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Back ampere-turns, also known as demagnetizing ampere-turns, refer to a component of the armature ampere-turns in an electric machine (such as a generator or motor) that produces a direct demagnetizing effect on the main magnetic poles.

Key Concepts:

  • Ampere-Turns: A unit of magnetomotive force (MMF) that represents the product of the current (in amperes) flowing through a coil and the number of turns in the coil.
  • Armature: The rotating part of an electric machine, typically containing coils or windings where electromotive force (EMF) is induced.
  • Main Poles: The primary magnetic poles in the machine that establish the main magnetic field.

Demagnetizing Effect:

  • Cause: The back ampere-turns result from the armature reaction, which occurs when the magnetic field produced by the armature current interacts with the main field produced by the main poles.
  • Effect: This interaction can partially oppose the main field, reducing its strength and thus demagnetizing the main poles to some extent.

Importance in Electric Machines:

  • Performance Impact: The demagnetizing effect of back ampere-turns can reduce the overall efficiency and performance of the machine by weakening the main magnetic field.
  • Design Considerations: Engineers must account for this effect when designing electric machines to ensure that the main field remains sufficiently strong for efficient operation.

Mathematical Representation:

\[ \text{Demagnetizing Ampere-Turns} = k \cdot I_{\text{armature}} \cdot N \]


  • \( k \) is a constant that depends on the machine’s geometry and winding configuration.
  • \( I_{\text{armature}} \) is the armature current.
  • \( N \) is the number of turns in the armature winding.


  • Compensating Windings: Additional windings can be added to counteract the demagnetizing effect.
  • Pole Design: Optimizing the design and placement of the main poles can reduce the impact of back ampere-turns.

Back ampere-turns or demagnetizing ampere-turns are a critical consideration in the design and operation of electric machines, influencing the overall magnetic field and performance.

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