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WorksheetsTransformer
Total questions: 108
Worksheet time: 54mins
A transformer will work on
AC Only
DC only
AC as well as DC
None of the above
A transformer transfers electrical energy from primary to secondary usually with a change in
Frequency
Power
Voltage
Time period
A transformer is an efficient because it
Is a static device
Uses capacitive coupling
Used inductive coupling
Uses electric coupling
The Primary and secondary transformer are _____ coupled
Electrically
Magnetically
Electrically magnetically
None of the above
The voltage per turn of the primary of a transformer is ______ the voltage per turn of the secondary.
More than
Less than
The same as
None of the above
The winding of the transformer with greater number of turns will be ____.
High voltage
Low voltage
Either high or low voltage winding
None of the above
A transformer does not posses ____ changing property.
Impedance
Voltage
Current
Power
The iron-core is used to ____ of the transformer.
Increase the weight
Provide tight magnetic coupling
Reduce core losses
None of the above
If a transformer core has air gaps, then ______.
Reluctance of magnetic path is decreased
Hysteresis loss is decreased
Magnetising current is greatly increased
Eddy current is increased
The maximum flux produced in the core of a transformer is _____.
Directly proportional to the supply frequency
Inversely proportional to supply frequency
Inversely proportional to primary voltage
None of the above
The flux in the core of a single-phase transformer is ____.
Purely alternating one
Purely rotating one
Partly alternating and partly roaring
None of the above
A transformer is so designed that the primary and secondary have _____.
High leakage resistance
Large resistance
Tight magnetic coupling
Good electric coupling
When the primary of a transformer is connected to a dc supply, ____.
Primary draws small current
Core losses are increased
Primary leakage reactance is increased
Primary may burn out
An ideal transformer is one which ______.
Has no losses and leakage reactance
Has same number of primary and secondary turns
Does not work
None of the above
In a practical transformer with no-load, the no-load current is In_____.
is in phase with Vp
Lags Vp by 90°
Lags Vp by an angle less than 90°
Leads Vp by 90°
The no-load primary current (In) is about ______ full-load primary current.
3-5%
30-40%
15-30%
Above 40%
The no-load pf of a transformer is small because ____.
Iron loss component of (In) is large
Magnetising component of (In) is large
Magnetising component of (In) is small
None of the above
The no-load input power to a transformer is practically equal to _____ loss in the transformer.
Iron
Copper
Eddy current
None of the above
In a practical transformer, the primary induced emf (Ep) and secondary induced emf (Es) are _____.
180° out of phase
90° out of phase
In phase
None of the above
The no-load primary current (In) of a practical transformer is ____.
Wholly reactive
Wholly active
Partly active and partly reactive
None of the above
In a practical transformer, the applied voltage Vp and secondary induced emf Es are _______.
in phase
180° out of phase
90° out of phase
none of the above
In a practical transformer, the secondary induced emf Es _____.
lags Φm by 90°
leads Φm by 90°
is in phase with Φm
none of the above
In a practical transformer, the voltage transformation ratio is given by ____
Np/Ns
Ep/Es
Ns/Np
none of the above
In an equivalent circuit of a practical transformer on no-load, the element Rc accounts for ______.
core loss in the transformer
copper loss in the transformer
magnetic leakage in the transformer
none of the above
In an equivalent circuit of a practical transformer on no-load, the element Xm accounts for _______.
core loss
copper loss
permeability of core
leakage reactance
Two things which are the same for primary and secondary of a transformer are
ampere-turns and voltage per turn
resistances and leakage reactances
current and induced voltage
none of the above
A transformer operates poorly at very low frequencies because ____.
permeability of core is increased
magnetizing current is abnormally high
primary reactance is too much increased
none of the above
If a power transformer is operated at very high frequencies, then _____.
primary reactance is too much increased
primary will draw large power
core loss will be excessive
none of the above
The primary leakage flux links _____.
primary winding only
secondary winding only
both primary and secondary winding
none of the above
The effect of leakage flux in a transformer is to ____.
increase copper losses
cause voltage drop in the windings
decrease copper losses
none of the above
Leakage flux in a transformer occurs because ______.
applied voltage and frequency are constant
air is not a good magnetic insulator
applied voltage is sinusoidal
transformer is not an efficient device
The mutual flux in a transformer remains constant at all loads because ____.
applied voltage and frequency are constant
leakage flux is small
iron-core is used
losses are small
Cores of large transformers are built up to nearly circular cross-section in order to reduce ____.
leakage reactance
iron loss
eddy current loss
copper loss
The primary and secondary windings are wound on the top of each other in order to reduce ____.
