Armature Reaction In DC Machines

In DC machines the term armature reaction describes the distortion of the magnetic field brought on by the current flowing through the armature winding. The machine’s overall magnetic field is affected by the magnetic field produced by the armature as it carries electricity. The machine’s performance especially in terms of voltage regulation & commutation may be impacted by this interaction which may result in changes to the neutral plane. For DC machines to operate steadily and effectively the armature reaction needs to be taken into account and compensated for by engineers.

In a DC machine, there are two types of magnetic forces present one is the armature flux & the main field flux. When the armature flux affects the main field flux it is called an armature reaction.

MNA And GNA

When armature conductors cross magnetic field lines an electromagnetic field (EMF) is created. There is an axis or plane where these conductors do not cross the flux lines as they go parallel to them. This axis where armature conductors moving parallel to flux lines do not generate an electromagnetic field (EMF) is known as the Magnetic Neutral Axis (MNA). Because brush placement helps the current in the armature conductors to reverse, brushes are positioned uniformly along the MNA. Conversely, the stator field axis is perpendicular to the Geometrical Neutral Axis (GNA).

Diagram For MNA and GNA(showing Distortion of the main Field Flux due to the armature Flux)

Leading Pole Tip And Trailing Pole Tip

These two describe the locations of the magnetic poles with respect to the armature rotational direction in the context of armature reaction in DC machines.

Leading Pole Tip: This is the pole tip that is ahead of the neutral plane in the direction of rotation. The combined action of the armature reaction and the main field produces a greater magnetic field at the leading pole tip this increasing flux density.

Trailing Pole Tip: In the opposite direction of rotation the trailing pole tip is situated behind the neutral plane. The main field and armature reaction combine to produce a weaker magnetic field which lowers the flux density.

Analyzing the impact of armature reaction on commutation and overall machine performance in DC machines requires an understanding of the positions of the leading and trailing pole tips. With this knowledge engineers may ensure stable operation and build compensating techniques.

Cases Armature reaction

In DC Generators

Armature reaction in a DC generators are refers to how the magnetic field created by the current in the armature winding affects the main field flux. This effect is consistent in both lap and wave wound machines. While in devices like the amplidyne in this armature reaction is intentionally used in DC generators and motors it leads to unwanted effects.

Let’s consider a two pole machine for simplicity but these effects apply similarly to each pair of poles. Armature reaction impacts all modes of operation for DC motors and generators.

In dc generators

• Geometrical Neutral Plane (GNP)
• Magnetic Neutral Plane (MNP)

Figure displays the primary poles of a DC generator and how the magnetic flux moves through the airgap into the armature. This distribution of flux occurs when the generator operates without a load & it appears symmetrically aligned concerning the poles. The poles contain concentrated coils & when these coils carry current they create a rectangular distribution of magnetic force which results in a corresponding rectangular flux distribution. This happens because the magnetic force acts uniformly across the airgap, making the flux directly proportional to the force at any given point.

In DC Motor

Apart from deliberate weakening of the field the DC machines can also experience a weakening effect known as armature reaction. This effect happens due to the influence of the armatures magnetic force on the machines flux. In small machines it is not a big deal but in larger machines this unwanted field weakening becomes significant enough to require extra design features to counter it. In simple terms, armature reaction occurs because the armatures magnetic force operates perpendicular to the main flux axis. This creates a high reluctance in a direction where the flux has to cross larger air spaces resulting in a relatively small quadrature flux. Normally this wouldn’t impact the average value of the main flux but if the main magnetic circuit it is iron is close to saturation the armature reaction flux can cause a net reduction in the main flux.

Reducing the flux usually increases the speed of the machine. So in a machine with noticeable armature reaction, when the load increases & the armature current rises for more torque the field weakens and the motor speeds up. While not an instability this behavior is not generally seen as desirable.

To counter this effect larger motors often include extra windings in the pole-faces connected in series with the armature. These compensating windings create a magnetic force opposing the armature’s force, helping to reduce or eliminate the armature reaction’s impact.

