Armature reaction in DC Generator | Demagnetization and Cross magnetization

Armature reaction is the effect of magnetic flux produced by the armature current on the distribution of flux under the main poles of a DC generator. The armature flux produces two effects:

1. demagnetizes or weakens the main field flux(Demagnetization effect)
2. cross magnetizes or distorts the main flux(Cross magnetization effect)

We can also see this armature reaction effect in an alternator. In an alternator, there are three types of distorting effects: cross magnetizing effect, demagnetizing effect and magnetizing effect. Whereas in DC generator, we can see only two effects as listed above.

The armature current flowing through the armature winding induces an armature flux, which in turn disturbs the main flux produced by the field winding. Due to this, the armature winding will not cut the field flux effectively to induce the desired output voltage. This effect is called armature reaction. This effect will either reduce the generated voltage, due to the weakening of total flux or cause sparking at the brushes.

In this section, you can understand the magnetic flux by the field winding and the armature winding separately. Then, you will learn, what happens when both fluxes interact with each other under loaded conditions, which conductors are responsible for armature reaction and finally, you will know how to reduce the effects of armature reaction.

Flux produced by the field winding

Consider a two-pole machine. Here the brushes are touching the armature conductors as shown in the figure below. But in practice, they touch the commutator segments.

Under the no-load condition, the armature conductor does not carry current. So the main field flux is distributed symmetrically with respect to the polar axis. The polar axis is nothing but the line joining the center of the North and south poles.

The vector OF_m represents the magnitude and direction of MMF producing the main flux. Also, the MNA, that is Magnetic Neutral axis coincides with the Geometrical neutral axis(GNA).

Magnetic Neutral axis(MNA) is the axis along which no emf is induced in the armature conductors, as they move parallel to the magnetic field. It is an axis, which is perpendicular to the flux passing through the center of the armature.

The geometrical neutral axis (GNA) is the axis that bisects the angle between the centerline of adjacent poles. It is the axis of symmetry between two adjacent poles.

Flux produced by the armature winding

Now, consider the armature winding alone. The armature conductors to the left of GNA carry the armature current inwards(+) and the right of G.N.A. carry the current in an outward direction(-). This current will set up magnetic flux lines in the conductors, called armature flux. By applying the cork-screw rule, the direction of magnetic lines of force can be found.

It is clear that armature flux is directed downward parallel to the brush axis. The vector OF_a represents the magnitude and direction of MMF producing the armature flux.

Effects of Armature reaction in DC Generator

In the above discussions, we have considered both the fluxes separately, which is not the case in practice. When the dc generator is loaded, the main field flux and armature flux will act simultaneously as shown in the below figure. As you can see from the diagram, the interaction of flux distorts the uniformity in the field flux density.

The resultant MMF vector OF is the sum of the MMF vector producing the main flux(OF_m) and MMF vector producing the armature flux(OF_a). Since MNA is always perpendicular to the resultant MMF, the MNA shifts through an angle say θ.

The brush also gets shifted to a new position and lies along with the Magnetic neutral axis. The brush position shifts in the same direction as the direction of rotation of the armature. This will cause the redistribution of armature conductors and hence armature current.

Some armature conductors under the influence of the North pole will come under the influence of South pole and vice-versa. It is shown in the below figure.

Since MNA gets shifted, the armature MMF vector is also found to lie on the axis of MNA. The armature MMF vector OF_a is not vertical but inclined at an angle θ. The vector OF_a can be resolved into two components, OF_d parallel to the polar axis and OF_c perpendicular to the polar axis.

• The horizontal component OF_d is in direct opposition of OFm. This has a demagnetizing effect on the main field flux. Hence called a demagnetizing component or weakening component of armature reaction.
• The vertical component OF_c is at the right angle to OF_m. It distorts the main field and hence cross magnetization effect results, which is said to be the distorting component of armature reaction.

Demagnetizing and cross magnetizing conductors

Due to the armature reaction effect in a 2 pole DC Generator, the brush axis will give a lead of θ, so as to lie along the new Magnetic neutral axis. Now consider the armature conductors lying within the angles AOB = COD = 2θ at the top and bottom of the armature. They carry the current such that the resultant flux produced will flow from right to left.

These fluxes will act in direct opposition to the main field and hence called the demagnetizing conductor and constitute the demagnetizing ampere-turns of armature reaction.

Now consider the remaining armature conductors that lie between AOC and BOD. These conductors carry the current such that the armature flux thus produced will point vertically downwards, that is, at right angles to the main flux.

Since these fluxes are responsible for distorting the main field flux, the conductors are called cross magnetizing conductors and constitute the cross magnetizing ampere-turns of armature reaction.

Compensating winding

With every change in load, the cross magnetizing effect of armature reaction will have a serious issue in larger machines. So it is very important to minimize those effects. There comes the use of compensating windings.

The compensating winding is a winding embedded in slots in the pole shoes. It is connected in series with the armature, in such a way that the current flowing in these winding is opposite to that flowing in armature conductors directly below the pole shoes.

It should be designed in such a way that the winding should provide sufficient MMF to balance the armature flux. Thus the effect produced by the armature reaction is neutralized by the compensating winding.

Interpoles

Inter poles are auxiliary poles, which are long and narrow placed in between two field magnetic poles, that is in the inter-polar axis. The interpoles are connected in series with the armature winding. Inter poles are used in almost all medium and large-sized DC machines.

They are wounded in such a way that they have the same polarity as that of the main pole coming next in the sequence of rotation. The flux generated by the interpoles will have the same effect as that of the compensating winding.

The inter pole is designed to cancel the armature reaction mmf in the inter polar axis. Since interpoles are connected in series with the armature, the change in direction of current in the armature will be balanced by the effect produced by the interpoles. An increase in the load current increases the armature reaction, and hence the effect of interpoles also increases.

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