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Synchronous Motors and Generators

Synchronous Motors and Generators. Synchronous Motors. Constant-speed machine Propulsion for SS “Queen Elizabeth II” 44 MW 10 kV 60 Hz 50 pole 144 r/min. Synchronous Motors (continued). Construction Stator identical to that of a three-phase induction motor – now called the “armature”

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Synchronous Motors and Generators

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  1. Synchronous Motors and Generators

  2. Synchronous Motors • Constant-speed machine • Propulsion for SS “Queen Elizabeth II” • 44 MW • 10 kV • 60 Hz • 50 pole • 144 r/min

  3. Synchronous Motors (continued) • Construction • Stator identical to that of a three-phase induction motor – now called the “armature” • Energize from a three-phase supply and develop the rotating magnetic field • Rotor has a DC voltage applied (excitation) • Rotor could be a permanent-magnet type

  4. Synchronous Motors (continued) • Operation • Magnetic field of the rotor “locks” with the rotating magnetic field – rotor turns at synchronous speed

  5. Salient-Pole Rotor Excitation Windings

  6. Salient-Pole Rotor with brushless excitation

  7. Synchronous Motor Starting • Get motor to maximum speed (usually with no load) • Energize the rotor with a DC voltage

  8. Salient-Pole Motor operating at both no-load and loaded conditions Angle δ is the power angle, load angle, or torque angle

  9. Rotating Field Flux and Counter-emf • Rotating field flux f due to magnetic field in the rotor. A “speed” voltage, “counter-emf”, or “excitation” voltage Ef is generated and acts in opposition to the applied voltage. • Ef = nsfkf

  10. Equivalent Circuit of a Synchronous Motor Armature (One Phase)

  11. Phasor Diagram for one phase of a Synchronous Motor Armature

  12. Synchronous Generators

  13. Motor-to-Generator Transition

  14. Motor-to-Generator Transition (cont) • Begin with motor driven from the infinite bus and the turbine torque in the same direction as the motor torque. • The motor operates normally, driving the water pump.

  15. Motor-to-Generator Transition (cont) Phasor Diagram VT = Ef + IajXs

  16. Allow the Turbine to take part load Motor becomes a generator as δ becomes > or = zero Excitation voltage is not changed and the vector traces an arc The power angle decreases to zero and then becomes positive

  17. Motor Action Power angle is negative

  18. Motor to Generator Transition Power angle is now = 0

  19. Generator Action Power angle is positive Note: Iacosθi is reversed!

  20. Generator Action (cont) • In order for Ia to reverse direction, voltage Ef must become a source voltage • Ef > VT Ia

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