Armature Reaction in a DC Generator

Armature Reaction is the effect of magnetic flux set up by armature current upon the distribution of flux under the main poles. The 2 pole DC generator is shown in the figure below.

armature-reaction-in-DC-generator-fig-1

When there is no load to the generator, the current in the armature conductors is zero. As a result, there exists only the mmf of the main poles, which produces the main flux (ϕ). The flux is distributed symmetrically with respect to the polar axis, that is, the central line of the north and south poles. The Magnetic Neutral Axis (MNA) or plane is perpendicular to the axis of the flux.

The magnetic neutral axis coincides with the geometrical neutral axis (GNA) in the armature reaction. Brushes are always placed along the magnetic neutral axis line. Hence, MNA is also called as the Axis of Commutation. The figure below shows the armature conductor carrying current with no current in field coils.

armature-reaction-in-DC-generator-fig-2

The direction of current in the armature conductor is determined by Flemings right hand rule. The conductors underlying on the same poles have the same direction of flow of current.

The combination of mmfs of the conductors produces a flux ϕA through the armature. The direction of the flux is downward and shown by an arrow pointing downwards. The left hand side of the armature shown in the figure above carries the current in the direction into the paper, whereas, the right hand side of the armature conductor carries current in the direction out of the paper.

The figure below shows the condition When the field current and the armature current are acting simultaneously.

Armature reaction in DC generator fig

This condition takes place when the generator is loaded. The flux produced by the main field poles of the generator and by the current in the armature conductors combines to form a resultant flux ϕR. The field flux entering the armature is shifted and distorted. This distortion produces in crowding of flux, i.e. increase in density in the upper and lower pole tip of the north and south pole respectively.

Similarly, there is a decrease in the flux density of the lower tip of the north pole and the upper tip of the south pole. The direction of the resultant flux is shifted in the direction of rotation of the generator.

The nonlinear behavior and saturation of the core, the increase in flux in one pole tip is less than the decrease in flux in the other pole tip and as a result, the main pole flux decreases.Since, Eg = kNϕ, the reduction in the field flux ϕ decreases the terminal voltage of the generator with the increase in load.

Effect of Armature Reaction

The effects of Armature Reaction are as follows:-

  • The magnetic flux density is increased over one-half of the pole and decrease over the other half. But, the resultant flux produced is slightly reduced. Hence, the terminal voltage is also reduced slightly. Therefore, this reduction in the total flux because of the Armature Reaction is known as Demagnetizing Effect.
  • There is distortion in the flux wave and a shift in the position of the magnetic neutral axis in the direction of rotation of the generator.
  • Armature reaction establishes a flux in the commutating zone. This flux induces the conductor voltage which in turn creates the commutation problem.

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