AC generator – working principles, construction, parts, and induced EMF

AC generator


Imagine you have a smartphone and your smartphone is about to be discharged and you are notified that electrical authorities are gonna be power cut for 3 days in your locality due to a major short circuit in the transformer. After listening to this news, how do you feel? Maybe you become sad. You may feel sad because You can’t able to charge your phone during these three days’ intervals. For today’s generation electricity become a lifeline. Without electricity, peoples start feeling bored and they feel that they miss something.

But have you ever thought about where this electricity came from? How does it produce? Electricity is produced by several means, some are listed below –

  • Hydroelectric power plant – by rotating turbines
  • Geological heat
  • Windmills
  • Nuclear Power plant
  • DC Generators
  • AC Generators
  • Burning of coal etc.

You will have seen that when the electricity goes, peoples start Generators to fulfill the requirement of electricity. So have you thought about what are these Generators? How does its work? If you didn’t think about then don’t worry. Today we are going to talk about AC generators in detail.

What is an AC generator?

AC Generator is a machine, which converts mechanical energy into electrical energy by rotating the coil in a magnetic field by some external means. AC Generator is a famous electricity-producing machine all over the world. And it is far far better than DC Generator. AC Generators are also called alternators.

Principle behind AC generators

The principles behind the working of an AC Generator are – whenever the magnetic flux linked with the coil changes, an EMF is induced in the coil and this induced EMF gives the induced current in the coil. And the phenomenon by which EMF is induced in the coil by changing the magnetic flux linked to the close circuit is called electromagnetic induction.


The construction of an AC Generator is not very complicated, anyone can make AC Generator at home, but it is necessary that the person know some elementary knowledge of electromagnetism. Here you can see the basic construction of the AC Generator.

AC generator - working principles, construction, parts, induced EMF
Diagram of an AC generator

Parts of an AC generator

The main basic parts of an AC Generator are –

  • Two permanent magnets
  • Soft iron core
  • Armature coil
  • Slip rings
  • Brushes
  • Prime mover

Now let’s talk about their functions in detail –

Two permanent magnets

This is a very very important part of an AC Generator. You can’t imagine an AC Generator without these magnets. This magnet is curved in shape and attached to the wall of the stator (A stator is the stationary part of the AC Generator in which the armature coil rotates). The main purpose of these two permanent magnets is to create a magnetic field.

Soft iron core

A soft iron core is a material that protects and supports the coil. The binding of coils is done on this soft iron core. It is done because a soft iron core can concentrate the magnetic field as much as 50,000 times more intensely than an air core. So it is used to increase the strength of the magnetic field.

Armature coil

It is the coil of copper that is bound to the soft iron core. A soft iron core with copper coiling forms an armature of cylindrical shape which rotates in between the stator and cut magnetic field lines. This armature coil is also called the rotor. Due to magnetic flux change induced EMF and current formed in this coil.

Slip rings

These are hollow rings that are connected to the endpoint of the armature coil. Slip rings are the point of transmission of induced current through the brushes to the external electrical equipment. It is insulated from the inside and fixed on the axle or shaft and it helps in the smooth rotation of the armature or rotor.


Brushes are made up of stationary graphite and metal contact (brush). It is connected to the slip rings and used for collecting the induced current from it. When armature rotates in between the stator then slip rings also rotate and when slip rings rotate these brushes rub on its outer curved surface and collect the induced current.

Prime mover

A Prime mover is an external machine having a diesel or kerosene tank, which is used for rotating the rotor (armature coil) in the stator. This prime mover can be diesel engines or electric motors. When prime mover starts, it creates a very loudly noisy sound.

Working of an AC generator

AC generator – working principles, construction, parts, induced EMF
Working of an AC generator

Now let’s understand the working of the AC Generator, basically, the AC Generator works on the principles of Michael Faraday’s laws of electromagnetic induction.

When the armature coil or rotor rotates in the magnetic field by the external machine like diesel engines or electric motors. The magnetic flux linked to the coil changes continuously. This continuously changing magnetic flux in the coil give the induced current or EMF. And this induced current is transmitted by the slip rings through the brushes to run the electronic machines.

This induced current is formed due to the changing magnetic flux linked to the coil, if any current is formed due to changing of magnetic flux then this type of current is also changing, it means changing magnetic flux induced only by changing current, and this changing current is called alternating currents. Then we can say that any currents which are formed due to the process of electromagnetic induction are alternating currents.

Watch this video for more understanding.

Derivation of induced EMF and current

According to the Michael Faraday laws of electromagnetic induction. Whenever the amount of magnetic flux linked to the circuit changes an EMF is induced in the circuit. Mathematically, it is given as – $$\mathcal{E}= = -\frac{d\phi}{dt}$$ If $N$ is the number of turns in the coil then – $$\mathcal{E} = -N\frac{d\phi}{dt}\qquad ….(1)$$ Magnetic flux $(\phi)$ is given as $$\phi = B.A = BA\cos\theta \qquad…(2)$$ Put the value of $\phi$ in equation …(1) $$\mathcal{E} = -N\frac{d}{dt}(BA\cos\theta)\qquad…(3)$$ Here, θ is the angular displacement of the rotor, and it is given as the product of angular velocity and time. $$\theta = \omega t$$

Put the value of θ in equation …..(3) $$\begin{aligned} \mathcal{E}& =-N\frac{d}{dt}(BA\cos\omega t)\\ & = -NBA\frac{d}{dt}(cos\omega t)\\& = NBA\omega \sin\omega t\\ \mathcal{E}& = \mathcal{E}_0\sin \omega t \end{aligned}$$  where $\mathcal{E}_0 = NBA\omega$ , called peak alternating voltage. Thus, $\displaystyle{\mathcal{E} = NBA\omega \sin\omega t}$ is the induced EMF in the armature coil. If $R$ is the resistance of the coil, then current is given as: $$I = \frac{\mathcal{E}}{R}$$ Or $$I = \left(\frac{NBA\omega}{R}\right)\sin\omega t$$ $\displaystyle{I = I_0\sin\omega t}$, where $\displaystyle{I_0 = \frac{NBA\omega}{R}}$ called peak alternating current.

Stay tuned with Laws Of Nature for more useful and interesting content.

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