When a synchronous generator loses excitation, what does it operate as?

When a synchronous generator loses excitation, it operates as an induction motor. This transition occurs because the rotor's magnetic field that is required to maintain synchronous operation disappears, thus making the stator's alternating current (AC) magnetic field the primary influence on rotor motion.

In the absence of excitation, the synchronous generator can no longer maintain a constant speed at synchronous frequency; instead, it begins to slip relative to the rotating magnetic field of the stator. As the rotor becomes influenced by the stator's magnetic field, it starts to behave like an induction motor where rotor currents are induced by the rotating magnetic field without the need for a separate excitation source.

The loss of excitation effectively creates conditions where the rotor will follow the rotating magnetic field produced by the stator, similar to how an induction motor operates, where the rotor is induced to turn by the magnetic field, often resulting in a decrease in efficiency and an increase in current draw from the grid.

This understanding highlights the electrical principles that enable synchronous machines to operate under different conditions, demonstrating their versatile nature in power systems.