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Transformer with open iron core?

  1. Apr 21, 2015 #1
    Hello everyone, I'm not sure if the title makes sense, but I'll try to explain it a bit better here. So, if we have a transformer with an open iron core (U shaped iron core, with coils wrapped around each side, same number of coils, same thickness), why is the electromagnetically induced emf in the second coil weaker than when there is a closed iron core (O with corners shaped)? I think it's because magnetic field permeates the iron core better than the air, but please explain if I'm wrong.

    The iron core I was talking about: http://oi62.tinypic.com/2q8st90.jpg

    First time it's open (weaker induced emf), and then we close it (a lot stronger induced emf).
  2. jcsd
  3. Apr 21, 2015 #2


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    If the core is closed, the magnetic flux will circulate inside this core, as the permeability is about 1000 times better here, than outside the core. So all of the flux passing through the primary coil will pass through the secondary coil (allmost same emf in the two coils).

    If you open the core, you introduce an airgap, and in an U shape there is a lot of airgap, not only at the top of the U, but also between the vertikal parts. So why wait to cross the airgap at the top, when shorter ways are to be found (just horizontally across the U in the middle)? Using this alternative route, all the flux through all the primary turns will not pass all of the secondary turns, resulting in a weaker secondary emf. The flux will cheat, not following the "intended" route. Some of the flux will be missing through some of the turns.
  4. Apr 21, 2015 #3
    I don't understand a part of it. How can the flux pass accross U in the middle, when both sides of U have coils around them, how can the flux pass through the coils? Look at this video for me please:

    That's exactly what I'm trying to describe. Now, just imagine there's no upper side, so we get an U shape, I think it would still make a similar path through the air, as it's shown in the video for iron. Since air is not as good conductor as iron, it will weaken the magnetic flux, thus, resulting in weaker induced ems in the second coil.
  5. Apr 21, 2015 #4


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    This is a difficult subject but one relevant factor in transformer design is to ensure that as much as possible of the flux due to the primary passes through the secondary. A continuous magnetic path through a high permeability core will ensure this. If you only use a 'U' shaped core (or a simple bar) then a significant proportion the flux can bypass the secondary and there will be less emf induced. (Flux Leakage) There are various designs of core and winding arrangement to maximise the flux linkage and the 'toroidal' system is very good with windings interleaved. However, it is harder to build because the windings have to be threaded around the ring shaped core. Double C cores and E and I laminations can be would and the core assembled round the windings, giving a certain amount of leakage because of the gaps.

    This is just one of many links that will tell you details about transformer design.
  6. Apr 21, 2015 #5

    jim hardy

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    you are correct

    You should see current in driving coil decrease substantially when you close the core. or, if you're driving with fixed current the voltage should increase.

    What pushes flux is called "magnetomotive force", it's analogous to "electomotive force", and is measured in amp-turns which is as name implies number of turns X number of amps.

    As you said, an amp-turn will push a lot more magnetic flux through an inch of air than through an inch or iron.

    Look up "right hand rule "

    Magnetic fields aren't constrained to iron the way current is constrained to wires. But iron conducts magnetic flux maybe a thousand times better than air, as opposed to copper wire conducting current millions of times better than air.

    So the math of magnetic circuits is more oriented toward fields than is math of electric circuits.

    Good to see hands-on experimenters. I hope you post plots of your data. Measure amp and volts accurately, plot volts in both coils vs amps in driving coil with and without the U core closed.

    old jim
  7. Apr 23, 2015 #6


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    No, that's not how it works: It's correct that if you introduce an airgap (open the core) within 1 ns, the flux will be weakened, and as a result, so will the emf in the secondary coil and also the back-emf in the primary coil.

    Now, the current in the primary coil is driven by the difference between the supplied voltage and the back-emf in the primary coil. So as this difference is increased, more current will be driven through the primary coil, increasing the flux until the back-emf ≈ supply-voltage. Having this flux "balanced" again the emf in the secondary coil will have been increased again to almost "nominal" value. (the voltage drop in the primary coil has increased some due to the higher current through the coil).

    So the main-reason to lower secondary emf, with an open core, is Flux Leakage, (flux choosing an non-authorized path).
    Last edited: Apr 23, 2015
  8. Apr 23, 2015 #7

    jim hardy

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    Hesch has described how a closed core transformer works when connected to a source of pretty constant voltage that's capable of a lot of current.
    When they figured that out around 1880 it was a "Eureka!" moment. Closing the core made transformers practical.

    Remember your primary winding has resistance so as the current increases, so does the fraction of voltage dropped in resistance, leaving less to be opposed by counter-emf.

    Let's look at a screen shot of your video at 5:08, but with top chopped off his transformer.
    Flux no longer has the luxury of an all iron path so will redistribute itself through the air, as my hand-drawn blue arrows show.
    Note some of the flux bypasses some of the windings, as Hesch said.


    You didnt say how you are powering your transformer core.
    If it's plugged into a wall socket it should hum and get hot when you remove the top. Current goes way up trying to push flux through the air.

    If it's given a constant current you'll only see voltage in both windings decrease because of the permeability you mentioned in first post.
    That reduction in permeability both reduces flux as you said, and causes flux to re-arrange as Hesch said.

    Is that picture any help ?

    Attached Files:

  9. Apr 23, 2015 #8

    jim hardy

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    i notice that video showed a microwave oven transformer.

    do NOT ever energize a microwave oven transformer (MOT) until after you have physically cut out its high voltage winding.

    The one in that video will kill anybody who touches (even accidentally) the hv winding. That's because it is around 1,5KV at an amp and is tied to the transformer core which is earthed through the power cord. An amp is lethal, in fact 1/50 amp can stop a heart.

    Tinkerers are getting killed by MOT's.. at my age i really fear for tiny fingers that just don't know.
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