Interpole shims: Proper placement prevents arcing
Have you ever wondered about the purpose of the shims found under the interpoles in most DC machines? Those shims are used by the manufacturer to adjust the interpole strength. If they are lost, left out or mixed up, the result will be a DC motor or generator that arcs - especially when loaded. DC machines require interpoles to provide a magnetic flux equal, but opposite, to the armature flux. This controls distortion of the field flux through the load range, thus preventing arcing at the brushes.