Re: Sherline controller thermal protection

Worry not - I have thought this through a little further than the surface
(cut my programming teeth writing real-time control software to control
nasty chemical plant - see PS below). I don't plan to disable the motor
cutout unless at the same time I use the thermal sensor to drive some
alternative mechanism - such as an alarm or an E stop for the whole
system. I figure that the cutout is designed to trip with a reasonable
margin before burnout occurs - in other words, there should be sufficient
time at that point to take some reasonable set of actions to close down in
an orderly manner.

Thanks for your concern though ;-)

Regards,
Tony

P.S. - I have seen at first hand the consequences of getting these sorts of
safety systems wrong. One of the process control jobs I worked on a couple
of decades ago involved re-building the software for a system that
controlled an epoxy resin manufacturing plant. The original software
(nothing to do with me, I hasten to add) was trigger-happy - generated
loads of "phantom" alarms. So the operators had learnt to ignore all of the
alarms that the software generated - including the very small proportion of
alarms that they really needed to know about. Limit switches were notorious
for this - they would fail & generate spurious alarm signals indicating
valves stuck in the wrong position, etc.. when the valve actuators were
working perfectly well. One such alarm occurred one day - but this time,
the valve was actually stuck in the wrong position, meaning that a critical
reagent had not been distilled off from the vessel before the product was
washed with water (with which this reagent reacted rather interestingly).
The operator was in the middle of writing out the trouble ticket to order a
replacement limit switch when the wash water was pumped into the pressure
vessel. The resulting reaction proceeded so rapidly that it blew the
stirrer assembly out of the top of the vessel (about 1 ton of motor,
paddles & mounting plates) & launched it 1/2 mile across the site, where it
ended up embedded in the side of a large, and happily empty, product
tank. Because the operator was writing out a ticket at the time, he was
protected by the very robust steel control panel between him & the vessel;
the brick wall behind him was blown away by the blast. The shift was
changing at the time; for some reason, the other operators chose to use the
back stairs, rather than the main stairs that ran down the side of the
pressure vessel...

Not surprisingly, they wanted the system re-written so that it generated
alarms that the operators could take seriously.

At 20:31 06/03/2001 +0000, you wrote:

>Boy, must be my day to play safety leader :-)
>
>Tony,
>
>Have you considered the ALTERNATIVE to the motor shutting off due to
>thermal overload?!?!
>
>At best, a burned out motor (which will STILL wreck your part and/or
>machine!).
>
>At worst, a fire as the increased temperature sets off some oil or
>solvent vapor... Or just the motor itself catching fire...
>
>IT DOES HAPPEN! That thermal overload is there for a reason!