How it works
The equivalent circuit referred to the primary replaces the actual transformer with a single series branch (R1 + a²R2) + j(X1 + a²X2) and a shunt branch R0 ∥ jX0 across the input. Short-circuit (SC) test supplies reduced voltage (typically 5–8% of rated) to the HV side while LV is shorted; at rated current the wattmeter gives total copper loss Psc. For a 10 kVA unit the SC test might give Vsc = 18 V, Isc = 43.5 A, Psc = 450 W — from these, Zeq = 18/43.5 = 0.414 Ω, Req = 450/43.5² = 0.238 Ω, Xeq = 0.331 Ω.
Key points to remember
The approximate equivalent circuit neglects R0 and X0, moving the shunt branch to the input terminals — valid because no-load current is only 2–5% of full-load current. Voltage regulation VR = (V2NL − V2FL)/V2FL × 100%; for a resistive load regulation is low, for a lagging load it is highest, and for a leading load it can be negative. Regulation formula: VR ≈ εR cosφ + εX sinφ where εR = per-unit resistance drop and εX = per-unit reactance drop. Examiners routinely ask for regulation at 0.8 pf lagging.
Exam tip
Every Anna University exam includes a numerical where you calculate regulation and efficiency from OC and SC test data — practice the full procedure of finding Req, Xeq from SC test and R0, X0 from OC test before your exam night.