Question about transformers

Discussion in 'General Electronics Chat' started by Racketman333, Sep 7, 2012.

  1. Racketman333

    Thread Starter New Member

    Sep 7, 2012
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    hi All
    I'm new to electronics and have a general question about transformers - would really appreciate a little assistance.

    ok, so I understand that transformers can be used to step up voltage (and with this comes an accompanying step down in current). That figures and I can see how that works with Ohms law if resistance is theoretically consistent on both sides.

    What I don't really understand probably stems from my lack of understanding of the concept of voltage. If voltage drives the current in a wire, when the voltage is stepped up, why doesn't it induce an accompanying increase in current flow? OK, so I also appreciate this is a physical impossibility as you would have gained some power and broken a fairly fundamental principle...i just don't quite get that a voltage can be high and current low if resistance stays the same.

    The only analogy I could think that would clarify this for me was that the primary voltage might be akin to a motorbike pulling current along (fast but light push), and the secondary voltage becomes more like a bulldozer (slower but much stronger than the bike).

    Am I on the right track, or am I missing something very basic?!

    thanks in advance for any help here!
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    Just Google transformer basics. I did a check and it will show a lot of good material
     
  3. shubham161

    Member

    Jul 22, 2012
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    energy can neither be created nor be destroyed.

    btw, resistance don't stay the same. for step up, more winding is needed and so more resistance is there is the secondary winding. I am not sure though.
     
  4. cabraham

    Member

    Oct 29, 2011
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    1st bold, my response - voltage does not "drive" current.

    2nd bold, my response - voltage does not "pull" nor "push" current.

    Once these urban myths are dispelled, I suggest you refer to peer reviewed reference text books. I'd recommend 2nd year EE basic circuit texts, then for a deeper understanding, physics or EE 3rd year fields text books. Best regards.

    Claude
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    You are applying the concept of resistance and Ohms Law incorrectly when it comes to transformers.

    An ideal transformer has zero resistance. If you apply AC voltage to the primary windings and there is no load on the secondary windings, there will be zero current in both the primary and secondary windings.

    Any thoughts of resistance should be about an external load on the secondary windings. The value of the resistive load will determine the current for a given output voltage.
    If you increase the AC voltage on the primary windings, of course the current through the load will increase, and so will the current in the primary windings.

    What is conserved is power transferred. For a 100% efficient transformer the Volts x Amps in the primary will match the Volts x Amps in the secondary.

    The more voltage x current you take at the load will draw an equal amount of power from the primary.

    What you may be confused with is the rated power of the transformer. If a transformer is rated to deliver 12VAC @ 1A, that is saying do not exceed this power rating otherwise performance will suffer and you can cause the transformer to overheat and start a fire.
     
    Last edited: Sep 7, 2012
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  6. mlog

    Member

    Feb 11, 2012
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    Think of a transformer as a device that passes along power. If you ignore losses in the transformer -- which there should be little -- you have:

    Power Out = Power In

    Vout * Iout = Vin * Iin
     
  7. WTP Pepper

    New Member

    Aug 1, 2012
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    Of course it does. If not, what does?

    Thevenin worked this out many years ago.
     
  8. WTP Pepper

    New Member

    Aug 1, 2012
    21
    6
    Transformers are best thought of as a gear box to a layman. High torque low speed will drive a high speed but low torque. The opposite also applies.

    Think of a car engine. Revs at high speed ~ 6000rpm, but with little power and of little use at that speed. Through a gear box, it can pull you up a hill slowly with a heavy load in tow. You change the transfer rate via gears. Excluding losses in the gear box, the power from the engine is the same as that applied to the wheels.

    So on an ideal transformer, the power in = power out. Double the output voltage via the turns ratio and you halve the current. On an ideal gearbox, halve the speed via the gears and double the output power.

    That's what gears do and that's what transformers do.
     
