Understanding Armature Reaction in DC Machines: Effects, Causes, and Solutions

Introduction: In a DC machine, two types of magnetic fluxes are present: armature flux and main field flux. The interaction between these is known as armature reaction. This phenomenon influences machine performance, and understanding it is crucial for electrical engineers.

MNA and GNA: Key Axes in DC Machines

EMF is induced in the armature conductors when they move through the magnetic field lines - cutting the magnetic field lines. If you check the image below, you can see that the main field flux lines are horizontal, so when armature is rotating, there is a point in time when the armature conductor move parallel to this main flux - so not actually cutting the lines. At this point, no EMF will be induced.

• Magnetic Neutral Axis (MNA): The axis where no EMF is induced because conductors move parallel to magnetic field lines. Brushes are always placed along the MNA because reversal of current in the armature conductors takes place along this axis.
• Geometrical Neutral Axis (GNA): The axis that is perpendicular to the stator field axis.

What is Armature Reaction?

Armature reaction refers to the distortion and weakening of the main magnetic field due to the magnetic field generated by the armature current. The image below illustrates how this effect changes the flux distribution:

How it happens?

• Consider, no current is flowing in the armature conductors and only the field winding is energized (as shown in the first figure of the above image). In this case, magnetic flux lines of the field poles are uniform and symmetrical to the polar axis. The 'Magnetic Neutral Axis' (M.N.A.) coincides with the 'Geometric Neutral Axis' (G.N.A.).
• The second figure in the above image shows armature flux lines due to the armature current. Field poles are de-energised.
• Now, when a DC machine is running, both the fluxes (flux due to the armature conductors and flux due to the field winding) will be present at a time. The armature flux superimposes with the main field flux and, hence, disturbs the main field flux (as shown in third figure of the above image). This effect is called as armature reaction in DC machines.

The adverse effects of Armature Reaction:

1. Flux weakening: In a dc generator, weakening of the main flux reduces the generated voltage.
2. Flux distortion: Due to this, the position of M.N.A. gets shifted (M.N.A. is perpendicular to the flux lines of main field flux). Brushes should be placed on the M.N.A., otherwise, it will lead to sparking at the surface of brushes. So, due to armature reaction, it is hard to determine the exact position of the MNA

Note:

• In a loaded DC generator, MNA shifts in the direction of rotation.
• In a loaded DC motor, it shifts opposite to the direction of rotation.

How to reduce Armature Reaction?

Usually, no special efforts are taken for small machines (up to few kilowatts) to reduce the armature reaction. But for large DC machines, compensating winding and interpoles are used to get rid of the ill effects of armature reaction.

1. Compensating winding:

Now we know that the armature reaction is due to the presence of armature flux. Armature flux is produced due to the current flowing in armature conductors. Now, if we place another winding in close proximity of the armature winding and if it carries the same current but in the opposite direction as that of the armature current, then this will nullify the armature field. Such an additional winding is called as compensating winding and it is placed on the pole faces. Compensating winding is connected in series with the armature winding in such a way that it carries the current in opposite direction.

2. Interpoles:

Interpoles are the small auxiliary poles placed between the main field poles. Winding on the interpoles is connected in series with the armature. Each interpole is wound in such a way that its magnetic polarity is same as that of the main pole ahead of it. Interpoles nullify the quadrature axis armature flux.

Frequently Asked Questions

Q1: Why is armature reaction important?

A: It affects voltage generation, efficiency, and brush performance.

Q2: Do all DC machines require compensation?

A: Small machines (less than a few kW) often don’t need compensation. Larger machines do.

Q3: Can poor brush placement cause damage?

A: Yes. Misaligned brushes due to MNA shift can cause sparking and wear.

Conclusion

Understanding armature reaction is vital for the effective operation and maintenance of DC machines. Whether you're a student or practicing engineer, mitigating its effects using compensating windings or interpoles is essential for efficient machine performance.