Armature reaction refers to the impact of the magnetic field generated by the current flowing through the armature winding on the distribution of the flux in a machine. This phenomenon occurs in both AC and DC machines and results from the interaction between the armature flux and the main magnetic field, thereby altering the overall flux distribution. Armature reaction can lead to various operational challenges, such as distorted flux patterns, reduced efficiency, and potential damage to the machine if not properly managed. It essentially represents the reaction of the motor's or generator's magnetic field to the armature's influence, indicating a fundamental aspect of electromechanical energy conversion processes that must be carefully considered in the design and operation of electrical machines.

This article focuses on the armature reaction. We will discuss its basics, working, types, and effects in detail. The information in this article helps you extensively in your SSC JE Electrical,  RRB JE, and GATE Electrical preparation journey.

What is Armature Reaction?

Armature reaction is a phenomenon in electrical engineering that describes the effect of the armature magnetic field on the main field in a motor or generator. It occurs when the current through the armature windings interacts with the main magnetic field, causing a distortion in the flux distribution within the machine. 

This interaction can affect the machine’s performance, leading to potential issues like decreased efficiency, increased heat generation, and mechanical stress. Understanding armature reaction is crucial for the design and operational control of electrical machines to ensure they deliver optimal performance and reliability under varying load conditions.

MNA and GNA

• MNA (Magnetic Neutral Axis): is the axis where armature conductors do not cut magnetic flux lines as they move parallel to flux lines hence no EMF is induced. Brushes are placed along MNA for smooth commutation.
• GNA (Geometric Neutral Axis):  is the axis perpendicular to stator field axis. Due to armature reaction, MNA shifts from GNA.

In DC machines, armature reaction manifests as distortion and weakening of the main magnetic field due to the magnetic field produced by the current-carrying conductors in the armature. 

This effect can cause a shift in the neutral plane (the position where the generated voltage is zero), leading to sparking at the brushes and reduced machine efficiency. The extent of armature reaction depends on the current load; higher loads exacerbate the distortion. Proper design and using compensating windings or interpoles are strategies to mitigate its impact, maintaining operational stability and performance.

Fig- Main Field Flux

The effect of armature flux (ϕₐ) on main field flux (ϕₘ) is called armature reaction. The armature reaction mmf produces two undesirable effects on the main field flux and these are:

• (1) Net reduction in the main field flux per pole
• (2) Distortion of the main field flux wave along the air gap periphery.

The armature mmf in a dc machine is stationary with respect to field poles but rotating with respect to armature.

Fig- Armature Flux

Armature reaction of unsaturated DC machine results in cross–magnetizing effect.

• Armature reaction distorts the main flux, hence 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 the armature reaction, it is hard to determine the exact position of the MNA.
• For a loaded DC generator, MNA will be shifted in the direction of the rotation.
• While for a loaded DC motor, MNA will be shifted in the direction opposite to that of the rotation.

Fig- Distortion of Main Flux Due To Field Flux

Armature Reaction in Alternator

In alternators, armature reaction is critical due to its role in voltage regulation and stability. The armature reaction can be either demagnetizing or magnetizing, depending on the power factor of the load. 

• For lagging loads, it's predominantly demagnetizing, weakening the main magnetic field and reducing the terminal voltage. 
• Conversely, for leading loads, it's magnetizing, potentially causing excessive voltages. The alternator's design often incorporates features such as damper windings to stabilize the magnetic flux and maintain voltage levels within desired ranges, safeguarding against the adverse effects of armature reaction.

Effect Of Armature Reaction

The effect of armature reaction in electrical machines includes:

• Flux distortion 
• Altered inductance, 
• And potential operational instability. 

It impacts the electromagnetic torque, leading to inefficiencies and, in some cases, mechanical sesses on the system. For generators, armature reaction can result in output voltage variations, making voltage control challenging. In motors, it may lead to uneven magnetic pull on the rotor, affecting smooth operation. Recognizing and mitigating the effects of armature reaction are essential for designing efficient and reliable electrical machines.

How to Reduce Armature Reaction

Reducing armature reaction involves several strategies aimed at minimizing its detrimental effects on machine performance. In DC machines, employing compensating windings or interpoles helps to neutralize the armature's magnetic field, thereby maintaining a stable flux distribution. For alternators, adjusting the pitch of the winding or incorporating damper windings can mitigate the impact of armature reaction. 

Additionally, maintaining an appropriate air gap and employing advanced design techniques that account for armature reaction effects during the initial design phase can significantly enhance machine reliability and efficiency. By effectively managing armature reactions, the longevity and performance of electrical machines can be significantly improved.

Following methods are used to reduce armature reaction in large DC machines:

• Adjust the Brush Position: Slightly shifting the brush position can optimize commutation process and reduce impact of armature reaction.
• Modify the Ends of the Poles: Modifying pole shoe shape and reducing pole arc can control flux distortion due to armature reaction.
• Compensating Winding: Additional winding on pole faces carries current opposite to armature current to nullify armature flux.

This article summarises all the information related to Armature reaction, which helps in propelling your preparation for various AE/JE and ESE examinations. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examination, like SSC JE, GATE, ESE, RRB JE, and state AE/JE Exams.