characteristics of dc shunt generator: Working Principle, EMF Equation, Use

DC Shunt Generator

A DC Shunt Generator is a special type of DC generator that is commonly used because it works on its own without needing any outside power to get started. It gives a stable voltage, whether you’re using it a little or a lot.

Let’s understand characteristics of dc shunt generator how it works in an easy way.

Imagine you have a wheel (called an armature) that is spun by a motor or engine (called the prime mover). As this wheel starts to turn, it cuts through tiny invisible lines of magnetic force. These magnetic lines are already present inside the machine, even before it starts spinning. They are called residual flux – kind of like leftover magnetism from before.

As the wheel turns, a small amount of electricity (called emf) is created in the wires inside. This electricity flows into another set of coils called the shunt field winding. These coils create even more magnetic lines that go in the same direction as the original leftover magnetism.

This makes the magnetic field stronger, which then makes more electricity, which again makes the magnetic field stronger—and the cycle keeps going. This building-up process continues until the machine naturally balances itself. Once that happens, the voltage becomes steady and does not keep increasing.

This point of balance depends on three main things:

1. The shape of the machine’s voltage graph (called open circuit characteristic)
2. The critical resistance (the maximum resistance where voltage can still build up)
3. The critical speed (the lowest speed at which the generator can still start producing voltage)

To understand the flow of electricity inside, we can use a simple rule called KVL (Kirchhoff’s Voltage Law), which helps us calculate how voltage behaves in a closed loop.

Working Principle

1. Magnetism Creates Power:
   • When the generator starts rotating (by a motor or engine), the armature moves inside a magnetic field.
   • This movement cuts the magnetic lines, and according to Faraday’s Law, electricity is generated in the armature.
2. Exciting the Field Winding:
   • In a shunt generator, the field winding gets power from the same generator. It is connected across the output terminals.
   • As soon as the armature produces a small voltage, a tiny current flows into the field winding.
3. Stronger Magnetic Field Builds Up:
   • This small current creates a magnetic field in the shunt winding.
   • This magnetic field helps the armature to generate more voltage.
4. Voltage Builds Up Gradually:
   • The more the voltage, the stronger the field gets. The stronger the field, the more voltage is produced.
   • This continues until the generator reaches its full rated voltage.
5. Stable Output:
   • Once running, the DC shunt generator gives a smooth and steady voltage output, which makes it suitable for many uses.

Understanding the DC Shunt Generator

Let’s learn how a DC shunt generator works, step by step, in the easiest way possible.

⚙️ The Main Parts in the Diagram:

• Ish → This is the shunt field current. It flows through the field winding and helps to create a magnetic field.
• Ia → This is the armature current. It flows in the armature (the main part that rotates).
• IL → This is the load current. It goes to the device or machine that is using the electricity.

🔌 Some Basic Terms:

• Ra → The resistance inside the armature winding.
• V → The voltage you get at the terminals (called terminal voltage).
• Vbr → The small voltage drop across the brushes (where the current is collected).

🔄 How the Currents Work Together

The current that flows inside the armature is the sum of two things:

Ia = IL + Ish

This means the armature sends current to both the load and the field winding.

⚡ Shunt Field Current:

The current through the shunt field winding is found like this:

Ish = V / Rsh

Where Rsh is the resistance of the shunt field winding.

🔋 Terminal Voltage Equation

The voltage at the output terminals is a little less than the voltage created inside the generator (because of some loss). It is given by:

V = Eg - Ia × Ra - Vbr

Where:

• Eg is the generated voltage (EMF) inside the armature.
• Ia × Ra is the voltage drop inside the armature.
• Vbr is the brush drop.

In many simple cases, Vbr is very small and sometimes not counted.

⚡ Power Equations

1. Power Developed Inside the Generator:

Pg = Eg × Ia

This is the total electrical power made inside the generator.

2. Power Given to the Load:

PL = V × IL

This is the power that actually goes to run your devices.

📘 EMF Equation

The voltage generated (called Eg) inside a DC shunt generator is:

Eg = (P × Φ × Z × N) / (60 × A)

Where:

• P = Number of poles
• Φ = Magnetic flux per pole
• z = Total number of armature conductors
• N = Speed of rotation in RPM (revolutions per minute)
• A = Number of parallel paths in the armature

🧠 Final Recap (In Short):

• The armature current splits into load current and shunt field current.
• The voltage at the output is slightly less than what’s generated inside, because of some resistance and brush drop.
• Power is made inside the armature and sent to the load.
• A special formula tells us how much voltage is produced based on how fast it spins and how strong the magnetic field is.
  
