Rewinding a Transformer Simplified

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05:58:58

Okay i noticed someone wanting to make a transformerless power supply, and he has a microwave oven transformer with a removed secondary. It couldn't be easier to wind yourself a 1:1 transformer using a microwave transformer. They often have a large core, and require less turns to operate correctly. They may require as much as 200-300turns, which is a half an hour worths of work, having the right materials and skills. Okay so how it's done?  You take a simple insulated wire and wind 10-15 turns for the secondary. You take a measurement of the output voltage. In this case we have 12V by winding 15 turns. So now we determine the winding ratio by dividing the primary voltage to the secondary. You can work with either the peak value or the RMS value it doesn't matter. Let's choose the peak value here. So our winding ratio is 320:12=26.66. Now that we know that, we just multiply the winding ratio by the windings of the secondary which we made (15). So 26.66*15=400. So you need to make 400 turns to turn the transformer into a 1:1 isolation transformer. If you want a different voltage, for instance let's say 50V. Then you divide 50 to the voltage we got from the 15turns (12V). 50:12=4.16. Now we take that number and multiply it by the turns of the secondary (15). 4.16*15=62.4, so after 62.4 turns you get 50V of output. Now you have to determine how much current can the wires safely pass, and with that the VA rating of the new transformer. The formula is d=0.02*SQRT(I) where I is in mA and d in mm. So if you want 5A at the primary (around 1kW of energy) the wire should be d=0.02*sqrt(5000) 1.41mm. Accounting for active resistance and the slight skin effect, choose a wire 5-10% thicker. So the ideal wire for the secondary is 1.5mm. For lower voltages for the secondary like the 50V example keep in mind that the current of the secondary will be raised as much times as the voltage is reduced, so if you have 5A at the primary you'll have around 21A at the secondary, (5A*4.16) so the wire thickness for the secondary should be d=0.02*sqrt(21000)=2.89mm. Again applying the 5-10% rule the wire should approximately be 3mm. You have to comply with the physical dimensions of the windows in which you'll wind the windings, so always keep that in mind too. Now lastly you need to calculate the VA rating of the transformer. Most of the times and especially for single phase/high power the formula is the same as for Watts. So VA=V*I VA=220*5=1100VA. With that you've finished designing your transformer and you can start building it. Take care, though because this still works at mains potential and it's dangerous if the given precautions are not used.  

P.S. If you want to build a transformer from scratch, with just having the wires and the core, the calculations are a bit difficult and vary from core to core, and from power rating to power rating. 

published 9 years ago

KingId

9 years ago

Very informative. I appreciate your work.

faceblast

9 years ago

microwave cores are flux dampened as well, look at the holes in the core. This makes them saturate much too easily to make decent power transformers. if you want to make a good power transformer, hang out around music shops and ask for broken guitar amplifier power supplies; musicians are always toasting them

thebugger

9 years ago

I'm pretty sure they'd make excellent power transformers. I've seen some online videos where they use a reconfigured microwave transformer as a spot welder, which often requires a dozen amps of current. But as a whole i personally prefer solid iron cores than lamination stacked cores, like the ones that most microwave transformers use. They saturate harder, and carry less heat losses. Anyway what I've made here is just an example. I'm pretty sure you'd need larger cores to maintain a continuous 20A at 50V. Although most microwave transformers are specified at above 1kVA it won't be wise to keep them at full power for continuous operation.

thebugger

9 years ago

And moreover they lack the window dimensions to make such windings.

jpoulin0901

9 years ago

Solid iron cores have LESS heat losses than laminated? Not that I've ever tested it personally, but that makes no sense. The whole point of the laminations is to mitigate eddy current losses in your core. I have never even seen a solid iron core transformer or inductor. They are ubiquitous;

jpoulin0901

9 years ago

MOT's are also somewhat different from conventional power transformers in that usually, the core is made of two parts held together with only a single pass weld on either side. This creates a small, but significant air gap which increases the reluctance of the magnetic circuit. Conventional transformers typically interleave the laminations of the core at each corner to create an unbroken magnetic circuit. The fact that MOT's are built this way makes them wonderfully simple to disassemble. Once the bottom is removed the remaining wound part can be used as a powerful electromagnet, as a leakage-induction based ballst, or easily scrapped for copper or as a source of magnet wire. The windings can be removed intact and used with other cores or any number if other experiments.

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