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Unformatted text preview: Name: 50 («A "(7’0 VLS Section (Circle One) Overbye ECE 430
Exam #1
Fall 2004 October 6, 2004 Closed book, closed notes One 8.5" by 11" inch note sheet and calculators allowed /, l. /25
2. /35
3. ‘ /25
4. /15 Total / 100 Krein 1. (25 points) A delta connected load with each Z = 843 0°12 is supplied from a balanced three phase wye
connected generator through lines having series impedance of 0.25 130° Q. The lineto—line
voltage measured at the load is 208 volts (for the angle, please use load Vat, as the reference, such
that Vab = 20840" volts). a. Calculate the three load phase currents (magnitude and angle).
b. Calculate the magnitude of the linetoIine voltage at the generator.
c. Calculate the power factor at the generator (be sure to indicate leading or lagging). a“ £3 0" VI“ 208 C) ‘ )
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4 V“; ': 12,735 40” \/ C3 33$: we “50": OQScQIQS 2. (35 points) A 480/240 V, 4.8—kVA, 60112, singlephase transformer is used to supply a 4.8 kVA load with a
power factor of 0.8 lagging at rated voltage of 240 V. a) If you assume the transformer is ideal, what would be the magnitude of the expected
primary side (480 Volt side) current? b) Again, if you assume the transformer is ideal, what is the equivalent impedance of the load
viewed from the primary side of the transformer? Next, the transformer is tested to determine its parameters. The parameters, all referred to the
high side, are found to be Reg; = 3 Q, Xeql = 4 Q, Rc = 6 k9, and Xm = 3 k9. Using the approximate equivalent circuit and assuming the same load as mentioned above c) What is the primary (high) side terminal voltage magnitude needed to supply the load at a
voltage of 240 V and what is its % voltage regulation? (1) What is the transformer’s efﬁciency (11) for this load?
e) Finally, assume that the load is replaced by a short circuit with the supply voltage adjusted to be equal to 480 V. Calculate the secondary short circuit current again using the
approximate equivalent circuit. ‘9 *3
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~ 5 AM 2,’ ‘1:st : \E 3. (25 points) Three loads are connected in parallel as follows: Load 1 (wired in delta) consumes 8 kW at a
lagging power factor of 0.8, load 2 (wired in wye) consumes 10 kVA at a lagging power factor of
0.9, load 3 (wired in delta) consumes 4 + 3]" kVA. The system lineto—line voltage is 480 V. a) Compute the total line current (magnitude only) that supplies these loads. b) Draw a perphase equivalent circuit for this problem. Show the actual impedance values
for each of the three loads. c) How much capacitance (in kvar) must be connected in parallel with these three loads to
correct the total power factor to 0.95 leading? a) .ILt'?
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Clamp: (3.3o+o.°1 :‘20.’Z’U WAR i 4. (Short answer, 15 points total) (6 pts) a) Draw the circuit model for the real transformer discussed in class, and explain the
physical signiﬁcance of each circuit component in the model. (Le ' Xv: ‘ _
{VA/r ﬂat I (a ‘ 4' QZ 2‘1 .9 R‘ 5 k») . v\ (N VWI>
i  (Ll I Z 1 y£$ (é 9"de H (it; c C70(t~v [03915 (Hg{g+ep¢§€‘$ 4 EMY Cumff/A‘R) KM :‘ maﬁwﬂi'zt'rtg mat/Kama!) «.2ka JV“ Quail? ewe panama?!“ ( 1+7 (6 pts) b) A 200 turn coil is wrapped around a toroid that has an average radius of 10 cm, a
crosssectional area of 50 cm2 and a relative permeability of u, = 500. Assuming
that all ﬂux is within the core, how much current must ﬂow in the coil to generate a ﬂux (in the core) of 0.005 Weber? Recall that U0 = 47: x 10'? 11/111. , 7'1 \_~~cz‘f_/\
¢ " $8,971 \KLACQ C‘ viii/A b Lii (3 pts) c) Put dot markings on the coils ...
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 Spring '12
 PeterSauer

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