Inter-turn fault (ITF) of stator winding is a very sensitive and critical fault in permanent magnet motors. If this fault is not diagnosed on-time, it can quickly expand and generate serious damage and even full damage to the motor. So, realisation of this fault, its impact on the motors and finally its diagnosis methods are essential and has high priorityfor users of the motors. This study presents the trend of the negative impacts of the fault on the performance of the machine. Available indexes for this fault detection are introduced and described in details. By analysing these indexes, their merits and drawbacks, limitations and ambiguous points are extracted. Finally, these indexes are compared and the best and worst indexes are introduced. Moreover, one of the most used index and the simplest one will be simulated in order to show capability of a new current-based ITF diagnosis index.