PCB transformer rated power less than specified

[danv]

We are using the transformer 3 VA 12V PCB transformer BVEL305 2052 .
When measuring voltage current and power on a load of about 54 ohms I get a maximum power of 3W monetarily but when I leave the load for 5 hours to settle down
the max power is only 2.74W instead of 3W as notes in the data sheet even trying to vary the load to get the maximum power

Is it Ok, why is the rated power less than specified
Thank you

[nerdyHippy]

Power = V^2/R. 12^12/54 = 2.7. Change your load and you will get a different amount of power.

[danv]

Thank you for the replay
But the max power delivered from the transformer was adjusted through a series of resistors to get the maximum power. So the point is that the transformer gives the rated power when cool, when it warm up the power decreases form 3 W to about 2.5W I would think it is to do with
A short circuit-proof transformer is a transformer for which the temperature does not exceed established limit values when the transformer is overloaded or short-circuited and which continues to fulfil all the requirements of the Standard listed above once the overload or the short circuit has been eliminated.
•   An unconditionally short circuit-proof transformer is a short circuit-proof transformer without a protective device for which the temperature does not exceed the established temperature limit values in cases of overload or short circuits which can continue to be operated after the overload or the short-circuit has been eliminated. Note: Due to physical limitations, this kind of transformer permits only structural designs with low levels of rated power of up to circa 4 VA. The open-circuit voltage factor can thereby take on a value of up to 2.00. The shape of the curve of the output voltage can deviate from the sinusoidal form. It is not mandatory that unconditionally short circuit-proof transformers need be permanently short circuit-proof.
•   A conditionally short circuit-proof transformer is a short circuit-proof transformer with a built-in protective device, which opens the electrical circuit or limits the electricity in the input or output circuit when the transformer is overloaded or short-circuited. Note: Examples of protective devices are fuses, overload releases, temperature fuses, automatic and non-automatic resetting temperature limiters, posistors and automatic mechanically-triggered protective switches.
A non-short circuit-proof transformer is a transformer which is intended to be protected against excessive temperatures by means of a protective device which is not built into the transformer