Transformers

Problem 633: When 120 V, 60 Hz is applied to the primary winding of a transformer, the core loss is 240 W. When 50 V, 25 Hz is applied the core loss is 50 W. Determine the core loss when 230 V, 50 Hz is applied to the primary winding.

Problem 634: 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.

Problem 635: In a 400 V, 50 c/s transformer, the total iron loss is 2500 W. When the supplied p.d. is 220 V at 25 c/s, the corresponding loss is 850 W. Calculate the eddy current loss at normal frequency and p.d.

Problem 636: 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 25 Hz and the magnitude of the voltage is made such as to maintain the same maximum flux density as before, the core loss falls to 75 W. Calculate the hysteresis losses at 60 Hz.

Problem 637: A 100-kVA, 4160/230 V, 60 Hz single-phase transformer has percentage impedance of 1.75 + j 4.0. Its efficiency when operating at rated conditions at 75% pf lagging is 97%. Determine the efficiency, if the transformer is loaded at rated load and voltage but at 50 Hz. Assume the ratio of eddy current losses to hysteresis losses at 60 Hz to be 2.5.

Problem 638: 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%. Determine the full load efficiency at 0.8 pf lagging.

Problem 639: 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 open circuit test and short circuit test. Open circuit test: I = 6.5 A, E = 230 V, P = 187 W. Short circuit test: I = 21.7 A, E = 115 V, P = 570 W. Determine the efficiency of the transformer at half load, unity pf.

Problem 640: The iron loss of a 25-kVA, 60 Hz single-phase distribution transformer is 1% of the rated output at unity power factor and the full load copper loss is 360 W. Determine the efficiency when operating one half of the rated output at 70% power factor lagging.

Problem 641: Given that full load copper losses are exactly twice the iron losses in a 50-kVA transformer, and that the quarter load efficiency is 96.5%. Calculate the full load efficiency at unity power factor.

Problem 642: A 15 kVA, 2300/230-volt transformer is under test. With the low side supplied with rated voltage and the high side open circuited, input power through a wattmeter reads 245 W. With the high side supplied with 65 V while the low side short circuited, input power at rated current reads 350 W. Determine the maximum efficiency that can be expected from this transformer at unity pf.

Problem 643: A 10-kVA, 2300/230 transformer has an equivalent resistance referred to the secondary 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?

Problem 644: The core loss 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?

Problem 645: A 100-kVA transformer has 94% efficiency at full load and at 50% full load with unity pf in both cases. Determine the maximum efficiency of the transformer at unity pf load.

Problem 646: A 100 kVA transformer has a maximum efficiency of 98% at 10% underload and at unity pf. Evaluate the efficiency at quarter load, 0.80 pf lagging.

Problem 647: A 1 MVA, 66/11 kV, 60 Hz transformer has an equivalent series impedance of 1.0 + j 5 ohms referred to the low voltage side and a no-load loss of 5500 watts at rated terminal voltage. What is the maximum efficiency at 0.8 pf lagging?

Problem 648: 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 the rest of the day. If the full load copper loss at unity pf is 400 watts, calculate the core loss.

Problem 649: 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 transformer?

Problem 650: Find the all-day efficiency of a transformer having a maximum efficiency of 98 percent at 15 kVA at unity pf and loaded as follows: 12 hours — 2 kW at 0.5 pf lagging; 6 hours — 12 kW at 0.8 pf lagging; 6 hours — at no-load.

Problem 651: A 30 kVA 2400/240 V, 60 Hz transformer has a full load power of unity over the period of 24 hours. The maximum efficiency is 95% and it occurs at full load. Calculate the all-day efficiency if loaded as follows: 6 hours at full load, 6 hours at quarter load.

Problem 652: A 1000 kVA, 6,600 /400 volts 60 cycle single phase core type 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 for the transformer to be most economical for the above loading.