A little background: When electric motor laminations are originally manufactured, after stamping, but before assembly and magnet wire insertion, the individual lamination steel plates are carefully annealed to relieve stresses and reduce hysteresis losses.
“Annealing” as defined by Wikipedia:
“In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time. And then allow slow cooling.”
Pulling old magnet wire from lamination slots, if NOT done carefully, lamination teeth bend (e.g. splayed)
When core-loss tests are conducted the primary conductor (typically MCM welding cable) is placed through the lamination center, end teeth may be splayed from the motion and weight of such a large cable
If a failed winding is not removed carefully, lamination teeth are sometimes splayed. Splaying or bending teeth reintroduces stresses that OEM annealing originally relieved. After the old winding is removed, bending lamination teeth back into place will likely not return teeth to their original uniform tight location. If not tight, lamination tooth gaps introduce magnetic zigzag losses and reintroduced stresses of bending teeth, both contributing to motor efficiency losses.
During motor end-user training, I often ask attendees, “What’s most important, motor efficiency or reliability.” The answer overwhelmingly is, “Reliability”. Apparently, efficiency isn’t a big deal to most electric motor end-users, but reliability is almost unanimous most important.
“Reliability and efficiency are often interconnected”
Consider for a moment, if motor lamination teeth are splayed from magnet wire extraction, resulting in magnetic zig-zag effect and reintroduced metal stresses, the new losses or inefficiencies manifest themselves as heat. The added internal motor heat may affect both electrical insulation, and bearings along with their vital lubrication. Over-time, if the added heat exceeds the motor’s heat dissipation design, internal motor components may become an issue. More to the point, “mean-time-between-failure” (reliability) may be compromised.
EASA Accredited and Green Motor Initiative motor service centers are very aware and work to NOT splay lamination teeth, as splayed teeth will not comply to either program’s identification requirements. From an overall industry perspective, motor service centers who rewind induction motors, should take the time and care to maintain pristine laminations. This is particularly true when working with very thin lamination core plate found in NEMA Premium® motors.
Recommendation: A picture is worth a thousand words, pre and post lamination pictures documenting a lamination’s tooth condition before and after winding removal should be considered as a part of a motor service center’s rewind documentation and quality assurance program. If splayed teeth exist from a previous rewind, it would be wise to inform the customer of the condition and concerns. In addition to lamination pictures, pre and post core-loss results with less than 20% watt-loss per pound rise after roasting and less than 4 watts loss per pound when complete, all add up to assurance of a processed lamination’s integrity.
EASA/AEMT, The Effect of Repair/Rewinding on Motor Efficiency (2003)
AEMT, The Repair of Induction Motors, Best Practices to Maintain Energy Efficiency (1998), (available at scribd.com)
EASA Accreditation Program Checklist (Version 3.0) w/Explanations
©2020 Green Motors Practices Group, all rights reserved