Electronics > Projects, Designs, and Technical Stuff
Noob plays with transformers
soldar:
You want to make a 1:1 isolation transformer with a core and bobbin you already have. 120V, 60 Hz I assume. Here is what I would do:
First of all measure the characteristics of the core by making a coil with a number of turns, say 100, and putting a 60 Hz current through it. Measure voltage and current and determine L value for 100 turns. L is proportional to the square of the number of turns so 200 turns would have 4 times higher L. That gives us the characteristics of the core we need.
Now measure the window of the bobbin, the space where the wire will go. Of the total area let us subtract 10% as safety margin, for insulation tape, etc.
The secondary will have, say, 10% more turns than the primary (and several taps). I assume primary and secondary will have same diameter wire. So, of the total bobbin window area Wa, the primary gets 43% and the secondary 47%.
Now you have an exact window area to work with. How many turns of wire you can fit depends on the wire diameter. Bigger wire, fewer turns. Assume a diameter and work out the number of turns you can fit in the primary allocated area. Work out the inductance and the resistance. Then the magnetizing current and power dissipation in the primary. If not enough inductance then you need more turns of thinner wire.
This way you are using all the bobbin space to the best advantage.
GeorgeOfTheJungle:
--- Quote from: rebelrider.mike on November 14, 2019, 09:11:22 pm ---I think that's why there's about 2.5A being used even without load.
--- End quote ---
Yep. That means the core is entering saturation. 120 times a second. Not good! You have to either put back the irons, or add turns to the primary. The no load losses ought to be max. 5% (less is better) of the transformer rated power. Connect it unloaded to a variac and decrease input voltage until you're at less than that 5% figure, then do the math: if for example rated Vin is 120 and you've got to set the variac to, say, 80V, then the number of turns you need in the primary is 120/80 => 1.5x the turns it's got now.
soldar:
Don't design it for exactly 120 V but give it at least 10% margin. One of the first transformers I designed, ages ago, was so tight that it would easily saturate and heat up. When the standard was raised from 220 to 230 V that was the end of it.
Note also that load current is irrelevant to saturation. Only the magnetizing current counts and the magnetization does not change from no load to great load.
So, basically, you need to design a primary that under no load has enough inductance to keep the current well inside the working park of the curve and not reach saturation. If that condition is met then the core will not saturate under load.
For a one time project like this my advice is to give wide margins in every step. You might not get the absolute last drop of power out of the thing but you can be sure it will work reliably and safely at the first try.
I have had to re-wind more than one transformer because I was too optimistic in the amount of copper wire I could fit in the bobbin.
rebelrider.mike:
Wow, thanks for all the replies! I guess I'm not getting email notifications, because I just now saw them.
I really appreciate the explanation between the isolation and auto transformers. That makes sense.
Some of your responses have a lot of info that I'll have to go back an re-read a couple times to make sure I get it all straight.
So the core's ability to handle the power going through it is really only dependent on how many turns you can physically fit inside... I think I get that.
Looking at the equation I've been using: T/V = 1 / 4.44 x 0.0001 x CA x B x f the turns per Volt will go down as the core size goes up since it's 1/CA (and all the other stuff). So either more turns are needed, or there needs to be more metal in the core. Seems the problem with CA (the core cross section area) is that it seems to assume almost no distance in between the plates for insulation. I wonder which is worse: eddy-currents from lack of insulation, or not enough core material? I've already decided to thin out the insulation I put on the plates so I can fit more in.
I figured there would be some Voltage drop anyway, and even if it were to drop to 115V, I think that anything I'm using if for would be fine. I like the tip on how to estimate how much wire will actually fit in the transformer, based on the cavity size.
I don't have a Variac, but I do have that little transformer that has lots of taps. Anyway, you all gave me lots to think about! Hopefully I'll return with good news. :)
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