University of Jordan Faculty of Engineering & Technology Electric Power Engineering Department EE373: Electrical Machines-M&M Second Semester 2014-2015 Tutorial # 2: Transformers Question #1 A 25-kVA, 440/220-V, 60-Hz transformer has the following parameters: R1 = 0.16 R2 = 0.04 Rc1 = 270 X2 = 0.08 Xm1 = 100 X1 = 0.32 The transformer delivers 20 kW at 0.8 power factor lagging to a load on the low-voltage side with 220 V across the load. Find the primary terminal voltage. (Use the exact equivalent circuit). Question #2 A 150-kVA, 2400/240-V transformer has the following parameters referred to the primary side: Re1 = 0.5 and Xe1 = 1.5 . The shunt magnetizing impedance is very large and can be neglected. At full load, the transformer delivers rated kVA at 0.85 lagging power factor and the secondary voltage is 240 V. Calculate: a) The voltage regulation b) The efficiency assuming core losses amount to 600 W. Question #3 A50-kVA, 2400/240-V, 60-Hz single-phase transformer has a short-circuit test performed on its high-voltage side. An open-circuit test is performed on the low-voltage side. The following test results were obtained: Open-circuit test, with the high-voltage winding open: Voc = 240 V Ioc = 5.4 A Poc = 186 W Short-circuit test, with the low-voltage winding shorted: Vsc = 48 V Isc = 20.8 A Psc = 620 W a) Determine the equivalent circuit of the transformer referred to the low-voltage side. b) Determine its voltage regulation, and efficiency at rated load, 0.8 power factor lagging, and rated voltage at the secondary terminals. Question #4 A single-phase, 10-kVA, 440/110-V, two winding transformer is connected as an autotransformer to supply a load at 550 V from a 440 V supply. Calculate the following: a) kVA rating as an autotransformer b) Apparent power transferred by conduction c) Apparent power transferred by electromagnetic induction. Question #5 Three identical single-phase transformers are each rated 10-kVA, 2400/120 V, 60 Hz. They are connected to form a three-phase 4160/208-V, Y-Y transformer bank. The equivalent impedance of each single-phase transformer referred to the primary side is 10 + j25 . The transformer bank delivers 27 kW at 208 V and 0.9 power factor leading. a) Draw the three-phase schematic diagram showing the transformer connection. Draw a perphase equivalent circuit. b) Determine the primary current, primary voltage, and power factor. c) Determine the voltage regulation.