TRANSFORMER: by TOP 1, UNDERGROUNDS
1. A single-phase transformer has 300 primary turns and 750 secondary turns. The net cross sectional area of the core
is 64 sq. cm. If the primary voltage is 440V, 60 Hz, find the maximum flux density in the core.
Ans. 0.86 T
2. An ideal 220/110-V transformer carries a (6+j8) ohms load at 110 V side. Under this condition calculate the current
in the 220V side.
Ans. 5.5A
3. A 10 kVA single phase transformer designed for 2000/400 V has the following constant, R1 = 5.5Ω, R2 0.2Ω, X1 = 12Ω,
X2 = 0.45Ω. Solve the approximate value of secondary voltage at full load, 80% lagging, when the primary supply
voltage is 2000V.
Ans. 377.46V
4. The no-load current of a transformer is 4 A at 0.25 pf when supplied at 250 V, 60 Hz. Find the resistance
representing the core loss.
Ans. 250 ohms
5. A 2200/200 V, 60 Hz, transformer takes 0.5A at power factor 0.3 on open circuit when supplied at 200 V. Find the
magnetizing current.
Ans. 0.477 A
6. A 500 kVA, 11/2.3 kV, 60 Hz transformer has been tested for short circuit test and gives the following test data:
600V, 45.4 A, 6kW. Determine the equivalent reactance referred to the secondary side.
Ans. 0.563 ohm
7. A 30 kVA, 2000/200V, single-phase transformer has primary resistance of 3.5 Ω and reactance of 4.5 Ω. The
secondary resistance and reactance are 0.015 Ω and 0.02 Ω respectively. Find the full load copper loss. Ans. 1125W
8. The full load copper loss of a transformer is 6400W. Solve the copper loss at half load.
Ans. 1600W
9. The hysteresis and eddy current loss in a 6600V, 60Hz transformer is 480 W and 280 W respectively. What will be
the losses when the transformer is connected to a 6900V, 40 Hz source.
Ans. 657.33W, 306.03W
10. A 50 kVA 4600/230V, 60 Hz has a full load copper loss of 615W and core loss of 285W. Calculate the percent
efficiency when the transformer is delivering quarter load at a pf of 0.9.
Ans. 97.205%
11. A 15-kVA, 60 Hz. 240/120 V transformer has a core loss of 350W and a full load copper loss of 650 W. Solve the
primary current when the transformer operates at maximum efficiency.
Ans. 45.83 A
12. A single-phase transformer rated 30 kVA, 440/220 V, has a maximum efficiency of 95% and it occurs at rated load,
unity power factor. Calculate the all-day efficiency if loaded as follows: 6hours at full load, 6 hours at quarter load,
no load the rest of the day.
Ans. 90.37%
13. During the short circuit test of a 25 kVA, 480/120 V transformer, 17.85V causes the rated current in the primary
winding. Solve the percent impedance.
Ans. 3.718%
14. The per unit impedance of a system is 0.7 pu. The base kVA is 300 and the base voltage is 11 kV. What is the per unit
impedance referred to the 400 kVA and 38 kV base values.
Ans. 0.0782 pu
15. A 7.2 kVA, 1200/120V, single phase transformer has the following winding parameters: R1 = 0.8 Ω, X1 = 1.2 Ω, R2 =
0.01 Ω, and X2 = 0.01 Ω. Determine the voltage regulation of the transformer when it is delivering rated load at 0.8
pf lagging.
Ans. 1.38%
16. Two transformer A and B are operated in parallel to share a load of 125 kVA at 0.8 pf lagging. Transformers are rated
as follows: A: 100 kVA, 0.9% resistance, 10% reactance; B: 50 kVA, 1% resistance, 5% reactance. Find the kVA share
of transformer B.
Ans. 62 kVA
17. A 480/120V, 5 kVA, two winding transformer is to be used as an autotransformer to supply power at 480 from 600V
source. Solve: a. new kVA rating as an autotransformer , b. power transformed, c. power conducted.
Ans (25kVA, 5kVA, 20 kVA)
18. A CT and a PT are connected to the current and voltage coils, respectively of a wattmeter measuring power going
into the load. The wattmeter reading is 240 W and the turns ratios of CT and PT are 100:5 and 10:1 respectively.
What is the actual power supplied to the load?
Ans. 48 kW
19. Three transformers are connected delta on the primary side and Y on the secondary side, step the voltage down
from 13,200 V to 460 V. Calculate the ratio of transformation of each transformer.
Ans. 49.7
20. Three 30:1 step down transformers are connected wye-delta for stepping down 132 kV three-phase transmission
voltage. Determine the secondary line voltage.
Ans. 2.54 kV
21. Three 25 kVA transformers are connected closed delta. a. What total rated kVA can the bank deliver? b. To what kVA
should be reduced if one of the transformers is removed so that the bank operates open delta?
Ans. 75, 43.3 kVA
22. An open delta bank delivers a 3 phase balanced load that draws 100 kW at 0.8 pf. Calculate the minimum kVA rating
of each transformer.
Ans. 72.168 kVA