What does a rheostat control in the field circuit of a shunt generator?

A rheostat is a type of variable resistor used to control the flow of electric current in a circuit. In the context of a shunt generator, its primary function is to adjust the current flowing through the field windings. This adjustment directly influences the magnetic field strength generated by the field coils, which in turn affects the generator voltage.

By varying the resistance within the field circuit using a rheostat, the field current can be controlled, allowing the operator to raise or lower the voltage output of the generator. When the field current is increased, the magnetic field becomes stronger, leading to an increase in the output voltage. Conversely, reducing the field current decreases the magnetic field strength and, subsequently, the voltage.

Thus, using a rheostat in this manner is crucial for voltage regulation in a shunt generator, making it the correct answer to the question about what is controlled in the field circuit of this type of generator.