iron losses
copper losses
leakage reactance
winding resistance
The amount of copper in the primary is _____ that of secondary.
about the same
greater than
smaller than
none of the above
The open-circuit test on a transformer is always made on ____.
low voltage winding
high voltage winding
either low or high voltage winding
none of the above
We can find _____ of the transformer with open circuit test.
copper losses
turn ratio
total equivalent resistance
total equivalent leakage reactance
In the short-circuit test on a transformer, we generally short circuit _____.
high-voltage winding
low-voltage winding
either low or high voltage winding
none of the above
The open-circuit test on a transformer gives ____.
copper losses
friction losses
iron losses
total losses
When the secondary of the secondary of the transformer is short-circuited, the primary inductance ______.
is decreased
is increased
remains unchanged
none of the above
When load on a transformer is increased, the eddy current loss ____.
is increased
remains unchanged
is decreased
none of the above
When load on a transformer is decreased, ____ loss is decreased.
eddy current
hysteresis
copper
friction
The eddy current loss in a transformer is directly proportional to _____.
thickness of core lamination
square of thickness of core lamination
the supply frequency
the flux density in the core
A transformer will have zero efficiency at ____.
full-load
no-load
half full-load
none of the above
The approximate efficiency of a large transformer is
65%
50%
80%
95%
The efficiency of transformer will be maximum when ____.
leakage reactances of the two windings are equal
resistances of the two windings are equal
copper loss is equal to core loss
none of the above
In a practical transformer, copper losses account for about ___ of the total losses.
10%
85%
25%
50%
By laminating the core of a transformer, we decrease ____.
leakage resistance
hysteresis loss
eddy current loss
copper loss
The all-day efficiency of a distribution transformer is _____ the commercial efficiency.
the same as
more than
less than
none of the above
The core-type transformer provides _____.
much longer magnetic path
shorter magnetic path
lesser average length per turn
none of the above
Transformers having ratings less than 5 kVA are generally ____.
oil cooled
natural air cooled
water cooled
none of the above
The transformer that should never have the secondary open-circuited when the primary is energized is ______.
power transformer
voltage transformer
autotransformer
current transformer
The temperature rise of a transformer is directly proportional to _____.
apparent power
reactive power
leakage reactance
none of the above
In an autotransformer, the primary and secondary are _____ coupled.
only magnetically
only electrically
magnetically as well as electrically
none of the above
What would happen if you connect a transformer to a dc circuit with a voltage of 20% of the nameplate ratings after a steady state condition is reached?
No voltage is registered at secondary
Rated no-load current flows to the secondary
Primary current is equal to voltage over equivalent primary impedance
Voltage is established at secondary
Magnetic induction can be expressed equally well except one of the following
lines/sq. m.
gauss
maxwells/sq. cm.
magnetic field
When V-V system converted into ∆-∆ system, increase in the capacity of the system is ____ percent.
86.6
66.6
73.2
50
Which is not correct in a wye connected transformer?
lower phase voltage
power cable can tolerate 100% insulation level
provide neutral ground
vulnerable to ferroresonace
What is the normal secondary circuit current of a current transformer?
15A
20A
5A
10A
Name of the transformer that provides an almost continuously variable stepless output voltage from zero to rated line voltage.
booster
UPS
AVR
variac
Instrument transformer are used in indicating and metering and with protective devices, they are used for ____.
measuring
detecting
relaying
sensing
Its primary function is to provide ground source
zig-zag transformer
sectionalizer
surge arrester
tertiary transformer
With the primary CT energized, the secondary must never be made open to prevent this to happen
build up magnetic bias
burning of primary winding
burning of secondary winding
absence of induced voltage
In transformer, the purpose of breather is to:
extract moisture of the air
to take insulating oil from conservator
to provide cooling to the winding
to provide insulation to the winding
If three transformers in ∆-∆ are delivering rated load and one transformer is removed, then what is the overload on each of the remaining transformer in percent?
173.3%
73.3%
57.7%
33.3%
What type of transformer bank is used to convert 2-phase to 3-phase power?
open-delta
scott-T
wye-delta
delta-delta
Transformer are rated in kVA instead of kW because
load power factor is often not known
kVA is fixed whereas kW depends on load
total transformer depends on volt-ampere
it has become customary
For power transformer, the conservator tank is intended to minimize
heating
internal pressure
oil sludging
breathing
What is the main use of zig-zag transformer?
to step down voltage
to stabilize transformer sequence currents
to provide path to zero sequence currents
to step down current
Laminated iron is used in AC magnetic circuits for the following reasons. Which is the best answer?
to increase rate of dissipation
to make assembly
to reduce permeability
to reduce eddy currents
Device use by transformer to adjust load voltage if the line supply is above or below terminal voltage?
thermal trip
tap changer
differential relay
bucholz relay
What is the typical use of autotransformer?
control transformer
variable transformer
isolating transformer
toy transformer
Three transformers are delta connected, one connection has been cut, what is the percent reduced.
