Neets Module 2 Assignment 5

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CVN68
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165441
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Neets Module 2 Assignment 5
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2012-08-09 16:20:42
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  1. What is meant by "transformer action?"
    The transfer of energy from one circuit to another circuit by electromagnetic induction.
  2. What are, the three basic parts of a transformer?
    • Primary winding
    • Secondary winding
    • Core.
  3. What are three materials commonly used as the core of a transformer?
    • Air
    • soft iron
    • steel.
  4. What are the two main types of cores used in transformers?
    Hollow-core type; shell-core type.
  5. Which transformer windings are connected to an ac source voltage and to a load, respectively?
    Primary to source; secondary to load.
  6. A transformer designed for high-voltage applications differs in construction in what way from a transformer designed for low-voltage applications?
    Additional insulation is provided between the layers of windings in the high-voltage transformer.
  7. What is meant by a "no-load condition" in a transformer circuit?
    A voltage is applied to the primary, but no load is connected to the secondary.
  8. What is meant by "exciting current" in a transformer?
    Exciting current is the current that flows in the primary of a transformer with the secondary open (no load attached).
  9. What is the name of the emf generated in the primary that opposes the flow of current in the primary?
    Self-induced or counter emf.
  10. What causes a voltage to be developed across the secondary winding of a transformer?
    The magnetic lines generated by the current in the primary cut the secondary windings and induce a voltage into them.
  11. What is the phase relationship between the voltage induced in the secondary of an unlike-wound transformer and the counter emf of the primary winding?
    In phase. Remember, the cemf of the primary is 180 degrees out of phase with the applied voltage. The induced voltage of the secondary of an unlike-wound transformer is also 180 degrees out of phase with the primary voltage.
  12. What is "leakage flux?"
    Lines of flux generated by one winding which do not link the other winding.
  13. What effect does flux leakage in a transformer have on the coefficient of coupling (K) in the transformer?
    It causes K to be less than unity (1).
  14. Does 1:5 indicate a step-up or step-down transformer?
    Step up.
  15. A transformer has 500 turns on the primary and 1500 turns on the secondary. If 45 volts are applied to the primary, what is the voltage developed across the secondary? (Assume no losses)
    Es = 135V (Es = 45V x 1500 turn / 500 turns)
  16. A transformer has a turns ratio of 7:1. If 5 volts is developed across the secondary, what is the voltage applied to the primary? (Note: ES is given, what is EP ?)
    Ep = 35V (Ep = 7 turns x 5V / 1 turn)
  17. A transformer has 60 volts applied to its primary and 420 volts appearing across its secondary. If there are 800 turns on the primary, what is the number of turns in the secondary?
    St = 5600 turns (St = 420V x 800 turns / 60V)
  18. A transformer with a turns ratio of 1:3 has what current ratio?
    3:1 current ratio (Turns ratio and current ratio have an inverse relationshop) (Np/Ns)
  19. A transformer has a turns ratio of 5:1 and a current of 5 amperes flowing in the secondary. What is the current flowing in the primary? (Assume no losses)
    Ip = 1A (Ip = Ns x Is / Np)
  20. What is the mathematical relationship between the power in the primary (PP) and power in the secondary (Ps) of a transformer?
    Ps = Pp - Pl
  21. Name the three power losses in a transformer.
    • Copper loss
    • eddy-current loss
    • hysteresis loss.
  22. The input power to a transformer is 1,000 watts and the output power is 500 watts. What is the efficiency of the transformer, expressed as a percentage?
    Eff = 50% (Eff = Pout / Pin x 100)
  23. Why should a transformer designed for 400 hertz operation not be used for 60 hertz operation?
    The inductive reactance at 60 hertz would be too low. The resulting excessive current would probably damage the transformer.
  24. List five different types of transformers according to their applications.
    • Power transformer
    • Autotransformer
    • Impedance-matching transformer
    • Audio-frequency transformer
    • Radio-frequency transformer
  25. The leads to the primary and to the high-voltage secondary windings of a power transformer usually are of what color?
    • Primary leads-black
    • secondary leads-red.
  26. What is the cause of most accidents?
    Carelessness.
  27. Before working on electrical equipment containing capacitors, what should you do to the capacitors?
    Discharge them by shorting them to ground.
  28. When working on electrical equipment, why should you use only one hand?
    To minimize the possibility of providing a path for current through your body.

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