What is Armature Reaction
The current in the armature winding will also produce magnetic flux (called armature flux). The armature flux distorts and weakens the main flux posing problems for the proper operation of the d.c. generator. The action of armature flux on the main flux (Flux of Field winding) is called armature reaction.
The armature reaction produces two effects
(1) Demagnetizing effect
(2) Cross magnetizing effect
Demagnetizing effect leads to reduction in generated voltage and cross magnetizing effect leads to sparking at the brushes.
Consider the two pole DC generator as shown in figure (1). When no current flows in the armature conductors, the flux produced by the main poles are symmetrical with respect to polar axis ( Axis joining the centers of two poles). The magnetic neutral plane (MNP) coincides with the geometrical neutral plane (GNP). Let Magneto-motive force (mmf) produced by Main flux is represented by vector OFm
In the figure (2) below only armature flux exits, the field coil being unexcited. By applying the Fleming’s right hand rule , the direction of armature currents in conductors under north pole is “in” (\times) and “out” (\cdot) in conductors under south poles.
The direction of magnetic flux of force in the armature can be found by cork screw rule. The armature flux is directed downward parallel to the brush axis through the armature. The magneto-motive force (m.m.f.) producing the armature flux is represented in magnitude and direction by the vector OFA .
we have considered main flux and armature flux separately. Under actual load conditions they act together as shown in figure (3) below .The resultant flux in the armature is no longer uniform and symmetrical about the poles axis, rather it is been distorted. It is crowded at the trailing pole tips and but weakened at the leading pole tips.
The resultant flux (OF) is found by the vector sum of OFm and OFA. Since the MNA (or MNP) is always perpedicular to the resultant flux. The MNP (or MNA) gets shifted through angle “θ” in the direction of rotation. As shown figure (3) below.
In order to achieve sparkless commutation , brushes must lie along the MNP (Or MNA). So the brushes are given the forward lead of “θ” to lie along the new MNP (or MNA). Due to brush shift, the mmf OFA is also rotated through the same angle “θ”. It is because some of the conductors which were earlier under N-pole now come under S-pole and vice-versa. The result is that armature m.m.f. OFA will no longer be vertically downward but will be rotated in the direction of rotation through an angle “θ”. As shown in figure (4) below.
Now OFA can be resolved into rectangular components OFc and OFd. as shown in above Figure. Now compare the figure (3) and Figure (4) Rectangular component OFd is in direct opposition to the m.m.f. OFm due to main poles. It has a demagnetizing effect on the flux due to main poles. For this reason, it is called the demagnetizing or weakening component of armature reaction. It leads to reduced generated voltage.
The component OFc is at right angles to the m.m.f. OFm due to main poles. It distorts the main field. For this reason, it is called the cross magnetizing or distorting component of armature reaction. It leads to sparking at the brushes.
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