[AZ-Observing] Re: MSW inverters

Hi Dan,
I believe the little clock motors in the older synchronous motor drives 
can be successfully operated from just about any of the small DC to AC 
power inverters.  No need for one of the monster capacity ones, either.  
One of the small 50 watt units designed to plug into your cigarette 
lighter should work.
Although those small inverters don't typically output pure sine wave AC, 
the issue is mainly the >frequency< of the AC.  That is, whether or not 
the unit is adjusted to give exactly 60 Hz output.  Now, most of those 
older telescope clock drives will run at a >solar< rate from "wall 
current", that is, exactly 60 Hz.  The older separate drive inverters 
had controls to change the frequency appropriately to give an exact 
 >sidereal< rate, by adjusting the output frequency to a bit less than 
60 Hz. 
So IF you were electronically enabled (or know someone who is), it is 
possible to go inside the power inverter and make an adjustment for that 
-- (voiding the warranty on the inverter, of course).  Then the clock 
drive could be operated at sidearal rate.
The non- pure sine wave AC will result in a bit of extra "heat" from the 
clock drive motor, (should not be any problem, as the clock motor only 
draws a couple of watts of electricity), and the exact 60 Hz output from 
the inverter will mean that the telescope will track a bit fast, that 
is, at a rate exact for the sun instead of the stars.  For visual 
observing, this should not be a problem.

For reference, I have "reverse-engineered" my old Orion drive inverter 
(I re-packaged it many years ago) -- the circuit is pretty simple, and 
 >>could<< be duplicated pretty easily at moderate cost by an 
electronics tech.  (In fact, that unit does NOT put out a "pure sine 
wave" -- rather, pretty much a square wave AC, and operates synchronous 
clock drives just fine.)  Also, there are several web pages that show 
circuits for such units, some more involved than others.  For instance, 
look at Dr. Michael Covington's web pages on his modern "ALCOR" inverter 
design:
http://www.ai.uga.edu/~mc/alcor/alcor.pdf

Cheers,

Gene Lucas
(17250)
Dan Heim wrote:
> Fellow Stargazers,
> I trying to find a way to power my old (ca. 1985) Orion drive system remotely 
> and lose the need for an extension cord.  It's a single RA drive, with a 
> synchronous 120 VAC motor.  What I'm wondering is, has anyone used a modified 
> sine wave inverter to power these things?  True sine wave inverters are 
> prohibitively expensive.  I'm concerned about tracking accuracy, of course, 
> and possible damage to the motor.  My online search provided conflicting 
> advice.
>
> Dan Heim
>   

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