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How to calculate Toroidal Core Maximum VA capacity

  1. Jun 22, 2016 #1
  2. jcsd
  3. Jun 27, 2016 #2
    Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
  4. Jun 27, 2016 #3


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    What are the specs of the transformer you want to wind on this core? What frequency? What input and output characteristics? What gauge wire(s) do you plan to use for Primary and Secondary?

    Are you using the toroidal core for noise reasons? How will you wind the Primary and Secondary? Will they be sector wound, or full-circumfurential (bifilar?)?
  5. Jun 28, 2016 #4
    My line voltage is 230VAC/50Hz I wanted single phase step down transformer to make from this core. But how to determine Core maximum VA capacity. is it 100VA or 200VA how to calculate that. Is Core wet is important in calculating VA .This core is M4 CRGO.Or I have to first wind Primary winding but how to test core maximum VA through that please help.
  6. Jun 28, 2016 #5


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  7. Jun 28, 2016 #6


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    Staff: Mentor

    Why are you wanting to use a toroidal core for an AC Mains step-down transformer? And what experience do you have working with AC Mains projects? What safety considerations are you keeping in mind in this project? What will the transformer be used for?
  8. Jun 28, 2016 #7

    jim hardy

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    anorlunda's link looks to me like a good one.

    As you read through it
    here's the direction you need to be heading -

    1. How many turns of wire must you have to in order for your core to support the voltage you want?
    That depends on the cross sectional area of the core. I cannot read that test you posted, too blurry and small.
    Look for a volts per turn rating.

    2. Your toroid has area of X in which you can place those turns of wire.
    They call it 'Window Area 'in transformer circles.
    What is maximum size wire of which you can physically fit your required number of turns into that area ?

    Ampacity of that wire X voltage your core can support is your maximum VA.
  9. Jun 28, 2016 #8
    • Because I wanted to build toroidal transformer.If toroidal core is used Transformer wet is much lower than same VA EI transformer And low humming high efficiency.I make few mains EI transformer and they working good And I lived without any accident. want to use toroidal transformer for amplifier.Can you help me now.
  10. Jun 28, 2016 #9
  11. Jun 29, 2016 #10
    Dear hardy sir
    • You have to zoom that image . This report says 3.30 Turn per volt for 1.5T. suppose we assume 3 turn per volt so 230*3=690 primary turn is required but how to calculate window area of toroidal core and how to chose wire diameter for primary if I chose 0.80A(dimeter-0.67mm,used-750 kcmil/A) supported wire size for primary. Than input VA of transformer is 184VA.Can I use this wire if it fits in Half Window Area of toroidal core without considerations of Core VA capacity.
  12. Jun 29, 2016 #11

    jim hardy

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    Why do you think the core cares how many VA it is transporting ?
    Flux is proportional to magnetizing current not load current.

    Its VA capacity is limited by how much wire you can fit through the window. The bigger the core cross section the fewer turns you need.

    That's your constraint - does it fit ? Will the winding run cool enough ?

    I'd go a little under 1.5T not over. Look at the magnetizing currrent !

    230 X 3.3 = 759
    and do you really have 230 V or closer to 240 ?
    Last edited: Jun 29, 2016
  13. Jul 1, 2016 #12
    Dear Hardy sir

    I found following link.They calculate Maximum power handling capacity of toroidal core.as bellow

    VA=5.0*J*Bm*f*Ac min*ID²*10-7

    VA =Power handling capacity (VA)
    J =Current density (A/ mm2)
    Bm =Maximum induction ( Tesla)
    f =Frequency ( Hz)
    ID =Inside diameter (mm)
    Ac min =Effective cross sectional area (mm2)
    If I enter My core values in that equation result is as follows.

    VA= (5.0*3.1*1.5*50*1000*3600)*0.0000001=418.5
    Is that means I can make toroidal transformer up to 418 VA.this website has all information about toroidal core calculation.

    EILOR MAGNETIC CORES - Calculation Data
  14. Jul 1, 2016 #13

    jim hardy

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    There you go - Well Done !
    That's a practical page !

    I do a reasonableness check on formulas before i trust them

    Let's see here
    J X ID2 tells how many amps i can get through the window for a given current density, which will define amps X turns
    Bm X Acmin tells how much flux i can get through the core , which when multiplied by frequency defines the volts per turn

    amps X turns X volts/turn = VA
    so the form of the equation passes my sanity check.
    They collected their constants in those two numbers, 5.0 & 10-7
    and i didnt check them. This is just a sanity check after all .


