According to Faraday's law the net flux in the core of a transformer will induce an EMF in both coils, and that magnetic flux generated by each induced current in the coils (on-load) will oppose any change to the net flux.

So how does this work to stabilize the transformer? Essentially, any change to the net flux will change the induced EMF in both coils by the same amount. I think the point of interest is that if the secondary EMF decreases so will the secondary current (and hence the secondary flux will decrease), but if the primary EMF decrease the primary current will increase, thereby increasing the primary flux (and raising the primary EMF again). Isn't this how a transformer remains stable?

Assuming this is so, my question is why the induced EMF opposes the current in the primary. (This is the supply current, right?) and secondly why doesn't the induced EMF in the primary affect the supply voltage. Thank you


1 Answer 1


The magnetic flux due to secondary load current is completely cancelled by the associated magnetic flux in the primary due to that secondary load current. The flux that remains is the primary magnetization flux and remains the same whether there is a secondary load or not.

If you prefer: the ampere turns in the secondary (which of course can only be due to load current in the secondary) produce equal and opposite "load" ampere turns in the primary. This means that only the ampere turns due to magnetization current are present in the core.

Ampere turns associated with load currents cancel.

  • \$\begingroup\$ OK, so flux from the load current cancels with flux from the current it induces in the primary. Shouldn't there also be self-inductance in the primary by the primary magnetization flux? This is where I'm getting stuck here, thanks \$\endgroup\$
    – sunra
    Commented Jul 26, 2021 at 16:17
  • \$\begingroup\$ The primary has self inductance. It's called magnetization inductance. \$\endgroup\$
    – Andy aka
    Commented Jul 26, 2021 at 16:46
  • \$\begingroup\$ Current isn't induced in magnetic components; voltage is induced. \$\endgroup\$
    – Andy aka
    Commented Jul 26, 2021 at 17:11
  • \$\begingroup\$ Thanks, essentially the change in primary flux when we attach a load is equal and opposite to the secondary flux. \$\endgroup\$
    – sunra
    Commented Jul 26, 2021 at 18:32
  • \$\begingroup\$ No, the change in primary flux doesn't equal the magnitude of the secondary flux. The two fluxes are equal and opposite. This of course discounts the "unalterable" magnetization flux i.e. the statement applies to currents/flux associated with a secondary load. \$\endgroup\$
    – Andy aka
    Commented Jul 26, 2021 at 18:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.