Not easy to do. Try the
steps described here.
Edit:
An idea for a more pragmatic approach:
It seems that the standard EN 61558-1 allows a max. temperature of the transformer windings of 100 °C for material of class A1, more for higher quality materials. Since we do not know what materials your transformer is made of, 100 °C (~210 °F) seems to be a safe limit.
You could determine the max. current by:
- apply a reasonable load current (depending on the size of the transformer use 50 % of the rated current of a similarly sized transformer), by adding power resistors to the secondary side.
- wait for e.g. 30 minutes and measure the temperature of the coil (measure at a few different points to get a valid reading).
- If the temperature of the transformer is below 100 °C, increase the load current in increments of e.g. 10 % (or more if the transformer is really cold) by adding more resistors (in parallel). Again wait, then measure. Continue until you reach a winding temperature of ~100 °C. That should set the max. output current for reliable operation.
If the temperature is above 100 °C, decrease the load and perform essentially the same steps as described before in the reverse direction (lower current).
Add a fuse on the primary side with a current rating 1/10 of the load current (or a bit above) for overload protection.
Be aware of the temperature of the environment while doing these measurements. If you want to operate the transformer in a closed housing, possibly badly vented, the environmental temperature of the transformer will be higher than during your initial measurements. Accordingly the winding temperature will also rise to a higher value. To avoid exceeding 100 °C, take this effect into account and lower the current accordingly,
