Meassuring transformer turns ratio without the core?

[ELS122]

So I am winding an L-core transformer but I forgot to meassure the voltage ratio before taking it apart, and I dont want to put it back together so as to not break the insulating layer on the core plates too much.
could I just power the primary with a lower voltage, and then wind some wire around it? just having an air core transformer?

[Benta]

With the core out, lower voltage and higher frequency will also work for getting the pri/sec ratio.

[gbaddeley]

Put a loose stack of lams in the primary core and power it using a low voltage eg 12v 60hz. Wrap 10-20 turns of hookup wire as a dummy secondary with a 100 R load. This will give a good idea of the T/V ratio.

[The Electrician]

So I am winding an L-core transformer but I forgot to meassure the voltage ratio before taking it apart, and I dont want to put it back together so as to not break the insulating layer on the core plates too much.
could I just power the primary with a lower voltage, and then wind some wire around it? just having an air core transformer?

You say that you are "winding".  Will you be using a new bobbin?  Do you mean "re-winding" on the old bobbin? Just what do you plan to do?  Are you going to keep the primary and re-wind a new secondary?  Or are you going to replace both windings?

Have you already removed the existing seconday winding?  If you haven't, then you can count the secondary turns and knowing the rated secondary voltage and primary voltage, you can calculate the turns ratio.

[ELS122]

So I am winding an L-core transformer but I forgot to meassure the voltage ratio before taking it apart, and I dont want to put it back together so as to not break the insulating layer on the core plates too much.
could I just power the primary with a lower voltage, and then wind some wire around it? just having an air core transformer?

You say that you are "winding".  Will you be using a new bobbin?  Do you mean "re-winding" on the old bobbin? Just what do you plan to do?  Are you going to keep the primary and re-wind a new secondary?  Or are you going to replace both windings?

Have you already removed the existing seconday winding?  If you haven't, then you can count the secondary turns and knowing the rated secondary voltage and primary voltage, you can calculate the turns ratio.

L core -- 2 bobbins

[The Electrician]

Is that all you have to say to my questions?  Can't you figure out how to substitute "bobbins" for "bobbin" in my post?  Have you destroyed the existing secondary(s)?

[NiHaoMike]

You can put a Hall sensor into the hole and use that to determine the approximate turn count of each winding by measuring how much current it takes to change the output of the sensor.

[ELS122]

You can put a Hall sensor into the hole and use that to determine the approximate turn count of each winding by measuring how much current it takes to change the output of the sensor.

Wow, that's a cool idea! but I dont think it would be very accurate, due to probably a large difference in the needed flux density to engage a hal sensor from another hal sensor, and also all of the magnetic flux wouldnt be perfectly focused onto the hal sensor of course...
or am I wrong?

[The Electrician]

Don't worry about the insulating layer on the laminations.  At the very least they will have a coating of steam oxide.  The better laminations have a coating of Carlite, which is magnesium silicate.  You should see how laminations are shipped to transformer manufacturers.  They come by motor freight, and they're stacked in a long cardboard box with nothing between the individual lams.  They are subject to a lot of shaking and vibration, and they are not damaged by all that rubbing against one another.  If the secondary is still there you could just put about 90% of them back in and that way you wouldn't have to jam the last few in.  That would be enough core to let you measure turns ratio in the usual way.  The insulating layer on the lams is much more rugged than you might think, and it won't be hurt by re-inserting then several times.

[ELS122]

Don't worry about the insulating layer on the laminations.  At the very least they will have a coating of steam oxide.  The better laminations have a coating of Carlite, which is magnesium silicate.  You should see how laminations are shipped to transformer manufacturers.  They come by motor freight, and they're stacked in a long cardboard box with nothing between the individual lams.  They are subject to a lot of shaking and vibration, and they are not damaged by all that rubbing against one another.  If the secondary is still there you could just put about 90% of them back in and that way you wouldn't have to jam the last few in.  That would be enough core to let you measure turns ratio in the usual way.  The insulating layer on the lams is much more rugged than you might think, and it won't be hurt by re-inserting then several times.

this transformer is from at least <1990. I used a multimeter and there are plenty spots where I can get continuity on the lamination layers.

well that's the thing I already unwound it