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

1. Mar 26, 2012

### Kal_Electri

The inductor is initially an air cored one.When the Iron core is introduced,the resistance is found to increase considerably.Please explain.

2. Mar 26, 2012

### Bob S

Resistance represents power losses; I e., Z(ω) = R + jωL. It is a real component of the inductor impedance.

An iron core can have both eddy current (skin effect) and magnetic loop hysteresis losses.

3. Mar 26, 2012

### eemichael83

Assuming you are referring to Impedance and not Resistance, when the core is introduced, the varying magnetic field of the inductor induces eddy currents in the iron core. These eddy's have their own field which opposes the primary (inductor's) field, therefore increasing the impedance.

4. Mar 26, 2012

### yungman

Why is the resistance which is the real part increase?

5. Mar 26, 2012

### Staff: Mentor

Losses.

6. Mar 26, 2012

### yungman

Like how? When you measure at DC, it is only the resistance of the wire and is constant no matter what. How is the induction of core change the DC resistance?

7. Mar 27, 2012

### Staff: Mentor

DC? DC hasn't been mentioned.

8. Mar 27, 2012

### jim hardy

DC resistance doesn't change.
But if you measure AC impedance and phase angle and do a polar to rectangular calc you get a real component that is different from what a DC ohm-meter would report.

That's because the core losses are energy losses which can only show up as resistance.
And they're only present when excitation is AC.

9. Mar 27, 2012

### davenn

Yes exactly, sounds like the OP is confusing resistance and impedance and said resistance instead of the other

Cheers
Dave

10. Mar 27, 2012

### davenn

DC was inferred as the OP spoke of resistance, not impedance

D

11. Mar 27, 2012

### Staff: Mentor

AC was inferred because the OP spoke of inductor, not solenoid.

12. Mar 27, 2012

### jim hardy

I dont know whether he meant impedance or resistance,, but to notice a core he had to use AC.

It's an interesting experiment. One should try diverse measurements for they always turn up some unexpected jewel of insight.

AC resistance and inductance are both affected by temperature of core which at first is strange,
because there's no temperature term in either L= N$\Phi$/I or $\Phi$=μNIA/$\iota$ength

It's the core's resistivity affecting eddy currents which both cancel flux and absorb energy.
Inductance will go up slightly with core temperature because eddy currents go down..
For Resistance you have competing effects between iron core and copper windings - i dont know which way it will go.

It's most noticeable in un-laminated cores as were used in early PWR control rod position sensors.

old jim

13. Mar 27, 2012

### sophiecentaur

Poor old OP.
I bet he's scuttled off with his tail between his legs. It was his first post, too!

Come back and tell us what you meant, Kal_Electri. You will get an answer once we really know what your question is.

14. Mar 28, 2012

### yungman

But R is the DC value only!!! Or are you implying the skin effect cause the R to go up and this has nothing to do with the inductance. It is pure resistance that increase with frequency!!! That actually makes sense!!! I think I answer my own question!!!

15. Mar 28, 2012

### jim hardy

Yungman - check my thinking

Iron losses in the core come out as heat so must cause an in-phase component of current. Else there'd be no net electrical energy transfer into core. P=VICos(Theta) and theta cant be 90 degrees if there's any watts heating the core.

So they will appear to be another resistance in parallel
which will show up as series in theveniin equivalent

and the DC ohms will differ from the ohms in real component of complex Z.

am i on track?

old jim

16. Mar 28, 2012

### sophiecentaur

Skin effect and core loss are two different sources of additional Resistance. They can both be embarassing and will add to the DC resistance effect or even dominate.

17. Mar 28, 2012

### jim hardy

What Sophie and Bob S said.

18. Mar 28, 2012

### 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.

19. Mar 28, 2012

### yungman

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

20. Mar 28, 2012