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Connecting the primary and secondary windings of transformer in series

  1. Feb 1, 2008 #1
    What happens if you connect the following in this order:

    1) from a battery to the primary in
    2) from the primary out to the secondary in
    3) from the secondary out to a motor
    4) from the motor to the battery

    I know that transformers usually step up or step down the voltage, but it shouldn't be possible to feed a stepped up voltage and add it to the total voltage right? So what would REALLY happen?

    Say if it was a step up transformer, the voltage would go up and the current would go down. But in my example, the current that goes through the primary windings would also go through the secondary windings. This complicates things I know, but does anyone have a good answer as to what would happen? Thanks.
    Last edited: Feb 1, 2008
  2. jcsd
  3. Feb 2, 2008 #2


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    I believe you've just made your transformer a very long piece of wire.
  4. Feb 2, 2008 #3


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    A transformer works on the principle of inductance. i.e. A CHANGE in magnetic flux though a loop of wire will cause a current to flow in the loop. So the transformer must see a voltage that VARIES with time (alternating current) inorder for there to be induction. Now with your set-up, you have a "steady" voltage from the battery (direct current) so you will not see the motor working any different from just having long wire attached as LURCH wrote.

    That being said; if you are using a motor with bruches, then there may be some small voltage spikes produced from arcing which may cause a little induction.

    If you are really interested in learning how transformers work, then check out http://en.wikipedia.org/wiki/Transformer.

  5. Feb 4, 2008 #4


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    Depending on the polarity of how you connect the primary to the secondary, I believe you will either see the sum of the magnetizing inductances of the primary and secondary (Lmp+Lms), or the sum of the leakage inductances (Lkp+Lks). Can you tell us why?

    EDIT -- I think this statement of mine above is incorrect, at least for the coupled inductors on the same coil:
    "I believe you will either see the sum of the magnetizing inductances of the primary and secondary (Lmp+Lms"
    Last edited: Feb 7, 2008
  6. Feb 7, 2008 #5

    But even if I put same current through both the primary and the secondary, wouldn't the difference in windage create a difference in voltage between them (because of transformers ability to direct magnetic flux from the coils)?
  7. Feb 7, 2008 #6


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    It sounds like you are a bit confused about how transformer action works. Try reading through this recent thread to see if it helps:

  8. Feb 7, 2008 #7
    So what if I did not want to work with my transformer as intended? Let's assume for the sake of argument that the following is safe, so we can focus on what happens if we, "Connect the primary and secondary in series when they have different numbers of turns."

    Fact: When the primary and secondary have been become part of the same circuit and are connected in series they would be required to have the same current (per previous information handed to me in this forum).

    Fact: If the current is the same between them, then the primary and secondary (previously assumed to have different numbers of turns) will have different fluxes. This violates the intended purpose of a transformer.

    Fact: If the fluxes are different, there would have to be additional counter flux, corresponding to the circuit current times the difference in the number of turns between the primary and the secondary.

    Question: What happens if the primary and secondary windings are connected in series where the ends normally associated with output are connected to each other through the load?

    Visual for the following underlying questions: "+" of voltage source ----wire---- Intended entry point of secondary winding ----wire---- Intended exit point of secondary winding ----unspecified load---- Intended exit point of primary winding ----wire---- Intended entry point of primary winding ----wire---- "-" of voltage source

    Underlying question A: What happens to the current in the backwards oriented primary if current moves forward from the forward oriented secondary into the load?

    Underlying question B: What happens to the current in the forwards oriented secondary if current moves forward from the backwards oriented primary to "-" of the voltage source?
    Last edited: Feb 7, 2008
  9. Feb 7, 2008 #8
    Your first post said "...from a battery...". If the source of power is a battery, then no "transformer action" will take place. The effect will be the same as if you simply unwound all the wire from the transformer and put it in series with your battery.

    Transformers don't "step up" or "step down" direct current, which is what you get from a battery.
  10. Feb 7, 2008 #9
    Then I am not talking about a direct current source anymore.
  11. Feb 7, 2008 #10


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    If you connect the two windings in series, you either add their effects, or they partially cancel each other's effects. The inductance goes as the number of turns squared, so if the two coils have the same number of turns, you either get 4x the inductance (I was wrong above when I said Lmp+Lms -- that's only true for non-coupled inductors), or zero inductance (actually Lkp+Lks), depending on the polarity of the series connection. You are correct that the current in the series coils has to be the same, and the flux in the core seen by each coil is the same
  12. Feb 7, 2008 #11
    Thanks. Now to examine a specific series connection more closely:

    Now for the questions:

    According to my knowledge, this would create a potential difference from the load into the "-" of the battery.

    According to my knowledge, this would create a potential difference from the "+" of the battery into the load.

