The complete opposite of the no-load test: we short-circuit one side and gradually raise the voltage until only the rated current flows. At that point, nearly everything the wattmeter measures is the copper loss, and from these measurements we extract the nameplate's treasure: the impedance Z%.
Test Method
- The low-voltage side is short-circuited (a sturdy short-circuit link across its terminals).
- The high-voltage side is energized at a very reduced voltage, gradually raised until the current reaches its rated value.
- The voltage required for this is a small fraction of the rated voltage (it is approximately the transformer's impedance %).
- The input power, current, and voltage are measured.
What Do We Extract from the Measurements?
| Measurement | Represents |
|---|---|
| Input power | The copper loss at full load (the core loss is negligible because the voltage — i.e., the flux — is very small) |
| Ratio of test voltage to rated voltage | The transformer's impedance Z% written on the nameplate |
| From calculations | The equivalent resistance and reactance of the windings |
Why from the High-Voltage Side?
Because its rated current is smaller, so it can be reached with a small test source and ordinary measuring instruments. Had we energized the low-voltage side, we would need currents of thousands of amperes.
The no-load test gives you the core loss, and the short-circuit test gives you the copper loss and the impedance — together they let you calculate the transformer's efficiency and its voltage regulation at any load without actually loading it. This is the elegance of test engineering.
Sample answer: Because the applied test voltage is a very small fraction of the rated voltage (around the transformer's impedance, e.g., 5-10%), and the magnetic flux is proportional to the voltage, while the core loss is approximately proportional to the square of the flux, so it drops to a negligible value. Meanwhile, the current in the windings is at its full rated value, so the copper loss I²R is at its full value — making the wattmeter reading effectively the copper loss.
Confusing the two tests: remember it this way — the no-load test uses full voltage and a tiny current, so it measures the core loss (which follows the voltage), while the short-circuit test uses full current and a tiny voltage, so it measures the copper loss (which follows the current).
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