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Armature Reaction in a DC Generator
The current flowing through the armature conductors creates a magnetic field, which is called as armature flux. This armature flux distorts and weakens the magnetic flux produced by the main poles. This effect of armature flux on the main flux is known as armature reaction.
Consider a two pole generator on no-load. Thus, the current in the armature conductors is zero. Under this condition, there is only the main flux (φm) in the machine which is produced by the main poles. This main flux is distributed symmetrically with respect to the polar axis (i.e. centre line of field poles).
The magnetic neutral axis (MNA, which is a plane perpendicular to the axis of flux) coincides with the geometrical neutral axis (GNA). The brushes are always placed along MNA, hence the MNA is also called as axis of commutation.
Now consider the armature carrying current with no current in the field coils. The direction of flux produced by the current in the armature conductors may be determined by cork-screw rule. Refer the figure, the conductors under the N-pole carry current in the direction into the plane of paper. Thus, the flux produced by the conductors under the N-pole is in the downward direction.
Similarly, the conductors under the S-pole carry current in the direction out of the plane of paper. These conductors also produce a flux which is directed downward. Therefore, all the armature conductor produces a flux through the armature in the downward direction. This flux is known as armature flux (φA).
This case shows the condition when the field current and armature currents are acting simultaneously. Hence, there are two fluxes inside the machine, one is produced by the main field poles of the generator and the other by the current in the armature conductors. These two fluxes combine to give a resultant flux (φR).
From the above discussion, it can be seen that the main flux entering the armature is shifted and distorted. The distortion increases the flux density in the upper pole tip of the N-pole and in the lower pole tip of the S-pole. Similarly, there is a decrease in the flux density in the lower pole tip of the N-pole and in the upper pole tip of the S-pole. Therefore, 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, hence the MNA is also shifted. Due to the non-linear behaviour and saturation of the core, the increase in the flux in one pole tip is less than the decrease in the flux in the other pole tip. This results in, the main flux is decreased. Consequently, the generated emf (Eg ∝ Nφm) is decreased with the increase in load.
Effects of Armature Reaction
The armature reaction in a DC generator causes the following adverse effects
- As the total field flux produced by each pole is slightly reduced, which reduces the generated EMF.
- Due to the shifting of the resultant flux axis, the MNA is also shifted in the direction of rotation of the generator.
- Due to the armature reaction, a flux is established in the commutating zone or neutral zone. This flux in the neutral zone induces conductor voltage that causes the commutation problems.
Remedies to the Armature Reaction Effect
There are four methods to reduce the armature reaction problem
Adjust the Brush Position
In this method, rotate the brush mechanism to find the correct neutral zone position. This can be applied only fixed load current.
Modify the Ends of the Poles
In this method, the field pole tip is to be modified so that high flux cannot exist on the ends because of the high reluctance path.
The effect of armature reaction can be reduced by placing a set of interpoles or commutating poles between the main poles of the DC generator. The polarity of interpole must be that of the main pole just next of it in the direction of rotation. The interpole windings are connected in series with the armature so that respective fluxes rise and fall together with the changes in the load current.
The heavy duty operations produce very sudden changes in the armature reaction. In such generators, the interpoles do not adequately neutralise the armature flux. Hence, to overcome this problem, the compensating windings are used.
The compensating winding is an auxiliary winding embedded in the slots of the main poles. The compensating winding is connected in series with the armature in such a way that the direction of the current in the compensating conductors in any one pole face will be opposite to the direction of current through the adjacent armature conductors. Hence, the compensating windings produce a flux equal and opposite to the armature flux and thus completely neutralise the armature reaction.
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