What is used to offset field distortion from the armature reaction?

The correct answer is compensating windings. These are additional windings placed in the field system of a DC machine that help neutralize the effects of armature reaction. Armature reaction refers to the distortion of the magnetic field caused by the magnetic flux produced by the armature winding when current flows through it. This distortion can lead to a decrease in the machine's efficiency and performance.

Compensating windings are designed to create a magnetic field that counteracts this distortion, allowing for a more uniform magnetic field and improved performance of the motor or generator. By providing a controlled field reaction, compensating windings enhance the stability and efficiency of the electrical machine under varying load conditions.

In contrast, field coils generate the main magnetic field, while armature windings are responsible for producing torque in motors or generating electric power in generators. Shunt resistors typically serve to limit current or stabilize voltage but do not specifically address the issues related to armature reaction.