AC MACHINE MCQ 17

 

AC MACHINES 17

1. A distribution transformer should be selected on the basis of
a) efficiency
b) all day efficiency
c) voltage regulation
d) none of these
ans. b

2. In a transformer the iron losses do not vary with load current because
a) iron losses are equal to copy losses
b) core flux remain constant
c) core area is constant
d) none of these
ans. b

3. It is economical to use auto-transformer when the transformation ratio is
a) low
b) high
c) neither too high nor too low
d) none of these
ans. a

4. The no load test on a transformer is usually carried out by exciting the low voltage winding because
a) the required power input is low
b) the no load current becomes significant and can be conveniently measured
c) it is not advisable to work on the high voltage side
d) none of these
ans. b

5. During a short circuit test, the iron losses are negligible because
a) the mutual flux is small
b) the power factor is low
c) the current is high
d) none of these
ans. a

6. The transformer principle is based on
a) self-induction
b) mutual-induction
c) electric induction
d) the Lenz law
ans. b

7. Hysteresis loss in a transformer depends upon the
a) applied voltage
b) the type of core material
c) number of laminations
d) reactance of the winding
ans. b

8. What is the efficiency of transformers when compared with that of electrical motors of the same power ?
a) much smaller
b) somewhat smaller
c) about the same
d) much higher
ans. d

9. If in a transformer we double the secondary turns and at the same time reduce the primary voltage by half , then the secondary voltage will
a) he halved
b) be four times as high
c) not change
d) be doubled
ans. c

10. What type of core is used for a high frequency transformer?
a) open iron core
b) closed iron core
c) shell type iron core
d) air core
ans. d

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AC MACHINE MCQ 16

 

AC MACHINES 16

1. Which of the following does not change in an ordinary transformer?
a) voltage
b) current
c) frequency
d) phase angle
ans. c

2. The color of fresh dielectric oil for a transformer is
a) pale yellow
b) dark brown
c) while grey
d) colourless
ans. a

3. In a transformer if in place of sine wave, a peaked voltage is fed to the primary,
a) the iron losses will be less
b) the iron losses will be more
c) the copper losses will be less
d) the noise level will be low
ans. a

4. The path of the magnetic flux in a transformer should have
a) low reluctance
b) low resistance
c) high reluctance
d) high resistance
ans. a

5. The function of breather in a transformer is
a) to filter the transformer oil
b) to provide oxygen to the cooling oil
c) to provide cooling air
d) to arrest the flow of moisture when outside air enters the transformer
ans. d

6. Transformer works on
a) ac only
b) dc only
c) both ac and dc
d) pulsating dc
ans. c

7. Iron losses in a transformer occur in
a) core
b) winding
c) insulating
d) main body
ans. a

8. Transformers use laminated core
a) to reduce eddy current loss
b) to reduce eddy current and hysteresis loss
c) to reduce copper loss
d) none of these
ans. a

9. In a transformer with closed magnetic coupling between primary and secondary windings.
a) efficiency will he high
b) all day efficiency will be high
c) regulation will be good
d) none of these
ans. c

10. The number of turns in the primary winding of a transformer depends on
a) input voltage
b) input current
c) both input voltage and current
d) none of these
ans. a

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AC MACHINE MCQ 15

 

AC MACHINES 15

1. Current transformers for meters and relays usually have
a) 10-A secondaries
b) 5-A secondaries
c) 10:1 ratio
d) 1:1 ratio
ans. b

2. The main purpose of using an inter-phase transformer with a multi-anode rectifier is to
a) permit only one anode to fire at a time.
b) cause the anodes to overlap in firing.
c) shorten the period of firing of each anode.
d) act as an insulating transformer between the power transformer and load
ans. b

3. Dust should never be allowed to accumulate on the windings and are of a dry-type transformer because
a) may short-circuit the windings
b) tends to corrode metal surface
c) reduces dissipation of heat
d) absorbs oil and grease
ans. c

4. The relative polarity of the windings of a transformer may be determined by
a) short-circuit test
b) open-circuit test
c) phasing out
d) back-to-back test
ans. c

5. The most common cause of contamination by water of oil used in transformers located indoors is
a) condensation of moisture from air in the upper part of the tank
b) decomposition of organic matter in the oil.
c) leaky bushings
d) use of filter blotters that have absorbed moisture from the air
ans. b

6. Part of the transformer which is most subject to damage from overheating is
a) winding insulation
b) frame or case
c) iron core
d) copper winding
ans. a

7. In general, the most important point to keep under constant watch during the operation of a transformer is its
a) primary voltage
b) core loss
c) exciting current
d) temperature
ans. d

8. The most essential feature of an auto-transformer is that it
a) is more suited for single-phase supply
b) is completely automatic in its operation
c) uses only one winding
d) none of the above
ans. c