How to Reduce Armature Reaction

In smaller machines typically up to a few kilowatts there is usually no special action taken to minimize armature reaction. However for larger DC machines specific measures like compensating windings & interpoles are employed to counter the adverse effects of armature reaction. Measures for Larger DC Machines: In larger DC machines, steps are taken to counteract armature reaction. These include

• Compensating winding: Armature reaction arises from the armature flux generated by the current in the armature conductors. To counter this a secondary winding placed close to the armature winding can carry an opposing current to canceling out the armature field. This additional winding are known as the compensating winding sits on the pole faces and is connected in series with the armature carrying current in the opposite direction.
• Interpoles : These are auxiliary poles situated between the main field poles. The winding on each interpole is connected in series with the armature & is wound to match the magnetic polarity of the main pole ahead of it. Interpoles serve to neutralize the armature flux along the quadrature axis.
• Brush Shift: Another method involves adjusting the position of the brushes. By slightly shifting the brushes the commutation process can be optimized or reducing the impact of armature reaction on the machines performance.

Effects of Armature Reaction

• The influence of the magneto motive force (mmf) created by the armature winding or current on the main field flux generated by the field or stator winding is termed armature reaction. This effect has two unfavorable outcomes first is it distorts the primary field and diminishes its overall flux. When distortion occurs there is a cross magnetizing effect while reduced flux results in demagnetization. In essence armature reaction alters the distribution of the main field around the air gap periphery and induces demagnetization within the machine. The demagnetizing impact of armature emf typically reduces total pole flux by around 1 to 5% from no load to full load.
• Iron losses in the pole shoes and teeth depend on the highest flux density they handle. When the main field flux distorts the maximum density under load surpasses that at no load. Consequently the iron losses become more noticeable during operation compared to when the machine is idle.
• Armature reaction leads to a rise in the peak gap flux density causing an increase in the maximum voltage across neighboring commutator segments during operation. If this voltage surpasses 30 V sparking might occur between these adjacent commutator segments.
• Armature reaction moves the brush axis away from GNA, causing a non-zero flux density along the interpolar axis. Consequently, coils undergoing commutation experience induced e.m.f. aiming to sustain the original current direction. This challenge makes commutation harder and results in delayed commutation.

Applications of Armature Reaction

Applications of Armature Reaction

• Compensating Windings: In larger DC machines there is a compensating windings strategically placed to counteract the effects of armature reaction. These additional windings produce a magnetic field opposite to the armature field &mitigating its adverse impact on the main field flux.
• Interpoles: Interpoles are auxiliary poles installed between the main poles of a DC machine. The purpose is to counteract the effects of armature reaction which are particularly the distortion of the main field. They aid in maintaining a more stable & uniform field distribution.
• Motor Speed Control: Understanding and managing armature reaction allows for better control of motor speed. By manipulating the armature current & field flux which engineers can fine tune the motor performance particularly its speed regulation under varying loads.
• Generator Voltage Regulation: In generators armature reaction affects the terminal voltage. Engineers can utilize this phenomenon to regulate and control the output voltage of the generator under different load conditions. This knowledge helps in designing systems that maintain steady voltage levels.
• Advanced Machine Design: Armature reaction is a crucial consideration in the design phase of electrical machines. Engineers use simulation tools and modeling techniques to predict & optimizess the machines performance accounting for armature reaction effects. This ensures more efficient and reliable machine operation in real-world applications.

Conclusion

Armature reaction that is present in DC machines which results from the interaction between the armatures magnetic field & the main field flux impacting performance in generators and motors. It causes distortion in the primary field that is leading to demagnetization and increased iron losses during operation. This phenomenon influences commutation challenges and higher voltages between commutator segments affecting machine stability. Strategies like compensating windings and interpoles help counter these effects. Understanding and managing armature reaction are crucial for optimizing machine performance, enabling better motor speed control, voltage regulation & improved machine design for efficient real world applications.

FAQ on Armature Reaction In DC Machines

What is armature reaction in a DC machine?

Armature reaction refers to the magnetic field distortion caused by the current flowing in the armature windings of a DC machine. This distortion impacts the main magnetic field in the machine.

Does armature reaction vary with the size of the DC machine?

Yes when armature reaction is more pronounced in larger DC machines due to higher currents & larger windings. Small machines might experience negligible armature reaction compared to larger ones.

Is armature reaction always a disadvantage in DC machines?

While armature reaction poses challenges that it also offers insights for controlling motor speed which regulating generator voltage & influencing machine design for improved performance under specific conditions. Understanding and managing armature reaction are crucial for optimizing DC machine operation.