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Forum Index : Solar : Toroidal transformer for DIY Inverter
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| Dev7 Newbie  Joined: 15/10/2025 Location: South AfricaPosts: 5 |
Hi all, I am currently designing a 1kVA 24V to 230V low frequency inverter. I have managed to pick up two faulty 1kVA Variacs that both use a toroidal autotransformer with a variable brush to adjust the voltage from 0-250V. I have unwound one of the transformers so that it is just the bare toroid now. I want to know if it would be possible to rewind the toroid into a 14V:230V 1kVA isolation transformer that I could use in my inverter design? Pictures uploaded below. .jpeg) .jpeg)  |
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| Godoh Guru  Joined: 26/09/2020 Location: AustraliaPosts: 678 |
HI Dev, it should work fine. Plenty of folk here have used second hand torroids to make their inverters. Most are much bigger than 1kva. I have made a couple for my off grid system that are 24 volt 4kva. I used two torroid cores and stacked them and glued them together. The bigger the core is the less turns you need to use. There are torroid calculators on the web that are pretty good. I have used aliexpress boards for the ones I built, others here have designed their own boards, I suggest that it is good to leave some headroom for regulation under load, i generally wind my transformers for 13 to 250 volts, that way they seem to behave well under load. good luck Pete |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 678 |
Oh I forgot to say we wind the transformers for a flux density of 1 Tesla. That seems to work well Pete |
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disco4now Guru Joined: 18/12/2014 Location: AustraliaPosts: 1133 |
See this post for a calculator to work out how many turns required to get 1 tesla. You need to workout the cross section of your toroid in cm and then adjust the number of turns until you get around 10,000 gauss. Note: 1 Tesla=10,000 gauss Frequency is in MHz so 50Hz is .00005 F4 H7FotSF4xGT |
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| Dev7 Newbie Joined: 15/10/2025 Location: South AfricaPosts: 5 |
Thanks guys, very helpful. I made some core measurements and used the calculator to get the following: OD = 135mm ID = 60mm Height = 55mm Cross sectional area = 20.6cm^2 230V winding = 500 turns 12V winding = 26 turns I am slightly concerned, because these core measurements seem too small for a 1kVA transformer. The datasheets of the transformers that are being sold online have diameters and heights slightly larger than this. If I have to, I could probably stack my two identical cores for one very tall core. I am also most likely going to check in with a rewinding shop during the week to see if I can get a quote and get their opinions. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 678 |
Hi Dev, I would stack the cores if I were you, that gives you much more headroom and also it is a lot less stress on the transformer and it will run cooler. Winding the cores is easy, but takes time. If you stack the cores then the turns will reduce too which makes the job easier. I used a piece of 32mm conduit and bent it into a hoop to load the wire onto to wind my cores. From memory I had about 220 turns on the high voltage side of my transformers. As your inverter will only be around 1kw then the cable for the low voltage side won't be as big so a 60mm hole in the centre should work. Most here aim for around 90mm centre to fit the big wire for the low voltage side. I ended up using flat glass insulated bar that was for rotor winding on my transformer. It was about 30mm wide and 3mm thick. It worked well but then I only had about 13 turns. Getting a winding shop to wind the cores could be expensive, but check them out. The wire is pretty expensive too, many folk unwind their cores carefully and re use the high voltage winding wire. For low voltage wire either welding wire or battery cable work pretty well. good luck Pete |
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