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Old 16th March 2010, 04:17 PM   #111
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Quote:
Originally Posted by bentl View Post
Hi,

I just triet to wind another transformer, after having rebuilt my winding jig with a stepmotor and a control that stops the motor at a preset count.

Interestingly, I ran into the exact same problem as with the smaller core; I went out of winding capacity on the coilformer.

The goal was to have 70 primary turns, 8.750 secondary turns for a 1:125 step-up ratio.

I used 0.71mm for the primary witch was exactly 1 layer, then insulation and then I had room for about 4200 turns of 0.25mm before the coilformer was 105% full.

So I ask again; is there a formula or table that can give a gestimate on how much wire a coilformer can hold?

Regards

bent

Hello Bent,

Can you tell us some of the dimensions of your transformer?
1) What is the core area?
2) What are the dimensions of the coil former? (length, available winding height)

Also, it would help those trying to give advice if we knew:
1) Do you want to use your transformer for full range, or hybrid use. If hybrid, what crossover frequency.
2) What is the capacitance of your panel? If you aren't sure, knowing the size and diaphragm to stator spacing should be enough for a good estimate.

Depending on the core size and your intended use, you may be able to use fewer primary turns which would require proportionally fewer secondary turns. I would also suggest with that large number of secondary turns you should use insulation between each layer of wire, or wind the secondary in multiple sections like Statics Man did(see post #59 in this thread). Either of these methods will help reduce the winding capacitance.
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Old 16th March 2010, 08:10 PM   #112
bentl is offline bentl  Norway
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Hi,

regarding the panels:
Dimensions: 1,0 X 0,35 meters
D/S distance: 2mm

Should give a capacitance of 7.011 * 10^-10 farad

I intend to first try the transformers at full range, and then cut the low frequency with a crossover if necessary.

The transformers I have available have the following core sizes:

Small: EI84, 29X29mm stacking height 40mm - coulde be doubled to 80 mm.
40mm holds about 4400 turns of 0.25mm wire. Window height is 14mm - material thickness = 12mm

Big: EI120, 40X40mm stack height 60mm, could also be made 40X20mm height 120mm
Window height is 20mm - material thickness = 18mm

The coilformers I make to measure as I have access to a cnc router and all sorts of plastic.

A double stacked core would possibly allow for sectioning the secondary windings.

Regards,
Bent
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Old 17th March 2010, 03:45 PM   #113
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Default Transformer Core & Winding Dimensions

Quote:
Originally Posted by bentl View Post
Hi,

regarding the panels:
Dimensions: 1,0 X 0,35 meters
D/S distance: 2mm

Should give a capacitance of 7.011 * 10^-10 farad

I intend to first try the transformers at full range, and then cut the low frequency with a crossover if necessary.

The transformers I have available have the following core sizes:

Small: EI84, 29X29mm stacking height 40mm - coulde be doubled to 80 mm.
40mm holds about 4400 turns of 0.25mm wire. Window height is 14mm - material thickness = 12mm

Big: EI120, 40X40mm stack height 60mm, could also be made 40X20mm height 120mm
Window height is 20mm - material thickness = 18mm

The coilformers I make to measure as I have access to a cnc router and all sorts of plastic.

A double stacked core would possibly allow for sectioning the secondary windings.

Regards,
Bent
To check understanding of dimensions, see attached picture of transformer core.
The descriptive names for the dimensions are the terms I have heard used.

