Armature Reaction in DC Generator: Armature reaction is the effect of magnetic flux set up by armature current upon the distribution of flux under the main poles.
When there is no load connected to the generator, the current in the armature conductors is zero. Under these conditions there exist in it only the mmf of the main poles which produce the main flux Φ. The flux is distributed symmetrically with respect to the polar axis, that is, the centre line of the north and the south poles. The direction of Φm is shown by an arrow. The magnetic neutral axis or plan (MNA) is a plane perpendicular to the axis of the flux.
The MNA coincides with the geometrical neutral axis or plan (GNA). Brushes are always placed along MNA. Hence MNA is also called the axis of commutation.
Armature conductors carrying current with no current in field coils. The direction of current in the armature conductors may be determined by Flemings right - hand rule. The current flows in the same direction in all conductors lying under one pole. The direction of flux produced by armature conductors may be determined by cork - screw rule. The conductors on the left - hand side of the armature carry current in the direction into the paper.
These conductors combine their mmfs to produce a flux through the armature in the downward direction. Similarly, the conductors on the right - hand side of the armature carry current in the direction out of the paper. These conductors also combine their mmfs to produce a aflux through the armature in the downward direction. Thus, the conductors on both side of the armature combine mmfs in such a manner as to send a flux through the armature in the downward direction. The armature flux produced is analogous to that produced in the equivalent iron - cored solenoid with its axis along the brush axis.
It is seen that the field flux entering the armature is not only shifted but also distorted. The distortion produces crowding of the flux (increase in the flux density) in the upper pole tip in the N - pole and in the lower pole tip in the S - pole. Similarly there is a reaction of flux (decresed flux density) in the lower tip of the N - pole and in the upper pole of the S - pole. The direction of the resultant flux has shifted in the direction of rotation of the generator. Since the MNA is always perpendicular to the axis of the resultant flux, the MNA is also shifted.
Because of the nonlinear behaviour and saturation of the core, the increases in flux in one pole tip is less than the decrease in the flux in the other pole tip consequently, the main pole flux is decresed. Since Eg = kNΦ, the reduction in the field flux Φ decreses the terminal voltage of a generator with increased load.
Effects of Armature Reaction
The effects of armature reaction are summarized below :
1. Magnetic flux density is increased over one half of the pole and decresed over the other half. But the total flux produced by each pole is slightly reduced and, therefore, the terminal voltage is slightly reduced. The effect of the total flux reduction by armature reaction is known as demagnetizing effect.
2. The flux wave is distorted and ther is shift in the position of the magnetic neutral axis (MNA) in the direction of rotation for the generator and against the direction of rotation for the motor.
3. Armature reaction establishes a flux in the neutral zone (or commutating zone). Armature reaction flux in the neutral zone will induce conductor voltage that aggravates the commutation problem.