How does an Alternator work ?

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Alternators are the workhorse of the power generation industry.It is capable to generate AC power at a specified frequency. They are also referred as Synchronous generators. This video gives a detailed and illustrative introduction on working of alternators.
Detailed webpage version of of the video is given below.

The Basic Principe

Electricity is produced in alternators by electromagnetic induction. To generate electricity in a coil either the coil should rotate with respect to a magnetic field or a magnetic field should rotate with respect to the coil.

Fig.1 Two methods to produce electricity: Rotating coil and Rotating magnetic field concept
In the case of alternators the latter approach is used. The reason behind rotating magnetic filed approach will be discussed in coming sessions.

Main Parts and Working

Rotor and Armature coils are the 2 main parts of an alternator. Rotor produces a rotating magnetic field. Armature coils are stationary and rotating magnetic flux associated with the rotor induces electricity in the armature coils.

Fig.2 Rotor and Armature coils are the 2 main parts of an alternature
The 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 4 poles. Rotor coils are excited with a DC power source. Magnetic field produced around it would be as shown.
Fig.3 A 4 pole salient pole rotor and magnetic filed produced around it when excited by a D.C power supply
The rotor is made to rotate by a prime mover. This makes the 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. This will generate an alternating E.M.F across the winding.

Fig.4 When rotor is made to rotate electricity gets induced in armature coils

Frequency of Induced E.M.F

Since 4 pole rotor has got 2 pairs of N-S pole, 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 and rotor speed. It can be easily established that frequency of induced E.M.F f(Hz), rotor speed N(rpm) and number of poles P are connected through the following relationship.

It is clear from this relationship that, frequency of electricity produced is synchronized with mechanical rotational speed.

Production of 3 Phase Electricity

For producing 3 phase A.C current, 2 more such armature coils which are in 120 degree phase difference with the first is put in the stator winding.

Fig.5 For producing 3 phase electricity 2 more armature wingdings which are 120 degree apart from the first is introduced
Generally one end of these 3 coils are star connected and 3 phase electricity is drawn from the other ends. Neutral cable can be drawn from the star connected end.

When to use a Salient pole rotor ?

It is clear from the equation above that in order to produce 60 Hz electricity a 4 pole rotor should run at following a speed of 1800 RPM. Such huge RPM will induce a tremendous centrifugal force on poles of the rotor and it may fail mechanically overtime.

Fig.6 Rotors with less number of poles require high RPM, this in turn induces huge centrifugal force on poles of the rotor
So salient pole rotors are generally having 10-40 poles; which demands lower rpm. Or salient pole rotors are used when the prime mover rotates at relatively lower speed (120 - 400 RPM), such as water turbines and I.C engines.

Pole core & Stator core

Pole core is used to effectively transfer magnetic flux and they are made with fairly thick steel lamina. Such insulated lamina reduces energy loss due to eddy current formation. At the stator side also core lamina are used to enhance the magnetic flux transfer.

Fig.7 Pole and stator core enhances magnetic flux transfer and they are made of laminated steel lamina

Self Excited Generator

DC current is supplied to rotor via a pair of slip rings. This is the reason why rotating magnetic field approach is used in alternator. If rotating coil method were used, slip rings have to fitted along with the armature coils in order to collect electricity. But transferring such high voltage electricity via slip ring is rather impractical. It is quite possible to transfer low voltage DC excitation current via slip rings.

This DC current is supplied either from an external source or from a small DC generator which is fitted on the same prime mover. Such alternators are called self excited.

Fig.8 Slip rings are used to supply DC current to the rotor coil; this DC current could come from an inbuilt DC generator
With variation of load generator terminal output voltage will vary. It is desired to keep the terminal voltage in a specified limit. An automatic voltage regulator helps in achieving this. Voltage regulation can be easily achieved by controlling the field current. If terminal voltage is below the desired limit AVR increases the field current, thus the field strength. This will result in increase in terminal voltage. If terminal voltage is below the specified limit the reverse is done.

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