The operation frequencies for most power systems in the world are either 50 or 60 Hz. A hydroelectric generator is usually equipped a salient-pole rotor,because hydraulic turbines work at low speeds. Relatively large number of poles is required to produce the desired frequency,and salient-pole structure is brtter adapted to this situation mechanically.However,steam turbines work at relative high speeds,so that the generators in these cases are two- or four-pole cylindrical-rotor machines.