Transformer winding turns on low and high frequency/voltage designs - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 12, 2019, 02:25:59 am
content: I am trying to understand a bit better how to construct a transformer.I come up with a table that does all the calculation based on what I learned here:http://www.giangrandi.ch/electronics/trafo/trafo.shtmlAnd I do have a few questions.1 - At 50hz and high voltage seems I need heaps of turns for my primary side. But when at higher frequencies and lower voltages the turns are pretty much ZERO.2 - Also higher the power required thicker is the wire and that makes total sense. But then also less turns are required which I do not understand why.3 - Also what would happen if I deliberately reduce my turns on scenario 50W @ 50Hz to a third?

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 12, 2019, 06:39:49 am
content: It all has to do with the magnetizing field that the core of your xformer is capable of handling. For laminated EI cores this field is around 1T (one tesla). More or less than that depending on your application and core material. For ferrite cores, this can be around 0.4T, for example.One trick is to design your xformer to maintain this field within specs.The field is directly proportional to the primary voltage, and inversely proportional to the transversal area of the core, the number of turns and the frequency.For standard EI cores, the core area also defines the area of the two wire windows, since their proportion is fixed.So, more power, more area (thicker wire). More area, less need for turns. Higher frequency? The same thing. Less need for turns. Or area.Another trick is to find the right core size so the windings can just fit inside the windows.Here's a video I did explaining how these calculations are made.Please, excuse the computer generated voice. I'm too shy.

author: soldar
author_rank: Super Contributor
timestamp: March 12, 2019, 07:55:10 am
content: Quote from: gkmaia on March 12, 2019, 02:25:59 amYou did notice the title of the page is "Calculating mains frequency transformers"? Right? ... You know what "mains frequency" means?Calculating and designing transformers is an art and many, many rules of thumb and approximations go into it. A transformer for higher frequencies is not calculated in the same way as a mains power transformer.Quote from: gkmaia on March 12, 2019, 02:25:59 amThat is how it works.Quote from: gkmaia on March 12, 2019, 02:25:59 amBecause you have more magnetic material in the core which means the same number of turns has greater inductance.Quote from: gkmaia on March 12, 2019, 02:25:59 amSomething is gonna give. Hopefully the circuit breaker.Designing and building mains frequency power transformers is relatively straightforward. Designing and building audio transformers that have a wide frequency range response is much more complicated.Basically, it is the wire what limits the amps and therefore power. As that page saysQuoteToo small a core and the required wire won't fit but if you could find wire with almost zero resistance then a small core could handle much more power.

author: gkmaia
author_rank: Frequent Contributor
timestamp: March 12, 2019, 08:04:45 am
content: Quote from: soldar on March 12, 2019, 07:55:10 amThat is all good I know what it mains frequency means. I did not know high frequency transformers design calculations were not to use the same equations.Quote from: bsfeechannel on March 12, 2019, 06:39:49 amWhat is not clear is why at higher the frequencies less turns are required?Something else I noticed with experiments is that on the same transformer in test if I increase the frequency the voltage on the secondary increases proportionally. Is that for the same reason as higher the frequency less turns are required?

author: soldar
author_rank: Super Contributor
timestamp: March 12, 2019, 08:52:12 am
content: Quote from: gkmaia on March 12, 2019, 08:04:45 amBecause the inductance and the voltage induced in the secondary is proportional to thechangein magnetic field and higher frequency changes faster. This is basic magnetism so you might want to study some magnetism if you want to understand what is going on. When I first studied all this I was fascinated by seeing that inductance is like the mirror image of capacitance and you can pretty much exchange voltage for current and inductance for capacitance and the same concepts and equations apply.If you just want to design and build small transformers I recommend Bernard'sCoil Design and Construction Manualwhich I have used for some decades now. It can be found cheaply in Amazon, ebay, abebooks, etc. (E.T.A.: It is an old book and has instructions on how to build audio transformers for vacuum tube radios and similar outdated things but it has mains power transformers too as well as RF coils.)https://www.ebay.com/itm/202616471372Quote from: gkmaia on March 12, 2019, 08:04:45 amIt shouldn't but as I have no idea what the setup is I cannot guess as to what is going on.

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2019, 08:33:32 pm
content: For a given transformer core, if the voltage is doubled or the frequency is halved, then the inductance must be doubled to maintain the same peak flux which means sqrt(2)=1.41 times the number of turns. However flux is also proportional to the number of turns so the number of turns actually needs to be doubled for 4 times the inductance.However doubling the frequency also doubles (at least) loss in the core lowering the maximum permissible peak flux if the same loss is to maintained so even more turns are required to raise the inductance and lower the peak flux; at some point the core will run out of winding window. This factor depends on the core and is why cores are rated for peak flux at a given frequency and at some point, increasing the frequency does not result in increased power capability because the core loss increases even more.Flux is proportional to the current and number of turns and inversely proportional to the inductance. Inductance is proportional to the square of the number of turns. So doubling the number of turns quadruples the inductance and halves the flux allowing twice the voltage to be applied.