Electrical Machines Operation in Variant - Speed applications, such as: water pumps, centrifugal fans air compressor and wind turbines is one the most important subject in the industry. Doubly-Fed Induction Machines, using fractional convertor, are able to control speed. So this limited power to 1/3 of rated power, involve with cost and loss reduction in power electronic equipment. Brushless doubly-fed induction machine with nested-loop rotor, not only has the fractional convertor advantage, but also has feature of removing brush and slip-ring. This advantage is behind of using nested-loop rotor, which has concentrate and integrated structure like squirrel-cage rotor. This feature will increase the reliability, and decrease the maintenance cost. Due to above mentioned features; there is now a growing interest in using this machine. There are two, three-phase winding set with different pole-pairs in the machine stator. They are known as power winding which is connected directly to the grid, and control winding which is supplied by power electronic convertor. In the healthy condition of the machine, difference in pole-pairs, will cause lack of direct magnetic coupling between two stator windings. Nested-loop rotor, used in this machine, consist of some nest and loop, that playing the roll of indirect coupling creator between the power and control windings, which is known as cross magnetic coupling ability of the nested-loop rotor. \\indent Electrical machines are involve with thermal, mechanical and electrical environmental conditions. Stator faults consist of the winding and the core faults have the majority of electrical machine faults. These faults start from inter-turn short circuit fault, which have not been diagnosed in incipient stages and have been spread. One of the fault diagnostic methods, is known as motor current signature analysis and is applicable in different fault diagnosis due to its non-invasive and easy implementation. In the frequency spectrum of stator winding currents, there are definite frequency components, which are dependent on the rotor structure, supply frequency and slip, which are known as rotor slot harmonics. Using these frequency components and their variation as an indicator in stator winding current frequency spectrum of the motor, will result in inter-turn short circuit fault diagnosis. In this thesis, the first goal is modeling of the brushless doubly-fed induction machine with nested-loop rotor with fault detection intention. And the ultimate goal, is offering an indicator using the analysis of stator winding current spectrum in simulated machine model in order to detect stator inter-turn short circuit fault. To this aim by using analytical method, machine magnetic fields, have been studied and nested-loop rotor slot harmonic components have been extracted. Also, winding function method has been extended to model the machine. This model has been used to calculate machine winding inductances. Beside machine equations containing stator, rotor, shorted inter-turn coil voltage and mechanical equations, have been provided in the both healthy and faulty condition. Then the validity of the winding function model has been verified using 2D- Finite Element model. simulating faulty condition , nested-loop rotor slot harmonic extracted equation, has been analyzed and emerging requirement of these frequency components in the stator winding current spectrum have been determined. Furthermore a suitable approach has been proposed in order to detect inter-turn short circuit in power and control windings and also faulty winding has been isolated based on nested rotor slot harmonic components in power winding current spectrum. Finally using fault feature extracted in this thesis, power and control winding Inter-Turn short circuit in induction, cascade and synchronous mode has been studied experimentally. Key word: 1- Brushless doubly-fed induction machine with nested-loop rotor , 2- Inter-Turn short-circuit fault detection , 3- Nested-loop rotor slot harmonic 4- Motor current signature analysis .