How do we determine a transformer's core loss without loading it at all? We leave it unloaded and energize it at its rated voltage: nearly everything it draws then goes to magnetizing the core and its associated losses. This is the idea behind the open-circuit test.
Test Method
- One side is left open (unloaded) — usually the high-voltage side.
- The other side (usually the low-voltage side) is energized at its full rated voltage and rated frequency.
- The following are measured: input power (wattmeter), no-load current (ammeter), and voltage (voltmeter).
What Do the Measurements Represent?
| Measurement | Represents |
|---|---|
| Input power at no load | Approximately the core (iron) loss (hysteresis + eddy current losses), since the copper loss is negligible (the no-load current is small) |
| No-load current | The magnetizing current plus a loss component, a small fraction of the rated value |
| From calculations | The parameters of the magnetizing branch in the equivalent circuit |
Why from the Low-Voltage Side?
Because its rated voltage is small and available from ordinary test sources, and standard-range measuring instruments suit its values — energizing a 33kV side, for example, at full voltage in a laboratory is impractical. The core loss is the same regardless of which side is energized, as long as the voltage and frequency are at rated values.
The result of this test is the "No-Load Loss" value written in transformer catalogs — the loss you pay for 24 hours a day as long as the transformer is connected to the network, even with no load at all.
Sample answer: Because the transformer is unloaded, so only the small no-load current (a tiny fraction of the rated value) flows through the windings, and the copper loss is proportional to the square of the current, making it negligible. Meanwhile, the voltage and frequency are at rated values, so the flux is full and the core loss is at its full operating value. Thus the wattmeter effectively reads the core loss (hysteresis and eddy current losses).
Performing the test at a voltage lower than rated 'for safety' and then adopting the result. Core loss depends on the flux, i.e., on the voltage; a lower voltage gives a core loss lower than the true value and a misleading reading.
Want to master electrical transformers step by step?
Follow trainer Fahad Refai's Electrical Transformers course — a practical walkthrough from the fundamentals to testing and reading catalogs.
Browse Fahad Refai's Courses