Alternator, How it works?

How does an Alternator Work?

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. Electricity is produced in alternators by electromagnetic induction. The way the alternator generates  electricity is from two types of alternator.

Types of Alternator

Type – 2          Type 2 is a rotating armature with a stationary magnetic field. Most alternators use are rotating magnetic field with a stationary armature.

Alternator Construction

How does an alternator work?

Kind of rotor shown here is known as salient pole rotor. For gaining better insight of its working let’s consider a rotor with just four poles. Rotor coils are excited with a D.C. power source.

The rotor is made to rotate by a prime mover, this makes that rotor flux also rotate along with it, at the same speed. Such revolving magnetic flux now intersects the armature coils which is fitted around the rotor. Magnetic flux will generate an alternating electromagnetic force across the winding.

Since four pole rotor has got two pairs of north and south poles, so when the rotor turns a half revolution the induced E.M. F. Takes one complete cycle so it is clear that frequency of the induced E.M.F. is directly proportional to the number of Poles in rotor speed

It can be easily established that frequency of induced E.M.F. rotor speed and number of Poles are connected through the following relationship.

      f(Hz), N(rpm), P

f = PN/120

It is clear from this relationship that frequency of electricity produced is synchronized with mechanical rotational speed. For producing three phase AC current two more such armature coils which are in one hundred twenty degree phase difference are star connected. Also It is clear from past equation that in order to produce 60 hertz electricity a four pole rotor should run at following speed of 1800 rpm. Such huge R.P.M. Will induce a tremendous centrifugal force on poles of the rotor and it may fail mechanically over time.

f = PN/120, 1800 rpm

Generally having ten to forty poles which demands lower rpm. Used when the prime mover rotates at relatively lower speed from 120 to 400 rpm. Pole core Is used to effectively transfer magnetic flux and they are made with fairly thick steel lamina. Such insulated lamina The reduces energy loss due to any current formation.

D.C. current is supplied rotor with a pair of slip rings. This is the reason why rotating magnetic field approaches used an alternator. If rotating coil method were used slip rings have to fit it along with the armature coils in order to collect electricity but transferring such high voltage electricity with sliprings rather impractical it is quite possible to transfer low voltage D.C. excite current with slip rings. This D.C. current is supplied either from an external source or from a small D.C. generator which is fitted on the same prime mover such alternators are called self excited. With variation of load generator terminal output voltage will vary it is desired to keep the terminal voltage in a specified limit automatic voltage regulator help seen achieving this voltage regulation can be easily achieved by controlling the field current. Terminal voltages below the desired limiter regulator increases the field current this will result with increasing terminal voltage if terminal voltages are below the specified limit the reverse is done.