measuring the power choke inductance confusion

[MaciekP]

Hello,
I bought the LCR hand meter (Escort 133A) and tried to measure my chokes inductance. I used a standard 100Hz frequency and on lundahl LL1638 20H/75mA choke I measured about 11H, Lundahl 10H/150mA choke - about 6H and other 20H/80mA choke has 14H. Thinking that something was wrong with a meter I tried another meter MOTECH 4080 with regulated measuring voltage. Changing form 250mV to 1V I got only minor inductance change. Escort uses 0.6V for masuring. I tried also serial and parallel measuring method - same result.
So, what's going on?
Is it a problem with to low measuring voltage or a problem with large DC air gap, so some DC is needed to "start" a choke?
Maybe a manufacturer is a bit to optimistic.
Anyone had similar experiences?
Maciek

[Sch3mat1c]

Mu changes a lot with amplitude, bias and so forth. Don't worry about 20% or more... it's just a choke after all.

Tim

[anatech]

Hi MaciekP,
The inductance is at a specified DC current. You may also need to compensate your test leads. I use an HP 4263A with Kelvin clips. I do recalibrate the test leads unless I just want a go / no go type reading. Make sure there is no other ferrous metal around when making measurements. (like nails or screws in your wood bench top).
-Chris

[smoking-amp]

Steel laminations have low permeability or Mu at low current levels. L/C meters don't use enough signal to get rated L readings for any but the very smallest chokes. Ferrite cores will read good on an L/C meter because they have high initial permeability.
Nickel/iron laminations also have high initial permeability, the main reason they are so good for output transformers.

To measure chokes, you need a variac, ammeter and volt meter. You will find the L starts low at low currents, then builds up at higher currents below the rated current, then starts to drop off again at higher currents due to iron magnetic saturation. An air gap in the core will tend to minimize this variation, but it is still there.

Don

[raypsi]

The Germans reinvented saturable reactors, switches. The key was to saturate the core with enough DC, and it would lose it's inductance, because the core was taken out of the equation, being it is saturated and can't sustain flux variations. Saturable reactors are just laminated iron core chokes.

An iron core inductor will read higher henries at lower current. At higher currents the core loses its permeability, and inductance goes down when permeability is lowered.

The caveat is swing chokes which use an air gap in the core.

I used a microwave oven power transformer as a choke, on my dmm it read 10 Henries. At low currents say in a preamp 10 henries is still the rating. But draw more current thru the choke and the Henries drop.

[diy-everest]

Hello,

I have a same expérience whith Lundahl chokes and Escort LCR meter !
The reason of is différence : when Lundahl make a mesure he do that at 50Hz 220V !!! it's why the value is the double at 100Hz !

For me it's not just, because we use at 100 Hz

Conclusion what Lundahl say, divise by 2 the value

Bye