Quote:
Originally Posted by Mike Betts
Chris,
While I am humbled among the likes of Al and the Good Doctor, I'll give you my 2 cents...
A motor at stall is delivering no mechanical power. All of the electrical power, even constant current, is converted to heat. Most external cooling that teams implement is over the case and not forced into the motor. The heat will build up in accordance with the thermal resistance of the internals of the motor.
This will likely happen very fast.
That little fan inside the FP is directly cooling the motor windings. If that little fan stops, my experience is that failure will occur very quickly.
In my opinion, the PTC added to the FP in recent years is to attempt to save a child's extremity or to mitigate an actual fire in the toy for which the motor was designed. It may not be fast enough to limit damage to the motor.
I would not design a system where the motor could stall.
JMHO,
Mike
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Yes, we are all humbled
I think from the discussion, that the FP motor will trip the PTC if it is stalled, although, I have yet to see any test data. I would propose a test where the motor is stalled and voltage increased until the PTC trips to see what the max continuous current is (at least for the lab conditions) , except I don't want to risk a motor for the test. My hair dryer has tripped a few times when the dust builds up over the fan exhaust, but it has always recovered. I would hope the FP would recover too. Has anyone actually tripped an PTC yet?
I really am not sure how to design a fixed roller that sucks up a ball to ensure a motor will not stall. It would seem that this requires limiting the normal force on the roller which is difficult when a ball is squished between a wall and a robot. Even with a frictionless back bar, it seems the ball can deform and push hard against the roller. If the roller is at all sticky getting a normal force equal to the output torque/radius doesn't seem too difficult.
We are using a FP with a banebot 16:1, a 1.6 in dia roller. This can deliver about 30 lb of tangential force to the ball and getting a normal force 30/u_roller seems plausible for any reasonable u_roller.
Squirrel and others seem to be able to spin the ball on the rug which implies u_roller>u_rug. Using a slippery roller can keep from stalling probably , but degrades ball magnet performance.
So, we limit the normal force on the roller and actually shut down the motor when the normal force lifts up the roller. I still would be very happy to know if we do stall that the motor is not damaged.