Synchronous generator is an a.c. machine with its speed proportional to the frequency of the grid (network) on which it is connected, and the speed does not vary with the load. The synchronous generators are identical with synchronous motors regarding their construction.
Hydro Generator is a synchronous generator with salient poles. It is called Hydro Generator because usually its driven motor (primary mover) is a hydraulic turbine. Salient poles are used when the number of pairs of poles is equal or larger than 3. Considering the relation below, we can draw the conclusion that the speed of a hydro generator is lower than 1000 rpm at a frequency of 50 Hz, or 1200 rpm at 60 Hz. The diameter of the rotor of a hydro generator is larger than its lenght.
The inductor of the hydro generator, supplied with direct voltage (by the excitation system), produces a magnetic field, which (due to the relative displacement related to the stator) induces in its windings an e.m.f.
The basic relation of a hydro generator is: