The Cold Facts — A Case of Overheating Motors

Motor failures can be a mystery, especially when their causes originate outside a facility's walls. That's the conundrum that faced a small motor shop recently, when a spate of failures sparked an investigation of the seemingly random breakdowns. The machinists called for assistance from the motor-rewind shop that had been repairing the failed models—a not-unusual resource for those facin...

06/01/2004


Motor failures can be a mystery, especially when their causes originate outside a facility's walls. That's the conundrum that faced a small motor shop recently, when a spate of failures sparked an investigation of the seemingly random breakdowns. The machinists called for assistance from the motor-rewind shop that had been repairing the failed models—a not-unusual resource for those facing motor meltdowns.

The winding-shop supervisor began his investigation by searching for patterns that could tie the failures together. Age wasn't an issue, as problems seemed to be affecting both old and new units. However, all the affected motors had been carrying full loads, and their windings showed they'd been overheating.

The machine shop hadn't undergone any changes since the motors started failing, but an adjacent building had taken on a significant new tenant—and insurance company—and the two operations were served by the same utility transformer.

The rewind-shop supervisor's prime suspect became possible nonlinear loads within the insurance building, and he proceeded in the best deductive fashion to prove his theory, with the help of a handheld power-quality analyzer.

He started by connecting the unit phase-to-phase at the machine shop's main service panel, and discovered a flat-topped voltage waveform with a total harmonic distortion (THD) of 7.8%. Another key discovery: the 5th harmonic was the dominant bar on the harmonics display. The supervisor also used the analyzer to compare the current drawn by fully loaded motors to the full-load-amp values on the units' nameplates, and found the motors were drawing more than the rated values by a small amount.

This collection of observations helped the supervisor nail down the cause of his customer's problems. The 5th harmonic on a three-phase induction motor produces a magnetic field that tries to run the motor backwards. As a result, the motor draws more fundamental current to offset this effect, which results in overheating. In this case, the boosted 5th harmonic was caused by the nonlinear loads generated by computers and office machines in the adjacent insurance building.

The utility, when notified about the problem, agreed to install a separate transformer to serve the machine shop alone. Again, the handheld analyzer came in handy, measuring distortion on the new transformer was measured at an acceptable 3.3%.





No comments
Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
integrated building networks, NFPA 99, recover waste heat, chilled water systems, Internet of Things, BAS controls
40 Under 40; Performance-based design; Clean agent fire suppression; NFPA 92; Future of commissioning; Successful project management principles
BIM coordination; MEP projects; NFPA 13; Data center Q&A; Networked lighting controls; 2017 Product of the Year finalists
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Commissioning electrical systems; Designing emergency and standby generator systems; VFDs in high-performance buildings
Tying a microgrid to the smart grid; Paralleling generator systems; Previewing NEC 2017 changes
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me