Small:
A= 40mm (window width)
B= 14mm (window height)
C= 29mm (tongue width)
D= 29mm (stack height)
Core Area = C x D = 841mm^2 = 8.41 cm^2
Winding Height = B - (bobbin material thickness) = 12mm
Winding Width = A - (bobbin material thickness x 2) = 36mm

Big:
A= 60mm (window width)
B= 20mm (window height)
C= 40mm (tongue width)
D= 40mm (stack height)
Core Area = C x D = 1600mm^2 = 16.00 cm^2
Winding Height = B - (bobbin material thickness) = 18mm
Winding Width = A - (bobbin material thickness x 2) = 56mm
Attached Images
File Type: jpg core_dims.jpg (8.6 KB, 411 views)
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Old 17th March 2010, 04:02 PM   #114
AndrewT is offline AndrewT  Scotland
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round wire wound very neatly, to minimise air area, will be somewhere around 60% to 70% copper.
Wound untidily and the copper could drop below 50% of the window area.
Layers of insulation film will take up some of your window area.
12mm * 36mm = 432sqmm.
4400 turns of 0.25 overall diameter insulated copper wire is 216sqmm.
you have less than 50% of insulated copper in the window. Take off the insulation of thin wire and that percentage will get even smaller.
Go down to 0.1mm diameter copper and your numbers will be considerably worse.

Theoretically, infinitely neat winding with infinitely thin insulation can get to just less than 78.5% copper. You can never approach that.
__________________
regards Andrew T.

Last edited by AndrewT; 17th March 2010 at 04:08 PM.
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Old 17th March 2010, 05:39 PM   #115
bentl is offline bentl  Norway
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Hi,

how about if I cleaved the laminations in half; reducing D by 50% and increased A with 100%

The core cross-section will be halved, but the winding capacity will be doubled and the volume of the core is unchanged.

Will the power handling/saturation limit be halved as well?

Regards,

Bent
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Old 17th March 2010, 07:46 PM   #116
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Quote:
Originally Posted by bentl View Post
Hi,

how about if I cleaved the laminations in half; reducing D by 50% and increased A with 100%

The core cross-section will be halved, but the winding capacity will be doubled and the volume of the core is unchanged.

Will the power handling/saturation limit be halved as well?

Regards,

Bent
I think what you are suggesting is to use just the E sections of the laminations and put them face to face as shown in the attached pic.
This would give you a bobbin shape more like that seen with C-core transformers.
This may have advantages if you add laminations to keep the original stack height.

For the case you described, since your core area (cross-section) is halved because D is halved, your power handling/saturation limit would be halved. You could easily compensate by doubling the number of primary turns since you have twice the winding width. But, capacitance between windings would increase unless you double the thickness of insulation between sections. Also, leakage inductance increases by the square of the number of turns. This is partially compensated by the longer winding which reduces the leakage inductance.

Core Sugguestion
If you can make your own bobbins, I might suggest an alternative. Many audio transformer designers increase power handling by simply increasing the stack height of the standard EI core. For instance increasing D from 40mm to 60mm. This increases the core area by 50%. You could then reduce your number of primary windings from 70 to 47. Your secondary turns could then be reduced from 8400 to 5640. This will make it easier to fit the turns into your 18mm winding height.

Winding space difficulties
As others have mentioned, your difficulty in fitting all the turns on to the bobbin when simply height and width calculations make it look like it should be easy are probably due to:
1) Not winding perfectly smooth layers with each wire flush with the adjacent wires and no crossed wires. Winding perfect tight layers with small wire takes practice and patience. In the beginning it might take you 30 minutes to do a single 200 turn layer.
2) Swelling on the flat sides of the bobbin. As you wind, you will notice the more layers you put on the more the shape loses its flat sides as the windings swell to form a more cylindrical shape. A general rule of thumb is that you can lose 10%-20% of winding height due to swelling.
3) If your winding is sloppy, or random, you quickly lose precious winding volume on the bobbin...easily eating up 50% or more of the available space.


Leakage Inductance
In case you have not found them before, there are some formulas to estimate leakage inductance. Here is one I have found to work well.

L(leakage) = [0.417 x Np^2 x TL x ((2 x n x c ) + a)] / [1e9 x n^2 x b]

Lleakage = leakage inductance in Henrys
Np = primary turns
TL = average turn length around bobbin
n = number of dielectric junctions between Primary & secondary windings.
c = thickness of the dielectric between winding sections
a = total winding height on the bobbin
b = width of windings on the bobbin
(all dimensions in mm)

Notice that leakage inductance is reduced by the square of the number of interleaved sections.


