Re: acmi accuracy...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Terry, Paul, 

A few comments intersperced: 

Tesla list wrote: 
>
> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net> 
>
> I have finally gotten the program and have used it.  Very nice!  Many 
> thanks for the DOS version for us non-UNIX folks 0:-) 
>
> I see that the error seems to increase when the primary and secondary are 
> aligned at the base or when the coupling is greatest.  I think there are 
> six possible causes for this:

I think there is another possibility (probability): When dimensions are entered
into acmi, if one of the dimensions are off, the error follows the ratios of
measured vs. calc'd. I think the error growth is proportional to dimensional
error. 

The more I use acmi, the more I see the importance to enter dimensions that
calc to measured inductances in acmi. This is not "making" acmi work, but
simply giving acmi accurate information. It is taking out human error when
determining dimensions or it can be viewed as entering measured information
directly into acmi. When acmi calc's inductances as measured, the mutual
inductance and coefficients are very accurate. (except for my coil and
therefore I expect it is "my" error somewhere). 
>
> 1.      Floor too close - In my measurements, the primary was only 4 inches
> off 
> the concrete floor.  Perhaps this is causing the error.

My secondary base was about 30" off the floor. 
>
> 2.      Primary to secondary capacitance - Perhaps when the coils are close,
> the 
> primary to secondary capacitive effects are a factor.

My spacing is about 2 1/8". 
>
> 3.      Winding direction - perhaps related to #2, the winding direction may 
> tend to add or subtract from the coupling.  This may explain why Bart and 
> my measurements trend in opposite directions.

My primary and secondary are wound in opposite directions. Looking down on the
primary, it is wound clockwise. The secondary relative to primary is
counter-clockwise. 
>
> 4.      Measurement error - Perhaps there is something in the technique of 
> taking the measurement that is causing the error.  Also, the meters I used 
> at the time were probably 1% accurate.  I now have two true RMS meters. 
> One 4 digit and the 6 digit HP34401A.  I could probably stretch the 
> accuracy out by two more digits now.

I am not comfortable with my current readings. Next measurement I do will use
both current and volt readings. I need to measure inductances as well. I feel
much better about acmi's predictions than my own measurements. 
>
> 5.      The program - Coding causing the error but the measurements are
> fine...
> 6.      Unused primary turns - I use 15 of 19 turns.  Perhaps the unused
> turns 
> are having some auto transformer or capacitive effect.

Yes, I thought about this last week. I have about 2 extra turns. If this is
causing some discrepancy, the full winding measurements can be compared to the
tap point measurements. 
>
> To check into these I could try the following tests: 
>
> 1.      Measure the 1000Hz primary to secondary capacitance at the 15
> heights. 
> 2.      Raise the coil off the floor by a foot or so and retake the
> measurements. 
> 3.      Use the better meters and big fixed ceramic resistors for current 
> limiting to reduce pure measurement error. 
> 4.      I can't think of a way to change winding direction.  If you turn the 
> secondary upside down it stays the same winding direction.  Turning the 
> primary over is awkward at best in my case and I don't think I would trust 
> that. 
> 5.      Try using all the primary turns to eliminate the effects of the
> unused ones. 
> 6.      ??? Can anyone else think of others or why one of these is not
> possible?? 
>
> I have a request: 
> It would be nice if acmi could input and output the numbers to many more 
> digits, at least while were trying to split hairs here.  The 0.001 
> resolution for k seems rather course (wow, aren't we picky :-)).  Also, if 
> it could do primary turns to the nearest 100th or 1000th, I could put in my 
> 15.125 primary turns without worry.  With higher resolution, some of these 
> errors would be easier to quantify and "err%" my be much better.  So I 
> would ask that the outputs have more digits for the time being.  My free C 
> compiler gave some error (LCC didn't like "strncasecmp") or I would do this 
> myself. 
>
> Cheers, 
>
>         Terry