I was looking at how some transformers are used at different times as step up or step down but it seems that they are generally designed to be used in one direction though they may work the opposite way and IDK if they loose some efficiency or just have more or less windings than what is "adequate".

Let's take a transformer that is a step up from 120v to 480. The primary has 100 windings and the secondary has 400 giving a 1:4 ratio - IDK what gauge as I guess that is dependent upon amperage/wattage needed.

Now I'm wondering what would happen if the windings were increased by 30 or 50% to 130: 520 (P:S) or 150:600 (P:S) both have a 1:4 ratio. On the other side, if 70:280 were used, less windings a 1:4 is achieved as well.

Some people were saying they were going to use a step up transformer (metal halide transformer) which is about a 1:2.5 steup up to go from 120 -> 270. going the other way you would get something like 120->50. The thing is when this is done, the secondary has A LOT more turns than the primary. I think the one I have is close to 200:500 - so in this case the primary needs 200 windings. When going to step down, the primary would have 500 - or 2.5x the number of windings than the original primary winding. Sure the ratios are the same but it seems a lot more wire is used. If the wire gauge used is 20g for the primary and 24 or 26 for the secondary could the following be better: 200 turns of 24-26g (same wire as original secondary) and 80 turns of 20g for the secondary (same as the original primary for the steup up converter of 200 turns). Both ratios are the same for all the transformers, the same gauge for the primary or secondary (relative to original) - but the only difference is the number of turns. So it would seems, to me, this step up transformer if re-engineered as a step-down could provide enough material for 2 - 2.5 transformers of the same wattage - or as a step- down the original will not put out the same wattage due to limitations of wire size??

Now I understand that you probably wound't get great performance with 25 turns primary and 10 turns secondary for step down, or 10 turns primary and 25 turns secondary for step up (using proper gauge of course). So I hope you see the question I am asking here. What is the advantage/disadvantaage of using more turns in a transformer.

I watched a video on building toroidal transformers and in one formula (I have great difficulty with formulas - with many variables - corresponding to the proper terms/functions) where a constant of 42 was used where I believe they were figuring out the number of turns needed for a transformer - but they never explained where that came from or what it does. Why not 30 or 21 or 86? It obviously effected the number of turns needed in the transformer. Most sites have only shown the ratio correlation as to how "turns" effects the voltage of the transformer.

I hope I gave enough info to get the question across.

 

Consider a 1 turn primary. It would have very low inductance, so at powerline frequencies it would look a lot like a dead short.

Now consider a primary with a million turns on a ferrite core. Great- the magnetizing current (The current through the inductance) would be very low, but wait -now the resistance is very high so now you have to take some turns off. Transformer design has many such tradeoffs.

 

You usually want the minimum number of turns so that the largest wire to fill up the winding area can be used to minimize winding resistance.
The lower turn limit is having enough turns so that the core doesn't saturate at the worst-case highest voltage and lowest frequency of the primary waveform.
This lower limit establishes the magnetizing current.
(If you go below that and the core saturates, then the inductance suddenly drops to essentially zero and the primary current is limited by only the primary winding resistance, letting out the magic smoke. )

So once you've established the lower limit on the number of primary turns, then the number of turns for the secondary(s) follows from the desired secondary voltage(s).

The core size is determined by how much current the transformer must carry.
Obviously a high current will require larger wires which then require a larger core to accommodate the larger wire area for a given number of turns.

 

In practice I have found the Turns/volt number for transformers in the the low Va range up to around 2Kva run around 2.5 to 4 turns/v for EI style, Toroids generally lower at 1.5 - 2 t/volt.
Max.