Induction and synchronous motors are electromechanical rotary energy converters that have the operating principle based on the generation of rotating fields and, for this reason, are sometimes called rotating field machines.
The electric current and voltage are alternated in the stator and rotor windings in the induction machines and, in turn, in the synchronous machines the stator is excited in alternating current and the rotor in direct current motors 182tc 3 hp. Polyphase windings produce spinning magnetomotive forces (fmm) relative to the physical structure where it is placed and these have approximately sinusoidal distribution along the air gap.
The windings used in the rotor of the doubly excited synchronous machines produce magnetomotive forces stationary relative to the physical structure of the rotor as they are excited by direct current but are spinning in space by the rotation of the rotor.
What is important in these two types of electromechanical energy converters is that the rotating fmm waves are stationary with each other. We must design the windings of spinning field machines to produce magnetic flux with distribution in the space closest to the sinusoidal form, because, thus, we can obtain sinusoidal voltages over time.