Electronics > Beginners
EI375 steel laminate primary transformer calculation
The Electrician:
--- Quote from: gkmaia on August 05, 2019, 01:12:13 am ---So if you look at the table bellow you will see that I am using your calculations to generate the results I need to convert my primary from 120v to 240v using the same core.
That was the whole idea as I have one of those Fluke 8012 multimeters that is 120 and has not voltage switch. I have to rewind the transformer to get it working on 240.
--- End quote ---
It's too bad you didn't say from the beginning that you are simply rewinding an existing transformer to use 220V that uses 120V as is.
All you have to do is unwind the 120V primary (which you have apparently done) and count the turns. Then you rewind the primary with twice as many turns of wire having 1/2 the cross-sectional area. For example, if the 120V primary is wound with 36 gauge wire (measure it with a micrometer), then you rewind with twice the turns of 39 gauge wire. Wire cross-sectional area halves with an increase of 3 gauge sizes. Wire gauges that are odd numbers, like 39, can be difficult to find. :( If the transformer was conservatively designed and doesn't get very hot in use with 120V, you might get away with using a wire gauge 4 times larger--40 gauge in this example.
It appears that Fluke designed the transformer to be usable on 50 Hz as well as 60 Hz, and if you just double the primary turns, that will still be the case. That's probably why they used (apparently) more turns than your calculations say would be necessary.
You don't need to know the permeability of the laminations, or the turns per volt, etc.
Don't try to modify the surface of the laminations by spraying them with clear coat. If you do that, you won't be able to get all the laminations back in place.
Transformer steel laminations come from the factory with an insulating coating. That insulation can be as simple as steam oxide, or better, a coating of magnesium silicate (Carlite): https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=101
Also: https://www.aksteel.com/sites/default/files/2018-01/carlite201208_1.pdf'
If you are careful not to allow the laminations to scrape against one another much when you're disassembling the transformer, the insulating coating, whatever it is, will probably be good enough to just reassemble the laminations without doing anything more to provide an insulating coating. The only effect of a little degradation of the existing coating will be a slight increase in core loss.
gkmaia:
The Electrician, thanks for your answer.
Not that bad. I knew that I had to double the windings but I also wanted for a long time to understand better how cores are calculated. Although there is still lots to learn the time JWillis toke to share his knowledge was incredible.Lots os things I did not know before now make much more sense.
Also knowing the why is actually better than just doing it.
Jwillis:
The Electrician is correct about not really needing to coat the individual laminates. But if you do ,Use the paint sparingly.I sometimes will use a lite wash of epoxy mixed with denatured alcohol as I do a final assembly just to keep the laminates from vibrating. Then a brush over the outside of the core for aesthetics and vibration control. But this can be messy. There is proper transformer epoxy but It's to expensive for a simple hobbyist like me.
More turns is not a bad thing because of the increased Magnetic filed .https://en.wikipedia.org/wiki/Inductance Explains " inductance describes the tendency of an electrical conductor, such as coil, to oppose a change in the electric current through it. When an electric current flows through a conductor, it creates a magnetic field around that conductor. A changing current, in turn, creates a changing magnetic field. From Faraday's law of induction, any change in total magnetic field (magnetic flux) through a circuit induces an electromotive force (voltage) across that circuit, a phenomenon known as electromagnetic induction. From Lenz's law, this induced voltage, or "back EMF" in a circuit, will be in a direction so as to oppose the change in current which created it. So changes in current through a conductor will react back on the conductor itself through its magnetic field, creating a reverse voltage which will oppose any change to the current. Inductance,"
So having an increased magnetic field will oppose any change in current more.This keeps the transformer from heating up a little better.
Learning can be fun and Now you know more than most.I apologize for my crappy math but the formulas are correct. Just plug in the known variables and you can create any transformer you want.
The Electrician:
--- Quote from: Jwillis on August 08, 2019, 08:58:35 pm ---More turns is not a bad thing because of the increased Magnetic filed .https://en.wikipedia.org/wiki/Inductance Explains " inductance describes the tendency of an electrical conductor, such as coil, to oppose a change in the electric current through it. When an electric current flows through a conductor, it creates a magnetic field around that conductor. A changing current, in turn, creates a changing magnetic field. From Faraday's law of induction, any change in total magnetic field (magnetic flux) through a circuit induces an electromotive force (voltage) across that circuit, a phenomenon known as electromagnetic induction. From Lenz's law, this induced voltage, or "back EMF" in a circuit, will be in a direction so as to oppose the change in current which created it. So changes in current through a conductor will react back on the conductor itself through its magnetic field, creating a reverse voltage which will oppose any change to the current. Inductance,"
So having an increased magnetic field will oppose any change in current more.This keeps the transformer from heating up a little better.
--- End quote ---
More turns on the primary of a 50/60 Hz transformer does not give more flux in the core for the same applied primary voltage.
In another thread: https://www.eevblog.com/forum/beginners/tap-on-secondary-of-transformer/msg2377359/#msg2377359
you give the standard formula: N=(10000 * Vp) / (4.44 * f * B * A)
Doing some algebra we get: 4.44 * f * B * A = (10000 * Vp)/N
Which becomes B = (10000 * Vp)/(N * 4.44 * f * A), so if N increases, B decreases.
This is because the primary of a transformer is typically excited by a voltage source rather than a current source. In that case, we use Faraday's law to determine the flux in the core. More turns means fewer volt-seconds applied to the core, and thus, less flux results. With a constant voltage applied, more turns means higher inductance, which means higher reactance, which means less exciting current. Since the inductance increases as N^2, reactance increases as N^2. So if you double the primary turns, the inductance is 4 times larger and the exciting current is 1/4 as much. Doubling the primary turns would double the ampere-turns if the current were the same as before increasing the primary turns, but the current is only 1/4 as much, so the net result is half the flux in the core.
If the primary were excited by a constant current, then more turns would mean more ampere-turns, hence more flux.
soldar:
I have not calculated and designed a transformer in ages but back in the day I did quite a few and I believe this is the kind of thing that starts out looking quite complex and as you practice you realize it is much simpler than it might look at first sight.
I would start by selecting a standard core and bobbin by rough calculation or even by eye.
Then I would calculate the wire windings necessary for that core. If they fit in the bobbin I was done. If not then select a slightly larger core and bobbin and start over again calculating the wiring. It was quite simple and mechanical.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version