2730.ppt
- 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
- 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 = nsfkf
- 10. Equivalent Circuit of a Synchronous Motor Armature (One Phase) ( ) T a a a l a ar f s l ar T f a a s T f a s V I R I jX I X E X X X V E I R jX V E I Z
- 11. Phasor Diagram for one phase of a Synchronous Motor Armature
- 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) VT = Ef + IajXs Phasor Diagram
- 16. Allow the Turbine to take part load The power angle decreases to zero and then becomes positive Excitation voltage is not changed and the vector traces an arc Motor becomes a generator as δ becomes > or = zero
- 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