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Unformatted text preview: ELECTRICAL ENGINEERING DEPARTNIEN T
California Polytechnic State University BB 255 Assignment #9' Problem 1 A 480V, 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? b. What must be done to change the power factor to 0.8 lagging? 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 lagging? Problem 2
A 480—V, 60—Hz, 400—hp, 0.8 PF—leading, six—pole, delta—connected synchronous
motor has a synchronous reactance of 1.1 and negligible armature resistance. Ignore its friction, windage , and core losses for the purpose of this problem.
a If this motor is initially supplying 400 hp at 0.8 PF—lagging, what are the magnitudes and angles of EA and IA? b. How much torque is this motor producing? What is the torque angleﬁ ? How
near is this value to the maximum possible induced torque of the motor for
this ﬁeld current setting? 0. If IEAI is increased by 15%, what is the new magnitude of the armature current? What is the motor’s new power factor? Problem 3 A 208V, Y—connected synchronous motor is drawing 40 A at unity power factor from a 208V power system. The field current ﬂowing under these conditions is 2.7 A. Its synchronous reactance is 0.8 . Assume a linear open —circuit Characteristic. a. Find the torque angle 6 . b. How much ﬁeld current would be required to make the motor operate at 0.8
PF lagging? c. What is the new torque angleé‘ ? Problem 4
A synchronous has a synchronous reactance of 2.0 per phase and armature
resistance of 0.4 per phase. If E; = 460/: — SW and = 48040°V, is the machine acting as a motor or a generator? How much power P is this machine
consuming from or supplying to the electric system? How much reactive power Q
is this machine consuming from or supplying to the electric system? Problem 5 A 480—V, 50112, 100KW, fourpole, Y—connected synchronous motor has a med
power factor of 0.85 leading. At full load, the efﬁciency is 91%. he armature
resistance is 0.08 and the synchronous reactance is 1.0 . Find the following
quantities for this machine when it operating at fullload: a. Output torque. b. Input power 0. The speed of the motor (1. The armature current e. The induced voltage f. The converted power g. The mechanical loss + the core loss + the stray loss smpuiaiion Pad f“ rs
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This homework help was uploaded on 04/07/2008 for the course EE 255 taught by Professor Shaban during the Fall '05 term at Cal Poly.
 Fall '05
 Shaban

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