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Need a little help (transformer)

  1. Dec 21, 2016 #1
    I am a student of class 12. I am making a transformer for a project. I took an iron rod about 3 inches long and cross sectional area about 0.5 square centimetres as the core. The primary coil has 100 turns and the secondary has 25. I used an ac source of 16 volt. I connected the primary coil to the ac source and measured the voltage across both the primary and secondary coil. I found that voltage across the primary coil is about 2 volt and there is no voltage across secondary coil. I used a multimeter for taking readings.
    I'm pretty sure I didn't scratch the insulation anywhere in the wires except at the ends, the secondary is insulated from the primary and the primary from the iron core by polyester paper, the coils are wound pretty tight, the ac source and multimeter are fine (I checked), the core gets magnetised, the connections were made well, the resistance of the wires are not that high(primary is about 6 ohms and secondary, about 3)......i can't see where the problem lies. Please help.
  2. jcsd
  3. Dec 21, 2016 #2
    You say the primary voltage is 2 , this must also be the source voltage ...check your voltmeter is set to measure AC Volts ... make sure the source is AC volts ...connect voltmeter across source ..it should read 16 ....then connect to the primary, the voltage should drop , but not too much.....try this first.
  4. Dec 21, 2016 #3


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    Try oz93666's suggestion.

    What is the length occupied by the primary coil on the rod? The magnetizing inductance may not be large enough, so it may be drawing a high current and causing a larger voltage drop across the source impedance. Assuming your primary coil is 1 inch long (2.5 cm) , I get a magnetizing reactance of about 8 ohm, which is not that big. If the coil length is about 3 inches, the magnetizing reactance reduces to about 2.7 ohm, which is definitely very small and is the reason for the reduced voltage across the primary.
    Last edited: Dec 21, 2016
  5. Dec 21, 2016 #4
    I'd connect the source to the secondary and measure V across the terminals of both sides. I may end up with a clue.
  6. Dec 21, 2016 #5
    The primary occupies about an inch and a half
    Last edited by a moderator: Jan 4, 2017
  7. Dec 21, 2016 #6
  8. Dec 21, 2016 #7
    I did. The source voltage is 16
  9. Dec 21, 2016 #8
    Did that too. In that case, voltage across secondary is 0.5 volt and there is no voltage across primary
  10. Dec 21, 2016 #9
    What's magnetic reactance?
  11. Dec 21, 2016 #10


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    The reactance responsible for establishing magnetic flux in the core.
    In this case, it is the self inductance of the primary coil. You have no of turns=100, area of the coil=0.5 sq cm, length of the coil=1.5 inch, and relative permittivity of iron=1000. Inductance L=μ0μrN2A/l. The inductive reactance then becomes XL=2πfL, where f is the supply frequency (50Hz I assume). With an inch and a half long coil, magnetizing reactance is around 5.3 ohm. You should check the internal impedance of the source. At least check the internal resistance using a multimeter.
  12. Dec 21, 2016 #11
    Thanks. This was helpful
  13. Dec 21, 2016 #12
    Hi Ruby,

    If the supply is on the 100 turn coil, the power output on the 25 turn coil should resemble .25 of the supply.
    If the supply is on the 25 turn coil, the power output on the 100 turn coil should resemble 4 x supply.
    It is worth noting that the coil connected to the supply is the primary, the other (output) coil is the secondary.

    The supply voltage should travel through the first coil generating a magnetic flux field, which must be transferred to the second coil through the core, which will in turn induce a reciprocal time shifted voltage charge in the 2nd coil.

    I am not convinced that using a single rod material is the best choice for this step-up transformer you have built, as it may be awkward to have 2 magnetic fields existing in the same section of the material. This alone could explain much of the voltage loss in that; the time shifted voltage from the second coil would be cancelling out a large portion of the voltage of the first coil, because the 2 magnetic fields are in the same space, where they would be better placed in 2 separate but over-lapping spaces.

    Having each coil wrapped around it's own iron core material may be a better option, or Imagine a square of iron, where one coil is wrapped on the east side and the other coil wrapped on the west side, of the iron square.

    e.g. http://www.electronics-tutorials.ws/transformer/transformer-construction.html
  14. Dec 21, 2016 #13
    Our project requires that we do it on the same core. Thanks for the suggestion anyway
  15. Dec 21, 2016 #14
    It seems clear from what you said the supply transformer is not up to the job ...you have an open circuit voltage of 16 , but when connected to primary it drops to 2 and when connected to secondary drops to 0.5 .. this can only mean it has a very high internal resistance/reactance ...insanely high ...
    Give details of supply transformer (it may be fried)/try a different one.
  16. Dec 21, 2016 #15


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    Well, Wee-Lemm made a fine point and it turns out that my caculations for the magnetizing inductance are valid for a square core where most of the flux is contained in the core , and not a rod. For the rod, the magnetizing reactance is even smaller because of the air-path. If you are using a rod, the flux completes its path through the air and that wouldn't be effective.
  17. Dec 21, 2016 #16
    I'll try that
  18. Dec 21, 2016 #17


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    100 turns doesn't sound nearly enough. Try 400 turns for the primary.

    Can you attach a photo of what you have constructed?

  19. Jan 4, 2017 #18
  20. Jan 4, 2017 #19

    jim hardy

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    cnh's point about a square core, more specifically one with a closed all iron path for magnetic flux, is the key to your dilemma.

    You understand no doubt Kirchoff's Current Law, to effect '...current must get back to where it started from....'
    Magnetic circuits are similar to electrical ones, magnetic flux too must get back to where it started from .


    Observe flux that gets pushed out the top of the coil comes back to its bottom. I only showed two lines, in theory some of the lines go clear out past Alpha Centauri.
    Now it is very difficult to push flux through air.
    Current is what pushes flux,
    and your source is incapable of delivering enough current to push enough flux through the air surrounding your core to make the voltage that you want in the coils.

    Try surrounding your transformer core with a path of iron and see if voltage goes up.
    (should be no air gaps - i'm just not very good with graphics)
    Be aware the C-Clamp won't work at frequency above a few hundred hz so try line frequency.

    Look up statement "There are no magnetic monopoles" ...
    and call this experiment your 'introduction to ferromagnetism' .

    By inserting paper between core end and clamp you will be able to show how tremendous is the effect of air gap.

    old jim
    Last edited: Jan 4, 2017
  21. Jan 4, 2017 #20


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    He would get a better transformer if he reduced the current on the coil by winding a longer length of wire. And/or fixing the current from the supply transformer. I get that.

    Would he also get a better transformer if he wound the primary on the clamped iron rod in Jim's picture and the secondary around the back of the C-clamp? That looks more like the transformers I see.
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