transformer
Transformers have hundreds of turns on both the secondary and primary winding, and as a result use very thin copper wires for each. But, why do they not just use fewer turns on each winding and get the same voltage ratio?
More importantly, why not use fewer turns of a thicker wire for an increased VA?
(instead of 1000:100 turns of 22 awg wire, why not 100:10 turns of 16 awg wire if this would increase VA)
1) Winding direction doesn't matter as long as you keep it consistent. If you are adapting a lathe or drill as a winder, the winding direction will be determined by the machine.
2) If the small windings are at approximately the same potential they can share a layer, otherwise you must provide sufficient insulation between them. Sometimes you may have to separate layers to reduce capacitance between them, or interleave them to reduce leakage inductance, but that is too complex to discuss here.
3) Winding coils side by side is simpler and provides better voltage isolation especially when the sections are different bins on the bobbin. That is essential for high voltage supplies and makes mains voltage safety approval easier.
However it increases the leakage inductance, and that matters for high frequency applications like SMPS. The only reasonable way to know if you can do this is to wind one that way and measure the leakage inductance. If it's out of spec (or even close to it) wind another in layers and compare them.
This question is highly related to this one, raised by the same person and my answer is virtually the same so, I expect one or the other to be marked as duplicate.
The main problem with putting transformer primaries in series is that even with supposedly identical units, you cannot expect the magnetization inductance to be the same. This means, that on light or no-loads, the primary voltages could be 50% different when wired in series. You have to treat the transformer as it's equivalent circuit demands: -
Forget the secondary for now. In ignoring the secondary you can ignore the perfect transformer symbol at the heart of the equivalent circuit. This means you are left with Xp, Rp, Rc and Xm. Because Xp and Rp are insignificant under no-load conditions you are left with the primary of the transformer looking like Xm in parallel with Rc.
These are the magnetization inductance and core losses respectively and you: -
CANNOT EXPECT THESE TO BE IDENTICAL for any two transformers that otherwise have the same model number.
This basically means the formulas in the question are too simple to merit further consideration. You can only use formulas like this when both transformer windings share the same core but, that then becomes a single transformer scenario.
Best Answer
When you apply voltage to the primary winding of a power transformer, some current will flow, even when the secondary is open circuit. The amount of this current is determined by the inductance of the primary coil. The primary must have a high enough inductance to keep that current reasonable. For 50 or 60 Hz power transformers, this inductance is pretty high, and you typically cannot get there with a small number of turns in the winding.