1. A method for detecting an insulation defect in a generator core, comprising: flowing an alternating excitation current at a first excitation frequency through an excitation winding adjacent the generator core to induce first eddy currents between laminations at the defect; measuring a first potentiometer voltage indicating magnetic flux caused by the first eddy currents induced at the first excitation frequency; flowing an alternating excitation current at a second excitation frequency through the excitation winding adjacent the generator core to induce second eddy currents between the laminations at the defect; measuring a second potentiometer voltage indicating magnetic flux caused by the second eddy currents induced at the second excitation frequency; and determining a severity of the defect and a depth of the defect using a semi-empirical relationship between the potentiometer voltages, the excitation frequencies, the severity of the defect and the depth of the defect.