I think that everyone who is familiar with the old Duo-Art motors
agrees that they do get quite hot when run for an extended period. If
the spill valve is adjusted correctly and everything is in good
condition, this is not a problem. The original posting implied that
the motor was not running hot before "reconditioning" but that it was
afterwards. It was not specifically stated that a three-hour test was
done before and after; so it may be that the motor got just as hot
before but was never tested for three hours. I expressed concern that
something may have been done to the motor to cause it to heat more,
because of the implication that it was fine before.
It is important to know what was actually done to the motor and how
hot it is really getting. If it was rewound with modern magnet wire,
it can get almost red hot without the insulation failing, depending on
what insulation was used. I do not have my references with me and am
too lazy to look on the internet, but the wire generally used for
modern motors (class B, F, or H) has a temperature rating that is much
higher than the original wire, which is class A (55 degrees C). This
is the temperature the wire can withstand, which is very different from
the temperature of the outside of the motor. Another concern is local
hot spots, which can be caused by defects in the wire or by areas that
are not as well cooled as other places. It all gets quite complicated.
The bottom line is that it is not good to guess what is safe, especi-
ally if the original wiring is being subjected to extremes. Also it is
not good to assume that the motor is fine because it was reconditioned.
It may actually be worse than it was before. A competent motor shop
that has experience with these old motors should be able to evaluate
one; but most shops have little experience with them.
I am very concerned about motor heating because I sell a device which
allows people to play their instrument for many hours. For this
reason, I encourage the use of a thermal protector and/or rewinding of
Here is a website with information about insulation classes: