The synchronous speed refers to the constant rotational speed at which the frequency of the output voltage remains same. It is important to maintain the rotor speed precisely at synchronous speed for proper synchronization.

Increasing the field current strengthens the main magnetic field, which increases the induced emf. However, beyond a point further increase in field current leads to saturation and then emf starts decreasing. This behavior is represented by the V-curve of a synchronous generator.

Permanent magnet synchronous generators have advantages like higher efficiency, reduced size, absence of separate DC excitation system and elimination of copper losses in field windings. However, the output voltage cannot be controlled.

The emf equation of a synchronous generator is E = Kφω, where E is the induced emf, K is a constant, φ is the magnetic flux through one turn of the coil, and ω is the angular velocity.

Cylindrical rotors are preferred for large generators as they can withstand higher mechanical stresses due to uniform air-gap flux distribution compared to discrete poles in salient pole rotors. They are also easier to manufacture for higher power ratings.