[amsat-bb] Re: G5500 Azimuth Rotator Update

David Palmer KB5WIA kb5wia at amsat.org
Tue Mar 13 20:40:24 PDT 2012

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

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.greg at 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

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