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 !