AC Alternators

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  1. What is the definition of an AC alternator? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    A synchronous machine that produces AC current.
  2. Portable Alternators are usually called generators, though a generator produces what type of current? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    DC.
  3. A rotor has ________ and an armature has ________. NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    • slip rings.
    • a commutator.
    • Although an armature is in both a generator (DC) and an alternator. The armature is not always the revolving part but always has the voltage induced into it. (AC) The power-producing component of an electrical machine. In a generator, alternator, or dynamo the armature windings generate the electric current. The armature can be on either the rotor or the stator. -Wikipedia
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    • The commutator is the ring divided into segments, The armature by definition does not necessarily turn (in AC) but is the place where generated voltage is collected.Image Upload
  4. The rotor in an AC alternator may also be referred to as? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    an alternator.
  5. What turns the rotor shaft and creates a potential in the stator? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    • Prime mover.
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  6. In a motor the rotor and stator are the core, but in an alternator the core is what? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    • many thin laminated sheets of iron pressed together.
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  7. The number of poles, combined with the ac line frequency (Hertz, Hz), are all that determine the no-load revolutions per minute (rpm) of the motor. So, all four-pole motors will run at the same speed under no-load conditions, all six-pole motors will run at the same speed, and so on.

    The mathematical formula to remember in helping make this calculation is?
    Source: https://www.achrnews.com/articles/84983-how-to-determine-speed-for-an-ac-induction-motor
    The mathematical formula to remember in helping make this calculation is the number of cycles (Hz) times 60 (for seconds in a minute) times two (for the positive and negative pulses in the cycle) divided by the number of poles.

    Therefore, for a 60-Hz system, the formula would be:

    60 x 60 x 2 = 7,200 no-load rpm /number of poles.

    For a 50-Hz system, the formula would be:

    50 x 60 x 2 = 6,000 no-load rpm / number of poles.
  8. In a 3 phase and two phase alternator, the windings consist of coils of copper wire placed how far apart electrically? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    120 degrees in 3 phase or poly phase and 90 in two phase.
  9. Coils in an alternator are replaceable in the field? T or F. NJATC ATP, Motors 2nd edition, AC alternators, AC alternators, PP 56
    Yes, they are usually individually bolted together so that you can replace any one.
  10. The stator in an alternator consists of what? Does it move? NJATC ATP, Motors 2nd edition, AC alternators, PP 56
    • Core and windings. No it is not the moving component.
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  11. A rotor uses _________ to allow current to flow through the rotor circuit. NJATC ATP, Motors 2nd edition, AC alternators, PP 57
    brushes and slip rings.
  12. Slow moving alternators with multiple poles use what to take advantage of low centrifugal force? NJATC ATP, Motors 2nd edition, AC alternators, PP 57

    At what speed are these no longer used?
    salient poles: poles that project out from the rotor, by definition. Salient: prominent or conspicuous

    • 1800 RPM.
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  13. An alternator where the magnetic field is created in the rotor defines what kind of alternator? NJATC ATP, Motors 2nd edition, AC alternators, PP 58
    revolving-field alternator.
  14. In a revolving field alternator, what kind of power is used to supply power to the rotor, resulting in the need for what standard components associated with this kind of power? NJATC ATP, Motors 2nd edition, AC alternators, PP 58
    D.C. power to the rotor which results in the need to use a brushless exciter.
  15. A brushless exciter is what? NJATC ATP, Motors 2nd edition, AC alternators, PP 58
    a brushless exciter is a rectifier assembly mounted on the main rotor shaft along with the exciter generator, on a revolving field alternator.
  16. An exciter generator is what? NJATC ATP, Motors 2nd edition, AC alternators, PP 58
    an assembly consisting of a small three phase alternator used to supply current to an alternator rotor, on a revolving field alternator
  17. In a revolving field alternator, the exciter rotor spins as a result of what? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    • In this motor the exciter rotor spins as a result of DC field supplied by a stationary set of coils that provide the flux. The exciter may or may not spin, and is referred to as the exciter. NJATC, AC Theory 2009, PP 40
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  18. A fixed magnetic field is created in the stator, with the rotor turning within the stator, defines what? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    revolving-rotor alternator, using slip rings and brushes.
  19. A revolving rotor alternator supplies power how? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    A revolving rotor alternator supplies AC power through the rotor slip rings and brushes.
  20. What is the downfall of a revolving rotor alternator and thus the reason they are rarely used today? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    Revolving-rotor alternators work well for lower voltages and currents but cannot handle large voltages and currents because of the difficulty of insulating the slip rings.
  21. A voltage that is too low can cause what? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    inductive loads to draw excessive current.
  22. The three factors controlling the output voltage of an alternator are what? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    • length of wire
    • speed of rotor
    • strength of the magnetic field
  23. Voltage in an alternator is directly proportional to what? NJATC ATP, Motors 2nd edition, AC alternators, PP 59
    The simplest answer is the number of turns.  It is also dependent upon the voltage regulator, amount of magnetism, and speed of the rotor, and Tortola length of wire.
  24. Define field windings. NJATC ATP, Motors 2nd edition, AC alternators, PP 59

