While the stator windings of the synchronous and induction machines are the same, 7.5hp 213t motors their rotors differ from each other in both the geometric shape and the winding type. While the rotor of the induction motor is cylindrical with polyphase winding, the rotor of the synchronous machine with excitation coils may be of smooth poles or protruding poles. The first is a distributed winding, while the second is concentrated, this usually happens, but not necessarily, the rotor magnets are formed by the direct current excitation of the rotor windings, from an external source.
These magnets can also be established by permanent magnet materials attached to the rotor structure, consequently, there is no need for the external DC source. The distribution of rotor flux formed by continuous excitation or by permanent magnets is stationary in relation to the rotor and, for this reason, the synchronous machine only develops torque when the rotor speed is equal to the synchronous one. By verifying the rotor types of the synchronous machines we observe that the air gap is constant for the rotor of smooth poles and variable for that of poles protruding.