  

Characteristics of DC Shunt Generator

What is Critical Resistance (Rc)?

Let’s understand this with a very simple idea.

Imagine a DC shunt generator is like a small factory that makes electricity on its own — we call this self-excitation. But for this to happen, the machine needs a little help from a part called the field winding. This part has a certain resistance, just like how some pipes resist the flow of water.

Now here comes the important part:

👉 Critical resistance is the highest resistance that the field winding can have — and still allow the generator to start making electricity by itself at a given speed.

If the resistance becomes more than this critical value, then the machine just won’t excite. It won’t be able to build up its voltage properly. It might give a tiny voltage due to leftover magnetism, but it won’t work as it should.

🌀 Why does this happen?

Let’s think about a few different speeds: N1, N2, and N3. At each speed, the generator behaves a little differently.

Now suppose we draw curves (like hills) showing how the generator builds up voltage at each speed. These are called OCC curves — just think of them like “voltage building” shapes.

At the same time, we also draw straight lines that represent different values of resistance in the field winding. These lines can be more or less slanted depending on the resistance.

Now watch what happens:

• At a certain speed (say N1), the straight line just touches the OCC curve. That’s the critical point — this line shows the critical resistance at speed N1.
• If the line goes above (steeper), it misses the curve. That means: resistance is too high, and the machine won’t start by itself.
• If the line is below (less steep), it cuts the curve, meaning: the generator will work fine and generate voltage.

📌 In Simple Words:

• Critical Resistance (Rc) is the limit.
• It tells us how much resistance we can allow in the field winding for the generator to just start working on its own.
• If resistance goes above this limit, the machine fails to excite.
• Rc changes with speed. Each speed has its own Rc value.
  
  

Critical Speed (Nc) of a DC Shunt Generator

To understand critical speed, let’s start with a simple idea.

A DC shunt generator can make its own electricity — this is called self-excitation. But for this to happen, the generator’s rotor (the rotating part) must spin fast enough. If it spins too slowly, it won’t be able to build up voltage on its own.

Now, here comes the main point:

Critical speed is the minimum speed at which the generator’s rotor must rotate so that the generator can just start self-exciting.

If the rotor turns slower than this critical speed, the machine won’t produce voltage on its own — no matter how long it runs.

How it works (in simple words)

• Inside the generator, there’s a field winding with some resistance.
• When the rotor turns, it creates a small voltage using the residual magnetism left in the machine.
• This small voltage flows into the field winding and helps boost the magnetic field.
• As the magnetic field grows, the voltage also grows — this is a loop.
• But this loop only works if the rotor is spinning fast enough.

Why speed matters

Imagine you’re trying to pump water up a hill using a bicycle. If you pedal slowly, the water won’t go up. But if you pedal fast enough, the water starts moving. The same thing happens in a generator — if the rotor isn’t fast enough, the voltage just won’t grow.

Important things to remember

• Critical speed depends on the resistance of the field winding. If the resistance is high, you may need a higher speed to self-excite.
• If the speed drops below the critical speed, the generator will not build voltage.
• So, to make sure a DC shunt generator works properly, it must always run at or above its critical speed.
  
  

Critical Resistance and Critical Speed

Imagine a machine that makes electricity. This is called a DC generator. In this activity, we are going to learn how we test and understand something called its Critical Resistance and Critical Speed.

We use a special setup. A device called a variable rheostat is connected in a line (series) with the field winding of the DC generator. This helps us control how much current goes into the field winding.

Then, a voltmeter is connected across the armature terminals. This voltmeter tells us how much voltage the generator is producing.

Now, the generator’s rotor (the rotating part) is turned at a constant speed – the normal speed it should run.

Next, we slowly change the current in the field winding by adjusting the rheostat. As we do this, we write down:

• The voltage shown by the voltmeter.
• The field current going through the winding.

We then draw a graph by putting field current on the horizontal axis and terminal voltage on the vertical axis. This graph is called the Open Circuit Characteristic (OCC).

This graph helps us understand how the generator behaves when it is not connected to any load.

