Online capacitance and dissipation factor monitoring of AC motor stator insulation

A new on-line technique for monitoring the insulation condition of AC motor stator winding is proposed. The approach uses a newly developed, High-Sensitivity Current Transformer (HSCT) to precisely and non-invasively measure the differential current (e.g, the insulation leakage current) of each phase winding from the motor junction box. Conventional differential current transformers (CT) used for fault protection can be replaced with the new HSCT to measure the winding insulation leakage current with higher sensitivity and accuracy. The HSCT can serve both motor health monitoring and motor protection functions. Presently, indicators for insulation condition such as capacitance (C), dissipation factor (DF), or insulation power factor (PF) are only obtainable off-line. The new approach can provide a low cost solution for on-line motor insulation condition assessment. Validation of the new HSCT technology is carried out during an accelerated life testing of a 460 V, 100 HP, 1200 RPM form wound induction motor. The motor used in this work was aged at high temperature (255°C) as the load cycled between 0% and 200% every 5 minutes.