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Tham khảo tài liệu 'electric machinery fundamentals power & energy_9', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 7 Induction Motors 7-1. A dc test is performed on a 460-V A-connected 100-hp induction motor. If VDC 24 V and IDC 80 A what is the stator resistance R1 Why is this so Solution If this motor s armature is connected in delta then there will be two phases in parallel with one phase between the lines tested. De R1 Therefore the stator resistance R1 will be V R1 R1 R1 2 R 1 DC R1 R1 R1 3 1 R1 3 3 24V 0.45 Q 1 2 IDC 2 80 A 7-2. A 220-V three-phase two-pole 50-Hz induction motor is running at a slip of 5 percent. Find a The speed of the magnetic fields in revolutions per minute b The speed of the rotor in revolutions per minute c The slip speed of the rotor d The rotor frequency in hertz Solution a The speed of the magnetic fields is 120 fe_ 120 50 Hz nsync 1 Pe ----- 2 3000 r min b The speed of the rotor is nm 1 - s nsync 1 - 0.05 3000 r min 2850 r min c The slip speed of the rotor is nslip snsync 0.05 3000 r min 150 r min d The rotor frequency is n 150 r .ninX2 2.5 Hz r 120 120 7-3. Answer the questions in Problem 7-2 for a 480-V three-phase four-pole 60-Hz induction motor running at a slip of 0.035. Solution a The speed of the magnetic fields is 171 120 fe_ 120 60 Hz sync p- ----- 4 1800 r min b The speed of the rotor is nm 1 - -0 nsync 1 -0.035 1800 r min 1737 r min c The slip speed of the rotor is nslip -nsync 0.035 1800 r min 63 r min d The rotor frequency is n i P 63 r rn.n 4 2.1 Hz A 120 120 7-4. A three-phase 60-Hz induction motor runs at 890 r min at no load and at 840 r min at full load. a How many poles does this motor have b What is the slip at rated load c What is the speed at one-quarter of the rated load d What is the rotor s electrical frequency at one-quarter of the rated load Solution a This machine has 8 poles which produces a synchronous speed of 1204 120 6Q Hz 900 r min sync P 8 b The slip at rated load is - n c - nm x 100 900- 840 x 100 6.67 nsync 900 c The motor is operating in the linear region of its torque-speed curve so the slip at lA .
