Table of Contents
Armature Reaction – Definition
When a motor or generator is running, there are two types of magnetic fields inside it. One magnetic field comes from the main field winding (which is supposed to be there), and the second one comes from the armature winding (the rotating part).
Now, when the armature winding carries current, it also produces its own magnetic field. This new magnetic field mixes with the main field and changes its shape and direction a little. This change is called Armature Reaction.
Simple Definition:
“Armature reaction is the effect of the magnetic field produced by the armature winding on the main magnetic field inside a motor or generator.”
Part of armature reaction in dc machine
Whenever a motor or generator runs, something interesting happens inside it. Let’s break it down step by step in very simple words.
🌀 What is Armature Flux?
Inside a motor or generator, there is a part called the armature. When current flows through this armature, it creates its own magnetic power. This magnetic power is called armature flux (∅ₐ).
Think of it like this:
Just like the sun gives out sunlight, the armature gives out magnetic lines. These invisible lines of magnetism are called flux.
🧲 What is Main Flux (∅ₘ)?
Apart from the armature, there is also a main magnet (or field winding) inside. It also creates its own magnetic flux. This is called the main flux (∅ₘ). This main flux is the one we normally use to run the motor or generate electricity.
⚡ What is Armature Reaction?
Now here’s the fun part.
When the armature starts carrying current (when we give load), it produces armature flux. This new flux interferes with the main flux. It can either help it or oppose it, or sometimes even disturb its shape.
👉 This effect of armature flux on the main flux is called Armature Reaction.
It’s like two people talking at the same time—one may disturb the other’s voice. In the same way, the armature flux changes the way the main flux behaves.
🔍 No-Load vs Load Condition:
- At No-Load:
When there is no current in the armature (like when the machine is not doing any work), the armature flux is zero (∅ₐ = 0). Only main flux (∅ₘ) is present. This is shown in Fig. 1.23. - Under Load:
As soon as we connect a load, armature current flows. Now the armature produces its own flux ∅ₐ. This changes the magnetic field inside.
Fig. 1.24 shows the pattern of armature flux alone, when main flux is not there (∅ₘ = 0).
Armature Reaction in DC Machine (Motor or Generator)
In a real DC machine — whether it’s a generator or a motor — two types of magnetic fields work together at the same time.
- One is from the main field winding,
- The other comes from the armature current (the current flowing in the rotating part).
When both of these fields are present, they combine and create a new magnetic field. This new field is called the resultant flux.
Now, something interesting happens because of this armature current:
- If the machine is working as a DC generator, this resultant magnetic field shifts slightly forward, in the same direction in which the rotor (the moving part) is turning.
- But if it is running as a DC motor, the same flux shifts backward, in the opposite direction of the rotor’s rotation.
You can check this shift using the right-hand thumb rule or Fleming’s rules — they help us understand directions in motors and generators.
Because this magnetic field has shifted, something else also moves — the Magnetic Neutral Axis or MNA. This is an imaginary line where there’s no magnetic field — it’s like a calm zone between the magnetic forces. But when the armature current disturbs things, this calm zone also shifts.
To keep the machine running smoothly, we need to adjust the brushes (the parts that pass current to the rotating part):
- In a generator, brushes should be moved forward — in the same direction as the rotor.
- In a motor, they should be moved backward — opposite to the direction of rotation.
This adjustment helps keep the brushes exactly on the new MNA. The angle by which we move the brushes is shown in a figure (Fig. 1.26) and is usually marked with the symbol θ (theta).
Another thing to notice is what happens near the pole shoes (the ends of the field magnets):
- On the leading edge (the front part in the direction of rotation), the magnetic field becomes weaker.
- On the trailing edge (the back part), the magnetic field becomes stronger.
This whole effect is called armature reaction, and it plays a big role in how well the machine works.
When Brushes Shift in a DC Machine
When we move the brushes in a DC machine from their original place, something important happens inside the machine.
The magnetic power created by the armature (which is the rotating part of the machine) also moves. We call this magnetic power armature flux.
Now imagine this magnetic power as a force named Fa. This force can be split into two smaller forces:
- Fd – This one goes exactly opposite to the main magnetic field of the machine. Because it tries to reduce or cancel out the main field, we call it the de-magnetizing force.
- Fc – This one goes at a right angle to the main magnetic field. It doesn’t fight the field directly, but it bends or twists it. That’s why we call it the cross-magnetizing force.
So, when we shift the brushes, the armature winding (the wires in the rotating part) acts like it is divided into two groups:
- One group weakens the main magnetic field (de-magnetizing).
- The other group bends and distorts the field (cross-magnetizing).
The conductors (wires) that fall between angles marked as POQ and XOY mainly create the de-magnetizing effect. The rest of the wires cause the cross-magnetizing effect.
Effect of Armature Reaction?
When a machine like a generator or motor is running, the armature (the rotating part) creates its own magnetic field. This magnetic field mixes with the main magnetic field that the machine already has.
Now, here’s what happens because of this:
- On one side of the pole, the magnetic power (called flux) becomes weaker, and on the other side, it becomes stronger.