    In a AC alternator, the field windings allow what?
    electromagnetic coils that provide the DC for the exciter.

    They allow the voltage regulator to control the field at the field winding and thus the output voltage of the alternator.
  25. Most of the alternators in service today are what? NJATC ATP, Motors 2nd edition, AC alternators, PP 60
    self excited meaning they get the power for the exciter from their own stator.
  26. The frequency output of an AC alternator must be kept _______. NJATC ATP, Motors 2nd edition, AC alternators, PP 61
    constant.
  27. The two factors controlling the output frequency of an alternator are what? NJATC ATP, Motors 2nd edition, AC alternators, PP 60
    number of poles and speed of the rotor
  28. AC generator (actually an alternator) speed is calculated using what formula? NJATC ATP, Motors 2nd edition, AC alternators, PP 60
    S= (120 x F)/ poles is wrong... this formula should assume HZ x 60 seconds x 2/ number of poles. A very old description of alternating current systems sometimes give the frequency in terms of alternations per minute, counting each half-cycle as one alternation; so 12,000 alternations per minute corresponds to 100 Hz.
  29. Each rotation in a single-phase AC alternator produces what? NJATC ATP, Motors 2nd edition, AC alternators, PP 61
    one phase of electricity. Or a complete cycle.
  30. Three phase AC power is generated by? NJATC ATP, Motors 2nd edition, AC alternators, PP 63
    rotating three coils through the magnetic field.
  31. The three phases in an AC alternator are electrically 120 degrees apart because? NJATC ATP, Motors 2nd edition, AC alternators, PP 64
    Because the rotor windings are 120 degrees mechanically apart.
  32. The prime mover in an alternator is what? NJATC ATP, Motors 2nd edition, AC alternators, PP 64
    The power source used to create the relative motion between the coil and the magnetic field.
  33. Alternators are rated in what and at what power factor? NJATC ATP, Motors 2nd edition, AC alternators, PP 64

    For this reason the KVA rating of an alternator is equal to what?
    KW at a power factor of 80 percent.

    125 percent of the KW rating.
  34. AC alternators may be cooled by what? NJATC ATP, Motors 2nd edition, AC alternators, PP 65
    Air, water, or hydrogen.
  35. Alternators are placed in parallel or series to carry added load? NJATC ATP, Motors 2nd edition, AC alternators, PP 65
    Parallel.
  36. Common methods for manual synchronization include? NJATC ATP, Motors 2nd edition, AC alternators, PP 65
    the use of a synchrosocope or the use of synchronizing lights.
  37. With automatic synchronization, a controller monitors what? NJATC ATP, Motors 2nd edition, AC alternators, PP 67
    Frequency, phase angle, and voltage of the incoming source and the busbar. This must be done before a generator can be switched into the circuit.
  38. Typically how much extra output power is recommended for a generator? NJATC ATP, Motors 2nd edition, AC alternators, PP 69
    25 to 50 percent.  Generators are provided with an 80% Power Factor (PF) for this reason.  They are over-rated on purpose.
  39. A single phase alternator and a three phase alternator use how many coils to generate a single or 3 phases?NJATC ATP, Motors 2nd edition, AC alternators, PP 70 Summary
    Single phase is one coil and three phase is three coils.
Author:
johnbowens
ID:
116940
Card Set:
AC Alternators
Updated:
2018-01-28 00:05:57
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AC alternators
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Description:
4th year JATC IBEW class: source ISBN 978-0-8269-1982-3
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