The all-day efficiency of a distribution transformer depends mainly on

 Q401. The magnetizing current of a transformer is usually about

A) 1–2% of full-load current
B) 5–10% of full-load current
C) 20–30% of full-load current
D) 40–50% of full-load current
Answer: B

Q402. The iron loss of a transformer mainly depends on
A) Load current
B) Frequency and voltage
C) Power factor
D) Efficiency
Answer: B

Q403. Hysteresis loss in a transformer can be minimized by using
A) Thick laminations
B) Cold-rolled silicon steel
C) High resistance core material
D) Aluminum core
Answer: B

Q404. Eddy current loss can be reduced by
A) Increasing core area
B) Laminating the core
C) Increasing core thickness
D) Using solid iron core
Answer: B

Q405. The main cause of noise in a transformer is
A) Magnetic flux leakage
B) Cooling fan
C) Magnetostriction effect in core
D) Humming of windings
Answer: C

Q406. The efficiency of a transformer is maximum when
A) Iron losses = Copper losses
B) Iron losses > Copper losses
C) Copper losses > Iron losses
D) Both losses are minimum
Answer: A

Q407. A 100 kVA transformer has full-load copper loss of 800 W and iron loss of 600 W. Its efficiency at full load and 0.8 pf lagging is
A) 96.6%
B) 97.4%
C) 98%
D) 95.5%
Answer: A

Q408. The open-circuit test of a transformer gives
A) Iron loss and magnetizing current
B) Copper loss
C) Leakage reactance
D) Efficiency
Answer: A

Q409. The short-circuit test of a transformer gives
A) Iron loss
B) Copper loss
C) No-load current
D) Efficiency
Answer: B

Q410. In a transformer, voltage per turn in primary and secondary is
A) Different
B) Same
C) Proportional to load
D) Inversely proportional
Answer: B

Q411. The leakage flux in a transformer produces
A) Humming noise
B) Voltage regulation
C) Leakage reactance
D) Eddy currents
Answer: C

Q412. A transformer cannot raise or lower the voltage of a DC supply because
A) DC flux does not link the secondary winding
B) DC has no frequency
C) There is no mutual induction
D) All of these
Answer: D

Q413. The efficiency of a large power transformer on full load is approximately
A) 60%
B) 75%
C) 98%
D) 85%
Answer: C

Q414. The all-day efficiency of a distribution transformer depends mainly on
A) Load factor
B) Power factor
C) Regulation
D) Core loss
Answer: A

Q415. Transformer efficiency can be improved by reducing
A) Copper losses
B) Leakage flux
C) Magnetizing current
D) All of these
Answer: A

Q416. When secondary of a current transformer is open-circuited, it may
A) Burn out
B) Saturate
C) Damage insulation
D) All of these
Answer: D

Q417. The function of breather in a transformer is
A) To filter the oil
B) To remove moisture from air entering the conservator
C) To cool the oil
D) To provide insulation
Answer: B

Q418. Silica gel in breather changes color from
A) Blue to pink
B) Pink to blue
C) White to black
D) Red to brown
Answer: A

Q419. In oil-immersed transformers, oil is used for
A) Cooling
B) Insulation
C) Both cooling and insulation
D) None
Answer: C

Q420. The tap changer in a transformer is provided to
A) Vary voltage ratio
B) Improve efficiency
C) Reduce losses
D) Maintain constant power factor
Answer: A

Q421. The tertiary winding in a three-winding transformer is mainly used for
A) Load balancing
B) Voltage regulation
C) Neutral stability
D) Harmonic suppression
Answer: D

Q422. Auto-transformer efficiency compared to two-winding transformer is
A) Higher
B) Lower
C) Same
D) Cannot be compared
Answer: A

Q423. The inrush current in a transformer occurs due to
A) Switching on the transformer
B) Overload
C) Fault
D) Magnetizing current saturation
Answer: A

