I have found it very difficult to find what actually happens with primary and secondary generated flux in transformers, I understand the normal transformer theory but it doesn't add up in reality! I am hoping someone can shed some light. Simply put, if magnetic lines of flux cannot cross or actually cancel does this mean that the secondary coil is an air core?

  • 2
    \$\begingroup\$ Could you explain why you believe the secondary coil is air-cored. Both primary and secondary are wound on the same core and the same magnetic flux flows through both windings. \$\endgroup\$
    – Chu
    May 5, 2015 at 22:50

3 Answers 3


You could begin an argument about transformers like this: -

  • A transformer has magnetizing flux due to current flowing into the whole of the primary winding when the secondary is open circuit i.e. the transformer is an inductor.
  • When extra current flows in the primary due to loading of the secondary this flux increases
  • The current flowing in the secondary winding also increases flux (incorrect, it totally cancels the ampere turns in the primary due to loads on the secondary)

However, the massive flux generated by the primary due to loading the secondary is totally cancelled by the reverse magnetic flux due to current flowing in the secondary. Consider these scenarios: -

enter image description here

  • Scenario 1 is just the primary winding with Imag flowing
  • Scenario 2 is two windings wound close together - Imag (the same as above) is shared by the two windings
  • Scenario 3 is the 2nd winding open circuit - it has the same voltage as the primary winding
  • Scenario 4 is the secondary load - Iload currents are in opposite directions and their fluxes cancel.
  • \$\begingroup\$ Scenario 4: the flux cannot cancel in reality, it still requires a path. \$\endgroup\$ May 5, 2015 at 22:12
  • 1
    \$\begingroup\$ @StevenCarr No, you are incorrect, the load associated fluxes cancel completely leaving just the flux due to magnetization. This is why transformers can sometimes trip breakers when unloaded - they are close to saturation due to the mag current. With a load current present there is a slight volt drop due to leakage inductors and this slightly reduces mag current and hence slightly reduces saturation. Most people don't find this intuitive but nevertheless it is true. \$\endgroup\$
    – Andy aka
    May 6, 2015 at 7:27
  • \$\begingroup\$ Looking closer at this issue, I think no additional flux is generated by the secondary (maybe a small amount on initial loading) as only secondary field intensity increases and this is cancelled by an equal and opposing primary field intensity, then looking at the BH curve this would then leave the primary flux almost the same under load . \$\endgroup\$ May 6, 2015 at 9:41
  • \$\begingroup\$ @StevenCarr Yup, that's what I said. Ampere turns on primary and secondary cancel except for mag current. \$\endgroup\$
    – Andy aka
    May 6, 2015 at 9:47

The flux lines (mostly) link the two coils- lines pass through the primary and through the secondary. The closed loops are a consequence of the lack of magnetic monopoles- unlike electric charges there are no sources and sinks of magnetic fields. Real transformers have a certain amount of flux that does not link the coils- that is called leakage inductance.

Magnetic field lines are a visualization aid to help understand intuitively a vector field. The flux line density is related to the magnitude of the field and the tangent to the field direction is the line direction.

Another way to visualize this in situations where most of the field is within a core is to think of it like current in a circuit- like a loop with a certain voltage applied (magnetomotive force) through an impedance (reluctance) or conductivity (permeability) and results in a current (flux) and current density in the conductor (flux density).

  • \$\begingroup\$ Ok, so for the flux lines to link, the coils must have similar poles if on the same core space i.e if both primary and secondary are wrapped around the same core space and at one instant one end of the primary coil is North at the same instant the same end of the secondary coil will be North. This scenario would seem to be suggesting both coils been additive to the field not opposing/cancelling as transformer theory implies? If the scenario is opposite then the core must be of suitable area to accept flux in both directions, so half domains aligned one way and vice versa? \$\endgroup\$ May 5, 2015 at 22:08
  • \$\begingroup\$ Any induced secondary current must be set up in a direction to oppose the changing flux that created it - Lenz's law \$\endgroup\$
    – Chu
    May 5, 2015 at 23:21
  • \$\begingroup\$ Yes I understand Lenz's law, I was looking to understand practically which path the secondary flux takes. Looking at the BH curve more closely I think no extra flux is created by the secondary coil, as the field intensity is increased in the secondary this doesn't imply that flux has to flow, if an opposing (primary) field of equal intensity is cancelling it. That seems to make sense to me now. @Spehro \$\endgroup\$ May 6, 2015 at 9:27

I'd like to try to summarize: When the primary windings are energized (voltage applied), current begins to flow and will produces magnetizing flux in the core. The flux induces a BEMF on the primary winding that opposes the applied voltage, which limits the current flow in the primary. On the secondary side, the flux in the core will induce a voltage, about the same if N_turns is 1:1. If the secondary circuit is completed (shorted or loaded) secondary current will flow. This current will then produce flux that counters what... the initial magnetizing flux? Has to if there was no load current/flux in the primary to start with. So what else? It seems that this is an ongoing iterative process. As the magnetizing flux is opposed by the secondary, the BEMF in the primary is reduced and the applied voltage can drive more current (load current) in the primary. This current then contributes about the same amount of flux that was previously countered by the secondary (i.e. flux in the core ends up about about the same, no net change). On the secondary the load current continues to increase based on the load characteristics, assuming the secondary voltage is constant or increasing, and the iterative process continues: increased secondary current, counter secondary flux, reduced primary BEMF, increased primary current, increased primary "counter-counter" flux until the secondary side settles out to satisfy good ole Ohms law. But only at an instant in time since the input is AC and it never really settles. This is sort of what happens with levitating super conductors; things move, fields and currents are induced, and equal counter fields are produce instantaneously so it all balances out. The primary and secondary fluxes except the for the initial magnetizing flux, which requires the core to link the flux with the windings, i.e., an air core will not work very well.


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