# Transformer: primary side & secondary side current 180 degree out of phase

I am a novice in electrical engineering. I notice that in transformer the secondary side current & current referred to as primary are 180 degree out of phase from each other.

But why it is so, I don't know.I did some theoretical analysis on this topic but no phasor analysis(why 180 degree). So, please give me suggestions.

I also thought a phasor analysis but I need 10 reputations.

Consider an ideal transformer for simplicity (from Wikipedia).

(the voltages and currents shown in the picture are phasors)

If you consider the above circuit (transformer) as a block in a larger circuit, because it is an ideal transformer you will not have any energy loss in the block. So you can write:

$$I_P \times V_P + I_S \times V_S=0$$

and also, the magnetic flux is constant in the core, and it doesn't have any leakage, so: $$\frac{V_P}{V_S}=\frac{N_P}{N_S}$$

from two above equations you can arrive at: $$\frac{I_P}{I_S}=-\frac{N_P}{N_S}=-a<0$$

which is equivalent to a 180 degrees phase difference between input and output currents. (The Wikipedia article does the same calculations, with $I_S$ current reversed)

There is a very intuitive way to understand why this must be so.

The ideal transformer does not dissipate energy; there is no energy loss (and certainly no energy gain).

Thus, if power is delivered (by an external circuit) to the primary, it must be supplied (to another external circuit) by the secondary.

If follows that if the primary and secondary voltages have the same polarity, the primary and secondary currents must have the opposite polarity.

For example, if the primary current enters the positive terminal of the primary, power is delivered to the primary.

Thus, the secondary must be supplying power which means that the secondary current must exit the positive terminal of the secondary, i.e., the secondary current is opposite the phase of the primary.

A much simpler answer to this question is that the current induced in the transformer secondary windings (assumed not an 'autotransformer') derive from an application of Lenz's law, which is to say, when the magnetic field in the common iron core of the transformer is forced to collapse by current being reversed and transitioning through zero in the primary windings, a current in the opposite direction ('180 degrees out of phase') is induced in the secondary windings.

In an autotransformer (not part of the question, but added for safety sake), one side of the secondary is connected directly to the secondary. It makes a big difference which phase of the transformer secondary is used, because choosing the wrong one would mean transferring all of the voltage on the primary side directly to the load. In most circuits, the transformer and the load would fail or burn.

bcause the emf induced in the secondary is known to be back emf...which opposes in such a way to show it lik a spring when gets compressed..it bounces with opposite phase.yoga....

• Could you edit your answer (mainly improve grammar), since your current answer does not make much sense, probably because english is not your primary language. – fibonatic Mar 9 '14 at 20:02

THE ANGLE BETWEEN PRIMARY AND SECONDARY 180 Deg Due to the flux from primary to secondary winding. so current flows in the primary and secondary entirely opposite direction