On-Line Diagnostics of Large Induction Motors

Abstract

The on-line monitoring techniques which have normally been used to assess the condition of large HV squirrel-cage induction motors have been dedicated to detecting rotor dynamic problems and bearing wear [1]. This is usually achieved by sensing the shaft movement using eddy current displacement probes in the bearing housings. The technique has proved to be successful for detecting drive misalignment, a bent shaft, oil whirl, bearing eccentricity and wear [2]. Unfortunately, an HV induction motor can fail due to other fault mechanisms which are not simply mechanical or electrical but are often interrelated and complex [3]. For example, frequent direct-on-line (DOL) starting can cause broken rotor bars or end rings due to the large starting currents producing high mechanical and thermal stresses on the cage winding [4]. DOL starting also produces large end winding forces and core vibration during the initial transient. This can cause mechanical damage to the end winding insulation and electrical failure.