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.