A Proposal for Insulation Diagnosis of 3 KV Motor Stator Windings

This paper presents a proposal for insulation diagnosis of 3 kV induction motor stator windings of mica splittings and solvent-type thermosetting resin insulation. Nondestructive and destructive tests were conducted on 29 windings it operation for 15 years or more. The average breakdown voltage (BDV), Vave of the coils was estimated using 10 to 12 coils from each winding. The relationship between Vave and Vmin, the BDV of the winding and the BDV of the worst coil in the winding, was determined. The criterion for insulation renewal, generally Vmin,7 kV (the withstand test voltage), could also be expressed in terms of Vave, with Vave≤l5 kV. Several nondestructive measurements were also examined for their ability to forecast Vave or Vmin. Although the dissipation factor tip-up (Δtanδ) should be related to Vave, no useful correlation was found. Partial discharge starting voltage (Vi) and the first turning point voltage (Pil) on the I-V curve in an ac current test proved to have comparatively good correlation with Vmin. A criterion for insulation diagnosis of that insulation is proposed, i.e. when Vi or Pil has decreased to 3 kV, the rated voltage, the insulation of the winding should be replaced.