Evaluating Motor Ground Insulation for Improved Operating Reliability

Attention is directed toward the evaluation of motor insulation by a series of ac power factor and dc voltage-leakage tests at progressively higher voltages. A careful testing procedure is outlined that points out the need for uniform test conditions. Use of power factor tip-up, polarization indices, and voltage-leakage slopes are discussed as indicators of insulation quality before the dielectric ground test. Contamination of ground wall insulations by the ingress of such foreign elements as water, lubricating oil, cleaning solvents, and dirt is thought to be a leading cause for the maldistribution of high potentials across ground wall thickness. A brief theoretical discussion of voltage distribution across ground wall thicknesses is included to explain this phenomenon that is pictured as the main reason for mysterious ground failures during ground testing of otherwise sound insulation.