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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-0 PowerPoint Slides to accompany Electric Machinery Sixth.

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Presentation on theme: "Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-0 PowerPoint Slides to accompany Electric Machinery Sixth."— Presentation transcript:

1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-0 PowerPoint Slides to accompany Electric Machinery Sixth Edition A.E. Fitzgerald Charles Kingsley, Jr. Stephen D. Umans Chapter 6 Polyphase Induction Machines

2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-1 Squirrel-Cage 6.1 INTRODUCTION TO POLYPHASE INDUCTION MACHINES Wound Rotor Two types of motor:

3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display INTRODUCTION TO POLYPHASE INDUCTION MACHINES How does an induction motor work? 1.Apply AC three-phase current to stator winding to produce rotating magnetic field. 2.Rotating magnetic field induces voltages in rotor windings resulting with rotor currents. 3.Then, rotor currents will create rotor magnetic field. 4.Constant speed stator magnetic field will drag rotor magnetic field. nsns n n s : Synchronous speed (the speed of stator rotating field in rpm). n : Rotor speed (rpm). SLIP: It is defined as the difference between synchronous speed and the rotor speed divided by synchronous speed.

4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display INTRODUCTION TO POLYPHASE INDUCTION MACHINES The rotor speed Mechanical angular velocity The speed of rotor magnetic field with respect to rotor is The relative motion of stator flux and the rotor conductors induces voltages of frequency (f r is called slip frequency)

5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Typical induction-motor torque-speed curve for constant-voltage, constant-frequency operation INTRODUCTION TO POLYPHASE INDUCTION MACHINES Breakdown torque

6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-5 Developed rotor winding of an induction motor with its flux-density and mmf waves in their relative positions for (a) zero and (b) nonzero rotor leakage reactance. 6.2 CURRENTS AND FLUXES IN POLYPHASE INDUCTION MACHINE

7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-6 Reactions of a squirrel-cage rotor in a two-pole field. Figure CURRENTS AND FLUXES IN POLYPHASE INDUCTION MACHINE

8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-7 Stator equivalent circuit for a polyphase induction motor. 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Counter emf generated by the resultant air-gap flux

9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-8 Rotor equivalent circuit for a polyphase induction motor at slip frequency. 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT

10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-9 Single-phase equivalent circuit for a polyphase induction motor. 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Models the combined effect of rotor resistance and shaft load

11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Alternative form of equivalent circuit. 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Electromechanical power is equal to the power delivered to this resistance

12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display ANALYSIS OF THE EQUIVALENT CIRCUIT P mech is not the net power but it includes the losses such as friction, windage. Output power and torque from the shaft is

13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display TORQUE AND POWER BY USE OF THEVENIN’S THEOREM (a) General linear network and (b) its equivalent at terminals ab by Thevenin’s theorem.

14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display TORQUE AND POWER BY USE OF THEVENIN’S THEOREM Equivalent circuits with the core-loss resistance R c neglected.

15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Induction-motor equivalent circuits simplified by Thevenin’s theorem. 6.5 TORQUE AND POWER BY USE OF THEVENIN’S THEOREM

16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Induction-machine torque-slip curve showing braking, motor, and generator regions. Figure 6.14

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display The End of This Chapter

18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Computed torque, power, and current curves for the 7.5-kW motor in Examples 6.2 and 6.3. Figure 6.15

19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Induction- motor torque- slip curves showing effect of changing rotor-circuit resistance. Figure 6.16

20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Electromechanical torque vs. speed for the wound-rotor induction motor of Example 6.4 for various values of the rotor resistance R 2. Figure 6.17

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Deep rotor bar and slot-leakage flux. Figure 6.18

22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Skin effect in a copper rotor bar 2.5 cm deep. Figure 6.19

23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Double-squirrel- cage rotor bars and slot-leakage flux. Figure 6.20

24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Typical torque- speed curves for 1800-r/min general-purpose induction motors. Figure 6.21

25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Connections of a one-step starting autotransformer. Figure 6.22

26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Interconnected induction and synchronous machines (Problems 6.7 and 6.8). Figure 6.23

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Induction-motor equivalent circuits simplified by Thevenin’s theorem. Figure 6.13


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