In principle, how is an alternating current waveform generated by a hydro-electric generator?
A hydro-electric generator takes the rotational energy from the turbine shaft (mechanical energy), to spin a set of field windings carrying direct current, thus creating a rotating magnetic field that will induce an alternating current in the stator. (electrical energy)
Because the machines are three phase synchronous generators, three phase windings are required to establish the 3 phase alternating voltage waveform. In this case the phase windings are spaced 120o apart which generates three separate voltages