No load Test on Transformers; Open Circuit test on Transformers

Open Circuit (No Load) Test on Transformer

The Open Circuit Test, also known as the No Load Test, is performed on a transformer to determine its core losses (iron losses) under no load conditions.

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Purpose of the Test

• To measure core losses (hysteresis and eddy current losses) in the transformer.

• To calculate parameters of the equivalent circuit (shunt branch).

• To determine the magnetizing current.

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Why High Voltage Side is Kept Open?

• In the open-circuit test, one winding of the transformer is left open.

• The high-voltage (HV) winding is kept open, and the test is performed from the low-voltage (LV) side.

• This is because:

  • At rated HV, the current is very small (hard to measure accurately).

  • Instruments required for HV would need higher insulation and safety.

  • So, LV side is used for practical measurement.

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Test Connections

• A Voltmeter, Ammeter, and Wattmeter are connected on the LV side.

• The HV side is left open.

• Supply is applied to the LV side using a Variac (to gradually increase voltage).

• The applied voltage is increased until it reaches the rated LV value.

📌 Circuit Diagram (Conceptual)

      LV Side (Test side)                   HV Side (Open)
   ┌─────────────┐
   │             │
   │   Ammeter   │
   │             │
   └─────┬───────┘
         │
      ┌──┴───┐
      │      │   Voltmeter
   Variac    ├────────────┐
      │      │            │
      └──────┘        Transformer LV winding → HV winding open
         │
   Wattmeter

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Readings Taken

• Voltmeter (Vo): Applied voltage (rated LV value).

• Ammeter (Io): No-load current (very small, < 5% of rated current).

• Wattmeter (Po): Power input to transformer under no load = Core loss.

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Theory

Since:

• Secondary is open, → No load current flows in HV side.

• Copper losses (I²R) in LV side are negligible (Io is very small).

• Therefore, Wattmeter reading ≈ Core losses.

The input power is expressed as:

$$P_o = V_o^2 / R_m$$

Where:

• $P_o$ = Core loss (from Wattmeter)

• $V_o$ = Applied LV rated voltage

• $R_m$ = Shunt resistance of equivalent circuit

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Conclusion

• The Open Circuit Test gives the core losses of the transformer at rated voltage.

• It also helps to calculate the magnetizing branch parameters (Rm & Xm) of the equivalent circuit.

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👉 Related: Short Circuit Test on Transformer

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