Electrical Machines: Unit II: D.C. Generators : Two Marks Questions with Answers
Two Marks
Questions with Answers
Q.
1 State the basic
components required for generator action to exist.
Ans. :
A
generating action requires following basic components to exist, i) The
conductor or a coil ii) The flux iii) The relative motion between conductor and
flux.
Q. 2 State Fleming's right hand rule.
Ans. :
If
three fingers of a right hand, namely thumb, index finger and middle finger are
outstretched so that everyone of them is at right angles with the remaining
two, and if in this position index finger is made to point in the direction of
lines of flux, thumb in the direction of the relative motion of the conductor
with respect to flux then the outstretched middle finger gives the direction of
the e.m.f. induced in the conductor.
Q. 3
What is the nature of the induced e.m.f.
in a d.c. generator? How it is converted to d.c. ?
Ans. :
The
basic nature of the induced e.m.f. in a d.c. generator is alternating (a.c.).
It is converted to d.c. using a device called commutator.
Q. 4
What are the functions of yoke? What is the choice of material for the yoke?
AU: Dec.-17, May-03
Ans. :
1.
It serves the purpose of outermost cover of the d.c. machine. So that the
insulating materials get protected from harmful atmospheric elements like
moisture, dust and various gases like SO2, acidic fumes etc.
2.
It provides mechanical support to the poles.
3.
It forms a part of the magnetic circuit. It provides a path of low reluctance
for magnetic flux.
It
is prepared by using cast iron. For large machines rolled steel, cast steel,
silicon steel is used which provides high permeability.
Q. 5 Why pole shoe has been given a
specific shape?
Ans. :
It
is necessary that maximum area of the armature comes across the flux produced
by the field winding. Pole shoe enlarges the area of armature core to come
across the flux, which is necessary to produce larger induced e.m.f. To achieve
this, pole shoe has been given a particular shape.
Q. 6
State the function of field winding in a
d.c. machine.
Ans. :
To
carry current due to which pole core, on which the field winding is placed
behaves as an electromagnet, producing necessary flux.
Q. 7 What are the functions of armature
core? What is the choice of material for the armature core?
Ans. :
1.
Armature core provides house for armature winding i.e. armature conductors.
2.
To provide a path of low reluctance to the magnetic flux produced by the field
winding.A
As
it has to provide a low reluctance path to the flux, it is made up of magnetic
material like cast iron or cast steel.
Q. 8 State the functions of armature
winding.
Ans. :
1.
Generation of e.m.f. takes place in the armature winding in case of generators.
2.
To carry the current supplied in case of d.c. motors.
3.
To do the useful work in the external circuit.
Q. 9 State the functions of commutator.
AU: Dec.-06
Ans. :
1.To
facilitate the collection of current from the armature conductors.
2.
To convert internally developed alternating e.m.f. to unidirectional (d.c.)
e.m.f.
3.
To produce unidirectional torque in case of motors.
Q. 10 What is the function of brushes?
Why brushes are made up of soft material?
Ans. :
The
function of brushes is to collect current from commutator and make it available
to the stationary external circuit. It connects the staionary external circuit
to the rotating commutator. Brushes are stationary and resting on the surface
of the commutator just making a contact with it. Thus as commutator rotates
there is friction between brushes and commutator. To avoid wear and tear of
commutator which is costly, the brushes are made up of soft material like
carbon.
Q. 11 Which are the two types of
armature windings used in d.c. machines? Explain in brief.
Ans. :
The
two types of armature windings used in d.c. machines are lap winding and wave
winding. In lap winding, if connection is started from conductor in slot 1 then
connections overlap each other as winding proceeds. It produces P number of
parallel paths. In wave type of connection, winding always travels ahead
avoiding overlapping. It travels like a progressive wave hence called wave
winding. Irrespective of number of poles it always produces 2 number of
parallel paths.
Q.12
Compare lap and wave type of windings.
