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ELEC 3164 HW 5 Spring 2012

ELEC 3164 HW 5 Spring 2012 - University of Colorado at...

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Unformatted text preview: University of Colorado at Denver and Health Sciences Center College of Engineering and Applied Sciences Department of Electrical Engineering EE 3164: Energy Conversion Homework Assignment # 5 —- Transformers 1. A l—phase, 2-winding transformer is rated at 50 kVA, 7200 V (primary) ~ 240 V (secondary). Calculate the full-load primary and secondary currents. The transformer is delivering 80% of rated load at 0.9 lagging power factor. Calculate the power output. Also calculate the secondary load impedance (Z2) and the corresponding impedance value referred (or reflected) to the primary side (Z’2). Ans: 6.94 A, 208.3 A, 36 kW, 1.44 A 25.80 Q, 1296 A 25.80 9 2. A l—phase, 2—winding transformer is rated at 10 kVA, 200 V (primary/source) — 400 V (secondary/load). The transformer efficiency is 98.6% when the transformer is delivering 20% above the rated load at 0.9 (lagging) power factor. Calculate the power input and the losses in the transformer. Calculate the load current. Ans: 10.953 kW, 153 W, 30A 3. A 1-phase, 2-winding transformer is supplying the following three loads, connected to the secondary side which is rated at 240 V: o 5 kVA @ 0.8 (lag) power factor 0 2 kW of heating and lighting load 0 3 small, 1—phase induction motors totaling 6 HP with an average efficiency of 0.8 and a power factor of 0.8 (lag) Determine a suitable size of a transformer. If the primary voltage is 7,200 V, calculate the primary and secondary load currents. Assuming a transformer efficiency of 98.5%, calculate the power input for this loading condition. Ans: 141<VA (rounded up), 1.90 A, 56.9 A, Pi = 11.78 kW 4. The secondary winding of a single~phase transformer has 180 turns. When the transformer is carrying full load, the secondary current is 18 amps at 60 HZ. At this operating point, the peak mutual flux ((Dm) is 20 me. Determine the value of induced voltage in the secondary winding. Ans: 959 V ...
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