Author Topic: Noob plays with transformers  (Read 7591 times)

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Offline rebelrider.mikeTopic starter

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Re: Noob plays with transformers
« Reply #25 on: January 11, 2019, 10:37:44 am »
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.
 

Online Ian.M

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Re: Noob plays with transformers
« Reply #26 on: January 11, 2019, 12:00:41 pm »
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.

 

Offline rebelrider.mikeTopic starter

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Re: Noob plays with transformers
« Reply #27 on: January 12, 2019, 04:49:52 am »
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!
 

Offline rebelrider.mikeTopic starter

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Re: Noob plays with transformers
« Reply #28 on: November 06, 2019, 03:38:39 pm »
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.
 

Offline rebelrider.mikeTopic starter

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Re: Noob plays with transformers
« Reply #29 on: November 14, 2019, 09:11:22 pm »
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...
 

Offline soldar

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Re: Noob plays with transformers
« Reply #30 on: November 14, 2019, 10:23:15 pm »
Well, I'm no further trying to predict how much power an unknown transformer core can handle.

I know you are having fun and I don't want to spoil it but "an unknown transformer core" can "handle" no definite amount of power because the core does not "handle" power. I understand what you are trying to get to but you are looking at the problem from the wrong angle.

The entire transformer handles power and you cannot really put a theoretical limit on what a core can handle and it depends also on what you mean by "handle". 

A separate primary and secondary are very different from an auto-transformer and the power they can handle will be different.

But, the way I look at this problem is the other way around. It is not "how much power can the core handle?" because I look at it more as the conductors being the ones that handle the power so I ask more "how much power can the conductors I can fit in that core handle?"

When designing transformers I would start out with a core and bobbin chosen almost by eye and then design the coils. If the coils did not fit in the bobbin I would switch to the next standard size up and start over again.

As you already know, Al wire will handle less power than copper wire. If you could get a wire with much less resistance than copper the same core could handle much more power.

So, you have a core, you need to determine how many turns you need, then what diameter wire will fit with those turns and the copper resistance will be what mostly determines the power it can handle.

So, you have a core and bobbin and want to build a 1:1 transformer. You can either measure the characteristics of the core or just assume a safe, low value.

If you give us the dimensions of core and bobbin I can take a stab at it.

If you can measure the properties of the core that would be even better. Assemble the core with, say 100 or 200 turns and measure inductance. at mains frequency by putting in series with a light bulb.

You want to determine how many turns you need to have a suitably small magnetizing current. Once you determine that you just put that number of turns with wire as thick as will fit.
« Last Edit: November 15, 2019, 01:13:44 am by soldar »
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Offline Cliff Matthews

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Re: Noob plays with transformers
« Reply #31 on: November 15, 2019, 01:26:53 am »
Careful with bolt isolation. I had some 500VA hot bolt issues and got it sorted-out with Kapton tape. Good luck!
 

Offline exe

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Re: Noob plays with transformers
« Reply #32 on: November 15, 2019, 08:08:29 am »
I think what limits power handling is rise of temperature. As long as a tranny remains within the temperature limit it should be fine. Afaik Rod Elliot wrote that he could get more from trannies by forced cooling: https://sound-au.com/xfmr2.htm?

> Fan cooling can increase the effective VA rating of a transformer significantly, but does not improve regulation.  Large power distribution transformers are almost always oil cooled, and they are now starting to use vegetable oils because they are less inclined to catch on fire, and pose minimal environmental impact should there be a coolant leak or other major fault.


Having said that, by looking at the size of your tranny, it should have a better regulation, more than 80, imho (you can see some figures for similar-sized trannies to get a rough idea how much it can take). So, this makes me thinking you pushed it well beyond what it was designed for.

Also, due to high mass the temperature may raise quite slowly. This may give a false impression that it can sustain current load. Don't get fooled by this, in a real device the tranny is unlikely to be in open air, so cooling will be much worse. Also, I suggest doing performance testing for at least half an hour with closely monitoring the temperature. Trannies are good at handling short overloads, but it in the long run they may tend to overheat if constantly abused.
 

Offline The Electrician

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Re: Noob plays with transformers
« Reply #33 on: November 15, 2019, 11:31:41 am »
 

Online Zero999

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Re: Noob plays with transformers
« Reply #34 on: November 17, 2019, 03:03:46 pm »
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.
Well if you've gut the core size, it might be saturating slightly which will increase losses. If the voltage drop is an issue, then wind extra turns on the secondary, but accept the off-load voltage will be a little on the high side. The output voltage of transformers is normally specified at full load, so the off-load voltage will be significantly higher, but you might want to go for 5% higher than desired voltage off-load and 95% of the desired voltage on-load.

