# Why does the current in a transformer's primary coil change when a load is attached to the secondary coil?

I don't really know if the question is suitable to be submitted here or in the Physics StackExchange. The problem is I don't quite understand this sentence from "Practical Electronics for Inventors" Book, p.377

When a load is attached to the secondary, the secondary current sets up a magnetic field that opposes the field set up by the primary current. For the induced voltage in the primary to equal the applied voltage, the original field must be maintained. The primary must draw enough additional current to set up a field exactly equal and opposite to the field set up by the secondary current.

The explanation says that when a load is attached to the secondary, the current in the primary must change to keep the applied voltage field the same. I have no idea what that means and why does this phenomenon have to occur in the primary, when there is a load in the secondary.

Furthermore it's assumed that the magnetizing current will be very small in comparison to the current after the circuit being loaded, which I also don't understand.

I need some more in-depth explanation. I have quite good background in physics of electricity from first year of engineering college, so feel free to give a deep explanation about what is happening.

• the primary produces flux, the secondary drains it away. Sep 3, 2020 at 20:34
• Notice that flux generates the back emf (voltage) in the primary winding to counter the input voltage. But when the secondary current starts to flows, the flux is reduced, resulting in a corresponding reduction of the back emf in the primary. The primary current then increases until the flux is again high enough to counter the applied primary voltage.
– G36
Sep 3, 2020 at 21:59
• TANSTAAFL in two entirely different ways: (a) If you take more power out of the secondary, it has to come from the primary. The load decreases the impedance of the system as a whole. (b) The DRY principle (don't repeat yourself) applies to knowledge as well as to code. It is counterproductive for us to replicate stuff just so you don't need to learn to search. "Give a man a fish ..". Sep 4, 2020 at 9:43

If you connect a nominal voltage to primary, then a magnetizing current starts to flow, which has a 90 degrees shift. Now, we could say that the nominal magnetic flux is present and the secondary voltage equals to primary with regard to transfer ratio Np:Ns.

Once you load the secondary, the current would cause otherwise to increase the magnetic flux but this is not going to happen, since the primary current also increases and cancels that extra flux.

I have no idea what that means and why does this phenomenon have to occur in the primary, when there is a load in the secondary.

Me neither, but it's the way it works. You will hardly find any human readable explanation on that even if you are a doc.

Furthermore it's assumed that the magnetizing current will be very small in comparison to the current after the circuit being loaded, which I also don't understand.

If the transformer is nominally loaded then we could say that this magnetizing current is very small compared to total primary current, but it is held constant regardless of the load current. If the secondary is unloaded, then this is the only current.

EDIT:

While beginning a study on a transformer, it is simpler that you imagine that primary, secondary, tertiary, ... voltages are induced due to magnetic flux change, and that flux is a cosine wave The voltages are all sine waves and perfectly in phase.

Next step is to add the magnetization current, this is taken from point of energy transfer, which not necessarily means that you have only one primary winding.

A more basic physics explanation is simply that conservation of energy holds*, and you can only stuff so much energy into the magnetic field of a transformer.

So if you're taking energy out of the transformer in the form of secondary current (and voltage), then you have to put energy into the transformer in the form of primary current (and voltage). It's just inescapable, which is why perpetual motion machines are for con men and fools.

You can do an amazing amount of power electronics design just by remembering that conservation of energy holds, BTW. Power in = power out. Motors, generators, power supplies -- they all obey.

* On any human scale. Shortly after you could build even a solar-system scale perpetual motion machine, you'd be ripped apart by runaway expansion of the universe. But that would be after every proton in the universe has evaporated into photons. So -- just take it that conservation of energy holds.

When a load is attached to the secondary, the secondary current sets up a magnetic field that opposes the field set up by the primary current.

If the secondary didn't do this, consider what might happen. Let's say the "net" field increased. If it increased then there would be more induced voltage in the secondary and the secondary current would increase and thus you have a vicious spiral ending in the universe collapsing (or something like that).

Say that the "net" field reduced, what would be the effect - if the field reduced then the secondary voltage would fall and there would be less secondary current and that means the field must restore. Do you see the problem?

For the induced voltage in the primary to equal the applied voltage, the original field must be maintained. The primary must draw enough additional current to set up a field exactly equal and opposite to the field set up by the secondary current.

That's what happens. The extra field created by the load current is wholly balanced by the opposing field that the primary generates due to load current. That's what causes the primary to take current when there is a secondary load.

Furthermore it's assumed that the magnetizing current will be very small in comparison to the current after the circuit being loaded, which I also don't understand.

The magnetization current can be any value without affecting transformer action. Clearly though, large mag current is undesirable for several reasons.

I need some more in-depth explanation.

This is a question and answer site and isn't geared-up for pages of explanation on request or otherwise but, maybe this picture will help: -

Because $$\I_P\$$ and $$\I_S\$$ have to be in opposition then the load-orientated fluxes must cancel.

• Do you have any books in mind to recommend If I really need that explanation ? Sep 3, 2020 at 17:59
• @MahmoudSalah no, I don't refer to books for transformers. I've added a picture that might help. Sep 3, 2020 at 18:01
• You get introduced to this in a basic 2nd-year Circuits course in college, but it's not much less hand-wavy than your electronics for inventors book. You really get this information in spades in a course in electric machines in college -- but that's going to be a junior or senior-level course, and it's not going to make a lot of sense unless you've taken (or are simultaneously taking) electrodynamics. Sep 3, 2020 at 20:38

When the secondary solenoid subjects to input flux, the opposing current sets in the solenoid to produce the reverse magnetic flux in order to balance the flux caused in it by primary winding. (Lenz law and Faraday explanation.)

This balance keeps on untilthe load is subjected to secondary and draws current from it .

This drawn current tends to lower the voltage of secondary by I X R = V.

Dropped voltage causes lowered current and lower opposing flux towards primary posed flux.

This dis-balances the equilibrium between primary flux and secondary flux.

Now more flux is drawn into the secondary because of its lowered opposition which in turn draws current into the primary from the main supply.

This all continues until the new current in the secondary is set which is necessary to form new opposing flux and the new balance between secondary and primary flux.

Secondary becomes the 'primary' of the first when it generates the equal and opposite symmetrical field under load as per Lenz's law. When current is drawn, emf is generated in the primary which adds to the input voltage. V/L=di/dt More voltage means increase in current.