Transformer turns ratio

[Tonewood]

Hi all-
When determining the turns ratio of old OTs, I put 1vrms AC across the secondary and read the voltage across the primary (plate to plate for push pull-is this correct?) I noticed that when I set the unloaded Variac to 1 volt then hook it to the xfmr, the voltage across the input drops to around 0.8 and even 0.5 for some transformers.
It seems like I should be setting the Variac to 1 volt when it is loaded but when I did this on a xfmr that listed its info, the numbers didn't jibe with the numbers listed on the xfmr.
Any thoughts?
Thanks

[g1]

Use the numbers you get from your meter with the variac loaded. Sometimes the numbers on the variac are only accurate when you have a specific line voltage. My variac is rated for 120V input and the numbers on it are only accurate when that is the case. For my unit, 120V on the dial actually means 100% of line voltage.
Regardless of the actual unit, with such low levels as 1 volt, the numbers on the variac dial will not be very accurate.

[Enzo]

What he said^^^

The numbers on the variac are only fractions. If it expects 120v in but sees 125v, then all the "settings" will be 5% high.

Your transformer under test provides a turns ratio, you need to know what the voltages are going in and coming out. The setting on the variac doesn;t mater at all. Pretend it is chinese. You measure the voltage across the transformer that you are feeding it. Whatever it says on the dial doesn't matter. 1v in ans 100v out sounds very appealing, but I imagine you have a pocket calculator, so 1.24v in and 99.6v out is no problem to punch into the calculator and get 80.3 to 1.

But even if you are doing that, the transformers do have resistance in their wires. And that will load your variac.

[Tonewood]

Thanks Enzo & G-One.

I will read the input voltage only when the variac is loaded.
btw, I never use the dial numbers on the variac, just use my voltage meter. I should have made that clear.

Thanks again.

[Chuck H]

For determining the turns ratio you want to use the loaded voltage measurement for the calculation. Yes, the input signal voltage drops when loaded by the transformer. Yes, some transformers do it more than others. No, the numbers will not usually add up to the listed transformer specs, but it will be close. Usually the calculation will be a little low. Just guessing, I attribute this to a likely lack of transformer efficiency when operated at such low voltage and current.

[jpfamps]

The Variac has to drive the very low impedance of the secondary, so the voltage will be loaded down when connected (the Variac will have low, but not zero output impedance).

As explained above, use the loaded voltage for your calculations.