Armature reaction

One of the most interesting phenomena that occurs in a DC Motor is armature reaction. Current flowing through the armature creates its own magnetic flux called the armature flux. The action of this armature flux on the main flux set up by the field is known as armature reaction. Let us explain this through a series of diagrams. The excitation of field winding sets up magnetic flux from North Pole to South Pole. When the current flows through the armature winding, the direction of current under one pole face is same and opposite to that of other. It can be clearly seen that the resultant armature flux strengthens the main flux on the top left and bottom right side and weakens it on the other. The resultant flux in the armature region is as shown.
Let us take a count on the effects Armature reaction produces.

• Vector addition of the armature flux on both side of the brush gives a resultant flux along the Q-Axis. In other words it cross magnetises the magnetic field.


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• Secondly the magnetic neutral axis shift away from its original position due to change in direction of flux lines.

• And the last one is the weakening of the magnetic field. Now the question might pop up in your brain “how weakening” when there is both weakening and strengthening in equal proportion.  Here I would take the opportunity to bring the B-H curve of the core into the scene. Most of the machines are designed at the knee point (B₀ ,H₀) of the B-H curve of the material. Below the knee point the B-H curve is almost linear but beyond there is saturation. At equal distance from H₀ the decrease in B is much more than the increase in B or strengthening of magnetic field is less than weakening. Hence the ultimate result is the weakening of magnetic field.

 Let us take a look at the detrimental effects it can cause.

• The distortion of magnetic flux leads to increase in flux density at the teeth. This increases the iron loss.


• The change rate of change of flux along the brush axis no longer remain zero. So there is induction of voltage in the commutation zone which is not desirable.


• When the motor is heavily loaded, the armature current increases, so the distortion also increases and the flux line crosses over the brush axis resulting in sparking. If the voltage is high enough it could result in a ring of fire.

To overcome these effects, what we can do is to increase the reluctance of the path of the armature flux or introduce extra flux which opposes the armature flux. Based on these concepts the various methods used for reducing the armature reaction are:

1. Chamfered or eccentric poles
2. Staking of lamination alternately
3. Rectangular holes in the cores

All these increases the reluctance of the path by introducing airgap. To produce the opposition flux we have

• Interpole winding
• Compensating winding

Interpole winding is placed along the Q-axis in series with the armature winding to oppose the cross magnetising effect of the armature reaction.




The compensating winding are housed in the pole shoe and placed in series with the armature to reduce the effect of armature reaction. The commutation problem is solved by shifting the brushes to the new MNA.