The effect of flux produced by the armature current on the flux produced by the main pole is known as armature reaction.
Let us consider a two poles dc generator which is shown above. When no load is connected to the generator, then the current of the armature conductor becomes zero. Only the MMF of the main pole exists in the generator which produced the main flux. The main flux is distributed symmetrically along with the polar axis. The polar axis is nothing but a line between the centers of the north pole and south pole. The arrow in the above fig. shows the direction of magnetic flux φM.
The magnetic neutral axis (MNA) is a plane perpendicular to the axis of the flux. The MNA coincides with the geometrical neutral axis. The brushes of the dc generator are always placed along the MNA, hence MNA is known as the axis of commutation.
Now consider a similar two-pole generator (as shown in fig 2.) in which the armature conductor carries current and no current flows into the field coil. The current flows in the same direction in all conductors lying under one pole.
The left side armature conductor carries current in the direction into the paper. The armature conductor combines their MMF to produce flux through the armature in the downward direction. Similarly, the right side of the armature conductor carries current in the direction which goes out of the paper. The armature conductor on the right side also combines their mmfs to produce the flux through the armature in the downward direction.
Thus, the armature conductor on both sides combines their mmfs in such a manner so that their flux goes through the armature in the downward direction. The flux induced in the armature conductor is represented by φA is shown in the figure.
You can find the direction of current induced in the conductor by fleming’s left-hand rule. Also, the direction of flux produced by the armature conductor may be decided by the corkscrew rule.
Fig 3. Shows the condition when the field current and the armature act simultaneously. This happens when machines run on-load conditions. Now there are two fluxes produced in the machines. The field flux is produced by the main field pole of the generator and the armature flux is induced by the current in the armature conductors. These two fluxes combine and produced the resultant flux φR as shown in fig 3.
When the field flux enters the armature, it is seen that the flux is shifted as well as distorted. The distortion produces an increase in the flux density in the upper tip of the north pole and the lower tip of the south pole. Similarly, there is some decrease in the flux density in the lower tip of the north pole and the upper tip of the south pole. The direction of resultant flux induced in the generator is shifted in the direction of rotation of the generator. As you know the magnetic neutral axis( MNA) is always perpendicular to the axis of resultant flux, the MNA is also shifted.
Effect of armature reaction in dc generator
Armature reaction has few effects on the dc generator which are written below:
- Armature reaction establishes flux in the neutral zone. The armature reaction flux will induce conductor voltage that causes the commutation problem.
- Armature reaction affects the flux density in such a way that the flux density over one-half of the pole increases and decreases over the other half of the pole. But the total flux produced by each pole is slightly reduced therefore the terminal voltage is reduced. This effect of reduction in total flux due to armature reaction is known as a demagnetizing effect.
- The resultant flux is distorted therefore there is a shift in the position of the magnetic neutral axis in the direction to the resultant flux ( in case of the generator) and opposite to the direction of resultant flux ( in case of the motor).