Transformers: relation between their current, voltage and resistance

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I am not able to digest the fact that on applying more voltage across a circuit, current decreases.

This isn't a correct picture of transformer operation.

For concreteness, assume the secondary circuit (load) is a single resistor of resistance $R_L$ and assume the primary is connected to an AC voltage source of magnitude $V_p$.

Now, if the primary voltage is increased (decreased), the current through $R_L$ increases (decreases) just as you expect it would.

However, for a step up transformer, the load resistance appears smaller to the source. So, in fact, while the voltage on the secondary is larger, the current in the primary is larger than it would be if the source were connected directly to the load.

Put another way, the source 'sees' a resistance smaller than $R_L$ and, thus, must supply more current to the primary than if the $R_L$ were connected directly to the source.

So, in fact, the increase in secondary voltage results in an increase in primary current. This is the nature of transformer action.

The analogy I want to use

The proper mechanical analogy to use is force (voltage), velocity (current) and lever (transformer).

For a step up (down) transformer, the lever is longer (shorter) on the force source side of the fulcrum.

For example, a sinusoidal force (voltage) applied to the long side of the lever, will result in a larger force (voltage) on the short side but a lower velocity (current). This is, I believe, intuitive.

What I want in the answer

I appreciate a direct approach. Assuming you do too, be aware that you are owed nothing and those that take the time to answer here do so in the expectation of trade of value for value.

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Mahathi Vempati
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Mahathi Vempati

Updated on December 15, 2020

Comments

  • Mahathi Vempati
    Mahathi Vempati almost 3 years

    My current understanding

    Transformers are used to step up and down voltage keeping power constant. Hence, for example, if I step up some voltage, the current will decrease in the secondary circuit.

    The issue I have

    I am not able to digest the fact that on applying more voltage across a circuit, current decreases. To me it seems that the extra potential difference should make the electrons zoom faster.

    My guess

    It occurred to me that maybe, for example during step up, due to the fact that there are more coils in the secondary circuit, the resistance of the secondary circuit increases and this increase in resistance reduces the current, although there is a higher voltage.

    But then I thought that if we use another material in the secondary coil, that has lesser resistivity, such that even on making more number of turns, it has a resistance equal to that if the primary coil, then what happens?

    The analogy I want to use

    10 joules of energy can apply 10N for 1m or 5N got 2m. But it does the same amount of work because the first object is obviously heavier.

    I would really appreciate it if someone could:

    1. Clarify how there is increase in voltage but decrease in current in secondary coil.

    2. Explain why my guess is correct or wrong

    3. Relate my analogy to the problem.

    4. Correct anything wrong in my understanding.

    Thank you


    Edit:

    There is one more similar question on the site, but it has a mathy answer. I'm looking for something more intuitive. (However, please include the relevant formulas.)

    • Admin
      Admin over 7 years
      That's it! You got answers!! All the best :D
    • Whit3rd
      Whit3rd over 7 years
      Your question includes 'increase in voltage but decrease in current' in a way that suggests a component has lessened current when applied voltage is increased (i.e. negative resistance) but your 'increase in voltage' isn't associated with voltage stress on a component, rather it is associated with a circuit component change (altering the turns ratio of a transformer). It is misleading phrasing, to describe a turns ratio change as though it were negative resistance.
    • ManRow
      ManRow almost 4 years
      @Mahathi Your confusion stems from misconception that both sides of a transformer are "separate circuits". A transformer does not "separate" circuits, it "magnetically couples" them. So you have one large circuit, with a different primary and secondary section. Any circuit analysis ignoring this primary-secondary coupling is likely to be quite inaccurate.
  • Mahathi Vempati
    Mahathi Vempati over 7 years
    +1, thanks for the great analogy. I put 'What I want in the answer' hoping to make my question clear and easier to answer, but I guess it looked cocky or seemed like an order. I'll reword that.
  • Andy aka
    Andy aka almost 5 years
    This is a very poor answer and incompatible with physics and electrical engineering.