[Refer section 3.4.3] AU: May-14,
Q.13 State the e.m.f. equation of a d.c.
machine.AU:
Dec.-03,13,16, May-05
Ans. :
The
e.m.f. equation of a d.c. machine is E = ϕ PNZ / 60A where P = Number of poles,
N = Speed im r.p.m.m A= Number of parallel paths, Z = Number of conductors and ϕ
= Flux per pole.
Q. 14
State the advantages of double layer
winding.
Ans. :
1.
It provides neat arrangement as all coils are identical.
2.
Greater flexibility can be achieved with double layer winding as coil span can
be easily selected.
Q. 15 Define a pole pitch. AU: Dec.-05
Ans. :
It
may be defined as the distance between the two adajcent poles i.e the periphery
of the armature divided by the number of poles. It may also be defined as the
number of armature conductors or number of armature slots per pole.
Q. 16
Define back pitch and front pitch
Ans. :
The
distance which is measured interms of armature conductors i.e. between top and
bottom coil sides of a coil measured around the back of the armature i.e. away
from the commutator is called back pitch and is denoted by yb.
It
is defined as the distance measured between two coilsides which are connected
to the commutator segment. It is denoted by yf.
Q. 17 Define winding pitch and
commutator pitch.AU:
Dec.-05
Ans. :
It
is defined as the distance between the starts of two consecutive coils measured
in terms of coilsides. It is denoted by Y. For Lap winding, Y = Yb -
Yf while for Wave winding, Y = Yb + Yf It is
defined as the distance between the two commutator segments to which the two
ends i.e. starts and finish of a coil are connected. It is measured in terms of
commutator segments and it is denoted by Yc
Q. 18 What is armature reaction? What
are its effects?
AU: Dec.-05, 09, 11, May-10, 13, 17
Ans. :
When
the generator is loaded, the armature carries the current. The armature current
sets up its own magnetic field. The effect of this armature flux on the
distribution of main field flux is called armature reaction. Its two effects
are,
i)
It reduces the generated voltage by distorting the main flux.
ii)
It causes sparking at the brushes.
It
has two effects on the main flux. One effect is to produce the flux which is in
direct opposition with field flux reducing it. This is called demagnetising
component. Second effect is to produce component which is at right angles to
the main field flux distorting it. This is called cross magnetising component.
Q. 19 What
is MNA and GNA?
Ans. :
The
axis along which there is no e.m.f. induced in the armature conductors is
called Magnetic Neutral Axis (MNA). The Geometric Neutral Axis (GNA) is nothing
but the axis of symmetry between the poles.
Q. 20
How to reduce the effects of the armature
reaction? [Refer section 3.11]
AU: Dec.-03, 05
Q. 21 Define commutation and commutation
period. AU:
May-12, Dec.-12, 14
Ans. :
A
process by which current in the short circuited coil is reversed while it
crosses the MNA is called commutation. The time during which the coil remains
short circuited is known as commutation period.
Q. 22 What is linear, under and over
commutation?
Ans. :
In
commutation reversal of current takes place. If current varies uniformly along
straight line then the commutation is said to be linear commutation. If current
takes more time than linear commutation then it is called under or delayed
commutation. If reversal of current in the coil is faster than ideal or linear
commutation then it is said to be over commutation or accelerated commutation.
Q. 23
State the methods of improving
commutation. AU: May-05, 06, Dec.-18
Ans. :
These
methods are, 1. Resistance commutation 2. Giving a brush shift 3. Use of
interpoles. The methods 2 and 3 are the parts of E.M.F. commutation.
Q. 24 Draw the circuit model of d.c.
shunt, series and compound generators.
[Refer sections 3.19, 3.20 and
3.21]
Q. 25 How the generators are classified
based on method of excitation ?
Ans. :
There
are two methods of excitation used for d.c. generators,
1.
Separate excitation 2. Self excitation.
In
separately excited generator, a separate external d.c. supply is used to
provide exciting current through the field winding.
