Electrical Sciences DC Generators

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Author:
ereim
ID:
29623
Filename:
Electrical Sciences DC Generators
Updated:
2010-08-11 14:40:38
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DC Generators
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DC Generators
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  1. The voltage that is delivered across the load
    Applied voltage
  2. Provides the energy conversion in a DC machine.
    Armature
  3. The mechanical conversion from AC to DC at the brushes of a DC machine
    Commutation
  4. Has a field winding in parallel with the generator armature and a field winding in series with the generator armature
    Compound generator
  5. Power lost as heat in the windings; caused by the flow of current through the coils of the DC armature or DC field.
    Copper loss
  6. Induced voltage that acts counter to applied voltage
    Counter-electromotive force (CEMF)
  7. If the two fields of a compound generator are wound so that their magnetic fields aid one another
    Cumulatively-compounded
  8. If the two fields of a compound generator are wound so that their flux fields oppose one another
    Differentially-compounded
  9. Circulating currents within the iron core of the armature. A loss of energy
    Eddy-currents loss
  10. Direct current winding through the field winding flowing through electromagnet conductors to produce a magnetic field. In order for a DC generator to operate properly, the magnetic field must always be in the same direction.
    Field excitation current
  11. The heat produced by this friction from rotation of magnetic domains in the iron of the armature.
    Magnetic hysteresis loss
  12. Caused by bearing friction, brush friction on the commutator, or air friction (windage), which is caused by the air turbulence due to armature rotation.
    Mechanical losses
  13. Provides the rotating element in a DC machine.
    Rotor
  14. Field excitation current that comes directly from the output of the generator
    Self-excited generator
  15. Field excitation current that comes from a separate DC source external to the generator
    Separately excited generator
  16. The field winding of a DC generator is connected in series with the armature
    Series-wound generator
  17. The field winding of a generator is connected in parallel with the generator armature
    Shunt-wound generator
  18. Part of a motor or generator that is stationary. In DC machines, the purpose of the stator is to provide the magnetic field.
    Stator
  19. The voltage that can be measured at the output of the generator
    Terminal voltage
  20. The voltage that can be measured at the output of the generator.
    Terminal voltage
  21. Define counter-electromotive force (CEMF) as it applies to a DC machine
    This induced voltage opposes the applied voltage; it counteracts some of the applied voltage, which reduces the current flow through the armature.
  22. Describe the effects of commutation in a DC generator
    Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction. More simply stated, commutation is the mechanical conversion from AC to DC at the brushes of a DC machine

    1. State the purpose of the following components of a DC machine:

    2. Armature

    3. Rotor

    4. Stator

    5. Field

    • The purpose of the armature is to provide the energy conversion in a DC machine
    • The purpose of the rotor is to provide the rotating element in a DC machine
    • The stator is the part of a motor or generator that is stationary
    • The purpose of the field in a DC machine is to provide a magnetic field for producing either a voltage or a torque.
  23. List the three conditions necessary to induce a voltage into a conductor
      • A conductor
      • A magnetic field
      • Relative motion between the two
  24. Given the direction of the magnetic field and motion of the conductor, determine the direction of the current induced into a conductor using the left-hand rule for generators
  25. Describe how terminal voltage of a DC generator is adjusted
    • DC generator output voltage is dependent on three factors
    • The number of conductor loops in series in the armature, armature speed, and magnetic field strength.

    The strength of the magnetic field, however, can be changed quite easily by varying the current through the field winding. This is the most widely used method for regulating the output voltage of a DC generator
  26. State the basis behind each of the four DC generator ratings
      1. Voltage: Voltage rating of a machine is based on the insulation type and design of the machine.
      2. Current: The current rating is based on the size of the conductor and the amount of heat that can be dissipated in the generator.
      3. Power: The power rating is based on the mechanical limitations of the device that is used to turn the generator and on the thermal limits of conductors, bearings, and other components of the generator.
      4. Speed: Speed rating, at the upper limit, is determined by the speed at which
  27. List the four internal losses found in a DC generator
    • Copper losses
    • Eddy-current losses
    • Hysteresis losses
    • Mechanical losses
  28. Describe the differences in construction between a shunt-wound and a series-wound DC generator with respect to the relationship between the field and the armature
    When the field winding of a generator is connected in parallel with the generator armature, the generator is called a shunt-wound generator

    When the field winding of a DC generator is connected in series with the armature, the generator is called a series-wound generator
  29. Describe the relationship between the shunt and series fields for cumulatively-compounded and differentially-compounded DC generators
    The compound generator has a field winding in parallel with the generator armature (the same as a shunt-wound generator) and a field winding in series with the generator armature (the same as a series-wound generator)

    If the two fields are wound so that their flux fields oppose one another, the generator is said to be differentially-compounded

    If the two fields of a compound generator are wound so that their magnetic fields aid one another, the generator is said to be cumulatively-compounded.
  30. Describe the voltage vs. load current characteristics for a flat-compounded, over-compounded, and under-compounded DC generator.
    In practical compounded generators, the change in output voltage from no-load to full-load is less than 5 percent. A generator with this characteristic is said to be flat-compounded

    For some applications, the series winding is wound so that it overcompensates for a change in the shunt field. The output gradually rises with increasing load current over the normal operating range of the machine. This type of generator is called an over-compounded generator.

    The series winding can also be wound so that it under compensates for the change in shunt field strength. The output voltage decreases gradually with an increase in load current. This type of generator is called an under-compounded generator.

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