One thing to note is that auto-transformers can be much smaller than isolating transformers, given the same power rating. An auto-transformer with a 2:1 ratio will have half the copper and iron of an isolation transformer of the same power rating. If your transformer is rated to 1kVA, then it'll be rated to no more than 500VA, when re-wound as an isolating transformer. Indeed, it's possible to convert an isolating transformer into a 2:1 auto-transformer, with double the power rating, just by connecting the windings in series. Think about a 1:1 120V:120V transformer, with the secondary winding connected in series with the primary winding. Assuming the phasing is correct, the voltage on the secondary will be double the primary voltage. This is how auto-transformers work.
 

Offline soldar

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Re: Noob plays with transformers
« Reply #35 on: November 17, 2019, 04:11:47 pm »
You want to make a 1:1 isolation transformer with a core and bobbin you already have. 120V, 60 Hz I assume. Here is what I would do:

First of all measure the characteristics of the core by making a coil with a number of turns, say 100, and putting a 60 Hz current through it. Measure voltage and current and determine L value for 100 turns. L is proportional to the square of the number of turns so 200 turns would have 4 times higher L.  That gives us the characteristics of the core we need.

Now measure the window of the bobbin, the space where the wire will go. Of the total area let us subtract 10% as safety margin, for insulation tape, etc.

The secondary will have, say, 10% more turns than the primary (and several taps). I assume primary and secondary will have same diameter wire. So, of the total bobbin window area Wa, the primary gets 43% and the secondary 47%.

Now you have an exact window area to work with. How many turns of wire you can fit depends on the wire diameter. Bigger wire, fewer turns. Assume a diameter and work out the number of turns you can fit in the primary allocated area. Work out the inductance and the resistance. Then the magnetizing current and power dissipation in the primary. If not enough inductance then you need more turns of thinner wire.

This way you are using all the bobbin space to the best advantage.

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Offline GeorgeOfTheJungle

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Re: Noob plays with transformers
« Reply #36 on: November 17, 2019, 04:53:31 pm »
I think that's why there's about 2.5A being used even without load.

Yep. That means the core is entering saturation. 120 times a second. Not good! You have to either put back the irons, or add turns to the primary. The no load losses ought to be max. 5% (less is better) of the transformer rated power. Connect it unloaded to a variac and decrease input voltage until you're at less than that 5% figure, then do the math: if for example rated Vin is 120 and you've got to set the variac to, say, 80V, then the number of turns you need in the primary is 120/80 => 1.5x the turns it's got now.
« Last Edit: November 17, 2019, 05:23:05 pm by GeorgeOfTheJungle »
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Offline soldar

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Re: Noob plays with transformers
« Reply #37 on: November 17, 2019, 06:44:12 pm »
Don't design it for exactly 120 V but give it at least 10% margin.  One of the first transformers I designed, ages ago, was so tight that it would easily saturate and heat up. When the standard was raised from 220 to 230 V that was the end of it.

Note also that load current is irrelevant to saturation. Only the magnetizing current counts and the magnetization does not change from no load to great load.

So, basically, you need to design a primary that under no load has enough inductance to keep the current well inside the working park of the curve and not reach saturation. If that condition is met then the core will not saturate under load.
 
For a one time project like this my advice is to give wide margins in every step. You might not get the absolute last drop of power out of the thing but you can be sure it will work reliably and safely at the first try.

I have had to re-wind more than one transformer because I was too optimistic in the amount of copper wire I could fit in the bobbin.
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Offline rebelrider.mikeTopic starter

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Re: Noob plays with transformers
« Reply #38 on: November 25, 2019, 09:24:34 am »
Wow, thanks for all the replies! I guess I'm not getting email notifications, because I just now saw them.

I really appreciate the explanation between the isolation and auto transformers. That makes sense.

Some of your responses have a lot of info that I'll have to go back an re-read a couple times to make sure I get it all straight.

So the core's ability to handle the power going through it is really only dependent on how many turns you can physically fit inside... I think I get that.

Looking at the equation I've been using: T/V = 1 / 4.44 x 0.0001 x CA x B x f  the turns per Volt will go down as the core size goes up since it's 1/CA (and all the other stuff). So either more turns are needed, or there needs to be more metal in the core. Seems the problem with CA (the core cross section area) is that it seems to assume almost no distance in between the plates for insulation. I wonder which is worse: eddy-currents from lack of insulation, or not enough core material? I've already decided to thin out the insulation I put on the plates so I can fit more in.

I figured there would be some Voltage drop anyway, and even if it were to drop to 115V, I think that anything I'm using if for would be fine. I like the tip on how to estimate how much wire will actually fit in the transformer, based on the cavity size.

I don't have a Variac, but I do have that little transformer that has lots of taps. Anyway, you all gave me lots to think about! Hopefully I'll return with good news. :)
 


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