Normally Supply voltage is applied on ___________ side during Open Circuit test on the transformer.
A
LV
B
HV
C
Both side (one by one)
D
All of these

Question

Normally Supply voltage is applied on ___________ side during Open Circuit test on the transformer.

Examoogle · Accepted Answer

LV

Explanation: In an Open Circuit (OC) test, the rated supply voltage is applied to the Low Voltage (LV) winding while the High Voltage (HV) winding is kept open-circuited. This is done primarily for safety reasons and to ensure that the measuring instruments (voltmeter and wattmeter) can operate within their standard voltage ranges.

Working Principle: The primary objective of the OC test is to determine the core losses (hysteresis and eddy current losses) and the shunt branch parameters of the transformer equivalent circuit. Since the secondary is open, no load current flows, and the primary current represents only the no-load (exciting) current, which is very small (typically 2-5% of rated current). Applying voltage to the LV side allows using standard laboratory equipment without requiring expensive high-voltage rated instruments.

Key Formulas: $P_{oc} = V_{lv} I_0 \cos\phi_0$ — Power measured represents core losses; $R_0 = \frac{V_{lv}^2}{P_{oc}}$ — Shunt branch resistance; $X_0 = \frac{V_{lv}}{I_m}$ — Magnetizing reactance

Key Points: The OC test is conducted at rated voltage and rated frequency.; The LV side is used to match standard low-voltage instrumentation.; Core losses are practically constant regardless of the load.; The secondary current is zero, so $I^2R$ copper loss is neglected.

Advantages: Safe for laboratory personnel; Uses standard low-range equipment; Directly identifies core loss parameters

Disadvantages: Cannot determine full-load copper losses; Requires stable frequency supply

Applications: Transformer efficiency calculation; Determination of no-load power factor; Verification of core design

Additional Info: HV side is kept open to avoid high voltages that would require expensive and potentially dangerous test equipment.. Option B is incorrect because applying rated voltage to the HV side would require a high-voltage variable supply and high-voltage measuring instruments, which is impractical.. The transformer core is excited by the rated flux, ensuring the core loss measured is the actual operating core loss.

Answer: A is correct — The Open Circuit test is performed on the LV side to utilize rated voltage safely using standard laboratory instruments.

Exam Tip: Always remember: OC test for Core Loss (LV side), SC test for Copper Loss (HV side).

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