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, 2winding transformer is rated at 50 kVA, 7200 V (primary) ~ 240 V
(secondary). Calculate the fullload 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 1phase, 2winding 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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 Spring '12

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