This article investigates the production mechanism of the open-circuit back-electromotive force (back EMF) in fractional-slot concentrated winding permanent magnet (FSCW-PM) machines based on the air-gap field modulation (AFM) theory. The relationship between spatial PM field harmonics and temporal back EMF harmonics is discovered by using a general model, which is suitable for both stator-PM and rotor-PM machines. The analysis results reveal that the FSCW-PM machines produce their open-circuit back EMF on the basis of AFM, which means that each back EMF harmonic is induced not only by a specific single spatial field harmonic but also by several different spatial field harmonics. Furthermore, a general winding factor calculation formula for FSCW-PM machines is derived together with the general calculation formula of all back EMF harmonics. Finally, the analysis is verified numerically by the finite element method and experimentally.