GATE EE 2025 Set 1 — Question 43
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Electrical Machines → Three-Phase Transformers → Connections & Vector Groups
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Question
The transformer connection given in the figure is part of a balanced 3-phase circuit where the phase sequence is “abc”. The primary to secondary turns ratio is 2:1. If , then the relationship between and will be
Correct answer
(A) I_A I_(ad) = 12√(3) and I_(ad) lags I_A by 30^.
Solution
Connection. The primary is star connected, so the primary line current is also the primary winding current of that limb. The secondary is delta connected, so , , are the secondary winding (phase) currents and , , are the secondary line currents.Step 1 - ampere-turn balance on one limb. With ,and is in phase with .Step 2 - delta line current. At the secondary terminal , KCL givesFor the sequence take , then and (consistent with ). HenceSo the secondary line current is times the winding current and lags it by .Step 3 - combine.Thereforeand, since is in phase with , lags by .Options (C) and (D) invert the magnitude ratio, and option (B) has the wrong sign of the phase shift.Answer: (A)
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