In real life, there is almost no difference between the OCC of a DC separately excited generator and a DC shunt generator. Both give the same kind of graph.

External Characteristic of a DC Shunt Generator

Let’s understand this in a very easy way.

The external characteristic of a DC shunt generator shows how the voltage at the terminals of the generator changes when we increase or decrease the load current.

Imagine a generator is running, and we are connecting different-sized loads (like bulbs, fans, or machines) to it. The generator is spinning at the same speed, and the field winding (which creates the magnetic field) stays unchanged. Now, as we add more load, the current increases, and we start to notice something:
👉 The voltage at the terminals begins to drop.

But why does this happen?

There are three main reasons for this voltage drop:

1️⃣ Ohmic Drop in Armature Winding

Inside the generator, current flows through wires. All wires have some resistance, even if it’s small. When current flows through them, a little voltage is lost in the form of heat.
👉 This is called the ohmic voltage drop.

2️⃣ Effect of Armature Reaction

As more current flows, it creates its own magnetic field inside the generator. This field disturbs the main magnetic field, making it weaker.
👉 Because of this, the voltage that the generator produces also drops.

3️⃣ Drop in Field Current (Important for Shunt Generator)

This is a special point for DC shunt generators. The field winding is connected across the same terminals from which we take the output. So when the output voltage drops (due to the first two reasons), the voltage across the field winding also drops.
👉 This means less current flows in the field winding, making the magnetic field even weaker.
👉 And that further reduces the voltage the generator can make!

So, this cycle continues, and the voltage keeps dropping more and more as load increases.

Load Test on DC Shunt Generator

🌟 What is a Load Test?

A load test is like giving a machine a small job and slowly increasing the work to see how well it performs. Just like we test a bicycle by riding it on different roads, we test a DC shunt generator by giving it more and more load and watching how it behaves.

⚡ What is a DC Shunt Generator?

A DC shunt generator is a special kind of electric machine. It makes direct current (DC) electricity. The word “shunt” means that the field winding (which creates the magnetic field) is connected in parallel with the armature (the rotating part). Both work together to produce electricity.

🛠️ Why Do We Do Load Testing?

We do a load test to check:

• Is the generator working properly?
• Does it give the correct voltage when we increase the load?
• Is it strong enough to handle more work?
• Is it safe and reliable?

🔌 How Do We Do the Load Test?

Here’s a simple step-by-step way to do the test:

1. Start the generator without any load. This is called no-load condition.
2. Measure the voltage and current. Write it down.
3. Slowly add load (like connecting bulbs or heaters).
4. For each load, note the voltage, current, and power.
5. Keep increasing the load little by little.
6. Stop when the voltage starts dropping a lot or when the generator reaches its maximum safe limit.
7. Draw a graph to show how voltage changes as load increases.
   
   
   
   

Where Do We Use DC Shunt Generators?

DC shunt generators may look like small machines, but they do some very useful work in our everyday life. Let’s see where and how they help us:

• Used in Electroplating
  This generator is used to help coat one metal over another. It keeps the process smooth by giving a steady flow of electricity.
• For Charging Batteries
  It gives a steady voltage which is just right for charging batteries safely.
• When We Need Steady Voltage
  In places where we need the voltage to stay the same and not jump up and down, this generator works perfectly.
• For Lights and Alternators
  It helps in lighting up small systems and also gives power to start big machines like alternators.
• In Trains for Braking
  In electric trains, it helps slow down the train safely by giving power back during braking. This is called regenerative braking.
• Where We Need to Control Speed
  It helps run DC motors where we need to change or control the speed easily.
• As Small, Portable Generators
  These are also used as small generators in places where only a little power is needed, like for camping or in small workshops.
• In Toys, Motorcycles, and Shaving Machines
  It is even used in small things like toy cars, motorcycles (as dynamos), and electric shavers.
• For Welding
  In arc welding, where we need strong and steady electric current, these generators are very useful.

What is a DC Shunt Generator?

A DC shunt generator is a type of machine that makes electricity. Inside it, two important parts—the field winding and the armature—are connected side by side (in parallel). This way, the electricity made by the machine goes both to the machine itself and to the thing it’s powering.

If it doesn’t have a built-in magnet, it needs help from a small battery to start working. This battery gives it the first push of power, and then the generator keeps going on its own.

FAQ