Short Answer
Armature reaction in DC machines is the effect of the magnetic field produced by the armature current on the main magnetic field. This effect disturbs and weakens the original field created by the field winding.
Due to armature reaction, the distribution of magnetic flux becomes uneven. It can cause problems like reduced efficiency, sparking at brushes, and poor performance of the machine.
Detailed Explanation:
Armature reaction in DC machines
Meaning of armature reaction
Armature reaction is an important concept in DC machines. It refers to the effect of the magnetic field produced by the armature current on the main magnetic field created by the field winding. In simple words, when current flows through the armature conductors, it produces its own magnetic field. This field interacts with the main field and changes its shape and strength.
In an ideal condition, the magnetic field produced by the field winding should be uniform and symmetrical. However, when the machine is in operation and the armature carries current, its magnetic field disturbs this uniform distribution. This disturbance is known as armature reaction.
Effects on magnetic field
The armature reaction causes distortion and weakening of the main magnetic field. This happens because the armature magnetic field opposes the main field in some parts and strengthens it in other parts.
As a result, the magnetic neutral axis (MNA), which is the position where no voltage is induced, gets shifted from its original position. This shift can create problems in commutation, as brushes are usually placed along the neutral axis.
The distortion of the magnetic field leads to uneven flux distribution in the air gap. This affects the performance of the machine and reduces efficiency.
Demagnetizing and cross-magnetizing effect
Armature reaction has two main effects: demagnetizing effect and cross-magnetizing effect.
The demagnetizing effect reduces the strength of the main magnetic field. This happens when the armature magnetic field directly opposes the main field. As a result, the total flux decreases, which can reduce the generated voltage in generators.
The cross-magnetizing effect distorts the main magnetic field. Instead of directly opposing the field, it changes the direction and shape of the magnetic flux. This causes uneven distribution and affects smooth operation.
Problems caused by armature reaction
Armature reaction creates several practical problems in DC machines.
One major problem is sparking at the brushes. Due to the shift in the magnetic neutral axis, the brushes may not be in the correct position. This causes improper commutation and leads to sparks, which can damage the commutator and brushes.
Another problem is reduction in efficiency. Since the main field is weakened, the machine cannot perform at its full capacity. In generators, the output voltage decreases, and in motors, the torque may reduce.
It can also lead to overheating and increased losses if not properly controlled.
Methods to reduce armature reaction
To reduce the effects of armature reaction, some methods are used in DC machines.
One method is the use of interpoles (commutating poles). These are small poles placed between the main poles. They produce a magnetic field that neutralizes the effect of armature reaction in the commutation zone.
Another method is the use of compensating windings. These windings are placed in the pole faces and carry current in such a way that they cancel the armature magnetic field.
Proper brush position adjustment can also help in reducing sparking and improving performance.
Conclusion
Armature reaction is the effect of armature magnetic field on the main field in a DC machine. It causes distortion and weakening of the magnetic field, leading to problems like sparking and reduced efficiency. Proper design and control methods are used to minimize its effects.