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If transformer is coil (primary and secondary), then when we connect ac source across primary winding which is a nothing but a coil, so entire coil impedance will come across ac source and it will be short circuit condition at input winding ( considering low resistance of primary winding ) and at secondary even if secondary is open then ac line is short circuited by low resistance primary winding.

Does it actually do short circuit of ac mains?

Please someone elaborate , I have this basic question.

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The "coil impedance" is not the short circuit that you imagine it to be. It is mostly an inductive reactance, due to the fact that it is wound around an iron core. This limits the amount of current drawn by the primary, when nothing's connected to the secondary.

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  • \$\begingroup\$ IMO this is the only answer that properly answers the question. \$\endgroup\$ – Rev1.0 Apr 29 '15 at 10:30
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Not sure if I get the question (due to my low experience), but if you only have a circuit with a coil then the only "resistance" you have is the cable's resistance and the special ability of the coil. The current through the coil creates a magnetic field and because of this field, the coil induces a voltage (or "emf") that stops your current. This only lasts for a very short while, but since it will happen everytime the alternative current changes direction I guess you're permanently protected from short circuiting.

schematic

simulate this circuit – Schematic created using CircuitLab - physics 2nd year student

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The transformer will generally have a high inrush current, and the larger the transformer kVa wise, the larger the inrush current, but the high current is only on the first AC cycle. As the core magnetizes, the idle current drops off to just about zero. It isn't a short across the mains, since the magnetic field builds up in the core nearly instantaneously. It is just that applying power initially, there is no magnetic flux to oppose the AC.

Often, on single phase transformers, you can reduce or eliminate the inrush surge by powering them up at a zero crossing

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    \$\begingroup\$ Nope. Worst possible inrush is caused by switching at zero crossing with the wrong remanence (from how it was shut off). \$\endgroup\$ – Spehro Pefhany Apr 28 '15 at 17:14
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What you are describing is essentially performing an OC (open circuit) test on a transformer. During an OC test we can apply rated voltage to the transformer. (Applying voltage over the rated voltage will result in the degradation of the insulating material.)

If you think of it purely are once circuit, yes the AC source is short circuited. But it isn't one circuit.The primary coil is magnetically coupled to the secondary which as you said is open circuit. This initial current which runs through the circuit is high, but it is not nearly as high as what the transformer can run under full load.

The only current the transformer will pull is the current required to overcome iron losses and magnetizing current. This current is usually 2-10% of the rated current.

The designer of the transformer will choose an appropriate gauge of wire to handle this current.

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  • \$\begingroup\$ But isn't inrush current normally MUCH higher than the rated current? You seem to imply the opposite. \$\endgroup\$ – Rev1.0 Apr 29 '15 at 10:31
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For all learning intents and purposes, you can view it as this;

enter image description here

It only short circuits if you short circuit the secondary, otherwise it simply acts as if it's a circuit with a coil on either line, i.e. like a normal output, but with the effect of the transformer ratio. So if it's 1:2, you put in 110VAC @ 2A and get around 220VAC @ 1A.

You shouldn't do anything with mains unless you isolate yourself with a 1:1 transformer, a fuse, and gloves. You don't have the basic stuff quite down yet.

For a more complex view;

enter image description here

Neither end of the primary connection is really touching in terms of a typical circuit. You can view it as the above image if the concept doesn't agree with you. However, there is coupling in both cases of my separation examples. If you short the secondary, you short the primary. They depend on each other, treat the secondary like you'd treat the primary.

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    \$\begingroup\$ Nice illustration, but IMO it does not hit the nail on its head regarding the original question (WHY the primary low resistance winding isn't a short by itself). \$\endgroup\$ – Rev1.0 Apr 29 '15 at 7:30

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