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During the short-circuit test of a synchronous machine, which of the following are accounted for in the short-circuit test loss? (i), I2R loss in the armature winding due to the flow of short-circuiting (AC) current (ii) Local core loss caused by armature leakage flux (iii) Core loss due to resultant air-gap flux. (iv) Windage and friction loss

During the short-circuit test of a synchronous machine, which of the following are accounted for in the short-circuit test loss? (i), I2R loss in the armature winding due to the flow of short-circuiting (AC) current (ii) Local core loss caused by armature leakage flux (iii) Core loss due to resultant air-gap flux. (iv) Windage and friction loss Correct Answer (i), (ii), (iii) and (iv)

Short Circuit test:

  • The machine is driven at rated synchronous speed and armature terminals are short-circuited through an ammeter.
  • Now, field current gradually increased from zero, until the short-circuit armature current reached its safe maximum value, equal to about 125 to 150% of the rated current.
  • Latter readings may be taken in a short time to avoid armature overheating.
  • A graph is drawn between short-circuit current isc and field current if as shown in the figure.
    Additional Information

    Open Circuit Test:

    • This test is used for determining the synchronous impedance.
    • The alternator is running at the rated synchronous speed, and the load terminals are kept open.
    • The excitation current may be increased to get 25% more than the rated voltage i.e. up to 125% of the rated voltage.
    • A graph is drawn between the open circuit phase voltage Eg and the field current If. The curve so obtained is called Open Circuit Characteristic (O.C.C).
    • The linear portion of the O.C.C is extended to form an air gap line.

     

    The Open Circuit Characteristic (O.C.C) and the air gap line is shown in the figure below

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