of the power drawn from it</p> Signup and view all the answers

    What happens to the terminal voltage and frequency of the system when the governor mechanism set point of a generator is increased?

    <p>Both the terminal voltage and frequency remain constant</p> Signup and view all the answers

    What is the voltage regulation of an alternator defined as?

    <p>The change in terminal voltage from no-load to full-load</p> Signup and view all the answers

    What happens to the terminal voltage when the load on a synchronous generator is increased at a leading power factor?

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

    What is the effect of increasing the mechanical input power to the prime mover on the power angle of the generator?

    <p>It increases the power angle</p> Signup and view all the answers

    What is the condition of the generator when it is operating at a slightly leading power factor?

    <p>It is supplying negative reactive power</p> Signup and view all the answers

    Why should a stationary alternator not be connected to live bus-bars?

    <p>Because the induced e.m.f. is not zero</p> Signup and view all the answers

    How can the generator be adjusted to supply some reactive power to the system?

    <p>By increasing the field current of the alternator</p> Signup and view all the answers

    What is the effect of increasing the load on a synchronous generator at unity power factor?

    <p>The terminal voltage decreases</p> Signup and view all the answers

    What is the role of the governor set point in a generator operating alone?

    <p>It controls the operating frequency of the power system</p> Signup and view all the answers

    What is the condition for paralleling an alternator with infinite bus-bars?

    <p>The alternator must be synchronized</p> Signup and view all the answers

    What is the sign of voltage regulation for a lagging load?

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

    What is the effect of increasing the field current on the reactive power output of the generator?

    <p>It increases the reactive power output</p> Signup and view all the answers

    What is the condition of the system when a generator is operating alone and supplying the system loads?

    <p>The real and reactive power supplied by the generator are equal to the load demand</p> Signup and view all the answers

    What is the effect of increasing the load on a synchronous generator at a lagging power factor?

    <p>The armature reaction voltage increases</p> Signup and view all the answers

    Study Notes

    Synchronous Generator: Equivalent Circuit

    • A synchronous generator has an internal generated voltage (Eo) which is produced in one phase.
    • The induced e.m.f (E) is the induced voltage after allowing the effect of armature reaction.
    • The terminal voltage (V) is the voltage at the generator terminals.
    • Synchronous reactance (XS) is the sum of armature leakage reactance (XL) and reactance of armature reaction (XAR).

    Phasor Diagram of a Synchronous Generator

    • The phasor diagram shows the relationship between the internal generated voltage (Eo), terminal voltage (V), and current (I) of a synchronous generator.
    • The phasor diagram is different for different power factors (UPF, lagging, and leading).
    • At unity power factor (UPF), the current is in phase with the terminal voltage.
    • At lagging power factor, the current lags the terminal voltage.
    • At leading power factor, the current leads the terminal voltage.

    Power Relationships for S.G.

    • Not all mechanical power (Pin) going into a synchronous generator becomes electrical power (Pout) out of the machine.
    • The difference between input power and output power represents the losses of the machine.
    • The input mechanical power is the shaft power in the generator.
    • The rotor velocity must remain constant to maintain frequency for a stable system.

    Power and Torque in Synchronous Generators

    • The maximum generated power occurs when the internal generated voltage (Eo) is equal to the terminal voltage (V).
    • The maximum power is called the static stability limit of the generator.

    The Effect of Load Changes on Operating of Synchronous Generator

    • When the load increases at the same power factor, the armature current increases, and the armature reaction voltage is larger than before.
    • At lagging power factor, the terminal voltage (V) decreases as the load increases.
    • At unity-power factor, the terminal voltage (V) decreases as the load increases.
    • At leading power factor, the terminal voltage (V) increases as the load increases.

    Voltage Regulation of Alternator

    • Voltage regulation is defined as the change in terminal voltage from no-load to full-load divided by full-load voltage.
    • For lagging loads, VR would be positive.
    • For leading loads, VR would be negative.
    • For unity loads, VR would be positive.

    Conditions for Paralleling Alternator with Infinite Bus-bars

    • The r.m.s line voltages of the incoming alternator and the infinite bus-bar must be equal.
    • The frequency of the generated voltage of the incoming alternator must be slightly higher than the frequency of the running system.
    • The incoming alternator and the infinite bus-bar must have the same phase sequence.

    Operation of Generators in Parallel with Large Power System

    • When a generator is connected in parallel with a large system, the generator will be essentially “floating” on the line, supplying a small amount of real power and little or no reactive power.
    • The behavior of alternators connected to an infinite bus-bar is controlled by the governor mechanism and the field excitation.
    • Increasing the mechanical input power to the prime mover will not change the speed ultimately, but it will increase the power angle.
    • Increasing the field current causes increasing the reactive power output of the generator.

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    Description

    This quiz covers the equivalent circuit of synchronous generators, a crucial topic in electrical engineering.

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