# What is the difference between applied voltage and induced voltage at primary side of transformer specifically?

I am not able to understand difference between applied voltage and induced voltage. Whenever voltage is applied at primary side of transformer there will be primary current producing flux using primary winding and that flux will again interact with primary winding to establish a current that will oppose the main current or the main flux will interact with primary winding to establish a emf(back emf) that will produce back current opposing main current. So what is induced voltage? if anyone can explain.

• It's probably best to read Chapter 16, Vol 2, Feynman Lecture Series to get a quick feel of the idea. When you feel comfortable with that chapter (and only then) you can try reading Chapter 17, Vol 2, Feynman Lecture Series for more detail. But the upshot is that when charges move, they set up a magnetic field. If the current changes then the magnetic field itself changes and does so in a way to oppose the change (resist it.) This acts like a back emf.
– jonk
Commented Oct 3, 2021 at 3:36
• Jonathon, you appear to be saying either (a) or (b) but what you write is confusing. Can you be clear about what you want to say. If all you feel you can validly say is "where does induced voltage come from" then just say that. If you feel you have to ask about what you appear to be asserting then ask; don't assert. Personally speaking, back emf in the coil that is driven is a bit of a red-herring. Sure, it exists (just as an induced voltage on a secondary winding exists) but it is of no consequence to any analysis that I know of. Commented Oct 3, 2021 at 10:08
• The question title is fair and seems to be related between two transformer tests: "induced voltage" and "applied voltage" and it is a frequent doubt among new transformer designers. But, in fact, ends up in a doubt related to confusing between the flux/voltage causing. Commented Oct 3, 2021 at 12:29

Volts per turn is the voltage across a single turn of a conductor looped around an area that encloses a time-varying flux. That area is the cross-sectional area of your core. At least, that's a good approximation in a high permeability cored inductor or transformer.

Going round once gives you volts per turn. Going round N times gives you N times that voltage, which allows you to make multiple windings all producing different voltages.

This is just a relation between the voltage and the flux. Just as Ohm's Law describes a relation between voltage and current at a resistor, and one of Newton's Laws describes the relation between force and acceleration.

When we start trying to establish what causes what, we run into conceptual problems. We say that a varying flux causes an induced voltage or a back emf across the winding. We equally well say that an applied voltage causes a time varying flux in the core.

That's why you're confused about the 'difference'. It's the same voltage, the same relation with the flux, but we describe it with different words.

It turns out that cause is not an appropriate concept to apply as we know it. If I kick a ball, and it breaks my neighbour's window, then I can say that the former caused the latter, because the reverse was certainly not true, the broken window didn't cause me to kick the ball.

We cannot use cause in this sense with voltage and flux. They both exist and vary together, one does not cause the other.

The primary terminals of a transformer do not care whether you have hooked up a low impedance voltage source to them (so are controlling the voltage), or a high impedance current source (so are controlling the current). The voltage across the terminals, and the current in and out of them, will still follow the same relation.

What is the difference between applied voltage and induced voltage at primary side of transformer specifically?

The applied voltage of a transformer primary is the voltage you would measure across the primary terminals. The induced voltage is the electromotive force induced by the changing magnetic flux in the core of the transformer.

The applied voltage is the vector sum of the induced voltage and the resistive voltage drop of the primary. Or put another way, the induced voltage is the applied voltage minus the resistive voltage drop. In many situations, the resistive voltage drop is neglected and the induced voltage is approximated to be the applied voltage.

[Note that the induced voltage has two parts, that part which is associated with "stray" inductance and that part which is associated with mutually coupled inductance with the secondary. Often another simplifying approximation is made and the stray inductance is ignored. However, precisely, it is only the induced voltage associated with mutually coupled inductance that is reflected in the secondary according to the turns ratio. (And this again does not fully appear on the terminals of the secondary, because there is resistance and stray inductance on the secondary as well.)]