I recall reading about a capacitance regulated induction motor generator.
Basically the capacitors were switched in and out of the circuit at the peak voltage of the sine wave, rather than the zero crossing. The capacitors when they were switched off had a diode and resistor keeping them charged at the peak voltage of the generator, so there were no current spikes during switching. the capacitors were some multiple of 1,2,4,8 uf iirc, i think there were 8 of them.
another option is a servo controlled variable transformer, but that is.. old school.