A short/long answer appears to hover around improvements in electrical insulation, the reduction in available
physical space required for additional material, and the winder’s individual discretion related to a winding repair.
The original question has blended two separate terms together. Phase-to-phase insulation related to isolating
adjacent coil phase groups, and “end winding insulation” which pertains to the entire wrapping of the coil end.
Here’s a fabricated mix of theory about the two topics based on present day shop floor observations,
and coupled with what’s gathered from long ago publications written on the subject.
The lack of phase-to-phase insulation in certain types of manufactured motors is certainly tied to an
economy-of-means as mentioned. Changing needs and circumstances have also eroded away at earlier design “over kill”.
The application and kind of environment the motor is designed to operate in, dictates insulation requirements.
For example, consider how smaller universal tool motors no longer require individual pieces of slot insulation
to electrically protect the winding. Newer, more robust and economical methods of isolating the winding from the
laminated iron core have developed.
Pertaining to end winding insulation, it’s likely linked to economics like the above, “but” it also seems to have
an additional story behind its intended purpose.
Early motors and generators had little to no physical protection surrounding coil windings.
As apparatus enclosures developed, housings were designed with multiple large openings for providing an ample
flow of cooling air. The purpose of “taped” end winding insulation originally appears to be for the protection of
stator coils from dirt.
In the United States, perhaps less frequent practice by manufacturers to include end winding insulation in
general purpose poly-phase motors may have had its beginning with NEMA’s 1952 frame Rerate, and the practice
likely accelerated with the 1964 Rerate.
(A meandering guess that others here may debunk or contribute further authenticity toward.)
One could say that when electric motor designs began being produced in physically smaller frame dimensions
(providing the same horsepower ratings of earlier generation open-frame motors), these constructions resulted in
less space remaining internally for additional insulation material.
Inside some of today’s totally enclosed fan cooled motors, there’s very little room for additional insulation material.
Winders nervous, concerned, or proud about the electrical integrity of their winding jobs will insert what’s required to
protect neighboring phase coil groups, and also to protect coils from intimate, physical contact with the surrounding
stator enclosure.
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John
