This paper investigates the terminal voltage distortion in the interior permanent magnet machines with fractional slot concentrated windings, with particular emphasis on the influence of stator slot N-s and rotor pole 2p combinations. The 12-slot/10-pole machine is first employed to show the voltage distortion phenomenon. Using the frozen permeability method, the mechanism is then investigated, which reveals that the variation of armature flux paths due to rotor saliency is the origin of such phenomenon, especially when the current advancing angle beta approaches 90 degrees. Further, the terminal voltage distortion of the six machines are compared, i.e., Ns +/- 1 = 2p (9-slot/8-pole, 9-slot/10-pole), N-s +/- 2 = 2p (12-slot/10-pole, 12-slot/14-pole), N-s/2p = 3/2 (12-slot/8-pole), 3/4 (12-slot/16-pole), which shows that the machines with N-s < 2p suffer more severe voltage distortion than their counterparts with N-s > 2p. In addition, considering the inevitable machine saturation, a design trade-off is proposed by selecting proper N-s/2p combinations to minimize the influence. Finally, prototypes are fabricated and tested to validate the analyses.