After reading all the discussion of possible reasons for motors
(especially duo-art) catching fire, it seems obvious that the
original motor and original protection systems are just not adequate.
I was aware that a badly adjusted spill valve could cause motors to
overheat but I never considered the scenario of the stalled music roll.

This possibility alone makes it clear that the original motor is just
not safe unless rebuilt with modern enameled wire that can stand hot
running.  Even a motor in good condition could suffer this failure if
it has original cotton covered wire.

As lovely as these old motors are, they are just not safe and should
not be used unless fully attended.  Thermal protection can help but it
can not protect against local hot spots.

A motor with a high resistance in the wiring, whether original or due
to subsequent mechanical or electrical damage, can catch on fire well
before the motor case overheats.  This is not the case with modern
wire.  I don't know if the HTML chart below will be readable but cotton
covered wire is class A.  Modern wire is F or H which is rated at much
higher temperatures.

 [ See the HTML charts at http://en.wikipedia.org/wiki/Insulation_system 
 [ & http://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html 
 [ Hot spots in electric motors is explained at
 [ http://www.leeson.com/TechnicalInformation/hottopic.html  -- Robbie

If you are just too cheap to rewind your motor and are not willing to
use a modern one in a remote pump, an arc fault interrupter sounds like
the best protection when used together with a thermal shut off device.
Old cotton covered wire is likely to be electrically leaky so it could
trip a ground fault interrupter making it not possible to use when
connected to one.

Arc fault interrupters are usually (at least the ones I have installed)
are also differential circuit breakers (ground fault interrupters),
although not rated or recommended for use as such, so they can have
this problem as well.  They are also very sensitive and might trip
falsely but that is better than a fire.  With adequate protection and
a motor that was not rewound you may find it too much trouble to keep
it running and finally decide to so the right thing and rebuild it or
use a modern motor instead.

Another option would be to install foot pedals to provide the vacuum.

Best regards,
Spencer Chase
Garberville, California
http://www.spencerserolls.com/