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Tham khảo tài liệu 'electric machinery fundamentals power & energy_8', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 6 Synchronous Motors 6-1. A 480-V 60 Hz four-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 Solution a If this motor is assumed lossless then the input power is equal to the output power. The input power to this motor is PIN J3vt ILcos 0 J3 480 V 50 A 1.0 41.6 kW The output torque would be _ _POU 41.6 kW 7load m 1 min m- 1800 r min 2n rad 221N m 1 r In English units tload 7.04 POUT nm _ 7.04 41.6 kW 1800 r min 163 lb ft b To change the motor s power factor to 0.8 leading its field current must be increased. Since the power supplied to the load is independent of the field current level an increase in field current increases Ea while keeping the distance EA sin Ỏ constant. This increase in EA changes the angle of the current IA eventually causing it to reach a power factor of 0.8 leading. c The magnitude of the line current will be Il ự3 VT PF V3 480 VW0.8 62.5 A 6-2. A 480-V 60 Hz 400-hp 0.8-PF-leading six-pole A-connected synchronous motor has a synchronous reactance of 1.1 Q and negligible armature resistance. Ignore its friction windage and core losses for the purposes of this problem. 149 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 angle J How near is this value to the maximum possible induced torque of the motor for this field current setting c If E AI is increased by 15 percent what is the new magnitude of the armature current What is the motor s new power factor d Calculate and plot the motor s V-curve for this .
