[amsat-bb] Re: G-5500 stuck problem

Fri Jan 2 10:01:09 PST 2009

To All,
Antenna rotator motors are capacitor run types. All induction motors shift
the phase of the current. The current is nearest to being in phase only when
the rotor is locked.  An unloaded motor draws almost no power as the current
is out of phase with the voltage. As the antenna rotators age, the run
capacitor looses capacity causing a reduction in torque from the motor. This
capacitor is in the control box, is low cost and the first thing that should
be replaces if the rotator is slowing down. Typically the run capacitor only
lasts for 10 years of service. TV antenna rotators did not last that long
but the Amateur Radio antenna rotators will often outlast the run capacitor.

Art, KC6UQH 

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Greg D.
Sent: Thursday, January 01, 2009 11:09 PM
To: wa6fwf at sbcglobal.net; amsat-bb at amsat.org
Subject: [amsat-bb] Re: G-5500 stuck problem

Hi Kevin,

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?

Greg  KO6TH

> 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.
> 
> 73
> 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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