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Cadacio, Jerico James C. 1. The maximum flux density in the core of a 3000/240-V. 50 Hz single-phase distribution transformer is 1.25 Tesla. IF the vo ltage induced per turn is 8 volts, determine the cross sectional of the core in cm2 2. The current densities in the primary and seco ndary windings of a step-down transformer are 1250 and 1500 A per square inch, respectively. The ratio of transformation is 10:1 and t he mean length per turn of the primary is 15% greater than that of the secondary. If the prmary winding resistance is 20 ohms. Calculate the resistance of the secondary windings. 3. A certain transformer has a ratio r atio of 8:1. The primary winding resistance is 0.00283 o hm per turn and has a1155 more turns than the secondary winding. FI the copper losses in the primary is 20% less than in the secondary, find the resistance of the seco ndary winding. 4. A 150 kVA, three -phase transformer supplies 100 kVA at 50% lagging power factor. The transformer’s winding impedance is 2% resistance and 5% re actance, based on 150 kVA. Determine the voltage regulation of t he transformer at this load. Assume 100% voltage. 5. A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc= 137 volts; Psc= 192W; Isc= 4.34 A . Calculate the percent regulation r egulation at a lagging power factor of 0 .707. 6. The eddy current and hysteresis losses of a 4,400-V. 60-Hz single-phase distribution transformer are 280 and 560 watts, respectively Determine the core loss when the transformer is connected across a 4,600-volt. 50-Hz source. 7. When a 220 V. 60 Hz is impressed on a certain transformer at no load, the total core loss is 200 W When the frequency of the impressed voltage is changed to 2 Hz and the magnitude of the voltage is made such as to maintain the maximum flux density as before, the core loss falls to 75 W. Calculate the hyste resis losses at 60 Hz 8. The full load copper loss of a 75 kVA single-phase transformer is thrice the iron losses. At half load unity pf. the efficiency is 97%. Determ ine the full load efficiency at 0.8 pf lagging. 9. A 50 kVA. 2300/230 volt. 60-cycle transformer is tested in the laboratory so that its characteristics maybe determined. The standard test requires an o pen circuit test and short circuit test. Open circuit test I=6.5 A E = 230V P =187 Short circuit test I=21.7 A E= 115 V P= 570W Determine the efficiency of the transformer at half load unity pf. 10. The iron loss of a 25-kVA, 60 Hz single-phase distribution transformer is 1% of the rate d output at unity power factor and t he full load copper loss is 360 W. Determ ine the efficiency when operating one half of the r ated output at 70% power factor lagging. 11. A 10-kvA. 2300/230 transformer has an equivalent resistance referred to the se condary winding of 0.05 ohm. The core loss of this transformer is 75 W. At what kVA load will this transformer operate at maximum efficiency? 12. The core toss of a 5 kVA single-phase transformer with normal voltage applied to the primary is 75 watts. The maximum efficiency occurs at 60% of full load kVA.' What is the full load efficiency of the transformer at 0 .80 power factor? 13. A 100 kVA transformer has a maximum efficiency of 98% at 10% underload and at unity pf. Evaluate the efficiency at quarter load, 0 .8 pf lagging.
14. A 1 MVA, 66/11 kV. 60 Hz transformer has an equivalent series impedance 1.0 + I 5 ohms referred to the low voltage side and a no-load loss of 5500 watts rated terminal voltage What is the maximum efficiency at 0.8 pf lagging. 15. The all day efficiency of a 10 kVA single-phase transformer is 94.7% when loaded as follows. full load unity pf for 4 hours and no load t he rest of the day if the full load copper loss at unity pf is 400 watts, calculate the core loss. 16. A 25 kVA distribution transformer operates at 20% overload. 0.3 pf lagging for 1 hour; at full load, 0.9 lagging pf for 3 hours, 50% load at unity pf for 8 hours and 10% load at 0.8 leading pf for the rest of the day. If core loss is 150 watts and full load copper loss is 500 W. what is the all day efficiency of the tr ansformer? 17. A 1000 kVA, 6,600/400 volts 60 cycle single phase core t ype transformer has the following average daily load: Full load at 0.8 pf for 8 hours Half load at 0.707 pf for 10 hours No-load for 6 hours Calculate the ratio of full load copper loss to the iron loss of the transformer to be most economical for the above loading. 18. The full load copper loss and iron loss of a 7 5 kVA transformer are equal to 1 kW, During a given day, the transformer is loaded as follows: full load at unity pf for 8 hours. one-half load at unity for 8 hours and no-load the rest of the day. Calculate the all-day efficiency. 19. A 60 Hz alternator generates a single-phase voltage of 120 V and has_.! reactance of 0.25 ohm. The alternator feeds a line. which is e ssentially resistive with an impedance of 0.1 ohm The line is terminated with a 2 :1 step up transformer. What should be the load impedance to be connected to the secondary side to maximum power transfer to it? 20. A transformer consists of a primary winding with 500 turns and two secondary windings of 125 turns and 36 turns. The 125-turn secondary winding has 60 ohms connected to its terminals and the 36-turn secondary winding has 3 ohms connected to its terminals. If the primary is connected to a 120-V. 60 Hz source, determine the current in the primary windings.
