GATE 2014 EE (EE1) – Question 23
A star-connected 400 V, 50 Hz, four-pole synchronous machine has open-circuit line voltage 400 V rms at field current 2.3 A, and short-circuit phase current 10 A rms at field current 1.5 A. Find per-phase synchronous impedance at rated voltage (Ω).
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Correct answer: 14.5 to 15.5
Explanation
The short-circuit characteristic is linear in field current, so at If=2.3A, Isc=10×2.3/1.5=15.3333A. At that field current the open-circuit phase voltage is400/√3=230.940V. Hence Zs=230.940/15.3333=**15.0613Ω**. Use equal field current for the two test values.