When to treat transformers as non-ideal?
This doesn't constitute and answer and will be deleted or converted to an answer when the OP comes clean on the experiment.

If you are using a standard AC transformer in reverse you can expect strange things given that the output impedance of your signal is 50 ohms and that you ran the transformer up to 810 Hz.

Firstly, you could make an argument for the normal primary magnetization inductance being about 10 henries. It's going to be around this figure for a power transformer. If the turns ratio is 50:1 you can then say that when driving the secondary winding, it will have a magnetization inductance of 10 H/ 2500 = 4 mH and at 8 Hz, this will be an impedance of 0.2 ohms.

None of this seems to tally with your results so come clean on what you are doing and properly explain your experiment with significant details about the transformer.