Novel frequency domain based technique to detect incipient stator inter-turn faults in induction machines

Traditionally, for medium and high voltage motors and generators, condition based monitoring (CBM) of stator faults are performed by measuring partial discharge (PD) activities. For low voltage machines, negative sequence impedance or currents are measured for the same. Such prognosis schemes should be carefully implemented as supply voltage unbalance, manufacturing related asymmetry, etc. also produce negative sequence voltages. A few motor current signature analysis (MCSA) based approaches have already been proposed to detect stator inter-turn faults. However little or no physical insight was provided to explain the occurrence of certain harmonics in the line current or the influence of voltage unbalance on these harmonics. Also, in at least one of the papers, a large portion of the stator winding was shorted to emulate the faults. The method proposed in this paper monitors certain rotor slot related harmonics at the terminal voltage of the machine once it is switched off. In the absence of supply voltage, issues such as voltage unbalance and time harmonics do not influence the measurements except as initial conditions; a very desirable feature when the machine is fed from an adjustable speed drive. Satisfactory simulation and experimental results have been obtained with only about 1.5% (5/324) of the total number of turns shorted.