Armature reaction , what is armature reaction in dc machine

In this article we are studied  about armature  reaction or what is armature reaction in dc machine ( motor or generator).

As in dc generator, armature reaction also occurs in adc motor. This is expected because when current flows through the armature conductors of a dc motor, it produces flux (armature flux) which lets on the flux produced by the main poles. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed.

1. In a generator, the armature current flows in the direction of the induced EMF (generated EMF) whereas in a motor, the armature current flows against the induced EMF (which is back EMF). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator, Hence, instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. , armature reaction in a dc generator weakens the flux at leading pole tips and strengthens the flux at trailing pole tips while the armature reaction in a dc motor produces the opposite effect.

2. In case of a dc generator, with brushes along G.N.A.  And no commutating poles used, the brushes must shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case of a dc motor, the brushes are given a negative lead. They are shifted against the direction of rotation. With no com mutating poles used, the brushes are given a forward lead in a dc generator and back ward lead in a dc motor.

3. By using com-mutating poles ( compoles), a dc machine can be operated with fixed brush  positions for all conditions of load.  Since com mutating poles winding carry the armature current, when a machine changes from generator to motor (with consequent reversal of current), the polarities off com mutating poles must be of opposite sign. Therefore, in a dc motor, the com mutating poes must have the same polarity as the main poles directly at the back of them. This is the opposite of the corresponding relation in a dc generator.

We can reduced the effect of armature reaction by use of  interpoles.

By using  of interlopes or compoles neutralizing reactance of voltage is done. They produce reversing voltage to neutralize the reactance voltage. These are small poles fixed to the yoke and spaced mid-way between the main poles as shown in fig .

They are wound with comparatively few turns and connected in series with the armature so that they carry armature current. Their polarity is the same as the next main pole ahead in the direction of rotation for a generator as shown in fig.

Hence in this way we can control armature reaction of dc machine