What is an AC Generator : Construction & Its Working Principle

An AC generator is a device that converts mechanical energy into alternating electrical energy for appropriate use. Based on the type of power input, there are two types of generators – AC generator and DC generator. Slip rings are used in AC generators to produce alternating current, while direct current is used in DC generators. AC generators are used in power plants, electric scooters, sailboats, bicycles, and so on. The input to the AC generators is usually mechanical energy which is supplied by steam and gas turbines &  internal combustion engines. AC generators are useful in wind turbines, small hydro-power plants, or in decreasing gas streams of higher pressure into lower pressure.

What is AC Generator?

Definition: The AC generator is a machine that converts mechanical energy into electrical energy in the form of alternative emf. A simple AC generator works on the principle of Faraday’s Law of Electromagnetic Induction. It has a coil of wire that rotates in a magnetic field.

Working Principle

AC generator working principle is, these are commonly referred to as alternators which work on the principle of Faraday’s Law of Electromagnetic Induction. The movement of a conductor in a uniform magnetic field changes the magnetic flux linked with the coil, thus inducing an emf.

Simple AC Generator
Simple AC Generator

The parts of the AC generator consists of a coil, slip rings, brushes, and a strong magnetic field as its main components.

Working of AC Generator

The coil is rotated in the magnetic field to produce a strong magnetic field. As coil on one side moves up through the magnetic field, an emf is induced in one direction. As the rotation of the coil continues and this side of a coil moves down and another side of the coil moves up, an emf is induced in the reverse direction. Fleming’s right-hand rule is used to determine the direction of the induced emf. This process repeats for every cycle and the emf produced is of alternating type.

Different Positions of a Coil
Different Positions of a Coil

The output of an AC generator is shown above with a graph.

  • A – When the coil is at 0 degrees, the coil moves parallel to the direction of the magnetic field and hence induces no emf.
  • B – When the coil is at 90 degrees, the coil moves at 90˚ to the magnetic field and hence induces maximum emf.
  • C – When the coil is at 180 degrees, the coil again moves parallel to the magnetic field and hence induces no emf.
  • D – When the coil is at 270 degrees, the coil again moves at 90˚ to the magnetic field and hence induces maximum emf. Here, the induced emf is opposite to that of B.
  • A – When the coil is at 360 degrees, the coil has completed one rotation and it moves parallel to the magnetic field and induces zero emf.

Consider a coil of rectangular shape with ‘N’ turns that rotates in a uniform magnetic field ‘B’ of an angular velocity ‘ω’. The angle between the magnetic field ‘B’ and normal to the coil at any time ‘t’ is given by, θ = ωt.

In this position, the magnetic flux is perpendicular to the plane of a coil and is given by B Cos ωt.

The magnetic flux linked with a coil of N turns is ɸ = B Cos ωt A, where A is the area of a coil.

The induced emf in the coil is given by Faraday’s Laws of Electromagnetic Induction, which is

ε = – dØ/dt

= – d (NBA Cos ωt)/dt

ε = NBA ω | sin ωt —— (i)

When the coil rotates through 90˚, the value of sine becomes 1 and the emf induced will be maximum, the above equation (i) reduces to,

ε0 = N Bm A ω = N Bm A 2πf ——- (ii)

Where Bm refers to the maximum flux density in Wb/m2

‘A’ refers to the area of a coil in m2

‘f’ = frequency of rotation of a coil in rev/second.

Substitute (ii) in (i),

ε = ε0 sin ωt

Induced alternating current is given by, I = ε/R = ε0 sin ωt/ R

Construction of AC Generator

The simple AC generator has two main parts – Rotor and Stator. The rotor is a rotating component and the stationary part of a machine is a stator.


The stator is a stationary component that efficiently holds armature winding. The armature winding purpose is to carry current to the load and the load can be any external equipment that consumes electrical power. It consists of three main parts:

  • Stator Frame – It is an outer frame that is used to hold stator core as well as armature windings.
  • Stator Core – It is laminated with steel or iron to reduce eddy current losses. Slots are made on the interior part of a core to hold armature windings.
  • Armature Windings – Armature windings are wound on the slots of armature core.


The rotor is a rotating part of an AC generator. It consists of magnetic field windings. DC supply is used to magnetize magnetic poles. Each end of magnetic field windings is attached to slip rings. This combination is connected to a common shaft on which the rotor rotates. The two types of the rotor are salient pole rotor and cylindrical pole rotor.

Salient Pole Rotor

The salient pole rotor type is shown in the figure below. In this type of rotor, the number of poles gets projected, known as salient poles with their bases pinned to the rotor can be observed. They are used in low and medium speed applications.

Salient Pole Rotor
Salient Pole Rotor

Cylindrical Pole Rotor

Cylindrical type rotors consist of an unruffled and robust cylinder with slots arranged on the outer surface of a cylinder. It is used in high-speed applications. The diagram of the cylindrical pole rotor is shown below.

Cylindrical Rotor
Cylindrical Rotor

Types of AC Generator

AC generators are of two types. They are

Asynchronous Generators

Asynchronous generators are also known as induction generators. In this type of generator, slip helps the rotor to rotate. Rotor always attempts to match the synchronous speed of a stator but fails. If the rotor matches the synchronous speed of a stator, the relative velocity becomes zero, and hence the rotor experiences no torque. They are suitable to run wind turbines.

Synchronous Generators

The synchronous generator is a type of AC generator that rotates at a synchronous speed. It works on the principle of Faraday’s law of electromagnetic induction – an emf is induced when a coil rotates at a uniform magnetic field. They are mainly used in power plants to generate high voltages.


The applications of the ac generator mainly include generating power from windmills, hydro-electric dams, and many more.


1). What is the difference between the AC generator and the DC generator?

In the AC generator, the electrical current reverses its direction periodically to become alternating current. In the DC generator, the electrical current flows in a single direction.

2). Do car alternators have AC or DC?

Primarily, the AC current is generated in the rotating armature and uses a commutator and brushes to convert to DC.

3). AC generator works on which principle?

It works on the principle of Faraday’s Laws of Electromagnetic Induction.

4). Name the types of AC Generators.

Synchronous and Asynchronous AC generators

5). Are batteries AC or DC?

Batteries are DC as they conduct current only in one direction.

In this article, we discussed the AC generator and its working principle. The reader can gain insights on AC Generator, types, construction, and applications. Here is a question for you, what is the function of the AC generator?

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