    According to my knowledge, the transformers should not be able to increase voltage without reducing the current usage, yet is seems like the original current from the batteries can be used to add to the load with the seeming prospect of circuit voltage gains from the transformer. What assumptions am I missing?
    Last edited: Feb 7, 2008
  13. Feb 7, 2008 #12
    This cries out for an experiment. Buy a 12 V CT transformer from DigiKey and use a short-proof power supply for the battery. Let us know what you found out.
  14. Feb 7, 2008 #13
    I already bought a 70 volt constant voltage transformer (rated at 4 watts) from http://www.fullcompass.com/product/303987.html

    I plan to use it in my motor as soon as it ships in. I already have a multimeter in which I can test the current and voltage going into it.
  15. Feb 7, 2008 #14
    That's an audio transformer and probably not what you want to run any kind of motor circuit. I also don't think it is a constant voltage transformer; those use ferroresonance and I can't imagine doing that in an audio unit (unless you like the hum "Say, why does your speaker hum?" "Cause it doesn't know the words.":rofl:)

    What exactly are you trying to accomplish in terms of this motor? A whole bunch of us on here have a lot of experience and would be glad to point out some ways to achieve your goal.

    In general, most times you run dc through a transformer, it quits being a transformer because you have saturated the core and now have a permeability near air. The only way around that is to use balanced "bucking" dc currents (thus my suggestion you buy a CT unit where you can do that).
  16. Feb 7, 2008 #15

    I have a mechanical commutator so I will get varying current and voltage going into the primary windings one I get this transformer.
  17. Feb 7, 2008 #16
    If you connect the primary and secondary as you have described, then you will have converted the transformer into an inductor. It will not be a very good inductor. The core will saturate sooner than you would expect with a purpose-designed inductor, because when it was used as a transformer, the flux caused by the primary and secondary tended to cancel, but this effect is not nearly so pronounced when it's wired as an inductor. You would need to introduce an air gap in the core assembly if you wanted it to be a good inductor.

    Even if you use a mechanical commutator to convert the DC from the battery into pulsating DC, which will therefore have an AC component, whatever voltage "step up" or "step down" may be going on internally between the windings will not be apparent outside the rewired transformer. You will have nothing more than an inductor in series with the motor and commutator. The inductor will provide opposition (impedance) to the current, and will decrease the effective voltage applied to the motor.

    The pulsating DC caused by the commutator will have an equivalent DC component and AC component, unless the commutator is wired as a bridge, in which case there won't be a DC component. Then the AC component will just be a square wave AC voltage. But if there is a DC component, the inductor (rewired transformer) will saturate easily, and when it does, its inductance will decrease, perhaps a lot, and the remaining effect will be just the resistance of the wire in the windings in series.

    If you tell us what you're trying to accomplish, we may be able to give you suggestions that will more directly get your desired result.
  18. Feb 8, 2008 #17
    I am aiming for better for better motor output than what I have right now. I rate this as that the product of rpms, torque, and time I can use to run the rotor (permanent magnet) from a battery power supply. Now the voltage in the transformer is a function of how fast the magnetic field changes. The faster the circuit is opened and closed by the commutator, the higher this voltage will be, and the turns do not change. I intend to somehow take current from the batteries and push the majority of them through the entire coil, instead of just the primary windings. I am trying to avoid the situation of limited current at the secondary windings. At the same time, I want someway to take advantage of the transformer's ability to produce voltage. However, I do not understand if it is possible to increase the voltage as readily if the current in transformer is forced to be the same in both in the primary and the secondary. However, even the tiniest increase in voltage in the secondary going into the coil would help to improve the efficiency of the motor.
  19. Feb 8, 2008 #18
    It sounds to me like you want to increase the voltage applied to the motor to be above what is available from your battery. Is that what you want?

    If this is so, why can't you just use a higher voltage battery? If you can't do that, then you will have to use a DC-DC converter. If you do that, the audio transformer you ordered will not be suitable for the purpose.



  20. Feb 8, 2008 #19
    I think the meaning of the phrase is that it is a line transformer which is meant to be driven at a constant voltage. Once long ago, when the world was still young, I used those to drive horn speakers at public events.

    The commutator will certainly help, but you will have to watch the frequency. Remember that the back end of a square wave is about 0.1 the fundamental frequency.

    The idea of adding windings in addition or subtraction certainly works ("buck-boost" line transformers do this all the time), but you're always stuck with a power output that is less than the power input (somewhere between maybe 80% and 99%).

    I think the point that might be throwing you off is the idea that it is current that causes the magnetic flux in a transformer core. In fact, as noted earlier by others, you should look at a transformer in terms of the flux being proportional to the changing voltage. There are typically three currents then that you see: a magnetizing current; a losses current; and a load current. The load current, which is what flows through your motor, is pretty much irrelevant to the core (or at least should be).

    Can you provide a sketch of what you're doing?
  21. Feb 8, 2008 #20
    Here ya go.

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