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Backward shift,” also known as “backward lead,” refers to a technique used in commutator machines (such as DC motors and generators) to adjust the position of the brushes relative to the commutator. This adjustment helps to minimize sparking and maintain efficient operation under varying load conditions. Here’s a detailed explanation:

Key Concepts:

  • Commutator Machines: These machines rely on brushes and a commutator to convert alternating current (AC) induced in the armature windings into direct current (DC) output or vice versa.
  • Neutral Position: Initially, the brushes are set in a neutral position where they align with the commutator segments, ensuring minimal contact resistance and sparking during operation.
  • Armature Reaction: When current flows through the armature windings, it creates a magnetic field (armature reaction) that interacts with the main magnetic field (flux) in the air gap.
  • Effect of Load Current: Under load, the armature reaction causes a rotation of the neutral axis of the air-gap flux relative to the mechanical position of the commutator and brushes.
  • Reducing Sparking: To prevent sparking, especially when the load current rotates the neutral axis, the brushes can be shifted backward (in the opposite direction to rotation) relative to the commutator.

Operational Details:

  1. Brush Positioning: Initially, the brushes are positioned at the neutral axis, aligning with the commutator segments to maintain minimal contact resistance.
  2. Load Impact: As load current increases, it causes the armature reaction, shifting the neutral axis.
  3. Backward Shift: To compensate for this shift and to avoid short-circuiting segments with significant induced voltage (zero emf conductors), the brushes are moved backward. This adjustment ensures that the brushes make contact with the appropriate commutator segments throughout the operational cycle.
  4. Sparking Reduction: By implementing a backward shift or lead, sparking is reduced because the brushes are properly aligned with the commutator segments that have minimal induced voltage, thus minimizing arcing and wear.


  • Improved Efficiency: Minimizes energy loss due to sparking, ensuring efficient conversion of electrical energy in commutator machines.
  • Extended Component Life: Reduces wear and tear on brushes and commutator segments, prolonging the lifespan of these critical components.

Backward shift or backward lead is a technique used in commutator machines to adjust the position of brushes relative to the commutator, particularly under load conditions where armature reaction affects the distribution of magnetic flux. By strategically shifting the brushes backward, operators can optimize performance, reduce sparking, and enhance the overall efficiency and reliability of DC motors, generators, and similar devices.

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