The
d.c. generator produces d.c. voltage. If this generated voltage itself is used
to excite the field winding of the same d.c. generator, it is called self
excited generator. Based on how field winding is connected to the armature to
derive its excitation, this type is further divided into following three types:
i) Shunt generator ii) Series generator and iii) Compound generator.
Q. 26
Explain how self excited d.c. generator
builds its voltage ?
Ans. :
Thus
when the generator is started, due to the residual flux, it develops a small
e.m.f. which drives a small current through the field winding. This tends to
increase the flux produced. This in turn increases the induced e.m.f. This
further increases the field current and the flux. The process is cumulative and
continues till the across its generator develops rated voltage armature. This
is voltage building process in self excited generators.
Q. 27 State the causes of failure to
excite self excited generaotor and remedies for it.[Refer section 3.22.1] AU: May-03,
08, 14, 17, Dec.-05, 06, 07, 10
Q. 28 Why shunt generator load
characteristics turns genyo back when overloaded? [Refer
section 3.25]
Q. 29 State the applications of various
types of generators. AU:
Dec.-07
Ans. :
Shunt Generators:
Commonly used in battery charging and ordinary lighting purposes. Series
generators: Commonly used as boosters on d.c. feeders, as a constant current
generators for welding generator and arc lamps.
Cumulatively compound generators:
These are used for domestic lighting purposes and to transmit energy over long
distance.
Differential compound generators:
The use of this type of generators is very rare and it is used for special
application like electric arc welding.
Q. 30
Define critcal field resistance. AU: Dec.-17
Ans. :
The
critical field resistance as that resistance of the field circuit at a given
speed at which generator just excites and starts voltage building while beyond
this value generator fails to excite.
Q. 31 Define critical speed.
Ans. :
The
speed for which the given field resistance acts as critical resistance is
called the critical speed, denoted as NC.
Q. 32
Why generators are operated in parallel?
Ans. :
The
generators are operated in parallel to obtain,
1.
Continuity and reliability of service without interruption
2.
Ease from maintenance and repair point of view.
3.
Operating each generator efficiently.
4.
Increase in plant capacity.
Q. 33 The series field winding has low
resistance while the shunt field winding has high resistance. Why? AU : May-07
Ans. :
The
series field winding is always connected in series with the armature and hence
has to carry the armature current which directly gets decided by the load. Thus
the current passing through series field winding is of high level. The voltage
drop across series field winding gets added to the voltage drop across armature
winding while deciding the back e.m.f. This voltage drop must be very small.
Hence as the current through series field winding is high, in order to keep
voltage drop across it to a small value, its resistance is very low. The shunt
field winding is directly connected across the rated supply voltage hence to
limit current through it, resistance is very high.
Q. 34 What are the conditions for
parallel operation of D.C. generators ?
AU: May-07, Dec.-08
Ans. :
i)
The voltages of both the generators must be equal.
ii)
The polarities of the generators must be same or the connections must be
interchanged till they become same.
iii)
The change of voltage with change of load should be of same character.
iv)
The prime movers driving the generators bal should have stable and similar
rotational speed characteristics.
v)
The positive nad negative terminals generators must be connected properly to
the positive and negative of busbar.
Q. 35 What are the major parts of a D.C.
generators ?AU:
May-09
Ans. :
The
major parts of d.c. machines are yoke, poles, field winding, armature, commutator
and brushes.
Q. 36 What is a commutator ?
AU: May-09, Dec.-14
Ans. :
The
commutator armature winding, is a device which converts alternating induced
e.m.f. in a generator to d.c. e.m.f. In case of motor it produces
unidirectional torque.
Q. 37 Why the air gap between the pole
pieces and the armature is kept very small? AU:
May-06
Ans. :
The
air gap between the pole pieces and the armature is kept very small to keep the
field strength at its maximum value. This is because the flux lines can pass
through iron very easily than the air gap. The small air gap generates high
voltage. Larger the air gap means stronger must be the magnetising force to
produce the required voltage hence the air gap is kept very small.
