Medium Induction Motor Winding Insulation Protection System Reliability Evaluation and Improvement Using Predictive Analysis MAHFOUD CHAFAI, LARBI REFOUFI , HAMID BENTARZI Signals and Systems Laboratory (SisyLab) DGEE, FSI, University of Boumerdes, Algeria - Abstract : The paper presents a quantitative reliability evaluation of a widely used protection system for medium power cage induction motors. This evaluation effort is based on a predictive analysis using ETA and FMECA in conjunction with field data regarding dominating failure mechanisms and causes of windings insulation breakdown. Dominant failure modes and failure mechanisms are analysed and then initiating causes of motor failures and their weighted contribution factors determined. An Event Tree Analysis is then developed for predicting the probability of the protection system outcomes. An FMECA is developed in order to provide the guidelines for actions in the framework of reliability improvement and a preventive maintenance program with particular attention given to environmental factors such as found in a cement plant. Key words: Protection, reliability, failure mechanisms, even tree, FMECA, improvement. 1 Introduction The induction motor is the workhorse of industry. Any operational failure, due to several impressed stresses, will cause considerable economic losses therefore there is a pressing need to maximise the protection and the availability of these ac machines. However, in the light of field data, dominant failure modes and failure mechanisms of some motor system part, the initiating causes of motor failure and their weighted contribution factors are determined. An Event Tree and FMECA Analyses are then developed for reliability evaluation and improvement of this protection system. 2 Induction motor stator winding failure mechanisms Industrial surveys on machine reliability shows [3] that the stator winding insulation is one of the most vulnerable components used in an AC electric machine. The failure of stator winding can be divided into: - Insulation degradation and hence breakdown. - Open circuit failure in the windings. 2.1 Insulation Failure Mechanisms (IFM) The stator winding insulation is always subjected to the combined thermal, electrical mechanical and environmental stresses during the long-term operation [7]. 2.1.1 Thermal stress Over time the insulation will deteriorate due to the normal thermal aging process; but the occurrence of premature failures, which are predominant, are a direct result of an over-current caused generally by an overload, a supply voltage unbalance and/ or over-voltage. 2.1.2 Electrical stresses Most of electrical failures are caused by a combination of over-voltage spikes and normal deterioration. This fast over-voltage can be caused by start-up switching, lightning, surges and VFD to propagate through the material, thus leading to a reduced time to breakdown [5]. 2. 2 Winding wire open circuit failure This failure, which rarely occurs, is generally due to quality of wire as well as the level of electromechanical and environment stresses pressed on the winding wire . The open circuit failure may happen at the terminal connection of the motor. The failure mechanisms sequence of the induction motor is summarized in the Fig.2 6th WSEAS International Conference on CIRCUITS, SYSTEMS, ELECTRONICS,CONTROL & SIGNAL PROCESSING, Cairo, Egypt, Dec 29-31, 2007 156