Electric frequency conversion from very low frequencies
If the driving power is very small, you will lose it all in power conversion. You may think it is not practical to build a 60 pole generator, but you may find that works where nothing else does.

Here are some 60-pole generators that have been operating for a bit over 100 years. Note the relative length vs. diameter. These are rated about 2 MVA each. All you need is to make a scale model of one of these. I have been thinking of asking if I can get the drawings to do that.

I would not be surprised if these generators are more than 90% efficient. Over the course of their history, they have been converted from 25 Hz to 60 Hz. Some of them may have been rewound more than once. The efficiency of a smaller version would depend a lot on just what the power level is and how well it is designed. How much power can be generated by a given set of magnets will depend a lot on the magnetic flux path, particularly the air gap. What is used for stator and rotor iron will be important, but I don't know how to quantify that.

With a 60-pole motor, I doubt that square magnets will be a huge issue. That make an irregularity in the air gap that will affect the waveform. There again, quantifying will be difficult.