Deck 3: Transformer: Resistance, Oil, Relays, Harmonics, and Core Properties

Question 1

In a transformer the resistance between its primary and secondary is

A)zero
B)1 ohm
C)1000 ohms
D)infinite

Answer 1

infinite

Question 2

A transformer oil must be free from

A)sludge
B)odour
C)gases
D)moisture

Answer 2

moisture

Question 3

A Buchholz relay can be installed on

A)auto-transformers
B)air-cooled transformers
C)welding transformers
D)oil cooled tr

Answer 3

auto-transformers

Question 4

Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds

A)50°C
B)80°C
C)100°C
D)150°C

Question 5

The main reason for generation of harmonics in a transformer could be

A)fluctuating load
B)poor insulation
C)mechanical vibrations
D)saturation of core

Question 6

Distribution transformers are generally designed for maximum efficiency around

A)90% load
B)zero load
C)25% load
D)50% load

Question 7

Which of the following property is not necessarily desirable in the material for transformer core ?

A)Mechanical strength
B)Low hysteresis loss
C)High thermal conductivity
D)High permeability

Question 8

Star/star transformers work satisfactorily when

A)load is unbalanced only
B)load is balanced only
C)on balanced as well as unbalanced loads
D)none of the above

Question 9

Delta/star transformer works satisfactorily when

A)load is balanced only
B)load is unbalanced only
C)on balanced as well as unbalanced loads
D)none of the above

Question 10

Buchholz's relay gives warning and protection against

A)electrical fault inside the transformer itself
B)electrical fault outside the transformer in outgoing feeder
C)for both outside and inside faults
D)none of the above

Question 11

The magnetising current of a transformer is usually small because it has

A)small air gap
B)large leakage flux
C)laminated silicon steel core
D)fewer rotating parts

Question 12

Which of the following does not change in an ordinary transformer ?

A)Frequency
B)Voltage
C)Current
D)All of the above

Question 13

Which of the following properties is not necessarily desirable for the material for transformer core ?

A)Low hysteresis loss
B)High permeability
C)High thermal conductivity
D)Adequate mechanicalstrength

Question 14

The leakage flux in a transformer depends upon

A)load current
B)load current and voltage
C)load current, voltage and frequency
D)load current, voltage, frequency and powerfactor

Question 15

The path of the magnetic flux in transformer should have

A)high reluctance
B)low reactance
C)high resistance
D)low resistance

Question 16

Noise level test in a transformer is a

A)special test
B)routine test
C)type test
D)none of the above

Question 17

Which of the foJIowing is not a routine test on transformers ?

A)Core insulation voltage test
B)Impedance test
C)Radio interference test
D)Polarity test

Question 18

A transformer can have zero voltage regulation at

A)leading power factor
B)lagging powerfactor
C)unity power factor
D)zero power factor

Question 19

Helical coils can be used on

A)low voltage side of high kVA trans¬formers
B)high frequency transformers
C)high voltage side of small capacity transformers
D)high voltage side of high kVA rating tr

Question 20

Harmonics in transformer result in

A)increased core losses
B)increased I2R losses
C)magnetic interference with communication circuits
D)all of the above

Question 21

The core used in high frequency transformer is usually

A)copper core
B)cost iron core
C)air core
D)mild steel core

Question 22

The full-load copper loss of a trans¬former is 1600 W. At half-load, the copper loss will be

A)6400 W
B)1600 W
C)800 W
D)400 W

Question 23

The value of flux involved m the e.m.f. equation of a transformer is

A)average value
B)r.m.s. value
C)maximum value
D)instantaneous value

Question 24

Silicon steel used in laminations mainly reduces

A)hysteresis loss
B)eddy current losses
C)copper losses
D)all of the above

Question 25

Which winding of the transformer has less cross-sectional area ?

A)Primary winding
B)Secondary winding
C)Low voltage winding
D)High voltage winding