re:transformer theory q

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Peter,

On 20 Nov 2001, at 18:01, Tesla list wrote:

> Original poster: "Peter Lawrence by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <Peter.Lawrence-at-Sun-dot-com>
> 
> 
> Many thanks for all the responses so far to the transformer theory question.
> I think the answer to question 1 can be summarized as:
> 
> 	fewer turns in the primary lowers its inductance and causes the
> 	primary current to increase (at least when the secondary is unloaded,
> 	not so sure about the loaded case). the effect is rather large,
> 	a 10% decrease in primary turns can cause 5x primary current in the
> 	unloaded condition.

That figure was for a particular transformer. It may be more or less 
in other transformers depending on how the manufacturer designed and 
rated them. 

    Assuming the winding window is packed tight with copper in both 
the following cases:

The case with fewer primary AND secondary turns to maintain the same 
output voltage: Magnetizing current increases, copper losses in the 
primary due to magnetizing current increases (dramatically and out of 
proportion to the decrease in primary turns), losses in both windings 
due to load current decreases (assuming you use a larger wire size to 
compensate for fewer turns)

> I still have not heard a clear answer to part 2, what happens (to the primary
> current) when there are more than the recommended primary turns (take +10%
> as an example).

The unloaded case with more primary turns (and secondary turns to 
maintain the same output voltage): Magnetizing (no load) current 
decreases, copper losses due to magnetizing current decrease 
somewhat, copper loss in both windings due to load currrent increases 
due to the smaller wire size in both windings.
 
> I am thinking of trying many taps on a test primary and finding the one that
> minimizes the primary current (in the unloaded secondary condition), does
this
> make sense? If I do this what happens when I put the shunts back in (or some
> fraction of the shunts), or should I do the experiment with the shunts?

It should be clear from the cases stated that there is an optimum 
winding configuration for a transformer loaded to its ratings 
continuously. In Tesla use however, more often than not, transformers 
are pushed beyond their ratings for intermittent use (which is OK as 
long as the windings are not allowed to get too hot for the 
insulation, bobbin etc.).

       In the case of the transformer where I reduced the primary 
turns, that was the only thing I changed. The primary heated a great 
deal more (quickly too) and the output voltage was 10%+ higher under 
load. In fact, load current in both windings increased as I was 
pulling more power due to the higher secondary voltage with the same 
load impedance and secondary heating due to load current increased as 
well.

       I later rewound the primary with thicker wire and the fewer 
turns to alleviate the extra heating in that winding somewhat.

Regards,
malcolm