- What Are OC and SC Tests?
- Open Circuit Test (No-Load Test)
- Procedure of Open Circuit Test
- Calculations from OC Test
- Short Circuit Test (Impedance Test)
- Procedure of Short Circuit Test
- Calculations from SC Test
- Determining Efficiency and Regulation
- Comparison: OC Test vs SC Test
- Advantages of These Tests
- FAQs
- Related Articles
What Are Open Circuit and Short Circuit Tests?
Open circuit (OC) and short circuit (SC) tests are indirect methods used to determine the equivalent circuit parameters, efficiency, and voltage regulation of a transformer without actually loading it. These tests require very little power — only enough to supply the losses — making them practical for testing large power transformers where full-load testing would be expensive and wasteful.
Together, the OC test gives core loss parameters (R₀, X₀) and the SC test gives winding impedance parameters (R₀₁, X₀₁), which fully define the transformer's equivalent circuit.
Open Circuit Test (No-Load Test)
The open circuit test determines the iron (core) losses and the no-load branch parameters of the transformer equivalent circuit. It is performed on the low-voltage (LV) side while the high-voltage (HV) side remains open.
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| Open Circuit Test — Circuit Diagram |
Procedure of Open Circuit Test
- Connect a voltmeter (V), ammeter (A), and wattmeter (W) on the LV side of the transformer.
- Keep the HV side completely open (no load connected).
- Apply rated voltage at rated frequency to the LV winding.
- Record the readings: V₁ (rated voltage), I₀ (no-load current), and W₀ (core loss).
Since the secondary is open, no current flows in the HV winding. The primary draws only a small no-load current I₀ (typically 2–5% of rated current). Copper loss in the primary is negligible (I₀²R₁ ≈ 0), so the wattmeter reads only core losses (hysteresis + eddy current losses).
Calculations from OC Test
Iw = I₀ × cos φ₀ (working/core-loss component)
Iμ = I₀ × sin φ₀ (magnetising component)
R₀ = V₁ / Iw (core-loss resistance)
X₀ = V₁ / Iμ (magnetising reactance)
These parameters R₀ and X₀ form the shunt branch of the transformer equivalent circuit.
Short Circuit Test (Impedance Test)
The short circuit test determines the copper losses at full load and the series impedance parameters (R₀₁, X₀₁) of the equivalent circuit. It is performed on the HV side while the LV side is short-circuited.
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| Short Circuit Test — Circuit Diagram |
Procedure of Short Circuit Test
- Short-circuit the LV winding using a thick copper conductor or ammeter.
- Connect a voltmeter (V), ammeter (A), and wattmeter (W) on the HV side.
- Apply a reduced voltage from a variable supply to the HV side.
- Gradually increase voltage until the ammeter reads full-load current (I₁ rated).
- Record: Vsc (short-circuit voltage), Isc (full-load current), and Wsc (copper loss).
The applied voltage Vsc is typically only 5–10% of rated voltage. At such low voltage, the flux in the core is very small, so core losses (proportional to flux²) are negligible. The wattmeter reads only full-load copper losses.
Calculations from SC Test
R₀₁ = Wsc / Isc² (equivalent resistance)
X₀₁ = √(Z₀₁² − R₀₁²) (equivalent reactance)
These parameters define the series branch of the equivalent circuit. To refer them to the LV side, divide by the transformation ratio squared (K²).
Determining Efficiency and Voltage Regulation
Once both tests are complete, transformer performance at any load can be predicted:
Where x = fraction of full load (0 to 1)
% Regulation = (Isc × (R₀₁ cos φ ± X₀₁ sin φ)) / V₁ × 100
Maximum efficiency occurs when iron losses equal copper losses, i.e., when x = √(W₀/Wsc).
Comparison: OC Test vs SC Test
Advantages of OC and SC Tests
- Low power consumption — only losses are supplied, not the full load power.
- Convenient for large transformers — no need for heavy loads or load banks.
- Complete equivalent circuit — both shunt and series parameters are obtained.
- Predict performance at any load — efficiency and regulation curves can be plotted.
- Safe testing — SC test uses reduced voltage, minimising insulation stress.
Frequently Asked Questions
1. Why is the open circuit test performed on the LV side?
The LV side is used because rated voltage on the LV side is lower and safer to apply. Also, the measuring instruments on the LV side handle lower voltages, reducing cost and risk. The HV side is left open since it draws no current.
2. Why is the short circuit test performed on the HV side?
The HV side has lower rated current, so the variable supply needs to provide less current. Also, the reduced voltage required (5–10% of rated) is easier to control on the HV side. The LV side is shorted because it carries the reflected full-load current safely.
3. Can we determine transformer efficiency without loading it?
Yes. By combining OC test (core losses W₀) and SC test (copper losses Wsc at full load), efficiency at any load fraction x can be calculated using: η = (x × kVA × cos φ) / (x × kVA × cos φ + W₀ + x²Wsc). This is the primary advantage of these indirect tests.
4. What is the condition for maximum efficiency of a transformer?
Maximum efficiency occurs when variable losses (copper losses) equal constant losses (core losses). The load fraction at maximum efficiency is x = √(W₀/Wsc). For most power transformers, this is designed to occur between 50–75% of full load.
5. What happens if we apply full voltage during the short circuit test?
Applying full voltage with the secondary shorted would cause dangerously high currents (10–20 times rated current) to flow, potentially burning the windings and damaging insulation. This is why only a small reduced voltage (5–10%) is applied — just enough to circulate full-load current through the winding impedance.