Armature reaction refers to the effect of the magnetic field produced by the armature current on the main magnetic field of the DC machine. A DC machine has two magnetic fluxes: armature and main field. The effect of armature flux on the main field is called armature reaction.
What are MNA and GNA?
When the armature conductors cut the magnetic field lines, EMF is induced. However, there is a particular axis or plane where the armature conductors move parallel to the flux lines and do not cut them. This results in no EMF being generated in the conductors. This axis is called the Magnetic Neutral Axis (MNA). The brushes are always placed along the MNA because the reversal of current in the armature conductors occurs along this axis.
The Geometrical Neutral Axis, commonly abbreviated as GNA, can be defined as an axis that is positioned perpendicular to the stator field axis. In other words, this axis passes through the center of the stator cross-section.
Armature Reaction
The figure below clearly illustrates the impact of the armature reaction.
When the field winding is energized, and no current flows through the armature conductors, the magnetic flux lines of the field poles are aligned symmetrically with the polar axis. The ‘Magnetic Neutral Axis’ (M.N.A.) is aligned with the ‘Geometric Neutral Axis’ (G.N.A.).
However, when current flows through the armature, the armature flux lines appear, and the field poles are de-energized. The image below displays the armature flux lines due to the armature current.
When a DC machine operates, the fluxes – one due to the armature conductors and the other due to the field winding – are present simultaneously. The armature flux overlaps with the main field flux, which causes a disturbance in the main field flux. This phenomenon is known as the armature reaction in DC machines, as shown in the image below.
The Adverse Effects Of Armature Reaction:
- Armature reaction weakens the main flux, reducing the voltage generated in a DC generator and the torque in the DC motor.
- Armature reaction causes the main flux to distort, which results in a shift in the position of the M.N.A. (M.N.A. is perpendicular to the flux lines of the main field flux). To avoid sparking at the surface of brushes, it is necessary to place them on the M.N.A. Due to the armature reaction, it becomes difficult to determine the exact position of the MNA accurately.
For a loaded DC generator, the MNA will shift in the direction of rotation. Conversely, for a loaded DC motor, the MNA will shift in the opposite direction of rotation.
How To Reduce Armature Reaction?
Compensating winding and interpoles are utilized in large DC machines to mitigate the harmful effects of armature reaction, and no special measures are taken for small machines with a capacity of up to a few kilowatts.
Compensating winding: It is known that the armature reaction occurs due to the presence of armature flux, which is produced by the current flowing through armature conductors. To nullify the armature field, a compensating winding can be placed near the armature winding and carry the same current but in the opposite direction. This winding is located on the pole faces and connected in series with the armature winding to carry the current in the opposite direction.
Interpoles: Interpoles are small auxiliary poles that are placed between the main field poles. The winding on the interpoles is connected in series with the armature. Each interpole is wound so that its magnetic polarity is the same as that of the main pole ahead of it. The purpose of the interpoles is to nullify the quadrature axis armature flux.