Rotating transformer, how to achieve maximum efficiency?

Question

Electronics

In order not to use brushes to transfer energy to a rotating part, a rotating transformer without a core is sometimes used, the efficiency is therefore much lower, but something can be obtained at the output. If anyone has met with this or worked, please describe the parameters, what is the optimal frequency of the generator, the number of turns, etc. According to my experiment, I received 300-350mA, 5-6 volts, and 12-15V at the input at a frequency of 3.5-4kHz.