My Wikipedia suggestion for this problem is Faraday's law of induction. They sum it up in pretty much a single quote.
The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.
There are lots of technicalities of motors and generators, but they're not necessary for this problem. The fundamental principle is that there is a wire spinning while in a magnetic field. The EMF, I'll denote $V$ for voltage, is quantified as follows with $r$ being the rotating radius of the coil assuming it's rectangular (as well as rotating in the right direction), $l$ is the other dimension of the rectangular loop, $B$ is the magnetic field, $\omega$ is the speed of rotation.
$$V = B r l \omega$$
If any single one of these factors had unlimited potential to increase then a motor could deliver infinite voltage. Of course they are all limited. The most obvious way to scale up power is to make a bigger machine.
There is one missing piece, which is that EMF refers to the voltage that can be either produced or converted into a mechanical action. That does not say anything of current, so taken at face value, such a simple coil rotating in a constant magnetic field would allow infinite power conversion if there were infinite current. Current in any wire or bundle of wires, is, of course limited by resistive heating limits. You can go find plenty of information about these limits but I will not cover them here. Yes, it is possible to use superconducting wires for both the primary coils as well as the magnetic field generating coils, but they also do not allow infinite energy conversion, and yes, there are companies that sell these.
I'm not familiar enough with the technology to say for sure, but I believe that the problem is still resistive heating. Superconductors generate much less heat, but each unit of heat they produce is much more expensive to remove if it's a low temperature superconductor. The 2nd law of thermodynamics gives a direct penalty on a heat flux out of a refrigerated system.