The purpose of field winding is to produce magnetic field main flux , whereas the purpose of armature winding is to carry armature current. Although the armature winding is not provided for the purpose of producing a magnetic field, still the current in the armature winding also produces a magnetic flux armature flux. The armature flux is distorts and weakens the main flux. The weakening of flux due to armature reaction in a dc generator also depends on the position of the brushes. GNA Geometrical Neutral Axis may be defined as the axis which is perpendicular to the stator field axis.
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The effect of armature flux on the main field flux is called as armature reaction. There is an axis or, you may say, a plane along which armature conductors move parallel to the flux lines and, hence, they do not cut the flux lines while on that plane. MNA Magnetic Neutral Axis may be defined as the axis along which no emf is generated in the armature conductors as they move parallel to the flux lines.
Brushes are always placed along the MNA because reversal of current in the armature conductors takes place along this axis. GNA Geometrical Neutral Axis may be defined as the axis which is perpendicular to the stator field axis.
Armature reaction The effect of armature reaction is well illustrated in the figure below. Consider, no current is flowing in the armature conductors and only the field winding is energized as shown in the first figure of the above image. In this case, magnetic flux lines of the field poles are uniform and symmetrical to the polar axis. The second figure in the above image shows armature flux lines due to the armature current.
Field poles are de-energised. Now, when a DC machine is running, both the fluxes flux due to the armature conductors and flux due to the field winding will be present at a time.
The armature flux superimposes with the main field flux and, hence, disturbs the main field flux as shown in third figure the of above image. This effect is called as armature reaction in DC machines. The adverse effects of armature reaction: Armature reaction weakens the main flux. In case of a dc generator , weakening of the main flux reduces the generated voltage. Armature reaction distorts the main flux, hence the position of M. Brushes should be placed on the M. So, due to armature reaction, it is hard to determine the exact position of the MNA For a loaded dc generator, MNA will be shifted in the direction of the rotation.
On the other hand, for a loaded dc motor , MNA will be shifted in the direction opposite to that of the rotation. How to reduce armature reaction? Usually, no special efforts are taken for small machines up to few kilowatts to reduce the armature reaction. But for large DC machines, compensating winding and interpoles are used to get rid of the ill effects of armature reaction.
Compensating winding: Now we know that the armature reaction is due to the presence of armature flux. Armature flux is produced due to the current flowing in armature conductors. Now, if we place another winding in close proximity of the armature winding and if it carries the same current but in the opposite direction as that of the armature current, then this will nullify the armature field.
Such an additional winding is called as compensating winding and it is placed on the pole faces. Compensating winding is connected in series with the armature winding in such a way that it carries the current in opposite direction. Interpoles:Interpoles are the small auxiliary poles placed between the main field poles. Winding on the interpoles is connected in series with the armature.
Each interpole is wound in such a way that its magnetic polarity is same as that of the main pole ahead of it. Interpoles nullify the quadrature axis armature flux. Related Posts.
Armature Reaction in DC Machine
In other words, the armature reaction represents the impact of the armature flux on the main field flux. The armature field is produced by the armature conductors when current flows through them. And the main field is produced by the magnetic poles. The armature flux causes two effects on the main field flux. The armature reaction distorted the main field flux It reduces the magnitude of the main field flux. Consider the figure below shows the two poles dc generator.
Armature Reaction in a DC Generator
Want create site? Find Free WordPress Themes and plugins. An armature, when rotating in its magnetic field, can have varying values of current flowing in its coils. The current can vary from zero at no load to the designed maximum, the actual value depending on the size of the machine. The armature, in carrying the current, sets up a magnetic field of its own. This field combines with the main field, producing a resultant field, and the process is called armature reaction. The resultant field is twisted in either the direction of rotation or against it, depending on whether the machine is being used as a generator or a motor.