Coupled Inductor measurements

[drumhellerw]

I have a coupled inductor that I have been measuring in the lab with a setup as shown in the attached picture. I apply a DC constant current through one winding and then measure the inductance of the second winding using a standard LCR meter.

I guess I expected the inductance measured on the LCR meter to increase initially with increased DC current in the first winding since the flux would 'add' into the second winding.

What I found however was that at some point, the inductance "crashed" when the enough DC current was passed through the first winding. It was just interesting to me that the flux would show up when it started to saturate but did not cause an increase in inductance with the added DC flux.

Is it true that for a coupled inductor that only AC flux is coupled to the other second winding?
Why then does the DC flux cause saturation to show in the second winding?

 

[FvM]

Core is magnetized by the sum of ampere turns. But why do you assume that inductance measurement is affected by DC field?
inductance = induced voltage/rate of current change

 

[KlausST]

I apply a DC constant current through one winding and then measure the inductance of the second winding using a standard LCR meter.

Inductance is an AC measure. DC has nothing to do with it.
Unless you want to measure core saturation behaviour.

The inductance surely does not increase with DC current, I rather expect the opposite, because of core saturation / unlinearities.

Klaus

 

[drumhellerw]

Inductance is an AC measure. DC has nothing to do with it.
Unless you want to measure core saturation behaviour.

The inductance surely does not increase with DC current, I rather expect the opposite, because of core saturation / unlinearities.

Klaus

Inductance = Webers / Amp

If there was additional Webers in the magnetic circuit then I presumed that the inductance would also increase.

 

[Easy peasy]

you are conflating a DC case with an AC case - for some powdered iron cores the Ui does indeed go up a little with a small DC bias

however in general - the higher the DC bias the lower the slope of the BH loop ( the Ui. Ur )

until finally the core saturates - and you get the air cored inductance

 

[KlausST]

@drumhellerw:

I never said: " Inductance=Webers/Amp"

Klaus

 

[Easy peasy]

there are some items in post #1 that need addressing

1) " since the flux would 'add' into the second winding. " unfortunately it is changing flux that couples for a secondary side effect.

and

2) " Is it true that for a coupled inductor that only AC flux is coupled to the other second winding?
Why then does the DC flux cause saturation to show in the second winding? "

As for 2) above - it is a general principle that for induction to occur the current in one wdg must be changing ( albeit slowly in some cases )

Once you apply sufficient ampere turns to a core to get near saturation the Ur of the core falls.

kind regards.

 

[BradtheRad]

The magnetic amplifier looks similar to the principles in your setup Small current in one winding affects greater current in the other winding. Increase one to decrease the other. Flux saturation might figure into it somewhere.

A magnetic amplifier is a seldom-used method, though it works. It draws enough curiosity that it's demonstrated in Youtube videos.