It is extremely important to be aware that the position of the collectors must always be in a specific region called the neutral line, where the voltage must be zero, but due to changes in the mechanical load of the motor, its armature current will be changed, https://www.mrosupply.com/couplings-collars/elastomer-couplings/478655_7300060_rexnord/ a factor that allows a neutral line to be displaced, giving rise to larger arcs in the brushes, which can result in considerable damage to the collector, as well as an excessive expense on the brush. In the case of small-sized engines, there is a kind of mechanical device, which may allow the process of moving the brushes to the new operating point, in which the zero voltage condition may occur.
Therefore, in the case of machines with high dimensions, it is indicated that smaller motors connected in series with the armature chain are inserted between the main poles, which are called interpolates, in which their function is basically to ensure the neutral line even under conditions of numerous armor chains, also called load conditions. This effect of the armature current that may affect the neutral line is called the armature reaction.