How does an alternator work?

How does an alternator work?

An alternator also called as AC generator is an electromechanical device which converts energy from mechanical domain to electrical domain. It is most widely used generator for generating the 3 phase power worldwide. When an AC generator is used along with other generators, its operation is in synchronism with them. Under this condition they are also called Synchronous generator.

Working Principle

Basic principle of operation of alternator is similar to that of DC generator i.e on the Faraday’s law of electromagnetic induction which states that

• Whenever the flux linking through a coil changes, an emf is induced in it.

• The amount of emf induced in the coil is directly proportional to the rate of change of flux and number of turns in it. Mathematically it can be summarized as

E= -N (dφ/dt) here (–)ve sign is due to opposing nature of emf induced (Lenz's law)

To satisfy Faraday’s law of electromagnetic induction, alternator should have 3 essential constructional requirements :

• A field system to produce the required working flux.

• A system of conductors or coils to which flux can link and emf can be induced.

• A relative motion between these two system.

Alternators having small voltage and power rating can be constructed in either way (rotating field type system or stationary field system).But alternators having larger voltage and power rating are invariably constructed with rotating field type system i.e. field winding is placed on rotor with DC supply provided to it and armature winding is placed on stator through which electrical output is withdrawn. Advantage for rotating field system are :

• Field system require about 2% of the power capacity of machine which can be easily supplied via slip rings and brushes.

• It is easier to provide the necessary insulation for stationary system of conductors.

• The insulation provided to the stationary system of conductors is not subjected to any kind of mechanical stress due to centrifugal action.

Working

The rotor of alternator is coupled with a prime mover mainly turbine which provides mechanical input to the rotor. Further rotor is also provided with DC excitation. Hence a synchronously rotating magnetic field is created in the air gap. This flux links to the stationary 3 phase armature conductors placed in stator part. As per Faraday’s law, an emf is induced in the conductors. This emf is used to drives the load connected to the output terminal of the alternator.
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