nice to see that the best EI material can be run as high as 1.5T
And that square stacks ( the central core of the EI is near square) are the way to build right.
1.5T on M6 or M3 cores are fine as long as you don't run your traffo to saturation under load...
since traffo primary has a fixed number of primary turns and a fixed idle current, ampere-turns increases with load and with it the flux...proceed with caution....
since traffo primary has a fixed number of primary turns and a fixed idle current, ampere-turns increases with load and with it the flux...proceed with caution....
EI transformer question
Hi All
I want design transformer of 250 watt I have EI core bobbin size 4.5cm*3.8cm how to select secondary wire gauge if i want 30 0 30 6A on output of transformer.can consider gauge for 6A or 3A.If I consider 6A 60v*6A=360W or 30V*6A=180w. please help.
Hi All
I want design transformer of 250 watt I have EI core bobbin size 4.5cm*3.8cm how to select secondary wire gauge if i want 30 0 30 6A on output of transformer.can consider gauge for 6A or 3A.If I consider 6A 60v*6A=360W or 30V*6A=180w. please help.
I think you should be designing your primary FIRST !
When you have that part of the design completed and tested, you will find that the secondary is easy.
If you aim for a target efficiency of 94%, then the 250VA (output at the secondary) transformer needs a primary that takes 250/0.94 = 266VA.
For a supply voltage of 230Vac you need the primary to pass 1.16Aac
@ 31.A/mm² you need a cross section of 0.373mm²
0.7mm diameter copper gives 0.3848mm²
How many turns of that wire will fit into half your window area?
Will those turns give sufficient inductance to operate at a sufficiently low core flux without saturating?
When you have that part of the design completed and tested, you will find that the secondary is easy.
If you aim for a target efficiency of 94%, then the 250VA (output at the secondary) transformer needs a primary that takes 250/0.94 = 266VA.
For a supply voltage of 230Vac you need the primary to pass 1.16Aac
@ 31.A/mm² you need a cross section of 0.373mm²
0.7mm diameter copper gives 0.3848mm²
How many turns of that wire will fit into half your window area?
Will those turns give sufficient inductance to operate at a sufficiently low core flux without saturating?
Modern transformers you think are toroidal with higher iron performance, much lower wire length, lower thickness of winding and higher effective cooling area, this because higher current density is allowed while they also perform better.
india uses 230 volts 50hZ right?
so:
1 3/8 x 1 3/4 inches good for 198 va....
primary is 720 turns of # 24 awg wire,
secondary is bifilar 94 turns of #19 awg wire...
note that in india, british swg is standard,
so make your adjustments accordingly...
scrapless cores are assumed....
Sorry but this method is basically wrong.
There are 2 strategies, designing for allowable temperature, and designing for given drop.
(Allowable) current density should be calculated from trafo geometry, Bmax and allowable temperature rise. These 3 parameters also determines maximum power rating, based on this the apropriate core can be selected. From geometry and Bmax V/turn can be calculated, hence turn nr is approximately given. Wire cross section area must be chosen to fit window evenly. Now you can calculate wire resistance, drop. If it is acceptable, then you can adjust secondary number of turns according to drop. Otherwise a bigger core is needed.
Design for drop: for a given core (geometry and Bmax) drop is given, proportional to power. You can chose a core which is apropriate in these aspect. Calculate number of turns and wire length! Based on length and resistance(calculated from drop) a wire cross section can be calculated. Done.
Tables with permissible power and drop should be provided by core manufacturer, but can also be calculated with more or less precision.
We at least agree that the primary has to be designed first.
AJT got it.
The EI core is already fixed, he does not need to find a different EI.
what's the difference between my statement and
One has to get the primary ampere turns right before one can proceed.
AJT got it.
design the primary first. But 24awg is a bit thin for 266VA primary. 21 or 21½awg suits better if 720 turns fits 50% of the window.
The EI core is already fixed, he does not need to find a different EI.
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Please forget ampere turns! I didn't tell anything about it, for a good reason. You don't need to calculate magnetic force.
If you mean just turns, then my answer is: 1 reason: nr. of turns can be calculated easily, no need for guessing current density. V=4.4*n*A*Bmax*f. (SI units.) 2nd reason: do you know what sufficient inductance is? Generally it is unknown. Depends on too many factors, and basically unspecified. You already experienced recently that it depends also on measuring voltage.
And your method is unfinished. You told 94 % efficiency, but I can't see how do you plan to check if it is achieved, or even viable. What if there is not enough turns fit in? What if inductance is "high enough"? Can you say you found the correct value? No, because resistance can be too high. Probably it will be in this case too. You said core is chosen, but you didn't use any parameter of it. How can you tell if it's appropriate or not?
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Andrew!
You said core is chosen, but you didn't use any parameter of it. How can you tell if it's appropriate or not?
BTW:
You didn't calculate ampere*turns, neither AJT.
Bmax is about 1.1 Tesla in that table.
Magnetising current is not increasing with load. (Actually it is decreasing a little.) When you load a trafo, primary and secondary magnetising forces are pointing to the opposite direction, and magnitudes are almost perfectly equal. Magnetic balance makes a trafo: trafo. This is because it is so sensitive for DC. A very small current is enough to go saturation (and for DC there is no cancellation on the other side). If magnetic effect of primary current wasn't cancelled by secondary current (multiplied by turns of course) in the core, then a tiny load would have drived it to saturation.
You said core is chosen, but you didn't use any parameter of it. How can you tell if it's appropriate or not?
BTW:
You didn't calculate ampere*turns, neither AJT.
Bmax is about 1.1 Tesla in that table.
Magnetising current is not increasing with load. (Actually it is decreasing a little.) When you load a trafo, primary and secondary magnetising forces are pointing to the opposite direction, and magnitudes are almost perfectly equal. Magnetic balance makes a trafo: trafo. This is because it is so sensitive for DC. A very small current is enough to go saturation (and for DC there is no cancellation on the other side). If magnetic effect of primary current wasn't cancelled by secondary current (multiplied by turns of course) in the core, then a tiny load would have drived it to saturation.
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agreed, for me it is temperature rise that is priority, coming from a very warm country.....
if i do not know the source and therefore the class of core, i just assume it to to be lowest grade, H50....
magnetizing current is not increasing with load, the primary ampere-turns is...
here is my recently built traffo using H50 cores, no load testing showed 60mA no load current,
and 15 watts of core loss, and with 9kgs of core material, that is 1.6 watts per kilogram...
with an effective radiating surface of about 59 sq inches, this traffo is running cool...
core is a 1 3/4 inch center leg with 4 inch stack....
will do short circuit testing shortly....
Attachments
This is true, but flux is determined by the sum of primary and secondary ampere*turns (=magnetising force = magnetising ampere*turns). (Note: pimary and secondary currents flow the opposite direction.) A core doesn't care which is primary which is secondary, current is current.
But actually we can't measure current at such high precision that is needed for telling flux. Instead we can tell flux from the fact that it induces voltage. Induced voltage equals to no-load voltage * (1 - primary drop), so (main) flux must also decreased with (resistive) load.
Looks nice!
and i guess there is no point to being too precise, being in the ball park is good enough for me......😉
i use the clear air drying polyurethane varnish where i dip(submerge)
the traffo for about 15 minutes, then pull out to drip, then back in again in a
process that takes almost an hour....
then under the hot manila sun to dry, takes about a week...
sun baking, these traffos get hot to about less than 50*C....
He already stated the EI core size.
And It gets him into the ballpark.
AJT appears to have used that core size to arrive at his suggested 720turns.
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