Induced emf supposed to oppose the applied voltage?

Discussion in 'General Electronics Chat' started by Silhorn, Apr 15, 2013.

  1. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    Hello,

    From what I know,

    When an alternating voltage is applied to an air coil, a current flows which lags by 90 degrees. A flux will develop which is in phase with the current. This changing flux will develop an emf which opposes the applied voltage meaning 180 degrees phase difference.

    Now if I place another coil next to the original coil, a emf will be induced into that coil. That as stated is 180 from the applied voltage but on your website, it says that it will be in phase with the applied voltage...?

    [​IMG]
     
  2. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    Hold on, I just found a website that says the phase relation induce in the second coil depends on the direction it is wound, is that why?
     
  3. crutschow

    Expert

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    Yes. The phase of the voltage is determined by the direction of the winding.
     
  4. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    Ok got that cleared up, now this raises a few more questions...

    1. For the secondary to draw current from the primary winding the secondary back emf must oppose the primary emf. But if both secondary voltage are in phase then both their back emfs must also be in phase with each other so they will not be able to oppose each other?

    2. Let's say if I have only an inductor in an AC circuit with 5Volts supply and the inductance of the inductor produces a 2V back emf would the inductor voltage now be 3Volts?
    Part of me will say yes it should!
    Part of me gets confused and knows the voltage across the inductor should be 5 Volts because the sum of all voltage drops in a circuit must equal the supply voltage of 5 Volts...

    Even though I have textbooks, google, and this forum, still very hard to understand something I cannot see!...
     
    Last edited: Apr 16, 2013
  5. crutschow

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    1. The primary and secondary back EMF don't oppose each other, they are part of the same back EMF. The back EMF of the primary with no load is generated by the primary inductance. This back EMF appears on the secondary with a voltage determined by the turns ratio.

    If you apply a load to the secondary, a current starts to flow due to the secondary voltage. This tends to reduce the magnetic field in the core, causing the primary back EMF to drop and more primary current to flow. The primary current increases until the magnetic field from the secondary current is cancelled. Thus the back EMF is always sufficient to just oppose the primary supply voltage while still supplying a primary current flow as determined by the secondary current and turns ratio.

    2. If you apply 5Vac to an inductor then its back EMF must also be 5V. If not, the inductor is saturated and the current increases until the rest of the EMF is being supplied by the inductor resistance and any other resistance in series with the inductor
     
  6. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    1.
    So how does the secondary current reduce the magnetic field in the core? I don't think it does this by countering it with it's own magnetic field or else it would still be in phase with the core magnetic field right?
    2.
    I thought the magnitude of the back emf created by the inductor depends on value of it's inductance?
    But let's just say it's back emf is 5v, then doesn't that mean it will then completely cancel out the supply voltage so it will be 0 Volts across the inductor?
     
  7. crutschow

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    1. If you look at the direction of the current generated by the secondary voltage into a load and determine the direction of the magnetic flux as determined by the right-hand rule, you will see the flux is opposite to the flux generated by the primary. So this flux is 180 degrees out of phase with the primary flux and reduces the primary flux. That is why the primary current has to increase to counter the secondary flux and maintain a flux sufficient to provide a matching counter-EMF for the primary voltage.

    2. You are putting the cart before the horse. The magnitude of the inductor impedance depends upon it's inductance and the frequency of the applied voltage. But, by definition, the back EMF has to be equal to the applied voltage (assuming the inductor core doesn't saturate). So a higher applied voltage simply increases the inductor current --- the voltage drop across it always equals that voltage.

    The back EMF in a inductor is equal to the voltage across the inductor. Don't know what you mean by "cancel out the supply voltage"? It's the same as having a voltage across a resistor. The resistor will provide a resistance (back EMF) equal to the supply voltage and the voltage across the resistor is equal to the supply voltage.
     
  8. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    Ok, so I know the polarity induce in the secondary coil depends on the way it is wound. This polarity will also determine the direction of current. And to find the direction of mmf we use the right hand rule.

    In order to oppose the main mmf, the secondary mmf must be of opposite direction to the main mmf.

    [​IMG]

    The top diagram shows 2 setups. The top one shows the same polarity induced in the secondary having the currents both in the same direction. Using the right hand rule you can see the mmf's go in opposing directions so the secondary mmf can oppose the primaries mmf.

    The bottom diagram shows an opposite polarity induced in the secondary having the currents flowing in opposite directions. Using the right hand rule it shows that both mmf's to be flowing in the same direction. So if they are flowing in the same direction, they cannot oppose each other right?
     
