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Power Transformers

Discussion in 'Hobby Electronics' started by tony.r, Sep 23, 2003.

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  1. tony.r

    tony.r Guest

  2. Hello Tony,
    I had a look at your web site but I am still a little confused.
    What went wrong with the transformer originally?

    You say also that you rewound the transformer and it works
    but gets too hot. I assume that you rewound it with the same
    number of turns and wire gauge as was there before.

    Assuming 240V input, If your transformer was at my place,
    I would connect the transformer primary up to my variac
    and slowly increase the voltage applied and also monitor
    the primary current. If the current stays in the 10s of milliamps
    region with 240 V applied. I would be pretty happy and
    then carry on with load testing the secondary.

    Tony, with unloaded secondary windings. How much
    primary current does your rewound transformer draw
    with 240 Volts applied. (assuming its meant for 240V use)

    John Crighton
  3. tony.r

    tony.r Guest

    Hi John

    At the maximum the amp should draw no more than 1 Amp.
    At idle I am not sure ?
    The primary did have a fault (short) and the blown fuse and low resistance
    reading showed that.

    1000 Primary windings of 0.25 mm Enam/Copper wire is obviously to few
    windings and the gauge of wire could be heavier to reduce losses and heat.

  4. Hello Tony
    If this idling current or magnetising current is high, then your
    transformer will get hot just sitting there with no secondary loads.
    Measure the primary idling current and tell us what it is.

    Was the 1000 turns accurately counted from the old winding, no

    When you use a variac on the input and slowly turn up the applied
    voltage, you can see the primary current rise almost proportionally.
    When the primary current starts to rise fast with just a small
    increase in the applied volts then you know there are not enough
    turns for that particular applied voltage.

    Borrow a variac and have a bit of play, you will see what I am
    going on about. Heh heh heh.....
    John Crighton
  5. Phil Allison

    Phil Allison Guest

    ** For an E-I tranny - yes. But not a for a toroidal.

    Those buggers show SFA primary current until the saturation limit is
    reached, then all hell breaks loose !

    ** Well, ....... maybe.

    Some mains txfmr designers like to stear well clear of saturation while
    others love to WALLOW in it !!!!

    The only real physical limit is * HEAT * - ie how many I squared Rs are
    there ??

    As long as that number is not more than 5% or 10 % of the rated VA all is
    "cool" as far as being a useable design is concerned.

    ** Maniacal laughter from a transformer designer is a WORRY !

    He'll be visiting graveyards for spare parts soon .........

    ............ Phil
  6. Hello Phil,
    Good, I am glad you are here, I was wondering why you
    hadn't jumped in sooner to help Tony out with his transformer.

    From the pictures at the web site I thought I saw a bunch
    of E & I laminations. Tony will put us straight as to what
    he has got. Anyway, I would still like to know what the
    idling current is when Tony gets round to measuring it.

    Over to you Phil, to sort Tony's transformer out. Heh heh heh
    It is getting late, I will see your words on my screen tomorrow.
    John Crighton
  7. tony.r

    tony.r Guest

    Hi Phil , John

    Yes it the old wasteless "E" and "I" Laminations and these examples came
    from the troublesome trans.

    When I rebuild it I will run some more detailed tests.

    M6 G.O.S.S How many Lines per Square Inch ?

    4.5 volts per turn gives me 1080 pri and 855 sec turns
    but what is the lines per Sq inch ?

  8. Phil Allison

    Phil Allison Guest

    "tony.r" <

    ** Ask a question that makes sense.

    .......... Phil
  9. tony.r

    tony.r Guest

    O.K School master Phil !!!

    M6 Grain Oriented Silicon Steel

    What is the max. flux density for this material ?

    A coefficient in lines per square inch would be acceptable.

  10. Phil Allison

    Phil Allison Guest

    ** Go ask a transformer lam supplier - I am not one.

    ........... Phil
  11. tony.r

    tony.r Guest

    Ha Ha

    I have actually, smartass. Hammond are getting back to me.

    It will be easier to build a new bugger from the start and with the correct
    silicon steels and tec specs to calculate windings correctly.

    Or I could try the yanks on A.G.A would not get me far though before it
    turned into a slinging match.

    Every little point is debated.

    Looney bin time
  12. Phil Allison

    Phil Allison Guest

    ** That is a load of bull.

    It is not simply a matter of "calculating" the correct turns per volt or
    saturation point - you need to do tests on the actual core stack and
    measure the magnetising current as the primary voltage increases. Then
    allow that the load on the secondary side actually pulls the core OUT of
    saturation when in use due to primary voltage drop.

    A 300 VA tranny on 240 volts AC may have as much as 1 amp off load
    magnetising current and be fine.

    ............. Phil
  13. Morning Gents,
    Tony, here is the phone number of my local transformer Guru,
    John Cox of Magcom, 2/17 King Rd, Hornsby 02/94762723.
    This man is very helpful. He will tell you all about the different
    grades of laminations and their different flux densities.

    He might even mention his bible.
    Design Shortcuts and Procedures for Electronics by Ordean Kiltie
    ISBN 0 916 512 274.

    Out of that book, John Cox gave me this formula . You may
    like to compare it with your own books just for interest.

    Flux density in KG (Kilo Gauss) = 70 times V divide by A times T

    A = area in square inches
    V = volts
    T = turns

    John Cox told me,
    using a flux density 10 to 13 Kilo Gauss is acceptable.
    Over 14 KG is too high.
    If using cheap lamination material use a figure of 10 KG
    If using top quality lamination material use a figure of 13 KG

    Ring up John Cox, he will point you in the right direction.

    John Crighton
  14. D.Castles

    D.Castles Guest

    Phil you have an attitude problem....get a life...the world is not one
    big transformer.......or is it?
  15. Phil Allison

    Phil Allison Guest

    "D.Castles" <>

    Phil you have an attitude problem....get a life...the world is not one big transformer.......or is it?


    ** My gawd !!!

    Dave Castles is back from the dead - and still fooling them he is a tech down at Latrobe Uni.

    As usual, he is unable to comprehend a single, simple idea re transformers - or anything else.

    .............. Phil
  16. tony.r

    tony.r Guest

  17. Phil Allison

    Phil Allison Guest

    ** Says a blind man about to step off a cliff ........

    ............. Phil
  18. tony.r

    tony.r Guest

    Hammond Electronics Pty Ltd

    The lamination in the 369EX is a 26 gauge, non grain oriented silicon
    steel. The nominal flux density at 50 Hz ( worst case ) is 15,200 gauss or
    98.07 lines per inch. This steel was chosen due to the fact that power
    transformers for audio applications have lower flux density's to avoid stray
    field and in turn yield lower core losses. Also distortion is less critical
    for power supply applications. We do use 29 gauge grain oriented silicon
    steel laminations for all of our output transformers where distortion is

    I hope this helps you. If not , please contact me.

    Doug Hutt
    Hammond Engineering


    I would like to know what type of silicon steel was used to make the 369EX
    and what the specs of that steel is.
    Lines per square inch / G.O.S.S

  19. tony.r

    tony.r Guest

    Blind Bart and the white canes.

    Now that I have the specs from the manufacturer it was just a matter of
    running it through the formula.

    The final turns per volt proved the point.
    The original heater wind was 35 turns and my calculations came to 34.818 .

    I have rewound the trans and will test it tomorrow.

    Thanks for all your help Aus.Electronics.

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