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Learning Outcome

ECA1212 Introduction to Electrical & Electronics Engineering Chapter 12: Introduction to Electric Machines by Muhazam Mustapha, December 2011. Learning Outcome. Be able to theoretically explain the various types of electric motors

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Learning Outcome

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  1. ECA1212Introduction to Electrical &Electronics EngineeringChapter 12: Introduction to Electric Machinesby Muhazam Mustapha, December 2011

  2. Learning Outcome • Be able to theoretically explain the various types of electric motors • Be able to theoretically explain the various types of electric generators • Be able to mathematically solve some parameters of DC motors By the end of this chapter students are expected to:

  3. Chapter Content • Electric Machines in General • DC Machines • Synchronous Machines • Induction Machines

  4. Electric Machines CO1

  5. Rotating Machines • Electromechanical machines are commonly rotational in nature • The machines require one part to be static and the other one to be rotating • Stator: stationary • Rotor: rotating • Both stator and rotor produce magnetic winding whose field will try to align each other – this produces mechanical motion CO1

  6. Rotor and Stator × × · · Current going in Stator winding Rotor winding Stator Field Rotor Field Current coming out Rotor Stator CO1

  7. Commutator Action Commutator reverses current in coil every half cycle There can be more than 1 pair of commutators CO1

  8. Windings • Two types of magnetic windings: • Armature: the winding connects to load • Field: the winding only to produce field • Either armature or field winding can be located as rotor or stator • The location of field and armature determines the type of the machine CO1

  9. Machine Types (Generator & Motor) CO1

  10. DC Machines CO1

  11. DC Machines • DC Machines are hard to construct, but easiest to discuss and analyze • Hence all our mathematical discussion on machines will be on DC machines • Other machine type will be covered as theory CO1

  12. Configurations • DC Machines can be constructed in a few configurations depending on series or parallel structure or the availability of a second power source Ra Ia La Va Rf Lf If Vf Separately Excited CO1

  13. Configurations Lf Ra Ra Vf Rf Ia La La Va Va Vf Lf If Shunt Connected Series Connected CO1

  14. Configurations Ra Series Winding Ra Series Winding Ia Ia La Va La Va Shunt Winding Shunt Winding Short-Shunt Compound Long-Shunt Compound CO1

  15. Steady State Equations • Referring to the following DC machine model, we can deduce some formulas for motor and generator at constant speed Is If RS Ra LS Rx Ia VLor Vs La Rf Eb, ωm Lf CO1

  16. Steady State Equations • Generator CO1

  17. Steady State Equations • Motor CO1

  18. Machine Constant • The armature constant of ka p = number of magnetic polesN = number of conductors per coilM = number of parallel paths in armature winding CO1

  19. Conversions n = round per minute, r/minωm = radian per second, rad/s 1 horse power = 746 watts CO1

  20. Synchronous Machines CO1

  21. Alternator • Just another word for AC generator • Normally a permanent magnet or a DC powered electromagnet will be placed at rotor to generate AC current • Stator would be wound with solenoid that carries the generated energy – there can be more than one windings hence it can generate more than 1 phase of electricity CO1

  22. Alternator × × × · · · N Single Phase S Three Phase Coils at stator CO1

  23. Synchronous Motor • Virtually identical to alternator • Needs a DC voltage exciter at rotor to start • Called synchronous because it spins at the same rate as the AC frequency used to drive it CO1

  24. Induction Machines CO1

  25. Induction Motor • The stator part is almost identical to synchronous motor • AC current (single or multi-phase) will be fed into stator – produces spinning field • There is no power or permanent magnet placed in the rotor • Rotor and stator are electrically separated • Then how mechanical force is applied to the rotor? CO1

  26. Induction Motor • Mechanical motion is possible by the induction process that is identical to the one in transformer • The changes in the magnetic flux from stator will induce current into the rotor winding and causes magnetic attraction or repel between stator and rotor poles CO1

  27. Induction Motor • The changes of the magnetic field need to involve the cutting of the rotor coils (Faraday’s Law) • This cutting is what called ‘slip’ between the rate of stator’s field rotation and the rate of rotor’s spin • Without the slip induction machine couldn’t work CO1

  28. Induction Motor • The ‘slippings’ also means that the rotation of rotor is not in-sync with the stator field rotation rate • This is the main electrical difference between synchronous machine and induction machine CO1

  29. Induction Generator • Makes use of the same induction concept in induction motor – slipping process • It requires a starting power at rotor to produce magnetic field for the induction process to start • After that, the power generated by the generator itself will be used to produce the needed rotor magnetic field CO1

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