The output voltage of a shunt DC generator is varied by adjusting the:

In a shunt DC generator, the output voltage can be adjusted by altering the current flowing through the field windings, which is facilitated by the field rheostat. The field rheostat is a variable resistor that regulates the amount of current in the field circuit. By increasing or decreasing this current, you can manipulate the strength of the magnetic field produced by the generator's field windings.

When the current through the field windings is increased, the magnetic field strength also increases, leading to a higher induced electromotive force (EMF) and, consequently, a higher output voltage. Conversely, if the current is decreased, the magnetic field weakens, reducing the output voltage.

Utilizing the field rheostat provides a straightforward way to control the generator's output and maintain desired operational parameters under varying load conditions.