Plain-rotor rotors are identified by having a uniform cylindrical shape and bundle of unsegmented, single-piece or laser-cut plates, usually attached to the shaft by “ribs”. The poles are constructed by grouping the sheet of the package and therefore are integral with the package of plates of the rotor and are not detachable. The field coils are constructed of insulated copper wires and secured in grooves of the sheet pack, with coil configuration being allocated without turning the poles at the periphery of the rotor. The bobbin heads are fastened by means of bandage or shaped steel sleeve on the coils.
The cage of this type of rotor is also symmetrically distributed. This rotor composition exhibits high symmetry, masses evenly distributed over the entire length of the rotor, making it cylindrical, compact and robust. Due to these peculiarities the synchronous motors with smooth poles rotor are most often used in machines of high rotation. Due to this rotor fabrication, the air gap is almost constant throughout the rotor circumference, with possible paths for equivalent magnetic flux through the interpolar regions, where there are also plates, which allows greater flow dispersion. https://www.mrosupply.com/motors/ac-motors/single-phase-motors/26120_l1319t_baldor/