AC MACHINES MCQS
1)
The basic function of a transformer is to change
a) the
level of the voltage
b) the
power level
c) the
power factor
d) the
frequency
ANS:
A
2) In an ordinary transformer which of the
following does not change
a) Voltage
b) Current
c) Frequency
d) All
of the above
ANS:
C
3) The
efficiency of the power transformer is
a) 50%
b) 60%
c) 80%
d) 95%
ANS:
D
4) In a transformer electrical power is
transferred from primary to secondary
a) Through
air
b) By
magnetic flux
c) Through
insulation medium
d) None
of these
ANS:
B
5) The two
windings of transformers are
a) Conductively
linked
b) Inductively
linked
c) Not
linked at all
d) Electrically
linked
ANS: B
6) Transformer
action requires a
a) constant magnetic flux
b) increasing magnetic flux
c) alternating magnetic fluxd
d) alternating electric flux
ANS:
C
7) The flux
created by the current flowing through the primary winding induces emf in
a) primary winding only
b) secondary winding only
c) transformer core only
d) both
primary and secondary windings
ANS:
D
8) The primary
and secondary windings of a power transformer always have
a) a common magnetic circuit
b) separate magnetic circuit
c) wire of same size
d) same number of turns
ANS:
A
9) The iron core
in a transformer provides a _______ path to a main flux
a) low reluctance
b) high reluctance
c) low resistance
d) high
conductivity
ANS:
A
10) If rated dc
voltage applied instead of ac to the primary of a transformer
a) secondary
of transformer will burn
b) primary
transformer will burn
c) secondary voltage will be excessively high
d) there will be no secondary voltage
ANS:B
11) A
transformer transforms
a) voltage b) current
c) voltage and current d) frequency
ANS:C
12) For an ideal
transformer the window should have
a) maximum
resistance on primary side and least resistance on secondary side
b) least
resistance on primary side and maximum resistance on secondary side
c) equal
resistance on primary and secondary sides
d) no
ohmic resistance on either side
ANS:D
13) The core
flux in transformer depends mainly on
a) supply voltage
b) supply voltage and frequency
c) supply voltage frequency and local
d) supply voltage and load but independent of
frequency
ANS:B
14) when voltage
is transformed from primary to secondary then it is__________
a)
multiplied by k2 b)
multiplied by k
c) divided by k2 d) divided by k
ANS: B
15) The applied
voltage of a certain transformer is increased by 75% while the frequency of the
applied voltage is reduced by 25%. The maximum core flux density will
a) increase
by seven times b) increase by three times
c)
reduce to one quarter d) remain the same
ANS: B
16) The low
voltage winding of a 400/230 V single phase 50 HZ transformer is to be
connected to a 25 HZ supply. In order to keep the magnetization current at the
same level in both the cases the voltage at 25 Hz should be
a) 230 V b)
460 V
c) 115 V d)
65 V
ANS:
C
17) A 50 Hz
transformer having equal hysteresis and eddy current losses at rated excitation
is operated at 45 Hz at 90% of its rated voltage. Compared to rated operating
point, the core losses under this condition.
