9-Assignment 9_Solution

9-Assignment 9_Solution - ELECTRICAL ENGINEERING DEPARTMENT...

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ELECTRICAL ENGINEERING DEPARTMENT California Polytechnic State University EE 255 Assignment #9 Problem #1 A 480-V, 60-Hz, 4-pole, Y-connected synchronous generator rated at 470-KVA and 0.85 PF lagging. Its armature resistance is 0.016 The core losses of this machine at rated conditions are 7 KW, and the friction and windage losses are 8 KW. a. Determine the electrical losses of this generator at rated conditions. b. If this generator is operating at the rated conditions, what input torque must be applied to the shaft? Express your answer both in Newton-meters and in pound- feet. Solution: a. Electrical losses KW 15.32 10 016 0 565 3 3 3 2 2 = = = = - x R I P P a a cu elec ) . ( ) ( . b. Input torque KW 82 429 8 7 32 15 85 0 470 . . . * = + + + = + + + = + core W F cu out in P P P P P s rad 5 188 4 60 * 120 * 60 2 s . . = = = π ϖ ϖ s in app P T ft - bl 1680 1800 429820 * 7.04 n 7.04 m - KN 28 2 5 188 82 429 s = = = = = in app app P T T . . . . . Problem #2 A 480-V, 60-Hz, 4-pole synchronous motor draws 50 A from the line at unity power factor and full load. Assuming that the motor is lossless, answer the following questions: a. What is the output torque of this motor? Express the answer both in Newton-meters and in pound- feet b. What must be done to change the power factor to 0.8 leading? Explain your answer using phasor diagrams. c. What will the magnitude of the line current be if the power factor is adjusted to 0.8 leading?
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