Why does the resistance of an inductor increase when an iron core is introduced?

Bob S

The theory of eddy current losses in transformer laminations is given in http://www.elect.mrt.ac.lk/EE201_em_theory.pdf
Page 7 shows that it increases as the square of frequency.

yungman

I have no idea, till yesterday, the DC resistance was the only thing I consider. I am learning.

jim hardy

@ Bob S - what a great link that is.

Interesting - hysteresis is to 1.6 power, same as Steinmetz measured ca 1890.
My old 1901 Sylvanus Thompson book speaks of him in present tense.
http://ieeexplore.ieee.org/Xplore/lo...hDecision=-203


Thanks !

sheshank

To say very simply and plainly... The Iron has drift current (because of earth's temperature) more than air. So, naturally more magnetic flux into it causes more aligned electrons with respect to the magnetic field applied. Magnetic flux is induced into it because inductor has twisted loops (Remember Biot-Sarvat Law which explains about the motion of electrons in a circular loop). Oriented electrons, formed in this manner, are always in a way that they oppose the flow of current through it - This is the basic principle of the inductor. Now as the no of electrons increase the total opposing flux through it increases (Self- induction) and so more opposition force followed by more impedance.

Remarkably when you place a similar coil near the same inductive coil, the same voltage is generated in it, which we call as a transformer. A transformer doesn't work in very low voltages just like an inductor, because the thermally drifted electrons are absent. This is as simple as that. Make it more generalized than specialized.

Hope you get it... :)

Averagesupernova

I would agree that in order for heat to be generated the current would have to be in phase with the voltage. Would we look at this as a parallel resistance or a series one?

jim hardy

It seems to me intuitive to call it parallel with a lossless inductor but that's just my mental laziness tying to keep it simple . Magnetizing and loss currents would simply add at 90deg.

But it could also be considered in series with an inductor of finite inductance

One could make thevenin and norton equivalents for the transformer that would help visualize.

To my tired old brain the polar to rectangular conversion of Z is the doorway to picturing it.
Admittances in parallel add. I think of losses as an admittance that absorbs power. Picturing it that way it's easier for me to arrive at the formulas, but some people are gifted with a different thought path - they can start with the formula and arrive at the picture..

When i can work something both directions is when i begin to feel confident about it.
I had trouble with magnetics because my first course was taught in English units, gilberts et al,
next one in CGS units, after that i picked up SI on my own. With my bad memory for names i still get confused.
Repeat of a post someplace else - Jack M Janicke's book "Magnetic Measurements Handbook" is a great reference for the home experimenter. I built one of his fluxgate magnetometers.

earlofwessex

in that (admittedly simplified) image, I think Rc are the core losses and Rp/Rs are the coil losses (DC resistance ofthe wire). its easy to see that its in parallel when you realise that its caused by the voltage rather than the current, as is the magnetising current (Xm).

not only does the value of Rc vary with frequency but also with amplitude for an iron core, once you start getting close to saturation.

jim hardy

Thanks EOW... that picture works perfectly for me and from it the formulas come naturally.

Rc also varies with temperature because resistivity of iron affects eddy currents.
But in a well laminated core they are small.

One good picture is worth soooo many words !