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
ZS = VOC / ISC
Where,
VOC is the open-circuit voltage per phase
ISC is the short circuit current
Calculation:
Given,
VOC(Line) = 1730 V
⇒ VOC (Phase) = (VOC(line) / √3) = (1730 / √3) = 998.82 V
ISC = 200 A
ZS = VOC / ISC
ZS = 998.82 / 200 = 4.99 ≈ 5 Ω