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Unformatted text preview: 62 ELECTRIC MACHINERY FUNDAMENTALS (d) Calculate the reactive power consumed or supplied by this load. Does the load consume reactive power from the source or supply it to the source? 119. Figure P114 shows a simple singlephase ac power system with three loads. The voltage source is V = 120 0 V, and the impedances of the three loads are Z 1 H11005 5 H11028 30 H9024 Z 2 H11005 5 H11028 45 H9024 Z 3 H11005 5 H11028 H11002 90 H9024 Answer the following questions about this power system. (a) Assume that the switch shown in the figure is open, and calculate the current I , the power factor, and the real, reactive, and apparent power being supplied by the load. N turns l c = 48 cm l r = 4 cm 4 cm Depth = 4 cm 4 cm 4 cm l g = 0.05 cm i N = ? FIGURE P113 The core of Problem 1 17. t (ms) 1 2 3 4 5 6 7 8 (Wb) 0.010 0.005 0.005 0.010 FIGURE P112 Plot of flux H9278 as a function of time for Problem 1 16. cha65239_ch01.qxd 10/23/2003 9:22 AM Page 62 64 ELECTRIC MACHINERY FUNDAMENTALS ( a ) If this bar has a load of 1 0 N attached to it opposite to the direction of motion, what is the steadystate speed of the bar? ( b ) If the bar runs off into a region where the flux density falls to 0. 30 T, what hap pens to the bar? What is its final steadystate speed? ( c ) Suppose V B is now decreased to 80 V with everything else remaining as in part b . What is the new steadystate speed of the bar? ( d ) From the results for parts b and c , what are two methods of controlling the speed of a linear machine (or a real dc motor)? REFERENCES 1. Alexander, Charles K., and Matthew N. O. Sadiku: Fundamentals of Electric Circuits, McGraw Hill, 2000. 2. Beer, F., and E. Johnston, Jr.: Vector Mechanics for Engineers: Dynamics, 6th ed., McGrawHill, New York, 1997. 3. Hayt, William H.: Engineering Electromagnetics, 5th ed., McGrawHill, New York, 1989. 4. Mulligan, J. F.: Introductory College Physics, 2nd ed., McGrawHill, New York, 1991. 5. Sears, Francis W., Mark W. Zemansky, and Hugh D. Young: University Physics, AddisonWesley, Reading, Mass., 1982. cha65239_ch01.qxd 10/16/2003 9:54 AM Page 64 TRANSFORMERS 147 2 8. A 200MVA, 15/200kV singlephase power transformer has a perunit resistance of 1.2 percent and a perunit reactance of 5 percent (data taken from the transformer s nameplate). The magnetizing impedance is j 80 per unit. (a) Find the equivalent circuit referred to the lowvoltage side of this transformer. (b) Calculate the voltage regulation of this transformer for a fullload current at power factor of 0.8 lagging. (c) Assume that the primary voltage of this transformer is a constant 15 kV, and plot the secondary voltage as a function of load current for currents from no load to full load. Repeat this process for power factors of 0.8 lagging, 1.0, and 0.8 leading....
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 Spring '05
 Shaban
 Flux

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