    I see where you got every number except the 3.1 for J
    and i got the same result as you when i multiplied

    so yes i think your 418.5 VA is the capacity of that core at 3.1 amps/mm2
    418 VA / 230V = 1.82 amps / 3.1 = 0.59mm2 wire , approximately #19 which seems reasonable
    Be aware it might take considerable patience to get enough turns on it when winding by hand instead of using a machine that lays them in neatly
    but it's really instructive to wind a core and run a saturation curve on it - plot magnetizing current versus volts per turn.

    I'm very much impressed with that Eilor tutorial you found. NIce Job !

    Have Fun . We learn 10X better by actually doing things than by just reading about doing them.

    Keep us posted ?

    old jim
    Last edited: Jul 1, 2016
  15. Jul 2, 2016 #14
    Hello hardy sir
    • Thanks for your replay and all your suggestion. This page have one table under power handling capacity in that table current density A/mm2 depend on toroidal core wet. my core wet is 1842 grams according to that table if core wet is under 3500 grams current density should be 3.1A/mm2.
    • I found one excel sheet of toroidal transformer winding calculation when i searching Toroidal core on ebay. Can you tell me is this excel sheet is right for calculation of toroidal transformer. https://drive.google.com/file/d/0B5ROBprWe44vTDVfZGFWQjBkMUk/view?usp=sharing
  16. Jul 2, 2016 #15

    jim hardy

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    How would i know? All i can do is sanity/reasonableness checks on some of the entries

    Most of the them look reasonable to me
    #19 wire is 0.9mm diameter plus insulation
    ~1mm wire wound around a 6 cm ID circle should be 60π turns, 188 and he got 191 very reasonable
    next layer should be a few less turns because ID is smaller and he shows that,
    1 amp would be 230 watts
    i figured about 3 ohms of primary resistance (791 turns at 5 + 5 + 2 + 2 cm per turn = 11 meters X )0.0264 ohm/meter = 2.9Ω )
    so i guess his entry for primary length must be centimeters not feet not meters
    and the primary resistance enty must be milli-ohms not ohms
    that'd be 3 volts of primary drop out of 230 applied, 3/230 = maybe 1.3% which falls between his Excel entries for 100 and 200 watts

    So i'd say it passes the sanity/reasonableness check
    though it sure would have instilled more initial confidence had he said what were his units
    and i wonder what is "final window" ?

    Try changing things like wire gage , core thickness, and see if all the numbers move in right direction

    That's all i can say from that screenshot.

    I'd have wound it based on pencil&paper calculations.
    That spreadsheet would give me confidence in them because somebody else got similar result.

    old jim
  17. Jul 2, 2016 #16

    Hello sir

    In that sheet instruction tap have all information about using worksheet "final window" is diameter of the remaining hole in the core center, before any final insulation layer is applied. in millimeters. Is wind transformer about 350VA using that calculation is good in real life.I attach instruction tab information in pdf form bellow.

    Attached Files:

  18. Jul 2, 2016 #17

    jim hardy

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    Curious, the guy mixes inches and millimeters

    but the calculations are straightforward enough
    i see you have to tell the program turns per volt
    so it's not doing any magnetic calculations just physical ones to fit your turns into the space available.
    It doesn't do any heat calculations either
    calculate your I^2R losses at rated power
    if they are less than 1 watt per square inch of surface i'd say you're in fine shape(old rule of thumb)
    otherwise it might be prudent to up wire by one AWG .

    i thought that was a screenshot but i see it's an active spreadsheet out there someplace

    old jim
  19. Jul 4, 2016 #18
    Hello Hardy sir
    No sir Turn per volt calculate spreadsheet automatically try changing core size value. And Can you tell me what is used of "Power load,W" column. Is for how mush regulation of transformer is given in particularly used VA rating.How to calculate I^2R losses.Tell me if you want this active spreadsheet.
  20. Jul 4, 2016 #19

    jim hardy

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    i dont know how to make it go. But that is to me amazing ... Handy program to know about!

    Will you be winding that transformer soon ? Keep us posted !

    old jim
  21. Jul 30, 2016 #20
    hello hardy sir

    Finally Done primary winding today of my toroidal core.Attach photo of transformer below.
    755 turn of 0.914 mm magnet wire.
    core size before winding OD-100mm, ID-60mm, Ht-50mm (M4,CRGO) as we discuss early.
    Sir when I test for secondary voltage I get 2.7 volt on 10 turn of magnet wire . why I get less volt per turn. Because before full primary winding when I wind 10 turn for testing perpose on core I get 3.2 volt.Please replay soon
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