Some transformer theory

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Original poster: "Matthew Smith by way of Terry Fritz <teslalist-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>

Hi All

Since MOT's came into vogue on this list, there have been various proposals 
for checking the secondary voltage in absence of an HV probe.  I did my 
calculations of the turns ratio by feeding the various transformers that I 
had lying around with 24V - 1/10th of the normal line voltage.

Do we really need to energise them at all?  We should be able to derive the 
turns ration from:

Square root (Zs / Zp)

I did this for a couple and found that I was within less than a percent of 
the value determined by the 24V energisation (I did the tests some time 
ago, but had been thoughful enough to write the ratios on the transformer 
bodies).  Measurement of resistance and inductance was by means of my 
faithful Wavetek 27XP which, I suppose, was energising the windings ;-)

I re-did the calculations and found that, with the numbers we are dealing 
with, you can actually get to less than half of a percent of the result 
above simply by taking:

Square root (Ls / Lp)

The greatest inaccuracy in my tests was some 14V out of over 2000V - not 
something that I'm going to lose sleep over!

Now for my questions:

I have seen comments that MOT's get hot when energised and under no 
load;  using V*V / Zp for one of the units gave me a whopping 600W+ - am I 
working this out right or are they really that inefficient?

Next question:

Can the root of the impedance ratio also be applied to flyback transformers 
- are they "classical" transformers, or are they in the TC bracket, where 
turns ratios cease to be entirely relevant?  I didn't have one with a known 
turns ratio to test, but measured one and the ratio *looked* sensible.

Cheers

M

-- 
Matthew Smith
IT Consultant - KBC, South Australia
http://www.kbc-dot-net.au