Synchronous Machines Overview
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Questions and Answers

What happens to the synchronous motor's power factor if the field current IF is changed while keeping the load constant?

  • The output power decreases
  • The power factor changes (correct)
  • The motor stops
  • The output power increases

    • A synchronous capacitor operates under load.

    False

    What are the two main types of induction motors?

    Cage rotor and Wound rotor

    When an induction motor is operating at synchronous speed, current flows through the rotor: _____

    <p>none</p> Signup and view all the answers

    Match the following components with their characteristics:

    <p>Damper windings = Used for starting a synchronous motor as an induction motor Synchronous motor = Operates with no load for power factor correction Slip = Difference between synchronous speed and actual rotor speed Pullout torque = Maximum allowable torque of an induction motor</p> Signup and view all the answers

    What occurs when an induction motor is rotated faster than synchronous speed?

    <p>It becomes a generator</p> Signup and view all the answers

    Which type of DC generator has a better terminal characteristic?

    <p>Cumulatively compounded DC generator</p> Signup and view all the answers

    A differentially compounded DC generator has a better terminal characteristic than a cumulatively compounded DC generator.

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

    The frequency of the rotor current in an induction machine is different from that of the stator frequency when slip is zero.

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

    What is the main drawback of a differentially compounded DC generator?

    <p>It has a poor terminal characteristic.</p> Signup and view all the answers

    How can the stator winding resistance of an induction machine be determined?

    <p>By performing a DC test</p> Signup and view all the answers

    If the load torque is removed in a shunt DC motor after it has started up, the machine would operate at ______ speed.

    <p>no-load</p> Signup and view all the answers

    Match the method of speed control with its application:

    <p>Adjusting field resistance = Used above rated speed Adjusting terminal voltage = Used below rated speed Armature voltage control = Used below rated speed Field resistance control = Used above rated speed</p> Signup and view all the answers

    What happens if the field is removed in a shunt DC motor after it has been started up?

    <p>The machine would speed up and try to reach infinite speed</p> Signup and view all the answers

    Armature reaction decreases the speed of a DC shunt motor.

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

    For what applications are series DC motors typically used?

    <p>Applications requiring very high torques.</p> Signup and view all the answers

    What is equivalent rotor resistance R2 in an induction motor model derived from?

    <p>RLR - R1</p> Signup and view all the answers

    The magnetic field caused by the armature current in a DC machine is known as armature reaction.

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

    List two ways to reduce armature reaction in DC machines.

    <p>Using interpoles, Using compensating windings</p> Signup and view all the answers

    The two types of armature windings suitable for DC machines are lap and ______.

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

    Match the following types of DC generators with their description:

    <p>Separately excited dc generator = Field current is supplied separately from the armature Shunt dc generator = Field winding is connected in parallel with the armature Series dc generator = Field winding is connected in series with the armature Cumulatively compounded dc generator = Series and shunt windings aid each other</p> Signup and view all the answers

    Which of the following factors does NOT affect the induced voltage in a machine?

    <p>The current in the armature</p> Signup and view all the answers

    Using lap armature windings is ideal for applications requiring high voltage and low current.

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

    What is the effect of armature reaction on the output terminal voltage V T of a separately excited DC generator?

    <p>Decreases voltage</p> Signup and view all the answers

    Which of the following is a cumulative compounded DC motor?

    <p>Combines features of shunt and series motors</p> Signup and view all the answers

    A differentially compounded DC motor has a main advantage of stability under varying loads.

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

    What is the primary initial source of field flux in a shunt DC generator when it is first turned on?

    <p>Residual flux in the poles</p> Signup and view all the answers

    What type of material is best suited for the poles of a permanent magnet DC motor?

    <p>Ferromagnetic material with high residual flux</p> Signup and view all the answers

    A brushless DC motor is a _______ motor that relies on a power electronic interface.

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

    A shunt DC generator can build voltage without any residual magnetic flux present.

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

    Match the type of compounded DC motor with its description:

    <p>Cumulatively Compounded = Combines features of shunt and series motors Differentially Compounded = Becomes unstable with increased load</p> Signup and view all the answers

    List one reason why a shunt DC generator might fail to build up voltage during starting.

    <p>There may be no residual magnetic flux in the generator.</p> Signup and view all the answers

    To flash the field of a shunt DC generator, you must connect the field directly to an ___ source.

    <p>external dc</p> Signup and view all the answers

    What is the main drawback of a differentially compounded DC motor?

    <p>It speeds up and becomes unstable as load increases</p> Signup and view all the answers

    Permanent magnet DC motors can operate with high torque similar to conventional DC motors.

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

    Which of the following is NOT a method to control the voltage of a shunt DC generator?

    <p>Change the load resistances</p> Signup and view all the answers

    What happens to the two field winding polarities when the direction of power changes in a cumulatively compounded DC machine?

    <p>They will be different.</p> Signup and view all the answers

    Why is the terminal characteristic of a series DC generator considered worse than that of a shunt DC generator?

