Greetings.
I have created a pair of modified transformers, but I am having trouble selecting a panel ammeter for the output because I cannot identify the secondary's maximum current. The transformers were originally GAL-700E-4 Microwave transformers but have had their secondaries removed and replaced with 10 turns of 10AWG battery cable. The input is 230-240Vac (from a UK plug socket). When plugged in and powered up, they make a real hum and the secondary output reads as 10Vac. My problem is that the transformers do not seem to have a datasheet available and therefore I cannot tell the gauge or number of turns in the primary, which is making it difficult to tell what the maximum current from the secondary would be. Would anyone with any experience with this sort of thing be able to give me some pointers?
Many thanks.
Hi,
As to the max primary current, if you can measure the wire diameter that would get you going. For a transformer like this I would think around 600 circular mils per amp would be a starting point. The max would be 300 cm/A but that's pushing it for a transformer like this.
What that gives you is the max current you should be using for operation for less than many 10 minutes because of the type of transformer. They usually have a high excitation current so the transformer would probably heat up. The fact that they hum a lot means high excitation current.
The real key to this is how hot the transformer gets and how fast. You can do some simple tests after you calculate that above. You would run it for the time you think you will need to run it for during use, and if it gets too hot you will just have to reduce the input current.
10 volts at 100 amps is 1000 watts, which will be about 4.4 amps input not including the excitation current, which you could measure with current meter. This means you should have a wire diameter of about AWG #16 or better, but with the excitation current maybe #14 which is rather thick wire.
Very short use runs however will not heat the transformer up much, as long as there is enough cool down time. Cool down time has to be at least 10 times the run time if you play to use it for a short run and they wait a while and use it for a 2nd, 3rd, etc., run.
The heat production comes mostly from the current in the primary and secondary due to the wire resistance, and that happens right away but still takes a while to heat up all that steel. The heat dissipation comes from the entire surface area and the type of cooling. Free air cooling does not work as well as forced air which would come from a high velocity fan. That means a fan could get you a lot more cooling which extends the continuous run capability.
The main key here is testing it. The more tests you do, the more you know about how well it will work. That's always the best way anyway, and in a commercial setting for some appliances they would be tested overnight at full load. There is insulation inside the transformers that can melt if it gets too hot, and it will drip out like thick goo. If you see any of that, it's of course no good, but before that you should just be measuring the temperature.
Now the output current is usually related to the input current by the turns ratio. If you have 200v in and 20v out that's 10 to 1, so the output current is normally 10 times the input current. Yours sounds like it would be more like 20 to 1. Unfortunately, the excitation current adds to the input current so it's more like Iout=(Iin-Iex)*20 but you can measure all that quite easily.
As someone else also said, you'll have to experiment with the welding technique as it may be a little different than when you use an actual welder. You might find some actual welder circuits online though so you can get a feel for what works well.
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