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# Re: [TCML] Determining a transformer's voltage and current

```Hi Bert,

Thank you for adding light to this cloudy issue!

I just ran an experiment to document the various results with a 15/30 NST.

Energizing the secondary with 120V yields 0.815V on the primary, indicating
a turns ratio of 147X.  Were I to believe this, if the primary were
energized with 120V, I would see 17.7KV on the secondary.

Energizing the primary with a variable AC source and metering the secondary
yields a more accurate ratio:
Vpri        Vsec    Ratio     Calculated-Vsec@120VAC
1.00    109    109X    13.08KV
2.00    2.34    117X    14.04KV
3.00    358    119.3X    14.3KV
4.00    482    120.5X    14.5KV
5.00    613    122.6X    14.7KV
6.03    728    124X      14.9KV

I don't have a means to measure the actual nominal output voltage but I'm
assuming it is 15.0KV  So it appears that energizing the secondary and
measuring the primary is inaccurate, and that energizing the primary with a
reduced voltage sufficient to give a safe and measurable secondary voltage
is a far more accurate technique, and that accuracy is best as the
excitation voltage gets higher.

Regards, Gary Lau
MA, USA

On Thu, Apr 21, 2016 at 3:56 PM, jimlux <jimlux@xxxxxxxxxxxxx> wrote:

> On 4/21/16 9:27 AM, Bert Hickman wrote:
>
>> Gary and all,
>>
>> Part of the problem (with NST's at least) may be due to the internal
>> current limiting shunts which reduce the effective coupling coefficient.
>> This will cause the measured voltage ratio to be somewhat different than
>> the actual turns ratio.
>>
>>
>> A more accurate approach might be to drive the LV side from a small
>> variac and then monitor the secondary voltage using the HV setting on a
>> DVM or a cheap analog meter. Or, use a simple resistive or capacitive HV
>> divider and a high-input impedance DVM. Driving the LV side at 10% of
>> its normal operating voltage should provide sufficient MMF to make the
>> core happy, while reducing the output to a lower, and safer, level.
>>
>>
> I use a 12.6V filament transformer for this kind of thing.
>
>  I started to build, but never finished, a little battery powered 60Hz
> source at about 10V for testing in surplus shops, scrapyards, and the
> like.  You see that unmarked transformer, and it looks about right from a
> size standpoint, but you need something to test with.  ALso handy for
> checking out big capacitors.
>
>
>
> Bert
>>
>
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