- This unbalance can cause something called magnetic saturation, which means the machine can’t hold any more magnetic power in that part.
- Because of this, the total magnetic force becomes less than what it should be. And if magnetic force is less, then the machine also produces less voltage (emf).
But that’s not all.
- Where the magnetic force becomes stronger, the iron inside the machine gets hotter, which causes more energy loss (called iron loss).
- Also, the voltage between the copper parts of the machine (called commutator segments) becomes higher when the machine is working under load.
- If this voltage goes above 30 volts, it can create sparks between those copper parts — and if that spark keeps growing, it can become a ring of fire, which is very dangerous.
Another problem is with commutation, which is how the machine switches the direction of current. Because of armature reaction, this switching gets slower and less smooth, which creates more sparks at the brushes — the small parts that touch the commutator.
All these things — sparking, heating, and uneven forces — cause the brushes and commutator to wear out quickly, meaning the machine gets damaged faster.
Reduce the Effect of Armature Reaction
When a motor or generator is running, the magnetic field created by the armature (the rotating part) can disturb the main magnetic field. This disturbance is called armature reaction. But don’t worry — we can reduce this effect using some smart methods.
Here’s how:
🔹 1. Move the Brushes Slightly
Normally, brushes sit along a line called GNA (Geometrical Neutral Axis). But if we shift them a little toward the MNA (Magnetic Neutral Axis), the disturbance from the armature becomes smaller. This helps the motor or generator run more smoothly.
🔹 2. Make the Air Gap Bigger at the Ends of the Poles
At the tips of the poles, the magnetic field can get crowded and create problems. If we increase the air gap there (make a little more space), it helps to control the crowding of the magnetic lines.
We usually make the air gap at the pole tips 1.5 to 2 times bigger than the gap at the center.
🔹 3. Increase the Resistance to Magnetism at the Pole Tips
If we make it a bit harder for magnetism to pass through the pole tips (this is called increasing reluctance), it helps reduce the extra magnetic crowding caused by the armature.
🔹 4. Use Compensating Windings
We can place special windings (coils of wire) in the pole faces. These are called compensating windings. They create a magnetic field that cancels out the armature’s disturbing effect in that area.
🔹 5. Add Interpoles Between the Main Poles
Interpoles are small poles placed between the big poles. They also have windings that help cancel out the effect of the armature reaction in the space between the main poles. This keeps the performance steady and clean.
GNA, MNA & Brush Axis
In a DC machine (like a motor or generator), there are big magnets called field poles. One is a North Pole, and the other is a South Pole. Between these two poles, there is a gap where magnetic lines (called flux) flow. These lines help the machine work.
Now, the middle line between the North and South poles is very important. This line is called the Field Axis or Polar Axis. Some people also call it the Direct Axis (d-axis). This line shows where the magnetic force is the strongest. It passes through the center of both the North and South poles.
Right in between the poles, but at a 90-degree angle to the Polar Axis, there is another line. This one is called the Geometrical Neutral Axis (GNA). Think of it like a road that crosses the Polar Axis exactly in the middle. There is no magnetic force along this line, and that’s why it’s called “neutral.” It is also called the Interpolar Axis.
So, in simple words:
- Field Axis / Polar Axis / d-axis: The main line that passes through the center of the magnets.
- GNA (Geometrical Neutral Axis): The line in the middle of the two poles, but at a right angle to the Polar Axis.
How EMF is Induced in a DC Generator
When the armature of a DC generator makes one full rotation (360 degrees), it moves through different parts of the magnetic field. This magnetic field is not the same everywhere. In some areas, it’s strong, and in others, it’s very weak.
Now, think of the armature wires as tiny travelers. As they move across the field axis (the line where the magnetic field is the strongest), they feel the full power of the magnetic field. Here, they cut the magnetic lines straight across, like slicing a rope. This action creates the maximum electricity (EMF) in the wires.
But when these wires pass through a special point called GNA (Geometrical Neutral Axis), something different happens. Here, the wires move almost in the same direction as the magnetic lines instead of cutting across them. So, they don’t feel much of the magnetic field, and as a result, no electricity is produced here.
That’s why this GNA is also called the Magnetic Neutral Axis (MNA)—because it’s like a resting place for magnetism, where no action really happens.
In short:
- Maximum EMF is made at the field axis.
- Zero EMF is made at the GNA.
- GNA is also called MNA, the magnetically quiet zone.
Brushes Are Placed on the MNA in a DC Machine
In a DC machine, we place the brushes on a special line called the Magnetic Neutral Axis (MNA). Do you know why? It’s to make the machine run more smoothly and reduce unwanted sparks between the brush and the rotating part called the commutator.
When a coil is about to change direction (we call this commutation), it passes through the MNA. At this point, the magnetic field around the coil becomes almost zero. Because there is no magnetic push or pull, the voltage created in the coil becomes very, very small. This helps the coil switch direction easily and without creating sparks.
The line that passes through both brushes is called the brush axis. Since it lies exactly at 90 degrees to the main magnetic line (called the d-axis or field axis), it is also called the q-axis (quadrature axis).
So, placing the brushes on the MNA helps the machine run better, with less noise, fewer sparks, and smoother performance.