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  9. cabraham

    Member

    Oct 29, 2011
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    What drives current is the power plant turbine energy conversion process, burning coal, nuclear fission, hydroelectric generation, wind, etc. THe ac power mains the xfmr plugs into is a CVS (constant voltage source because the power company forces it to be, not Mother Nature. The power company spins their turbines at constant speed resulting in constant voltage (& frequency). They could just as well spin at constant torque resulting in constant current & voltage would vary with load.

    For reasons beyond the scope of this thread, they don't do that. I suggest that the OP examine university sites as well as xfmr OEM sites to learn how xfmrs operate. The web is full of good & bad info intermixed. I urge the OP to review peer-approved sources.

    Claude
     
  10. shubham161

    Member

    Jul 22, 2012
    50
    3
    hey i remember you from physics forum. i read your interesting discussion on "theory of causality". I think it's your favorite topic and you have a done a great research on it.
     
  11. vk6zgo

    Active Member

    Jul 21, 2012
    677
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    This is a philosophical question,like "Why is a cat not a dog?"
    or "I know we have gravity,but why don't things fall up?"

    As you said,it is a physical impossibility to gain power between the primary & secondary of a transformer,& that is the crux of the matter!

    You only have whatever power you put into the primary,so as P=V*I,if V gets smaller,I gets proportionally larger.
    Transformers don't work at DC,so for the moment,forget the DC resistance of the windings,& concentrate on the idea that
    power in=power out.

    Of course,not all the primary power makes it to the secondary,some is used up in losses.making the transformer warm,but it is as near as "Damn" is to swearing.

    At your present level of knowledge,you need to accept some fundamentals without trying to find deeper meanings,otherwise,you will keep coming up against roadblocks of your own making.

    Later on,you will revisit some of this stuff & have a deeper understanding,but even if you become an expert on electromagnetic theory,ultimately,you will come up with currently unanswerable "Why is a cat.....?"type questions.
     
  12. cabraham

    Member

    Oct 29, 2011
    82
    30
    Thanks, actually I needed a refresher course. In August 2010, I took my Ph.D. qualifier exam for the 2nd time & I did pass it (had I not passed, I would have been dismissed from the Ph.D. program). One condition was that I had to take a course on signals & systems. We syudied causality. The key to causality was knowing that an output can respond only to past & present value of inputs, never the future value of input.

    If a quantity B is caused by another quantity A, then it is imperative that A takes place chronologically ahead of B. If A lags behind B, then A cannot be the cause of B. Since B takes place first, it might be the cause of A, but it might not.

    That is the crux of causality, events must be chronologically correct in order to establish one as the cause of the other. If one can show that A takes place after B, then A cannot possibly be the cause of B.

    Regarding OP question, I responded to a poster who says that "voltage drives current", which is not generally true. Examining the way generators operate reveals that energy conversion is required to establish & maintain both I & V. If the generator spins at constant speed producing constant voltage, then the load resistance is dropped to half, the current does not double while maintaining constant voltage. More fuel must be burned in order to preserve constant voltage value with doubled current. The steady voltage combined with halved resistance does not double the current.

    Thanks for the comments, & best regards.

    Claude
     
  13. vk6zgo

    Active Member

    Jul 21, 2012
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    Actually,we are not all stupid,& are well aware that our electricity sources ultimately come from power stations,where mechanical power from the burning of fossil fuels,or other source are converted into electrical power.

    It is,however,a convention ( & has been for many years),to represent a source of electrical power as a "perfect" voltage source,in series with an imaginary resistor representing the internal losses of the device,or a current source in parallel with a resistor also representing those losses,which include the considerations you referred to.

    This is a useful concept for most work in Electronics.
    OK,it doesn't represent the whole truth about what is going on,but most of us aren't aiming at a Ph.D.

    The OP is obviously having trouble in visualising how a transformer works,so is unlikely to get a handle on the matters you brought up.
    As I said to him,it is probably better at this point to just get an idea of how a transformer works in practice,& leave the "Why is a cat not a dog?"questions for later.
     