a)
reduced by 10% b)
reduced by 19%
c)
reduced by 14.5% d) remain unchanged
ANS:
C
18) A 1-ph
transformer when supplied from 220 V, 50 Hz has eddy current loss of 50 W. If
the transformer is connected to a voltage of 330 V, 50 Hz, the eddy current
loss will be
a) 168.75
W
b) 112.5 W
c) 75 W
d) 50 W
ANS:
B
19) 1 kVA, 230
V, 50 Hz, single phase transformer has an eddy current loss of 30 watts. The
eddy current loss when the transformer is excited by a dc source of same
voltage will be
a) 30 watts b) more than 30 w c) less than 30 w d)
zero watt
ANS: D
20) At 50 Hz
operation, a single phase transformer has hysteresis loss of 200 W and eddy
current loss of 100 W. Its core loss at 60 Hz operation will be
a) 432 W
b) 408 W
c)
384 W
d) 360 W
ANS: C
21) On no-load
phasor diagram of transformer, the core loss component of the current remains
in phase with
a) no-load
current b) primary supply voltage
c) core flux d) primary induced voltage
ANS: B
22) In a transformer core, 3rd and 5th
harmonic components of fluxes are
respectively 10% and 4% of the fundamental flux. The 3rd and
5th harmonic induced emfs in the winding, in terms of the
fundamental induced emfs are
respectively
a) 30%, 20% b) 10%, 12% c) 50%, 20% d)
50%, 12%
ANS: A
23) In a
transformer, the exciting current will be in phase quadrature with the
impressed voltage provided
a) only the
leakage impedance drop is ignored
b) only the core
loss is ignored
c) both the
leakage impedance drop and the core loss are ignored
d) only the
no-load copper loss is ignored
ANS: C
24) The power
factor of a power transformer on no load will be about
a) unity b) 0.75 c)
0.5 d) 0.35
ANS: D
25) The power factor of a transformer on no load is poor due to
a) magnetizing
reactance of the transformer
b)
open-circuited secondary
c) low primary
winding resistance
d) low no-load
current
ANS: A
26) Primary winding of a transformer comprises of two identical windings in parallel. If one winding is removed, magnetizing current will be
a) halved b) the same c)
doubled d) increased four times
ANS: C
27) The core
flux of a practical transformer with a resistance load
a) is strictly
constant with load changes
b) increases
linearly with load
c) increases as
the square root of the load
d) decreases
with increase of load
ANS: A
28) The
inductive reactance of a transformer depends on
a) electromotive
force b)
magnetomotive force
c) magnetic flux d) leakage flux
ANS: D
29) The flux in
transformer core
a) increases
with load b)
decreases with load
c) remains
constant irrespective of load d)
none of the above
ANS: C
30) The mutual
flux in a loaded transformer can be varied by varying
a) primary
current b) load
impedance
c) secondary
current d) reluctance of
the magnetic path
ANS: D
31) The primary
ampere-turns are counter balanced by
a) secondary
ampere-turns b) primary
flux
c) increase in
mutual flux d) increase
in secondary current
ANS: A
32) In a
transformer_________ decreases with the increase in leakage flux
a) primary induced
emf b) secondary induced emf
c) secondary
terminal voltage d) none of the
above
ANS: C
33) power
transformed from primary to secondary depends upon
a) number of
primary turns
b) number of
secondary turns
c) current
transformation ratio
d) magnetic
coupling b/w primary and secondary winding
ANS: D
34) A
transformer is supplying pure resistive load. The power factor on primary side
will be
a)
about 0.95(lead) b)
about 0.95(lag)
c)
zero d)
unity
ANS:
B
35)
The phasor diagram of a transformer on load can be drawn only if we know
a)
equivalent circuit parameters of the transformer
b)
load current
c)
load p.f.
