Armature Reaction | Armature Reaction in a DC Generator

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 OF_A .

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 OF_m and OF_A. 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 OF_A 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. OF_A 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 OF_A can be resolved into rectangular components OF_c and OF_d. as shown in above Figure. Now compare the figure (3) and Figure (4) Rectangular component OF_d is in direct opposition to the m.m.f. OF_m 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 OF_c is at right angles to the m.m.f. OF_m 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.

Read Also

1. Derive EMF Equation of DC Generator
2. Armature Winding | What is armature winding
3. Construction of DC Generator