# Will primary flux & secondary flux (back MMF) in a transformer attract or repel each other?

I couldn't find it on google or may be I am not using the right keywords, if my terminologies are not sounding accurate, please bear with me, I am beginner in electrical engineering.

When the primary coil of a loaded transformer is connected to AC current, it generates a flux that goes into secondary, the loaded secondary induces voltage that generates its own back EMF induced flux which cancels the flux generated by primary. What I don't understand is this cancellation happens when both fields have same polarity or different polarity? Will North-North flux cancels each other or North-South flux cancels each other? Also my question is whether primary flux and secondary back EMF induced flux attract each other or repel each, (meaning whether they have same polarity or different polarity)?

When the primary coil of a loaded transformer is connected to AC current, it generates a flux ...

loaded or unloaded, it doesn't matter. It can be conceptually easier to start with the unloaded case when trying to see how primary EMF controls the core flux.

that goes into secondary

the flux is in the core. It couples to both the primary and the secondary.

the loaded secondary induces voltage that generates its own back EMF induced flux

The voltage induced in the secondary, when applied across the load, causes a load current to flow in both the load and the secondary.

which cancels the flux generated by primary.

which current is of opposite sense to the primary current, so would tend to reduce the flux (not cancel) in the core. As the magnitude of the flux is determined by the magnitude of the primary voltage, it must stay constant. The primary current increases so that the total magnetising current, that is the algebraic sum of the primary and secondary current (which as the currents are more or less opposite in sign is often written as primary-secondary currents), remains constant.

What I don't understand is this cancellation happens when both fields have same polarity or different polarity?

The primary and secondary currents have opposite direction, so their individual fields would have different polarity.

Will North-North flux cancels each other or North-South flux cancels each other?

Talking about N and S poles is more useful when referring to bar magnets, or open solenoids. The field inside a transformer is closed. However, referring to the coils as solenoids, if the primary current produces a N pole at the top, the secondary current would produce a S pole.

Also my question is whether primary flux and secondary back EMF induced flux attract each other or repel each, (meaning whether they have same polarity or different polarity)?

Attraction and repulsion are terms that are more useful when physical objects can move. The primary and secondary coils are wound on the same former round the same coil. However, there is a sense in which the concept of attraction/repulsion can be used to get some insight into what's going on in a transformer.

Take a system like a stretched rubber band, whose two ends attract each other, or two magnets whose N poles repel each other, or a mass above the earth, that attracts it. Distort the system, pushing against the force you feel. In all cases, this is storing energy in the system. Now give it its freedom, stop distorting the system, and it will move to reduce the stored energy.

In a transformer or inductor core, the stored magnetic energy is proportional to the magnetising current squared. Not the primary current, or the secondary current, but the magnetising current, that is, their algebraic sum. If the secondary current was in the same phase as the primary current, their sum would increase, increasing the stored energy. That's not what energy storage systems do, they try to reduce their energy. This means that the secondary and primary current oppose each other, to reduce the net current, reduce the net magnetisation, and reduce the stored energy in the core.

• Good answer. Only comment I would add is that the stored energy is not limited to the magnetizing current. Energy is stored in the leakage inductance as well, right? Commented Aug 13, 2022 at 11:43
• @relayman357 given the obvious level-0 knowledge of the OP, I think we'll leave leakage inductance out of this, along with finite permeability and winding resistance, to keep the transformer as ideal as possible. I try to pitch my 'lies to children' at a suitable level. The more serious things I've omitted are why the primary voltage fixes the core flux, and why secondary current causes an additional primary current to flow, other than assert that they somehow do. I've hinted at the finite permeability thing by replacing 'cancel' with 'tends to reduce', so leaving a finite magnetising current. Commented Aug 13, 2022 at 13:09
• @Neil_UK I did one experiment with 3 phase transformer where I made the middle one primary and other two secondaries, one one secondary is loaded the load doesn't light up because the flux follows the path of other unloaded secondary, when both secondaries are loaded only then the both loads light up, by this I conclude that the primary flux is repelled by the back EMF induced flux when one secondary is loaded, that is why the primary flux follows the path of other unloaded secondary which is the path of least reluctance. That is why I asked whether the primary flux is repelled or attracted... Commented Aug 13, 2022 at 13:13
• @Yogie I've answered the question you apparently asked, about a single phase ideal transformer (a reasonable assumption for 'transformer' without any clarification). If you'd like to find out about a three phase transformer core driven in the unusual way you suggest, then ask a new question, preferrably with a diagram, and I may look at it. Briefly, the H field created by the secondary current opposes the primary, and the flux follows the path of least reluctance. Perhaps relayman357 was on to something when he mentioned leakage inductance, which your configuration has in spades! Commented Aug 13, 2022 at 13:32