Definition of Alternator

An alternator is an electrical generator that converts mechanical energy into electrical energy in alternating current. Most of the alternators use a rotating magnetic field with a stationary armature. The Alternator is used at an electrical generating station and other power generation sectors. An alternator is a machine that is used to generate electrical energy. For this reason, most peoples think that how does alternator operate to generate electricity. We will discuss this in detail below but the alternator or generator can be used for some other purposes.

What is Alternator Operation?

Alternator operation is the rotor winding is energized from the DC excitation and the alternator alternates N and S poles. These are developed on the rotor. When the rotor of the alternator is rotated in the anti-clockwise direction by a prime mover, the stator of the alternator or armature conductors are cut by rotor poles.

Then, e.m.f. is induced in the armature conductors due to the electromagnetic induction process. The induced e.m.f. is alternate from the N and S poles of the rotor and passes through the armature conductors. The direction of induced e.m.f. may be found by Fleming right hand rule, and frequency is given by,

  • f = NP / 120
  • P = number of rotor poles
  • Where N is the speed of the rotor in r.p.m.

The magnitude of the voltage induced in each phase depends upon the rotor flux, the number of conductors in the phase, and the rotor’s speed. When the rotor is rotated, then the 3-phase voltage is induced in the armature of the Alternator. The magnitude of induced e.m.f. depends upon the speed of rotation and the DC exciting current. The magnitude of e.m.f. in each phase of the alternator armature winding will be the same. However, they differ in phase by 120° electrical degree.

What is the Working Principle of Alternator?

Sometimes a question arises how does alternator operate to generate electricity. The working principle of an alternator is elementary. It is just like the basic working principle of a DC generator. It also depends upon Faraday’s law of the electromagnetic induction process. This says the current is induced in the conductor inside a magnetic field of Alternator when there is a relative motion between that conductor and magnetic field.

All of the alternators work on the principle of the electromagnetic induction process. Every alternator operates on the same fundamental working principle of DC generator. The alternator also has the same armature winding and armature field winding as a DC generator. But there is one crucial difference between the two such as. 

In a DC generator, the armature winding is placed on the rotor to convert alternating voltage generated in the armature winding to a direct voltage at the rotating commutator. The field poles are placed on the stationary part of the Alternator. Because no commutator is required in an alternator, it is usually more convenient and advantageous to place the field winding on the rotating part (rotor) and armature winding on the stationary part (stator).


An alternator has a 3-phase winding on the stator and a DC field winding on the rotor. This DC source (called exciter) is generally a small DC shunt or compound generator mounted on the Alternator’s shaft.

Rotor Construction 

There are two types of rotor construction in the Alternator, namely.

  • Salient pole type
  • Non-salient pole type

Salient Pole Type

In the salient pole type alternator, salient are mounted on a large circular steel frame fixed to the Alternator’s shaft.  

Non-Salient Pole Type

In a cylindrical pole type alternator, the rotor is made of a smooth solid forged-steel radial cylinder having several slots along the outer periphery.

What are the Types of Alternators?

From this standpoint, two types of alternators are widely used in electrical power system

  • Revolving armature type
  • Revolving field type.

The four different types of alternators include

  • Automotive alternators used in modern automobiles
  • Marine alternators are used in marine applications.
  • Brushless alternators are used in electrical power generation or Power plants as the primary source of electrical power. 
  • Radio alternators are used for low band radio frequency transmission.

What are the Applications of Alternator?

Alternators are using to generate electricity in power plants, in modern automobiles to charge the automobile battery and to power the electrical system. Until 1960, automobiles used DC dynamo generators with the commutator. Alternators are widely used in electrical power plants to generate electricity. We can say Alternator is the source of electricity.

Difference Between Alternator and Generator

The difference between the Alternator and generator is given here, which explains how they are different from each other concerning structure, output, efficiency, etc. These alternator generator differences may help to get a clear conception. Before starting with the differences, it is essential to know what a generator is and how it works. The differences between the Alternator and generator are included.