9. When secondary of the transformer is loaded, the primary current will
a) not be effected
b) draw more current
c) draw less current than the secondary current
d) be the vector sum of no load current and excessive current drawn due to the secondary current.
ans. d

10  shell type transformer is commonly used because
a) it has more magnetic flux leakage
b) it has less magnetic flux leakage
c) it has two magnetic paths
d) both b and c are correct
ans. d

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AC MACHINE MCQ 14

 

AC MACHINES 14

1.With increase in supply frequency of a transformer, there is
a) decrease in rating
b) increase in rating
c) no change in rating
d) none of these
ans. b

2. The essential condition for parallel operation of two single phase transformers is that they should have same
a) polarity
b) phase sequence
c) voltage ratio
d) percentage impedances
ans. a

3. Two transformers operating in parallel will share the load depending upon their
a) rating
b) leakage reactance
c) efficiency
d) per-unit impedance
ans. a

4. Which of the following method is used for cooling of transformer up-to a capacity of 10 MVA ?
a) oil natural cooling
b) Air blast cooling
c) Forced oil cooling
d) none of these
ans. a

5. Out of the following choices for polyphase transformer connections which one will you select for 3 to 2 phase conversion?
a) scott
b) star/star
c) double scott
d) star/double-delta
ans. a

6. When two identical transformers are available , then the best method to find their efficiency under load condition is
a) short-circuit test
b) back to back test
c) open-circuit
d) phase sequence test
ans. b

7. The open-circuit test on a transformer is meant for measuring its
a) copper losses
b) iron losses
c) insulation resistance
d) equivalent resistance
ans. b

8. Oil is invariably used in large transformers in order to
a) insulate the coils
b) insulate the core
c) lubricate the core
d) lubricate the coils
ans. a

9. The secondary winding if a current transformer whose primary is carrying current should
a) not be open circuited
b) not be short circuited
c) always be open circuited
d) not be connected to the current coil of a wattmeter
ans. a

10. Before disconnecting an ammeter from an energised current transformer circuit, the current tranformer
a) primary should be opened
b) secondary should be opened
c) primary should be shorted
d) secondary should be shorted
ans. d

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AC MACHINE MCQ 13

 

AC MACHINES 13

1. In a transformer
a) both oc and sc tests are conducted on lv side.
b) oc test is conducted on hv and sc test is conduted on lv.
c) oc test is conducted on lv and sc test is conducted on hv.
d) both oc and sc test are conducted on hv side.
ans. c

2. For successful parallel operation of two single phase transformers, the essential condition is that their
a)  percentage impedances should be equal
b) turns ratios should be exactly equal
c) polarities must be properly connected
d) KVA ratings should be equal.
ans. c

3. In a single phase induction regulator, the output voltage can be varied
a) both in magnitude and phase
b) in magnitude only
c) in phase only
d) in magnitude less than the supply voltage
ans. b

4. Transformer action requires a
a) constant magnetic flux
b) increasing magnetic flux
c) pulsating magnetic flux
d) alternating electric flux
ans. c

5. In an oil filled transformer, oil is provided for
a) cooling
b) insulation
c) lubrication
d) both cooling and insulation
ans. d

6. Transformer cores are laminated to reduce
a) eddy current loss
b) hysteresis loss
c) both a and b
d) ohmic loss
ans. a

7. CRGO laminations in a transformer are used to minimise

a) eddy current loss
b) hysteresis loss
c) both a and b
d) ohmic loss
ans. b

8. An Air-core transformer, as compared to iron core transformer, has
a) less magnetic core loss
b) more magnetic core loss
c) no magnetic core loss
d) less ohmic loss
ans. c

9. Transformer zero voltage regulation occurs at
a) unity pf
b) leading pf
c) lagging pf
d) zero pf leading
ans. b

10. Transformer maximum efficiency, for a constant load current, occurs at
a) 0.8 pf
b) zero pf leading
c) zero pf lagging
d) unity power factor
ans. d

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AC MACHINE MCQ 12

 

AC MACHINES 12

1. Winding to earth faults may be caused by
a) sludging of the oil
b) surge voltage only
c) current chopping only
d) both surge voltages and current chopping
ans. d

2. The two windings of a transformer are designated as
a) primary and secondary windings
b) primary and hv windings
c) secondary and lv windings
d) hv and lv windings
ans. d

3. The flux involved in the emf equation of a transformer has
a) rms value
b) average value
c) total value
d) maximum value
ans. d

4. The leakage flux in a transformer depends upon
a) the applied voltage
b) the frequency
c) the load current
d) the mutual flux
ans. c

5. Good voltage regulation of a transformer means that the difference between its no-load  voltage and full-load voltage is
a) very high
b) low
c) very low
d) none of these
ans. d

