Before studying the Armature Reaction in DC Generator, it is recommended to read the working of dc generator
in detail. It will help you to understand Armature Reaction and its effect in DC Generator.
During no load condition, current passing through the armature winding is zero. In that case, the only magnetic field present in the generator is produced by the stator field poles known as main magnetic field. Magnetic field lines of the poles are uniform and symmetrical to the axis.
Armature Reaction:
When load is connected and current starts to flow through the armature conductors, it creates its own magnetic field. Magnetic field produce by the armature current is called armature flux. The produce armature flux is going to react with the main flux and try to distort and weakens the main field flux. The distortion and weakening of main field flux by the armature flux is called armature reaction. Before going to the effects and solution of armature reaction, you need to know about the neutral zone and position of brushes in dc generator.
Neutral Zone:
The axis where there is no voltage is going to be induced in the armature coil is called neutral zone (where main field flux is minimum or zero). Let assume that the coil is rotating under given set of poles. When the coil passes through the magnetic neutral axis, rate of change of flux or emf induced in the armature coil is minimum. Hence midpoint between the main field poles is called neutral zone (magnetic neutral plane).
What should be the position of brushes in DC Generator?
As we know that during commutation, coil segments gets short circuited by the brushes. Brushes always short circuited one particular coil segment and draws current from the remaining coils. There could be huge flow of current in that particular short circuited coil if emf is induced in it. In order to prevent this, brushes should be place along where induced voltage in the coil segment is minimum.
Since emf induced in the armature coil is minimum along magnetic neutral axis, so the brushes are placed along the magnetic neutral axis or q-axis. However, if you ask practically, then the position of brushes in dc generator should be along d-axis or perpendicular to the magnetic neutral axis. (As in the below fig) Let me explain why?
When the coil is along magnetic neutral axis ( induce voltage in the coil is minimum), segments of armature coil at that instant is getting short circuited at d-axis. In that case, short circuit current produce in the coil segments will be less since rate of change of flux is minimum at this point. This simply means that brushes should be place along direct d-axis. In all other positions, there is certain value of induced emf which will result in the flow of high short circuit currents in the coil segments and ultimately huge sparking. Hence brushes should always place perpendicular to the magnetic neutral axis or we can say along d-axis.
It should be noted that in fig1(Ist fig. of the article). position of the brushes is along magnetic neutral axis which is just to show the direction of coil. Practically, position of the brushes should be along d-axis.
What will happen if neutral zone (magnetic neutral axis) has a induce voltage ?
If neutral zone (magnetic neutral axis) has a induced voltage, there will be circulating current in the coil which results in huge
losses. Circulating currents will also heat up the winding which results in huge sparking. Now, lets come to the effects of armature reaction.
Effects Of Armature Reaction:
The position of brushes should always be along d-axis in order to ensure good commutation. During no load condition, magnetic neutral axis is located between the mid-point of the field poles (orthogonal to the main field flux). Moreover, brushes are also perpendicular to the magnetic neutral axis.
The problem arises when generator is loaded. When dc generator is loaded, current flows through the armature coil due to which armature flux is produced and armature reaction takes place. Once this happens, magnetic neutral axis is shifted or changes its position due to the distortion of main field flux. Similarly, the position of brushes also varies. Hence there is need to shift the brush assembly ( should be perpendicular to the magnetic neutral axis) to avoid the flow of current in the coil.
Actually, the main problem is that load is varying quantity. When load varies, armature current varies which means that armature flux also varies. Change in the armature flux means that magnitude of distortion of main flux also changes. Similarly, magnetic neutral axis also shifted accordingly.
In simple words, we can say that magnetic neutral axis position is changing continuously. In that case, it is difficult to place the brushes at one position (along direct d- axis). Hence armature reaction which is produced due to the load current is causing problem.
Solution of Armature Reaction:
Interpoles:
In order to nullify the effect of armature reaction, commutating poles also known as interpoles are used. Poles flux which cancels out the armature flux is called commutating poles or interpoles. Interpoles are connected in series with the armature winding through brushes. Brushes are wound with these interpoles in such a way that the armature current flows from the brush to the interpoles and then to the external load. Once this happens, there are three fields in the machine.
Main Field:
Flux created due to the stator field poles (main flux).
Armature Field:
Flux produce due to the armature current flowing through the armature conductors (Armature flux).
Interpole Field:
Flux produced due to the armature current flowing in the commutating poles (interpoles).
Now, load carrying armature current flow from the brushes to the interpoles and then to the external load. Flux produce due to these interpoles is such that it is completely opposite and equal in magnitude to the armature flux. Actually, no. of turns on the interpoles are matched in such a way that it cancels out the armature flux produced due to the armature reaction. In that case, both interpole and armature flux cancels out the effect of each other due to which armature reaction is completely neutralized. Hence the only flux exist in dc machine is the main field flux.
Moreover, since interpoles are connected in series with the armature. Hence any increase or decrease in armature current (during load varying condition) also results in the increase or decrease of interpole flux. In simple words, interpole flux completely nullify the effect of armature reaction for all values of load current. This will result in no induced emf in the armature coils at neutral axis. Hence no sparking as well as no circulating current in the armature coil takes place. Moreover, there is also no need to shift the brush assembly.