page_title: Commutator (electric)
section_title: Further compensation for self-induction
hierarchical_section_title: Commutator (electric) / The commutating plane / Further compensation for self-induction
context_page_description: A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings on the armature are connected to the commutator segments.
  Commutators are used in direct current machines: dynamos and many DC motors as well as universal motors. In a motor the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force is produced. In a generator the commutator picks off the current generated in the windings, reversing the direction of the current with each half turn, serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit.
context_section_description: Self-induction – The magnetic fields in each coil of wire join and compound together to create a magnetic field that resists changes in the current, which can be likened to the current having inertia.
  In the coils of the rotor, even after the brush has been reached, currents tend to continue to flow for a brief moment, resulting in a wasted energy as heat due to the brush spanning across several commutator segments and the current short-circuiting across the segments.
  Spurious resistance is an apparent increase in the resistance in the armature winding, which is proportional to the speed of the armature, and is due to the lagging of the current.
  To minimize sparking at the brushes due to this short-circuiting, the brushes are advanced a few degrees further yet, beyond the advance for field distortions. This moves the rotor winding undergoing commutation slightly forward into the stator field which has magnetic lines in the opposite direction and which oppose the field in the stator. This opposing field helps to reverse the lagging self-inducting current in the stator.
  So even for a rotor which is at rest and initially requires no compensation for spinning field distortions, the brushes should still be advanced beyond the perfect 90-degree angle as taught in so many beginners textbooks, to compensate for self-induction.
caption: Brush advance for Self-Induction.[13]
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