Q424. A transformer’s impedance voltage is 5%. The short-circuit current will be
A) 5 times full-load current
B) 10 times full-load current
C) 20 times full-load current
D) Equal to full-load current
Answer: C

Q425. If frequency of the supply is increased, keeping voltage constant, the core losses will
A) Increase
B) Decrease
C) Remain same
D) Depend on load
Answer: A

Q426. The no-load current of a transformer lags the applied voltage by
A) 0°
B) 30°
C) 60°
D) 75°–80°
Answer: D

Q427. The polarity test in transformer is used to find
A) Phase difference
B) Polarity of terminals
C) Voltage ratio
D) Efficiency
Answer: B

Q428. The primary and secondary of a transformer are
A) Conductively coupled
B) Inductively coupled
C) Both
D) Neither
Answer: B

Q429. The cooling method for large power transformers is
A) Air blast
B) Oil natural air natural (ONAN)
C) Oil forced air forced (OFAF)
D) Both B and C
Answer: D

Q430. The regulation of a transformer at unity power factor is
A) Zero
B) Positive
C) Negative
D) Minimum
Answer: D

Q431. In a transformer, if secondary load decreases, primary current
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: B

Q432. The core of a transformer is laminated to reduce
A) Hysteresis loss
B) Eddy current loss
C) Copper loss
D) All losses
Answer: B

Q433. Transformer action requires
A) Alternating flux
B) Constant flux
C) Steady current
D) DC supply
Answer: A

Q434. The full-load copper loss of a transformer is 1200 W. The copper loss at half load will be
A) 600 W
B) 300 W
C) 1200 W
D) 150 W
Answer: B

Q435. The auto-transformer is most economical when
A) Voltage ratio is near unity
B) Voltage ratio is high
C) Power factor is low
D) Load is light
Answer: A

Q436. A 1-phase transformer has maximum efficiency at 0.8 pf when delivering 20 kVA. Its efficiency at full-load 0.8 pf is
A) Same
B) Lower
C) Higher
D) Unity
Answer: B

Q437. The main function of conservator in a transformer is to
A) Allow oil expansion
B) Prevent moisture
C) Provide cooling
D) Collect sludge
Answer: A

Q438. Core-type transformers are generally used for
A) High voltage
B) Low voltage
C) Small rating
D) Large rating
Answer: D

Q439. Shell-type transformers are preferred for
A) Low voltage, high current
B) High voltage
C) Large rating
D) Power transmission
Answer: A

Q440. The main advantage of using higher flux density in a transformer is
A) Smaller core size
B) Lesser loss
C) Improved cooling
D) Reduced leakage
Answer: A

Q441. Transformer oil should have
A) High dielectric strength
B) Low viscosity
C) High flash point
D) All of these
Answer: D

Q442. The dielectric strength of transformer oil is usually
A) 1 kV/mm
B) 30 kV/mm
C) 3 kV/mm
D) 10 kV/mm
Answer: B

Q443. In a distribution transformer, the efficiency is maximum at
A) 100% load
B) 50% load
C) 25% load
D) Full load
Answer: B

Q444. The resistance voltage drop of a transformer depends on
A) Power factor
B) Frequency
C) Voltage
D) Load
Answer: D

Q445. If a transformer is operated at voltage higher than rated, its
A) Core losses increase
B) Copper losses decrease
C) Efficiency increases
D) No-load current decreases
Answer: A

Q446. Transformer oil testing includes
A) BDV test
B) Moisture test
C) Acidity test
D) All of these
Answer: D

Q447. For a given load, copper loss varies as
A) Load
B) Load²
C) 1/Load
D) Constant
Answer: B

Q448. When a transformer is over-excited, it draws
A) High magnetizing current
B) Less current
C) Constant current
D) Zero current
Answer: A

Q449. The output of a current transformer is always
A) Constant voltage
B) Constant current
C) Variable voltage
D) Constant power
Answer: B

Q450. The efficiency of an ideal transformer is
A) 100%
B) 90%
C) 75%
D) 99%
Answer: A