d)
all of the above
ANS:
D
36)
When a transformer is operating on no-load, the primary applied voltage is
approximately balanced by
a)
primary induced emf b)
secondary induced emf
c)
terminal voltage across the secondary d) voltage drop across the resistance and reactance
ANS:
A
37)
In the transformer circuit mode, the core loss is represented as a
a)
series resistance b)
series inductance
c)
shunt resistance d)
shunt inductance
ANS:
C
38)
Full load voltage regulation of a power transformer is zero when power factor
of the load is near
a)
unity and leading b)
zero and leading
c)
zero and lagging d)
unity and lagging
ANS:
A
39)
In a transformer zero voltage regulation at full load is
a)
not possible
b)
possible at leading power factor load
c)
possible at lagging power factor load
d)
possible at unity power factor load
ANS:
B
40)
Positive voltage regulation is an indication of ____ load
a)
inductive b)
capacitive
c)
either inductive or capacitive d)
pure resistive
ANS:
C
41)
A 4kva, 400/200 V 1-ph transformer has resistance of 0.02 pi and reactance of
0.06 pu. Its actual resistance and reactance referred to hv side, are,
respectively
a) 0.2 ohm and 0.6 ohm
b) 0.8 ohm
and 2.4 ohm
c) 0.08 ohm and 0.24 ohm
d) 2 ohm and 6 ohm
ANS:B
42)
If the percentage resistance of a power transformer for secondary side is 2.5
percent and turn ratio is 1:10, the percentage secondary resistance referred to
primary will be
a) 25
b) 2.5
c) 0.25
d) 0.025
ANS:B
43)
10 KVA , 400V/200V single phase transformer with a resistance of 3 % and
reactance of 6% is supplying a current of 50A to resistive load.the voltage
across the load is
a) 194
V
b) 196
V
c) 198
V
d) 390
V
ANS:A
44)
In a power transformer iron losses remain practically constant from no load to
full load this is because
a) Core
flux remains constant
b) Leakage
flux remains contant
c) Both
a and b
d) Neither
a nor b
ANS: A
45) In a power transformer if in place of
sinusoidal wave a peaked wave voltage is fed to primary
a) copper
losses will be less
b) noise
level will be reduced
c) iron
losses will be more
d)
iron losses will be less
ANS: D
46)
On which of the following factors does hysteresis loss depend
a) flux
density
b) frequency
c) thickness
of laminations
d) time
select the correct answer using the code
given below
a) 2
and 3
b) 1
and 2
c) 1
and 4
d) 3
and 4
ANS:B
47)
The full load copper loss and iron loss of a transformer are 6400 W and 5000 W
respectively. The copper loss and iron loss at half load will be, respectively
a) 3200
W and 2500 W
b) 1600
W and 5000W
c) 1600
W and 1250 W
d) None
of these
ANS:B
48)
The main purpose of performing open circuit on a transformer is to measure its
a) Copper
losses
b) Iron
losses
c) Total
losses
d) Insulation
resistance
ANS: B
49)
Power transformer are designed to have maximum efficiency around -------- full
load
a) Nearly
b) 70%
c) 50%
d) 25%
ANS:A
50)
The transformer efficiency will be maximum at a power factor of
a) 0.8 lead
b) Unity
c) 0.8
lag
d) 0.5
lag or lead
ANS: B
51)
What is load at which maximum efficiency occurs in case of a 100 KVA transformer with iron loss of 1 KW and full
load copper loss of 2 KW ?
a) 100
KVA
b) 70.7
KVA
c) 50
KVA
d) 25
KVA
ANS:B
52)
A Two KVA transformer has iron loss of 150 W full load copper loss of 250 W the maximum efficiency of transformer would
occur when the total loss is
a) 500
W
b) 300
W
c) 200
W
d) 100
W
ANS:B
53)
The size of transformer core depends upon
a) Frequency
b) Permissible
flux density in core material
c) Area
of the core
d) Both
a and b
ANS:D
54)
The permissible flux density in cold rolled grain oriented steel is about
a)
1.8 T
b)
2.5 T
c)
3.5 T
d)
4.5 T
ANS:A
55)
Transformer oil must be free from
a) Moisture
b) Sledges
c) Gases
d) Sulphur
ANS: A
56) A 10 KVA 400V/200V single phase transformer
with 10% impedance draws a steady short circuit line current of
a) 50
A
b) 150
A
c) 250
A
d) 350
A
ANS:C
57) The efficiency of two identical transformers
under load conditions can be determined by
a) Back-to-back
test
b) Short
circuit test
c) Open
circuit test
d) Any
of the above
ANS:A
58)
Transformer will operate at maximum efficiency when
a) Hysteresis
= eddy current loss
b) Eddy
current loss = copper loss
c) Copper
loss = iron loss
d) Hysteresis
loss= copper loss
ANS: C
59)
Which part of transformer is subjected to maximum heating?
