Electrical Revolution: Armature Reaction in the DC Generator

The flux which is produced by field winding is called as main flux or field flux.
The armature conductors produce armature flux.
The effect of armature flux on main flux or field flux is known as armature reaction.
First we consider the main flux and the armature flux separately and latter combined effects of both on each other are considered.

Main flux ( Field Flux )

When the field winding is energized it produces the main flux whose direction is from N – pole to S – pole.
The flux is symmetrical distributed about main pole axis when the DC generator is at no – load.

The axis which is perpendicular to field flux or main flux is known as magnetic neutral axis ( MNA ).
The position of brushes always lies in MNA. There is no emf produced in the armature conductors which are under MNA.
The MNA is also called as "Commutation axis" because reversal of current takes place along this axis.
Vector PQ represents main flux, both in magnitude and direction also.

Armature Flux

Figure B shows the armature flux when the field winding is unexcited.
The armature conductors produce armature flux when the generator is actually loaded.
The direction of armature current is found by Fleming’s right hand rule. The current direction is downwards ( x ) in conductors under N – pole and upwards (·) in those under S – pole.
The direction of the armature mmf is found by cork – screw rule which is shown in the downward direction by PR.

Combined effect of armature flux and main flux

The resultant flux when both armature and main flux exist simultaneously ( Operate at same time ) due to rotation of armature in clockwise direction is shown in the figure C.
It is observed that the flux through armature is not uniform but it is distorted.
The resultant flux is crowded at the trailing pole tip but it is weaken at the leading pole tip. ( The pole tip which is first met during rotation of armature is known as leading pole tip and other as trailing pole tip ).
The PQ represents main flux, PR represents armature flux and PS represents resultant flux as shown in the Figure C.

The new position of MNA is always perpendicular to resultant flux PS.
It means that the MNA shifts by some angle q and hence the position of brushes also shift by angle q in the direction of rotation.
Now the armature current is redistributed. Some conductors which are under influence of N – pole come under influence of S – pole and vice versa.
The armature mmf is new represented by vector OF_a which is at angle q with GNA.

Demagnetizing component

The components OF_d which is direct opposition to main mmf and so it reduces the main mmf that is why it is called as De magnetising components of armature reaction.

Cross - magnetizing component

The components OF_c is at right angle to main mmf and so it distorts the main mmf. Therefore it is called as Cross - magnetizing components of armature reaction.
Therefore the armature reaction produces demagnetizing and cross – magnetizing effects which will increase with increase in armature current.

Describe the effect of armature reaction on iron losses?

The value of maximum flux density at given load increases above the no load value due to field distorting effect of armature reaction.
The iron losses depends upon the maximum value of flux density therefore its value increases considerably than on no load condition.

Method of Reducing the Effect of Armature

Reaction

The effect of armature reaction can be reduced by

(1) Increasing the numbers of field turns to produce additional main flux
(2) To increase the reluctance of the cross – magnetizing field path. The pole tips offer major reluctance to the flux path therefore by increasing the length of air gap at pole tips reduces the effect of armature reaction.

Compensating Winding

The function of the compensating winding is to reduce the effect of armature reaction.
These windings are embedded in axial slots in the Pole shoes.
The compensating winding is connected in series with the armature winding and carries current in opposite direction to that of adjacent armature conductors below the pole shoes.

To Compensate the effect of armature mmf under the pole shoes, it is necessary that the compensating ampere turns are exactly equal to armature amp – turns.

Number of armature turns / pole = Z / 2P

Number of armature turns / pole for compensating winding =

= Z / 2P × ( Pole arc / Pole pitch )

The compensating winding is used only in the DC machines due to higher cost, space requirement and copper loss of the compensating winding.
It is only used in the large DC machines. The compensating winding are also used for large DC machines which are subjected to large fluctuating of load i.e. rolling mill motors.
The fluxes will suddenly shifting forward / backward with change in load in the absence of compensating winding.
This flux will induce statically induced emf in the coils.
This induced emf may produce strike between consecutive commutator segments if its value may be very high.
As a result flash over around the whole commutator leads to short circuiting the whole armature.

Interpoles or commutating poles

The function of the interpoles is to neutralize the cross magnetizing effect of armature reaction.
They are placed at the midway between main poles.
They are small poles which are wound with heavy gauge of copper wires.
The interpoles winding is connected in series with the armature winding such that mmf due to interpoles is completely cancel by cross – magnetizing mmf due to armature.

The cross – magnetizing effect is completely cancel at any loads because both winding are connected series and carry same armature current.
The polarity of the interpoles is same as that of the main pole ahead in case of generator and it is same as that of the main pole behind in case of motor.

Electrical Revolution