.jpg "How to reduce armature reaction?")
Field coils create the primary field in a DC machine. The armature carries current and creates a magnetic field, known as the armature flux, in both the generating and motoring modes. The term "armature reaction" refers to the impact of armature flux on the main field.
What is armature reaction?
Effect of armature reaction.
- It demagnetizes or weakens the main flux.
- It cross magnetizes or distort it
- It demagnetizes or weakens the main flux. As a result of the armature flux's demagnetizing effect on the field flux, the induced emf is reduced, which lowers the generator's terminal voltage.
- The magnetic neutral axis (MNA) shifts away from the ground plane as a result of the armature reaction, which also affects how air-gap flux is distributed. Since brushes were present on GNA, the commutation action is affected if the brush axis is not adjusted to MNA.
- Iron losses typically remain consistent whether the machine is operating under load or not. But when the generator is loaded, the armature reaction has an impact on the flux density. As a result, iron losses are higher when the generator is loaded than when it is not.
The first effect leads to reduced generated voltage and the second one leads to the sparking at the brushes.
How to reduce armature reaction?
For small machines (up to a few kilowatts), reducing the armature reaction is typically not a priority. However, compensating winding and interpoles are employed in large DC machines to eliminate the negative consequences of armature reaction.
♦ Increasing brush contact resistance.
Sparking can be minimized by employing a high brush contact resistance with the commutator section being converted.
♦ Using compensating winding.
In large dc machines, the armature reaction's cross-magnetizing impact can be neutralized by utilizing an additional winding known as "Compensating Winding." This compensating windings are positioned in embedded slots on the main poles and face the armature conductors, which induce the cross-magnetizing effect. These windings are coupled in series with the armature windings.
♦ Using Interpoles or Commutating poles or Compols.
These are small auxiliary poles that contain a few turns of copper conductor coiled around them. They are positioned between the main poles and connected to the armature winding in series.
Interpoles produce a counter mmf in the interpolar region to offset the influence of armature conductor mmf, similar to compensatory winding (GNA).
