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Synchronous motors and generators operate by synchronizing the rotation of the rotor's magnetic field with the rotating magnetic field produced by the stator windings. Synchronous motors are used for constant-speed applications like propulsion for large ships. They work by locking the rotor's magnetic field to the rotating stator field. Synchronous generators can be produced by transitioning a synchronous motor driven by a turbine from motor to generator operation as the power angle shifts from negative to positive.

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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 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
Synchronous Motors (continued) Operation Magnetic field of the rotor locks with the rotating magnetic field rotor turns at synchronous speed
Salient-Pole Rotor Excitation Windings
Salient-Pole Rotor with brushless excitation
Synchronous Motor Starting Get motor to maximum speed (usually with no load) Energize the rotor with a DC voltage
Salient-Pole Motor operating at both no-load and loaded conditions Angle 隆 is the power angle, load angle, or torque angle
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
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
Phasor Diagram for one phase of a Synchronous Motor Armature
Synchronous Generators
Motor-to-Generator Transition
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.
Motor-to-Generator Transition (cont) VT = Ef + IajXs Phasor Diagram
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 隆 becomes > or = zero
Motor Action Power angle is negative
Motor to Generator Transition Power angle is now = 0
Generator Action Power angle is positive Note: Iacos慮i is reversed!
Generator Action (cont) In order for Ia to reverse direction, voltage Ef must become a source voltage Ef > VT Ia

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2730.ppt