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If, in a 3-ϕ alternator, a field current of 50 A produces a full-load armature current of 200 A on short-circuit and 1730 V on open circuit, then its synchronous impedance is ________ Ω
1. 3.46
2. 8.66
3. 5
4. 4

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Correct Answer - Option 3 : 5

Synchronous Impedance of alternator:

Open Circuit Test:

  • In the open-circuit test for determining the synchronous impedance, the alternator is running at the rated synchronous speed, and the load terminals are kept open.
  • This means that the loads are disconnected, and the field current is set to zero.


Short Circuit Test:

  • The field current should first be decreased to zero before starting the alternator.
  • Each ammeter should have a range greater than the rated full load value.
  • The alternator is then run at synchronous speed.
  • Same as in an open circuit test that the field current is increased gradually in steps and the armature current is measured at each step.
  • The field current is increased to get armature currents up to 150% of the rated value.


The synchronous impedance of the alternator is given by



VOC is the open-circuit voltage per phase

ISC is the short circuit current



VOC(Line) = 1730 V

⇒ VOC (Phase) = (VOC(line) / √3) = (1730 / √3) = 998.82 V

ISC = 200 A


ZS = 998.82 / 200 = 4.99 ≈ 5 Ω 

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