a) Frame
b) Core
c) Winding
d) Oil
ANS: C
60)
zero sequence current flows from a line into a transformer bank if the windings are
a) Grounded
star/delta
b) Delta/star
c) Star/grounded
star
d) Delta/delta
ANS:A
61)
magnetizing current in transformer rich in harmonics
a) 3rd
harmonic
b) 5th
harmonic
c) 6th
harmonic
d) 4th
harmonic
ANS:A
62)
Transformer fed from a fundamental frequency voltage source, the source of harmonic is the
a) Overload
b) Poor
insulation
c) Iron
loss
d) Saturation
of core
ANS:D
63)
The outer cage of a double squirrel rotor consists of
a) Manganese
brass
b) Red
copper
c) Bronze
d) Steel
ANS: A
64)
In the equivalent ckt of a double-cage induction motor, the two rotor cages can be considered
a) To
be parallel
b) To
br in series-parallel
c) To
be in series
d) To
be in parallel with stator
ANS: A
65)
The purpose of the starter is to
a) Limit
the starting current
b) Limit
the speed
c) Protect
against over voltage
d) Produce
back emf
ANS: A
66)
For a starting a 3-ph induction motor is about ____ of its full-load rated
current
a) Half
b) Twice
c) 5
to 7 times
d) 15
to 20 times
ANS: C
67)
Synchronous speed of an induction motor can be increased by
a) Reducing
mechanical friction
b) Increasing
supply voltage
c) Increasing
number of poles
d) Increasing
supply frequency
ANS: D
68)
while operating on variable frequency supplies, the ac motor requires variable
voltage as well in order to
a) Protect
the insulation
b) Avoid
the effect of saturation
c) Improve
the capabilities of the inverter
d) Protect
the thyristor from dv/dt
ANS: B
69)
In rotor resistance control of induction motor, the hardness of speed torque characteristics
a) Increases
b) Decreases
c) Remain
same
ANS: B
70)
During plugging of an induction motor
a) Phase
sequence is reversed
b) A
dc source is connected to stator
c) One
phase is open-circuited
d) Power
is fed back to mains
ANS: A
71)
The method that gives continuous speed control of a slip-ring induction motor
is
a) Rotor
resistance control
b) Secondary
foreign voltage control
c) Concatenation
or cascade operation
d) Line
voltage control
ANS: B
72)
In case of voltage injection method of speed control, the injected emf should
be of
a) Supply
frequency (f)
b) Slip
frequency(sf)
c) (1-s)f
d) (2-s)f
ANS: B
73)
Induction generators deliver power at
a) Leading
power factor only
b) Lagging
power factor only
c) Leading
as well as lagging power factor
d) Unity
power factor only
ANS: A
74)
The primary winding of a Schrage motor is located
a) In
stator
b) In
lower part of the rotor
c) In
upper part of rotor
d) Partly
in stator and partly in rotor
ANS: B
75)
linear induction motor is used in
a) Traction
b) Magnetic
attraction
c) Mechanical
workshops
d) Textile
mills
ANS: A
76)
In a split phase motor the ratio of number of turns on auxiliary winding to
that on main winding is
a) Unity
b) Less
than one
c) More
than one
d) Two
ANS: B
77)
why is a centrifugal switch used in a 1-ph induction motor
a) To
protect the motor from overloading
b) To
improve the starting performance of the motor
c) To
cut off the starting winding at an appropriate instant
d) To
cut in the capacitor during running conditions
ANS: C
78)
The torque speed characteristics of two phase induction motor is largely affected by
a) Voltage
b) Speed
c) x/r
ratio
d) supply
frequency
ANS: C
79)
The capacitor in a capacitor-start induction run ac motor is connected in
series with
a) starting
winding
b) running
winding
c) squirrel
cage winding
d) compensating
winding
ANS: A
80)
In a shaded pole motor, shading coils are using to
a) reduce
winding losses
b) reduce
friction losses
c) produce
rotating magnetic field
d) protect
against sparking
ANS: C
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