GATE Electrical Engineering - Complete Master Library

 

📘 Transformer OC & SC Tests – Complete Explanation

Open Circuit (OC) and Short Circuit (SC) tests are used to determine transformer parameters without loading it fully. Very important for GATE numericals.

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🔹 1. Open Circuit Test (OC Test)

Purpose:

• Find core loss
• Find Rc and Xm

Performed on: Low voltage side (for safety). Secondary kept open. Measurements:

• V₀ – Applied voltage
• I₀ – No-load current
• W₀ – Power input

Core loss:

P_core = W₀

Power factor:

cosφ₀ = W₀ / (V₀ I₀)

Calculate: I_c = I₀ cosφ₀ I_m = I₀ sinφ₀ Then:

Rc = V₀ / I_c Xm = V₀ / I_m

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🔹 2. Short Circuit Test (SC Test)

Purpose:

• Find copper loss
• Find R_eq and X_eq

Performed on: High voltage side (usually). Secondary short-circuited. Low voltage applied until rated current flows. Measurements:

• V_sc – Applied voltage
• I_sc – Current
• W_sc – Power

Copper loss at full load:

P_cu = W_sc

Equivalent impedance:

Z_eq = V_sc / I_sc

Equivalent resistance:

R_eq = W_sc / I_sc²

Equivalent reactance:

X_eq = √(Z_eq² − R_eq²)

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🔹 3. Example

OC Test Data: V₀ = 200 V I₀ = 2 A W₀ = 150 W Core loss:

P_core = 150 W

SC Test Data: V_sc = 20 V I_sc = 10 A W_sc = 100 W Equivalent impedance:

Z_eq = 20 / 10 = 2 Ω

Equivalent resistance:

R_eq = 100 / (10²) = 1 Ω

Equivalent reactance:

X_eq = √(2² − 1²) = √3 = 1.73 Ω

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🎯 GATE Important Points

• OC test gives core loss
• SC test gives copper loss
• SC voltage very small (5–10% rated)
• Use these parameters for regulation & efficiency

OC Test → Core Loss SC Test → Copper Loss