Electronics > Projects, Designs, and Technical Stuff
Noob plays with transformers
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rebelrider.mike:
Hi folks! It's been a long while, but I haven't given up yet. I've been re-rusting my plates, and though it's not as easy as YouTube suggested, it's still going well.

I got all the plates cleaned with acetone, etched in white vinegar, and the rusting process has been kick-started with hydrogen peroxide and salt.


It's not as uniform as I had hoped, maybe because I didn't get all the coating off like I thought I did. But they're still rusting quite nicely, and boiling them is definitely converting the orange rust into black rust.


I've found that now the rusting process has begun, I only need to let them set out for a few hours, and the bare spots start to rust on its own, and can be re-boiled. Here's a few that are part way through the blackening process:


At this point, it's taking longer because I don't have the time to sit and watch the "soup" boil more than once every couple days. But more than half the plates are just about done.
Ian.M:
You are getting there, so you might as well carry on, but IMHO zinc parkerising would have been easier as its a one step hot bath, rather than the rust, convert, and repeat ad-nauseum black oxide process you are using.

rebelrider.mike:
First 11 plates are done! I think they turned out well:


They got a light sanding and a thin coat of oil. Only 183 left!
rebelrider.mike:
Wow, my last post on this thread was in January! Where has the time gone? Oh yeah, other projects.

Anyway, I guess I never told you all that I finally got the plates done. That was a while ago. But I've been stuck trying to decide whether to reuse the aluminum wire, or buy new copper wire. The aluminum has the advantage of being free, but I'd have a much lower power rating on the transformer when finished. New copper would give me a much higher maximum power, but I have to buy it. With limited funds, and multiple projects competing for money, it was actually something I had to think about for a while.

Finally decided on new copper. Not only that, but also a new tool to help me keep track of the number of turns. But first I had to get the old wire back off.


I picked 16 AWG which will give me a maximum power of 360W. Good thing I didn't go any bigger, as the coils just barely fit. Here's the counting machine I got with the spool adapted to it.


The counter machine is supposed to let you do winding faster with the little handle on the side, but 16 AWG wire is really stiff, so I wound up (no pun intended) just turning the spool by hand, and using the counter to keep track. It's actually very accurate.

167 turns later, the first coil is done.


I used the original insulator between the coils, and did another 167 turns on the second coil. It also has a center tap at turn 84, because the box it goes in has three holes in one side, so may as well. Gotta remember though that the center tap will be 60V, and at 3A, that means it's only good to 180W, as opposed to the 360W using the end taps.


I did a few maths before and after I started. Knowing the circumference of the spool, I estimated that I'd use about 124 feet of wire. That includes about 10% extra for good luck. Not knowing the circumference of the finished primary coil, I took my best guess that 167 feet of wire would be enough. Adding more extra, I figured a 300 foot spool would cover it. Seems everyone sells copper wire by the pound instead of the foot. So I got 2.5 lbs. of wire estimated at 315 feet. It all worked out, as i have a fair amount left over.

Later, I did more calcs and figured that at 124 feet, my primary coil should have about 0.5 Ohms from end to end. That is what I got on my multi-meter. Measuring the resistance of the secondary coil, I got 0.6 Ohms. That roughly translated back to around 150 feet. Or about 120% of the length of the primary. So I over estimated how much wire I'd need by quite a lot, but better to have more than enough than less.
rebelrider.mike:
Well, I'm no further trying to predict how much power an unknown transformer core can handle. This transformer was rated at 1000W. One of the tricks I picked up was to square the surface area of the tongue cross section. In this case 22.44 sqcm x 22.44 sqcm = 503W. Half of the original rating. The other trick I found was to weigh the transformer. They are supposedly able to handle about 40W/kg. Mine weighed in at 4 kg, so 160W.


Moving along though, I put the plates back on the spool, and wired in a 12A braker, a mains Watt meter, and a 250W lightbulb for the load.




I also checked beforehand the various continuities and resistances to make sure nothing weird was happening. Everything was in spec.


So here are the results:
It was loud, but didn't trip the breaker.
121V in vs. 119V out, without load.
Under load, the input was 119V, 3.58A, 426W.
Output was 115.5V, 2A, 231W.


With Watts in / Watts out * 100 to measure efficiency, I've got a whopping 54.2% Impressive, no? No.


Here's what I think is wrong. In my enthusiasm to coat the plates with oxide, I think I made the coating too thick. It's certainly effective. The bare plates are almost no resistance, and the coated plates are thousands of Ohms. But not all the plates fit back in. I think I'm going to have to thin down the coating on all the plates until they do. Not enough iron mass in the core means the flux magnetic thing is wrong and it won't absorb enough energy from the mains current. I think that's why there's about 2.5A being used even without load.


Time to take it apart and do some sanding...
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