[Q:-01]
What is an alternator ?
Ans :
An alternator or a.c. generator is a
synchronous machine which converts mechanical energy into electrical energy and
produces alternating e.m.f.
[Q:-02]
On what principle does it work ?
Ans :
The alternator works on the same principle
of Faraday's laws of electromagnetic induction as in d.c. generator i.e.
whenever a conductor links with the magnetic field (either conductor is moving
or the field is moving) an e.m.f is induced in the conductor.
[Q:-03]
What are the different types of alternators ? Which is in common use ?
Ans :
Alternators are of
two types—
(i) Rotating armature
type,
(ii) Rotating field
and stationary armature type.
Rotating field type is
commonly used.
[Q:-04]
What are the advantages of stationary armature and rotating field ?
Ans :
The advantages are as
follows :—
(i) Rotating field
system is comparatively light and can be rotated with high speed.
(ii) High voltage
upto 33 kV can be generated due to high speed.
(iii) For high
voltage it becomes comparatively easy to insulate the stationary armature
winding than on a moving system.
(iv) For field
excitation the rotor requires only two slip rings to supply current to their
rotating field system at low voltage for which smaller size and capacity of
slip rings can be used.
(v) No slip rings are
needed for connecting the armature winding with the external circuit.
(vi) Rotating field
system experiences little difficulty in supplying a very few ampere current to
the field magnet with negligible sparking effect between slip rings and
brushes.
[Q:-05]
What are the main parts of an alternator ?
Ans :
The main parts of an alternator are
(i) Rotor (Salient
pole type and Cylindrical type)
(ii) Stator (Frame,
Core and Armature conductors and
(iii) Exciter.
[Q:-06]
What are the types of rotating field type alternators ?
Ans :
(i) Salient pole type & (ii) Smooth
cylindrical type.
[Q:-07]
What do you mean by the salient pole type rotor ?
Ans :
The salient pole type rotor means the low
and moderate speed rotor having large diameter and small axial length with
projected (salient) poles coming out of the rotor frame the outer surface of
which almost follows the inner cylindrical surface of the stator core.
[Q:-08]
What do you mean by the smooth cylindrical type rotor ?
Ans :
The smooth cylindrical type rotor means
the high speed smooth cylindrical shaped rotor having less diameter and more
axial length with poles on the rotor not projecting out from the rotor surface.
[Q:-09]
Which type of alternator is suitable for coupling with steam turbines and why ?
Ans :
Smooth cylindrical of non-salient pole
type alternator is suitable for coupling
with steam turbine because of its high peripheral speed .
[Q:-10]
Which type of alternator is suitable for hydro-electric plants and why ?
Ans :
Salient pole type alternator is suitable
for hydro-electric because of its low and moderate speed.
[Q:-11]
In which alternator the cylindrical type rotor is used ?
Ans :
High speed
alternator.
[Q:-12]
In which alternator the salient type rotor is used ?
Ans :
Low and medium speed upto 1000 r.p.m.
rotating field type small alternator.
[Q:-13]
How many poles are generally designed in turbo alternators and hydel
alternators ?
Ans :
The turbo alternators are designed with
the rotor field of two poles or four poles whereas the hydel alternators are
designed with the rotor field of any even numbers from 6 poles to 36 poles.
[Q:-14]
What type of alternator is used for high voltage generator ?
Ans :
Stationary armature type alternator is
used for high voltage generation.
[Q:-15]
Why are the stator cores laminated ?
Ans :
The cores are
laminated to minimise the eddy current losses.
[Q:-16]
What types of slots are provided in stator cores of alternators ?
Ans :
(i) Open type slots
& (ii) Semi closed type slots.
[Q:-17]
What are the types of windings used in 3-phase alternators ?
Ans :
(i) Single layer
winding and (ii) Double layer winding.
[Q:-18]
What is meant by breadth factor ?
Ans :
The breadth factor or the distribution
factor is the ratio of vector sum to arithmetic sum of the e.m.fs generated by
the conductors. It is denoted by Kd and is equal to
Kd= ((Sin
nα)/2)/(n.Sin(α/2))
where n = number of
slots/pole/phase and α angle between two adjacent slots.
[Q:-19]
What is meant by coil span factor ?
Ans :
The coil span factor or the pitch factor
is defined as the cosine of half the angle by which the coil span is shorter
than 180°. It is denoted by Kp and is equal to Kp = cos β/2,
where β is the angle by which the coil Span is shorter than full pitch.
[Q:-20]
What are the methods of connecting the windings of 3-phase alternators ?
Ans :
(i) Star connection
and (ii) Delta connection.
[Q:-21]
How are the field poles excited in the alternator ?
Ans :
The field poles are excited from an
external d.c. source through an exciter usually coupled to the alternator.
[Q:-22]
How is d.c. supply fed in the rotating field type alternator ?
Ans :
The d.c. supply is fed to the field
winding by means of two slip rings mounted on rotor shaft.
[Q:-23]
What do you mean by the term exciter ?
Ans :
Exciter means a d.c. shunt generator
supplying direct current to alternator field generally coupled to the
alternator shaft.
[Q:-24]
What do you mean by exciting current ?
Ans :
The exciting current
means the current given to the field.
[Q:-25]
At what voltage the field of an alternator is usually excited ?
Ans :
125 volts to 250
volts d.c.
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