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# Primary inductance change with frequency

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#### bowman1710

##### Full Member level 3
I have measured the primary inductance of a low frequency transformer at different frequencies, with 10Khz it is 2.2mH and at 100Khz it is 460uH. It has a resonant of about 800khz

With the following equations

L=n^2*AL
L=(3.2*n^2*u*A)/(10^8*l)

I guess I am right in saying the AL has a direct relationship with the permeability of the core material for the top equation to be correct, the frequencies must be to low for any capacitance to have any effect on the inductance. If I am right in saying so, what is the relationship with AL to the permeability is it?????

AL=(3.2*u*A)/(10^8*l)????

Don't know where you got the second equation and what it's supposed to mean. Why do you think that the transformer has "a resonant of about 800khz"? Parallel or series resonance? How did you determine it?

How did you measure the inductance? The main inductance of a laminated core transformer is strongly voltage dependant. A small signal inductance measurement is mostly useless to determine the magnetizing current in operation.

Don't know where you got the second equation and what it's supposed to mean.

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Why do you think that the transformer has "a resonant of about 800khz"? Parallel or series resonance? How did you determine it?

How did you measure the inductance?

I have a AP300 frequency response analyzer and a LCR meter for a backup, the core I am measuring is a silicon steel core, im yet to find out whether it is laminated or not. Does that other equation not apply then?

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Also why cant you measure laminated cores with an analyzer?

SO YOU wouLD BE CAUSING MUCH EDDY (sorry caps) current at 100khz, I wonder if that is something to do with it, somehow.

This is due to the flux skin effect on the steel core, causing a large drop in permeability.

Ferrite has an insulative binder between metallic particles and thus capable of higher currents.

This is due to the flux skin effect on the steel core, causing a large drop in permeability.

This what I was thinking. do you know if there is a direct relationship between AL and u at all?

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Also, i think the core might be laminated or tape wound, so does that mean that a frequency analyser and LCR wont work for measurements, if so, can someone explain in detail why?

Skin effect has all sorts of effects. As well as nobbling the core permeability it also also changes the Q because of the ski effect on the winding affecting straight resistance.

The first unrealistic assumption is that effective µr of a core is a constant. In reality it's field strength dependent, particularly for laminated steel cores. Measuring at different frequencie with constant voltage results in a variable field strength over frequency.

The measured frequencies are quite high, you have to look at loss factor D to determine if eddy currents or other losses affect the measurement. If D is > 0.1 or even 0.2, inductance values aren't accurate.

bowman1710

Points: 2