Designed with longevity in mind, electric motors are made to function safely, economically, and perpetually. In theory, proper care and use of these components will ensure many years of successful operation. However, the real world is a different story, where motors are subjected to numerous environmental and operational hazards that cause thermal stress on the windings. Thermal stresses degrade the insulation quality of the motor, thereby diminishing its life expectancy. That’s why monitoring the operating condition of motors has become such an important aspect of predictive maintenance programs.
For many years, maintenance and reliability engineers were limited to static or off-line testing of motors — determining a motor’s suitability for continued duty by using a handheld volt/ohm meter, clamp-on ammeter, or megohmmeters. But quite often these tools became forensic devices used mainly to diagnose “what happened” after the motor failed.
Although static testing remains an essential component in monitoring and trending motor health, the methods are limited and are mainly intended to define insulation quality. Modern online test equipment aids the technician in predicting imminent, insulation-related failures. The early detection of these potential problems enables him to schedule “downtime” and make scheduled repairs rather than perform emergency maintenance.
Online motor testing. Dynamic or online test equipment monitors motors in their natural environment. Dynamic testing provides details about power quality, the motor’s operating condition, and a very large amount of information related to the load. Modern test equipment collects data through three voltage probes and three corresponding current transformers, which are attached directly to the piece of equipment. More advanced equipment has the added ability to collect data through permanently mounted equipment located in the control cabinet of low-voltage motors or the low-voltage cabinet of medium- and high-voltage motors. Once recorded, the data is then calculated and interpreted by the test equipment and displayed for comparison, reporting, and trending.
The data collection process is non-intrusive, allowing the technician to collect a vast amount of information without interrupting operations or processes. Besides being non-intrusive, the dynamic tester is able to receive, store, diagnose, and trend information within seconds. Then, the technician can transfer the data to a desktop PC for further analysis and sharing with other maintenance personnel. Numerous areas of concern, like high temperature readings or overvoltage situations, previously unavailable to most predictive maintenance personnel and reliability technicians, are now quickly and easily captured and displayed with electronics-based equipment.
Five key indicators. To fully analyze a motor’s health, the technician must be able to see the complete picture, including all aspects that affect the normal operation of the motor. These areas of concern are grouped in five main categories: power condition, motor performance, motor condition, load and energy assessment, and operating efficiency.