On 2012-09-26, Michael A. Terrell wrote:

Jon Elson wrote:

Michael A. Terrell wrote:


The winding may have opened & arced, causing plasma which ignited the
oil. I used to see hundreds of bad clock motors a year, when one shop
did school clocks & fire alarms. Most had bad gears, but some had open
coils and burn marks.

Yup, I agree. An impedance-limited motor can't get hot enough to
ignite oil. But, if there was internal or external arcing, that could do
it. I'm still a little surprised internal arcing could do it, as these
motors are usually pretty well closed up. Either it arced internally
for quite a while, or until an outside wire burned off and dropped into
the usual oily mess on such pumps. It really seems a properly-sized
fuse should have cut this off before it got to that stage. Might be
something to install while rebuilding.


Some have vents to cool the coils, or open windings. Older motors
were often built with a brass or aluminum deep drawn cover that was
friction fit to the steel body, and could work loose. Some would pull
off with your fingers, and had enough gap to let a flame out if it was
in the right spot.

This is one of the teardrop shaped ones which contain the gear
train. The usual deep drawn cover was over the actual motor part, the
teardrop likely also had some enclosed lube, and I remember others of
the type had the whole gear train solder sealed into the teardrop, but
the motor winding was a different matter.

This is based on drawings downloaded. I'll have a better idea
when I get a chance to pull it out and take it apart outdoors. (If
there is enough left to judge from. :-)

Since the motors were designed for another use, the
thought of starting a fire likely wasn't considered. I once found a
particle board speaker cabinet that was set on fire when the wirewound
two watt 20 ohm pot caught fire. Someone had removed the line
transformer to get more volume, rather than find a short in the building
wiring. When I found the bad wire, the speaker could be heard all over
a 40,000 Sq foot room, for about a second. It had shorted to the
ceiling grid, which was grounded to the steel building and support
posts.

Ouch! I presume that the cone was tatters, as well as the voice
coil being toast. :-)

Enjoy,
DoN.

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