RE: [CAD_CAM_EDM_DRO] Servo heat-sensors or paranoia?

Abbey, these devices are commonly used as overtemperature sensors
on standard ac motors. The most common are the sicon carbide wafers,
but polymer devices are made as well. Thes are known as PTC thermistors.
I used to design them.

Ranges of snap action switches that use a bi-metallic element are also used.

I think they are commonly not used on servos because adequate protection
can be provided by simply setting the continuous current limit on the
servo amp at or below the max continuous stall current rating of the motor.
Usually short peaks above this value are ok and can be set in most amps as
well.

It is certainly not paranoia to be thinking a lot about e-stop sytems
though.

Such a system should:

1) work even if the control computer does not
2) protect against shorted amps and runaway
3) protect against a stalled spindle due to excessive feed or a damaged
cutter
4) cause a safe rapid shutdown in a power failure

and many other factors, if you want to discuss them.

Les
Leslie M.Watts
L M Watts Furniture
Tiger Georgia
(706) 212-0242

Main page:
http://www.lmwatts.com
Engineering:
http://www.lmwatts.com/shop.html
Cnc surplus for sale:
http://www.lmwatts.com/forsale.html
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http://www.lmwatts.com/signwp.html



-----Original Message-----
From: AbbyKatt [mailto:cnc@...]
Sent: Tuesday, December 07, 2004 3:12 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Servo heat-sensors or paranoia?



I was adding to my E-stop circuit for my servo system on my mill, when I
came across a nice little item on the electronics site I use. It's a
small temperature sensitive switch that drops in resistance from 100Mohm
to 100 Ohm when a certain temperature is reached. Different models come
tuned to different temperatures.... Like 50 and 75 degrees centigrade.

Cut to the chase: Has anyone attached these to their servos/steppers and
wired the whole chebang up to an E-stop circuit? How common is servo
burnout (blunt tool/loss of lube causes the motor to run at way too much
power maybe?) Would anyone recommend this? Or is the likelyhood if this
kind of failure so small that it's no worth my while implementing?

Thanks!
~Abby




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