transformer theory part2

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johnbowens
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62650
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transformer theory part2
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2011-12-29 14:25:04
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transformer theory
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NJATC transformer theory principals and applications
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  1. Explain how placing a load on the secondary affects the current flow in the primary? ATP & NJATC, Transformer Theory Principles & Applications, 2006 (NC SET 2) WB PP 6
    secondary flux opposes primary induction, canceling some of the flux. More I in primary.
  2. What is the turns ratio? Explain how it is written and what each of its numbers represents. ATP & NJATC, Transformer Theory Principles & Applications, 2006 (NC SET 2) WB PP6
    • Np= Ep = Is
    • Ns = Es =Ip

    • N=number of turns
    • P=primary
    • S=secondary
    • E=voltage
    • I=current
  3. Transformer mutual induction is also referred to as? ATP & NJATC, Transformer Theory Principles & Applications, 2006 (NC SET 2) WB 8
    coefficient of coupling.
  4. Are taps always located on the secondary side of a transformer? ATP & NJATC, Transformer Theory Principles & Applications, 2006 (NC SET 2) WB 8
    No. They are most commonly on the primary or high voltage winding. ATP & NJATC, Transformer Theory Principles & Applications, 2006 PP 25 (NC SET 2)
  5. The primary is always the high voltage side and the secondary is always the low voltage side? general class discussion. (NC SET 2)
    not always true.
  6. What are the 3 types of transformer most commonly used? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 8 (NC SET 2)
    toroidal, shell, and core type.



  7. An explosion vent is a pipe that extends a few feet above the cover of a transformer and is curved toward the ground at the outlet end of the pipe. It is atleast how big in diameter? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 10 (NC SET 2)
    4 inches.
  8. A transition device between underground cable and overhead lines is called what? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 10 (NC SET 2)
    A pothead.
  9. What must insulate a power circuit where they enter the transformer? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB10 (NC SET 2)
    bushings.
  10. Liquid transformers have a level gauge that has a mark at the correct level for fluid at what temperature point? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 11 (NC SET 2)
    25 degrees C / 77 F.
  11. Two things determine whether a transformer is step up or step down. What are they? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB11 (NC SET 2)
    • Turns ratio.
    • Primary winding's physical relationship to the core.
  12. Transformers are rated in what? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 11 (NC SET 2)
    KVA, versus generators in KW at a PF of 80 percent.
  13. Apparent power is what properties of a circuit? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 12 (NC SET 2)
    Circuit voltage and current. It includes the true power (resistive component) and Reactive power (inductor and capacitor components)
  14. The connected load to the transformer is one of what two ratings and cannot exceed what? The electrician is responsible for this. ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 12 (NC SET 2)
    apparent or true and cannot exceed the full current rating of the transformer.
  15. A generator is always rated in what with a power factor of what? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 12 (NC SET 2)
    KW with a power factor of 80%.
  16. Describe the power triangle using the terms opposite (vertical), adjacent (horizontal), and hypotenuese. ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 12 (NC SET 2)
    • kVAR- opposite
    • KW - adjacent
    • kva - hypotenuese
  17. True power can be produced only when current and voltage are both what? ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 12 (NC SET 2)
    both positive or both negative.
  18. An increase in VAR's have what affect on the power factor? ATP & NJATC, Transformer Theory Principles & Applications, 2006 PP 13 (NC SET 2)
    It causes the power factor to decrease. Think of the triangle. The larger the KVAR the less the cosine of theta. PF is inverse to cosine of theta.
  19. Describe the available voltages between ABC legs of a delta transformer. Draw it out if possible. ATP & NJATC, Transformer Theory Principles & Applications, 2006 WB 21 (NC SET 2)
  20. Cores are construced of what? ATP & NJATC, Transformer Theory Principles & Applications, 2006 PP 26 (NC SET 2)
    Legs and yokes. Vertical legs support the coils and upper and lower yokes connect the legs.
  21. Three phase and two phase core type transformers typically use what leg design? ATP & NJATC, Transformer Theory Principles & Applications, 2006 PP 26 (NC SET 2)
    Three phase use three leg and two phase use two leg designs.
  22. For a step down transformer the _________ coil is wound first and for a step up transformer the _______ coil is wound first. ATP & NJATC, Transformer Theory Principles & Applications, 2006 PP 26 (NC SET 2)
    • Step down- secondary is wound first
    • step up- primary is wound first
    • Remember: the larger coil is wound first

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