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Transcript
EEE1012 Introduction to Electrical & Electronics Engineering Chapter 9: Introduction to Electric Machines by Muhazam Mustapha, October 2010 Learning Outcome By the end of this chapter students are expected to: • 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 Chapter Content • • • • Electric Machines in General DC Machines Synchronous Machines Induction Machines Electric Machines Rotating Machines • Electromechanical machines are commonly rotational in nature • The machines require one 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 Rotor and Stator Current going in Stator winding Rotor winding × × · Stator Field · Rotor Field Current coming out Stator Rotor Commutator Action Commutator reverses current in coil every half cycle There can be more than 1 pair of commutators 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 Machine Types (Generator & Motor) Type DC Synchronous Induction Winding Type Location Current Type Armature Rotor DC Field Stator DC Armature Stator AC Field Rotor DC Primary Stator AC Secondary Rotor AC DC Machines 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 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 Rf Lf If Vf Separately Excited La Va Configurations Lf Ra Rf Ia Vf Ra La Va Vf La Lf If Shunt Connected Series Connected Va Configurations Ra Ia Series Winding La Shunt Winding Ra Series Winding Ia Va La Shunt Winding Short-Shunt Compound Long-Shunt Compound Va Steady State Equations • Referring to the following DC machine model, we can deduce some formulas for motor and generator at constant speed Is LS If Ra Rx RS Ia VL or Vs La Rf Eb, ωm Lf Steady State Equations • Generator Eb k am T P m Eb I a m k aI a VL Eb I a Ra I S RS Ia IS I f Steady State Equations • Motor Eb k am T P m Eb I a m k aI a VL Eb I a Ra I s RS Is I f Ia Machine Constant • The armature constant of ka pN ka 2M p = number of magnetic poles N = number of conductors per coil M = number of parallel paths in armature winding Conversions 60 n m 2 n = round per minute, r/min ωm = radian per second, rad/s 1 horse power = 746 watts Synchronous Machines 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 Alternator × × N × · Single Phase S · · Coils at stator Three Phase 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 Induction Machines 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? 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 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 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 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