Fault occurrence in insulation coating of stator core lamina causes creating hot spot, growing the fault and expanding its area. Consequently, winding insolation breakdowns occur due to excessive temperature rise, which causes electrical machine to burnout. This paper aims to present a non-invasive method with online performing capability to detect stator core interlaminar insulation fault in three-phase induction motors. Finite elements method is used to simulate the induction motor in healthy and faulty conditions. Then, frequency spectra of induced torque and stator current are compared in healthy and faulty conditions. The comparison shows creating or intensifying some harmonics due to the fault. Examining effects of the fault severity, motor load level and supply voltage unbalance on amplitudes of the harmonics, a specific harmonic next to 15th harmonic of the stator current is selected as the fault index. Occurring a rather weak fault, normalized amplitude of this harmonic rises from -88.11db to -55.57 db, and then, to -49.90 db by a small increase in the fault severity. From no-load to full-load, depending on the fault severity, amplitude of this harmonic may change up to 3.9 db. Stator voltages unbalance intensifies this harmonic in the healthy state, too, but not as much as its intensification due to the fault occurrence. Under fault condition, unbalance voltages effect on the amplitude of the selected harmonic is up to 2.5 db. Similar fault is created in a real induction motor and required tests are performed on it. The experimental test results approve the simulation results.