Sl. No.Differentiating PropertyAlternatorGenerator
ADefinitionAn alternator is a device that converts mechanical energy into AC electrical energy.A DC generator is a mechanical device that converts AC or DC electrical energy.
BOutput CurrentAn alternator always induces an alternating current.A generator may generate alternating or direct current.
CEnergy EfficiencyAlternators are very efficient.Generators are considered less efficient.
DOutputAlternators have a higher output power than generators.Generators have a lower output power when compared with an alternator.
EEnergy ConservationAlternators use only the required amount of energy and thus, it conserves more energy.Generators use all the energy that is produced and so, they conserve less energy.
FPolarization After InstallationPolarization is not required in the case of alternators.Generators need to be polarized after installation.
GMagnetic FieldThe rotating magnetic field is inside the stator of an alternator.In the case of a generator, the magnetic field is stationary or fixed where the armature winding spins.
HArmature MovementThe armature of an alternator is stationary.The armature of a generator is rotating.
IInput SupplyThe alternator takes the input supply from the stator.The generator takes the input supply from the rotor.

What is the main Difference Between Synchronous and Asynchronous Motor?

DefinitionA synchronous motor is a machine whose rotor speed, and the stator magnetic field are equal.
N= NS = 120f/P
An asynchronous motor is a machine whose rotor rotates at less than the synchronous speed.
N < NS
TypeThe Brushless motor, Variable Reluctance Motor, Switched Reluctance Motor, and Hysteresis motor is the synchronous motor.Asynchronous Motor is called Asynchronous Motor.
SlipThe synchronous motor does not have a slip. The value of the slip is zero.Asynchronous motors have slip therefore the value of slip is not equal to zero.
Additional power sourceIt requires an additional DC excitation to initially rotate the rotor near the synchronous speed.It does not require any additional exciting current.
Slip ring and brushesSlip ring and brushes are requiredSlip rings and brushes are not required.
CostSynchronous motor is costly as compared to Asynchronous motorIt is less costly
EfficiencyEfficiency is greater than Asynchronous motor.It is less efficient
Power factorBy changing excitation the power factor may be adjusted accordingly as lagging, leading, or unity.Asynchronous motor runs only at a lagging power factor.
Current supplyCurrent is given to the rotor of the synchronous motorThe rotor of the Asynchronous motor does not require any current.
SpeedThe Speed of the motor does not depend on the variation in the load. Speed is constant.The Speed of the Asynchronous motor decreases with the increasing load.

Difference Between Asynchronous and Synchronous Generator

The main difference between synchronous and asynchronous generators is rotor synchronism. When the rotor rotates slower than synchronous speed the generator acts as a motor. When the rotor rotates at a more speed than synchronous speed then the rotor acts as a generator.

Synchronous generators are one of the most commonly used alternators. The modern power generation industry is widely used in hydroelectric power, thermal power plant, nuclear power generation, and diesel power generation. The asynchronous motor that operates as a generator is one of the most commonly used alternators.

The asynchronous generator is an alternator that utilizes an air gap rotating magnetic field between a stator and a rotor to interact with an induced current in a rotor winding. According to the working principle, it is also called an Induction generator. The speed of the generator is a little bit higher than the synchronous speed. The output power of the asynchronous generator increases or decreases with the slip rate.

Why do We need Parallel Operation of Alternators?

Alternator parallel operation is an important issue. Now we are going to discuss the parallel operation of alternators. Suppose there are four alternators are installed in any power plant, and they are operating at the same load. If somehow load demand is increased than the rated load, and meeting the extra load demand is not possible then another alternator is needed to connect in parallel to meet the extra load demand which is called alternator parallel operation. It is noted that both alternators must be at the same power and will also rotate at the same (r.p.m.) speed, phase sequence will be the same, and incoming voltage must be the same.

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