A 500 kVA, 34.5/13.8 kV, 3-phase transformer is delta-wye connected. The primary and secondary winding resistances are 35 ohms and 0.876 ohm per phase, respectively. Calculate the percent efficiency at full load and unity power factor if the core loss is 3 kW.
A97.8
B98.7
C96.4
D97.2
#162📜 EE Pre-Board August 2018📘 PowerhouseEasy
A 75 KVA, 13,200/240, 60Hz, single-phase transformer has 1% resistance and 3% reactance. Find the copper loss of the transformer at 3/4 of full-load and unity power factor.
A155 W
B422 W
C368 W
D508 W
#163📜 EE Pre-Board August 2018📘 PowerhouseHard
In a transformer, the core loss is found to be 60 watts at 30 Hz and 90 watts at 40 Hz measured at the same maximum flux density. Compute the eddy current and hysteresis losses at 60 Hz in watts.
A90, 75
B85, 60
C95, 69
D88, 72
#164📘 EE Pocket Reviewer RojasEasy
Which of the following is the reason why the core of a transformer is heating up when the primary windings is connected to a supply voltage?
AReluctance of the core
BPermeability of the core
CHysteresis loss
DFerromagnetism
#165📘 EE Pocket Reviewer RojasEasy
A transformer will work on what type of voltage?
Adc only
Bac only
CBoth dc and ac
DAny kind of supply voltage
#166📘 EE Pocket Reviewer RojasEasy
For successful parallel operation of two single phase transformers, which one is the most essential condition required?
APercentage impedance are equal
BTurn ratios are equal
CkVA ratings are equal
DPolarities are properly connected
#167📘 EE Pocket Reviewer RojasEasy
Which one is true about an ideal transformer?
AThe windings has no resistances
BThe core has no losses
CThe core has infinite permeability
DAll of these
#168📘 EE Pocket Reviewer RojasEasy
Which of the following are the same for both primary and secondary windings in a transformer?
ACurrents and induced emfs
BAmpere-turns and voltage per turn
CResistances and leakage reactances
DNone of these
#169📘 EE Pocket Reviewer RojasEasy
The regulation of a transformer is least affected by changes in frequency at which loading?
AUnity power factor
BLeading power factor
CLagging power factor
DAll of these
#170📘 EE Pocket Reviewer RojasEasy
What is the reason why transformers are rated in kVA instead of kW?
ALoad pf is normally not known
BThe kVA is fixed while the kW is dependent on load pf
CTotal transformer loss depends on volt-ampere
DIt is already customary that the rating is kVA, based on early transformers.
#171📘 EE Pocket Reviewer RojasEasy
What is the reason why the kVA rating of an ordinary two-winding transformer is increased when connected as an autotransformer?
ATransformation ratio is increased
BSecondary voltage is increased
CSecondary current is increased
DEnergy is transferred both inductively and conductively
#172📘 EE Pocket Reviewer RojasEasy
At what load is the efficiency of a transformer be zero?
ANo load
BNear full load at unity pf
COverload at leading pf
DUnderload at lagging pf
#173📘 EE Pocket Reviewer RojasEasy
When a V-V bank is converted to a Δ-Δ bank, what is the percentage increase in its capacity?
A66.7%
B73.2%
C173.2%
D141.4%
#174📘 EE Pocket Reviewer RojasEasy
Which one is the main cause of the noise in transformer?
AMagnetic flux
BAlternating current in the windings
CCooling oil which is heating up
DAll of these
#175📘 EE Pocket Reviewer RojasEasy
At what loading condition is the efficiency of a power transformer maximum?
ANo load
BHalf load
CNear full load
DA little more than full load
#176📘 EE Pocket Reviewer RojasEasy
How does the voltage per turn in the primary windings related to the voltage per turn in the secondary windings?
AEqual
BLess than
CGreater than
DEither B or C depending on step-up or step down applications
#177📘 EE Pocket Reviewer RojasEasy
The eddy current loss in a transformer are reduced by laminating the core, the laminations being insulated from each other by which of the following?
AThick paper
BThin sheets of mica
CLight coat of core plate varnish
DThin sheets of copper
#178📘 EE Pocket Reviewer RojasEasy
Which of the following transformer connections of a transformer will give the highest secondary voltage?
ADelta primary, delta secondary
BDelta primary, star secondary
CStar primary, star secondary
DStar primary, delta secondary
#179📘 EE Pocket Reviewer RojasEasy
Which one is true about an autotransformer?
AIt has two separate windings connected in series
BIt has no secondary windings
CIt is best suited in stepping up the voltage only
DIt has only one winding
#180📘 EE Pocket Reviewer RojasEasy
Which of the following is true in a distribution transformer?
AThe all-day efficiency is more than the commercial efficiency
BThe all-day efficiency is less than the commercial efficiency
CThe all-day efficiency is equal to the commercial efficiency