33%
57.7%
67%
42%
Which of the following is common to insulator, bushing, potential transformer and transformer winding?
noise level
insulator
current rating
BIL rating
What is the ratio of capacity of open delta bank to the capacity of close delta bank?
72%
63.2%
57.7%
6.67%
In an ideal transformer, what is the efficiency?
100%
90%
80%
70%
When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of a magnetic material distributed around its periphery, the transformer is termed as:
core-type transformer
regulating transformer
grounding transformer
shell type transformer
The safety precaution to follow when using a potential transformer is to
ground the primary
ground the secondary
insulate for high current
connect it in series with the line
The copper losses of the transformer will
not be affected
decrease with the lowering of the power factor
increase with the increase of the power factor
increase with the lowering of the power factor
In the open circuit test of the transformer the wattmeter reading is equal to the
eddy current loss
current loss
hysteresis loss
core loss
An autotransformer used for starting induction and synchronous motors to reduced the current drawn from the supply lines.
motor starter
reducer transformer
starting compensator
four-point starter
The load of an instrument transformer consisting of delicate moving elements of ammeter, voltmeter, and wattmeter is termed as
transformer power
burden
instrument impedance
meter load
Which transformer has only one winding?
distribution
power
autotransformer
isolated winding transformer
Copper loss of a transformer are determined by means of the
polarity test
short circuit
open circuit
inductive circuit
The principle of working of a transformer is
static induction
dynamic induction
mutual induction
self induction
The laminations of transformer is made from
low carbon steel
silicon steel sheet
nickel alloy steel stampings
chrome steel sheets
What is common in the two windings of a transformer?
electric circuit
magnetic circuit
winding wire gauge
none of these
Preferably, the resistance between the primary and the secondary of a transformer should be
as low as possible
as high as possible
low or high depending upon whether it is step up or step down respectively
high or low depending upon whether it is step up or step down respectively
Ideal transformer assumptions do not include
zero reactance of windings
zero resistance of windings
no leakage flux
no saturation of core
The reactance of transformer is determined by its
leakage flux
common core flux
size of the core
permeability of the material of the core
The resistance of the low voltage side of a transformer
is equal to resistance of its high voltage side
is more than resistance of its high voltage side
is less than the resistance of its high voltage side
none of the above
The basic property of the transformer is that it changes the voltage level of an AC signal
without changing the power
without changing its shape
without changing the frequency
without changing power, frequency or shape
At every instant the direction of secondary current in a transformer much be such as to oppose any change in flux. This is in accordance with
Faraday's laws
Joule's law
Lenz's law
Coulomb's law
The magnitude of mutual flux in a transformer is
low at low loads ang high at high loads
high at high loads and low at high loads
same at all levels
varies at low loads and constant at high loads
What type of core is used for high speed frequency transformer?
air core
closed iron core
aluminum core
open iron core
The no-load current of a transformer in terms of full load current is usually
1 to 3%
3 to 9%
9 to 12%
12 to 20%
Which of the following is a correct statement given below is true about autotransformer?
It has two separate windings connected in series externally
It can only step down the voltage
It has only one winding
It most suitable for power transformer
In any transformer the voltage per turn in primary and secondary remains
always different
always same
always in ratio of K
sometimes same
Power transformers are designed to have maximum efficiency at
no load
half load
near full load
little more than full load
As compared to an amplifier, a transformer cannot
increase output power
increase output current
increase the output voltage
none of these
Two transformers when operating in parallel will share the load depending upon their
magnetizing current
leakage reactance
per unit impedance
efficiency
No load primary input is practically equal to the iron losses in the transformer because
primary current is small
secondary current is small
both the currents are small
no load in put is not equal to iron losses
If the supply frequency to the transformer is increased, the iron loss will
decrease
increase
do not change
none of the above
The size of a transformer core will depend on
frequency
flux density of the core material
area of the core
both frequency and flux density of the core material
Bulchoz relay cannot be used on
three-phase transformer
air cooled transformer
500kV transformer
1000kV transformer
What is the power factor of a potential transformer
0.97
0.95
0.98
0.99
For differential protection, when are the current transformers of the distribution transformer are connected wye-delta?
delta-wye
delta-delta
wye-wye
open delta-wye
What is the moisture limit that should be specified when selecting a transformer?
0.5%
1.5%
1.0%
1.75%