    <p>It saturates more easily under heavier loads, resulting in lower output voltage.</p> Signup and view all the answers

    An overcompounded cumulatively compounded generator has ___ at full-load compared to no-load.

    <p>VTfull-load &gt; VTno-load</p> Signup and view all the answers

    Match the type of DC generator to its characteristic aspect:

    <p>Shunt DC Generator = Best voltage regulation Series DC Generator = Worse voltage regulation Cumulatively Compounded Generator = Variable voltage performance based on load Undercompounded Generator = VTfull-load &lt; VTno-load</p> Signup and view all the answers

    Which of the following losses are not considered when accounting for the difference between mechanical power applied to the rotor and electrical power produced in a synchronous generator?

    <p>Stray losses</p> Signup and view all the answers

    A 60 Hz synchronous generator can be operated at 50 Hz without any changes.

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

    What is one method to determine the synchronous reactance XS in a synchronous generator?

    <p>Perform an open circuit test and get the internal generated voltage EA.</p> Signup and view all the answers

    The voltage generated by a synchronous generator can be increased by increasing the speed of rotation and by increasing the ______.

    <p>field current</p> Signup and view all the answers

    What happens to the terminal voltage compared to the internal generated voltage in a synchronous generator?

    <p>Terminal voltage is less than internal generated voltage</p> Signup and view all the answers

    The terminal voltage versus field current curve flattens due to rotor saturation.

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

    Name one way to start a synchronous motor.

    <p>Reduce the speed of the stator magnetic field.</p> Signup and view all the answers

    Match the terms related to synchronous generators with their descriptions:

    <p>Open circuit test = Measures internal generated voltage EA Short-circuit test = Measures short-circuit current IASc Synchronous speed = Speed at which the rotor locks with the magnetic field Field saturation = Limits voltage increase at high field current</p> Signup and view all the answers

    Study Notes

    Synchronous Machines

    • Losses in Synchronous Generators: Not including stray losses, losses include copper losses, core losses, friction losses, and windage losses. These account for the difference between applied mechanical power and generated electrical power.

    • Operating at Different Frequencies: A 60 Hz synchronous generator can operate at 50 Hz, but the operating voltage must be derated to 83.3% of its original value.

    • Determining SynchronousReactance is the opposition that inductors and capacitors offer to the flow of alternating current, impacting the efficiency of electrical circuits. (Xs): An approximate method involves these steps:

      • Perform an open-circuit test to get internal generated voltage (EA) from the open-circuit characteristic at a given field current.
      • Perform a short-circuit test to get short-circuit current (IAsc) from the short-circuit characteristic at the same field current.
      • Calculate Xs by dividing EA (from step 1) by IAsc (from step 2).

    • Determining Winding Resistance: An approximate winding resistance can be determined by applying a DC voltage to the stationary windings and measuring the resulting current flow.

    • Increasing Voltage in Synchronous Generators: Voltage can be increased by either increasing rotor speed or increasing field current.

    Other Synchronous Machine Information

    • Terminal Voltage vs. Internal Voltage: In a synchronous generator, the terminal voltage (VΦ) is typically less than the internal generated voltage (Ea).

    • Terminal Voltage vs. Field Current: The terminal voltage vs. field current curve flattens as field current increases. This is because the rotor saturates, meaning further increases in field current produce little increase in magnetic field strength.

    Induction Machines

    • Types of Induction Motors: The two main types are cage rotor and wound rotor induction motors.

    • Rotor Current at Synchronous Speed: At synchronous speed (zero slip), no current flows through the rotor. This is because no flux is being cut in the rotor.

    • Rotor Current Frequency: The rotor current frequency (f₁) is related to the stator frequency (fe) by the slip (s). The formula is f₁ = sfe

    • Pullout Torque: The maximum allowable torque of an induction motor.

    • Induction Motor as a Generator: If connected to another induction machine, and spun slightly faster than synchronous speed, the motor becomes a generator.

    • Determining Stator Winding Resistance: Stator winding resistance can be determined by performing a DC test.

    • Losses in Induction Machines: Not including stray losses, losses include stator copper losses, core losses, rotor copper losses, friction losses, and windage losses.

    DC Machines

    • Armature Reaction: Armature reaction is the magnetic field produced by current flowing in the armature (rotor). In DC machines, this reaction results in a weaker field.

    • Types of DC Generators: Three types include the separately excited, shunt, and series DC generators. Others are cumulatively and differentially compounded generators.

    • Controlling DC Generator Voltage: Voltage can be controlled by changing rotor speed or adjusting the field current.

    • Armature Reaction Effect on Voltage: Armature reaction decreases the output terminal voltage of a separately excited DC generator because it leads to less field current and thus a reduced induced voltage.

    • Various methods for controlling the speed and voltage of various DC machines.

      • Using interpoles (for armature reaction reduction).
      • Using compensating windings (to counteract armature reaction).

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    Description

    This quiz explores key concepts related to synchronous machines, focusing on losses in synchronous generators, operating frequencies, and methods for determining synchronous reactance and winding resistance. Understanding these principles is essential for electrical engineering students dealing with generator systems.

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