| Electronics > Beginners |
| Transformer Winding |
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| MagicSmoker:
--- Quote from: ArthurDent on July 15, 2019, 04:05:46 pm ---...The input voltage on the primary winding is first ‘converted’ and stored as a magnetic field in the core. Next, as this magnetic field collapses, the magnetic energy is ‘converted’ or induces a voltage into the secondary winding where the turns ratio of primary to secondary determines the output voltage. ... --- End quote --- This is a common misconception: the energy delivered to the secondary from the primary in a true transformer is not actually stored in the core, as the flux from the secondary amp*turns exactly cancels out the flux from the primary amp*turns. Some energy does get stored in the core as a result of magnetizing it, but this energy is automatically returned to the supply every cycle and doesn't cause a net power draw, anyway, (except for various losses) because the magnetizing current is phase shifted from the supply voltage by 90 degrees (it is purely reactive, in other words). One prominent exception is the flyback transformer - energy from the primary is stored in the core before transferring to the secondary - because the primary and secondary don't conduct at the same time. As a result, flyback transformers have to be significantly larger to transfer a given amount of power than their conventional counterparts. EDIT - and they need an air gap, which does most of the actual energy storage, oddly enough. Another exception is the transformer in an LLC converter, which both stores energy and acts as a true transformer, depending on conditions at any given time, but this - and other resonant mode topologies - is a bit beyond what the OP was asking about... :P |
| tautech:
--- Quote from: soldar on July 15, 2019, 03:06:22 pm ---........................... Both mention https://ludens.cl/Electron/trafos/trafos.html --- End quote --- This ^^^ OP, to get a good idea of what's involved study it. |
| bob91343:
In order to design a transformer one needs to figure the flux density first. Looking in catalogs of core materials one sees that a particular material saturates at some specified density. So you pick a density considerably lower than that. Reason: if you approach saturation, the inductance drops and the input current climbs rapidly. Once you decide on a density, allowing for normal variations in materials and input voltage, you can select a primary number of turns for a given cross section area of core. That will insure that you can apply a given voltage at a given frequency without excessive primary current. If you choose a small core, you will need more turns of wire, which takes up more space. If you use finer wire you can't handle as much current. So the cross section area of the core has a major influence on the amount of power you can transform. In addition, a particular core provides a window area to accommodate the windings. Again, too small a core gives insufficient room for enough wire. An optimum design allocates about half the window to the primary and half to the secondary. So we pick a core, decide how many primary turns we need to keep the flux density low enough and still not take up more than half the window. So that determines wire size. If that results in 100 turns, then the secondary can be the same (1:1 transformer) or more or less depending on the ratio we want. Fewer secondary turns allows for larger wire size, etc. In general, a conservative design runs perhaps 500 circular mils of wire per Ampere. That keeps the copper losses low and avoids a hot transformer. There are other factors one needs to take into consideration, such as insulation. On an iron core for typical power line transformers, it's not important to make a neat winding other than making it space efficient. I could ramble on. I have designed and built many transformers over many years and am aware of the pitfalls and tradeoffs. I don't intend for this message to be taken too literally as a design guide but more as an introduction to the subject, allowing the reader to decide how to proceed. The literature is out there, and has been out there for many years. Yes it's theoretical, but it has to be for a safe and efficient design. In other words, it's not for beginners. |
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