Interleaving & insulation suggestions
With a panel capacitance of 700pf (probably closer to 800pF once you account for dielectric of the spacers) and your winding capacitance will probably be another 500pF - 800pF you will be driving a total capacitive load of around 1200pF - 1600pF. With this in mind, and the goal of a step-up ratio of 120 you will need to keep your leakage inductance below 3uH(40mH reflected to secondary) if you want to keep the transformer bandwidth above 20kHz. This is a tough task and will require at least 3 to 4 interleaved sections. For a first attempt you might try 3 primary layers in parallel. One at the bottom of the secondary winding, one in the middle, and one on the top. To keep winding capacitance down, you will need to put some insulation between each layer of the secondary. I would suggest 0.1mm or so. You will probably want double that (0.2mm) between each primary-to-secondary section breaks.


What Others Have Done
If you look at the step up transformers for most full range ESLs (Audiostatic, QUAD, Final, Capaciti, etc) you will see that instead of one transformer, they have used two(Audiostatic, QUAD ESL63, Final) or perhaps two sections of windings on separate legs of the transformer core(Capaciti, Audio4). Instead of trying to build one transformer with a step up ratio of 120 and full audio bandwidth, you will find it easier to wind two transformers with step up ratios of 60 and put their primaries in parallel and secondaries in series to get the desired 120 ratio.
Attached Images
File Type: jpg Core-doubleE.jpg (5.0 KB, 395 views)

Last edited by bolserst; 17th March 2010 at 08:09 PM.
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Old 24th March 2010, 04:11 AM   #117
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I finished my bais supply and tried stacking transformers.
Works great!
I can get fullrange at a low level and down to 200hz with 18 turns (1:77 ratio) and 400hz with 9 turns (1:154 ratio)at full power.heck I have ten of these things tomorrow I'll stack 1 or 2 more.I'm hoping 3 will get me full range at full power (100watts) but 4 will defintely do it.
Also just as I suspected the ratios of the two identical transformers were not the same.
As much as 10% difference was found between the two of them.
Luckiley all of the windings are seperated from each other and could be wired with a balanced eqeal output per stator.
Attached Images
File Type: jpg original transformer setou.jpg (60.7 KB, 367 views)
File Type: jpg new transformer setup.jpg (54.1 KB, 368 views)
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Old 24th March 2010, 07:18 AM   #118
bentl is offline bentl  Norway
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Hi geraldfryjr!

Great job on those toroids - I never thought of doing that with toroids.

I have just completed 1 transformer based on EI120 laminations; core size 40mm X 20mm double stacked so the stack height is 126 mm

Windings are
45 turns X 3 in parallel on primary side
1100 X 2 X 3 in series (1100 turns of double wire in 3 sections)

Step up ratio 1:146

I made the coilformer out of 2mm polypropylene; my girlfriend made made all the parts on the cnc router and I just had to glue all parts together.

I have not tested it yet, I am waiting for my new oscilloscope and some terminals to connect all the wiring to.

Regards
Bent
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Old 24th March 2010, 12:45 PM   #119
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bentl,can't wait to see your results! last night i found a supplier for some raw toiridal cores. Toroidal Cores Stock I'm going to give them a call later.Their prices seem very reasonable compared to a compleatly wound core.
I don't mind winding one if it is big enough to get the spool of wire through the center.
You know bigger is better.
pn#120 for $22.52 at 1867VA come on! Maybe something even bigger.
I guess I have to lay out some guidelines because the sky is unlimited now. jer
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Old 24th March 2010, 01:23 PM   #120
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And here you can see a DIY toroid winding machine:

The Water Fuel Cell :: View topic - Automatic Toroid Winder
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