[amsat-bb] Re: G-5500 stuck problem
ko6th_greg at hotmail.com
Thu Jan 1 23:08:58 PST 2009
So, my take-away here is that it's not the stall itself that is the problem; the motor is practically designed to self-destruct. Simply running the rotor back and forth enough times will do the same thing. In fact, probably worse, given the higher draw.
One thing just occurred to me... AC current measurements assume a specific phase relationship between voltage and current - power factor, I think they call it. This is a highly inductive load, and the phase-shift capacitor throws another unknown into the puzzle. I wonder if the two measurements you took are real? It could be that you think that the running current is higher, when it may just be a measurement error. How were they measured?
> From: wa6fwf at sbcglobal.net
> To: amsat-bb at amsat.org
> Date: Thu, 1 Jan 2009 20:51:47 -0800
> Subject: [amsat-bb] Re: G-5500 stuck problem
> Hi Greg,
> these transformers and motors are rated for intermittent duty so getting them stuck and left that way kills them, if memory serves
> they pull about 1.7 amps running, and drop to about 1.2 amps stalled, 1.2amps x 25v = 30 watts and over time that heat builds up and
> fries the windings.
> It's just a oddity of this particular type of ac motor that it behaves this way, if it was another type or a dc motor you would see
> the current rise that you expected and fuses would pop.
> I found this out when I was making a add on card to my LVB tracker to replace the G-5500 control box, I thought I would be slick
> and add one of the PPTC resettable fuses on the motors, so I needed to know the running current and locked current so I could pick
> the right one, I was also surprised when the current dropped a little instead of going up, and then doing some reading on the web
> about dual winding AC motors that use a capacitor to phase shift the other winding explained why this happens.
> Kevin WA6FWF
> ----- Original Message -----
> From: "Greg D." <ko6th_greg at hotmail.com>
> To: <wa6fwf at sbcglobal.net>; <amsat-bb at amsat.org>
> Sent: Thursday, January 01, 2009 7:04 PM
> Subject: RE: [amsat-bb] Re: G-5500 stuck problem
> Ha, really... Thanks, I was "sure" it was the other way around.
> So, if the current drops a little on a jam, why would it burn up either the transformer or motor?
> Greg KO6TH
> > From: wa6fwf at sbcglobal.net
> > To: amsat-bb at amsat.org
> > Date: Thu, 1 Jan 2009 18:42:15 -0800
> > Subject: [amsat-bb] Re: G-5500 stuck problem
> > Hi Greg,
> > Actually you need to measure the current while it is moving, then if it drops a little then its a jam, if it goes to zero then
> > its the limit switch.
> > I know this goes against reasoning, you expect a locked motor to pull more current, but these are split winding AC motors with a
> > capacitor and they act differently.
> > This is also why when you get a cable snag you burn up the motor or the transformer or both before you blow the fuse, that fuse I
> > think just saves you from a mis-wired or shorted cable.
> > 73
> > Kevin WA6FWF
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