Hi Greg, Hey, thanks!
Good question about how long it takes the windings to degrade. Haven't tested this myself, but I'd seen a post a while back where someone measured current draw on the motor, and found that it's actually almost the same between normal operation and a full stall. That also could be the reason why the 1-Amp fuse I installed in the controller (which was barely enough to run the motors without blowing - a 0.75-Amp fuse I tried blew) never blew throughout my motor stall. So .... with similar current and voltage in a stall as compared to normal use, that should mean the power consumed (and heat generated) would be similar whether the motor is running or stalled. When I was bench testing the rotator, after maybe 5 full trips to 450 degrees and back (perhaps 10-15 minutes of continuous motor operation) the thermal switch rated at 75C tripped open. So that means the switch is sensing the motor heat, and that it seems in normal operation the motor can go from ambient to 75C (or more) in 10+ minutes. Hard to tell what the failure temp of the enamel wire in that motor is, but let's guess 120 degrees to be conservative. That would give a very rough ballpark of 20 mins stall (or 20mins continuous back-and-forth with no stops) to raise the temp by 100C to cook the motor. You're right about thermal mass and winding temps -- certainly the windings will be hotter than the motor chassis. Will cutting power to the motor when the chassis gets to 75C protect it? Don't know for certain, but after my last experience, I'm adding this insurance, since it's better than no protection! :-) (doesn't beat common sense tho -- if I had more of that, I would have checked antenna clearance before letting the computer run the sat array automatically!) 73 de Dave KB5WIA! On Wed, Mar 14, 2012 at 2:41 AM, Greg D <ko6th.g...@gmail.com> wrote: > Hi Dave, > > How long does one have to stall the motor before the windings' insulation > starts to degrade? I seem to recall it's not very long (less than minutes). > > With all that thermal mass, and the insulation from the heat shrink tubing, > is the thermal switch really going to be effective mounted that way? I would > have thought it needed to be buried in the windings themselves somehow > (though I can't imagine getting good thermal contact there either). > > Great pictures, by the way. > > Greg KO6TH > _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
