What happens to the rotor of a synchronous generator when it loses excitation?

When a synchronous generator loses excitation, the rotor loses its magnetic field, which is essential for maintaining synchronous speed. In this situation, the mechanical power input from the prime mover (like a turbine) continues to drive the rotor. However, since the rotor is no longer producing the necessary magnetic field, the generator cannot maintain synchronization with the rotating magnetic field created by the stator windings.

As a result, the rotor experiences an imbalance between the mechanical power it is receiving and the electrical power it is supposed to generate. This causes the rotor to accelerate beyond synchronous speed because the mechanical input continues while the magnetic interaction that keeps the rotor in sync is lost. When this occurs, the rotor will overspeed until either the prime mover is controlled, or the generator is disconnected from the grid to prevent damage.

This phenomenon illustrates the importance of excitation control in synchronous generators, as proper excitation levels are crucial for maintaining stability and synchronism in the power system.