  9. studiot

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    If I interpret your diagrams correctly you have drawn two single wires side by side, in each case.

    When considering magnetic effects you need to consider a coil or loop. Such a loop has two wires with currents transiting in opposite directions and the formulae for magnetic effects includes the area enclosed by this loop.

    A single wire does not enclose any area.
     
  10. crutschow

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    Suppose you have two concentrate coils wound on a core in the same direction. If you now apply a voltage to one coil (positive top), that will cause a current to start flowing into the top (top to bottom) in that coil as determined by its inductance. At the same time the mutual inductance will induce a positive voltage at the top of the second coil due to the applied primary voltage. If have a load across the secondary, then a current will flow out of the top (bottom to top) due to this voltage.

    Thus you can see that the current flow of a transformer is in the opposite direction between primary and secondary for coils wound in the same direction. The inverse would be true if the coils were wound in the opposite direction. Thus the secondary flux due to a load current always opposes the primary flux.
     
  11. studiot

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    Please note crutschow has wound his primary and secondary coils on a core.

    This is important in the theory of transformers as the core removes the direct link between the two coils.
    That is the primary induces magnetic flux in the core and this flux acts on both primary and secondary to induce a back emf in each which is why the secondary winding direction is reversed.
     
    Last edited: Apr 20, 2013
  12. crutschow

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    Whether the two concentric coils have a magnetic core or an air core has no effect on the relative direction of the voltages and currents. It does affect the inductance of the transformer and the coupling efficiency between the primary and secondary.

    I'm not sure by what you mean by a "core removes the direct link between the two coils". :confused:
     
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  13. studiot

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    The flux \varphi in the core is induced by the primary EMF, Ep and is in phase with the primary current Ip. Owing to self induction Ip lags Ep by some factor depending upon the inductance, which in turn depends upon the core material.

    The secondary EMF, Es, is caused by the flux \varphi. It is proportional to

    \frac{{d\varphi }}{{dt}}

    so lags phi by ∏/2

    Again owing to the inductance Is lags Es by ∏/2, thus Is and Ip have opposite phases.
     
  14. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    Um, ok I have tried to take note of what has been said and over the last couple of days have been making a picture of a transformer showing everything i know to help me better understand transformer and to ask my question.

    The following diagram shows both winding wound in the same direction producing 2 flux paths which oppose each other which will in turn make the primary draw more current to strengthen it's flux. This I now understand.

    [​IMG]

    Now, this is the part I don't understand. The following diagram shows both windings wound in opposite directions to one another. This creates 2 flux paths which will flow in the same direction along each other in the core. If both flux from primary and secondary are supposed to oppose each other, how will this work for this situation?

    [​IMG]
     
  15. The Electrician

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    The way the windings are wound on the core is exactly the same in the bottom diagram as in the top diagram.

    In the top diagram, the top end of the left winding (labeled +) goes under the core first; it's the same in the bottom diagram.

    In the top diagram, the top end of the right winding (labeled -) goes over the core first, and it does the same in the bottom diagram.
     
  16. studiot

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    Silhorn, you are putting a lot of effort into your own version of transformer action, rather than trying to understand a conventional approach.

    Unfortunately your version is leading you into difficulties.

    Why are you including flux produced by the secondary coil?
    If this coil is open circuit then is there any current?
    Therefore is there any flux?

    Let us start with a single coil, connected to an AC generator and consider what happens.

    We can develop the theory from there if you like.
     
  17. studiot

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    You mentioned the right hand rule earlier.

    This rule is inappropriate for understanding transformers, you need Faraday's or Lenz's laws.

    You do not need calculus although a quite simple level helps.

    You might find the elihu experiment quite fun. It is a way of building understanding by stages and lots of fun to carry out.

    http://educypedia.karadimov.info/library/The jumping ring experiment.pdf
     
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  18. Silhorn

    Thread Starter New Member

    Apr 9, 2013
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    Ah that experiment knocked some sense into me.

    I always thought the right hand rule would not only determine the current direction but also the winding direction. I thought the a counter clockwise winding would allow current to flow only one way and vise versa.

    Amid the confusion now I know the concept. The primary will ALWAYS induce a mmf in the secondary that is of opposite direction. The secondary whether wound clock or anti-clock wise will still always have a mmf of opposite direction to the primary. The only thing the winding direction would do is affect the direction the current flows through it without changing mmf direction as can be proved with the right hand rule.

    Thankyou for all your help. I appreciate it very much.
     
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