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Armature reaction in DC Generator | Demagnetization and Cross magnetization

by | Last updated Jul 15, 2021 | DC Generator

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

In this section, let us understand the flux produced 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.

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 commutators.

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 North and south poles.

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

Flux produced by field winding

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.

Geometric 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.

Consider the armature winding alone. The armature conductors to the left of GNA carry the current inwards(+) and right of G.N.A. carry current in an outward direction(-). This current will set up magnetic flux lines in the conductors. 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 OFa represents the magnitude and direction of MMF producing the armature flux.

Flux produced by the armature winding

In the above discussions, we have considered both the fluxes separately, which is not the case in practice. When the 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.

Armature reaction in DC Generator

The resultant MMF vector OF is the sum of the MMF vector producing the main flux(OFm) and MMF vector producing the armature flux(OFa). 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 MNA. 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 OFa is not vertical but inclined at an angle θ. The vector OFa can be resolved into two components, OFd parallel to the polar axis and OFc perpendicular to the polar axis.

  • The horizontal component OFd 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 OFc is at the right angle to OFm. It distorts the main field and so-called cross magnetizing or distorting component of armature reaction.

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

Demagnetizing and cross magnetizing conductors

Now consider the remaining conductor that lies between AOC and BOD. These conductors carry the current such that the 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 as cross magnetizing conductors and constitute the cross magnetizing ampere-turns of armature reaction.

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.

Compensating winding to reduce armature reaction in dc generatoe

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.



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