A no-load test (also called open-circuit test) is performed on a transformer to determine:

1. Core (Iron) Losses

• Measures hysteresis and eddy current losses in the transformer core

• Represents power dissipated when energized with no load

• Important for calculating transformer efficiency

2. No-Load Current (Excitation Current)

• Determines the magnetizing current required to establish flux in the core

• Typically 2-5% of rated current for power transformers

• Helps identify core saturation or winding issues

3. Turns Ratio Verification

• Confirms voltage transformation ratio between windings

• Checks for shorted turns or incorrect tap settings

4. Core Quality Assessment

• Abnormal no-load current may indicate:

• Core lamination damage

• Inter-laminar shorts

• Poor core grounding

Test Procedure

1. Connection:

• Apply rated voltage to LV side (with HV side open)

• Use precise voltage source (variable AC supply)

2. Measurements:

• Input voltage (V₀)

• No-load current (I₀)

• Input power (P₀) → Core losses

3. Calculations:

• Core loss = P₀ (measured wattmeter reading)

• No-load power factor = P₀/(V₀×I₀)

• Turns ratio ≈ V_HV_open/V_LV_applied

Key Observations

Standards

• IEC 60076-1 (Power transformers)

• IEEE C57.12.90 (Test code)

This test is always performed at rated frequency with one winding open-circuited. For three-phase transformers, test is done phase-by-phase.

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