  14. cabraham

    Member

    Oct 29, 2011
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    But we should not confuse a newbie with statements that are not true. The Thevenin-Norton equivalence principle is a good foundation, but it simply states that a complex network can be replaced by a voltage source plus series resistor, or a current source plus parallel resistor. But the Thevenin-Norton theorem does not state that one of them "drives" the other.

    In the real world, voltage sources are less lossy than current sources since conductors are lossier than insulators. So the utility company spins their turbines at constant speed producing constant voltage. They could produce constant current as well by spinning them at constant torque.

    To visualize a xfmr operating, I'd recommend the mechanical gear set analogy. Torque is higher on one side but speed is lower. The product is power which is conserved. Beyond that, this question requires peer reviewed reference books for a sufficient answer. That is all I'm saying.

    I did not intend to insult anyone's intelligence. I'm sure people know that power plants transduce mechanical to electrical energy. But the bus is so good, steady, & well regulated, we just take the constant nature of the voltage for granted. It takes a lot of effort to maintain that steady voltage & frequency. BR.

    Claude
     
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  15. Wendy

    Moderator

    Mar 24, 2008
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    Actually not quite true, and we have had problem with that attitude in the past. Analogy is used often in electronics when teaching new concepts, starting with conventional flow and moving on to many other cases. It is a mistake to insist on perfection for beginners. Many times theory is taught is such way to get someone past a hump, so they can move on and learn other concepts. Later, when they have a deeper background, they can relearn the more correct explanations, and not be as confused by them. It is one of the core differences between teaching beginners and advanced students.

    The main component in transformers is counter EMF. It also works in motors and generators.

    When you spin a motor it also becomes a generator. The magnetic field generates a counter electric field that resists the flow from the main electric field. As you load a motor down the counter electric field is reduced, causing a greater current flow.

    With transformers it is somewhat similar. A counter electric field is created in the secondary, which feeds back to the main coil the same way the main magnetic field is feed to the second coil. This resists the current flow in the primary coil.

    When you tap into the field of the second coil this feedback to the primary is reduced, which causes current through the primary to increase proportionally.

    This directly feeds in conservation of energy, you are using the energy feed into the primary out the secondary, but when no energy is tapped from the secondary it will resist the flow from the primary. It is one of the reasons a transformer is so efficient. It is one of the few man made devices that can (not will, but can) exceed 95% efficiency.
     
    Last edited: Sep 11, 2012
  16. cabraham

    Member

    Oct 29, 2011
    82
    30
    My point was that someone mentioned the phrase "voltage drives current" & I was pointing out that it isn't that simple. The xfmr illustration you just gave supports my point that I & V are mutual & interactive. But it needs addition.

    When the secondary is loaded, the sec load current generates a B field that counters that of the xfmr core, generating a counter-counter-emf. This ccemf opposes the cemf, & the primary current draw increases. I know how xfmrs work.

    The original disagreement had to do with the notion that voltage is driving the current. I simply pointed out that either one can be the fixed independent quantity, but the power company, for good reasons, elects to hold the voltage constant, letting current vary with load.

    Yes I am aware that a motor produces generator action & vice-versa. That was not the topic under scrutiny. As far as analogies go, they can be useful when presented correctly. I just don't feel that a concept which is not entirely correct should be introduced to a beginner, thinking that they can move on later to more advanced studies and correct the flaw.

    I've found that many have great difficulty extending to the next level, a reluctance to discard the analogy that seems to work for them for so long. When a beginner is taught something partially true early, they may not be willing or able to discard it when they need to at a more advanced stage. So I like to be safe & not allow the beginner to adopt an analogy that is partially incorrect, out of fear that the damage may not be reversible later.

    When someone has believed something for a long time, it may be easier to move a mountain than to change their mind. You seem to believe that we can give a beginner a compromised analogy for now, then when they get to a more advanced stage, we can just tell them to discard it, or part of it. I'm saying that with some, that is not likely to happen.

    Just my $0.02. BR.

    Claude
     
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