If we apply a voltage at the primary terminals of a transformer then for this we can get a voltage at the secondary. But they have 180 degree phase difference. What is the reason behind this??

I want to know why the phase difference between primary voltage and secondary voltage is 180 degree.

  • \$\begingroup\$ Yes, it all depends on transformer construction. \$\endgroup\$
    – serge-k
    Commented Apr 4, 2016 at 1:20
  • \$\begingroup\$ 180 degrees is just a fancy way of saying the phase is reversed from what you expect. This could be due to manufacturing differences- they wound one way instead of the other, or chose to attach leads to the opposite terminals than you expected. Swap primary OR secondary connections and you'll be right as rain. \$\endgroup\$ Commented Apr 25, 2016 at 22:37
  • \$\begingroup\$ What happen if you swap your wire at the terminals? May phase turn to zero. \$\endgroup\$ Commented Feb 23, 2017 at 20:52
  • \$\begingroup\$ Perhaps the secondary side is wound in reverse direction to the primary side? \$\endgroup\$ Commented Apr 30, 2017 at 4:28
  • \$\begingroup\$ @Raihan Khalil please provide the diagram of the transformers. Different transformers have different phase shifts. \$\endgroup\$
    – alejnavab
    Commented May 16, 2020 at 6:56

4 Answers 4


Imagine you wound an inductor but instead of one thicker wire you used two thinner wires (this is commonplace by the way). An AC voltage is applied on both wires - lets call the AC voltage AC live and AC neutral and lets say neutral is connected to 0V for reference.

No problem so far?

Now, imagine you only applied AC live to one of those wires and left the other one open circuit (AC neutrals both at 0V still). What would be the induced voltage in the unconnected wire? Would it be: -

  • In phase
  • Out of phase by 180degrees
  • Same amplitude
  • Different amplitude

If the induced voltage on the unconnected wire were antiphase then joining it up to the AC live would create a seriously big problem and there would be sparks and breakers tripping. If the induced voltage on the unconnected wire were significantly different to the AC live and you joined them up then same problem as before - sparks and breaker trips.

This means if you wind a transformer with two wires in the same way there will be no phase shift but you can always swap the primary wires or secondary wires over and get 180 degrees.

Inherently there is no 180 degrees phase shift - you only get 180 degrees when you get into a muddle with the wires.


The phase difference exists or not depending on whether the primary and secondary turns are wound in the same direction or not, which will make which end of the secondary you compare with what end of the primary in phase or not.


I've not the strong answer, but I have an opinion.

If I draw the diagram of a transformer, and try to find-out the direction of current using Lenz's law; if I keep the coiling-pattern of primary and secondary coils Same (Say both as righthand-screw, or both as Lefthand-screw); I'm getting an 'opposite', i.e. 180-degrees phase-shifted currents in the 2 coils.

Tried finding out the direction of current in transformer coils

However I do-not know, in which-way Lenz's law acts in transformer; because the law states the induced current will try to hinder the cause. but when the secondary circuit of a step-up transformer turned On (closed) , so-far I've know , the current in primary-coil goes-up

In-fact I was going to ask a question about whether a transformer cause any phase difference. But I found already someone asked such a question here, already mentioning 180 degrees phase shift. So I've added my- thoughts here.

Any feedback is welcome.

  • \$\begingroup\$ The question talks about applying a voltage and not analysing the currents. As per my answer the voltages are in phase and simple logic tells you that if the secondary feeds a load, current into the primary (zero degrees) produces current out to the load (180 degrees). This is a direct result of zero degree phase shift between primary and secondary voltages. \$\endgroup\$
    – Andy aka
    Commented Apr 25, 2016 at 11:03

it will be phase shift of 90 degree as when cueent or voltage be maximum in primary coil be maximum then flux will be maximum and induced emf in secondry coil in secondry coil be zero. and when current be zero in primary coil then dI/dt be maximum so induced emf be maimum in secondry coil.

  • 3
    \$\begingroup\$ Welcome to EE.SE. This is incorrect. See Andy aka's answer. Please edit to correct or delete so you don't get downvotes. \$\endgroup\$
    – Transistor
    Commented Nov 25, 2020 at 14:26

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