When acting as a generator, the mechanical energy is generated and supplied to the machine by the application of a torque and by the rotation of the shaft / shaft thereof. Thus, the mechanical power source may be a hydraulic, gas or steam turbine. If the generator is connected to the mains, the voltage at its terminals is freely determined by the frequency of rotation and by the number of poles, resulting in the frequency of the three-phase voltage of the machine.
In order for the synchronous machine to effectively convert the mechanical energy applied to its shaft / shaft, the field winding found in the machine rotor must be supplied by a DC voltage pump motor ac source so that, when rotating, the Magnetic field produced by the rotor poles has a relative movement to the conductors of the stator windings.
By the relative movement between the magnetic field of the rotor poles, the intensity of the magnetic field circulating through the stator windings, can vary in time, and according to the ohm law, there will be a voltage induction to the terminals of the stator windings . Due to the spatial distribution and arrangement of the stator winding assembly, the voltages induced at its terminals will alternate three-phase sinusoidal.