Effects of Armature Reaction and Remedies to Armature Reaction Effect
Armature Reaction Effects
In this post we will discuss about the various effects of armature reaction and how to reduce the effects of armature reaction.
What is armature Reaction?
When the generator is loaded, the armature carries the current. The armature current sets up its own magnetic field. The effect of this armature flux on the distribution of main field flux is called Armature Reaction.
It reduces the generated voltage by distorting the main flux and it causes sparking at the brushes.
What are the Effects of Armature Reaction?
- The armature reaction will reduce the generated EMF due to decrease in value of flux per pole.
- The iron losses in the teeth and pole shoes are determined by the maximum value of flux density at which they work. Due to distortion in main field flux the maximum density at load increases above no load. Thus more iron losses are observed on load than no load.
- Due to this process the maximum value of gap flux density increases. This will increase the maximum voltage between adjacent commutator segments at load. If this voltage exceeds beyond 30V the sparking may take place between adjacent commutator segments.
- The armature reaction shifts brush axis from GNA. Thus flux density in the interpolar axis is not zero but having some value. Thus there will be an induced emf in the coil undergoing commutation which will try to maintain the current in original direction. This will make commutation difficult and will cause delayed commutation.
How to Reduce the effects of armature reaction?
Remedies to armature reaction effect:
The following methods are used in order to reduce the effect of armature reaction.
- The armature reaction causes the distortion in main field flux.
This can be reduced if the reluctance of the path of the cross-magnetising field is increased.
The armature teeth and air gap at pole tips offer reluctance to armature flux.
Thus by increasing length of air gap, the armature reaction effect is reduced.
- If reluctance at pole tips is increased it will reduce distorting effect of armature reaction.
By using special construction in which leading and trailing pole tip portions of laminations are alternately omitted.
- The effect of armature reaction can be neutralized by use of compensating winding.
It is always placed in series with armature winding.
The armature ampere conductors under pole shoe must be equal to compensating winding ampere conductors which will compensate armature mmf perfectly.
- The armature reaction causes shifting the magnetic neutral axis.
Therefore there will be some flux density at brush axis which produces emf in the coil undergoing commutation.
This will lead to delayed commutation.
Thus the armature reaction at brush axis must be neutralized.
This requires another equal and opposite mmf to that of armature mmf.
This can be applied by interpoles which are placed at geometric neutral axis at midway between the main poles.
Among these reduction methods, providing the compensating winding method is most popular in actual practice of DC machines.
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