Q. 38 Why does curving the pole faces in
a d.c. machines contributes to a smoother d.c. output voltage from it? AU: Dec.-10
Ans. :
For
having smoother output voltage from d.c. machines its commutation must be as
ideal as possible. In order to have good commutation the flux density in the
air gap must decrease gradually from maximum value under the centre of the pole
to zero on the interpolar axis. The flux distribution which drops abruptly from
maximum to zero leads to commutation difficulties and magnetic noise. Thus to
achieve good air gap flux distribution the pole faces are curved which improves
the commutation and contributes to a smoother d.c. output voltage.
Q. 39 What is meant by reactance voltage
?
AU: May-12
Ans. :
An
armature coil has certain inductance. When coil undergoes commutation, there is
change in current (di) in that coil and the commutation takes certain time
(dt). Thus there is self induced e.m.f. in a coil undergoing commutation given
by L di / dt This e.m.f. induced in the coil undergoing commutation is called reactance voltage.
Q. 40
Why the external characteristics of DC
shunt generator is more drooping than that of a separately excited generator? AU: May-14
Ans. :
In
seperately excited generator, Ia =IL and Ish
is not supplied by armature. In d.c. shunt generator, la =IL
+Ish hence the drop Ia Ra is more than in
seperately excited generator. Hence the external characteristics of d.c. shunt
generator is more drooping than that of a seperately excited generator.
Q. 41 Compare
shunt and series field winding used in d.c. motors.
[Refer section 3.20.1]
Q. 42 Specify the role of Interpoles in
DC machine?AU:
Dec.-16, 19, May-15
Ans. :
The
interpoles are used to induce the reversing e.m.f. in the coil undergoing
commutation required to neutralise the reactance voltage. The interpoles are
placed in between the main poles. The interpoles make the communication
sparkless and with interpoles the sparkless commutation upto 20 to 30 %
overload with fixed brush position can be obtained. They also help in
neutralising cross-magnetising effect of armature reaction.
Q. 43
What is meant by residual emf in DC
generator? AU : May-15
Ans. :
When
supply to the field winding in generators is removed, the field current becomes
zero. But still there exists some magnetic flux associated with the poles. This
is called residual flux. The e.m.f. induced in the generators by cutting the
residual flux by armature is called residual of e.m.f.
Q. 44
Draw various characteristic of D.C. shunt
generator. AU: May-16
Ans. :
Refer
Fig. 3.25.2 and Fig. 3.25.3 (a) of Section 3.25.
Q. 45 Why load voltage across DC shunt
generator is decreasing with increase in load current? AU: Dec.-18
Ans. :
As
load current increase, the armature current increases as field curren is
constant. Hence the drop IaRa increases which has to be
supplied by Eg. Also Ia produces its own flux which
affect the main field flux by distorting it. This is called armature reaction.
Due to this effect, the generated e.m.f. decreases. Hence the load voltage
across the dc generator decreases as the load current increases.
Q. 46 On what occasions dc generators
may not have residual flux ?
AU: May-19
Ans. :
1.
Wrong field winding connections due to which flux gets produced in opposite
direction to residual flux and cancelling it.
2.
Generator is driven in opposite direction which wipes out the residual flux.
Q. 47
How the critical field resistance of a dc
shunt generators is estimated from its OCC ?AU: May-19
Ans. :
Draw
the tangent to the initial part of the O.C.C. The slope of this line is the
critical resistance for the speed at which data is given.
Q. 48
Write down the formula for reactance
voltage of linear and sinusoidal commutation. AU Dec.-19
Ans. :
Reactance
voltage = L × 2 I /TC .................. Linear commutation
Reactance
voltage = 1.1 × L × 2I/ TC..............Sinusoidal commutation
Where
L = Coefficient of self inductance
Tc
= Time of commutation
Electrical Machines: Unit II: D.C. Generators : Tag: : D.C. Generators | Electrical Machines - Two Marks Questions with Answers
Electrical Machines I
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