In the case of large electric machines, other smaller motors connected in series with the armature chain should be placed between the main poles. We call these interpolates, in which their function is basically to guarantee the neutral line even under different armature currents, also called load conditions. This effect of the armature current that can affect the neutral line is called the armature reaction.

The position of the collector is located in a region called the neutral line, in which the voltage should be zero, however, as a function of the changes in the mechanical load of the motor, its armature current will be changed, and this factor allows a displacement of the neutral line. This causes larger arcs in the brushes, which can result in considerable damage to the collector, as well as excessive brush wear.  qd bushed

In the case of small electric motors we find a kind of mechanical device, which can guarantee the process of moving the brushes to the new operating point, in which the zero voltage condition can occur.