Armature Reaction in a DC Generator & its Effect

     In the last article, we have seen the working principle of dc generator and how we obtained the dc voltage at the output terminal. Now let us see the armature reaction of a dc generator.

What is Armature Reaction ?

     In a dc generator, the magnetic flux is produced by the field coils when it is excited by a dc source. This field flux will be uniformly distributed along the air-gap of the machine and links with the conductors placed on the rotating armature. Thus it causes an emf to induce in the rotating armature conductors ( according to faraday's law of electromagnetic induction ).

     When a load is connected across the generator terminal the emf causes a current to flow through armature conductors and to load. Now the current in the armature conductors also set up a magnetic flux called armature flux. This armature mmf flux always affects the distribution of field flux in the air-gap, this effect is known as armature reaction. The armature flux has two types of effects on the field flux.
  • It demagnetizes or weakens the main flux, or
  • It cross magnetizes or distorts the main flux.

     This armature reaction effect reduces the generated terminal voltage of the machine and decreases efficiency. Let us see the effect of armature reaction in a more detailed way.

Armature Reaction in DC Generator :

     Let us consider a bipolar generator ( two-pole ) with uniform flux distribution between the two magnetic poles ( field poles ) as shown below. When the generator is on no-load no current flows i.e, current in armature conductors or circuit is zero. The brushes are also shown touching the armature directly, but in practice, they touch commutator segments.

Armature Reaction in a DC Generator

It is seen that,
  • There is uniform flux distribution through the armature conductors with respect to the axis of the poles.
  • The geometrical neutral axis ( GNA a line between the center of two magnetic poles or axis of symmetry ) and magnetic neutral axis ( MNA is the perpendicular axis existing between the two magnetic poles at which zero flux or no emf is induced on the armature conductors ) are at the same position.

     For convenience, the brushes attached to the armature without a commutator is shown. The axis of brushes position is always kept along the MNA axis because the reversal of current in armature conductors takes place across this axis and thus achieving a sparkless commutation. Hence, the axis MNA is also called an axis of commutation. The vector OFm represents the direction ( i.e., top to bottom ) and magnitude of flux produced by the field coil, which is perpendicular to MNA.

     Now, when the generator is loaded the armature current starts circulating and produces its own flux called armature flux. But let us consider armature flux alone without field flux as shown below. It can be seen that the direction of armature flux under the field poles N ( represented by crosses ) and S ( represented by dots ) is towards the left ( found from Fleming's Right-Hand Rule ). Let Fa be the armature flux and the vector OFa which is parallel to the brush axis represents the direction and magnitude of armature flux.

Armature Reaction in a DC Generator

     So far the direction and magnitude of field flux and armature flux shown above are considered with different cases i.e. if field flux is considered armature flux is not considered and vice-versa. But practically in a dc generator, under actual load conditions, both field and armature fluxes exist simultaneously.

     If both the fluxes are considered at a time the uniformly distributed field coil flux in the air-gap gets distorted due to the effect of armature flux as shown below. The effect of armature flux on the field flux is in such a way that the strength of flux is more at trailing pole tips and less at leading pole tips.

Armature Reaction in a DC Generator

     This causes a reduction in the main field and a shift in the MNA axis to a new position at an angle θ. The brushes are also made to shift along this new MNA axis ( for sparkless commutation ). The shifting angle taken depends upon the magnitude of the load current. The resultant flux OFr after overcoming the armature reaction effect is found by vectorially combining the OFm and OFa.

Effect of Armature Reaction :

     When the MNA axis and brush axis are shifted to the new position from the GNA. Some of the armature conductors previously under N-pole come under the influence of S-pole and vice-versa as shown below.

Armature Reaction in a DC Generator

Depending upon these changes there are two types of armature reaction effects.
  • The armature conductors that lie in the angle θ on both sides possess a 'Demagnetizing Effect' on the main field and conductors called demagnetizing armature conductors.
  • The armature conductors that lie other than in the angle θ possess 'cross-magnetizing or Distorting Effect' and conductors called cross-magnetizing armature conductors.

     It should be noted that both the effects are proportional to load current ( i.e., magnitude of armature current ). Also, it is impossible in practice to shift the brush axis along the MNA axis for sparkless commutation. Alternative arrangements are made to the machine to reduce the armature reaction effect for better commutation.

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