6.7 Wattage (WATT)
At no load, with the secondaries open circuited, a transformer will still draw current and consume power. The current is typically only a few percent of the normal full-load current, and the I2R copper loss is thus negligible. The measured power is then the power dissipated by eddy current and hysteresis effects in the core and is known as the core loss. The core loss will vary with the chosen grade of core material and the quality of its assembly, especially where laminates are stacked together.
The wattage test is used on line frequency power transformers to measure core loss. This can confirm that the transformer has been assembled properly, with the appropriate number of turns, the right grade of magnetic material for the core, and the correct air gap if required. See also the WATX test for higher power measurements using an external AC source.