6. Distribution transformers have core losses
a) greater than copper losses
b) equal to copper losses
c) less than copper losses
d) equal to 1/2 (copper losses)
ans. c

7. Eddy current loss in a transformer depends on
a) both voltage and frequency
b) voltage alone
c) load current alone
d) thickness of core
ans. d

8. Hysteresis loss in a transformer depends upon
a) frequency
b) supply voltage
c) square of the frequency alone
d) square of the voltage alone
ans. a

9. The open-circuit test on transformer is conducted to obtain
a) the leakage impedances
b) the ohmic loss
c) hysteresis loss only
d) core losses
ans. d

10. The short-circuit test on a transformer is conducted to obtain
a) core losses at any load
b) ohmic losses at any load
c) hysteresis losses
d) both core and ohmic losses
ans. b

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AC MACHINE MCQ 11

 

AC MACHINES 11

1. A transformer may have two or more ratings depending on
a) the type of winding used
b) the type of cooling used
c) the type of core used
d) the type of insulation used
ans. b

2. Conservator is used
a) to take up the expansion of oil due to temperature rise.
b) to act as an oil storage
c) for better cooling of the transformer
d) none of these
ans. a

3. Buchholz relay is placed
a) in between the conservator and the breathe
b) in between the tank and the conservator
c) in between the h.v. winding and the bushing
d) in between the l.v.winding and the bushing
ans. b

4. Buchholz relay is a
a) voltage sensitive device
b) current sensitive device
c) frequency sensitive device
d) gas actuated device
ans. d

5. Buchholz relay causes
a) tripping for major fault and alarm for minor faults
b) alarm for major fault and tripping for minor faults
c) tripping for both major and minor faults
d) alarm for both major and minor faults
ans. a

6. Transformer oil is used as
a) insulant only
b) coolant only
c) both insulant and coolant
d) insert medium
ans. c

7. Sludging of transformer oil means
a) continuous expansion and contraction due to heating and cooling
b) formation of semi-solid hydro-carbon due to heat and oxidation

c) decomposition of transformer oil under the influence of power arcs
d) evaporation of transformer oil due to heating
ans. b

8. To avoid breakdown of transformers under surges any one of the following except one may be adopted
a) use of surge absorbers
b) use of surge diverters
c) use of metal shields
d) Reinforcement to end-turn insulation.
ans. d

9. Repeated switching of transformers located nearer the generating station may cause
a) surface flashover of the bushings
b) mechanical distortion of the windings and subsequent turn to turn insulation failure
c) excessive temperature rise
d) intense local eddy currents in the core
ans. b

10. If the drying out operation of a transformer is unduly shortened and normal voltage is applied, then there is possibility of
a) inter-turn failure
b) winding to earth failure
c) terminal to terminal flashover
d) excessive overheating of the transformer
ans. a

 

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AC MACHINE MCQ 10

 

AC MACHINES 10

1. For cold rolled grain-oriented silicon steel metered overlap is preferable for the core-yoke joint as
a) it improves mechanical strength
b) it makes better use of core space
c) it reduces magnetostriction
d) it reduces magnetizing current and also the core-losses
ans. d

2. The direction of central phase winding of a three-phase shell type transformer is reversed with respect to the outer phases to
a) save considerable amount of core material
b) reduce leakage fluxes
c) reduce short circuit forces
d) minimise eddy current loss
ans. a

3. Low voltage windings are placed nearer to the core in the case of concentric windings because it reduces
a) hysteresis loss
b) eddy current loss
c) insulation requirement
d) leakage fluxes
ans. c

4. Spiral winding is suitable only for windings
a) carrying very low current
b) carrying very high current
c) rated for high voltage
d) rated for low voltage
ans. b

5. Helical winding is suitable for
a) low voltage winding of small transformers
b) high voltage winding of small transformers
c) low voltage winding of large transformers
d) high voltage winding of large transformers
ans. c

6. Cross over winding is suitable for
a) low voltage winding of small transformers
b) high voltage winding of small transformers
c) low voltage winding of large transformers
d) high voltage winding of large transformers
ans. c

7. Continuous disc winding is suitable for
a) low voltage winding of small transformers
b) high voltage winding of small transformers
c) low voltage winding of large transformers
d) high voltage winding of large transformers
ans.d

8. In general , for all sizes of distribution transformer, it is preferable to use
a) plain sheet steel tanks
b) tubed tanks
c) corrugated tanks
d) radiator tanks
ans. b

9. For small power transformers, it is preferable to use
a) tubed tanks
b) corrugated tanks
c) radiator tanks
d) tanks with separate coolers
ans. a

10. For large power transformer, it is preferable to use

a) tubed tanks
b) corrugated tanks
c) radiator tanks
d) tanks with separate coolers
ans. d

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