The vast majority of power plants use three phase synchronous generators. Thermal (coal & gas fired) power plants use types which have two magnetic poles and rotate at 3600 or 3000 rpm (to respectively give 60 or 50 Hz). Nuclear plants often use four pole configurations rotating at 1800 or 1500 rpm. Hydro plants use relatively low speed generators with higher pole numbers. These speeds are dictated by the design of the associated turbines.
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