Original Date: 11/03/1996
Revision Date: 01/18/2007
Best Practice : Electrical Signature Analysis
Most industries rely on electric motor and generator operations. However, corrective and preventative maintenance practices are more costly solutions than condition-based servicing of rotating equipment. Motor total current analysis is typically limited by gross variations of loads, and vibration analysis can be a time-consuming task. To better predict motor and rotating machinery performance degradation at a decreased cost, Oak Ridge National Laboratory (ORNL) developed and patented several electrical signature analysis (ESA) techniques.
Traditional methods of analyzing load and performance with total current and power monitoring cannot provide the high resolution needed to describe evolving faults or small variations in the applied loading. The alternative is the more-costly, less-effective mechanical vibration analysis which uses multiple sensors. However, the increased cost and complications associated with monitoring prevent operators from detecting problems with the motors or generators located inside their manufacturing plants.
ORNL required proven methods to monitor and control its weapons and nuclear operations. With that initial goal, ORNL developed several signal conditioning and signature analysis methods that exploit the intrinsic abilities of conventional electric motors and generators to act as transducers. By using simple commercial-off-the-shelf (COTS) current and voltage probes (Figure 2-10), ESA detects and analyzes the small time-dependent load and speed variations generated within an electro-mechanical system and converts them to frequency signatures which are useful for both mechanical and electrical fault detection. ESA provides similar data from its one non-intrusive probe (current analysis for motors or voltage analysis for generators) as conventional vibration analysis does with its multiple sensors.
ORNL demonstrated the motor (current) analysis of the ESA method on an Army program which monitors a motor- conveyor system on an ammunition resupply vehicle (RSV). Similarly, Oak Ridge Centers for Manufacturing Technology (ORCMT) successfully completed generator (voltage) analysis for a test-stand demonstration on the internal drive generators of 757 and 767 aircraft engines. Furthermore, a prototype ESA is monitoring the 19 motors which drive the chilled water pumps at ORCMT and tracking the pump performance via current variation.
Applicable to any motor-load or prime mover-generator system, ESA methods work as a maintenance aid and method for tracking overall performance. ORCMT used ESA to track actual load requirements of a 350 hp, 1800 rpm motor. After reviewing the data, ORCMT decided to replace the motor with a smaller, 125 hp, 1200 rpm motor and realized an annual operating cost savings of $50,000. ESA’s most attractive benefit is its cost savings associated with condition-based maintenance in manufacturing and operational environments. ORCMT documented a 1989 study on the costs associated with various maintenance strategies used on rotating equipment. The study estimated the normalized cost per year at $18 per hp for corrective, $13 per hp for preventative, and $10 per hp for condition-based maintenance.
Figure 2-10. Electrical Signature Analysis
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