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  • Alternators are the workhorse of the power generation industry.

  • It is capable to generate AC power at a specified frequency.

  • It is also called as synchronous generator.

  • Electricity is produced in alternators by Electromagnetic Induction.

  • To generate electricity in a coil,

  • either the coil should rotate with respect to magnetic field

  • or a magnetic field should rotate with respect to the coil.

  • In the case of alternator the latter approach is used.

  • Rotor and armature coils are the two main parts of an alternator.

  • The rotor produces a rotating magnetic flux.

  • Armature coils are stationary

  • and rotating magnetic flux associated with the rotor

  • induces electricity in the armature coils.

  • This 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 DC power source.

  • Magnetic field produced around it would be a shown.

  • 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 a revolving magnetic flux

  • now intersects armature coils,

  • which is fitted around the rotor.

  • This will generate an alternating

  • EMF across the winding.

  • Here is a slowed down version of the

  • rotor stator interaction.

  • For this four pole system

  • when the rotor turns half revolution,

  • induced EMF takes one complete cycle.

  • It can be easily established that, frequency of induce EMF,

  • rotor speed and number of poles are

  • connected through following relationship:

  • It is clear here that,

  • frequency electricity produced is synchronized

  • with mechanical rotational speed.

  • For producing 3 phase AC current,

  • two more such armature coils

  • which are in 120 degree phase difference with the first

  • is put in the stator winding.

  • Generally one end of these three coils are Star connected,

  • and three phase electricity is drawn from the other ends.

  • It is clear from this equation that,

  • in order to produce 60-hertz electricity

  • a 4-pole rotor should run at following RPM.

  • Such huge rpm will induce a tremendous centrifugal force on polls of the rotor,

  • and it may fail mechanically over the time.

  • So salient polel rotors are generally having

  • 10 to 20 polls, which demands lower RPM.

  • Or salient pole rotors are used

  • when the prime mover rotates at relatively lower RPM.

  • Pole core is used to effectively transfer

  • the magnetic flux,

  • and they're made with fairly thick steel lamina.

  • Such insulated lamina reduce energy loss due to eddy current formation.

  • Armature winding of three-phase,

  • 12 poll system is shown here.

  • A stator core is used to enhance the magnetic flux transfer.

  • DC current is supplied to rotor via a pair of slip rings.

  • DC current is supplied from an external source

  • or from a small DC 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

  • An automatic voltage regulator helps in achieving this.

  • Voltage regulation can be easily achieved

  • by controlling the field current.

  • If the terminal voltage is below the desired limit,

  • AVR increases the field current,

  • thus the field strength.

  • This will result in an increase in terminal voltage.

  • If terminal voltage is high,

  • the reverse is done.

  • We hope you had a nice introduction

  • on the working of alternators.

  • Thank you !

Alternators are the workhorse of the power generation industry.

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B2 US rotor voltage magnetic flux rpm electricity

How does an Alternator Work ?

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    噹噹 posted on 2016/01/12
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