How can the output of a separately-excited AC generator be controlled while running at a constant speed?

The output of a separately-excited AC generator can be controlled by modifying the amount of field current. In a separately-excited generator, the field current is supplied independently from the armature and plays a crucial role in controlling the magnetic field strength. By increasing the field current, the magnetic field strengthens, which leads to an increase in the induced voltage and, consequently, the output voltage of the generator. Conversely, reducing the field current weakens the magnetic field, resulting in a decrease in output voltage.

Modifying the field current is a direct and effective way to regulate the generator's output without changing its speed. This allows for fine control over the output characteristics under varying load conditions while maintaining a constant rotational speed of the generator.

Adjusting the input voltage directly affects the generator's performance and stability. Changing the load resistance impacts the output but doesn't effectively control the source of induced voltage. Altering the generator speed would change the power output characteristics and is generally not a method used to regulate output at a constant speed. Therefore, modifying the field current stands out as the primary method for controlling output in a separately-excited AC generator.