is there an electrical reason that a rotor can spin too fast?
The coils are... well, "coils". They have an inductance. As frequency increases, inductive reactance increases. As inductive reactance increases, the effective internal resistance of the source increases, meaning the total current it can deliver reduces, so you get less power out of it.
If your machine has steel or iron cores, the magnetising and demagnetising effort required is approximately constant per event - more events per second means more wasted energy, more heat. So you get less power out.
If you use conventional diodes to rectify your AC for use (ie, if it's not being used as raw AC for example, to run a heater), diodes have a finite recovery time. The higher the frequency, the less efficient they are, so you get less power out.
If you have have lengthy cables from your machine to where the power is used, you have greater inductive (and capacitive) losses in the cables at higher frequencies, so you get less power out.
Short answer? Yes, you can "go too fast"
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